SYNTHESIS AND EVALUATION OF A NEW CLASS OF
POLYMERS FOR ASPHALT MODIFICATION
by
GLEN A. CROSSLEY
A thesis submitted to the Department of Chemistry in
conformity with the requirements for the degree of
Master of Science in Engineering
Queen's University
Kingston, Ontario, Canada
September, 1998
Copyright O Glen A. Crossley, 1998
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ABSTRACT
This thesis investigates the synthesis and evaluation of a new class of polymers
for asphait modification. A series of polyisoprene homopolymers were synthesized using
stable fke radical polymerization (SFRP) technology at the Xerox Research Centre of
Canada (XRCC). It was discovered that by using an XRCC proprietary rate enhancing
additive, polyisoprene could be solution polymerized ui ethyl acetate at L45OC to
approximately 40 % yield number average molecular weights up to 50000 daltons (based
on polyçtyrene standards), and polydispersities between 1.3 and 1.7. It was also
demonstrated that molecular weight could be controlled by adjusting the initial monomer
to initiator ratios in the polymerizations, and that polydispersity increased with reaction
tirne.
A chloromethylstyrene mimer was synthesized as a functional initiating adduct,
which \vas subsequently chah extended with isoprene to form a block copolymer with a
single unit of chloromethylstyrene followed by an elastomeric polyisoprene chain. It was
also demonstrated that the labile TEMPO end group on an SFRP polyisoprene couid be
near quantitatively removed by reduction with zinc/acetic acid.
Polyisoprenes of molecular weights of approximately17400 and 40800 daltons
were chain extended with the functional monomers 3-(trimethoxysilyl)propyl
methacrylate (TMSPMA) and 2-(dimethy1amioo)ethyl methacrylate (DMAEMA). These
matenals, dong with the correspondhg polyisoprene homopolymers, were evaluated as
asphalt binder additives to improve moisture resistance and low temperature performance
of asphalt concrete. It was shown that asphalt concrete modified with 3 - 5 wt %
polyisoprene-b-TMSPMA retained up to 90 % of its original tende strength after a
-.
modined TunniclifVRoot moisture conditionhg test, while unmodified and polyisoprene
modified asphalt concrete Iost approximately 50 % of their original tensile strength.
In low temperature thermal stress restraïned specimen tests (TSRST),the silane
modified polymes were shown to decrease the fracture temperature of asphalt concrete
by 7°C when added to the asphalt binder at concentrations as low a s 3
wt. %,
mode of failure changed fkom catastrophic fracture to gradual yielding.
while the
ACKNOWLEGEMENTS
I would fim like to thank my supervisor at Queen's, Dr. Simon Hesp, for his
guidance and support throughout the project. Dr. Charles "Howie" Honepan, my COsupervisor from Xerox Research Centre of Canada, provided a wealth of knowledge in
synthetic chemistry and becarne a trdy good firiend.
At Xerox, Dr. Michael Georges and his New Materiais Design and Synthesis
Group were al1 excellent teachers and very supportive throughout the project. Speciai
th&
go to Mike Georges, Barkev Keoshkerian, Paula MacLeod, Peter Odell, Nancy
Listigovers, Marion Quinlan and Gord Hamer, each of whom went out of their way to
share their knowledge of Stable Free Radical Polyrnerization technology. Their
enthusiasm made my stay at Xerox an unforgettable experience.
In my lab group at Queen's Darren Thom provided invaluable assistance in the
preparation and evaluation of hot mix asphalt. Laurentiu Lazar and Bob Campbell are
thanked for sharing their great senses of humour, and always sound technical advice.
Finally 1 would like to thank my fiends and family for their continuous support
during this time in my life.
TABLE OF CONTENTS
ABSTRACT
1
ACKNOWLEDGEMENTS
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
LIST OF ABBREVIATIONS
LIST OF SYMBOLS
f 11
1.0
2.0
3 .O
NTRODUCTION
1.1
Ove~ew
1.2
References
ASPHALT LITERATtTRE REVIEW
Moishire Damage in Asphalt Pavements
Factors Influencing Stripping
Review of Moishire Sensitivity Tests
2.3.1 Qualitative Moisture Sensitivity Tests
2.3.2 Quantitative Moisture Sensitivity Tests
Antistripping Agents
2.4.1 SilaneCouplingAgents
2.4.2 Polyamines and Amidoamines
2.4.3 Hydrated Lime
2.4.4 Other Antislripping Agents
Polymer Modified Asphalt Pavements
Low Temperature FaiIure of Asphalt Pavements
References
...
iv
vii
S..
v111
xi
a*.
x111
1
4
5
5
6
8
8
9
12
13
15
15
16
17
18
21
STABLE FREE RADICAL POLYMERIZATION LITERATURE
REVIEW
Introduction to Living Polymerizations
Living Ionic and Insertion Polymerization Methods
3.2.1 Living Anionic Polymerization
3.2.2 Living Cationic Polymerization
3.2.3 Group Tramfer Polymerization (GTP)
Living Radical Polymerizations
3.3.1 The Iniferter Process
3.3.2 Atom Tramfer Radical Polymerization
Stable Free Radical Polymerization
3.4.1 Overview
3.4.2 Mechanistic Considerations
3.4.3 Architectural Control
Summas, of Objectives
References
26
26
27
27
28
28
29
30
32
33
33
35
36
37
38
4.0
STABLE FREE RADICAL POLYMERIZATION EXPERIMENTAL
4.1
Materials
4.1.1 Monomers
4.1.2 Initiator, Nitroxide, Rate Enhancing Additive, Solvent
4.2
Homopolymerizations
4.2.1 Isoprene Polymerizations
4.2.2 Styene Polymerizations
4.2.3 Characterization
4.3
Functionalization Reactions
4.3.1 CMoromethylstyrene Unimer
4.3.2 Removal of Nitroxide
4.3 -3 Synthesis of Block Copolymers
4.4
References
5.0
A S P W T EXPERIMENTAL
5.1
Materials
5.1 .1 Asphalt Binder
5.1.2 Minerd Aggregate
5.1.3 Polymer Additives
5.2
Asphalt Concrete Preparation
5.3
Moisture Sensitivity Test
Themals
Stress Restrained Specimen Test
5.4
5-5
References
6.0
STABLE FREE RADICAL POLYMENZATION RESULTS AND
DISCUSSION
6.1
Isoprene Polyrnerizations
6.1.1 Preliminary Studies
6.1.2 Scaled Reactions
6.2
Styrene Polymerizations
6.3
Characterization
6.4
Functionalization Reactions
6.4.1 Chloromethylstyrene Unimer
6.4.2 Removal of Nitroxide
6.4.3 Synthesis of Block Copolymes of Polystyrene
6.4.4 Synthesis of Block Copolyrners of Polyisoprene
6.5
References
7.0
ASPHALT RESULTS AND DISCUSSION
7.1
Moisture Sensitivity
7.2
Low Temperature Performance
7.3
Other Considerations
7.3.1 Polymer Architecture
7.3.2 Cost of Modification
7.4
References
8.0
VITA
SUMMARY AND RECOMMENDATIONS FOR FUTURE WORK
8.1
Summary
8.2
Recommendations for Future Work
101
IO1
102
103
vii
LIST OF TABLES
Table 2.1
Cornparison of Quantitative Moisture Tests
Table 2.2
Common Asphalt Additives and Suppliers
Table 5.1 :
Percentages of Fine and Coarse Aggregates in an HL-3 Mix Design
Table 6.1 :
Summary of Exploratory Isoprene Polymenzations
Table 6.2
Sumrnary of Scaled Isoprene Polymerizations
Tabie 6.3
Formation of Polystyrene-b-TMSPMA
Table 6.4
Average Block Length of Functionalized Polyisoprenes
Table 7.1
Summary of Tende Strengths and Retained Strength Ratios for
Control and Polymer Modified Asphalt Concrete Mixtures
Table 7.2
Summary of TSRST Low Temperature Performance
Table 7.3
Summary of Asphalt Cernent Materials Costs
..-
VU1
LIST OF FIGURES
Figure 2.1
Typical Retained Strength of an Asphalt Mixture after Moistue
Conditioning as a Function of Void Content
Figure 2.2
Stripping Patterns for the Japanese Petroleum Institute Test
Figure 2.3
Formation of a Covalent Bond between an Organofunctional Silane
and a Siliceous Surface
Figure 2.4
Formation of Hydrated Lime fiom Limestone
Figure 2.5
Typical Thermal Restrained Specimen Test (TSRST) Profile for an
Unmodified Asphalt
Figure 3.1
Mechanism of Group Transfer Polymerization
Figure 3.2
Reaction Scheme of the Iaiferter Process
Figure 3.3
A Typical Inferter and Inferter-Temiinated Polymer
Figure 3.4
Mechanism of Atom Transfer Radical Polymerization
Figure 3.5
Mechanism of Stable Free Radical Polymerization of Styrene
Figure 5.1
Particle Size Distribution for Coarse and Fine Aggregates
Figure 5.2
Aggregate Mix Design and MT0 HL-3 Specifications
Figure 5.3
Viscosity of Polyisoprene Homopolymer Modified Binders
Figure 5.4
Viscosity of High MW Polyisoprene-b-TMSPMA Modified Binders
Figure 5.5
Viscosity of Low MW Polyisoprene-b-TMSPMA Modified Binders
Figure 5.6
Viscosity of High MW Polyisoprene-b-DMAEMA Modified Binders
Figure 5.7
Apparatus for Tensile Strength and TSRST Experiments
Figure 6.1
as a Function of Reaction
Number Average Molecular Weight (Mn)
Time for a Nitroxide Mediated Styrene Polymerization
Figure 6.2
as a Function of Conversion and
Number Average Molecular Weight (Mn)
Molar Ratio of Monomer to Initiator for a Nitroxide Mediated Styrene
Polymerization
'
Figure 6.3
H NMR Spectrum of SFRP Polystyrene Showing Characteristic
Resonances of Tempo (T) and Benzoyloxy (B) Endgroups, and Adjacent
Methylene and Methine Protons on the Polystyrene Chain
Figure 6.4
'H N M R Spectrum of SFRP Polyisoprene Showing Characteristic
Resonances of Tempo (T) and Benzoyloxy (B) Endgroups, and Adjacent
Protons on the Polyisoprene Chain
Figure 6.5
'H NMR Spectnim of Chloromethylstyrene Unimer
Figure 6.6
'H NMR Spectrum of Polyisoprene Initiated with Chloromethylstyrene
Unimer (Expansion)
Figure 6.7
'H NMR Expansions of Polyisoprene (a) and Polyisoprene Reacted with
Zinc/Acetic Acid @) in Regions Characteristic of TEMPO and Adjacent
Proton Resonances
Figure 6.8
Structures of 'Functional' Methacxylate Monomers
Figure 6.9
Dual Detection GPC Plot of Polystyrene and Polystyrene Chain Extended
With TMSPMA
Figure 6.10
Estimation of TMSPMA Block Length by 'HNMR Comparing
Polystyrene Arornatic Resonmces with TMSPMA Trimethoxysilyl
Group Resonances
Figure 6.11
'H NMR Spectra of Low Molecular Weight PST and Block Copolymer
Forrned by Extension with TMSPMA (region characterstic of styrene
protons adjacent to TEMPO and Benzoyloxy Group)
Figure 6.12
Formation of DMAEMA Functionalized Polyisoprene
Figure 7.1
Cornparison of Conditioned and Unconditioned Tensile Strengths of
Unmodified and Polyisoprene Homopolymer Modified Asphalt
Concrete Samples
Figure 7.2
Cornparison of Conditioned and Unconditioned Tensile Strengths of
Polyisoprene Block Copolymer Modified Asphalt Concrete Samples
Figure 7.3
Cornparison of Tensile Strength Ratios of Unmodified and Polyisoprene
Homopolymer Modified Asphalt Concrete Samples
Figure 7.4
Cornparison of Tensile Smngth Ratios of Polyisoprene Block Copolymer
Modified Asphalt Concrete Samples
Figure 7.5
Cornparison of Themial Stress Accumulation in Unmodified and
Polyisoprene-b-TMSPMA Modified Asphalt Concrete
Figure 7.6
Stress Accumulation in Silane Modified Asphalt Samples During
Thermal Stress Restrained Specimen Tests
Figure Al
'H NMR Spectrum of Conventional Polyisoprene
Figure A2
Expansions of Cbioromethylstyrene Unimer 'HNMR Spectnim
Figure A3
Determination of Relative TMSPMA Block Length in Low Molecular
Weight Polyisoprene-b-TMSPMA
Figure A4
Detennination of Relative TMSPMA Block Length in High Molecular
Weight Polyisoprene-b-TMSPMA
Figure A5
Determination of Relative DMAEMA Block Length in Low Molecular
Weight Polyisoprene-b-DMAEMA
xi
LIST OF ABBREVIATIONS
AAPT
Association of Asphalt Paving Technologists
AASHTO
Amencan Association of State Highway and Transportation Officiais
ASTM
Amencan Society for Testing and Materials
ATRP
Atom Transfer Radical Polymerization
BPO
Benzoyl Peroxide
CMS
Chloromethylstyrene
DMAEMA
2-(Dimethylamino)ethyl Methacrylate
ECS
EnWonmental Conditioning System
ESR
Electron Spin Resonance
EtOAc
Ethtyl Acetate
EVA
Ethylene Vinyl Acetate
GPC
Gel Permeation Chromatography
GTP
Group Transfer Polymerization
HL-3
Hot Laid Asphalt Mix Design
HMA
Hot Mix Asphalt
JPI
Japanese Petroleum Institute
LVDT
Linearly Variable Displacement Transducer
MeOH
Methanol
MT0
Ministry of Transportation Ontario
MW
Molecular Weight
MWD
Molecular Weight Distribution
NCHRP
National Cooperative Highway Research Program
PD
Polydispersity
PI
Polyisoprene
SB
Styrene Butadiene Block Copolymer
SBR
Styrene Butadiene Rubber
SBS
Styrene Butadiene Styrene Block Copolymer
SFW
Stable Free Radical Polymerization
SHRP
Strategic Highway Research Program
xii
TEMPO
2,î,6,6-tetramethyl- I -piperdinyloxy
TGA
Thermogravimetric Analysis
THI:
Tetrahy drofuran
TMS
Tetrarnethyl Sïiane
TMSPMA
3-(Trimethoxysily[)propy1 Methacrylate
TSR
Tende Strength Ratio
TSRST
Thermal Stress Restrained Specimen Test
XRCC
Xerox Research Centre of Canada
LIST OF SYMBOLS
concentration of monomer, m o n
rate constant for propagation, s-'
concentration of pseudo living c h a h , m o n
ratio of fonvard and reverse rate constants for TEMPO trapping reaction
concentration of TEMPO, m o K
asphalt concrete density from picnometry, &m3
asphalt concrete maximum theoretical density based on specific gravity of
aggregates and asphalt cernent, g/cm3
mass of a surface-dry, saturated asphalt concrete sample, g
mass of a asphalt concrete sample, g
specific volume of water at 2S°C, cm3/g
percent voids in an asphalt concrete sample
average tensile strength of a moisture conditioned set of asphalt concrete
samples, MPa
average tensile strength of a dry set of asphalt concrete samples. MPa
g l a s transition temperature of a mixture, K
glass transition temperature of component 1 and 2, K
mass fractions of components 1 and 2
number average molecular weight, daltons
weight average molecular weight, dahons
1.0 INTRODUCTION
1.1 Overview
Asphdt cernent is the world's highest volume hot melt adhesive, with over 30
million tons being used annually in the United States atone. Its resistance to acids, alkalis
and salts, combined with its waterproofing properties, make it an ideal material for
pavement constniction, and as such it is the binder used in over 90 % of North American
roads'. A typical pavement combines a load-bearing aggregate base-layer, topped with a
surface layer of asphalt concrete that is 10 to 20 cm in thickness. For the purpose of this
thesis, ody this asphalt concrete layer will be addressed.
It is usefùl to first claify some of the termhology used in asphalt engineering.
Asphait cernent, binder or simply asphalt refers to a semisolid, viscoelastic material that
is generally refined fiom crude petroleurn. It is thennoplastic and flows on heating, but is
solid at and below room temperature. Hot m k arphalt (HMA) is a composite material
mixed and laid at elevated temperatures, which combines coarse and fine mineral
aggregates coated and held together by a small amount (usually 4-7 wt. %) of asphalt
cernent Throughout this thesis, the terms asphalt pavement and asphalt concrete are
synonymous with W. MU- Design is the practice of optimizing the quantities and type
of materials used in an asphalt pavement design to give desired engineering properties.
A typical asphalt pavement mix design should meet the following requirements' :
-
1 ) Suflcienî Asphait Cernent for pavement durability
2) Adequate Stability - to endure traffic conditions without permanent deformation
3 ) Suncient Voids - to dlow some compaction with t h e , to prevent flushhg andor
bleeding of asphalt cernent fiom the pavement, and to prevent rutting
4) Maximum Voidî - to limit permeability of air and moisture, which can cause
premature aging and deterioration
5) Suflcient Workability - to enable the matenal to be laid with traditional equipment
and practices
-
6) Proper Aggregate Texture and Hardness to give a skid resisting surface layer under
adverse weather conditions.
The major problem with North America's highway infiastructure is that since
most of these roads were designed in the 1950s and 1960s for much lower t&~c loads
and volumes, the conventional asphalt pavements used in their construction are now
deteriorating faster than ever before3". RecogniPng that conventional pavement design
was no longer resulting in satisfactory roads and highways, the U.S.Govemment invested
US$ 150 million in 1987 to develop performance based specifications for asphait
pavements. This program, called the Strategic Highway Research Prograrn (SHFW),
focused on delivering specifications and test methods for evaluating asphalt binders and
asphalt bindedaggregate mixes. Two of the most problematic areas in pavement
performance were identified as m o i m e susceptibili$ and low temperature performance
of asphalt concrete mixtures6. W l e both of these problems can be partially addressed by
proper selection and design of binderlaggregate cornbinations, new additives are being
developed and specified by asphalt agencies to improve the durability of pavements under
harsh environmental conditions and heavy trafEc loading.
The objective of this thesis is to synthesize and evduate new polymeric materials
that could simdtaneously address the moisture sensitivity and low temperature
performance of typical asphait pavements. While there are some chernicals which can
improve the moisture sensitivity of an asphalt concrete, and certain polymers that can
enhance the low temperature properties of a pavement, there have been few materials
designed to address these issues together.
Recently, Stable Free Radical Polyrnerization (SFRP) techniques have been
developed by Georges et al. at the Xerox Research Centre of Canada (XRCC)', providing
a new means of synthesizing a variety of unique polymers. Unlike other living
polymerization techniques, SFJlP pennits the use of mild reaction conditions and a wide
range of monomers. The result is that end-functionalized and block copolymers can be
made more easily than with previous technologies. This discovery has expanded the
potential for precise polymenc design in an economically feasible mamer, and therefore
the ability to make new classes of materials for pavement modification.
4
1.2 References
The Asphalt Handbook, Manual Series No. 4 (MS-4), 1989 ed., The Asphalt Institute,
Lexington, KY, 1988.
Mk Design Methoh for Asphalf Concrefe and Other Hot-Ma Types,Manuai Series
No. 2 (MS-2), 6th ed., The Asphah Iastitute, Lexington, KY, 1988.
Van Voorst, B. "America'sAsphalt Bungle", Tirne, May 25, 1992.
Moon, P. "Drive at your own risk. Highway network going downhi11, threatening
lives", The Globe and Mail, No. 44934, Thompson Newspapen, Toronto, Dec. 18,
1993.
Scholz, T., Terrel, R., Al-Joaib, A., Bea, J. "Water Sensitivity: Binder Validation",
Strategic Highway Resemch Program Report: SHRP-A-402, National Research
Council, Washington, D.C.,1994.
Jung, D., Vinson, T. "Low-Temperature Cracking: Binder Validation", Strategk
Highway Research Program Report: SHRP-A-399, National Research Council,
Washington, D.C., 1994.
Georges, M., Veregin, R-, Kannier, P., Hamer, G. "Narrow Molecular Weight Resins
by a Free-Radical Polymerization Process", Macromofecules, 1993,26,2987.
2.0 ASPHALT LITERATURE REVIEW
2.1 Moisture Damage in Asphalt Pavements
Stripping of asphalt cernent fiom mineral aggregate has long been recognized as
one of the most serious causes of pavement failure and hundreds of studies and technical
papers have been devoted to causes and the means of preventing this problem. It is well
known that certain aggregates are highly resistant to stripping, however, the cost of
movhg these materials long distances is prohibitively expensive and contractors are
forced to design pavements using local aggregate. Consequently, there is the need for
effective methods of evaluating the moisture sensitivity of asphalt mixtures, as well as
additives to improve the adhesion between aggregate and asphalt cernent.
Two general phenomena are responsible for moisture damage of asphalt
pavements. Stripping or loss of adhesion involves disruption of the bond between
aggregate and asphalt cernent hthe presence of water. The Asphalt Institute describes
six types of stripping1including emulsïfïcation, detachment, displacement, film rupture,
pore pressure and hydrauiic scouring. Though each of these occurs by a different
mechanism, al1 are a direct consequence of water entering the asphalt pavement and
rnigrating to the aggregate/asphalt interface. Loss of cohesion is a result of detrimental
interactions between water and the binder itself, causing an overall loss in integrity of the
asphalt matrix. WMe the majonty of test methods are not capable of distinguishing
between damage caused by stripping and damage caused by l o s of cohesion, most of the
literature addresses stripping as the primary source of moisture damage in asphait
concrete.
2.2 Factors Inflnencing Stripping
The factors influencing the tendency for an asphalt pavement to strip include the
chernical and physical properties of the aggregate and asphalt, and the overall design
characteristics of the pavement itself.
Since al1 aggregates are considered hydrophilic and asphalt cernent is essentiaily
hydrophobie, water will preferentially wet the aggregate surface if it reaches the
interface. The chemical nature of the mineral aggregate is generally considered the most
critical factor in establishg a good asphalt/aggregate bond. Carbonic aggregates such as
Limestone are basic and tend to bind better with the polar constituents in the asphalt
cernent than acidic siliceous aggregates (Le. sand or granite). Aggregate surfaces must be
dry and clean of dust or clay particles to ensure proper adhesion. Surface texture and
porosity of the mineral aggregate also play a role in enhancing mechanical bonding at the
interface.
Several design critena need also be addressed when selecting a mix design for an
asphalt pavement. Obviously the subgrade shouid provide adequate drainage to allow
water to fieely drain fiom the system. Freshly crushed aggregate tends to have a higher
surface charge than aged materials, rendering it less wettable by asphalt cement, so
stockpiled aggregates should be used whenever possible. Increasing the amount of
asphalt cernent in the mix generally irnproves the coating of the aggregate and reduces
stripping, but may have a negative effect on other properties of the pavement leading to
problems such as rutting and shoving.
n i e percentage of air voids in asphalt concrete is of critical importance in
pavement performance. Terrel and Al-Swailmi introduced the concept of "Pessimurn
Voids" to help explain moisture damage2. At low voids content (< 5 vol. %) asphalt
concrete is essentially impermeable to water and resists moisture damage. At higher
voids contents (> 13 vol. %) the pavement becornes fÎee draining and water easily leaves
the system before moving to the asphalt/aggregate interface. The problem is that most
asphalt pavements in North America are constructed in the intermediate or pessimum
range where moisture damage is maximized (çee Figure 2.1).
'" I
*
impermeable
O
5
Pessimum Voids
1O
Free Draining
t5
20
Air Voids (%)
Figure 2.1:
Typical Retained Strength of an Asphait Mixture &er Moisture
Conditioning as a Function of Void Content ( d e r ref. 2)
Tnough there are numerous other factors influencing the rnoisture sensitivity of
pavements, there is general consensus that the problem can be controlled by proper
pavement design, including the identification of susceptible asphalt/aggregate
combinations and their treatment with effective antistripping additives.
2.3 Review of Moisture Sensitivity Tests
A nurnber of tests have been developed to predict the moishm sensitivity of
asphalt mixtures, to evaluate the effectiveness of &stripping agents, and to estimate the
service H e of a road. While several of these methods have been adopted by agencies
such as the Amencan Society for Testing and Materials (ASTM) and the Amencan
Association of State and Highway Officiais (AASHTO), there is still an ongoing debate
over the ability of each of these tests to accurately predict the field performance of an
actud
The tests can generally be separated into two categories: those that
provide quaIitative, and those that give quantitative assessments of moimire damage.
2.3.1 Qualitative Moisture Sensitivity Tests
The qualitative tests most often involve coating a 'standard' aggregate with
asphalt cement, static immersion of the loose aggregate in water, followed by a visual
evaluation of the degree of stripping. Usually a passlfail criterion is based on an
estimation of the percentage coating retained after immersion. Some of the more
common qualitative tests include:
Boil Tests (Texas Boil Test, ASTM ~3625')- An uncompacted asphalt mixture is
irnrnersed in boiling water for 1 minute, d e r which the amount of retained coating is
visually classified as "above 95 %" or "below 95 %". Some agencies speci& a longer, 10
minute boiling period.
Static lmmersion Tests (ASTM
D 1 6 6 4 ~AASHTO TI82')
- Aggregates are
screened between 3/8" and 114" sieves and combined with 5.5 wt. % asphalt cernent. The
coated aggregate is cured for 2 hours at 60°C, then immersed in room temperature water
9
(25OC) for 16-18 hours. A visual estimation is made of the retained coating and a 95 %
passlfail criterion is applied.
Jopanese Pe~oleuminstitutte (JP9 i
est'' - Similar to the static immersion test, a
standard aggregate ( 10- 13 mm) is selected and coated with 5.5 wt. % asphah cernent.
The immersion step is 30 minutes at 80°C. A more descriptive assessrnent of stripping is
made by classification of the aggregate coating into one of the five categories outlined
below :
NO.
o f Pottern
No stripping
Stripping on
Stripping on
widens,
Dot stripping
occufs. Edges edges and smatl edges
Conthuous
stripping
con be Light
occurs.
brOWh
dot stripping
occur.
occurs,
ASphdt
remains
in dots.
Appearance
Figure 2.2:
Stripping Pattern for the Iapanese Petroleum lnstitute Test (der ref. 10)
While these qualitative tests provide a quick means of i d e n t i m g coarse
aggregate - asphalt combinations that are susceptible to stripping, they not are capable of
identiwg stripping in the fme aggregate, which can lead to more catastrophic failure6.
A second concem is that the qualitative tests are not base on any sort of pavement
performance property, and therefore should not be used as a sole indicator for potential
moisture damage.
2.3.2 Quantitative Moisture Sensitivity Tests
Quantitative moisture sensitivity tests provide a much more reliable prediction of
moisture damage. Most of the methods involve measuring a ratio of properties between
conditioned and unconditioned compacted specimem. These may be tensile strength,
modulus, or number of cycles to failure (under some sort of stress). A minimum value of
the ratio is usually specified. The most commonly used tests are summarized below:
Table 2.1 :
Comparison of Quantitative Moisture Tests
Test Method
Property Measiwd
Immersion
Compression
ASTM D 1075
AASHTO TI 65"
Ratio of unconfïned
compressive strength
(ASTM D 1074) of
conditioned and
unconditioned samples
TunniclzffRoot
Ratio of indirect tensile
Method
strength (ASTM
ASTM ~ 4 8 6 7 ' ~ D4123) of conditioned
and unconditioned
samples
Lottman Test
Comparison of indirect
NCHRP 192 I4
tensile strength and/or
modulus of control
sarnples with mildly
and severely
conditioned samples
''
Modzped Loftman
Test
AASHTO T-283l5
Immersion
Marshall Test
M T 0 LS-283l6
Kennedy Texas
Freeze Thuw
Pedestal Tesr
AAPT 1982"
Ratio of indirect tensile
strength of conditioned
and unconditioned
samples
Ratio of Marshall
stability of conditioned
and unconditioned
samples
Number of fieeze-thaw
cycles to failure for a
set of specîmens on a
stress wdestal
Conditioning
Warm water immersion of samples
for 4 days at 49OC or 24 hours at
60°C. Test at 25°C
Vacuum saturation (25°C)of air
voids in specimens with water to 5580 vol. %, followed by 24 hour soak
at 60°C. Test at 25°C
Control subset - 5 hour soak at test
temp (24OC)
Miid - 30 minute vacuum saturation
at 660 mm of mercury, 3 hour soak
at 24°C.
Severe - saturation as above, fkeeze
at - 18°C for 16 hours, followed by
24 how soak at 60°C.Test at 24°C
Vacuum saturation at 25°C to 5580 % saturation, fieeze at - 18°C for
16 hours, followed by 24 hour soak
at 60°C. Test at 25°C
Vacuum saturation at 25*C,
followed by 24 hour soak at 60°C.
Test at 25°C
Freeze sample at - 12OC for 15 hours,
thaw 45 minutes at 24"C,then cure 9
hours at 49OC. Repeat cycle until
faiiure.
The Immersion Compression method was the nrst standardized retained strength
test but has traditionally had poor correlation with actual pavement performance, possibly
due to the mild nature of the moisture conditioning6. The Lotman Method proposed
measuring indirect tensile strength after vacuum saturation, a fieeze thaw cycle and warm
water immersion. T d c l i f f and Root found the fieeze thaw cycle too severe, but
Mproved on the Lottman test by specifj4ng limits on the degree of vacuum saturation
(55-80 %) in the conditioning phase of their method", and have had excellent success in
correlating lab results with test sections in the field'92o. The Modifed Loftmon
Procedure is similar to the TunniclZfRoot procedure, but adds at l e s t one fkeeze thaw
cycle to the conditioning phase, as was proposed in the original Lottman Method. B o t .
the Modified Lottman and T u n n i c l i ~ o omethods
t
are gaining widespread acceptance,
with agencies speciQing passing tensile strength ratios (TSR)of 70-80 %631U. In
Ontario the Immersion Marshall test is specified by the Ministry of Transportation, likely
due to their experience with this test ushg a broad variety of Ontario aggregates.
Though very popular in some areas of the southern United States, the Texas
Freeze Thaw Pedestal Test is cnticized for using only a smdl band of aggregate in
preparing the specimens. Several researchers question its ability to accurately predict
pavement performance, and are also reluctant to use the test since due to the length of
time needed to acquire results (one freeze-thaw cycle per day)?
Recently SHRP has proposed the Environmental Conditioning System (ECS)to
includes a complex environmental chamber
evaluate moisture d a ~ n a ~The
e ~ apparatus
~.
capable of forcing water through a specimen and measuring pemeability and resilient
modulus during a senes of warrn water immersions, and fieeze-thaw cycles. The system
is expensive, and is yet to prove to be a better simulation of field conditions that the
A final class of tests involves measuring rutting resistance
simpler tensile rnetho~ls~~?
of compacted, moisture-conditioned, specirnens using a wheel-tracking device. Though
the preliminary results seem promising2', there is not yet suffïcient data to warrant the
use of this method over established procedures.
With so many test methods available to the asphalt engineer, the selection of a
suitable test method becomes a difficult task. However, regardless of which method is
selected it is critical that the volume of air voids in the samples is consistent, and that
results from different test procedures are compared with caution.
2.4 Antistripping Agents
Most commercial antistripping agents are surfactants designed to be soluble when
mixed in asphalt cernent, but with a functionality that drives them towards the
asphaltlaggregate interface. The majority are liquids that are generally added at 0.2 to 1.O
W. % on the binder.
Lime, portland cernent, and a few others, are exceptions that are
used as aggregate surface treatments in concentrations of 1.O to 2.0 wt. % based on the
total coarse aggregate.
Since the effectiveness of an antistripping agent is very asphalt/aggregate specific,
it is impossible to quantitatively compare the effectiveness of the various antistripping
additives fiom studies in the literature. It is also very difficult to directly compare the
results of studies using different test methods. For example, the qualitative stripping tests
tend to favour liquid antistripping agents, while the tensile methods favour lime2'.
However, it is still useful to review some of the most promising chernicals in current use.
2.4.1 Silane Coupling Agents
Coupling agents are materials that Unprove the adhesive bond between two
dissimilar surfaces. Organofunctiod silanes have long been used to facilitate the
adhesion between polymers and inorganic materials and are used extensively in fiberglass
reinforced composites to strengthen the polymer/fibre interface. Trialkoxysilanes, for
example, hydrolize rapidly in the presence of water to give their corresponding silanols.
In a second but slower reaction the silanol functionalities can condense directiy with
silanol groups on a siliceous surface29:
Organofunctional
silanol
Organofunctional
silane
Hydroxylated
surface
Figure 2.3:
Organofunctional silanol
covalent1y bound to surface
Formation of a Covalent Bond between an Organofunctional Silane and a
Siliceous Surface
Since asphalt concrete can be considered a nonpolar, organic ma& (the asphalt
cement) that often contains large arnounts of silica-based mineral fiiler (sand, granite
etc.), silanes seem to be a logicd choice as antistripping agents. More than 40 years ago,
Sanderson evaluated the effectiveness methylchlorosilanes against stripping with a wide
range of aggregates with excellent results for granite and other silica based miner al^^^.
More recently, DiVito et al. tested an amino-functional silane as an aggregate
pretreatment with promishg results on two aggregate sources3'. Graf also showed that a
similar amino-silane, added to the asphalt cernent at as Iow a concentration as 0.05 wt. %,
doubied the number of cycles to failure in the Texas fieeze thaw pedestal ted2. Silanes
have also been demonstrated to increase the resistance of asphalt pavements to attack by
soi1 bactena, which are often identified in sections of severely stripped pavement33.
Aggregate pre-coating with silanes gave encouraghg resuits in the Iapanese
Petroleum Institute Test conducted by researchers in ~ a ~ a n ' ' Motivated
.
by these results,
an extensive study in Canada by Stolie et al. c o h e d the P
Itest results, but also
detemiined that silanes can reduce the dry strength of asphalt mixtures, and recommends
against their use as antistripping agentsJ4.
In a recent study on the low temperature failure of particdate filled asphalt
binders, Rodnguez et al. demonstrated that interfacial modification of glas spheres with
amino silanes, followed by in situ reaction with rnaleic anhydride functionalized
polybutadiene, can increase the fi-acturetoughness of an asphalt binder well beyond the
predicted improvement for unmodified glass Hier using Evans' Theory?
From the work of these researchers, it appears that low molecular weight silanes
can imprrwe interfaciai adhesion between asphalt and aggregate, but the effect they can
have on softening the asphalt matrix, resulting in a reduction in tensile strength, must be
addressed.
2.4.2 Polyamines and Amidoamines
The most commody used iiquid antistripping agents are amine derivatives, whose
basic hctionalities are thought to interact with acidic groups on the surface of
aggregates. Though there are literally himdreds of these on the market, the resuits of
studies on their effectiveness ranges fiom excellent to poor, as some have been shown to
act as emulsifjmg agents which accelerate the stripping process'.
2.4.3 Eydrated Lime
The use of several forms of chernical lime has been successful in preventing
smpping, especially when used as a pretreatment for coarse aggregates. Quicklirne, for
example, is produced by the hi& temperature calcination (buming) of lirnestone, which
can then be M e r reacted with water to form hydrated lime:
l irriestone
Figure 2.4:
quicklime
hydrated lime
Formation of Hydrated Lime fiom Limestone
The positive effects of hydrated lime are thought to be a due to the formation of
calcium salts on the aggregate surface, which are more compatible with asphdt cernent
than the unmodified aggregate. Most often hydrated lime is added dry as 1-2 wt. % of
the aggregate fiaction, but is also sometimes coated onto coarse aggregates fiom a slurry
which is then dned to remove the moisture. Though these methods of addition tend to
give promising results, lime's effectiveness when added directly to the binder remains
somewhat questionable36.
WhiIe primarily used to reduce moisture damage, hydrated Lime has also been
shown to reduce the age-hardening of some asphalt pavements37. Recovered asphalt
cernent f?om pavements modified with 1 W. % hydrated lime tends to have a lower
viscosity and higher penetration than corresponding unrnodifïed pavements, which
suggests that the sofier binder should be less susceptible to low temperature fdure.
Though hydrated lime is a relatively economical matenal, the costs associated
with drying sluny-coated aggregate c m be prohibitiveiy expensive3*. Safety concerns
have also been raised after workers involved in the addition of dry lime (a caustic
material) to aggregate streams have complained of moderate skin and respiratory
irritation39.40 . Consequently, there remains a strong incentive for the development of
effective new antistripping agents that can be added directly to the asphalt binder, and
which do not generate dust or toxic odours.
2.4.4 Other Antisîripping Agents
Iron napthanate has also been studied as an aggregate pretreatrnent and as such it
has given excellent stripping resistance. It is thought that üke chernical lime, the success
is due to the formation of water insoluble salts between positive iron ions and negative
ionic groups at the aggregate smface4'.
In a very new approach to controlling stripping, styrene butadiene rubber (SBR)
latex has been used as a spray coating for coarse aggregates42. Though the coating
procedure may slow production rates, the advantage of using higher molecular weight
polymers is that they produce Little odour at high temperature and that they are not
corrosive to equipment or workea.
2.5 Polymer Modified Asphalt Pavements
Polymen are known to increase the nuting resistance of asphalt pavements by
increasing the high temperature e e s s of the binder, and have more recently been
dernonstrated to improve thermal performance of pavements at low temperaturef3.
The cntical factors that determine the effect of polymer modification are the
physical properties of the polymer, and its compatibility with the asphalt binder.
Comrnon polymer additives can be loosely classified into two categories. Plastumers are
materials that increase the stifkess of the binder and therefore its resistance to plastic
deformation at hi& temperatures. Elustomers, as their name suggests, tend to improve
the elasticity of asphalt binden, and as such they may increase the failure strain of asphalt
concrete at low temperatures. Table 2.2 classifies some common polymenc asphalt
additives.
Table 2.2:
Type
Plastomers
Elastomers
Common Asphalt Additives and Suppliers
Polymer
EVA
Polyethylene
SB, SBS
SBR Latex
Natural Rubber Latex
Manufacturer
Dupont, Exxon
Dupont, Eastman Chernical
Shell, Fina, Koch Materials
BASF
Koch Materials
Styrene-butadiene di-block (SB) and tri-block (SBS) copolymen are by far the
highest volume polymers used in modifïed asphalt with 43 million pounds being used in
North Amenca, 124 &on
pounds in Europe, and 15 million pounds in Japan in 1994*.
However, SBS and SB may do littie to improve the moisture sensitivity of asphalt
pavements45,creating the incentive to h d new increasingly effective asphalt modifiers.
Since hot mix asphalt (HMA) is a relatively inexpensive material, usually costing
CDN $40-55 per tome, the price associated with polymer modification is critical.
Ponniah and Kemepohl have demonstrated through life cycle cost analysis that the use of
polymer additives c m be cost efficient if these materials extend the pavement sewice life
by 2 to 3 years, without increasing the cost of the asphalt cernent by more than 100
percent46.
2.6 Low Temperature Failure of Asphalt Pavements
Asphait pavements fail at low temperatures due to a combination of thermal and
fatigue induced stresses. As an asphalt pavement cools, the individual layers within the
pavement contract at m e r i n g rates, resulting in a build up of internai stresses and
fiction between the surface and base layee in the pavement. When these stresses exceed
the tende strength of the pavement, the structure cracks at reguiar intervals transverse to
the trafic direction to relieve the stress.
Severai factors can affect the low temperature performance of asphait concrete
mixtures including type and amount of asphalt cement, aggregate properties and
gradation, air void content, pavement age, and rate of cooling. Since the overall
pavement performance will depend on a combination of these factors, several standard
tests have been developed to relate low temperature performance to bdamental asphait
and asphalt concrete properties. The most widely accepted of these methods is the
19
thermal stress restrained specimen test (TSRST)~'.In a typical TSRST,a cylindrical or
rectangdar asphalt briquette is enclosed in an environmentai chamber and maintained at
constant strah. The chamber temperature is then cooled at a constant rate until the
accurnulated thermal stresses in the asphalt briquette hctures the sample. Usually
tensile stress is plotted vs. temperature, as is iIiusaited in Figure 2.5:
O
-5
-10
-15
-20
-25
-3 O
-35
Temperature ( O C )
Figure 2.5:
Typical Thermal Restrained Specimen Test (TSRST) Profile for an
Unrnodified Asphalt (after ref. 48)
From Figure 2.5 it can be seen that thermal stresses initially accumulate slowly
due to stress relaxation withh the asphalt briquette. As the sample is cooled M e r , the
ability of the asphalt to relax stress through viscous flow is decreased until the transition
temperature is reached, beyond which the sample can no longer relax by this mechanism,
and the stress increases linearly with temperature until failure. Due to the success of the
TSRST in predicting low temperature properties of asphalt concrete mixtures49*50*51
its
use, combined with a moisture sensitivity test, is often recommended as a standard part of
modem asphdt mix design.
21
2.7 References
Cause and Prevention of Stripping in Asphalt Pavements,Educational Series No. 10
(ES- i O), 2nd ed., The Asphalt Institute, College Park, MY, 1988.
Terrel, R-, Al-Swailmi, S. "Role of Pessimum Voids Concept in Understanding
Moisture Damage to Asphalt Concrete Mixtures", Tramportation Research Record,
1993,1386,31.
Shatnawi, S., Van Kirk, I. "Premature Asphalt Concrete Pavement Distress Caused
by Moisture-Induced Damage", Tramportation Research Record, 1993, 1417, 168.
Coplantz, J., Newcomb, D. "Water Sensitivity Test Methods for Asphalt Concrete
Mumires: A Laboratory Cornparisson", Tramportation Research Record, 1988, 1171,
44.
Parker, F., Gharaybeh, F. "Evaluation of Tests to Assess Stripping Potential of
Asphalt Concrete Mixtures", Tranportation Research Record, 1988, 1171, 18.
Hicks, G. "Moisture Damage in Asphalt Concrete", NCHRP Synthesis of f l g h a y
Practice 175, Tramportation Research Board, National Research Council,
Washington, DC, 1991.
"Standard Test Method for Effect of Water on Bituninous-Coated Aggregate - Quick
Field Test",ASTM D 3625-83,1988 Annual Book of ASTM Standards, vol. 04.03,
Amencan Society for Testing and Materials, Easton, MD, 1988,441.
"Standard Test Method for Coating and Stripping of Bitumen-Aggregate Mixtures",
ASTM D 1664-80,1988 Annual Book ofASTM Standards, vol. 04.03, Amencan
Society for Testhg and Materials, Easton, MD, 1988, 2 17.
"Standard Test Method for Coating and Stripping of Bitumen-Aggregate Mixtures",
AASHTO T 182-84, Standard Specifcations for Tranrportation Materials and
Methoh of Sampling: P m II Tests, 17" ed., American Association of State Highway
and 1'ransportation Officials, Washington, DC, 1995, 349.
10. Anzaki, Y ., Ikeda, T. "Improvement of Asphalt Pavement Durability by Surface
Treatrnent of Coarse Aggregates", Proceedings, Paving in Cold Areas Mini
Workshop 3 (PICA-3), 1987,2,622.
11. "Standard Test Method for Effect of Water on Cohesion of Compacted Bituminous
MDmires", ASTM D 1075-81,1988 Annual Book of ASTM S t a n d a d , vol. 04.03,
American Society for Testing and Matenals, Easton, MD, 1988, 181.
12. "Standard Test Method for Effect of Water on Cohesion of Compacted Bituminous
Mixtures",AASHTO T 165-9 1, Standard Speczjicattiomfor Tramportation Materials
and Methods of Sampling: P m II Tests, 17" ed., Amencan Association of State
Highway and Transportation Officials, Washington, DC, 1995,3 15.
13. "Standard Test Method for Effect of Moisture on Asphalt-Concrete Paving Mixtures",
ASTM D 4867,1988 Anmuil Book of ASTM Standards, vol. 04.03, Amencan Society
for Testing and Materials, Easton, MD, 1988, 568.
14. Lottrnan, R "Predicting Moisture-Induced Damage to Asphaltic Concretel*,NCHRP
Report 192, Transportation Research Board, National Research Council, Washington,
DC, 1978.
15. "Resistance of Compacted Bituminous Mixture to Moisture Induced Damage",
AASHTO T 283-89, Standard Specificatiomfor Tmportution Materials and
Methods of Sampling: Part LI Tests, 17med., Amencan Association of State Highway
and Transportation Officials, Washington, DC, 1995,751.
16. "Method of Test for Resistance to Stripping of Asphalt Cernent in Bihiminous
Mixhire by Immersion Marshall", MT0 LS-283, Laborutory Testing Manual, vol. 1,
Ontario Ministry of Transportation, Downsview, ON, 1987.
17. Kennedy, T., Roberts, F., Lee, K. "Evaluation of Moisture Susceptibility of AsphaIt
Mixtures Using the Texas Freeze-Thaw Pedestal Test", Proceedings of the
Associarion of Asphalt Paving Technologkts, 1982,s 1,327.
18. Tunnicliff, D., Root, R. "Use of Antistripping Additives in Asphaltic Concrete
Mixtures - Laboratory Phase", NCHRP Report 2 71, Transportation Research Board,
National Research Council, Washington, DC, 1984.
19. Tunnicliff, D., Root, R. "Use of Antistripping Additives in Asphaltic Concrete
Mixtures - Field Evaluation", NCHRP Report 373, Transportation Research Board,
National Research Council, Washington, DC, 1995.
20. Tunnicliff, D. "Performance of Antistripping Additives",Proceedings of the
Association of Asphalt Paving Technologists, 1997,66,344.
2 1. Seddik, H., and Emery, J. "Moisture Damage of Asphalt Pavements", Proceedings.
Paving in Cold Areas Mini Workrhop 6 (PICA -6), L 996,2,3 8.
22. Kandhal, P. "Field and Laboratory Investigation of Stripping in Asphalt Pavements:
State of the Art Report", Transportation Research Record, 1994, 1457,36.
23. Parker, F., Parker, F. "Evaluation of Boiling and Stress Pedestal Tests for Assessing
Stripping Potential of Alabama Asphalt Concrete Mixtures", Transportation Research
Record, 1986, 1096,90.
24. Schoiz, T.; Terrel, R;Al-Joaib, A.; Bea, J. "Water Sensitivity: Binder Validation ",
Sîrategic Highway Reseorrh Program Report: SHRP-A-402, Nationai Research
Council, Washington, D.C.,1994.
25. Terrel, R, Scholz, T., Al-Joaib A., AI-Swailmi, S. Validation of Binder Roperties
Used to Predict Water Sensitivity of Asphait Mixtures", Proceedings of the
Association of Asphalt Paving Technologists, 1993,62, 172.
26. Aschenbrener, T., McGennis, R., Terrel, R. "Comparison of Several Moisture
Sensitivity Tests To Pavements of Known Field Performance", Proceedings of the
Association of Asphoit Paving Technologists, 1995,64, 163.
27. Aschenbrener, T. "Evaluation of Hamburg Wheel-Tracking Device to Predict
Moistwe Darnage in Hot Mix Asphalt", Transportation Research Record, 1995,
1492,193.
28. Kennedy, T., Ping, V. "An Evaluation of Effectiveness of Antistrippuig Additives in
Protecting Asphdt Mixtures fiom Moisture Damage", Proceedings of the Association
ofAsphalt Paving Technologists, 1991,60,230.
29. Plueddemann, E. Silane Coupling Agents, 2nded., Plenum Press, New York, NY,
1991.
3 0. Sandenon, F. "Methylchlorosilanesas Antistripping Agents",Highway Research
Board Procedings, 1952, 3 1,288.
3 1. DiVito, J., Morris, G. "Silane Pretreatment of Mineral Aggregate to Prevent Stripping
in Flexible Pavements", Transportation Research Record, 1982,843, 104.
32. Graf, P. "Factors Affecthg Moishue Susceptability of Asphalt Concrete Mixes",
Proceedings of the Association of Asphalt Paving Technologists, L 986, 55, 175.
33. Brown, L., Pabst, G., Marcev, J. "The Contribution of Microorganisrns to Stripping
and the Ability of an Organofunctional Silane to Prevent Stripping", Proceedings of
the Association of Asphdt Paving TechnoZogists, 1990, 59,36 1.
34. Stoiie, D. "Silane Coupling Agents to Reduce Moisture Susceptibility of Asphait
Concrete", M T 0 Report: P A V-90-04, Ontario Ministry of Transportation,
Dowrisview, ON, 1990.
35. Rodriguez, M., Morrison, G., v d o o n , J., Hesp, S. "Low-Temperature Failure in
Particdate-Filled Asphalt Binders and Asphdt Concrete MUmires", Proceedings of
the Association of AsphaIt Pming Technologirts, 1W6,65, 159.
36. Stroup-Gardiner, M., Epps, J. "Four Variables m a t Affect the Performance of Lime
in Asphalt-Aggregate Mixtures",Tramportution Research Record, 1987, 1115, 12.
37. Petenen, J. "Lime-Treated Pavements Offer Increased Durability", Roua5 and
Bridges, Jan. 1988,85.
38. Hanson, D., Graves, R, Brown, E. "Laboratory Evduation of the Addition of Lime
Treated Sand to Hot Mix Asphalt", Trmporfation Research Record, 1994, 1469,35.
39. 'Evaiuation of Bituminous Test Sections; Volume 2: Surface Treatments", MT0
Report: M-128, Ontario Ministry of Transportation, Dow-ew,
ON, 1988.
40. DUnning, R., Schulz, G., Gawron,, W. "Control of Stripping with Polymer Treatment
of Aggregates", Proceedings of the Association of Asphalt Paving Technologists,
1993,62,223.
41. JO, M., Tarrer, A., Jeon, Y., Park, S., Yoon, H. "Investigation of the Effect of
Aggregate Pretreatment with Antistripping Agents of the Asphalt-Aggregate Bond",
Pe~oleumScience and Technology, 1997, 15,245.
Williams, T., Milmis, F., Schulz, J. "Aggregate Coating Reduces Asphdt Repair",
Better Roadr, Jan. 1998, 19.
Rodriguez, M. The Efects of Particulate Fillers on the Low Temperature Fracture
Behavior of Asphalt Binders and Hot MU.Asphah, M.Sc. Thesis, Queen's
University, Kingston, ON, 1995.
"SB Copolymers - Prime Candidates for Replacement", Rubber World, vol. 1 12,
1994.
45. Davidson, J-, Ernyes, J., "Stripping: A Laboratory Study", Proceedings of the
Canadian Technicul Asphalt Association, 1993,3 8, 77.
46. Ponniah, J., Kennepohl, G. "Polymer-Modified Asphalt Pavements in Ontario:
Performance and Cost Effectiveness", Transportation Research Record, 1996, 1545,
150.
47. Jung, D., Vinson, T. "Low-Temperature Cracking: Binder Validation", Strategic
Highwuy Research Program Report: SHRP-A-399, National Research Corncil,
Washington, D.C., 1994.
48. Jung, D., Vinson, T. "Low-Temperature Cracking: Test Selection", Sirategic
Highway Research Program Report: SHRP-A-400, National Research Council,
Washington, D.C., 1994.
49. Kanema, H., Vinson, T., Zeng, H. 'Zow-Temperature Cracking: Field Validation of
the Thermal Stress Restrained Specimen Test". Strategk Highwuy Research Program
Report: SHRP-A401,National Research Council, Washington, D.C.,1994.
Zubeck, H., Zeng, H.,Vinson, T., Janoo,C."Field Validation of the Thermal Stress
Restrained Specimen Test: Six Case Histories", Tranrportation Research Record,
1996, 1545,67.
King, G., King, H., Hardea, O., Arrand, W., Planche, P. "Infiuenceof Asphalt Grade
and Polymer Concentration on the Low Temperature Performance of PoIymer
Modined Asphalt", Proceedings of the Association of Asphait Paving Technologists,
1993.62. 1 .
3.0 STABLE FREE RADICAL POLYMERIZATION
LITERATURE REVIEW
3.1 Introduction to Living Polymerizations
With the increasing demand that modern technology is placing on polymeric
materials, polymer chemists and engineers are being driven to produce new polymers of
highly controlled molecular weight and structural architecture. One route to this end is
through the use of living polymerizations which can be used to prepare narrow
polydispersity resins, well-defined block copolymers, end-fùnctionalized materials, and a
variety of unique polymer structures.
These living polymerizations, under a strict definition, display the following
charactensticsl':
The polymerization reaction consists of initiation and propagation steps only.
Transfer and termination reactions are not present or are negligible in the overall
reaction scheme.
The number of chains after initiation remains constant, and the polymers remain
active even when al1 the monomer is depleted. Polymerization continues on addition
of fÎesh monomer.
Initiation is at least as fast as propagation, and the degree of polymerization can be
cdculated as a function of the monomer concentration divided by the initiator
concentration.
The molecular weight distribution
m)is a narrow, Poisson distribution.
Molecular weight (MW) increases linearly with conversion.
It should be mentioned that while few "living" polymerizations completely obey ail of
these d e s , several systems behave closely enough that they can be at l e s t considered as
'pseudo'-living or controlled polymerization methods.
The nature of the chah end in a living polymerization provides a means for
producing polymers mattainable by conventional step or chah growth processes. Block
copolymers c m be made by sequential addition of different monomers after each
rnonomer is consumed by the reaction. Mono and di-fiinctional initiators provide a
straight-fonvard means for producing AB and ABA block materials. Similady, mono- or
di- 'end-fuoctionalized' (telechelic) materials are easily produced by reacting the living
chah ends with a functional chain-capping agent.
In past years, ionic and insertion living polymerization methods have received the
most attention; the characteristics and limitations of these will be reviewed here briefly.
Living radical polymerizations, which are a more recent discovery, will be discussed in
considerably more detail in following sections.
3.2 Living Ionic and Insertion Polymerization Methods
3.2.1 Living Anionic Polymerization
Living anionic polymerization was the fim recognized living polymerization
mechanism3. In a typical polymerization, an &y1 lithium hitiator tramfers a negative
charge to initiate a growing chah, to which monomer is subsequently added. With the
use of an aprotic solvent and in the absence of impurities, there is no tennination or
transfer reactions and the polymerization proceeds until dl monomer is depleted, each
chah containing a living end. The polymerization has ail the characteristics outlined
above for a true 'living polymerization', and both block copolymers4and fiuictionalued5
resins can be easily produced. Styrene, butadiene, isoprene, acrylates and methacrylates
are polyrnerized commercially by this method. However this process is far fiorn ideal in
the industrial sense since it requires highly pure monomers, an aprotic solvent (which c m
be an environmental concern) and very low polymerization temperatures (usually < 80°C). Potential monomen are also limited to those that do nct contain acidic protons
(which couid be abstracted and temiinate growing c h a h ) or functionalities that could
react with the active centre.
32.2 Living Cationic Polymerization
There are two main types of living cationic polymerization. The first is a cationic
ring-opening reaction which can be used to polymerize a few specific cyclic monomers,
such as THF~.The rnechanism requires a stable counter-ion for initiation and is only
considered truly living if termination, tramfer and reversibility of the reaction c m be
suppressed7. A more common tyw of living cationic polymerization involves vinylic or
vinylenic based monomers, for which transfer is an issue due to the reactiviw of the
carbenium ion at the end of the polyrner chain. To compensate for this, living chah ends
are 'stored' as a stable covalent iodide or as a salt complex between insertions of
monomer'*'. A catalyst is used to increase the rate of polymerization, which is decreased
by the capping/uncapping reactions. This method is used for the commercial
polyrnerization isobutylene9and its copolymerization with isoprene to make butyl rubber,
but is not seeing widespread applications with other monomers.
3.23 Group Transfer Polymerization (GTP)
Group transfer polymerization is an insertion process discovered by Webster and
Sogahtoin 1983, and can be used for the polymerization of acrylates and methacrylates at
higher temperatures than used for anionic polymenzations of these monomers. The
& mechanism Uivolves a silyl ketene acetyl initiator, and the sequential inseaion of
monorner to form a growing chain''. 'ïhe silyl group migrates fiom monomer to
monomer during the reaction, and hence the narne Group Transfer Polymerization:
Figure 3.1 :
Mechanism of Group Transfer Polymerization (after ref. 12)
Like the other living methods, the degree of polymerization in GTP is governed by the
initial ratio of monomer to initiator charged to the reactor, and low polydispersities are
obtained. Although higher temperatures can be used and the reaction is easy to control,
there remain a number of concerns for the comrnercialization of the process:
1) The catalysts used are highly toxic and difficult to remove fiom the polyrner.
2) High molecular weights are difficult to achieve since impurities terminate the
reaction.
3) The polymerization must be conducted in atihydrous atmosphere.
4) The potential monomers for this process are Iimited to acrylates and acrylate-like
materials.
3.3 Living Radical Polyrnerizations
The polyrnerization methods described in the previous sections are each limited
by the types of monomer they can polymerize and al1 require stringent control of
monomer purity and the reaction environment Free radical polymerktions can
generally be performed under much more forgiving conditions and with a wide variety of
monomers. However, in classical fke radical polymerization, c h a h are initiated at
different tirnes during the reaction, resulting in a broad dispersity of molecular weights.
Chahs are also generaily short-living and are tenninated irreversibly (except in the case
of side reactions such as chah transfer to polymer), which makes the production of
hctionalized or block copolymers very difficult.
In an attempt to combine the advantages of living ionic/insertion polymerizations
with the flexibility of fkee radical polymerization, there has been a signifîcant amount of
research into the possibility of living or pseudo-living radical polymerization. Three such
polymerization methods will be described here, each is based on the concept of reversibly
tenninating the growing chah to control transfer and termination reactions.
3.3.1 The Iniferter Process
Probably the first example of a pseudo-living radical polymerization was reported
by Otsu" in 1982, and is based upon iniferters. The name iniferter originates fkom the
combination of roles the initiator plays in the reaction: initiator transfer agent and initiator
teminator. These polymerizations are similar to classical f?ee radical polymerizations
except that transfer to initiator (iniferter) and prirnary radical termination can not be
neglected. In fact, in the iniferter process these reactions are what give the
polymerization its living character. The general reaction scheme is given on the
following page, where IF is an iniferter hgment, and M is monomer14:
IF-IF + IF--t IF-
IF-+ M + IF-MlIF-Mi-+ M -t IF-M.,+,IF-Mg + IF-IF + IF-M,-IF + IFIF-w- + IF- + IF-w-IF
IF-M,-IF + IF-Mi- + IF*
Decomposition of inifeter
Recction of inverter with monomer
Propagation
Chriin tramfer to inifrter
Primary radicaI terminarion
Reinitiation of polymer chain
Reaction Scheme of the Merter Process
ui step (1) the i n i f e r decomposes (usually with heat or UV light), giving two f?ee
radicals. Initiation (2) and propagation (3) are analogous to standard fiee radical
polymerization. However, when the probabilities of steps (4) and (5) are high, transfer to
and termination by species other t h n the iniferter or inifexter fkgments c m be neglected.
In step (6) the polymer chain is reinitiated and chah growth continues. Repeated
addition of monomer between capping/uncapping steps is the mechanism for forming
high MW poiymer. Though the polymerization behaves in a living manner with
molecular weight proportional to conversion, MWDs are high due to a slow rate of
initiation in step (1)".
The structure of many common iniferters are based on akyl thiuram disulfides
which form labile carbon sulfûr bonds (S-C) with the monomer.
Figure 3.3 : A Typical Iniferter and hiferter-Temllnated Polymer (after ref. 16)
The iniferter process had been used to make a variety of block copolymers, and
' .
Y-;
recently Clouet has used functional iniferters to make block copolymen in which one
block is produced by radical polymerization and the other by step-growth extending fkom
the functional end of the iniferterf6. Since chah termination by recombination is never
completely eliminated and iniferter fragments are capable of initiating new c h a h ,
relatively broad polydispeaities and production of homopolymer during a block
copolymerization are always a problem.
3.3.2 Atom Transfer Radical Polymerization (ATRP)
A very recent addition to the class of living k e radical polymerizations is
ATRP", discovered by Matyjasweski at Carnegie Mellon University in 1995. The
process is based on atom transfer radical addition, an efficient method of carbon-carbon
bond formation in organic chemistry. A T W has k e n shown to produce vinyl polymers
with low poIy-dispersities and MW proportional to conversion throughout the reaction.
A typical atom transfer polymerization requires monomer, an initiator (usually an
akyl halide), and an atom transfer promoter (a metal coordination cornplex). A general
reaction scheme is given below:
Propagation:
Figure 3.4:
Mechanism of Atom Transfer Radical Polymerization ( d e r ref 17)
In a redox reaction, the copper(1) coordination complex (CU'LJ reversibly
abstracts a chlorine atom fkom the initiator (R-CI), resulting in a 1-
radical (R.) capable
of initiating polymerization. Monomer is added to the active site as the radical (R-M-) is
capped and uncapped by the transfened chlorine atorn. On a thermodynamic basis,
equilibrium is established between active and inactive chains, but must favour dormant
c h a h to reduce the possibility of chain coupling reactions. To maintain a low
polydispersity, the chains must grow at the same rate and therefore kinetics niggest that
the exchange between active and dormant sites is rapid".
Though the ATRP appears promising (yielding living polymers with
functionalitlies as low as 1.03'4, there is concem that resulting materials are generally
highly coloured and that the toxic metal complex remains within the polymer.
3.4 Stable Free Radical Polymerization (SFRP)
3.4.1 Overview
The living radical polymerization process currentiy receiving the most attention is
the SFRP process discovered2' and patenteda= by Georges et al. at the Xerox Research
Centre of Canada. The chemistry is based on the use of a stable nitroxide radical to
revenibly react with a propagating chain in a conventional fiee radical polymerizatïon,
limiting termination and transfer reactions.
Their first studies involved the bulk polymerization of styrene with benzoyl
peroxide (BPO)and the nitroxide stable radical 2,2,6,6-tetramethyl- 1-piperdinyloxy
(TEMPO).Nitroxides were selected over the sulfur radicals formed by dissociation of
iniferters13because the nitroxides were not known to initiate polymerization and were
expected to form weaker bonds with the propagating chainZO.The resdting polymers had
narrow MWDs (< 1.5) and MWs which increased proportiondly with conversion.
Kinetic studiePJ4 have shown that the mechanism of a styrene polymerization is as
illustrated below. Styrene, BPO and TEMPO are heated to -90°C, at which point BPO
dissociates, fonning a fiee radical which adds to styrene, which is subsequently capped
by reaction with TEMPO. The mixture is held at this t e m p e m e for a few hours and
then heated to -125"C, at which point styrene polymerization begins, and the growing
chains are reversibly terminated with the TEMPO radical. The trapping reaction by
TEMPO of growing polymers chains has been proven diffusion controlled. Propagation
continues and the resulting polymer contains a BPO initiator fkagment at one end and a
living TEMPO terminated chai. at the other.
-
Figure 3.5:
+
HP=M
+
3
O-N
Mechanism of Stable Free Radical Polymerization of Styrene
3.4.2 Mechanistic Considerations
Georges and coworkers have studied the kinetics of the SFRP process using ESR
(electron spin resonance) techniques, and have shown that the rate of polymerization can
be determined as24:
where:
Fr]
= concentration of monomer. rnom
16
=
[LJ
KL
m
rate constant for propagation, s"
= concentration of pseudo living chains moVL
= the ratio of forward and reverse rate constants for the
radical trapping reaction
= concentration of fieeTEMP0, mom.
The above equation, compared with the equation for the rate of polymerization in
conventional fiee radical polymerization, differs ody by the factor in the brackets
(assuming that the concentration of the pseudo living chah [LJ is equal to the
concentration of the growing chah in a conventional radical polymerization FI-]).
Since
KLis large (the formation of a TEMPO-radical bond is much faster than breaking such a
bond), the factor in the brackets is therefore less than unity. This is supported by the fact
that the living-polymerization is slower than the corresponding conventioaal radical
polymerization. Decreasing the TEMPO concentration in the polymerization can
increase the reaction rate, but at the cost of increased polydispersity in the final
Other possible rate enhancements include increasing the reaction temperature (and
therefore 4) and/or selecting a more labile nitroxide2' (ie.decreasing KJ.
The use of certain organic acids and salts in the nitroxide mediated fke radical
~~~
found
'
to cause a significant increase in the
polymerization has been s t ~ d i e d and
polymerization rate combined with a slight increase in polydispersity. The mechanism
proposed is that the organic acid or salt consumes nitroxide mdicals, which in tum shifb
the equilibrium (KJ between dormant and growing chahs. Georges and coworkers have
also discovered that though the polymerization rate can be increased by simply
decreasing the TEMPO concentration, the best compromise between polydispersity and
polyrnerization rate is achieved in the presence of an additive such as camphorsulfonic
acid (CSA)~'which decreases the concentration of fiee nitroxide after initiation.
3.4.3 Architectural Control in SFRP
Although the majority of the initial studies performed on stable fiee radical
polymerization were based on styrene homopolymerizations, considerable efforts have
been made with a variety of other monomers and initiating mechanisms to yield polymers
of a wide range of architectures and fbnctionalities. Georges et al. have made significant
progress in the polymerization of acrylate2' and chlorornethyl~tyrene~~
homo- and blockDel Nin has studied
copolyrners, with some work in the area of diene c~polymerization~~.
nitroxide-mediated homopolymerization of isoprene in dichlorobenzene claiming
conversions to 90 %, though less than 30 % of this polymer was ever recovered from the
reaction mixture32. Other researchers have focussed on polymerizations of functional
monomers such as 4-vinylpyridines3and N-vinyl carbazoleM. Hawker has shown that
unirnolecular initiators cm be used in the synthesis of dentritic, end-fiinctional and other
complex polymer
The robust nature of stable fiee radical polymerization has
resulted in a multitude of new chernistries for architectural control, with several exceiient
reviews of these available in the literat~re~~?
3.5 Summary of Objectives
Stable fiee radical polymerization is providing new methods of producing
materials previously available only through more costly and technically difncult ionic
and insertion processes. With this in mind, the objectives of this thesis were to:
1. Synthesize elastomers of different molecular weights using SFRP technology
2. Develop chemistry for the fhctionalization of these materials
3. Evaluate these fiinctional polyrners as asphait additives.
Webster, O. "Living Polymerization Methods", Science. 1991 , Z 1,887.
Sawamoto, M. Encyclopedia of Polymer Science ami Engineering, John Wiley & Sons: New
York, 1988; Vol. 9, p 139 & supp., p 380.
Rempp, P.; Memll, E. Polymer Swhesis; Huthig & Wepf: New York, 1991, 121.
Rempp, P.; Merrill, E. P o h e r Synthesis; Huthig & Wepf: New York, 1991, 138.
Quirk, EL;Yin, J.; Guo, S-H.;Hu, X-W.;
Summers, G.; Kim,J.; Zhu, L-F.; Schock, L.
"Anionic Synthesis of Chain End Functionalized Polymen", MabomoL. Chem... MacromoZ.
Symp. I990,32,47.
Rempp, P.; Memll, E. Polymer Synthesk; Huthig & WepE New York, 1991, 157.
Sawamoto, M. Encyclopedia of Poiymer Science and Engineering, John Wiley & Sons:
New York, 1988; Vol. 9, p 139 & supp., p 381.
Rempp, P.; Memll, E.. Polymer Sjmthesis; Huthig & Wepf: New York, 1991, 171.
Faust, R.; Kennedy, J. "Living Carbocationic Polyrnerization. N. Living Polymerization of
Isobutylene", JournaZ of Polymer Science Poi'er Chemisiy,. 1996,34,344 1
10. Webster, O.; Hertler, W., Sogah, D; Farnharn, W.; RajanBabu, T."Group Transfer
Polymerization. 1. A New Concept for Addition Polymerization with Organosilicon
Initiators", Journal of the America Chernical Society, 1993, 105,5706.
1 1. Webster, O . Encyclopedia of PoZymer Science and Engineering, John Wiley & Sons: New
York, 1988; Vol. 7, p 580.
12. Sogah, D.; Hertler, W.; Webster, O., Cohen, G."Group Transfer Polymerization.
Polyrnerization of Acrylic Monomers". Macromolecules l987,2O, 1473.
13. Otsu, T.;Yoshida, M. "Role of Initiator-Transfer Agent-Terminator (Iniferter) in Radical
Polymerizations: PoIymer Design by Organic Disulfides as Initiators", Makromolecuiur.
Chernistry Rapid Comrnunicatons. 1982,3, 127.
14. Rempp, P.; Memll, E. Polymer Synthesis; Huthig & Wepfi New York, 1991,86.
15. Otsu, T.;Yoshida, M. "A Model for Living Radical Polymerization", MakrornoiemZar .
Chemistry Rapid Communications, I982,3, 133,
16. Clouet, G. "Block Copolymers and End-Functionalized Polyrners through Living Radical
Polymerization", Polymer Preprints, l992,33, 1 , 199.
17. Wang, J-S.; Matyjaszeweski, K. "Controlled/"Living" Radical PolymerUation. Atom
Transfer Radical Polyrnerization in the Presence of Transition Metal Complexes", JournaI of
the Americmr Chemical Society, 1995 117,56 14.
18. Patten, T.; Xia, J.; Abernathy, T.; Matyjaszeweski, K. "Polymers with Very Low
Polydispeaities by Atom Transfer Radical Polymerization", Science 1996,272,886.
19. Wang, J.; Matyjaszeweski, K. "ControIled/"Living" Radical Polymerization. Halogen Atom
Transfer Radical Polymerization Promoted by a Cu(I)/Cu(II) Redox Process",
Macromolecules 199528, 790 1.
20. Georges, M.; Veregin, R.; Kazmaier, P.; Harner, G. "Narrow Molecular Weight Resins by a
Free Radical Polyrnerization Process", Macromolecules, 1993,26,2987.
2 1. Georges, M., Veregin, R., Kazmaier, P., Hamer, G., US.Patent 5,322,912, 1994.
22. Georges, M., Veregin, R., Kapnaier, P., Harner, G. US.Patent 5,401,804, 1995.
23. Veregin, R.; Georges, M.; Kaunaier, P.; Hamer, G. "Free Radical Polymerizations for
Narrow Polydispersity Resins: Electron Spin Resonance Studies of the Kinetics and
Mechanism", Macromolecules, 1993,26,53 16.
24. Veregin, R.; Georges, M., Hamer, G.; Kazmaier, P. "Mechanism of Free Radical
Polymerizations for Narrow Polydispersity Resins: Electron Spin Resonance and Kinetic
Studies", Poiymer. Preprint,. 194,35, 1, 797.
25. h i e r , P.; Moffat, K.; Georges, M.; Veregin, R.; Hamer, G. "Free Radical Polymerization
of Narrow Polydispersity Resins. Semiempirical Molecular Orbital Calculations as a
Criterion for Selecting Stable Free Radical Reversible Terminators", Macromolecules. 1995,
28, 1841,
26. Georges, M.; Veregin, R.; Kazmaier, P.; Hamer, G.; Saban, M. "Narrow Polydispersity
Polystyrene by a Free Radical Polymerization Process-Rate Enhancement", Macromoiecules,
1994,27,7228.
27. Odell, P., Veregin, R., Michalak, L., Georges, M. "Characteristics of the Stable Free Radical
Polymerization of Styrene in the Presence of 2-Flouro- 1-rnethylpyridinium p
Toluenesulfonate", Macromolecules, 1997, 30, 2232.
28. Veregin, R., Odell.; Michalak, L.; Georges, M. "Mechanism of Rate Enhancernent Using
Organic Acids in Nitroxide Mediated Living Free Radical Polymerizations",
Macromolecules, 1996, 29,4 161.
29. Listigovers, N.; Georges, M.; Odell, P.; Keoshkerian, B. "Narrow-Polydispersity Diblock
and Triblock Copolymers of Alkyl Acrylates by a "Living" Stable fiee Radical
Polymerization", Macrornolecwles, 1996,29,8992.
30. Kazmaier, P., Daimon, K., Georges, M., Hamer, G., Veregin, R. "Nitoxide-Mediated
"Living" Free Radical Polymerization: A Rapid Polymerizatopm of Chloromethyl(styrene)
for the Preparation of Random, Block, and Segmental Arborescent PoIyrners",
Macrornolecules, 1997,30,2228.
3 1. Macleod, P., Georges, M., Hamer, G. "Functional Polyrners by Stable Free Radical
Polymerization", Polymeric Materials Science and Engineering, 1997,76, 150.
32. Del Nin, J. ïhe Nitroxide Mediated Polymerizafion of Styrene and Isoprene, M.Sc. Thesis,
Queen's University, Kingston, ON, 1997.
33. Borisch, J., Wendler, U., Jaeger, W. "Controlled Radical Polymeritation of 4Viny 1pyridine", Mucromol e c h Rapid Communications, 1 997, 18,975.
34. Baethge, H., Butx, S., Gudryn, S. "Living Free Radical Copolymerization of Styrene and NViny I Carbazo le", Macromole~ZarRapid Communications, 1997, 18,975.
35. Hawker, C. "Living Free Radical Polymerization: A Unique Technique for the Preparation of
Controlled Molecular Architectures", Accounfs of Chernical Research, 1997,30,3 73.
36. Sivararn, S. "Controlled Free Radical Polymerization", Journal of Scietttificand Indwirial
Research, 1997, 56, 1.
4.0 STABLE FREE RADICAL POLYMERIZATION
EXPERIMENTAL
4.1 Materials
4.1.1 Monomers
The monomers used in this study included isoprene (Fluka, 99%), styrene
(Aldrich, 99 %), chloromethylstyrene (CMS, Dow, 98 %, mixture of 3- and 4- isomers),
3-(tri1nethoxysily1)propyl methacrylate (TMSPMA, Dow, 97 %), and 2-(dimethylamino)
ethyl methacrylate (DMAEMA, Aldrich, 98 %). AU materials were used as received.
4.1.2 Initiator, Nitroxide, Rate Enhancing Additive, Solvent
Benzoyl peroxide (BPO, Aldrich, 97 %) was used as the initiator in ail
homopolymerizations. The nitroxide 2,2,6,6-tetramethyl- 1-pipendinyloxy (TEMPO,
Nova Molecular Technologies, 98 %) was used as received. A proprietary rateenhancing additive was also employed in many of the isoprene polymerizations, but its
identity remains the intellectual property of the Xerox Research Centre of Canada, and
will be refened to as "additive" throughout this thesis. Ethyl acetate (EtOAc, Caledon,
99.5 %) was selected as a solvent in the isoprene reactions for several reasons. First, it
would reduce the viscosity of the reaction mixture at high conversions compared to a
b u k polymerization. Secondly, its high volatility (B.P. 78°C) would facilitate its
removal at the end of a polyrnerization reaction. Thirdy, ethyi acetate has a relatively
low chah transfer constant for the monomers being studied and was therefore not a
concem for maintahïng controlled polymerization conditions. Finally, the use of EtOAc
as a solvent would reduce the vapour pressure in the reaction vessel compared to a bulk
polymerization.
4.2.1 Isoprene Polymerizations
All of the initial isoprene polymerizations were performed in a 300 mL
pressurized bornb-type parrfMreactor, due to the volatility of isoprene under SFRP
reaction conditions. Typically the reaction volume was kept constant (100 mL), as was
the ratio of isoprene to ethyl acetate solvent (3:l). The monomer to initiator ratios were
varied throughout the experiments as were the ratio of initiator to nitroxide, and in some
cases the ratio of initiator to rate enhancing additive.
In a typicai polymerization with 1000: 1 monomer/iitiator molar ratio, isoprene
(75 mL,750 mmol), ethyl acetate (25 mL),BPO (182 mg, 0.75 mmol), TEMPO (152 mg,
0.98 mmol) were charged to the reactor at room temperature. The vessel was then seaied
nom the atmosphere, and purged while stirring with ten pressurize/depressurize cycles
fiom a 40 psi purified argon feed. M e r the final purge the reactor was depressurized,
then heated to 14S°C and maintained at this temperature for the duration of the reaction.
When the rate-enhancing additive was used, it was charged to the reactor with the
initiator and nitroxide at the start of the reaction. In the case of scaled isoprene reactions
(which were perforrned to provide sufficient materiais for asphalt modification) a 2 L
pkriM reactor was used with a 1.5 L reaction volume, and the same procedure used for
the smaller vessel was followed.
To prepare a small sample of 'conventional' polyisoprene for cornparison with the
controlled polymerization material, BPO (0.970 g, 4.0 mmol), isoprene (60 mL, 600
mmol) and ethyl acetate (40 mL) were added to the 300 EL parrTMreactor and heated to
80°C for 4 hours. The product was isolated by evaporation of solvent and residual
monomer.
4.2.2 Styrene Polymerizations
Since the goal of this research was to prepare functionalized materials for asphalt
modification, a series of low molecular weight polystyrene samples were made for
preliminary investigation into block copolymer formation. Polystyrene was chosen as a
'model' polymer because it has been extensively studied under SFRP conditions, and is a
relatively straight forward matend to p@
and characterize.
In a typical styrene polymenzation with 200: 1 monomer/initiator ratio, styrene
(50 mL,440 mmol), BPO (529 mg, 2.18 mmol) and TEMPO (443 mg, 2.83 mmol) were
charged to a 100 ml three-neck, roundbottom Bask fitted with a condenser. A sealed
mechanical stirrer was placed in the centre neck of the flask and the other necks were
sealed with rubber septa. The system was purged by bubbling argon through the solution
for 10 minutes. The flask was then placed in a preheated oil bath at 135°C for the
dwation of the polymerization.
4.23 Characterization
Unless otherwise specified, al1 polymers were characterized for molecular weight
@y injecting crude samples without precipitution into nonsolvent) using a Waters/
Millipore liquid chromatograph equipped with a Waters 510 pump, Ultmstyragel
columns of pore sizes 1 x 104,2 x 500, and 100 A, and a Waters 410 differential
refkactometer. Tetrahydrofuran was used as the solvent at a flow rate of 1.O mL/min, and
the column was calibrated with polystyrene standards. Data were acquired and processed
with ~ i l l e n i u m
2.12
~ ~software.
The conversion in the styrene polymerizations was monitored using
thermogravimetric analysis (TGA) of aliquots taken fiom the reaction vessel. In the case
of the isoprene polymerizations, the seaied bomb could not be sampled over the course of
the reaction. The low volatility of isoprene also made the use of TGA to characterize
conversion dificult. Consequently, for the isoprene polymerizations, a crude yield was
determined at the end of each reaction by measuring the mass of polymer isolated after
removing residual solvent and monomer in a vacuum oven at 60°C and 760 mm of
mercury until weight loss of the sample with time became negligible.
NMR spectra of polymers dissolved in CDC13with TMS as an intemal reference
were acquired at 2S°C on a Bruker DPX 300 spectrometer. Typicaily samples were
purified prior to analysis by repeated precipitation fiom dichloromethane into methanol
(10-fold excess of MeOH), followed by drying in a vacuum oven.
4.3 Functionalization Reactions
43.1 Chlormethylstyrene Unimer
As a means of producing an end-functional polymer, the 3/4-chloromethylstyrene
unimer was prepared as hitiating adduct which could be used for chah extension with
isoprene, and then for M e r chemistry on the chloromethyl group. A 200 mL
roundbottom flask was equipped with a condenser and a magnetic stir bar. Chloromethylstyrene (100 mL, 710 mmol), BPO (2.33 g, 9.63 m o l ) and TEMPO (1.40 g, 8.96
mmol) were added to the flask which was then purged with argon for 10 minutes. The
flask dropped into a 135°C oil bath for 5 minutes, during which the colour of the reaction
mixture changed fiom a dark orange to light yellow, indicating the disappearance of fiee
nitroxide. The product, an orange oil, was isolated by flash chromatography on a silica
gel column, eluting nrst with hexanes, then dichloromethane, and was then identified by
'HNMR spectroscopy. To demonstrate chah extension, CMS unimer (307 mg, 0.714
mmol) was added to isoprene (70 mL, 700 mmol) in a 300 mL parrTMreactor and heated
to 135OC for 3.5 hours. The polymer was isolated by evaporation of unreacted isoprene,
followed by precipitation fiom dichloromethane into methanol.
4.3.2 Removal of Nitroxide
A second effort at end-functionalization focussed on chemistry at the
nitroxide/polymer bond. Previously, Solomon et ai. reported that for low molecular
weight, nitroxide-capped, methyl acrylate oligomers, reduction with a zinclacetic acid
mixture cleaved the C-O bond, resuiting in quantitative yield of the hydroxyl-terrninated
oligornerL.To demonstrate this chemistry for the polyisoprene system, polyisoprene (Mn
2600,2.0 g), M c dust (2.0 g) and glacial acetic acid (50 mL) were added to a
roundbottom fiask fitted with a condenser and magnetic stir bar. The mixhue was heated
to reflux for 2 hours with and additional spike of zinc dust ( 1.O g) each half-hour. The
reaction mixture was then cooled and the acetic acid was evaporated ovemight. The
polymer was redissolved in ethyl acetate, Pnc was removed by filtration, and the organic
solution washed twice with dilute hydrochloric acid. The organic layer was then
separated, dried with magnesium s a t e and evaporated to give the purified product,
which was analyzed by 'HNMR spectroscopy.
433 Synthesis of Block Copolymers
The moa ideal fûnctionaluation reaction would involve a 'one-pot' synthesis in
which the polyisoprene was taken to very high conversion, at which point a
functionalization reaction would be perfonned on the unpurified material, yielding the
desired product. Block copolymer formation seemed like a logical choice for such a
process, since block copolymers could ideally be made by adding a second polymerizable
monomer to the reaction vesse1 after a first block had proceeded to high conversion. For
application as an asphalt additive, the second block need only be a short segment of a
reactive monomer. Since most 'functional' monomers have boiiing points significantly
higher than isoprene, it was decided to make the longer elastomer blocks first, so that
residual monomer could be removed if necessary.
Two fùnctionalities of interest in a polymeric asphalt modifier are amine and
trimethoxysilyl groups, which are thought to interact with the surfaces of common
mineral aggregates. Both of these functional groups are available as methacrylate
derivatives, which have the potential to be used as monomers in stable £iee radical
polymerization. In fact, at the tirne of this work there was some evidence for
rnethacrylate c h a h to be self-teminating after growing to a few thousand daltons under
SFRP conditions2. This would provide the desired short functional block at the end of a
longer elastomeric chah.
To demonstrate this chemistry in a well-characterized system, low molecular
weight polystyrene was extended with 3-(trimethoxysily1)propyl methacrylate.
Polystyrene (M. 7500, 1.O g) was dissolved in TMSPMA (10 mL) in a roundbottom flask
and heated with stimng to i4S°C for 4 hours. After the fïrst few minutes, the solution
changed fiom transparent to a luminescent, translucent white colour. Samples were taken
fiom the reaction mixture each h o u and injected into the gel permeation chromatograph.
A sample was also injected into a dual detection GPC equipped with a differential
refkactometer and W detector set at 254 nm. At the end of the reaction, the polymer was
recovered by precipitation in methanol, then purified by reprecipitation fiom
dichlormethane into methanol. A 'HNMR spectnun of the polymer in CDC13was
acquired. A similar experiment was performed with 2-(dimethylamino) ethyl
rnethacrylate.
To make usefùl materials for asphalt modification, this chemistry was extended to
the scaled polyisoprene systems. The reaction mixtures fiom the scaled polyisoprene
reactions were heated to 80°C under 760 mm of mercury vacuum to rernove residual
monomer. In a typical chain extension, polyisoprene (75 g) was dissolved in an equal
mass of TMSPMA or DMAEMA in a 500 mL roundbottom flask. The mixture was
heated to 145°C with stirring for 2 hours. The solution changed fiom transparent to
translucent, similarly to the polystyrene reactions. The system was then cooled to room
temperature and the polymer was recovered by diluting the viscous reaction mixture with
dichloromethane, followed by precipitation into a I 0-fold excess of methanol. This
procedure was repeated until no monorner could be seen in the 'HNMR spectrum of the
polymer. Purified samples of each attempted chain extension were injected into the GPC
and estimation of block length made by deteminhg the relative amounts of characteristic
polyisoprene and copolymer proton resonances ushg a Bruker AC 200 spectrometer.
4.4 References
1. Solomon, D., Waverly, G., -do,
1986.
E., Hill, W., Cacioli, P. US.Patent 4,581.429,
2. Steenbock, M., Klapper. M., Mullen, K., Pinhal, N., Hubrich, M. "Synthesis of Block
Copolymers by Nitroxyl-Controlled Radical Polymerization", Acta Polymer, 1996,
47,276.
5.0 ASPHALT EXPERIMENTAL
5.1.1 Asphalt Binder
The binders used in this study were both 85-1 00 penetration grade, typicd of
those used throughout Southem Ontario. For the stripping experiments, a South
Amencan crude (R754)refined by Imperia1 Oil in Montreal was selected on the basis of
its moisture sensitivity when combined with a high sand content mix design. This binder
was either used as received or modified with 3 or 5 % polymer additive. In the thermal
stress restrained specimen tests, a Bow River binder (TK110) fiom Petro-Canada's
refinery in Clarkson, Ontario, was used to facilitate a direct cornparison with previous
low temperature studies on this binder modified with various polyrners'2.
5.1.2 Mineral Aggregate
The aggregate used in preparing asphalt concrete samples was supplied by
Dibblee Construction Ltd. of Westbrook, Ontario. Sand, limestone screenings, and HL-3
limestone coarse aggregate (318")were sieved individually (Figure 5.1) and combined in
the percentages given below to fit an MT0 HL-3 mix design (see Figure 5.2). This
aggregate gradation was used for both the antistripping and low temperature experiments.
Table 5.1 :
Percentages of Fine and Coarse Aggregates in HL-3Mix Design
Aggregate
Shortall Sand
Limestone Screenings
HL-3 Limestone
Percentage in Mix
46.9
15.6
37.5
,+
0.0 1
o.1
1
10
1O0
Sieve Diameter (mm)
Shortad Sand +Lhestone Screenings +HL-3 Coarse Limestone
Figure 5.1 :
0.01
Particle Size Distribution for Coarse and Fine Aggregates
o.1
1
Sieve Diameter (mm)
10
+Mix Design -HL-3 Specification
Figure 5.2:
Aggregate MU<Design and MT0 HL-3 Specifications
1 O0
5.13 Polymer Additives
The homopolymers and block copolymers prepared at the Xerox Research Centre
of Canada were precipitated fiom dichloromethane Uito methanol (to remove solvent and
residual monomer), then dried in a vacuum oven at 60°C ovemight or u n d their weight
loss with time became negligible.
5.2 Asphalt Concrete Preparation
Aggregates were dried at 160°C for at Ieast 24 hours prior to use, then weighed
into stainless steel pans in the proportions as given above to give a total aggregate m a s
of 3600 g. The aggregate was then placed in the oven with the compaction moulds,
mixing bowl and mWng apparatus for at least 8 hours prior to sample preparation.
Asphalt binder (-500 g) was heated in a 1 quart paint c m to 150°C for one hour
prior to mixing. In the case of polymer modified samples, the can was placed in a
heating rnantle and the polymer was slowly added to the hot asphalt cernent and mixed
for approximately 15 minutes at 3000-4000 rpm by using a Polytron (Brinkmann) high
shear mixer. When the binder was well-mixed, a sample was taken and its viscosity
rneasured on a Brookfield Viscorneter using a #2 1 spindle at 50 rpm. A temperature was
then selected for mixing with aggregate so that the viscosity of the binder was close to
170 cP (See Figures 5.3 to 5.6). When this desired temperature was obtained, the binder
(190-230 g or 5-6 wt. % on the mixture) was added to the hot aggregate (3600 g) and
mixed for approximately 1 minute on a Hobart mechanical mixer until the aggregate was
unifomily coated. If necessary, the mixture was then rehimed to the oven to be reheated
for a short period of time d e r which it was spooned into a six inch diameter
Figure 5.3:
130
Viscosity of Polyisoprene Homopolymer Modified Binders
140
150
160
170
180
Temp (OC)
Figure 5.4:
Viscosity of High MW Polyisoprene-b-TMSPMA Modified Binders
140
150
160
180
Temp (OC)
Figure 5.5:
130
Figure 5.6:
Viscosity of Low MW Polyisoprene-b-TIMSPMA Modified Binders
140
150
160
Temp (OC)
170
180
Viscosity of High MW Polyisoprene-b-DMAEMA Modified Binders
mould for compaction. The mix temperature was monitored using a digital thermocouple
and compaction was started when it corresponded to a binder viscosity of approximately
280 cP. A Rainhart gyratory compactor was used to cornpress the samples by applying a
pressure of 400 psi with a rotatïng angled plate (1.25' tilt) to a sample height which
yielded cylindrical samples with a desired voids content (usually 7-8 % for the stripping
tests and 4-5 % for the low temperature tests). Two briquettes were made for each
binder. The rnoulds were cooled for about one hour before ejection of the samples after
which they were allowed to cure for at least another 24 hours before cutting took place.
Four rectangular asphalt briquettes (120x3 5x35 mm) were cut fiom each
cylindrical sample using a masonry saw equipped with a diamond-tipped blade. The
samples were air-dned overnight, then measured for voids content using picnometry.
The dry mass of each sample was recorded and its volume was measured by detemiining
the mass of water it displaced fiom a g l a s picnometer. Each sample's density was then
calculated and the voids content detemiined by the formula:
where:
~me~lred
Ptlteoreticoi mm:
-
-
sample density from picnornetry, g/cm3
maximum theoretical density based on the specific
gravity of the aggregates and asphalt cernent, @m3.
5.3 Moisture Sensitivity Test
In order to measure the moisture sensitivity of the asphait concrete samples, a
laboratory test was developed to provide an accelerated means of simulating moisture
damage. A modified version of ASTM D4867 (the so-called TunniclWRoot ~ e t h o d ~ )
was adopted since it seemed to be widely accepted in the practice and could be perfonned
(with some modifications) using the available equipment. The asphalt concrete samples
for these experiments were made with a relatively low asphalt binder content (5 wt. %
R754) and a relatively high voids content (7-8 vol. %) to increase the potential for
rnoisture damage.
After each pair of asphalt concrete briquettes were cut into rectangular tensile test
specimens, the samples were sorted into two subsets of three samples each (two were set
aside for other tests) such that the average voids content for each subset was
approximately equal. The first subset was simply epoxied onto thick steel test plates, the
adhesive cured for 24 hours, then cooled overnight in a refngerator at 5OC. The tensile
strength (in tension) of the each sarnple was measured using an MTS Sintech 2/G load
fiame equipped with a 2000 Ibs load ce11 and an environmental conditioning chamber to
maintain the sample at 5OC (see Figure 5.7). The crosshead speed was set at 25.4
mm/minute.
The second subset of samples were subjected to a moisture conditioning
procedure designed to simulate in an accelerated fashion moisture damage comparable to
that found in the field. Ln the first step, the samples were placed in an open beaker
containing water at 25OC. The container was then put in a sealed chamber and a vacuum
of
- 640 mm of mercury was applied for five minutes to partially saturate the void space
within the samples. Tests in other labs have shown that the degree of saturation is
strongly dependent on the magnitude of the applied vacuum and almost completely
independent of the duration of its application4. The samples were removed from the
water bath and were weighed when their surface appeared dry. The degree of saturation
was calculated as follows:
where:
M .
Mh
h o
~4m,~?
-
mass of the surface-dry, saturated asphalt sample, g
initial mass of the dry asphalt sample, g
specinc volume of water at 25OC, cm3/g
percent voids in asphalt sample.
An acceptable saturation value was established to be 55-80 %. Samples with lower
degrees of saturation were reconditioned under a slightly higher vacuum, those higher
than 80 % were deemed over-saturated (i.e. damaged) and were not used as suggested by
ASTM method D4867. The samples were then placed in a constant temperature bath at
60°C for 24 hours, surface dried, and cooled to room temperature (approx. 1 hour), glued
ont0 metal plates in an alignment fixhire, cured for 24 hours, and measured for tensile
strength immediately after stabilizing at 5OC overnight.
The tensile strength ratio (TSR)is used to evaluate the moisnire sensitivity of an
asphalt pavement and is defined as:
TSR = -.100
0,
where:
cm
-
crd
=
average tende strength of the moistute conditioned subset, MPa
average tende strength of the dry subset, MPa.
In most cases three samples in each subset gave smcient reproducibility, however, in a
few experiments, one or two additional tende tests were performed in order to reduce the
standard deviation of the subset. The error for each subset was calculated in tenns of a
90% confidence interval.
5.4 Thermal Stress Restrained Specimen Test
The thermal stress restrained specimen test (TSRST) provides a means of
evaluating the low temperature fracture strength and h c t u r e temperature of an asphalt
concrete, and it is considered to give a reasonable indication of an asphalt mixture's low
temperature
For these experiments, asphdt concrete samples were prepared using 6 wt %
binder (TK110) modified with 3-5 wt. % polymer, and compacted to give a voids content
of 4-5 vol. %. These values are more representative of a typical HL-3 hot mix asphalt
and were also used to permit cornparison with the results of previous studies on the same
base asphalt modified with different polyrners12.
The experimental setup is similar to that used for the moisture susceptibility tests.
The rectangular asphalt concrete samples were glued onto thick metai platens,
conditioned overnight at O°C, then mounted in the MTS Sintech 2/G Ioad fnune inside
the temperature controlled chamber (see Figure 5.7). Two linearly variable displacement
tramducers (LVDTs) were located on either side of the asphait briquette to measure
strain in the specimen. As the chamber was cooled fiom O°C at a rate of 10°C per hour,
the LVDTs measured the contraction of the sample and the ~eshvorksTMcornputer
software directed the load h
e to brhg the asphalt briquette back to its original length.
This process continued until the sample failed and the failure temperature and strength
were recorded. Further details on this rnethod can be found in the SHRP report by Jung
and vinson6.
Invar Rod
Asphalt Concrete
Sample
End Platen -
4--
0 Micrometer
Environmental
Chamber
Figure 5.7:
Liquid
Nitrogen
Apparatus for Tensile Strength and TSRST Experiments
60
5.5 References
Rodriguez, M . The Effects of Partimlate Fillers on the Low Temperature Fracture
Behavior of Asphalt Binders and Hot Mn Asphults, M.Sc. Thesis, Queen's
University, Kingston, ON, 1995.
Cai, H. "Recycled Polymer Modified Asphalt-Aggregate Mixes", Intemal Report:
Depmtment of Chemistry, Queen's University, Kingston, ON, 1996.
"Standard Test Method for Effect of Moisture on Asphdt-Concrete Paving Mixtures",
ASTM D 4867-88,1988 Annual Book of ASTM Standards, vol. 04.03, American
Society for Testing and Materials, Easton, MD, 1988.
Tunnicliff, D., Root, R. "Use of Antidpping Additives in Asphaltic Concrete
Mumires - Laboratory Phase", NCHRP Report 274, Transportation Research Board,
National Research Council, Washington, DC, 1984.
Kanerva, H., Vinson, T., Zeng,H. "Low-Temperature Cracking: Field Validation of
the Thermal Stress ReStrained Specimen Test". Strategic Highway Research Program
Report: SHRP-A-401, National Research Council, Washington, D.C., 1994.
Jung, D., Vinson, T. "Low-Temperature Cracking: Test Selection", Shategic
Highway Resemch Program Report: SHRP-A-400, National Research Council,
Washington, D.C., 1994.
6.0 STABLE FREE RADICAL POLYMERIZATION
RESULTS AND DISCUSSION
6.1 Isoprene Polymerizations
6.1.1 Preüminary Studies
Initial experiments on isoprene polymerization were focussed on obtalliing
reaction conditions which codd be used to produce polyisoprene with nmow
polydispersity (-1 -5) with the highest possible molecular weight and yield. It was
quickty discovered that a rate-enhancing additive would be needed to give materials of
hi& molecular weight in a reasonable polymerization t h e . Table 6.1 summ&s
the
reaction conditions and results of a senes of exploratory isoprene polymerizations.
Surnmary of Exploratory Isoprene Polymerizations
Table 6.1 :
Reaction
Number
Isoprenel
BPOa
TEMPO/
BPO/
Additivea
a
Molar ratio
Reaction
Time
b)
M n
M
W
(daltons) (daltons)
PD
Cnide
Yield
(%)
It should be noted that not only did the additive increase the yield and molecular
weight for a given reaction, there was also a broadening in polydispersity, and in some
cases a slight high molecular weight shoulder in the GPC trace of the additive-assisted
polymers. The additive is thought to work by reducing the amount of fiee nitroxide in
the system as the polymerization proceeds, compensating for an increase in fiee nitroxide
caused by the irreversible termination of some of the polymer c h a h . This enables the
reaction to continue at a reasonable rate, since the rate of polymerization in a nitroxide
mediated polymerization is inveaely proportional to the concenmtion of fiee nitroxide in
the systernl. However, the exact mechanism of the rate enhancement is not known.
From the data in Table 6.1 it can also be seen that by keeping the ratio of TEMPO/BPO/
Additive constant, molecular weight can be controlled by adjusting the monomer to
initiator ratio for a given polymerization tirne. It should also be noted that maintainhg
the ratios of TEMP/BPO and BPO/Additive close to 1.3: 1 and 1:1 respectively appears
necessary for maintaining a narrow polydispersity at longer polymerization times.
6.1.2
Scaled Reactions
From the initial experirnents, conditions were selected for the scale up of two
polyisoprenes syntheses to target materials with molecular weights (Mn's) of 2OK and
40K. The characteristics of the resulting polymers, which will be referred to as low
moleculm weighr and high molecular weight polyisoprene for the purpose of this thesis,
are given on the following page:
Table 6.2:
Summary of Scaled Isoprene Polymerizations
Material
Mn(daltons)
MW(daltons)
PD
Yield (%)
Low MW polyisoprene
17400
24800
1.43
40
High MW polyisoprene
40800
67800
1.66
39
The molecular weights and yields of the scaled reactions were as expected fkom the
preliminary studies, and the materials both displayed a very slight high molecular weight
shoulder as was characteristic of reactions involving the rate enhancing additive.
6.2 Styreoe Polymerizations
Styrene polymerizations were performed with monorner to initiator ratios of 100,
200,400 and 800 to 1 to provide materials mode1 studies on fùnctionalization. Figure 6.1
shows how that during a typicai SFRP styrene polymerization, molecular weight
increases with time and a narrow polydispersity is maintained during the course of the
reaction. Figure 6.2 illustrates how for each of the styrene polymerizations, number
average molecular weight increases linearly with conversion, characteristic of a living
polymerization system.
1 O000
Molecular Weight Idaltons)
Figure 6.1 :
Number Average Molecular Weight (M.) as a Function of Reaction
Time for a Nitroxide Mediated Styrene Polymerization
20
30
Conversion (%)
Figure 6.2:
Number Average Molecular Weight (M.) as a Function of Conversion and
Molar Ratio of Monorner to Initiator for a NitroXide Mediated Styrene
Polymerization
63 Characterization
Molecular weights reported in the previous sections and throughout the rest of this
thesis are based on polystyrene standards, and therefore the absolute values for the
polyisoprene samples are less meaningfid than the relative numbers between polymers.
To monitor the fimctionalkation reactions described in the following sections, it
is useful to k
t point out some of the characteristics of the
'HNMR spectra of SFRP
polystyrene and polyisoprene. SFRP polystyrene has been weil studied in the literature in
terms of the resonances characteristic of both the TEMPO and benzyloxy endgroups
(when initiated with BPO) and the methine and methylene protons adjacent to these
structures2. Figure 6.3 illustrates some of these signds for SFRP polystyrene. At 0.1,0.2
and 0.9 ppm are resoaances characteristic of TEMPO methyl groups. Between 3.9 and
4.6 ppm is a region of complex microstructure which due to the methylene and methine
protons adjacent to both TEMPO and BPO ,with the TEMPO group being rotationally
restricted at room temperature. From 7.4 to 8.0 ppm are three resonances which are
characteristic of the ortho, meta and para protons on the benzoyloxy initiator fragment.
An in depth study of SFRP polyisoprene end-groups and microstructure has not
been previously perforrned, and is complicated by the fact that while styrene tends to
polymerize in a head to tail conformation, isoprene polymerizes with a combination of
1,2-, 1,4-, 3.1-, cis and tram steriochemistries. However, some 'H NMR signals are still
worth identifjmg as they cm be used to monitor changes in stnicture at either end of the
polymer chain. Figure 6.4 is a 'H NMR spectrum of SFRP polyisoprene. At 1.1, 1.2 and
1.5 pprn are signals characteristic of a TEMPO end group. Similar to polystyrene,
resonances at 7.4,7.6 and 8.0 ppm are due to the benzoyl peroxide initiating fhgment. In
the expansion, two peaks are highlighted. At 4.1 and 4 3 ppm are resonances fkom
protons adjacent to the nitroxy group while the strong si@
at 4.7 ppm is characteristic
of protons on carbon adjacent to the benzoyloxy group.
A sample of conventional polyisoprene was synthesized (in the absence of
TEMPO) to give a low molecular weight polymer of M.
- 3800 and PD - 1.50. Figure
A l in the Appendix is a 'H NMR spectrum of this material, which was used to CO&
@y comparing polyisoprene methyl resonances identified in the ~iterature~*~)
that the
SFW polyisoprene has essentially the same microstructure as conventionally
polymerized matenal. It should also be noted that the spectnun contains the signal at 4.7
ppm fiom protons adjacent to the benzoyloxy group, but there is no signal at 4.1 or 4.3
ppm due to protons adjacent to a TEMPO end group.
4.5
Figure 6.4:
4.0
ppm
IH NMR Spectrum of SFRP Polyisoprene Showing Characteristic Resonances of Tempo (T) and
Benzoyloxy (B) Endgroups and Adjacent Protons on the Polyisoprene Chain
6.4 Functionalization Reactions
A series of different approaches were investigated to provide end-bctionalized
polymers for asphalt modification, and these are reported here in the order they were
attempted, combined with the reasoning behind moving from one chemistry to the next.
6.4.1 Chlormethylstyrene Unimer
Since several researchers had synthesized unimolecular initiating adducts based
on a single repeat unit ancVor initiator hgment capped with a labile nitroxide (a
the chloromethylstyrene unirner was synthesized to provide an initiating
~.nirner)~*~",
adduct which could be used for both polymerization initiation and for M e r
fimctionalization chemistry. The CMS unimer was synthesized at a 48 % yield, assurning
an initiahg efficiency of 0.5 for BPO and with TEMPO as the limiting reagent. The
product was purified by flash chromatography and its structure confimied by 'HNMR as
illustrated in Figure 6.5. The 'HNMR spectnim is similar to that of the styrene unimer
(BST) reported by Georges et al!, with three distinct regions in the spectrum. From 0.5 1.5 ppm the four broad singlets are characteristic of the methyl groups on the TEMPO
moiety in a restricted rotational environment caused by the styrene aromatic group.
Between 4.4 and 5.2 ppm are three mutliplets caused by the three aliphatic styrenic
protons. A pronounced difference fiom the BST spectrum is the presence of a pair of
-
singlets at 4.6 pprn, which are characteristic of the choromethyl protons of both the 3and 4- isomers of the CMS segment. The aromatic protons of both the styrenic and
benzoyl groups are found in the region between 7.2 and 7.9 ppm.
To demonstrate chain extension of the CMS uninier, the adduct was extended
-
with isoprene to give a polymer of M, 3700 and PD 1.25. Figure 6.6 shows an
expansion of the CMS initiated polyisoprene in the region characteristic of termi.mil
protons on the polyisoprene segment. At 4.1 and 4.3 ppm are the s i d s fiom isoprene
protons adjacent to the TEMPO end group. However, there is no strong signal at 4.7 ppm
which would be characteristic of isoprene protons adjacent to a benzyloxy group.
uistead, there is a signal at
- 4.6 ppm fiom the chloromethyl protons on the single CMS
unit which sits between the benzoyloxy group and polyisoprene chain. Though not
demonstrated in this thesis, the CMS functionality can be easily used for other chemistry,
such as quaternarization, which has been demonstmted for polymers with multiple CMS
units7.
6.4.2
Removal of Nitroxide
A simpler route to an end-functionalized material would be to synthesize a
polymer under SFRP conditions and then modiQ one of the ends afier the
polymerization. This wodd eliminate the step needed to synthesize and isolate a
hctional initiating adduct. Since asphalt is generally mixed at temperatures at which
the nitroxidel polymer bond is quite labile (150-1 60°C), it was thought that this would be
a logical location to attempt to functionalize the polymer, by replacing the labile nitroxide
with a covalently bound functionality. Polyisoprene of M.
- 2700, PD - 1.27 was
reacted with zinc in refluxing acetic acid in an attempt to reduce the O-Nnitroxide bond.
Figures 6.7 shows that the TEMPO resonances normally Iocated at 1.15, 1.2 and 1.5 ppm
are no longer present in the 'H NMR spectnim of the modified polymer. In the region
Figure 6.6:
NMR Spectrum of Polyisoprene Initiated with Chloromethylstyrene Unimer (Expansion)
r
l
l
l
~
l
1
5.5
Figure 6.7:
l
L
5.0
~
l
l
l
4.5
l
[
l
l
l
4.0
l
~
l
l
ppm
l
l
i i ~ l l l l ~ l l l ~ ~ " l l ~ l I l I ~ I I I l [ I I I I ~ 1 I I I
1.5
1.3
1 .I
0.9 ppm
lH NMR Expansions of Polyisoprene (a) and Polyisoprene Reacted with
ZindAcetic Acid @) in Regions Characteristic of TEMPO and Adjacent
Proton Resonances
characteristic of the protons adjacent to TEMPO,the signals at 4.1 and 4.3 ppm have
shifted to 4.4 and 4.55 ppm, which could be characteristic of protons next to a hydroxyl
group. Solomon et al. have demonstrated that this chemistry converts nitroide capped
methyl acrylate oligomers to hydroxyl terminated m a t e r i d . However, it was decided
that though this reaction shows potential for low molecular weight materials, higher
moleculm weight polyrners would necessitate the use of solvents to solubilize the
polymer during the reduction reaction, adding a further complication to the
hctionalization chemistry. Since the completion of this experimentd work, Pionteck
demonstrated a series of other chernistries for fiinctionalizing the polymerlnitroxide
bondg. These methods also required a series of steps, some with fairly toxic chemicals,
so a simpler method was sought.
6.4.3 Synthesis of Block Copoiymen of Polystyrene
SFRP polystyrene was selected as a mode1 compound for block copolymer
synthesis with the silane and amine fûnctionalized methacrylates TMSPMA and
DMAEMA, whose structures are illustrated below :
H
'c=c,
H'
.CH,
OCH,
1
C-O-(CHJrSi-OCH,
I
0"
OCH,
H
'c=c,
H'
.CH3
0"C-O-(CH2)rN,
TMSPMA
Figure 6.8:
1CH3
Structures of 'Functional' Methacrylate Monomers
DMAEMA
Ch
-
When polystyrene of Mn 7500 was heated to 145OC in TMSPMA and sampled
over four hours, rnolecular weight increased during the fîrst h o u as monitored by GPC,
then rernained essentially constant during the subsequent 3 hours, as illustrated in Table
Table 6.3 :
Formation of Polystyrene-b-TMSPMA
Reaction Time (hrs)
Mn (cialtons)
MW(daltons)
Polydispersity
O
7500
8500
1.14
1
8800
9900
1.12
2
8800
9900
1.13
3
9000
10000
1.12
4
9000
9900
1.1 1
While the absolute block length of the TMSPMA segment can not be estimated directly
fiom the GPC data (since the system is calibrated with polystyrene standards), the facts
that the polydispersity remains nmow and that the molecular weight does not increase
significantly d e r the first hour suggest that the reaction essentially stops after the chahs
extend a short length, and that there is some sort of terminahg mechanism for the living
for block- and
polymer chains. This result has been reported before by other~'~*"
homopolymerization of a variety of methacrylates. Moad et al. have proposed that under
normal SFRP reaction conditions, methacrylate c h a h tend to grow a few units then
terminate by a proton abstraction reactioo by the nitroxide, yielding an olefinic endgroup
on the methacrylate chah and the correspondhg hydroxyarnine of the nitroxide'
'.
Figure 6.9 is a dual detection GPC plot using rehctive index and U V detectors of
the initial reaction mixture and the polymer after chah extension. While rehctive index
alone is not sensitive for measuring block length, any polystyrene which has not chah
extended should appear in the UV-absorbance plot as a low moiecular weight shoulder,
since the UV detector set at 254 nrn should only see the aromatic protons on the
polystyrene block. While there is a very slight low molecular tail in the UV plot of the
block copolymer, it appears that most of the chah has extended.
Block copolymer formation can also be rnonitored by 'H NMR spectroscopy.
Figure 6.10 iilustrates how the TMSPMA block length c m be estimated based on
comparing the relative integrations of the aromatic polystyrene resonances (between 6.3
and 7.3 ppm) with the eimethoxysilyl resonance at
- 3.6 ppm.
Assuming that a
polystyrene of Mn= 7500 has approxirnately 70 repeat units, Uitegration of these peaks
suggests that the chains have extended on average by 5 units of Th4SPM.Aor a molecular
weight increase of
- 1250 daltons.
Figure 6.1 1 shows a 'H NMR overlay of the region characteristic of protons
adjacent to TEMPO and the benzoyloxy group in both the starting polystyrene and the
block copolymer. It can be seen fiom this diagram that the broad resonances fiom the
styrenic proton adjacent to TEMPO at 4.4 and 4.5 ppm have disappeared in the block
copolymer (as expected), while the signal for protons next to the initiator hgment at
4.28 pprn remains unchanged.
The same experiments were performed to produce DMAEMA modified
polystyrene, and the reaction was also found to stop after a short penod of growth. Based
on these results it was decided to chah extend the scaled polyisoprene reactions to
provide fùnctionalized higher molecular weight elastomers for asphalt modification.
1 O00
1 O000
100000
Molecular Weighî (daltons)
Figure 6.9:
Dual Detection GPC Plot of Polystyrene and Polystyrene Chain Extended
With TMSPMA
Figure 6.10: Estimation of TMSPMA Block Length by 'HNMR Comparing Polystyrene Aromatic
Resonances with TMSPMA Trimethoxysilyl Group Resonances
Figure 6.1 1 : 'HNMR Spectra of Low Molecular Weight PST and Block Copolymer Formed by Extension
with TMSPMA (region characterstic of styrene protons adjacent to TEMPO and Benzoyloxy Group)
6.4.4 Synthesis of Block Copoiymem of Polyisoprene
Functionalization of the scaled high and low molecular weight polyisoprenes was
performed by dissolving these materiais in TMSPMA and DMAEMA and heating to
145°C with stimng. The reaction mixture was sampled before and during the reaction,
and the samples were injected into a GPC. The materials reacted with TMSPMA showed
a very slight Nicrease in molecdar weight, but it was dinicult to distinguish from
molecdar weight variations produced by repeated injection of the initial reaction mixture.
However, when the higher molecular weight polyisoprene was reacted with DMAEMA,
there was a distinct decrease in molecular weight when measured by GPC in
tetrahydrofuran, as illustrated in Figure 6.12. This shifl to lower molecular weight is not
surprising since a polar DMAEMA block on the end of a polyisoprene chah should give
it Merent solution properties than the homopolymer in THF. As a rough estimate of the
average methacrylate block length (based on polyisoprene molecdar weight deterrnined
by GPC withpoljsfyrene standards), 'HN M R was used to measure the ratio of
characteristic polyisoprene and methacrylate resonances. For TMSPMA, this involved
comparing olefinic polyisoprene resonances between 4.5 and 6.0 ppm with the
trimethoxysilyl resonance at
- 3.6 ppm. For DMAEMA modified polyisoprene, the
olefinic polyisoprene resonances were compared to the resonance of the two DMAEMA
methylene protons adjacent to the ester oxygen. Samples were repeatedly precipitated
fiom dichloromethane into methanol to conhm that the magnitude of these peaks did not
change with extended purification (as would be the case if the methacrylate signals were
caused by low molecular weight methacrylate homopoiymer oligomrn or residual
monomer).
1 O00
10000
100000
Molecular Weight (daltons)
Figure 6.12:
Formation of DMAEMA Functionalized Polyisoprene
1000000
Table 6.4 n i m m k e s the average molecular weight increase for the modifieci
polymers as determined by 'H
and Figures A3, A4, and A5 in the Appendix show
the spectra and integrals used to determine these relative values.
Table 6.4:
Average Block Length of Functionalized Polyisoprenes
Modified Polyrner
LOWMW PI-b-TMSPMA
High MW PI-b-DMAEMA
a
Polyisoprene
TMSPMA
Block (dalton~)~ E310ckb(daltons)
17400
1200
40800
-
DMAEMA
~ l o c (daltons)
k~
4000
Determined by GPC with polystyrene standards
Detennined by 'HNMR relative to the polyisoprene blocka
These materials were tested as asphalt additives after purification by precipitation fiom
dichioromethane into methanol. Though it is unclear fiom this research exactly what
fiaction of polyisoprene chahs have extended with the functional monomers, the
materials appeared to have sufncient bctionality to warrant their evaluation in asphalt
as a prelirninary investigation into the performance of this type of functionalized
elastomers.
84
6.5 References
Veregin, R., Georges, M., Hamer, G., Kazrnier, P. "Mechanism of Living Free
Radical Polymerization with Narrow Poiydispersity: Electron Spin Resonance and
Kinetic Studies", Macromolecules, I 995,28,439 1.
Georges, M., Veregin, R., Kazmier, P., and Hamer, G. 'Taming the Free-radical
Polymerizatioa Process", Trenak in Poljmer Science, 1994,2,66.
Pham, Q., Petiaud, R. Spectres RMV des Polymeres, Vol 1., SCM,Paris, 1991, 126.
Hawker, C. b'Molecular Weight Control by a "Living" Free-Radical Polymerization
Process", Journal of the American Chernical Society, 1994, 116, 11185.
Moffat, K., Hamer, G., Georges, M., hzmier, P. Stover, H. "Stable Free Radical
Polymerization of Styrene Using a Combined Initiator Reversible Tenninating
Agent", Polymer Preprints, 1996,37(2), 509.
Braslau, R., Burrill, L., Siano, M., Nail, N., Howden, R., Mahal, L. "LowTemperature Preparations of Unimolecuiar Nitrox.deInitiators for ''Living" Free
Radical Polymerizations", Mucrumolecules, 1997,30,6445.
Listigovers, N., Georges, M., and Honeyman, C. "Synthesis of Diblock Copolymers
via a bbLiWig'y
Stable Free Radical Bu1.k Polymerization and Conversion to
Functionalized Amphiphilic Materials", Polymer Preprints, 1997,38(2), 4 10.
Solomon, De,Waverly, G., Rizzardo, E., Hill, W., Cacioli, P. US. Patent 4,581,429,
1986.
Malz, H., Komber, H., Voigt, D., Piontek, J. "Reactions for Selective Elimination of
TEMPO End Groups in Polystyrene", Macrornolecu[m Chemistry und Physics, 1998,
199,583.
10. Lokaj,J., Vlcek, P., Kriz, J. "S ynthesis of Pol ystyrene-Poly(2-(dimethy1amino)ethyl
methacrylate) Block Copolymers by Stable Free Radical Polymerization",
Macrornoleçule, 1997, 30,7644.
11. Moad, G., Ercole, F., Krstina, J-, Moad, C., Rizzardo, E., Thang, S. "Controlled
Growth Free Radical Poiymerization of Methacrylate Esters - Reversible Chain
Transfer vs. Reversible Tenninationy',Poljmer Preprints, 1997, 38(1), 744.
7.0 ASPHALT RESULTS AND DISCUSSION
7.1 Moisture Sensitivïty
The moisture sensitivity of a standard HL-3design asphalt pavement was evaluated
and the effect of polymer modification on the retained strength after moiconautioning was investigated. Two polyisoprene homopolymers, and these base
materials extended to block copolymers with TMSPMA and DMAEMA, were tested as
asphalt additives at 3 and 5 wt. % on the binder. Table 7.1 nimmarizes the tensile
strength meamrements on these materials using the modined Tdcliff-Root Method.
While two separate control samples of unmodified R754 binder were prepared and
evaluated, mauily Control Sample 2 will be used for cornparison as its voids content is
more similar to the voids measured in the modified samples. Control Sample 1 has a
significantly lower voids content then the rest of the samples, and therefore results in a
higher average conditioned and unconditioned tensile strength.
From Table 7.1 it can be seen that both control samples lose approximately half
their original dry strength after vacuum saturation and the 24 h o u rnoisture conditioning,
ve-g
that the aggregate/asphalt combination selected to represent a typical Southem
Ontario pavement was indeed moisture sensitive. Furthemore, examination of the
fiachire surface suggested that failure might be occtming due to either cohesive failure in
the binder or stripping of the fine aggregate, rather than stripping of the coarse aggregate,
which appeared to rem& well coated after moishue conditioning. This was as expected
since coarse limestone is known to resist rnoisture damage and perform well in stripping
tests, while sand, a highly sensitive fine aggregate, ofien leads to a more catastrophic
'
pavement failure .
Table 7.1:
Summary of Tende Strengths and Retained Strength Ratios for Control
and Polymer Modified Asphalt Concrete Mixtures
Binder
Control 1
('754)
Uncondirioned Sampies
N Voids Dry Strength
(%)
Wa)"
4
6.2
3.10k0.34
Conditioned Sumpies
n Voids Wet Strength
('w
Wa).
TSR
(%)
4
6.1
1.60 I0.08
52
3
7.3
2.67
* 0.36
3
6.9
1.52 i 0.2 1
57
5% Low MW
Polyisoprene
3
8.0
1.65 k 0.27
3
7.7
1.14 k 0.25
69
3%HighMW
Polyisoprene
5
7.0
2.56 k 0.26
3
7.1
1.29 t 0.1 1
50
3% Low MW
PI-b-TMSPMA
3
7.1
2.79 k 0.48
3
6.8
2.1 1 t 0.30
76
5% Low MW
5
8.4
1.93k0.30
3
8.3
1.48 k 0.20
77
3% High MW
PI-b-TMSPMA
5
8.7
2.11k0.23
3
8.6
1.82 k 0.34
86
5% High M W
3
7.6
2.31 k 0.22
3
7.5
2-08 k 0.08
90
3% Low M W
PI-b-DMAEMA
2
7.2
2.25 k 0.31
2
7.3
1.50 t 0.48
67
5% Low MW
3
7.8
1.88k0.18
3
7.6
1.27 +- 0.17
68
3% Low MW
Polyisoprene
Polyisoprene
a
90% Confidence interval
In the first set of polymer modincation experiments, R754 binder was modined
with 3 and 5 wt. % of the low (-25K)and higher (-40K)molecular weight wlyisoprene
homopolymers. Figure 7.1 shows the conditioned and unconditioned tensile strengths of
these materials compared to the unmodified binder. In each case the retained tensile
strength is significantly less than unconditioned strength, with tende strength ratios
ranging fiom 5û-69 %. Though the retained strength ratio for the 5 wt. % low MW
polyisoprene sarnple seems high (compared to the control and other homopolymer
modifïed samples), this is a result of the unconditioned tensile strength being low, likely
due to a plasticizing effect caused by the low molecular weight polymer being added at
higher concentrations to the binder.
Figure 7.2 sumarizes the tensile test results for the polyisoprene block
copolymers. Like the polyisoprene homopolymer, the 5 wt. % low molecular weight
silane block copolymer appears to have a reduction in dry strength compared to the
control, probably due to a similar plasticizing effect. However, al1 four of the silane
block copolymers have significantly higher retained strengths than their corresponding
homopolymer samples. The tensile strength ratios for the unmodified, homopolymer and
block copolymer samples are represented graphically in Figures 7.3 and 7.4 respectively,
which clearly show that the silane block copolymers outperform both the polyisoprene
homopolymers and the tertiary amine-functionalized block copolymers in the Tunnicliff
Root Test. The higher molecular weight silane polymer appears most promising with
retained strength ratios approaching 90%.
R754 Control R754 Control R754 + 3%
R754 + 5%
R754 + 3%
R754 + 5%
1
2
Low M W PI Low M W PI Hïgh M W PI High MW PI
Unconditioned Sarnples
Figure 7.1 :
Conditioned Sarnples
Cornparison of Conditioned and Unconditioned Temile Strengths o f
Unmodified and Polyisoprene Homopolymer Modified Asphalt Concrete
SampIes
Low MW PI- Low MW PI- High M W PI- High M W PI- High MW PI- High MW PI-
bTMSPMa
b-TMSPMa
bTMSPMa
b-TMSPMa b-DMAEMA b-DMAEMA
Unconditioned Samples ~ConditionedSamples
Figure 7.2:
Cornparison of Conditioned and Unconditioned Tensile Strengths of
Polyisoprene Block Copolymer Modified Asphait Concrete Samples
R754 + R754 + R754 +
Control1 Control2 3% Low 5% Low 3% High 5% High
R754
R754
R754 +
MW PI
Figure 7.3 :
M W PI
MW PI
M W PI
Cornparison of Tensile Strength Ratios of Unmodified and Polyisoprene
Homopolymer Modified Asphalt Concrete Samples
Low M W PI- tow M W PI- High M W PI- High M W Pi- High MW PI- High M W PIb-TMSPMa
Figure 7.4:
b-TMSPMa
b-TMSPMa
b-TMSPMa b-DMAEMA b-DMAEMA
Cornparison of Tende Strength Ratios of Polyisoprene Block Copolymer
Modified Asphalt Concrete Sample
It is somewhat surprising that there appears to be little difference between the
binders modified with 3 and 5 wt. % of the high molecular weight silane block copolyrner
However, had more niw material been available, an increased number of tests at each
concentration rnight be able to distinguish between the two levels of modification. It is
also unclear fiom the TunniclZf Root Test alone whether the improvement in moisture
sensitivity is a result of chernicd bonding between the silanol groups on the polymer with
surface fiinctionalities on the mineral aggregate, or whether the improved stripping
resistance is a consequence of an increase in binder viscosity due to crosslinking of the
polyrner during the moisture conditioning phase. Regardless of the mechanisrn of
irnprovement, the silane-modified polymers show promise as asphalt modifiers.
7.2 Low Temperature Performance
Since moistue sensitivity is only one of a number of factors that govem the field
performance of a pavement, the themal stress restrained specimen test (TSRST)was
selected to study the low temperature performance of the silane-modified polymers. A
different binder (TK110)at 6 wt. % on the asphaWaggregate mixture was used and voids
were kept in the 4-5 vol. % range. These values are more characteristic of a typical
pavement (the Tunnicliff Root conditions are selected to increase the susceptibility for
moisture damage), and also permit direct comparison with the results f?om another study.
Table 7.2 summarizes the TSRST fkcture strength and hcture temperatures of TIC110
control and 3 and 5 wt. % high MW polyisoprene-b-TMSPMA modified asphalt
concrete. Also included for comparison are TSRST results fiom previous research
comparing TIC110 with SB modified TKl IO binder using the same aggregate gradation2.
Table 7.2
Summary of TSRST Low Tempera-
Binder
Tests
Perfomame
Voids
Peak Stress
Failure or Yield
("/.>
Temp (OC)'
Failure
-3 1.3 2-8
Catastrophic
Mode of
-
ControI (TK110)
4
42
(MWa
2.51 f 0.70
3% High MW PIbTMSPMA
5% High MW PIb-TMSPMA
Controi ('T'Ki 1O)'
3
5.3
1.83 & 0.40
-38.3 f 0.5
Yield
3
5.3
2.31k0.35
-37.8 t 5.9
Yield
5
2.6
3.1 1 0.7
-32.1 f 1.6
Catastrophic
3% SBD
4
4.7
3.1 1 0.3
6% SBD
5
6.3
3.0 3- 0.7
*
+ 2.2
-4 1,2 + 0.5
-38.3
Catastrophic
Catastrophic
% Confidence interval
R e d t s nom reference 2 included for cornparison only
a 90
Typically, the control sarnples are able to undergo stress relaxation at
temperatures above about -22"C, but as the sarnples cool below this point, stress
increases Iinearly with temperature until the sample breaks catastrophically. While the
hcture temperature for TKI IO binder correlates well with the results from a previous
study, the fracture stress found in this thesis is lower. This can be explained by the fact
that though voids content has little effect on the h c t u r e temperature in a TSRST, a
higher percentage voids tends to result in a lower fiachue strength for a given binder?
For the PI-b-TMSPMA block copolymers, Figure 7.5 illustrates how the block
copolymer modified sample relaxes stress in the asphalt briquette more quickly than the
unrnodified binder (Le. the accurnulated thermal stress is lower at any given temperature).
Furthemore, it shouid be noted that at modification levels as low as 3 wt. %, the
temperature at which the peak stress is reached is 7°Cbelow that of the control sarnples,
and that the failure is not catastrophic. Instead the maximum stress is followed by a slow
decrease in intemal stress without hcture to temperatures as low as -50°C. Figure 7.6
shows three TSRST stress vs. temperature c w e s for TKI IO binder modined with 3 w t
% silane-isoprene block copoiymer. Although there is some variation in the peak stress
between samples, they all reach a maximum stress at around -38OC. The fact that none
of these samples fail below this temperature may be a result of the interaction between
the silane finctionality on the polymer with d a c e fiinctionalities on the polar
aggregates, combined with the rubberizing effect of the polyisoprene block.
It should be noted that the 7°C improvement in low temperature performance for
the 3 wt. % silane-modified polyisoprene binder is equivdent to the enhancement found
for 3 wt. % SB modification in the previous shidy (see Table 7.2). However, the SB
modified asphalt concrete is reported to fail catastrophicaliy, while the PI-b-TMSPMA
modified material does not appear to do so in this study. Styrene butadiene copolymers
are also not known to significantly improve pavement rnoisture sensitivity when added
directiy to the binder4 and often require the use of an additional antistripping agent in
moisture sensitive mix designs5.
O
Figure 7.5:
-10
-20
-3 0
Temperature (OC)
-40
-50
Cornparison of Thermal Stress Accumulation in Unmodified and
Polyisoprene-b-TMSPMAModified Asphalt Concrete
-20
-30
Temperature (OC)
Figure 7.6:
Stress Accumulation in Silane Modified Asphalt Samples During
Thermal Stress Restrained Specimen Tests
73 Other Considerations
Polymer Architecture
73.1
While several companies now sell speciaky SB and SBS block copolymers
specincdy designed for asphalt modification, the materials in this thesis were
synthesized for a prelunlliary investigation into the effectiveness of a short fimctional
block in an elastomeric asphalt modifier,and therefore bbck length and overail
molecular weight have not been optimized. Isoprene was selected to form the
elastomenc segment due to its ease of handLing compared to butadiene. However,
polybutadiene is known to have a significantly lower glas transition temperature than
polyisoprene, and therefore a polybutadiene based block copolymer would likely have
better low temperature performance than the polyisoprene based material used in this
study. For example, assuming that the elastomer and asphalt cernent form a completely
miscible mixture, the Fox ~ ~ u a t i can
o d be used to estirnate a glass transition
temperature, Tg,for the mixture:
- 1 - --
4 +-M2
Tg,,
Tg,
Tg2
where:
T&h
Ta,Tf3
Ml,
Mt
-
Glass transition temperature of the mixture, K
Glass transition temperatures of components 1 and 2, K
Mass fiactions of components 1 and 2.
For 5% polymer modification and substituthg approximate g l a s transition temperatures
for asphalt cernent, polyisoprene and polybutadiene of 0, -72 and -85°C respectively, the
use of a polybutadiene segment would likely r d t in a 102°C &op in the Tgof the
mixture compared to polyisoprene, which is significant in t e m of low temperature
performance.
73.2 Cost of Modification
It has been suggested by the Ontario Ministry of Transportation that for a polymer
modified asphalt pavement to be cost effective, the pnce associated with polymer
modification should not increase the cost of the asphalt cernent by more than 100 %7.
The following table summarizes typical costs of asphalt cernent and some potential
polymer modifiers:
Table 7.3: Siimniary of Asphalt Cement Materials Costs
Material
Approximate Cost (SCDN)
Reference
Asphalt Cement
$150 / tonne
MTO'
SBS
$1.10 / lb
Fina Chemicals
Polybutadiene
$1.00-1.15 11b
Bayer Rubber Inc.
Polyisoprene
$1.70 / lb
Goodrich Chernicals
Silane Monomer (TMSPMA)
$20.00 1 lb
Witco, Dow Coming
According to the M T 0 study, if 6 % polymer modification is required, then the polymer
additive should cost less than S 1.25 1 Ib. This is well below the potential cost of the
silane modified elastomers synthesized in study. However, if only 3 % silane modified
elastomer is required, obviously the polymer cost couid double to $2.50 / lb. While the
silane monomer is expensive, if the silane-polybutadiene block copolyrner was effective
at a silane block length of 5 - 10 wt %, and if the cost of the living radical process
became comparable to that of the anionic process used to make SBS, a silane modified
polybutadiene could be feasible, costing between $2.00 - 3.00 1 lb. This price would
drop M e r if the TMSPMA could be supplied on a buik scale (the pice quoted is based
on a 200 L d m ) .
From the results of the TunniclZf Root Test and the TSRST on the 3 wt. % higher
molecular weight polyisoprene-b-TMSPMA, silane modified elastomers may well have
potential as a new class of asphah modifiers. More research is needed to optimize the
polymer architecture and to develop a better understanding of the mechanism of
performance enhancement.
100
7.4 References
Hicks, G. "Moisture Damage in Asphalt Concrete", NCHRP Synthesis of Highway
Practice 175, Transportation Research Board, National Research Council,
Washington, DC, 1991.
Cai, H. Recycied Polymer Modijied Asphall-Aggregate Mixes, Intemal Report,
Department of Chemistry, Queen's University, Kingston, ON, 1997.
Jung, D., Vinson, T. "Low-Temperature Cracking: Test Selection", Strategic
Highway Research Program Report: SHRP-A-400, National Research Council,
Washington, D.C.,1994.
Davidson, J., Ernyes, J., "Stripping: A Laboratory Study", Proceedings of the
Canadian Technical Asphalt Associarion, 1993,3 8,77.
Won, M., Ho, M. "Effect of Antistrip Additives on the Properties of PolymerModified Asphalt Binders and Mixîures", Transportatim Research Record, 1995,
1436,108.
Sperling, L. Introduction to Physical Pol'er Science, John Wiley & Sons, Inc., New
York, NY, 1986.
Ponniah, J., Kennepohl, G. "Polymer-Modified Asphalt Pavements in Ontario:
Performance and Cost Effectiveness", Trumportation Research Record, 1996, 1545,
150.
8.0 SUMMARY AND RECOMMENDATIONS FOR
FUTURE WORK
8.1 Summary
The objective of this thesis was to synthesize and evaluate hctionalized
elastomers as asphalt additives for improving the rnoisture sensitivity and low
temperature performance of asphalt concrete. Using nitroide-mediated fiee radical
polymerization technology, polyisoprenes of various molecular weights were synthesized
by adjusting the initial monomer to initiator ratio and varying the reaction tirne for a
polymerization. A proprietary rate enhancing additive was necessary for achieving
higher fields and molecular weights, but was also s h o w to cause a slight broadening in
polydispersity .
Several approaches to polymer fhctionalization were demonstrated. The
choloromethylstyrene unimer was prepared as a functional, unimolecular initiahg
adduct, and reduction chemistry was presented for removal of the nitroxide ~ o the
m end
of a living polyisoprene chah Chain extension of polystyrene and polyisoprene with the
fùnctional methacrylates DMAEMA and TMSPMA was shown to proceed with only a
slight increase in molecular weight of the polymers.
Polyisoprenes reacted with DMAEMA and TMSPMA were evaluated as additives
for hot mix asphalt. Asphalt cernent modified with polysioprene-b-TMSPMA was shown
to considerably improve the moisture sensitivity of a standard MT0 HL-3 mix design, as
detemiined by a modified T u n n i c l i ~ o omoisture
t
sensitivity test. The same polymers
were also found to significantly decrease the failure temperature of asphalt concrete as
determined by a TSRST study.
8.2 Recommendations for Future Work
It is clear fiom this research that silane-modifieci elastomers show potential as a
new class of polyrners for asphait modification if they c m be produced at a reasonable
cost. Since this study involved a prelirninary investigation into the synthesis and
evaluation of a new type of matenal for use as asphalt additives, friture research should
focus on improving the yield and molecular weight control in both segments of the
polyisoprene block copolymers. Such improvement in architechiral control could be used
to optimize the polymers to give the most enhancement when added to hot mix asphalt.
Specific work could be focussed on methods of characterizing and controlling block
purity, as well as an investigation into the rnechanism of termination of SFRP
methacrylate block copolymers.
APPENDM
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.
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8.0
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Figure A 2
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Expansions of Chloromethylstyrene Unimer 'H NMR Spectnim
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Figure A4:
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