Chlorinated Paraffins as Effective Low Cost
Flame Retardants for Polyethylene
Daryl Stein and Don Stevenson
Dover Chemical Corporation
Dover Chemical Corporation
3676 Davis Road
Dover, OH 44622
Abstract
Chlorinated paraffins have long been used to flame retard rubber mine belts, adhesives, caulks,
urethane foams and fabrics. Due to improvements in manufacturing, chlorinated paraffins with improved
thermal stability are now available, which makes it possible to use these materials in polyolefins. For
example, chlorinated paraffins can be used in combination with other flame retardant additives such as
antimony trioxide, magnesium hydroxide, and even nanomers to yield flame retarded HDPE formulations,
which still retain good physical properties.
Introduction
Chlorinated paraffins are a low cost flame retardant with only the hydrated metal oxides being less
expensive. They are manufactured by reaction of a paraffin wax with elemental chlorine either neat, in a
solution, or in an emulsion. Product and property variations are induced by changes in raw materials,
duration and conditions of chlorination, work-up and stabilization. Based on bond dissociation energies,
chlorinated paraffins theoretically should have stability up to 300C. However, due to the presence of
imperfections in the molecule from the synthesis, the maximum recommended processing temperature for
chlorinated paraffins is 230C. Selection of the right feed stock, the optimum carbon chain length and the
correct chlorine content yields chlorinated paraffins with optimum properties for a given application. In the
case of solid flame retardants, the optimum carbon chain length is C22-C26 with the optimum chlorine
content at 73%, which is an average of one chlorine per carbon.
Chlorinated paraffins have a synergistic effect with Group V metal oxides, especially antimony
trioxide . In addition, since they lack chromophores, they have better UV stability than do brominated
aromatic fire retardants. Finally, chlorinated paraffins are additive flame-retardants that are melt processable
with partial solubility in thermal plastics. Therefore, they do not bloom and have an influence on physical
properties such as flex strength, heat deflection temperature and percent elongation.
1-3
Although chlorinated paraffins have not often been used in polyolefins due to concerns over thermal
stability, they can be part of a low cost flame retardant package as long as the processing conditions are not
too severe. One potential application is in HDPE for replacement of wooden pallets and crates, which is a
large market in which plastics in general have not penetrated to any great extent due to fire hazard and cost
issues. It would be advantageous then to have a low cost fire retardant package, which would allow an
HDPE pallet not to be any greater a fire hazard than a wooden one. At the same time, the flame retardant
package would preserve important physical properties such as flex and impact strength. Thus in this work
we are going to examine the use of chlorinated paraffins in combination with other fire retardants and
synergists in low cost flame retardant packages for HDPE.
Synthesis of Chlorinated Paraffins
In general, a wax is treated with molecular chlorine to yield a chlorinated paraffin and HCl as
shown in equation 1. The addition of one chlorine atom per carbon results in a product with a 73% chlorine
content. In practice the chlorine content for a solid product can range from 70-74% with 72% being a
typical value. The physical properties of the product depend on the chain length of the wax, the chlorine
content, and the manufacturing process.
C20H42 + 20 Cl2
C20H22Cl20 + 20 HCl
(1)
(73% Chlorine)
An example of the affect chain length and chlorine content on physical properties of a chlorinated
paraffin can be seen Table 1. Here the chlorinated paraffin with the longer chain length has a higher
softening point compared with one with the same chlorine content but shorter chain length.
In addition, as the chlorine content increased, the softening point for both the short and long chain
paraffins increased.
Table 1
The Effect of Chlorine Content on Softening Point
Wt% Chlorine
70
71
72
73
74
Softening Point °C
C 18-22
90
95
100
115
130
Softening Point °C
C 22-26
105
110
120
135
160
The manufacturing process can play an important part in determining physical properties and
stability of a chlorinated paraffin. As can be seen the Table 2, a change from the standard manufacturing
process to a new proprietary one, leads to a product with improved thermal stability. In figure 1 the standard
and improved process CP’s are compared in injection molding of polyethylene. In this example, a
formulation of 60% PE, 20% chlorinated paraffin (CP), and 20% antimony trioxide (ATO) were injection
molded at 200 °C. During this process, the injection molding was stopped and the hot material held in the
barrel of the injection molder for periods of five and ten minutes. When the injection molding was resumed,
the improved process CP showed significantly less color development.
Table 2
The Effect of the Manufacturing Process on the Properties of Chlorinated Paraffins
Standard Process
Improved Process
Chlorine Content, wt%
71-72
72-73
Softening Point, °C
105
130
Thermal Stability
HCl after 4h at 175°C
Color Stability
15 minutes at 200 °C
0.1
0.01
black
yellow
Mechanism of Flame Retardance
Studies have shown that upon heating, chlorinated paraffins release HCl, which in the condensed
phase can promote the formation of char and in the vapor phase can act a flame poison. The decomposed
chlorinated paraffin itself also forms a char-like residue, to further inhibit the release of volatile materials2.
Vapor phase inhibition is greatly enhanced when chlorinated paraffins are used with Group V metal
oxides, especially antimony trioxide. Although ATO alone does not work as a flame retardant, it has been
proposed that ATO reacts in with HCl in a series of reactions (eqs. 2-5) to form antimony trichloride, which
can act both as a radical scavenger to disrupt oxidation and as a vapor barrier above the condensed phase to
smother the flame1, 3. The mechanism is similar to the one proposed for brominated flame retardants.
Sb2O3 + 2HCl  2SbOCl + H2O
5 SbOCl  Sb4O5Cl2 + SbCl3
4Sb4O5Cl2  5Sb3O4Cl + SbCl3
3Sb3O4Cl  4Sb2O3 + SbCl3
ca. 250C.
245-280C.
410-475C.
475-565C.
(2)
(3)
(4)
(5)
Even though chlorine has a lower flame retardant efficiency than bromine, chlorinated paraffins can be
highly effective. They can also be used synergistically with brominated flame retardants to give an optimum
flame retardant package4.
Experimental
Raw Materials
The following raw materials were used:
Chlorinated paraffin, Chlorez 700 SSNP from Dover Chemical
Antimony trioxide from Laurel Industries
Magnesium hydroxide from Versamag
Expandable graphite, Grafguard 373 from Graftech, Inc.
Nanomer, I.44PA purchased from Nanocor
HDPE, M 6580 from Equistar, density 0.965, MI 8.4 purchased from General Polymers
Impact Modifier, Engage 8150 from DuPont Dow, density 0.868, MI 0.5 purchased from General
Polymers
Maleic anhydride grafted HPDE, Polybond 3009 from Crompton.
Preparation of the Nanomer Masterbatch.
The nanomer was not added directly to the flame retarded HDPE formulations, but instead it was
added as a masterbatch using a procedure recommended by Nanocor. A dry blend of HDPE (30), Polybond
(30), and nanomer (40) was prepared and was fed into a 27 mm Leistritz twin screw extruder running at 500
rpm. The heating zones were set from 140-160 °C. In general, the extruder was run as fast and as cold as
possible in order to generate the maximum amount of shear.
Sample Preparation
The flame retarded formulations were prepared in a lab size 1600 cc Banbury type internal mixer.
The mill was set at 99 °C, and the door was set at 93 °C. In general, the resin was added first and the shear
from the mixing caused the temperature to rise. When the temperature reached about 130 °C, the resin
began to flux. At this point the flame retardant compounds were added. The mixing was allowed to
continue until the resin temperature reached 160 °C. The total mixing time was about 15-20 minutes. The
sample was cooled, granulated, and injection molded into test specimens at 190 °C with a Battenfeld
injection molder.
Testing
The notched Izod impact strength was determined by the method described in ASTM D 256. The
flex modulus and flex strength was determined by the method described in ASTM D 790. The peak heat
release for 100 mm x 100 mm x 3.2 mm test plaques was measured at Ohio State University with a cone
calorimeter with a radiant heat flux set at 35 kW/m2.
The designed experiments and statistical analysis were carried out with Minitab statistical software program
from Minitab, Inc.
Discussion
Chlorinated Paraffins as Flame Retardants
An experimental program was undertaken with the improved grade of chlorinated paraffin to
develop flame retardant formulations for injection moldable HDPE, which is a difficult material to flame
retard, and still maintain acceptable physical properties. The goal was to develop a low cost formulation,
which would yield a cone calorimeter peak heat release that was similar to that for wood, which is about 250
kW/m2. The reason is that for safety purposes the pallet must pass an expensive full-scale burn test such as
UL 2335, and the cone calorimeter is a useful screening tool.
As can been seen in table 3, blending of a chlorinated paraffin into HPDE produced an increase in
flex modulus and flex strength but at the cost of a decrease in Izod impact strength.
Table 3
The Effect of Chlorinated Paraffins on Physical Properties of HDPE
Formulation
HDPE (nat)
10% CP
20% CP
1% Secant Modulus
MPa (psi)
827 (120,000)
806 (117,000)
889 (129,000)
Flex Strength
MPa (psi )
25.5 (3700)
26.9 (3900)
30.3 (4400)
Notched Izod
J/m (ft·lb/in)
64 (1.2)
37 (0.7)
27 (0.5)
In figure 2 are the heat release results for the chlorinated paraffin at 10 and 20% loadings. As can
be seen in the figure, only a modest reduction in heat release was obtained under the conditions used.
However, when the synergist antimony trioxide was used along with the chlorinated paraffin (CP/ATO 3:1),
a nearly 50% reduction in peak release was observed for the 20% loaded formulation. Therefore, a
combination of chlorinated paraffin and antimony trioxide can product a material with good flame retardant
properties and yet still maintain some good physical properties. Nevertheless, it is clear that additional
synergists are required to further improve the flame retardant and physical properties.
Use of Nanomers
Nanomers and the corresponding nanocomposites are currently the focus of intensive research,
because of the significantly improved physical properties of a polymer at low loadings. In contrast, there
has been relatively little work carried out on the use of nanomers as flame-retardants in polyethylene5. In
general, a 30 - 40% reduction in the rate of peak release in the cone calorimeter was observed for PE
nanocomposites6.
In figure 3 are the graphs for heat release versus time for different nanomer loadings. They
show a moderate decrease of about 23% for the rate of heat release. As for flame retardancy, it appeared
that little is gained by increasing the loading from 10 - 15%. In table 4 are our results for other physical
properties for the nanomer in HPDE. As can be seen in the table, the use of a nanomer alone caused a 20%
increase in stiffness, and a 16% increase in flex strength. However, there was a decrease in the notched
Izod impact strength.
Table 4
Physical Properties of HPDE with Added Nanomer
Sample
Peak Heat
Release
(kW/m2)
HDPE (natural)
712
5% Nanomer
679
10% Nanomer
568
15% Nanomer
549
1% Secant
Modulus
Mpa
(psi)
827
(120,000)
889
(129,000)
903
(131,000)
992
(144,000)
Flex Strength
Mpa
(psi)
Notched Izod
J/m
(ft·lb/in)
25.5
(3700)
28.2
(4100)
29.6
(4300)
30.3
(4400)
64
(1.2)
32
(0.6)
21
(0.4)
21
(0.4)
The next step was to use a combination of the chlorinated paraffin and the nanomer. For this work
we used a 22 factorial experiment with a center point. In this experiment there were two variables, which
were set at high and low levels. The first one was the chlorinated paraffin loading, and the second one was
the nanomer loading. In this experiment the ratio of CP to ATO ratio was fixed at 3:1 and the combination
of the two was used at 10 - 20 wt%. The nanomer was used at 5 - 15 wt%. The idea was to cover a broad
but still reasonable range of loadings. Advantages for this type of experiment are that the relative
importance of the variables on the responses such as peak heat release, flex modulus, and Izod impact can
be easily determined. In addition, interactions between variables and their influence on responses can be
observed.
The results for the heat release versus time are in table 5 and figure 4. A 28 - 58% reduction in rate
of heat release was found. Analysis of the data indicate that both CP and nanomer variables contributed to
the reduction in heat release. In fact of the two, the nanomer was the stronger contributor, which was not
expected based on the earlier results.
In addition to peak heat release, a 72% increase in flex modulus and a 24% increase in flex strength
were observed. However, the impact strength again showed a significant decrease. The analysis indicated
that both the CP and the nanomer loadings contributed to an increase in flex modulus and flex strength.
Again the nanomer was strong contributor. As for impact strength, both CP and nanomer caused a decrease.
Table 5
Results for the Combined use of CP and Nanomer in HDPE
Std Run
Order
CP+ATO
(wt%)
Nanomer
(wt%)
Peak Heat
(kW/m2)
1
10
5
480
2
20
5
512
3
10
15
320
4
20
15
298
5
15
10
375
Modulus
Mpa
(psi)
820
(119,000)
861
(125,000)
1171
(170,000)
1419
(206,000)
1006
(146,000)
Flex Str.
Mpa
(psi)
26.9
(3900)
28.2
(4100)
29.6
(4300)
31.7
(4600)
28.9
(4200)
Izod
J/m
(ft·lb/in)
32
(0.6)
21
(0.4)
16
(0.3)
32
(0.6)
16
(0.3)
Analysis of the data for interactions between the CP and nanomer, indicated that except for the case
of impact strength, that they were very small. It appears that the CP and the nanomer were strictly additive
in their effects at least as far as can be determined by the level of sensitivity for this design. Threedimensional plots of heat release and flex modulus versus CP and nanomer concentrations are given in
figure 5 and 6 and give a visual picture of these variables over the experimental range.
In contrast to the above results, we obtained some preliminary x-ray diffraction data (table 6), which
indicate that the chlorinated paraffin was aiding in the exfoliation of nanomer, as can be seen by the increase
in the d-spacing between the nanomer layers. The increase in separation (start of exfoliation) in the nanomer
layers may explain why the nanomer is now a strong contributor to physical properties in the presence of the
CP, especially for flex modulus. These results are not unexpected for it is known that chlorinated paraffins
can act as coupling agents in polyolefins that are filled with minerals such as mica and wollastonite 7.
Nevertheless, the extent of exfoliation (if any) in these samples will require additional work to determine.
This work is now in progress.
Table 6
XRD data for HDPE Samples
Wt% (CP + ATO)1
20
20
1
2
Wt% Nanomer
5
5
15
15
402
D-spacing (nm)
2.90
3.14
2.64
3.29
2.61
CP/ATO at 3:1 ratio
Masterbatch sample
Chlorinated Paraffins with Magnesium Hydroxide
Magnesium Hydroxide is a well-known fire retardant additive that works by releasing water upon
heating to cool the flame. It can be used at higher processing temperatures than can aluminum trihydrate,
and it often can impart better physical properties as well8.
In order to determine the efficacy of this combination, another 2 2 factorial experiment was carried
out. In this experiment, the following ranges were selected: magnesium hydroxide 20 - 40 wt% and (CP +
ATO) 10 - 20 wt% with the CP/ATO ratio fixed at 3:1. In addition, an impact modifier was used at 10 wt%
in each formulation in order to improve impact strength. In table 7 and figure 7 are the results of this
experiment. As can be seen from the data, the peak heat release has been effectively reduced to well below
the target of 250 kW/m2. At the same time, the stiffness and impact properties were maintained.
Table 7
Results for Chlorinated Paraffins and Magnesium Hydroxide in HDPE
Std Run
Order
(CP+ATO)
wt%
Mg(OH)2
wt%
Peak Heat
kW/m2
1
10
20
318
2
20
20
232
3
10
40
198
4
20
40
132
5
15
30
184
Modulus
Mpa
(psi)
675
(98,000)
758
(110,000)
779
(113,000)
799
(116,000)
841
(122,000)
Flex Str
Mpa
(psi)
20.7
(3000)
22
(3200)
21.4
(3100)
20
(2900)
22
(3200)
Izod
J/m
(ft·lb/in)
139
(2.6)
27
(0.5)
59
(1.1)
69
(1.3)
37
(0.7)
Further analysis of the results indicates that both the CP and magnesium hydroxide were important
variables for reducing the peak heat release. The CP appeared to have a somewhat larger effect than did the
magnesium hydroxide. There also was no interaction (synergy) between them so that the effects for the two
fire retardants are simply additive. A 3-D plot of these two variables versus peak release is given in figure
8.
As for the other variables the results for CP and magnesium hydroxide loadings can be summarized
as follows:
CP+ATO
Flex Modulus
+
Mg(OH)2
+
Flex Strength
+
+
Izod Impact
-
In general an increase in CP and the magnesium hydroxide loadings increased both modulus and flex
strength while they decreased impact strength. However, the decrease in impact strength can be offset with
the use of an impact modifier so that desired physical properties can be maintained.
When interactions were looked at, there were interactions between the (CP + ATO) and Mg(OH) 2
variables for modulus, flex strength and impact strength responses. These interactions may be the result of
the CP acting like a coupling agent for the Mg(OH)2. A 3-D plot of (CP+ATO) and Mg(OH)2 versus
modulus is given in figure 9. The interaction can be seen by the curvature in the plot.
Paraffins with Magnesium Hydroxide and Expandable Graphite
Expandable graphite is graphite in which sulfuric acid has been introduced between the layers of the
graphite structure. The acid is tightly bound and does not leach out. Upon burning the material can expand
to over 100 times its original volume, which makes it a very strong char former. The reason for using
expandable graphite was to reduce the overall loading in the formulation.
In this experiment a 23 factorial design was used (i.e. three variables at two levels). The (CP+ATO)
range was 10 - 20 wt%. The magnesium hydroxide range was 5 – 15 wt% while the range for expandable
graphite was 1.0 -10 wt%. In addition, the impact modifier was used at 20 wt% for all formulations.
The results of this experiment are in given in figure 10 and table 8. The one thing to note is that the
peak heat results are well below the target value for most of the formulations. In other words the formulations
are overloaded. Thus it should be possible to reduce the loadings and still have acceptable burn properties.
Another thing to note is that too much impact modifier was used for the samples are somewhat rubbery as can be
seen by the large Izod impact values. In fact most of the Izod values are either partial breaks or technically no
breaks (the crack do not penetrate far enough into the sample). Nevertheless the large range should still give
some insight into the affect of the variables on impact strength.
Table 8
Results for Chlorinated Paraffins with Mg(OH)2 and Expandable Graphite
Std Run
Order
(CP+ATO)
wt%
Mg(OH)2
wt%
EG
wt%
1
10
5
1
Peak
Heat
kW/m2
424
2
20
5
1
247
3
10
15
1
282
4
20
15
1
137
5
10
5
10
152
6
20
5
10
112
7
10
15
10
148
8
20
15
10
108
9
15
10
5.5
143
Modulus
Mpa
(psi)
537
(78,000)
758
(110,000)
654
(95,000)
1075
(156,000)
978
(142,000)
827
(120,000)
675
(98,000)
896
(130,000)
606
(88,000)
Flex Str
Mpa
(psi)
16.5
(2400)
18.6
(2700)
17.2
(2500)
28.2
(4100)
26.2
(3800)
20
(2900)
17.9
(2600)
19.3
(2800)
17.9
(2600)
Izod
J/m
(ft·lb/in)
614
(11.5)
416
(7.8)
582
(10.9
379
(7.1)
326
(6.1)
235
(4.4)
304
(5.7)
203
(3.8)
320
(6.0)
Analysis of the three variables on peak release indicates that all three reduce the peak heat release
and that expandable graphite has the greatest affect of the three. A 3-D plot of (CP+ATO) and expandable
graphite versus peak heat release is given in figure 3.
As for the influence of the three variables on the other responses, a summary is as follows:
Modulus
Flex Strength
Izod Impact
(CP+ATO)
+
+
-
Mg(OH)2
+
+
-
Exp. Graph.
+
+
-
In general the three variables have a positive influence on modulus and flex strength and a negative
influence on impact. In fact expandable graphite has a large negative influence on impact. Once again,
however, this negative influence can be overcome with the use of an impact modifier.
When interactions among the variables are considered, CP and expandable graphite appear to have a
negative interaction as far as the stiffness and impact properties are concerned. A 3-D plot of (CP+ATO)
and expandable graphite versus modulus is given in figure 1.
Conclusions
With the proper selection of raw materials and process conditions, an improved grade of chlorinated
paraffin can be produced. With improved thermal stability, it was possible to use this material in HDPE
along with nanomers, magnesium hydroxide, and expandable graphite to give an effective flame-retardant
package. The advantages of chlorinated paraffins as a flame retardant are that they are low cost, do not
bloom, and can act as a coupling agent.
Acknowledgements
The authors would like to acknowledge the generous help and assistance of Vic Lee, Joanne
Reneker, Shelly Meek, Tarang Shah, Pat Hudkins, Ryan Reese, and Jeremy Rine in the preparation of this
paper.
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