Water-blown Expandable Polystyrene. Improvement of the Compatibility of the Water
Carrier with the Polystyrene Matrix by In Situ Grafting
Water-blown Expandable Polystyrene.
Improvement of the Compatibility of the Water
Carrier with the Polystyrene Matrix by In Situ
Grafting
Part I. Mechanism of free radical grafting
Jan Pallay, Hugo Berghmans*,
Laboratory for Polymer Research,
Katholieke Universiteit Leuven,
Celestijnenlaan, 200F,
B-3001 Heverlee
Fax: +32-16-327990
Received: 20 November 2001 Accepted: 2 January 2002
SUMMARY
In the synthesis of water-blown expandable polystyrene, granular starch as a
carrier of water, the blowing agent, was grafted with polystyrene by radical
initiated polymerization. Organic peroxides, such as tertbutyl perbenzoate or
dibenzoyl peroxide were used as free radical initiators. The graft polymerization
reaction was confirmed by FT-IR spectroscopy and SEM. The reaction leads
to low level grafted starch. However, the amount of grafted polymer can be
significantly increased using maleic anhydride in the monomer feed. The effect
of concentration of maleic anhydride was studied. The reaction mechanism
was proposed.
INTRODUCTION
Expandable polystyrene (EPS), an important industrial commodity with
wide spectrum of applications, is generally prepared via suspension
polymerization of styrene. Pentane as the blowing agent is usually added
during the polymerization. Pre-expansion is realized by heating the
pentane-containing polystyrene beads above their glass transition
temperature(1).
Pentane is a volatile organic compound (VOC) that causes some problems
during processing and transportation. It is a highly flammable liquid with
explosion hazard and its emissions are affecting the quality of the air. As
a consequence it becomes evident to look for an alternative blowing
process making use of a less problematic blowing agent.
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A new process for the production of EPS has been developed, using water
as the blowing agent(2, 3). As the solubility of water in polystyrene is very
low, water is trapped inside the polystyrene matrix through the use of a
water swellable polymer that is introduced as a separate phase. The natural
polymer starch was chosen for this purpose as a promising material.
Starch, as natural polymer, is incompatible with polystyrene and this leads
to agglomeration. The hydrophilicity of starch results in washing-out from
the polymerizing system. It is well known(4) that compatibility between two
incompatible polymers can be improved e.g. by grafting of the polymer
macromolecules with the macromolecules of the matrix polymer.
Chemical modification of starch via graft polymerization of vinyl monomers
onto starch is widely used method for the improvement of the properties
of starch and therefore to increase its utility. Numerous monomers and
free radical initiating systems have been investigated and reviewed in the
literature(5,6).
Starch graft copolymers are prepared by generating free radicals on the
starch backbone and then allowing these macroradicals to react with the
monomer. A number of initiating methods may be divided into two broad
categories:
1)
initiation by chemical methods - most widely used method is the
reaction with ceric salts, such as ceric ammonium nitrate(7, 8, 9, 10, 11,
12,13) or with hydrogen peroxide in the presence of an activator(8, 14,
15) such as ferrous ammonium sulfate. However, initiation with
persulfates or manganese is also employed(2). Recently also initiation
with organic peroxides was patented(16).
2)
initiation by irradiation - generally
beam(19) techniques are used.
60Co(7, 17, 18)
or electron
The common sign of all the procedures reported in literature is that they
are carried out in an aqueous media and that hydrolyzed or gelatinized
starches are preferred.
Although the literature contains reports on the graft polymerization onto
starch of numerous monomers (acrylonitrile(5, 10, 11, 13), methyl acrylate(6,
7, 8, 9, 15, 16), methyl methacrylate(7, 12, 15), and many others(1, 2)), very few
processes for graft polymerization of styrene(14, 15, 16) have been reported.
Because styrene graft polymerizes poorly onto starch with the different
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Carrier with the Polystyrene Matrix by In Situ Grafting
chemical initiating methods, starch-g-polystyrene copolymers are
conveniently prepared by 60Co irradiation of the starch-styrene mixture.
A review of literature also indicates that, although the grafting of starch
with synthetic polymers has been known for about 40 years, very few
processes have led to commercialization.
An alternative approach, which has been reported in the literature, is to
form graft copolymers in situ during blend preparation by using polymers
containing reactive functional groups(20). The blending is performed
under the conditions that promote the reaction. The method is commonly
known as “reactive blending”. Synthetic polymers with functional groups
like carboxylic acid, anhydride, epoxy, etc., can react with hydroxyl
groups on the starch and form covalent bonds. From the point of view
of the reactivity it is to be expected that the cyclic anhydride group may
react more quickly than the other ones. Anhydride functionality can be
easily incorporated into a polymer by copolymerization or grafting of
anhydrides like maleic anhydride.
Recently we have reported(2) the improvement of the stabilization of the
fine starch-in-polystyrene dispersion. Sufficient stabilization effect and
improved compatibility of starch with polystyrene were reached by
making use of maleic anhydride in the polymerization of styrene with
dispersed starch granules. The compatibilizing effect could be ascribed to
the grafting of polystyrene onto the starch. In this paper we extend our
previous study and report the evidence that granular starch can be grafted
with polystyrene also in the conditions of free radical suspension
polymerization of styrene with dispersed starch.
EXPERIMENTAL PART
Materials
Styrene (99%) was obtained from Nova Chemicals (The Netherlands).
Maleic anhydride (99%), dibenzoyl peroxide (70%) and tert-butyl
perbenzoate (98%) employed in the synthesis were obtained from Aldrich
Chemical Co. and used as received.
Samples of starches were purchased by Cerestar R&D, company of
Eridania Bèghin-Say (Belgium). Starches were unmodified and used as
purchased. Suspension stabilizers hydroxy-ethyl cellulose and tricalcium
diphosphate were provided by Nova Chemicals (The Netherlands).
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Polymerization
The starch-g-polystyrene copolymers were prepared in the course of the
suspension free radical polymerization proceeded by the bulk prepolymerization of the styrene-starch mixture. The details of the
polymerization are discussed elsewhere(2).
Scheme 1 The different steps in the fractionation of the reaction product from
bulk pre-polymerization and suspension polymerization of WEPS
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Water-blown Expandable Polystyrene. Improvement of the Compatibility of the Water
Carrier with the Polystyrene Matrix by In Situ Grafting
Fractionation of the Reaction Product
For the characterization of the grafted starch and the polymer matrix, the
reaction products after bulk pre-polymerization or after suspension
polymerization were fractionated as shown in Scheme 1.
To separate the grafted starch from the matrix polymer the reaction
product was dissolved in toluene. In the typical experiment 10 g of dry
sample (drying at 60°C to the constant weight) was dissolved in 500 ml
of toluene at room temperature and stirred overnight. The dispersion was
then separated by centrifugation at 14000 RPM for 2 hours.
The polymer matrix was recovered from the solution by precipitation in
methanol. In a typical experiment 5 ml of the solution was diluted with
50 ml of fresh toluene and dropwise added to 500 ml of methanol.
To remove still present ungrafted polymer matrix, the crude grafted starch
was extracted in the Soxhlet extractor for 72 hours with toluene. The
minimum time necessary for a quantitative extraction was determined
through the extraction of a mechanical mixture of starch and polystyrene.
This mixture was prepared by mixing a dispersion of the starch granules in
a polystyrene solution at room temperature followed by precipitation in
methanol. From such experiments it was concluded that polystyrene could
be removed in a quantitative way form starch after 48 hours of Soxhlet
extraction. The presence of polystyrene was controlled by FT-IR spectroscopy.
To characterize the polystyrene, which was grafted onto starch, the
starch-g-polystyrene was subjected to an acid hydrolysis in order to
remove the starch. In a typical experiment 1g of the grafted starch was
dispersed in 250 ml of 1M HCl and the dispersion was refluxed for
6 hours. The insoluble residue was filtered, washed with distilled water
and dried to the constant weight.
Characterization
The graft copolymers and the polymers recovered after acid hydrolysis
were characterized by FT-IR spectroscopy using FT-IR Perkin–Elmer
System 2000.
Scanning electron microscope (SEM) images were obtained using JEOL
electron microscope JSM T220A.
The amount of bonded maleic anhydride was evaluated at Cerestar,
Belgium, using the standard procedure for determination of acids in di-
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starch adipates. The sample of esterified starch was dispersed in moderately
concentrated sodium hydroxide solution to hydrolyze fully the anhydride
from the starch. After acidification, the free acid was extracted with ethyl
acetate. The ethyl acetate was removed and the dry residue was silylated.
This solution was injected to the gas chromatograph accommodating a
capillary column and a flame ionization detector. Pimelic acid was used
as internal standard.
The molecular weights were determined at Nova Chemicals, The
Netherlands, by GPC using liquid chromatograph provided with an UV
detector at 254 nm.
The amount of maleic anhydride in the polymer matrix as well as in the
polymer grafts was determined by FT-IR spectroscopy using the calibration
curve that relates the band ratio to the maleic anhydride concentration in
the copolymer. The band ratio was calculated according equation 1:
Band ratio =
height of peak at 1780 cm-1
(1)
height of peak at 1600 cm-1
EXPERIMENTAL RESULTS AND DISCUSSION
Free Radical Grafting of Polystyrene onto Starch
It was reported in our previous study that the starch granules as the carrier
of the blowing agent have to be introduced into the polystyrene matrix
at the beginning or in the early stages of the polymerization. As starch has
strong tendency to be washed-out to the water phase during the
suspension polymerization the bulk pre-polymerization step was
incorporated. The aim of the pre-polymerization step was to increase the
viscosity of the matrix so that the mobility of starch is limited as much as
possible. To improve the compatibility of the starch with the polymer
matrix a small amount of maleic anhydride was added. It was proposed
that during the synthesis of the water-blown expandable polystyrene
(WEPS) starch granules are grafted with the matrix polymer.
In a typical bulk pre-polymerization experiment starch was dispersed in
styrene at room temperature. Maleic anhydride (0.5wt.-% related to the
polymer matrix) and a small amount of free radical initiator were added
and the mixture was polymerized under nitrogen atmosphere. After a
styrene conversion of about 30%, a sample was taken from the reaction
mixture and subjected to the fractionation represented in Scheme 1.
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Water-blown Expandable Polystyrene. Improvement of the Compatibility of the Water
Carrier with the Polystyrene Matrix by In Situ Grafting
The reaction mixture with fresh free radical initiator was afterwards
dispersed in water and the suspension was polymerized to the total
conversion of styrene. After the suspension polymerization was completed
the beads were collected, dried and subjected to the fractionation as well
(Scheme 1).
From the examination of the products that were obtained from the
fractionation of the reaction mixture resulting from the bulk prepolymerization or the beads resulting from the suspension polymerization
we can conclude that the starch granules are truly grafted with polystyrene.
Proof of Grafting
The infrared spectrum of starch - stage II in the fractionation according
the Scheme 1 - was taken as the proof of grafting. The typical pattern of
the infrared spectrum of such starch is shown in Figure 1. The spectra of
pure SMA copolymer and pure starch are shown for comparison. In the
spectrum of grafted starch (curve c) the absorption band of starch
together with the typical bands of the SMA copolymer (1780, 1600,
1490 and 1450 cm-1) can be observed. The proof that the starch granules
isolated from the polymer matrix after polymerization are truly grafted is
the infrared spectrum of a polymer graft - stage III in the fractionation
according the Scheme 1 - (curve d). The spectrum of the polymer graft
shows besides typical bands for grafted polymer also additional bands at
1125 and 810 cm-1 that are assigned to glucose. In the case of a physical
mixture, the insoluble fraction after acid hydrolysis should show the
spectrum of the corresponding polymer only, since starch is completely
removed by hydrolysis.
Additional proof of grafting was obtained by SEM. The images (Figure 2)
obtained with starch granules isolated from the polymer matrix - stage II
in the fractionation according the Scheme 1 - revealed that grafted starch
granules have a spherical shape in comparison with the angular shape of
native ones. This fully confirms the grafting since every starch granule is
embedded in a shell of anchored polymer chains.
The absorption of water can be also used to prove that starch, isolated
from the polymer matrix after free radical polymerization, is grafted. The
starch considered as grafted absorbs significantly less water at both 30%
and 75% of relative humidity (Figure 3), which is ascribed to the strongly
attached shell of hydrophobic polymer.
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Figure 1 FT-IR spectra: a) styrene-maleic anhydride copolymer; b) pure
native starch; c) starch isolated from the matrix after polymerization shows
absorption bands of SMA; d) SMA graft isolated from grafted starch shows
absorption bands of glucose
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Water-blown Expandable Polystyrene. Improvement of the Compatibility of the Water
Carrier with the Polystyrene Matrix by In Situ Grafting
Figure 2 SEM images of starch granules: a) native corn starch; b) corn starch
grafted with SMA copolymer
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Figure 3 Absorption of water by native and SMA grafted starch: ● native
starch at 30% RH; ❍ grafted starch at 30% RH; ■ native starch at 75% RH;
❏ grafted starch at 75% RH
Influence of the Presence of Maleic Anhydride and the Free
Radical Initiator in the Bulk Pre-Polymerization on the
Grafting of Starch
To study the role of maleic anhydride and the free radical initiator in the
grafting of granular starch in some experiments, maleic anhydride or the
free radical initiator or both were left out. The matrix polymers (stage I),
grafted starches (stage II) and polymer grafts (stage III) isolated by the
fractionation of the pre-polymerized mixture as well as the beads after the
suspension polymerization (see Scheme 1) were characterized. The
results are summarized in the Table 1.
Maleic anhydride that is added to the reaction mixture (exp. series 2)
copolymerizes with styrene. Besides the copolymerization it also takes
part in the esterification of starch as could be seen from the amount of
covalently bonded maleic anhydride on the starch after pre-polymerization
(Table 1). When an initiator is present in the pre-polymerization step
(exp. series 2b), the amount of bonded maleic anhydride on the starch
granules is lower, as a result of more effective copolymerization of styrene
with maleic anhydride in the presence of the free radical initiator.
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12.36
10.10
0.59
0.61
0.47
no
yes
yes
yes
2a
2b
0.29
9.34
0.86
yes
no
1b
0
11.55
0.52
0
no
no
1a
amount of grafted
polymer after prepolymerization step
[wt% (± 0.22)]
amount of covalently
bonded maleic anhydride
after pre-polymerization
step [wt% (± 0.15)]
initiator in the
prepolymerization
step
maleic
anhydride
Exp. series
Table 1 Amount of grafted polymer in the experiments with and without maleic anhydride
amount of grafted polymer
after suspension
polymerization [wt% (± 1.18)]
Water-blown Expandable Polystyrene. Improvement of the Compatibility of the Water
Carrier with the Polystyrene Matrix by In Situ Grafting
The amount of grafted polymer on the
starch granules after the bulk prepolymerization could be considered
as constant in all experiments within
the range of the experimental error.
However, the small amount of grafted
polymer in the experiments without
maleic anhydride (exp. series 1)
suggests that grafting is possible
directly on the starch backbone.
The amount of grafted polymer on the
starch after the suspension
polymerization has also to be
considered as constant in all
experiments. In spite of the fact that
all results are in the range of
experimental error, the results
obtained in the experiments with
maleic anhydride (exp. series 2) were
in all corresponding experiments
slightly higher than in the experiments
without maleic anhydride (exp.
series 1). From these results it is
proposed that the presence of maleic
anhydride in the system promote the
grafting onto the starch. Secondly, no
bonded maleic anhydride was found
in the samples of starch isolated from
the beads after suspension
polymerization. This suggests that
when maleic anhydride is present in
the reaction mixture, the grafting is
taking place on two different sites: the
starch back bone itself and the double
bond of maleic anhydride covalently
bonded (esterified) on the starch.
Consequently we can conclude that
an increase of the maleic anhydride
concentration will increase the total
amount of grafted polymer.
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The presence of a free radical initiator in the bulk pre-polymerization has
no strong influence on the total amount of grafted polymer. Nevertheless,
it was observed that in the particular experiments, in which a free radical
initiator was used in the pre-polymerization step, more reproducible
results concerning the amount of grafted polymer were obtained.
Influence of Starch Type
WEPS can be prepared making use of various types of starches. The basic
series of experiments was performed making use of five different starches
(corn, wheat, potato, rice and high amylose corn) and 0.5 wt.-% of maleic
anhydride (related to the polymer matrix). Analyzing the amount of
grafted styrene-maleic anhydride copolymer on starch we had to conclude
that despite the different properties of the starches (such as amylose
content, granule size, etc.) the obtained results are similar, respectively
they are in the range of the experimental error. The 95% confidential
interval for the total amount of grafted polymer is 9.6 - 15.2 wt.-%.
Therefore we can conclude that the type of starch has no significant effect
on the grafting copolymerization. For this reason only corn starch was
used in the later experiments.
Influence of the Maleic Anhydride Concentration
Table 2 illustrates the effect of the maleic anhydride concentration. To
examine this effect on the grafting of starch, the amount of bonded maleic
anhydride and the amount of grafted polymer were determined from the
starches isolated by the fractionation of both the pre-polymerized
reaction mixture and the beads obtained from the suspension
polymerization.
With increasing concentration of maleic anhydride in the monomer feed
the amount of maleic anhydride bonded onto starch after the prepolymerization step increases as far as the concentration related to the
starch reaches the value of about 35 - 40 wt.-%. A further increase of the
maleic anhydride concentration did not lead to an increase in the amount
of bonded maliec anhydride. This can be explained by the fact, that the
high amount of bonded maleic anhydride increases the probability of the
grafting reaction and consequently the amount of free bonded maleic
anhydride is reduced. This is in agreement with the results of the amount
of grafted polymer. The amount of grafted polymer after the bulk prepolymerization step is constant in the experiments in which the
concentration of maleic anhydride is lower than 25 - 30 wt.-%. A further
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6.78
12.79
3.87
3.85
52.01
8.89
23.39
0.80
3.60
8.26
0.59
0.24
0.51
8.34
0.86
0
1.14
amount of grafted polymer
after suspension
polymerization [wt% (± 1.18)]
amount of grafted
polymer after prepolymerization step
[wt% (± 0.22)]
amount of covalently
bonded maleic anhydride
after pre-polymerization
step [wt% (± 0.15)]
increase of the maleic anhydride
concentration causes significant
increase in the amount of the
grafted polymer on starch as a
result of the higher probability of
the grafting reaction on two
reaction sites, the starch backbone
and the double bond of maleic
anhydride bonded on the starch.
The same conclusions can be drawn
concerning the total amount of
grafted polymer after the
suspension polymerization.
From these results we can
conclude that the amount of
covalently bonded maleic anhydride
on the starch backbone has an
important influence on the total
amount of grafted polymer as it
increases the probability of a
successful grafting reaction.
49
0.053
3f
40
0.037
3e
20
29
0.014
0.024
3c
3d
0.005
3b
9
0
0
Mechanism of Grafting
3a
mole fraction of maleic anhydride
maleic
concentration
anhydride in the related to starch
monomer feed
[wt%]
Exp. series
Table 2 Dependence of the amount of grafted polymer on the maleic anhydride concentration
Water-blown Expandable Polystyrene. Improvement of the Compatibility of the Water
Carrier with the Polystyrene Matrix by In Situ Grafting
On the basis of the presented results
we can suggest the mechanism of
the free radical graft polymerization
of starch. The possible reaction
steps in the process are illustrated
in the Scheme 2.
As in other systems in which
grafting is accomplished via free
radical reactions, the present
process may be presumed to involve
formation of radical sites on the
backbone of starch. This may occur
either by reaction of a radical from
the initiator with the starch molecule
or by transfer of styrene radicals to
starch (Eq. 4a and 4b). The
presence of an anhydride group on
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Jan Pallay and Hugo Berghmans
Scheme 2 Reaction steps of the free radical graft polymerization of starch in
the preparation of WEPS
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Water-blown Expandable Polystyrene. Improvement of the Compatibility of the Water
Carrier with the Polystyrene Matrix by In Situ Grafting
the starch molecule, resulting from esterification reaction of the starch
with maleic anhydride (Eq. 5), produces a new possibility for the
formation of a radical on the starch molecule. The formation of a radical
on the esterified starch is easier because of the higher reactivity of the
double bond and may be achieved either by the reaction of a radical from
the initiator (Eq. 6a) or by the transfer from a styrene radical (Eq. 6b).
Considering the experimental results the thermal initiation occurs only to
a minor extent. Thus, the addition of initiators to the system increases the
probability of radical formation on the starch backbone.
In the bulk pre-polymerization step the grafting copolymerization
proceeds as “grafting from” according the propagation reactions (Eq. 8a
and 8b). The results on the amount of grafted polymer from the
experiments with a higher amount of maleic anhydride in the monomer
feed (Table 2) suggest that the growth of the polymer chain from the
active site formed by the double bond of the maleic anhydride group
(Eq. 8b) is favoured.
The situation is however changed, when the reaction mixture is
transferred to the suspension polymerization and new portion of free
radical initiator is added. Addition of initiators to the reaction system
increases the possibility of the formation of new radicals on the starch
backbone by the reaction of a radical from the initiator with the double
bond of bonded maleic anhydride. The results summarized in the
Table 3, reveal that the polymer grafts (stage III in the Scheme 1) isolated
by the fractionation of the beads after suspension polymerization are
more related, in the respect to the molecular weight and amount of
maleic anhydride, to the matrix (stage II in the scheme 1) isolated from
the pre-polymerization mixture than to the matrix (stage II in the Scheme
1) isolated after the suspension polymerization. This suggests that the
termination of the macroradicals of the polymer matrix formed during
the pre-polymerization and early stages of the suspension polymerization
step with the radicals on the starch backbone (Eq. 10a, b and 11a, b) is
preferred over the propagation like the matrix polymer. Thus, the
grafting copolymerization proceeds mainly as “grafting onto” as far as
the radicals from the double bonds of maleic anhydride are fully
consumed.
CONCLUSIONS
The results from this study reveal that during the synthesis of WEPS the
starch granules are grafted with the polystyrene. The polymer formation
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16
250
230
215
200
190
3b
3c
3d
3e
3f
Note:
a - polydispersity 1.8-2.0
b - polydispersity 1.9-2.0
c - polydispersity 2.7-3.1
d - polydispersity 1.9-2.3
250
matrix
a
70
200
grafted
polymerb
5.4
5.0
4.2
3.3
1.6
0
matrix
7.1
6.5
8.4
6.2
0
grafted
polymer
conc. of MA [wt% (± 0.4)]
after pre-polymerization
Molecular weight x10-3
3a
Exp. series
70
150
165
170
180
matrix
c
210
300
230
130
grafted
polymerd
Molecular weight x10-3
0.9
0.7
0.7
0.5
0
matrix
7.9
6.0
2.9
1.9
0
grafted
polymer
conc. of MA [wt% (± 0.4)]
after suspension polymerization
Table 3 Analysis of molecular weight and content of maleic anhydride in the matrix and in the polymer grafts
Jan Pallay and Hugo Berghmans
Cellular Polymers, Vol. 21, No. 1, 2002
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Water-blown Expandable Polystyrene. Improvement of the Compatibility of the Water
Carrier with the Polystyrene Matrix by In Situ Grafting
on the starch results mainly from the interaction of growing polymer
macroradicals with radicals formed on the double bond of maleic
anhydride attached to the starch backbone as a result of addition of free
radical initiators to the system at the beginning of the suspension
polymerization step. This reaction takes place when the pre-polymerized
reaction mixture is transferred to the suspension polymerization
The formation of the matrix polymer is still the dominant process. This
results directly from the heterogeneous nature of the polymerization and
can be concluded from the obtained results. One has to keep in mind that
the graft polymerization is only a side reaction in this particular system. The
main objective of this study was to show that the low-grafted starch is
possible to prepare making use of the usual free radical initiation systems,
such as organic peroxides. Moreover, it was shown that the starch could
be grafted in its granular form. Improving of the graft polymerization is
possible by enhancing the number of reactive sites on the starch backbone
by the addition of maleic anhydride to the reaction system.
ACKNOWLEDGEMENT
The authors wish to thank Nova Chemicals, the Fund for Scientific
Research, Flanders (FWO) and IUAP4/11 (Belgian Programme on
Interuniversity Attraction Poles initiated by the Belgian State, Prime
Minister’s office) for financial support.
JP wishes to thank H. Grinderbeek (Cerestar, Belgium) and F. Metsaars
(Nova Chemicals, The Netherlands) for the help with analysis.
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1.
H. F. Mark, N. G. Gaylord, “Encyclopaedia of Polymer Science and
Technology”, John Wiley &Sons, Inc., New York (1970)
2.
J. Pallay, P. Kelemen, H. Berghmans, D. Van Dommelen, Macromol.
Mater. Eng, 275(2000), 18
3.
H. Berghmans, I. Chorvath, P. Kelemen, E. Neijman, J. Zijderveld
United States Patent N° 6,127,439 (Oct.3, 2000), Nova Chemicals
(International) S.A.
4.
C. Koning, M. Van Duin, C. Pagnoulle, R. Jerome, Prog. Polym.
Sci. 23(1998), 707
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