Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR THE PREPARATION OF POLYMER PARTICLES
The present invention relates to a process for the
preparation of polymer particles containing polymer of a
vinylarene monomer and a foaming agent and to polymer
particles and to foamed articles.
Particles that contain such vinylarene polymer and
foaming agent are generally known as expandable polymers.
A well-known expandable polymer is expandable poly-
styrene. Expandable polystyrene is produced on a
commercial scale by suspension polymerisation. The
foaming agent is usually a low-boiling hydrocarbon, such
as a C3-C6 hydrocarbon, in particular pentane.-'fhe
expandable polystyrene is used for making foamed articles
that are produced by expanding the polystyrene particles.
In the expansion process the foaming agent is (partially)
Z5 released and may be emitted into the environment. Such
emissions are regarded undesirable and ways are sought to
reduce the amount of hydrocarbon foaming agent.
In US-A-5,096,931. expandable polystyrene is described
which contains a small amount of a polar polymer and some
water and some hydrocarbon foaming agent. This product is
manufactured by suspension polymerisation of a mixture of
styrene and the polar polymer in the presence of the
hydrocarbon foaming agent. The disadvantage of the
product obtained is that it still requires the presence
of a hydrocarbon foaming agent apart from minor amounts
of water.
It would thus be desirable if the amount of water
could be increased at the expense of the amount of
hydrocarbon foaming agent. One possible way to increase
the amount of water would be to enhance the amount of
polar polymer in the polymer particles. However, it would
. , ' ' ' .y.-
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be difficult to incorporate the polar polymer into the
polystyrene by suspension polymerization because the polar
polymer would wash out into the aqueous phase of the
suspension. The unsatisfactory result is that too little
s polar polymer is incorporated into the resulting particles
and, hence, that too little water has been taken up into
these particles.
Surprisingly it has been found that more water could
be incorporated into the polymer particles if the
1o polymerization is conducted in two steps. The amount of
water that can be used as foaming agent is such that one
may refrain from incorporating any amount of hydrocarbon
foaming agent.
Accordingly, the present invention provides a process
15 for the preparation of polymer particles containing the
polymer of a vinylarene monomer and having an average
particle diameter of 0.1 to 6 mm, which process comprises:
a) mixing a polar polymer capable of absorbing at
least 0.5 g of water per gram dry polar polymer, with a
2o vinylarene monomer, the amount of the polar polymer ranging
from 2.0 to 20%, by weight, based on the amount of polar
polymer and vinylarene monomer;
b) pre-polymerizing, via a bulk polymerization, the
vinylarene monomer in the mixture thus obtained to a
25 polymerization degree of 15 to 50% to obtain a pre
polymerized mass;
c~ suspending the pre-polymerized mass in an aqueous
medium in the presence of a protective colloid dispersant
to yield suspended particles; and
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d) polymerizing the suspended particles to complete
monomer conversion.
In another aspect of the invention, there is provided
polymer particles containing a polymer of a vinylarene
monomer, a polar polymer capable of absorbing at least 0.5
g of water per gram dry polar polymer, and 3 to 40 %wt
water, based on the weight of vinylarene, and which have an
average particle diameter of 0.1 to 6 mm; the amount of
polar polymer ranging from 2.0 to 20% wt., based on the
to polar polymer and vinylarene.
In still another aspect of the invention, there is
provided a foamed article containing a polymer of a
vinylarene monomer and a polar polymer capable of absorbing
at least 0.5 g of water per gram dry polar polymer, in
which the amount of polar polymer ranges from 2.0 to 20
%wt, based on the polar polymer and vinylarene.
In yet another aspect of the invention, there is
provided use of the polymer particles of the invention in
the preparation of a foamed article.
2o The present process is capable of yielding polymer
particles with satisfactory expandability properties that
do not contain an organic foaming agent. These polymer
particles can be separated from the aqueous mixture and
expanded to yield pre-expanded particles, which are
optionally treated further to obtain foamed articles.
Suitable vinylarene monomers to be used in the present
process are well known in the art and can suitably be
selected from styrene, cx-methylstyrene,
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chlorostyrene, dimethylstyrene, vinyltoluene and similar
styrenic derivatives. Preferably, the vinylarene is
styrene.
The polar polymers are defined as being capable of
absorbing at least 0.5 g of water per gram of dry
~ polymer. The absorption capacity is determined according
to ASTM method F 716-82. Suitable absorption capacities
range from 0.5 g water/g polar polymer to more than 200 g
water/g polar polymer. Although any polar polymer can be
used, it is suitably selected from polyvinyl alcohol,
polyvinyl acetate, polyacrylic acid, polyethylene g~ycols
and cellulose derivatives. Polyvinylpyrrolidone (PVP) is
a preferred polar polymer. This polar polymer is
completely miscible with water within the temperature
range of 0 to 120 °C. The absorption capacity is
therefore taken to be higher than 200 g water/g of dry
polymer.
Another preferred class of polar polymer is
constituted by starch and modified starches. The
modification of starch is suitably conducted by
esterification or etherification. The water absorption of
starch can be increased by gelatinisation. Starch may
also be modified by etherification of part of the
hydroxyl groups, e.g. from 0.1 to 100, with an alkyl
group, e.g. a C1-C~ alkyl group. Part of the hydroxyl
groups may also be esterified. It is possible to make
esters with a mono- or a dicarboxylic acid. Suitable
acids include acetic, propionic and butyric acid, and
malonic, malefic and succinie acid. Preferred acids are
succinic acids which contain an alkyl or alkenyl
substituent. The alkyl or alkenyl substituent has
suitably from 1 to 16 carbon atoms. The dicarboxylic
acids may be used in such amounts that from 0.1 to 10% of
the hydroxyl groups are esterified. Preferably the mono-
ester is formed; the remaining carboxylic group may be
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left acidic or be converted to a salt, e.g. an alkali
metal or ammonium salt.
The starch can be modified before being added to the
process, or the starch can be modified in situ. The '
latter can be achieved by contacting the starch with the
modifying compound during the preparation process,
preferably in process step a). Modifying compounds which
are preferably used for in situ modification are
(meth)acrylic acid and malefic acid. These compounds are
preferred because they can also polymerise with the
vinylarene monomer. Malefic acid is particularly
preferred.
Vinylarene strains can be crosslinked by using cross-
linking agents having two or more vinyl groups_,The most
convenient cross-linking agent is divinylbenzene. The
latter compound is very suitable because of its complete
compatibility with the vinylarene monomers. Preferably,
polymerisation is carried out in the presence of a
relatively small amount of cross-linking agent, e.g. from
0.001 to 0.1 %wt, based on amount of vinylarene,
preferably from 0.01 to 0.1 cwt. This amount of cross-
linking agent makes that the molecular weight of the
vinylarene polymer increases, while substantially no
cross-linking is observed. It has been found that such
polymer gives polymer particles of increased
expandability.
Cross-linking can also be achieved by using cross-
linked polar polymer, e.g. cross-linked starch. The
starch can have been cross-linked with a dibasic acid,
e.g. a,cu-dicarboxyl alkanes having from 2 to 10 carbon
atoms, such as adipic acid.
The polar polymer is suitably added as a polymer to ,
the vinylarene monomer. It is, however, also possible to
prepare the polar polymer in situ. An example of such an ,
_ in-situ preparation is the formation of a mixture of
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acrylic acid and vinylarene, in which acrylic acid is
polymerised in situ, prior to the pre-polymerisation of
the vinylarene.
The polar polymer may have molecular weights which
can vary within wide limits such as from 50 to
500,000,000. Suitable molecular weights (weight average
molecular weight) range from 50,000 to 750,000.
The present invention makes it possible to
manufacture polymer particles containing a relatively
high amount of polar polymer and water. These amounts are
suitably higher than the amounts employed in the polymers
described in US-A-5,096,931. Advantageously, the amount
of polar polymer ranges from 2.0 to 20, preferably from
3.0 to 7.5 cwt, based on the weight of the polar polymer
and vinylarene monomer, and the amount of water in the
particles produced, before prefoaming~ranges from more
than 3 to 40 %wt. If the content of polar polymer is too
low, the water-adsorbing capacity of the resultant
particle may remain unsatisfactorily low. If the amount
is too high, the mechanical properties of the foamed
article, made from the resultant particles, may be
adversely affected.
The pre-polymerisation step may be conducted in any
known manner. This includes anionic polymerisation, free-
radical polymerisation and thermal polymerisation. The
degree of monomer conversion can easily be controlled in
thermal polymerisation by increasing or decreasing the
temperature. Therefore, thermal polymerisation is
preferred for the pre-polymerisation step. Preferably,
the thermal polymerisation is effected by heating the
solution to a temperature of 60 to 180 C, preferably
from 110 to 130 C. When the desired conversion has been
achieved the temperature is reduced andthe polymerisa-
tion stops. It is most preferred to carry out the pre-
polymerisation step by thermal polymerisation in the
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presence of a relatively small amount of free-radical
initiator. A suitable amount is between 0.005 and 0.200
by weight of initiator, based on amount of vinylarene. It
has beenfound that the presence of the small amount of
initiator gives polymer particles of increased
expandability.
Optimal conversion degrees may vary if different
polar polymers are used. Preferably, the conversion
varies between 15 and 400 of the vinylarene monomer,
preferably between 25 and 400. It is believed that due to
the pre-polymerisation the mobility of the polar pol-ymer
in the pre-polymerised mass is reduced, thereby
facilitating a fine distribution of the polar polymer in
the pre-polymerised mass. It i.s believed that by this
fine distribution the water-uptake in the form of minute
droplets is favoured.
Subsequent to the pre-polymerisation step the pre-
polymerised mass is suspended in an aqueous medium. The
volume ratio between the aqueous medium and the pre-
polymerised mass may vary between wide ranges, as will be
appreciated by a person skilled in the art. Suitable
volume ratios include 1:1 to 1:5 (pre-polymerised
mass:aqueous phase). The optimal ratio is determined by
economic considerations.
The aqueous medium may contain one or more
conventional stabilizing agents, such as polyvinyl-
alcohol, gelatine, polyethylene glycol, hydroxyethyl
cellulose, carboxymethyl cellulose, polyvinyl
pyrrolidone, polyacrylamide, but also salts of
polyacrylic acid, phosphoric acid or pyrophosphoric acid,
malefic acid, ethylene diamine tetraacetic acid, and the
like, as will be appreciated by the person skilled in the ,~
art. The amount of the stabilizing agents may suitably
vary from 0.1 to 0.9 cwt based on the weight of the
- aqueous medium. For completeness' sake it is observed
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that during the suspension step c) and polymerisation
step d) water is incorporated into the pre-polymerised
mass. However, any polymeric stabilizing agent, e.g.
' polyvinyl pyrrolidone or hydroxyethyl cellulose, is
essentially not taken up by the suspended pre-polymerised
~ mass.
The polymerisation step d) is advantageously effected
by free-radical polymerisation by means of a free-radical
initiator. Thermal polymerisation is less preferred as it
would need to be carried out at elevated pressure in view
of the water present.
The free-radical initiator can be selected from the
conventional initiators for free-radical styrene poly-
merisation. They include in particular organic_peroxy
compounds, such as peroxides, peroxycarbonates and
peresters. Combinations of peroxy compounds can also be
used. Typical examples of suitable peroxy initiators are
C6-C2p aryl peroxides, such as decanoyl peroxide, benzoyl
peroxide, octanoyl peroxide, stearyl peroxide,
3,5,5-trimethyl hexanoyl peroxide, peresters of C2-C1g
acids and C1-C5 alkyl groups, such as t-butylperbenzoate,
t-butylperacetate, t-butylperpivalate, t-butylperisobuty-
rate and t-butylperoxylaurate, and hydroperoxides and
dihydrocarbyl (C3-C10) peroxides, such as diisopropyl
benzene hydroperoxide, di-t-butylperoxide, dicumyl-
peroxide or combinations thereof. Radical initiators
different from peroxy compounds are not excluded. A
suitable example of such a compound is a,a.~-azobisiso-
butyronitrile. The amount of radical initiator is
suitably from 0.01 to 5 %wt, based on the weight of the
vinylarene monomer. The process is suitably initiated by
heating the reaction mixture to elevated temperature,
e.g. in the range of 60 to 140 C.
The polymerisation process of thestep d) may
suitably be carried out in the presence of a chain
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_.
transfer agent. The person skilled in the art will
appreciate that these chain transfer agents can be
selected from mercaptans, such as C2-C15-alkyl
mercaptans, e.g. n-dodecylmercaptan, t-dodecylmercaptan,
n-butylmercaptan or t-butylmercaptan. Preferred are
aromatic compounds such as pentaphenylethane, and in
particular the dimer of cc-methylstyrene.
The free radical polymerisation is suitably carried
out at a temperature of -60 to 140 °C, preferably 80 to
120 °C, and a pressure of 0.3 105 to 6.0 105 Pa (0.3 to
6.0 bar) , preferably 2.5105 to 4.0105 Pa (2.5 to
4.0 bar). These react~.on conditions are well-known to the
skilled artisan.
The present invention enables the skilled artisan to
manufacture expandablepolymer particles with relatively
high contents of polar polymers and water. Accordingly,
the present invention also provides polymer particles
containing a polymer of a vinylarene monomer, a polar
polymer capable of absorbing at least 0.5 g of water per
gram dry polar polymer, and water, which polymer
particles contain from more than 3 to 400 of water, based
on amount of vinylarene and have an average particle
diameter of 0.1 to 6 mm. Preferably, the particles
contain from 4 to 16 awt of water. These particles are
wv 25 expandable without the presence or a C3-C6 hydrocarbon
foaming agent. This makes that the particles can contain
less than 0.5 owt of C3-C6 hydrocarbon, more preferably
less than 0.25 owt, based on amount of vinylarene. Most
preferably, the particles contain rio C3-C6 hydrocarbon.
The amount of polar polymer can be varied in accordance
with the desired amount of water. Advantageously the
particles contain from 2.0 to 20 °swt of polar polymer,
based on the polar polymer and vinylarene.
The polymer particles may further contain several
additives or coatings in effective amounts. Such
~;a:°;v ~;~~~~=.
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additives include dyes, fillers, stabilizers, flame
retarding compounds, nucleating agents, anti-static
MCS17/TS9085PCT
.ft' Ti~d:~-~ ' tft:
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compounds and lubricants. Of particular interest are
coating compositions containing glycerol- or metal
carboxylates. Such compounds reduce the tendency of the
' particles to agglomerate. Suitable carboxylates are
glycerol mono-, di- and/or tristearate and zinc stearate.
Examples for such additive compositions are disclosed in
GB-A-1,409,285. The coating compositions are deposited
onto the particles via known methods, e.g., via dry-
coating in a ribbon blender or via a slurry or solution
in a readily vapourizing liquid.
The particles have advantageously an average diameter
of 0.1 to 6 mm, preferably from 0.4 to 3 mm.
The expandable particles can be pre-foamed by
conventional methods, e.g. by using steam, to yield
l5 particles having a reduced density, e.g. from 80 to
140 kg/m3. It will be appreciated that in order to
vapourize the water included in the particles to effect
foaming, the temperature must be higher than used for
C3_C6 hydrocarbon foaming agents, which have a lower
boiling point than water. If steam is used the use of
superheated steam is required. Foaming can also be
effected by heating in oil, e.g. silicone oil, or by
microwaves.
The pre-foamed articles can be further converted into
foamed articles in any conventional way.
The present invention also relates to foamed articles
containing a polymer of a vinylarene monomer, and a polar
polymer capable of absorbing at least 0.5 g of water per
gram dry polar polymer, in which the amount of polar
polymer ranges from 2.0 to 20 %wt, based on the polar
polymer and vinylarene.
The particles obtained can be used in the preparation
of foamed articles.
The invention will be further illustrated by means of
the following examples.
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EX_A_MPT,F', 1
An experiment was carried out using polyvinyl-
pyrrolidone (PVP) with a weight average molecular weight
of 360,000. The water absorption capacity of the PVP used
was more than 200 g water/g dry PVP. The experiment was
conducted by stirring 50 g styrene and 5.5 g of PVP and 1 '
g of dibenzoyl peroxide (DBP) as initiator under nitrogen
for 1 hour. The mixture was heated to 80 °C for 0.5 hour
under nitrogen during intensive stirring. The resulting
highly viscous liquid (styrene conversion was 17%) was
suspended in a solution of 3 g PVP and 1 g hydroxy ethyl
cellulose (HEC) (to stabilise the suspension) in 300 g
water at 90 °C while stirring vigorously. The reaction
mixture was kept at 90 °C for 8 hours to complete the
reaction. Subsequently, the reaction mixture was cooled
to ambient temperature to yield polymer beads which were
separated from the aqueous phase by filtration. The beads
were foamed by subjecting them to hot silicone oil at
150 °C. The beads contained about 10% PVP as determined
spectroscopically by NMR. The water content of the beads
and their expandability when heated to about 140 °C
(expressed in ratio of volume of the expanded beads to
volume of the beads before expansion), are shown in
Table 1.
FXpMPT,E ~
A similar experiment was carried out as Example 1 but
with 0.5 g DBP as initiator. The pre-polymerisation
reaction was conducted for 45 min at 80 °C. In the
resulting viscous liquid the styrene conversion was 18%.
The pre-polymerised mass was suspended in an aqueous
phase consisting of 1.2 g HEC and 3 g PVP and 0.5 mg
potassium persulphate in 300 g water. The polymerisation
was continued for 6 hours at 90 °C to yield complete
polymerisation. The reaction mixture was cooled to ,
ambient temperature to yield polystyrene beads. The PVP
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content of the beads was again 10%w. Water content and
expandability are shown in Table 1.
FKA_M_PT,F 3
' A third similar experiment was carried out in which
55 g styrene and 5 g PVP was subjected to polymerisation
' in the presence of 0.55 g DBP and 0.37 g tert-butyl
perbenzoate as initiators. This mixture was stirred under
nitrogen for 12 hours. Subsequently, the reaction mixture
was heated to 80 °C under vigorous stirring to provoke
pre-polymerisation. The reaction mixture was kept at this
temperature for 45 min to yield a viscous liquid (styrene
conversion 18%). The liquid was suspended in an aqueous
medium consisting of 1.2 g HEC and 2 g PVP in 300 g water
at 90 °C. The reaction mixture was kept at 90 °_C for
5 hours to yield complete conversion. The reaction
mixture was cooled to ambient temperature and the beads
produced were filtered. The PVP content in the beads was
9%w. Water content and expandability are shown in
Table 1.
Example No. Water content, %wt on Expandability
total bead
1 25 4
2 25 4
3 20 4
50 g Styrene and 5 g PVP were homogenised. To this
mixture 0.5 g DBP was added. The resulting mixture was
suspended in 300 g water. Whilst stirring the suspension
was heated to 80 °C. The suspension turned into a white
latex without any distinct droplets being formed. After
4 hours at 80 °C the mixture is heated to 90 °C for one
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more hour. The mixture is cooled to ambient temperature
and the resulting product is a white latex without any
visible beads.
EXAMPLE 4
- A number of larger scale experiments were carried out
in which the amount of styrene was 1320 g, and the amount
of PVP was 79, 120 or 211 g. The initiators were 18 g DBP
and 8.9 g tert-butyl perbenzoate (TBPB). The pre-
polymerisation was conducted at 120 °C for 2 hours in the
absence of the initiators (Mode 1) or at 80 °C for 45 min
in the presence of the initiators (Mode 2). In the former
case the initiators were added to the mixture after
dispersion of the pre-polymerised mass in the suspension.
The suspension medium consisted of 43.6 g PVP,_26.1 g HEC
and 36.1 mg potassium persulphate in 6550 g water. The
suspension polymerisation was conducted for 4 hours at
90 °C and followed by heating at 120 °C for 2.5 hours.
The results of these experiments are indicated in
Table 2. The Table also includes expandability results of
the beads which results were determined by measuring the
volume expansion in silicone oil at 140 °C.
Exp. PVP, Pre-pol. Conversion Water Expan-
No. g after pre- content, dability
pol. %w
4.1 79 Mode 1 18 29 4
4.2 120 Mode 2 30 37 4
4.3 211. Mode 1 20 38 4
EXAM T~F
Styrene (79 pbw) was placed in a stainless steel
reactor and stirred under nitrogen. Starch modified with
- 5% sodium n-octenylsuccinate («CERESTAR 062E7'~ from
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Cerestar Benelux B.V.) in an amount of 5.3 pbw of
modified starch dispersed in 15 pbw styrene was added to
the reactor. The modified starch used had a water
~ absorption capacity of 14 g water/g dry modified starch.
A solution of 0_8 pbw DBP and 0.2 pbw TBPB in 6 pbw
styrene was added to the reaction mixture. The
temperature in the reactor was increased to 80 °C in
30 minutes and kept at this temperature for 155 mins.
Subsequently the temperature was lowered to 70 °C and
kept at this temperature for 130 mins. The styrene
conversion in the pre-polymerised mass was 48%. The mass
was subsequently transferred into a suspension poly-
merisation reactor containing 226 pbw of deionized water
and 0.9 pbw HEC. The suspension was heated for 240 mins
at 80 °C, 60 mires at 90 °C and 60 mires at 120 °C. Then
the reaction mixture was allowed to cool to ambient
temperature. The beads produced were filtered. The beads
had a water content of 7 cwt, based on the amount of
styrene, starch and water.
The beads, having a density of about 1050 g/dm3, were
exposed to an air stream of 135 °C. Expansion of the
beads with a particle diameter of 0.9-1.25 mm, resulted
in a foam with a density of 110-140 g/dm3.
Styrene (80 pbw), malefic anhydride (0.5 pbw in 3 pbw
styrene) and modified starch cross-linked with 0.4-°s
adipic acid ("CERESTAR 05309 from Cerestar Benelux B.V.)
(5.3 pbw in 11 pbw styrene) were stirred under nitrogen
at 30 °C. The cross-linked starch used had a water
absorption capacity of 19 g water/g dry cross-linked
starch. By heating of the mixture to 130 °C in 70 mires
and maintaining this temperature for 110 mires, styrene is
converted by thermal polymerisation. Subsequently, the
reaction mixture is cooled to 70 °C within 40 mires. The
styrene conversion is about 39%. A solution of 0.4 pbw
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DBP and 0.2 pbw TBPB in 6 pbw styrene was added and the
mixture was homogenised at 70 °C. The mixture was
subsequently dispersed in 139 pbw water with 0.6 pbw HEC.
The suspension was heated for 240 mins at 80 °C, 60 mins
at 90 °C and 120 minx at 120 °C to yield complete styrene
polymerisation. The beads obtained had a water content of
8 cwt and a starch content of 4.8 %wt.
The beads obtained showed the same foaming behaviour
as those in Example 5.
EXBMPLE ?
Styrene (75 pbw), malefic anhydride (0.5 pbw in ~ pbw
styrene), and modified starch as in Example 5 (5.3 pbw in
pbw styrene) were mixed at 30 °C. The temperature was
raised to 120 °C in 60 minx and the mixture was kept at
15 this temperature for 160 mina. Subsequently the'temper-
ature was lowered to 70 °C. The styrene conversion in the
pre-polymerised. mass thus obtained was 320. To the pre-
polymerised mass were added 0.4 pbw DBP, 0.2 pbw TBPB and
0.04 pbw divinylbenzene in 6 pbw styrene. The mass was
homogenised at 70 °C. The pre-polymerised mass was
suspended in 226 pbw water containing 0.9 pbw HEC. The
suspension was heated for 240 minx at 80 °C, for 60 mina
at 90 °C and for 120 mins at 120 °C to yield complete
styrene polymerisation. The beads obtained had a water
_. content of 10 cwt and a starch content of 4.8 cwt.
When the beads were exposed to an air stream of
135 °C, an expanded foam was obtained with a density of
80-120 g/dm3.
FXAM_PT_,F: $
Styrene (77 pbw). malefic anhydride (0.5 pbw in 3 pbw
styrene), TBPB (0.025 pbw in 3 pbw styrene), and the
modified starch as used in Example 5 (5.3 pbw in 11 pbw
. styrene) were stirred at 30 °C. Polymerisation was
started by increasing the temperature to 120 °C in
60 mins and keeping it at this value for 50 mins. The
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temperature was subsequently decreased to 70 °C, while
the styrene conversion in the pre-polymerised mass
obtained was 280.
A solution of lauroyl peroxide (0.66 pbw), TBPB
(0.2 pbw) and divinylbenzene (0.02 pbw) in styrene
(6 pbw) was added to the pre-polymerised mass and, after
homogenisation, the resulting mixture was suspended in an
aqueous medium consisting of 139 deionized water,
0.57 tricalciumdiphosphate and 0.1 pbw Natrosol (a
cellulose c'ierivative). The suspension was heated at 80 °C
for 240 minx, at 90 °C for 60 mins and at 120 °C fog
120 mins. The polystyrene beads thus obtained were
separated from the suspension by filtration. The beads
contained 12 cwt water and 4_8 %wt starch, based on
amount of polystyrene.
When exposed to air of 135 °C the beads yielded a
foam with a density of 80-100 g/dm3.
. . ~ . . r, °. .. . ,
