Preparation of styrene polymer foam particles

Styrene polymer foam particles of from 10 to 30 kg/m.sup.3 in density are obtained by foaming styrene polymers which, per kg, contain from 0.7 to 3 moles of blowing agent mixture composed of from 60 to 90 mol % of water and from 10 to 40 mol % of the C.sub.4 - or C.sub.5 -hydrocarbon with the aid of microwaves.

The present invention relates to the preparation of foam particles from 10 
to 30 kg/m.sup.3 in density from styrene polymers by a novel, environment 
friendly process using only a small amount of an organic blowing agent. 
The preparation of foam particles from styrene polymers is known. The 
starting point in the art is an expandable styrene polymer which contains 
a C.sub.4 - or C.sub.5 -hydrocarbon blowing agent and is foamed up by 
heating with steam. Expandable styrene polymers are customarily prepared 
by suspension polymerization of styrene in the presence of the blowing 
agent. The foam particles are subsequently further expanded by renewed 
heating in a perforated pressure resistant mold with superheated steam, 
when they become fused together to form a molded foam. 
It is an object of the present invention to reduce the amount of organic 
blowing agent in the production of styrene polymer foam particles and 
thereby to cut environmental pollution. 
We have found that this object is achieved by replacing some of the organic 
blowing agent by water and by foaming with the aid of microwaves. 
The present invention accordingly provides a process for preparing styrene 
polymer foam particles from 10 to 30 kg/m.sup.3 in density, which 
comprises foaming styrene polymer particles which, per kg, contain from 
0.7 to 3 moles of a blowing agent mixture composed of from 60 to 90 mol % 
of water and from 10 to 40 mol % of one or more than one C.sub.4 - or 
C.sub.5 -hydrocarbon with the aid of microwaves. 
DE-A-3,532,347 discloses the preparation of foamed plastics which contain 
hydroxyl, thiol, alkylamino, amino or carboxyl groups with the aid of 
microwaves. The blowing agents used are chlorofluorocarbons. 
GB-B-922,547 discloses the foaming of expandable styrene polymers which 
contain 10% by weight of toluene sulfonamides or tetrachlorobiphenyl and 
of chlorinated polystyrene having a chlorine content of 25% by weight with 
the aid of microwaves. The blowing agent used is petroleum ether. 
Expandable styrene polymers in which the organic blowing agent has been 
wholly or partly replaced by water show completely unsatisfactory foaming 
characteristics if conventionally foamed with the aid of steam or hot air. 
Surprisingly, foaming with the aid of microwaves produces a significantly 
lower density. This is very surprising to a skilled worker, since, if 
organic blowing agents are used alone, foaming with the aid of steam, hot 
air or microwaves produces in each case foam particles of approximately 
the same density. 
Styrene polymers for the purposes of the present invention are polystyrene 
and copolymers of styrene with other .alpha.,.beta.-olefinically 
unsaturated compounds which contain not less than 50 parts by weight of 
styrene as copolymerized units. Suitable copolymerization components are 
for example .alpha.-methylstyrene, ring-halogenated styrenes, 
ring-alkylated styrenes, acrylonitrile, esters of acrylic or methacrylic 
acid with alcohols of from 1 to 8 carbon atoms, N-vinyl compounds, such as 
vinylcarbazole, maleic anhydride or even small amounts of compounds which 
contain two polymerizable double bonds, such as butadiene, divinylbenzene 
or butanediol diacrylate. It is also possible to use copolymers and graft 
copolymers of from 40 to 90% by weight of polystyrene and from 10 to 60% 
by weight of polyethylene or polypropylene. 
The styrene polymers contain, per kg, from 0.7 to 3 moles of a blowing 
agent mixture composed of from 60 to 90 mol % of water, present in very 
finely divided form, and from 10 to 40 mol % of a C.sub.4 - or C.sub.5 
-carbon, such as butane, n-pentane, isopentane and/or neopentane, present 
in dissolved form. In general, the blowing agent mixture is composed of 
from 1 to 3% by weight of water and from 1 to 3.5% by weight of pentane. 
The water is dispersed in the styrene polymer by the presence of an 
extremely small amount of a customary wetting agent. A suitable wetting 
agent is for example an ethoxylated tallow fat alcohol. The effect of the 
wetting agent is to prevent the exudation of water during the storage of 
the expandable styrene polymer. The wetting agent is in general used in an 
amount of from 0.001 to 0.1% by weight, based on the styrene polymer. 
The expandable styrene polymer particles can be present in bead form, in 
the form of cylindrical granules or in the form of crumbs. The particles 
advantageously have a diameter of from 0.1 to 6 mm, in particular from 0.4 
to 3 mm. They mainly contain further additives in the form of customary 
dyes, fillers, stabilizers, flame retardants, synergists, etc. in 
customary amounts. 
They are prepared in a conventional manner, advantageously by suspension 
polymerization of styrene in an aqueous phase in the presence of both the 
blowing agent and the wetting agent. After the polymerization has ended, 
the beads are separated from the aqueous phase, washed and briefly dried 
to remove adherent surface water. 
To prepare the foam particles, the expandable styrene polymer particles are 
foamed up with the aid of microwaves. The water present in a very finely 
dispersed form in the interior of the beads absorbs the microwave 
radiation and becomes vaporized, and the simultaneous vaporization of the 
hydrocarbon and the softening of the styrene polymer causes the beads to 
expand. The foam particles obtained are from 10 to 30 kg/m.sup.3 in 
density. After settling down, they can be further expanded with the aid of 
steam or hot air and welded together in the conventional manner in 
pressure resistant perforated molds to give molded foam products.

In the Examples, parts are by weight. 
EXAMPLES 1 to 6 
In a pressure resistant stirred kettle, a mixture of 150 parts of water, 
0.1 part of sodium pyrophosphate, 100 parts of styrene, pentane and 
wetting agent (ethoxylated tallow fat alcohol) in varying weights, based 
on styrene, 0.15 part of tert-butyl benzoate and 0.45 part of benzoyl 
peroxide were heated to 90.degree. C. with stirring. After 2 hours at 
90.degree. C., 4 parts of a 10 % aqueous solution of polyvinylpyrrolidone 
were added as suspension stabilizer, and the mixture was stirred at 
90.degree. C. for a further 2 hours, than at 100.degree. C. for 2 hours 
and finally at 120.degree. C. for 2 hours. 
The resulting polymer in bead form having an average particle diameter of 
about 1 mm was separated from the aqueous phase, washed and briefly dried 
with air at from 40.degree. to 70.degree. C. to remove adherent surface 
water. Thereafter the water content (internal water) of the beads was 
determined. 
The beads were then irradiated in a conventional microwave cooker (power 
rating 650 W) with microwaves for 6 minutes, and they expand. For 
comparison, the beads were foamed for 6 minutes with a steam flow. The 
results are summarized in the table. 
TABLE 
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Density after foaming [kg/m.sup.3 ] 
Wetting 
Internal 
with micro- 
Example Pentane 
agent 
water 
waves with steam flow 
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1 (comparison) 
-- 0.005 
0.5 415 800 
2 (comparison) 
-- 0.010 
2.1 350 790 
3 2.3 0.020 
2.4 29 130 
4 3.2 0.010 
2.3 15 90 
5 (comparison) 
2.3 -- &lt;0.1 200 135 
6 (comparison) 
3.2 -- &lt;0.1 150 92 
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