CA2148991A1

CA2148991A1 - Expandable styrene polymers

Info

Publication number: CA2148991A1
Application number: CA002148991A
Authority: CA
Inventors: Rolf Henn; Klaus Hahn; Andreas Deckers; Graham Edmund Mckee; Hermann Tatzel; Rudolf Drumm; Jurgen Fischer
Original assignee: Individual
Current assignee: BASF SE
Priority date: 1994-05-13
Filing date: 1995-05-09
Publication date: 1995-11-14
Also published as: EP0682076A2; ES2105809T3; EP0682076B1; DE4416863A1; KR950032316A; EP0682076A3; DE59500626D1; JPH0859929A

Abstract

The invention relates to expandable styrene polymers for elastic polystyrene foams, comprising a) from 75 to 99 % by weight of polystyrene and/or a styrene copolymer containing at least 50 % by weight of copolymerized styrene, b) from 0 to 24 % by weight of at least one styrene-soluble elastomer, c) from 1 to 25 % by weight of at least one graft copolymer having a core/shell structure, d) from 1 to 15 % by weight, based on the sum of a), b) and c), of a low-boiling blowing agent, and, if desired, e) conventional additives in effective amounts, and to foams and foam moldings produced therefrom.

Description

r B}~SF Illrt i ~nqc~91,Al 1 .9~hA ft 933482 0 . Z~ . 0050~4487 1 "''I _ 2~48~1 Expandable styrene polymers The present invention relates to expandable styrene polymers which are suitable for the productlon of elastic foams.
Foams based on polystyrene have achieved considerable industrial importance as thermal insulation and packaging materials. They are produced on an industrial scale by first preparing expandable 10 styrene polymers by suspension polymerization of styrene in the presence of a blowing agent, expanding these polymers by heating to give foam particles, and subsequently welding the particles in molds to give moldings.
Polystyrene foams are rigid. Their low elasticity is disadvanta--geous for many applications, for example in the packaging sector, since protection of the packaged goods against impact is only possible to an lnadequate extent, and the foam moldings used as packaging materials break even on only small deformation.
Attempts have therefore already been made in the past to increase the elasticity of polystyrene foams.
EP-A--561 216 describes a proces3 for elastifying polystyrene oams, in which foam slabs having a density from 3 to 12 kg/l are compressed to about 1/3 of their size in one direction and then released again. Boards cu~ fLom the slabs treated in this way have increased elasticity and are used, for example, for solid-borne sound insulation.
However, the terhn1~1ities of the process mean that this proce--dure is very difficult to apply to moldings and is therefore not carried out.
US-A-4,424,285 and US-A-4,409,338 describe foamable styrene poly-mers which are prepared by polymerization of a solution of from O . 5 to 4 . 0 96 by weight of styrene-butadiene or styrene-butadiene-styrene block copolymers ln styrene and which have a short mold cooling time.
However, this only increases the elasticity of the foams to an insignificant extent due to the small amount of rubber added.
In US-A-4,307,134 and US-A-4,333,970, shells of styrene-butadiene copolymers are polymerized onto polystyrene beads with partial grafting, and the resultant beads are impregnated with blowing agent and 3ubsequently expanded. However, the resultant foams ~ASY Aktiengesellschaft 933482 O.Z. 0050/44871 21~8~1 have an irregular 9hell structure and unfiatisfactory 1 Anir~l properties .
GB-A-1,220,611 describes a foamable polymer composition having increased oil resistance which comprises a styrene-acrylonitrile copolymer and a polybutadiene elastomer, where the styrene-acry-lonitrile copolymer $8 dispersed in the elastomer and the blowing agent is absorbed in the elastomer phase with swelling and partial dissolution. However, such foams have unsatisfactory 10 mechanical properties.
In all the prior-art processes described, the blowing agent diffuses out of the beads very rapidly. After only a few days, the 108s of blowing agent can be 80 large that proper foaming of the beads i5 no longer possible. In particular, thi~ effect bc-comes undesirably evident on addition of elastomer in proportions of greater than 5 % by weight, as necessary ~or achieving ade-quate elastificatlon.
20 It 18 an object of the present lnventlon to provide ~Yr~nA~hl e ~tyrene polymers which are sultable for the production of elastic foams, do not lose slgnificant amounts of blowing agent even after extended storage, and are recyclable.
We have found that this object is achleved by ~Yr~n~hl ~ styrene polymers for elastlc polys~yrene foams, comprislng a) from 75 to 99 % by weight of polystyrene and/or a styrene copolymer containing at least 50 ~ by weight of copolymerlzed styrene, b) from 0 to 24 % by weight of at least one styrene-soluble elastomer, c) from 1 to 25 ~ by weight oi at least one graft oopolymer having a core/shell ~uuLule, d) from 1 to 15 ~ by weight, based on the sum of a), b) and c), of a low-boiling blowing agent, and, if desired, e) conventional additives in effective amounts.
The present invention accordingly provide~ r~n~l~hl ~ styrene polymers for ela-tic polystyrene foams, comprising BA8iF Akt;on~Eo~ hDft 933482 O.Z. 0050/44871 ~ 8~91 a) from 75 to 99 % by weight of poly8tyrene and~or a styrene copolymer containing at lea5t 50 % by weight of copolymerized styrene, b) from 0 to 24 % by weight of at least one r~yL~ oluble e1astomer, c) from 1 to 25 % by weight of at lea3t one graft copolymer having a core/shell ~LLU~:~UL~, d) from 1 to 15 9~ by weight, based on the sum of a), b) and c), of a low-boiling blowing agent, and, lf desired, e) conventional additLve3 in effective amounts.
The present invention furthermore provides elastic polystyrene foams ' ~ i n~
a) from 75 to 99 % by weight of polystyrene and/or a styrene copolymer containing at least 50 9~ by weight of copolymerized styrene, b) from 0 to 24 % by weight of at least one ~LYL~ 301Ub1e elastomer, c) from 1 to 25 % by weight of at least one graft copolymer having a core/shell ~lLLul~:LuLe~ and, if desired, d) conventional additives in effective amounts.
The present invention rUL ' ' ~ provides ~L~ 55L5 for the preparation of the elastic styrene polymers and moldings produced from the elastic polystyrene foams.
c ~ L a) in the oYr~n~l~hl e styrene polymers ' ~o~ from 75 to 98 % by weight, preferably from 85 to 93 k by weight, of poly-styrene and/or a styrene copolymer containing at least 50 % by weight, preferably at least 80 % by weight, of copolymerized styrene. Examples of suitable ~ are c-methylstyrene, 40 ring-halogenated styrenes, ring-alkylated styrenes, acrylo-nitrLle, esters o~ acrylic or methacrylic acid with alcohols hav-ing 1 to 8 carbon atom3, N-vinylcarbazole, maleic acid and maleic anhydride. Irhe polystyrene t~d~ L~Iy~u~lly contains a small amount of a copolymerized cr~alink~n~ agent, ie. a compound containing more than one, preferably 2, double bonds, such as divinyl-benzene, butadlene or butanediol diacrylate. Ihe crn9~1;nk;n~

BASF Aktiengesellscha$t 933482 O.Z. 0050/44871 ` 21~8~1 4 agent is generally used in amounts of from 0.005 to O.OS mol%, based on styrene.
In order to achieve particularly high f-Yr~nd~h; l; ty, it is exre-dient for the styrene polymer to have a mean molecular weight Mw (weight average), measured by the GPC method, of from 100,000 to 200,000, in particular from 130,000 to 180,000. The foam has improved processing properties if the high-molecular-weight flank of the molecular-weight distribution curve measured by the GPC
10 method i8 80 steep that the difference between the means (M2+1-Mz) is less than 150,000. The GPC method is described in G. Glockler, Polymercharakterisierung, Chromatographi5che Methoden, Volume 17, Huthig-Verlag, l~ lh-rg~ 1982. These means are described in .G. Elias, Makromolekule, ~uthig-Verlag, llc.~ g, 1971, pages 52-64 .
Styrene polymers which have the abovementioned mean 1 rrl-l Ar weights can be obtained by carrying out the polymerization in the presence of regulators. Tha regulators used are expediently from 20 0 . 01 to 1:5 ~ by weight, preferably from 0 . 01 to 0 . 5 % by weight, of a bromine-free organLc compound having a chain--transfer constant E~ of from 0.1 to 50. Addit~on o~ the regulator during the polymerization is expediently delayed until a conversion of from 20 to 90 9~ has been reached in order to achieve a steep hiyll --1 er~ll Ar-weight flank of the molecular--weight distribution curve .
An advantageously high ~Yr--n~ n capacity can also be achieved if component a) contains from 0.1 to 10 ~ by weight, advantageously 30 from 0.5 to 10 % by weight, of a styrene polymer having a mean r-l~c-llAr weight (weight average) of from 500 to 5000.
Further detalls on r -1 I-rl-l Ar-weight regulation in the preparation of f~Yrnn~ hl e 8tyrene polymers are given in EP-B 106 129 .
Styrene polymers which contain from 0.1 to 2 % by weight, prefer-ably from 0 .15 to 1. 5 % by weight, of copoly ^r1 7ed acrylonitrile give foams which are distinguished by substantial absence of shrinkage. A mixture of from 95 to 99 . 5 % by weight of polysty-40 rene and from 0 . 5 to 5 9~ by weight of a styrene-soluble styrene-~lcrylonitrile ccpolymer also exhibits these properties if the total acrylonitrile content in the mixture is from 0.1 to 2 % by weLght, preferably from 0.15 to 2 ~ by weight.
Styrene polymers containing from 3 to 20 % by weight, preferably from 5 to 15 % by weight, of copolymerized acrylonitrile give foams having high oil resistance. A mixture of from 50 to 85 t by BA.`3F A~cti~ n~-sellc-h-ft 933482 o.z. 0050~44871 : 21~89~1 weight of polystyrene and from 15 to 50 % by weight of a styrene-soluble styrene-acrylonitrile copolymer also exhibits this advan-tageous property if the total acrylonitrile content in the mixture is from 3 to 20 % by weight, preferably from 5 to 15 ~ by weight. Such mixtures are prepared in a simple manner by dissolv-ing the proposed amount of styrene-acrylonitrile copolymer in styrene before the polymerization.
Styrene polymers containing from 2 to 15 % by weight, in parti-10 cular from 3 to 12 9~ by weight, of maleic acid or maleic anhydride a8 r r give foams which are distinguished by high heat distortion resistance. It i9 advantageous to use a mixture of polystyrene and a commercially available gtyrenc - 1 "; r anhydride copolymer having a maleic anhydride content of from 15 to 49 % by weight, which can easily be prepared by dissolving the copolymer in styrene before the polymerization.
Component b) is, in particular, a styrene--soluble elastomer hav-ing a glass transition temperature of below O~C, preferably below 20 -10C, in particular below -20~C.
The elastomer i~ generally essentially uncrosslinked, if desired only crt~l i nkt cl to the extent that the solubility in styrene i8 not impaired.
Preference i3 given for the novel styrene polymers to polybuta-diene rubbers, in particular those having a r-lt~rulAr weight (Mw) of from 200,000 to 300,000 and containing < 50 '6 of 1,4-cis structures and from 5 to 20 ~ of 1, 2-vinyl structures (medium-cis 30 structure) or from 50 to 99 % of 1, 4-cis structures and ~ 5 % of 1, 2-vinyl structures ( high-cis structure ) .
The elastomer phase is dispersed in the styrene phase in the form of cell particles in the polystyrene phase.
These cell particle3 should have diameters of from 0.1 to 10 llm, in particular from 0 . 5 to 5 llm.
The presence of component b) produces, in particular, better com-40 patibility betwe~n components a) and c) and a further increase in the elasticity of the foams.
However, it is al30 possible to omit component b) in the produc-tion of the novel products.

8A~iF Aktif'n~fellc~hDft g33482 O.Z. 0050/44871 ` ~ . 2148991 The core/3hell rubbers u8ed a8 component c) are preferably pre- =
pared by emul5ion polymerization with partial grafting.
In thi3 process, first olefinically unsaturated monomers are polymerized, usually in emulsion, and a Nshell" is then polymer-ized onto the resultant particles ( "coren ) by polymerization of other ~ f;nir~lly unsaturated monomers, again usually in emul-sion, with grafting taking place between core and shell.
10 For the preparation of the novel styrene polymers, component c ) is, in particular, a product having a core of a flexible polymer and a shell of a more rigid polymer. For the purposes of the present invention, the term "flexible polymer is taken to mean a polymer having a glass transition temperature of from 20 to -60C, preferably from 10 to -40C. The core material here usually com-prises products of the polymerization of mixtures of Cl-C8-alkyl acrylates and alkyl aromatic `, such as styrene, and ccn-ventional cr~slinkin~ agents and graft crosslinkin~ agents.
Preference ia given to mixtures of from 40 to 90 S by weight of 20 alkyl acrylate and from 10 to 60 " by weight of alkyl aromatic The shell material here preferably ccmprises product3 of the polymerization of mixtures o~ alkyl methacrylates and styrene, where the styrene content is preferably from 80 to 99 S by weight and the alkyl methacrylate content is preferably from 1 to 20 9 by weight.
The proportion of the core is from 40 to 80 ~ by weight and that 30 of the shell is from 60 to 80 ~ by weight, in each case based on tha total weight of the monomers.
The core/shell rubber is usually precipitated after the polymer-ization by a method known to the person skilled in the art, for example by adding magnesium salts to the emulsion, washed, dried and comminuted.
~owever, it is also pos~iibl~ to convert the rubber into a readily conveyable form by spray drying. It is less common, but just as 40 possible to meter the aqueous emulsion mixture directly into an extruder together with the poly3tyrene.
A description of such polymers and their preparation is given, for example, in EP-A-0 376 096.

E~ASF Akti~n~es~l 1 schaft 933482 O.Z . 0050/44871 ~ 8 9 ~ 1 Component c) is in the polystyrene phase in the form of capsule particles having diameters of from 50 to 300 nm, in particular from 100 to 200 nm.
Information on the morphology of elastomer-modified 6tyrene poly-mers is given in: Echte, Rubber-Toughened Styrene Polymers, Advance~ in ChemiYtry Series l~o. 222, 1989.
A~ component d), the ~YrAn~Ahle styrene polymers contain, in 10 homogeneous distribution, from 2 to 15 t by weight, preferably from 3 to 10 ~ by weight, of a low-boiling blowing agent. The blowing agent should not disaolve the polystyrene, but should be soluble in polystyrene. The boiling point ~hould b~ below the softening point of the polystyrene. Examples of suitable blowing agents are propane, butane, pentane, hexane, cyclopentane, cyclo-hexane, octane, dich1orodifluoromethane, trifluorochloromethane and l,l,l-difluorochloroethane. Pentane is preferred.
The ~YrAnr1Ahle styrene polymers may furthermore contain effective 20 amounts of conventional additives, such as dyes, fillers, stabi-lizers, flameproofing agents, synergists, nucleating agenta, lubricants, antistatics, substances which have a non-stlck action during foaminq, and agents for shortenLng the t~ nq time on n ~ n .
Other suitable additives are poly ~ 2, 6-dimethyl ) -1, 4-phenylene ether and poly-1,4-phenylene sulfide. In amounts of from 1 to 20 9~ by weight, based on component a), these additives increase the heat distortion resistance of the foam.
The novel styrene polymers are preferably prepared by mixing oom-ponents a), b), c) and, if used, d) in the melt, usually in an extruder, where, during addition of d), the extrudate must be cooled ao rapidly after extruaion that foaming does not occur.
The resultant styrene polymer is subseguently comminuted, usually by granulation.
If the blowing agent d) is not added to the styrene polymer during extrusion, it must be added after granulation.
It is furthermore posaible to dissolve component b) in styrene and to polymerize this solution, in which case the polymerization is preferably carried out in bulk.
In this procesa, component b) is dissolved in styrene and this solution is polymerized by processes known per se, usually with addition of free-radical initiators or by the supply of heat.

BASF Aktif-n~^cel1Frh~t 933482 o.Z. 0050/44871 '' ~ 21489!11 However, it i8 al80 po88ible to carry out the polymerization in bulk to a conversion of about 30 ~, to su~pend the resultant pre-polymer in a known manner and to comelete the polymerization in suspension .
Particularly favorable results are achieved if the styrene polymers are prepared by bulk polymerization of a solution of component b) in ~tyrene in the above-described manner and mixing the polymer with component c ) .
The mixing of the polymer of a) and b) with component c) is pre-ferably carried out in the melt, in particular by extrusion.
E~owever, it i~ also possible to add the dried component c) together with component b) to the styrene before tha polymeriza-tion and then to polymerize the mixture as described above Ln order to save additional extrusion and granulation steps. In this f.mhr~ t, it ig algo possible to add the blowing agent during the polymerization and thus to save an additional step for addi-20 tion of blowing agent.
Usually, however, the blowing agent is added by the impregnation method. To this end, the novel polymers must be converted into particle form. This is expediently carried out by eYtrusion with subsequent granulation.
The granules are then usually in the form of particles, ie. in bead form or granule ~orm. Their mean diameter is preierably from 0.1 to 6 mm, in particular from 0.4 to 3 mm.
For the impregnation, the granules are suspended in a liquid, usually water, in th2 presence of conventional auxiliaries and additives in a pressure container, and the latter is rendered inert and brought to a temperature which is above the softening point, but below the melting point, of the polymer. The blowing agent i8 in~ected at this temper~ture. After cooling And decom-pression, the impregnated granules are separated off, purified and dried, preferably at room temperature, for example in a stream o~ ~ir.
Further details on conventional prep~ration proce~ses are given, for example, in Kunststoffh~n-lh~ h, Volume 5, Polystyrol, edited by ~. Vieweg and G. D~ 1 f.r~ ~4rl--E~anser-Verlag, Munich, 1969.
For the production of foams, the ~Yr~nfiilhle styrene polymers are expanded in a known manner by heating to temperatures above their softening point, for example by means of hot air or preferably by ~ ~AiF Ak~i~ ~, r^llc^h-ft 933482 O.Z. 0050/44871 21~89~i~
means of steam . The f oam particlea obtained can bo expanded further by re-heating after cooling and, if desired, after inter-im storage. They can sub5eqUently be welded to form moldings in a known manner in molds which do not seal in a gas-tight manner.
The foams obtained have den5ities of from 10 to 60 g/l. They are distinguished by high elasticity. Thus, they have a r^~ n~^ of up to 90 9~ on quadruple compression. They are thus clearly superior to conventional polystyrene particle foams.
The losses of blowing agent from the unfoamed beads are very low.
Even after storage for several wee3cs, foaming was still possible without problems.
The prefoamed beads have a uniform cell I~LU. LULe and weld during molding without formation of voids. The moldings produced in this way have excellent heat distortion resistance.
In addition, the novel foams have a surprisingly good thermal 20 insulation capacity of up to 10 % better than conventional poly-styrenes of the same density. The foams and moldings can be recycled without problems.
The invention is illustrated in greater detail with reference to the examples below:
Example 1 3500 g of ~ polystyrene having a viscosity number of 74 cm3/g and 30 a molecular weight (Mw) of 220,000, and 1500 g of a core/shell rubber having the composition: core 65 parts by weight (75 % by weight o~ n-butyl acrylate/25 ~ by weight of styrene), shell 35 parts by weight ( 95 ~ by weight of styrene/5 9~ by weight of methyl methacrylate ), were extruded ln a Werner und P~leiderer twin-screw extruder having a diameter of 30 mm at 190C and a throuyhput of 10 kg/h, giving a ~ , ~ mixture. The mixture was forced through a die assembly having a 1 mm bore, and the extrudate was passed through A water bath and, after cooling, was cut into pellets measuring 1 X 1 x 3 mm.
In this mixture, the rubber was in the form of capsule particles having a mean diameter of 100 nm. 6000 g o~ this blend were illLL~Jiu~ ed into a 50 1 stirred reactor together with 21,000 g of demineralizQd water, 76 g of sodium ~yL~ hv~ ate, 155 g of mag-nesium sulfate heptahydrate and 50 g o~ a 40 ~ strength by weight solution of an alkyl~ lf~^nAte (Mersolat~ ~ 30, 3ayer AG).
The reactor was ~losed, flushed twice wit~ 1 atm of nitrogen and BASF Akti~n,~s^l 1 crh--ft 933482 o.Z. 0050/44871 214~9~1 heated to 130C with stirring at 250 rpm. Nhen a temperature of 130C had been reached, 720 g of a mixture of 80 S by weight of n-pentane and 20 % by weight of isopentane were injected into the reactor over a period of 15 minutes, and the mixture was stirred at 130C for a further lO hours.
After cooling and ~ ssion, the reactor contents were dis-charged. The beads were collected, washed twice with demineral-ized water and dried in a suction filter by sucking through 10 ambient air at 23C.
The beads had a blowing agent content of 6.1 % by weight and an internal water content of 0.11 ~ by weight.
After open storage for one day, batch prefoaming for 10 minutes at 100C gave a bulk density of 11.4 g/l.
After open storage for fourteen day5, a bulk density of 11.8 g/l was achieved under the same prefoaming conditions.
~
In both cases, the foam had a b j ~~~lq~ fine--cell ~Lu~:LuL~.
Steam treatment of foam beads for 20 secondD at a Du~e~
pheric pressure of 0.7 bar in a mold measuring 20 X 20 X 4 cm which did not seal fully gave a board having a density of 21 g/l.
After quadruple compression by 70 S, this had a recovery of 88.5 &; at a density of 35.5 g/l, the elastic recovery in the same experiment was 84 . 5 S ( determined in accordance with DIN 53 577) .
The Poensgen thermal conductivity (DIN 52 616) was 7 S below the ~LLe D~Ilding value, determined under the same conditions, for the standard polystyrene 5~yLv~vL~D F 14 (BASF AG) of the same density .
Example 2 85 parts by weight of a polymer prepared by free--radical polymer-ization of a solution of 8 parts by weight of a polybutadiene 40 having a molecular weight (~Iw) of 250,000 and a medium--cis struc-ture in 92 parts by weight of styrene, and 15 parts by weight of the core/shell rubber of Example 1 were blended and granulated as ~-qrr; h-cl in Example 1, impregnated with the blowing agent mix-ture ~_qrr1 h~-l in Example 1 and foamed.

BASF Aict.if~n~s~l 1 c~-h~ft 933482 0. Z . 0050~44871 ~ 214L8~91 Af ter work-up, the granule9 had a blowing agent content of 5 . 8 by welght and an internal water content of 0 . 6 'c by weight.
After open storage for one day, batch prefoaming for 7 minutes at 100C gave a minimum bulk density of 10.6 g/l.
After open storage for three days, a minimum bulk density of 10 . 9 g/l was achieved under the same prefoaming conditions.
10 In both cases, the foam had a homogeneous, fine-cell structure.
A board having a density of 19.8 gi~l produced ~8 in Example 1 had a recovery of 92 % (determined in accordance with DIN 53 577) after quadruple compression by 50 9~.
The Poensgen thermal conductivity (DIN 52 616) was 7 ~ below the corresponding value, determined under the same conditions, for the standard polystyrene sLyLu~uL$ F 14 (BASF AG) of the same density .
Example 3 (comparison) The ~Lu~;eduLe was similar to that of Example 1, but the core/
shell rubber was not added to the polystyrene.
After work-up, the product had a blowing agent content of 8.2 9 by weight and a water content of 0 . 03 ~ by weight.
AftQr open storage for one day, batch prefoaming for 10 minutes 30 at 100C gave a bulk density of 12.1 g/l.
After open storng~ for fourteen days, a bulk density of 16.9 g/l was achieved under the same prefoaming conditions~
In both cases, the foam had ~ conrse structure.
A board having a density of 20 . 0 g/l produced as in Example l had a recovery of ~4.5 9~ after quadruple compressLon by 70 &; at a density of 36.5 g/cm, the elastic recovery in the same experiment 40 was 76.2 9~ (determined in accord~nce with DIN 53 577).

Claims

1. An expandable styrene polymer for elastic polystyrene foams, comprising a) from 75 to 99 % by weight of polystyrene and/or a styrene copolymer containing at least 50 % by weight of copoly-merized styrene, b) from 0 to 24 % by weight of at least one styrene-soluble elastomer, c) from 1 to 25 % by weight of at least one graft copolymer having a core/shell structure, d) from 1 to 15 % by weight, based on the sum of a), b) and c), of a low-boiling blowing agent, and, if desired, e) conventional additives in effective amounts.

2 . An expandable styrene polymer as claimed in claim 1, wherein component b) is selected from the group consisting of poly-butadiene, polyisobutylene and ethylene-propylene rubber.

3. An expandable styrene polymer as claimed in claim 1, wherein component b) is polybutadiene.

4 . An expandable styrene polymer as claimed in claim 1, wherein component c) is a graft copolymer having a core/shell struc-ture .

5 . An expandable styrene polymer as claimed in claim 1, wherein component c) is a graft copolymer having a core of a polymer having a glass transition temperature of from 20 to -60°C and a shell of a harder material.

6 . An expandable styrene polymer as claimed in claim 1, wherein component c) is a graft copolymer comprising from 40 to 80 %
by weight of core and from 20 to 60 % by weight of shell.

7 . An expandable styrene polymer as claimed in claim 1, wherein component c ) is a graft copolymer having a core of a copolymer comprising from 40 to 90 % by weight of an alkyl (meth)acrylate and from 10 to 60 % by weight of styrene, and a shell of a copolymer comprising from 80 to 99 % by weight of styrene and from 1 to 20 % by weight of an alkyl (meth)acrylate .

8. A process for the preparation of an expandable styrene poly-mer as claimed in claim 1, wherein component b) is dissolved in component a ), the resultant solution is polymerized by a process known per se, the polymer is worked up in a conven-tional manner, and mixed with component c) in the melt, and the blend is granulated and impregnated with blowing agent in a manner known per se during or after the granulation.

9. A process for the preparation of an expandable styrene poly-mer as claimed in claim 1, wherein component b) is dissolved in component a), component c) is dispersed in this solution, the dispersion is polymerized by a process known per se, and the polymer is worked up in the conventional manner and im-pregnated with the blowing agent by a process known per se.

10 . An elastic polystyrene foam, comprising a) from 75 to 99 % by weight of polystyrene and/or a styrene copolymer containing at least 50 % by weight of copoly-merized styrene, b) from 0 to 24 % by weight of at least one styrene-soluble elastomer, c) from 1 to 25 % by weight of at least one graft copolymer having a core/shell structure, and, if desired, d) conventional additives in effective amounts .

11. A process for the production of an elastic foam molding, which comprises expanding an expandable styrene polymer as claimed in claim 1 in particle form by heating at a tempera-ture above the softening point, and welding resultant foam particles to one another by heating in molds which do not have a gas-tight seal.