Europaisches Patentamt O 017 086 0») European Patent Office (1Î) Publication number: Office européen des brevets Bl

(12 EUROPEAN PATENT SPECIFICATION

(45) Date of publication of patent spécification: 24.08.83 ® Int. ci.3: C08J 9/20, B32B 27/08, B32 B 27/30, B 32 B 27/32 @ Application number: 80101440.8 (22) Date offiling: 19.03.80

(54) Expandable thermoplastic polymer beads and process for their production.

(§) Priority: 19.03.79 JP 32680/79 (73) Proprietor: KANEGAFUCHI KAGAKU KOGYO KABUSHIKI KAISHA 2-4 Nakanoshima 3-chome @ Date of publication of application: Kita-ku Osaka-shi (JP) 15.10.80 Bulletin 80/21 @ Inventor: Senda, Kenichi (45) Publication of the grant of the patent: 17-8095, 1-Ban, Miyanoue 24.08.83 Bulletin 83/34 Akashi-shi Hyogo-ken (JP) Inventor: Nishida, Tatehiko 269-14, Tai Aboshi-ku @ Design ated Contracting States: Himeji-shi Hyogo-ken (JP) BE DE FR GB Inventor: Nakagawa, Masao 442-21, Sone-cho Takasago-shi Hyogo-ken (JP) (56) Références cited: DE - A - 1 694 094 DE-A-2 755 005 (74) Représentative: Vossius-Vossius-Tauchner- DE-B-2525173 Heunemann-Rauh Patentanwalte US - A - 3 743 61 1 P.O.Box 860767 Siebertstrasse 4 D-8000 Munchen 86 (DE)'

Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shaH not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1 ) European patent convention). Courier Press, Leamington Spa, England. The present invention relates to expandable small particles having a large surface area. thermoplastic polymer beads which hardly lose Therefore, easy evaporation of the foaming any of a foaming agent absorbed within their agent takes place in the beads according to expandable form and maintain satisfactory elas- US-A-3 743 611. ticity and rigidity; the novel polymer beads are Under these circumstances, the present useful for the production of foamed shaped inventors have investigated ways to eliminate articles, wherein the polymer beads are pre- the foregoing drawbacks in these types of liminarily foamed (prefoamed) with heat and the products by improving their foaming agent prefoamed beads are then packed into a retention capacity. As a result, the present suitable metal mould to give a product of the inventors have created the expandable thermo- desired shape upon further heating. The pre- plastic polymer beads of the invention, which sent invention further concerns the production can readily be prefoamed by the manufacturers of expandable thermoplastic polymer beads of foamed articles and which additionally have having a core composed of vinyl polymer grains satisfactory elasticity and strength, as well as and a skin of an olefin polymer covering the excellent mouldability. The invention was finally grains. made complete by the discovery that the ideal Up to the present day a few types of expand- product can be obtained if grains of a vinyl poly- able polymer beads have already been pro- mer are covered or coated with a polyolefin duced on a commercial scale by adding a resin (olefin polymer), thereby causing the styrene-based monomer to a polyethylene- coated vinyl polymer to retain a foaming agent based resin in an aqueous medium, polymeriz- and imparting the necessary elasticity through ing these together and then impregnating the the polyolefin outer layer (skin). This invention, product with a foaming agent. These modified therefore, relates to novel expandable thermo- beads certainly give expanded articles (popcorn plastic polymer beads consisting of composite plastics) superior in elasticity to comparable beads having a core and a skin and containing a conventional foamed polystyrene and show a foaming agent; the core being composed of a relatively high degree of expansion compared to polymer formed from one or more vinyl mono- the conventional foamed polyolefins. In other mers; the skin being composed of an olefin words, by comparison with pure polyethylene polymer containing some or all of said vinyl resin expandable polymer beads of these types monomers; and the foaming agent being a gas can easily absorb a foaming agent and give rise or a liquid at ordinary temperature and pres- to a relatively highly expanded pre-foam. How- sure and having a boiling point lower than the ever, being prone to lose foaming agent, the softening point of the core material. beads must first be pre-foamed within a short The vinyl polymer used in this invention time after absorbing foaming agent and then denotes a polymer composed chiefly of styrene dispatched to the manufacturers of foamed and/or methyl methacrylate units. These poly- articles. mers are superior in their capacity for retaining Thermofoamable thermoplastic beads a foaming agent, as well as in their foamability consisting of an aliphatic olefin polymer, a and further advantageous physical properties. foaming agent and an organic material for The vinyl polymer which constitutes the core of enhancing preservation of the foaming agent the expandable thermoplastic polymer beads within said beads are described in US-A- consists of a polymer or polymers made from 3743611. The said organic material may be monomers and containing from 60 to 100% by another thermoplastic resin having an affinity to weight of styrene and/or methyl methacrylate said foaming agent, a solvent for the resin and from 0 to 40% by weight of monomers above room temperature or a copolymerizable other than styrene and/or methyl methacrylate, vinyl monomer and mixtures thereof. such as acrylonitrile vinyl acetate or mixtures The thermoplastic beads according to US- thereof. When the content of styrene and/or A-3 743 611 1 contain the aliphatic olefin methyl methacrylate is less than 60% by polymer as basic component. The organic weight, the resulting polymer does not have a material for enhancing preservation of the sufficient foaming agent retention capacity or foaming agent (which may be a vinyl polymer) is the physical properties required in the expanded simply mixed with the olefin polymer without moulding. any specific form or distribution of one compo- The olefin polymer used in this invention is nent or the other. However, since the olefin preferably polyethylene, an ethylene-vinyl polymer (e.g. polyethylene) is little compatible acetate copolymer, an ethylene-methyl meth- with the vinyl polymer (e.g. polystyrene), phase acrylate copolymer, an ethylene-vinylchloride separation occurs easily in the microstructure. copolymer, 1,2-polybutadiene or similar; these The vinyl polymer which is to serve as foaming may be used either singly or in combination agent preserving medium is not comprised in with each other. the beads according to US-A-3 743 611 in The vinyl monomer contained in the olefin the form of one large grain but in the form of polymer, i.e. in the skin of the expandable thermoplastic polymer beads of this invention, used. By the use of the vinyl monomers other is limited to those vinyl monomers which can be than the primary one (styrene), however, the absorbed into the olefin polymer. Once it has stability of the suspension mixture will be been absorbed into the olefin polymer, the vinyl improved; moreover, inasmuch as these vinyl monomer serves not only to lower the melt monomers are well grafted on or blended with viscosity of the olefin polymer to make the coat- the vinyl polymer (polystyrene), they can help to ing of the vinyl polymer easy but also improve control the mechanical properties of the core the strength of the skin when properly incor- polymer. porated. The vinyl monomer content in the The grains of the core polymer used have a olefin polymer is 5 to 50% by weight, and pre- size of 300 to 5000 micron (p) in the case of ferably 10 to 30% by weight. When the vinyl polystyrene. This particle size can also be used monomer content is less than 5% by weight, the in the case of the other polymers. effect and functioning are inferior; conversely, Given that the symbol A is the weight of when the content is more than 50% by weight, grains of the vinyl polymer, B the weight of the the skin formed from the polyolefin becomes powdered olefin polymer, and C the weight of too hard and brittle. one or a mixture of more than one type of vinyl The vinyl polymer formed from the monomer monomer, the weight ratio of (A+B) to C is in absorbed into the polyolefin is preferably the range of from 90:10 to 30:70. When the composed chiefly of polystyrene; however, a ratio of (A+B):C is greater than 90:10, the poly- small amount of polymethyl methacrylate poly- olefin coat on the vinyl polymer becomes inade- acrylonitrile, polyvinyl acetate or similar may be quate. Conversely, when the ratio is smaller mixed with the polystyrene. than 30:70, the suspension of the solution It is necessary that the weight ratio of the mixture becomes unstable. Moreover, the core to the skin be in the range of from 95:5 to proportion of A to B must be so determined that 30:70. When the weight ratio of core to skin the proportion of the core to the skin lies in the exceeds 95:5, the polyolefin can no longer be range of from 95:5 to 30:70 in weight. expected to have the desired effect. In view of the fact that the exact distribution By contrast, when the weight ratio is below of the vinyl monomers (their weight is C) 30:70, the foaming capacity to produce satis- between the vinyl polymer (its weight is A) and factory expansion becomes too low to allow the olefin polymer (its weight is B) cannot be practical application of the beads. accurately forecast, the final proportion of A:B The foaming agent used in this invention cannot be exactly fixed without considering C should generally not dissolve the vinyl polymer, When the weight of the vinyl monomers (C) is or at least not swell it excessively. The foaming small, the proportion A:B can be set at a value agent must be gaseous or liquid at room within the range of 95:5 to 30:70. When the temperature and atmospheric pressure. Thus value of C is large, however, in order to adjust lower aliphatic hydrocarbons can be used as the ratio of the core to the skin at a value foaming agents. In this case the foaming agent between 95:5 and 30:70, the proportion A:B is chiefly composed of such lower aliphatic will have to be determined as appropriate by a hydrocarbons as propane, butane, or pentane, simple preliminary experiment. and the balance of the composition may be As dispersing agent in the process of this replaced by hexane, heptane or cyclohexane. invention, water-soluble polymers such as poly- In the second place, this invention pertains to vinyl alcohol or methyl cellulose, and inorganic a novel process for the production of expand- compounds which are hardly soluble in water able thermoplastic polymer beads, which such as calcium phosphate or magnesium pyro- process comprises suspending grains of a vinyl phosphate are suitable. polymer, a powdered olefin polymer, one or In the present process, given the necessity more vinyl monomers and a cross-linking agent that the beads of the vinyl polymer adhere to in an aqueous medium containing a dispersion and be covered by the powdered olefin pol,,, agent, and polymerizing the vinyl monomers in mer, the affinity of the two polymers and their the presence of an initiator. A foaming agent combined effect on the melt viscosity become may be added to the dispersion mixture during important in a consideration of the covering or after polymerization, or beads or grains of the efficiency in relation to the beads. As the poly- vinyl polymer containing the foaming agent can olefin resin of which the skin is composed is be used as starting material. impregnated with the vinyl monomer, the melt The vinyl monomers other than the primary viscosity of the polyolefin resin is lowered and monomer (the primary monomer is the mono- the core is easily covered with melted poly- mer which constitutes the grains or beads) pre- olefin resin. From this standpoint the ethylene- ferably have good solubility for the grains of a vinyl acetate copolymer with a vinyl acetate polymer and poor solubility for the olefin poly- content of 15 to 50% by weight and a melt mer. In other words, when the primary vinyl index of 1 to 100 gives excellent results as monomer is styrene, methyl methacrylate, regards the covering efficiency and mechanical acrylonitrile or vinyl acetate can be used. How- property of the foamed articles. In this case, the ever, the olefin polymer will also satisfactorily powdered ethylene-vinyl acetate copolymer cover the polystyrene core if styrene alone is should preferably have a particle size of from 10 to 2000 µ, and this particle size can equally be articles have the improved strength and the applied to the other polyolefins. elasticity resulting from the double structure, In the present invention the covering and by which means shortcomings inherent in efficiency can greatly be improved when the each of the polymers are overcome; the con- grains of the vinyl polymer are impregnated ventional polyethylene polystyrene foamed with the foaming agent. In this case the reac- shaped articles, on the other hand, are only tion of the dispersion mixture needs to be designed to yield the intermediate properties of carried out under pressure using either an inert polyethylene and polystyrene. like the gas nitrogen or foaming agent itself, as The invention described in mentioned earlier. present so far general terms will be explained in more detail In order to polymerize one or more of types through the following examples. the vinyl monomers added to the reaction the conventionally used initiators for system, Example 1 the suspension polymerization are suitable. An dispersion was prepared by add- Typical examples of these initiators organic aqueous are ing 10.5 of tertiary calcium phosphate (3.5 peroxides such as benzoyl peroxide, lauroyl g parts by weight) as a dispersing agent and peroxide, 1,1 - bis(tert - butyl peroxy)3,3,5 - 0.126 g of a sodium alkyl (or alkene) sul- trimethyl cyclohexane, and similar, azo phonate (0.042 part by weight) to 1500 of compounds such as azobisisobutyronitrile and g water in a 3-litre autoclave. In the azobisdimethyl valeronitrile. pure aqueous dispersion were suspended 300 of In the of this invention, g a pow- process moreover, a dered ethylene-vinyl acetate copolymer having cross-linking agent is employed to cross-link the a melt index of 3 and a content of 25% vinyl vinyl polymers and the olefin polymers. Such acetate units (100 parts by weight) ("FLO-VAC cross-linking include bifunctional agents mono- K-2010", particle size 149-71 Ou) and 450 mers like divinyl benzene and similar, and g of polystyrene beads (150 parts by weight, organic peroxides like di-tert-butyl peroxide, particle size 840-1410 µ) containing 7% by dicumyl peroxide and similar. By the use of the weight of butane as a foaming agent. The mix- cross-linking the polymer beads are agent pre- ture was agitated with a stirrer at 300 To vented from shrinking, which might otherwise rpm. the well dispersed suspension are added 150 occur in the course of the pre-foaming and g of methyl methacrylate (50 parts by weight) and moulding or shaping process. 150 g of styrene containing 0.08 part by weight The expandable thermoplastic polymer beads of divinyl benzene, 0.28 part by weght of of the invention generally have a present benzoyl peroxide, and 0.15 part by weight of spherical or an ellipsoidal form; however, slight 1,1 - bis(tert - butyl peroxy)3,3,5 - tri - methyl deformation does affect their not practical cyclohexane (50 by weight). The poly- have size of 400 to 8000 parts application. They a µ; merization was conducted as described below. therefore conventional any moulding apparatus Nitrogen was introduced into the autoclave at utilizing polystyrene beads can be used without 15°C so that the internal pressure reached 9 the need to make modification. The core of any kg/cm2. Then the temperature in the autoclave the is well suited to hold the vinyl polymer was raised to 60°C for 1 hour, and finally to i.e. when the is made from foaming agent, core 90°C for 3 hours. Next the resulting product the retains the polystyrene, core foaming agent was heated at 105°C for 2 hours and subse- the itself; such as firmly as polystyrene affinity quently cooled. When the temperature had of the with the polystyrene core foaming agent fallen to 40°C the nitrogen was vented. 90 of be said be much than that of the g can to greater n-butane (30 parts by weight) were fed into the conventional binary polyethylene-polystyrene autoclave and the contents were heated to the beads with foaming agent. Consequently, 105°C for 6 hours. After cooling the autoclave the beads expandable thermoplastic polymer was opened and the product was removed and stand thus can long storage periods, enabling rinsed with water. The beads obtained had a the bead manufacturers send the beads of to spherical shape and contained 6.0% by weight the invention the manufacturers of foamed to of n-butane, and they proved to be coated uni- articles without which the bead pre-foaming, formly with about 30% by weight of the hitherto to do in the producers were obliged ethylene-vinyl acetate copolymer. The beads of conventional poly- case binary polyethylene were aged at 5°C for 24 hours before steam cored As result, the costs of styrene products. a was applied to foam them; as a result, be much reduced, thus expan- transportation can sion of as much as 61 times the original volume which should be of producing a saving great was observed. significance to industry. The foamed shaped articles produced from The beads pre-foamed by about 30 times the expandable thermoplastic polymer beads of were packed into a 20x25x10 cm frame and this invention excel in strength and elasticity on exposed for 20 seconds to super-heated steam account of the fact that every interface of the having a pressure of 0.8 kg/cm2. The resulting foamed beads is coated with a tenacious olefin shaped article, in which the beads were found polymer layer in which no cellular structure is to be closely fused, had a density of 0.035 present. Thanks to this, the foamed shaped g/cm3 and showed excellent strength and elasticity, as compared with conventional uniformly; however, in this case the beads foamed polystyrene. looked somewhat yellowish because acryloni- Separately, after being left to stand at room trile had been used instead of methyl methacry- temperature (about 15°C) to investigate their late. The beads expanded up to 64 times their capacity for retaining the foaming agent, the original volume when they were subjected to beads showed that the foaming agent had steaming in the same way as described in evaporated from the skin of the ethylene-vinyl Example 1. In a shaped article produced from acetate copolymer within two days, and the beads made to expand 30 times in the pre- retention capacity was 82% in total, extending foaming stage, the beads were seen to be fused from the second to the fifteenth day. This figure together. The density of the shaped article was clearly proves that there is substantially no dif- 0.032. The strength and the elasticity were both ference in the foaming agent retention capacity superior compared with the conventional of these binary beads and the polystyrene beads foamed polystyrene; however, these results having a similar particle size, whose retention were slightly inferior to those obtained in capacity is 89%. The beads which had been left Example 1. for 10 days after the absorption of the foaming The capacity for retaining the foaming agent, agent expanded up to 57 times their original measured by the same method as described in volume when steam was applied. Example 1, was 83%, whereas that of the conventional expandable polystyrene beads was Example 2 89%. After the beads had been left for 10 days In 300 g of styrene (100 parts by weight) after impregnation of the foaming agent, they were dissolved divinyl benzene, benzoyl showed expansion of up to 52 times their initial peroxide, and 1,1 - bis(tert - butyl volume. peroxy)3,3,5 - trimethyl cyclohexane, in the same amounts as in Example 1. Methyl Example 4 methacrylate was not used in Example 2. n- Example 1 was repeated but instead of Butane was used in an amount of 180 g (60 nitrogen gas, 60 g of n-butane (20 parts by parts by weight), which was as much as twice weight) were introduced into the autoclave that of Example 1. The other conditions under pressure at room temperature. After remained unchanged. The beads produced con- standing at 60°C for 1 hour, the mixture was tained 6.8% by weight of n-butane and proved polymerized at 90°C for 3 hours. After poly- to have a slightly deformed spherical form. The merization had taken place, the temperature beads were able to expand up to 57 times their was raised to 105°C and stirring was con- initial volume when heated in the same way as tinued for 2 hours. The product was then cooled described in Example 1. Two samples of shaped and taken out of the autoclave and rinsed with articles produced from the same beads which water. The resulting beads contained 5.1% by had been made to expand by about 30 times weight of n-butane and had a spherical form. and about 40 times in the pre-foaming stage They expanded up to 58 times when they were had a density of 0.032 g/cm3 and 0.021 g/cm3, subjected to the same steaming process as in respectively. In these samples the beads were Example 1. found to be closely fused together. These samples were superior to the conventional foamed polystyrene in their elasticity and strength. The elasticity of these samples 1. composite expandable thermoplastic proved to be better than that of the samples in polymer beads comprising an olefin polymer, a Example 1. The investigation into the foaming vinyl polymer and a foaming agent which is agent retention capacity (at 20°C) using the gaseous or liquid at ordinary temperature and procedure described in Example 1 gave a result pressure, characterized in that the beads consist of 75% for the expandable beads of this inven- of a core and a skin, the weight ratio of the core tion and 86% for the conventional expandable to the skin being in the rage of from 95:5 to polystyrene beads. After the beads had been left 30:70, the core being composed of 60 to 100% for 10 days after being impregnated with the by weight of styrene and/or methyl meth- foaming agent, they were able to expand up to acrylate, the skin being composed of the olefin 45 times their initial volume when subjected to polymer which contains 5 to 50% by weight of heat. the vinyl polymer, and in that the foaming agent has a boiling point below the softening point of Example 3 the core material. The production of the expandable thermo- 2. Polymer beads as claimed in claim 1, plastic polymer beads of this invention was wherein the core contains 0 to 40% by weight carried out in the same way as stated in of acrylonitrile, vinyl acetate or mixtures thereof. Example 1, except that methyl methacrylate was 3. Polymer beads as claimed in claim 1 or 2, substituted for acrylonitrile. The beads pro- wherein the olefin polymer which constitutes duced had a spherical form and contained 6.3% the skin is an ethylene-vinyl acetate copolymer. by weight of n-butane. The ethylene-vinyl 4. Polymer beads as claimed in claim 3, acetate copolymer was found to cover the core wherein the ethylene-vinyl acetate copolymer contains 15 to 50% of vinyl acetate monomer 5. Polymerisatperlen nach Anspruch 1 bis 4, units and has a melt index of 1 to 100. wobei das Vinylpolymerisat der Haut Polystyrol 5. Polymer beads as claimed in claims 1 to 4, ist. wherein the vinyl polymer in the skin is poly- 6. Verfahren zur Herstellung der Poly- styrene. merisatperlen nach Anspruch 1, dadurch 6. Process for the production of the polymer gekennzeichnet, daß man Körner eines Vinyl- beads according to claim 1, which comprises polymerisats, ein pulverförmiges Olefinpoly- suspending grains of a vinyl polymer, a merisat, ein oder mehrere Vinylmonomere, die powdered olefin polymer, one or more vinyl in das Olefinpolymerisat absorbiert werden kön- monomers capable of being absorbed into the nen, und ein Vernetzungsmittel in einem ein olefin polymer and a cross-linking agent in an Dispergiermittel enthaltenden wäßrigen aqueous medium containing a dispersing agent Medium in einem Verhältnis A:B von 95:5 bis in a ratio of A:B in the range of 95:5 to 30:70 30:70 und in einem Verhältnis von (A+B) zu C and in a ratio of (A+B) to C in the range of im Bereich von 90:10 bis 30:70 suspendiert, 90:10 to 30:70, where A represents the weight wobei A das Gewicht der Kömer des Vinylpoly- of grains of the vinyl polymer, B the weight of a merisats, B das Gewicht des pulverförmigen powdered olefin polymer, and C the weight of Olefinpolymerisats und C das Gewicht des oder the one or more vinyl monomers, and poly- der Vinylmonomeren ist, und die Vinylmono- merizing the vinyl monomers in the presence of meren in Gegenwart eines Initiators poly- an initiator, the foaming agent being added to merisiert, wobei man das Treibmittel dem the dispersion mixture during or after the poly- Dispersionsgemisch während oder nach der merization, or beads of the vinyl polymer Polymerisation zusetzt oder Perlen aus einem containing the foaming agent being used as das Treibmittel enthaltenden Vinylpolymerisat starting material. als Ausgangsmaterial verwendet. 7. Process as claimed in claim 6, wherein 7. Verfahren nach Anspruch 6, wobei man grains of an expandable polystyrene previously Körner aus einem expandierbaren Polystyrol, die impregnated with a foaming agent are used as vorher mit einem Treibmittel imprägniert starting material. worden sind, als Ausgangsmaterial verwendet. 8. Process as claimed in claim 6 or 7, 8. Verfahren nach Anspruch 6 oder 7, wobei wherein one of the vinyl monomers is styrene eines der Vinylmonomeren Styrol und das and the other is selected from methyl andere Methylmethacrylat, Acrylnitril oder methacrylate, acrylonitrile, and vinyl acetate. Vinylacetat ist. 9. Process as claimed in claims 6 to 8, 9. Verfahren nach Anspruch 6 bis 8, wobei wherein the olefin polymer is an ethylene-vinyl das Olefinpolymersat ein Äthylen-Vinylacetat- acetate copolymer. Copolymerisat ist.

1. Expandierbare thermoplastische Poly- 1. Perles expansibles composites en matière merisat-Verbundperlen, umfassend ein Olefin- polymère thermoplastique, comprenant un poly- polymerisat, ein Vinylpolymerisat und ein bei mère oléfinique, un polymère vinylique et un Normaltemperatur und Normaldruk gas- agent moussant qui est gazeux ou liquide aux förmiges oder flüssiges Treibmittel, dadurch température et pression ordinaires, carac- gekennzeichnet, daß die Perlen aus einem Kern térisées en ce que les perles sont constituées und einer Haut bestehen, wobei das Gewichts- d'un noyau et d'une pellicule, le rapport en poids verhältnis von Kern zu Haut im Bereich von 95:5 entre le noyau et la pellicule étant de l'ordre de bis 30:70 liegt, der Kern aus 60 bis 100 95/5 à 30/70, le noyau étant constitué de 60 à Gewichtsprozent Styrol und/oder Methyl- 100 parties en poids de styrène et/ou de methacrylat und die Haut aus dem Olefin-Poly- méthacrylate de méthyle, la pellicule étant con- merisat besteht, das 5 bis 50 Gewichtsprozent stituée du polymère oléfinique qui contient 5 à eines Vinyl-Polymerisats enthält, und das Treib- 50% en poids d'un polymère vinylique, et en ce mittel einen Siedepunkt unterhalb des que l'agent moussant présente un point d'ébulli- Erweichungspunktes des Kernmaterials hat. tion inférieur au point de ramollissement de la 2. Polymerisatperlen nach Anspruch 1, wobei matière de noyau. der Kern 0 bis 40 Gewichtsprozent Acrylnitril, 2. Perles en matière polymère suivant la Vinylacetat oder Gemische davon enthält. revendication 1, caractérisées en ce que le 3. Polymerisatperlen nach Anspruch 1 oder noyau contient 0 à 40% en poids d'acrylonitrile, 2, wobei das Olefin-Polymerisat, das die d'acétate de vinyle ou de melanages de ces com- Haut darstellt, ein Äthylen-Vinylacetat-Copoly- posés. merisat ist. 3. Perles en matière polymère suivant l'une 4. Polymerisatperlen nach Anspruch 3, wobei des revendications 1 et 2, caractérisées en ce das Äthylen-Vinylacetat-Copolymerisat 15 bis que le polymère oléfinique qui constitue la pelli- 50 Gewichtsprozent Vinylacetat-Monomerein- cule est un copolymère d'éthylène-acétate de heiten enthält und einen Schmelzpunkt von 1 vinyle. bis 100 hat. 4. Perles en matière polymère suivant la revendication 3, caractérisées en ce que le oléfinique en poudre et C le poids d'un ou de copolymère d'éthylène-acétate de vinyle con- plusieurs monomères vinyliques, et la poly- tient 15 à 50% d'unités monomères d'acétate mérisation des polymères vinyliques en pré- de vinyle et présente un indice de fusion de 1 à sence d'un agent d'amorçage, l'agent mous- 100. sant étant ajouté au mélange en dispersion pen- 5. Perles en matière polymère suivant l'une dant ou après la polymérisation, ou des grains des revendications 1 à 4, caractérisées en ce du polymère vinylique contenant l'agent mous- que le polymère vinylique dans la pellicule est sant étant utilisés comme matière de départ. du polystyrène. 7. Procédé suivant la revendication 6, carac- 6. Procédé de préparation de perles en térisée en ce que des grains d'un polystyrène matière polymère suivant la revendication 1, expansible précédemment imprégné par un caractérisé en ce qu'il comprend la mise en agent moussant sont utilisés comme matière de suspension, dans un milieu aqueux contenant départ. un agent de dispersion, d'un polymère vinylique 8. Procédé suivant l'une des revendications 6 en grains, d'un polymère oléfinique en poudre, ou 7, caractérisé en ce qu'un des monomères d'un ou plusieurs monomères vinyliques, vinyliques est du styrène et en ce que l'autre est capables d'être absorbés dans le polymère olé- choisi parmi le méthacrylate de méthyle, l'acrylo- finique, et d'un agent de réticulation, dans un nitrile, et l'acétate de vinyle. rapport A/B de l'ordre de 95/5 à 30/70 et dans 9. Procédé suivant l'une des revendications 6 un rapport entre (A+B) et C de l'ordre de 90/10 à 8, caractérisé en ce que le polymère olé- à 30/70, A représentant le poids des grains du finique est un copolymère d'éthylène-acétate de polymère vinylique, B le poids d'un polymère vinyle.