GB1577542A - Manufacture of foamable particles of olefin polymers - Google Patents

Description

(54) MANUFACTURE OF FOAMABLE PARTICLES OF OLEFIN POLYMERS (71) We, BASF AKTIENGESELL SCHAFT, a German Joint Stock Company of 6700 Ludwigshafen, Federal Republic of Germany, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a process for the manufacture of foamable particles of an olefin polymers, in which process the olefin polymer is mixed with a blowing agent (i.e.

a material which emits gas when heated above a decomposition point) to form a mixture, the mixture is heated below the decomposition point of the blowing agent and is extruded, and the resulting continuous foamable strand is then comminuted to give cylindrical foamable particles, which are then irradiated by means of ionizing radiation and optionally subsequently surface-coated with a material which prevents adhesion of the particles.

A plurality of processes for the manufacture of olefin polymer foams has been disclosed. For example, a foamed web has been manufactured by incorporating a solid blowing agent into an olefin polymer, extruding the mixture to give an unfoamed web, crosslinking this web by means of ionizing radiation and passing the web, hanging unsupported, vertically through a heating tunnel, thereby producing a foamed web with predominantly closed cells.

In another process, a foamable web is foamed by slowly passing it, on a hot air cushion, through a horizontal hot air tunnel, thereby allowing the web to foam up freely in all three dimensions.

In yet another process, foaming is effected by causing a foamable structure to expand whilst it floats on a heat transfer medium.

In all the above processes, expensive equip menr is usde to produce webs or sheets which are supplied, as bulky semi-finished goods, to the processor, who converts them to the ultimately used articles, which have a relatively simple geometrical shape, by punching, sawing, deep-drawing and other shaping processes.

The foam webs can also be comminuted by means of suitable cutters to give cubic or cuboid granules, which can be compressionmolded, by suitable methods, to give foam articles of any desired shape. However, because of the unfavorable close packing achievable with such rectangular foam granules, the moldings obtained suffer from the disadvantage of having a large void space and a low strength, resulting from the unfavorable conditions for surface adhesion between the foam particles, and therefore the above process gives unsatisfactory results at the molding stage.

Closer packing is achievable with spherical or cylindrical foam particles. The processes disclosed for the manufacture of such particles include some in which organic blowing agents which are gaseous or liquid at room temperature are worked into a polyolefin resin and the gel thus obtained is extruded at above the decomposition point of the blowing agent and above the softening point of the resin, to give foam strands, which are cut, whilst still hot, so as to produce cylindrical particles with an advantageous ratio of diameter to length. However, because of the low softening point of the resin and the consequent instability of the foam structure these particles cannot be compression-molded directly and instead must first be crosslinked by an after-treatment with ionizing radiation.

The above possbile methods of manufacturing olefin polymer foam particles of irregular shape exhibit the common feature that because of the expense of the method, the processor can only be provided with granules which have already been foamed. Because such granules are very bulky, they present transport problems which can only be overcome with difficulty and which would make it uneconomical to manufacture moldings from foamed polyolefin particles.

The present invention seeks to provide the manufacture with a material which can be prefoamed, and further processed, by means of suitable processes in one and the same location and therefore can eliminate the expensive transport of material which has already been foamed.

According to the invention there is provided a process for the manufacture of foamable particles of an oletin polymer, which includes mixing the polymer with a blowing agent to form a mixture, heating the mixture below the decomposition point of the blowing agent and extrudng it, and comminuiting the resulting continuous foamable strand to give cylindrical foamable particles, which are then irradiated with ionizing radiation, wherein the extruded continuous foamable strand is chopped, whilst still hot, perpendicular to the extrusion direction so as to form cylindrical, foamable particles od from 0.5 to 5, preferably from 1 to 3, mm diameter and having a length to diameter ratio of from 1.3:1 to 1.8:1 and these particles are irradiated substantially uniformly on all sides by means of ionizing radiation.

The irradiation of the particles from all sides may preferably be carried out whilst the particles are on a vibrating conveyor on which the particles are subjected to a continual change of position, in rapid time sequence. The conveying surface of the vibrating conveyor is preferably manufactured from a metal having a high reflectance for electrons.

The olefin polymers used for the manufacture of the products of the invention are suitably low pressure or high pressure polyethylene, polypropylene or a copolymer of ethylene with up to 50% by weight of vinyl acetate, butadiene, propylene or an acrylic ester. Minor amounts of polybutadiene or polyisobutylene may also be admixed to the polyethylene, to increase the modulus of elasticity.

The blowing agent is suitably an organic, normally solid compound which has a sufficiently high decomposition temperature that it does not undergo decomposition when incorporated into the olefin polymer and the resulting mixture is extruded. Specifically, the process of the invention may be carried out with p,p'-di-phenylsulfhydrazide, N,N'-dinitrosopentamethylenetetramine or azodicarboxamide. However, the use of azodicarboxamide is preferred.

To lower the decomposition temperature of the blowing agent, one or more activators may be added to the mixture, provided the processing temperatures permit this. In general, salts or oxides of polyvalent heavy metals are effective activators. However, zinc oxide and zinc stearate are particularly suitable. These compounds may be added in amounts of from 0.1 to 25 ,g, by weight based on the blowing agent employed. The concentration range of from 0.25 to 5% by weight has proved the most advantageous.

Suitable stabilizers for stabilizing the polymer against light, heat and/or oxygen may be added to the mixture.

The tendency of the particles produced according to the invention to adhere during foaming may make it necessary to surfacecoat the particles. Depending on requirements, this coating may be applied as a solid, a solution, a suspension or a dispersion. The coating can be affected by mixing the coating material with the foamable granules whilst they are cold or whilst they are at an elevated temperature, provided this is below the softening point of the olefin polymer resin.

Preferred coating materials adhere well to the granules but do not diffuse into the polymer and are either not in the molten state at the foaming temperature or, if they are in the molten state, are immiscible with the polymer. Coating with pulverulent inorganic compounds has proved the most advantageous.

For example, finely divided silica gel, zinc oxides, kaolin and talc all give good results.

It is also possible to apply organic resins which are incompatible with the olefin polymer. For example, sticking-together of the foaming particles can be effectively prevented by applying pulverulent nylon, polytetrafluoroethylene or polyvinyl chloride to the surface of the granules. In this case, an adhesion-promoting agent, e.g. paraffin oil or low molecular weight polyisobutylene may advantageously be present. Reactive mixtures of, for example, polyepoxy compounds and polyamines may also be used.

The amount required in order effectively to suppress the sticking-together of the granules may be from 0.05 to 5% by weight, based on the weight of the foamable granules, depending on the nature of the coating material and the effectiveness of its distribution.

The most advantageous amount of coating is, according to our practical experience, from 0.25 to 1% by weight.

In a preferred embodiment of the process according to the invention the cylindrical particles are crosslinked by passing them, by means of a rapidly vibrating conveyor, under a source of radiation in such a way that the particles repeatedly change their position in rapid sequence and as a result are exposed uniformly on all sides to about the same radiation dose, so that very homogeneous crosslinking is achieved. If an electron beam generator-which, as is known from experience, only gives a limited depth of penetra tion-is used as the source of ionizing radiation, then a vibrating conveyor must be used to provide the necessary exposure of the particles for achieving uniform crosslinking on all sides.

The vibrating conveyor can consist of a continuously circulating conveyor belt which in addition is caused to undergo periodic vibrations by means of a vibration generator; however, the vibrating conveyor can also consist of a fixed sheet-like structure which is suitably caused to undergo longitudinal vibrations and thus simultaneously provides a conveying action and brings about the desired change in position of the particles which are to be crosslinked.

The effect of uniform crosslinking on all sides can be additionally improved by manufacturing the conveyor belt or vibrating sheet of a metal which has a high reflectance for electrons, causing scattered radiation which impinges simultaneously from several directions on the particles to be crosslinked.

The proportion of insoluble gel--produced by the treatment with ionizing radiation-in the irradiated polymer should preferably be at least 5% by weight and at most 70% by weight. A particularly preferred gel content is between 25% by weight and 60% by weight.

The gel content is determined analytically in the conventional manner, in boiling toluene.

An essential characteristic of the invention relates to the shape the cylindrical granules.

We have in fact found that unsatisfactory results are achieved, when manufacturing irregular moldings, if the length to diameter ratio of the cylindrical foamable particles is either too large or too small. In both cases, moldings which exhibit inadequately close packing and little mutual surface adhesion of the foamed particles are obtained. If, on the other hand, the extruded strands of circular cross-section are chopped into such lengths as to produce particles which in the foamed state have a length to diameter ratio of at most 1.5 and at least 0.5, the particles can be used to manufacture irregular moldings with dose packing and good internal cohesion.

In a preferred embodiment, the length to diameter ratio is from 1.1 to 0.9.

The most advantageous shape of the nonfoamed particles cannot, however, be deduced directly from these statements, since deforma- tion of the continuous strands of circular cross-section in the semi-plastic state, after issuing from the extruder leads to frozen-in stresses which, after the treatment with ionizing radiation, are only released, at least partially, during foaming. If, therefore, the ad vantageous length to thickness ratio used above in connection with the foamed particles were also to be applied to the nen-foamed particles, shrinkage phenomena would, under unfavorable manufacturing conditions, give foam particles with a length to thickness ratio unfavorable for the manufacture od moldings.

In the process of manufacture, the extruded strands of circular cross-section are chopped, whilst still hot, i.e. plastic, perpendicular to the extrusion direction, and the product is cooled. Experiments show that such particles foam preferentially in the direction of the radius of the cylinder after they have been exposed to the treatment with ionizing radiation. The ratio of length to cylinder diameter of the non-foamed particles produced by transverse chopping of the hot strands should, according to the invention, be from 1.3:1 to 1.8:1. If such particles are foamed, foam particles having a length to diameter ratio of about 1 are obtained, which are exceptionally suitable for use for the manufacture of uniformly foamed moldings.

The Examples which follow illustrate processes within the invention.

EXAMPLE 1.

A mixture consisting of 88 parts by weight of high pressure polyethylene of melt index 2 g/10 min, 12 parts by weight of azodicarboxamide and 1 part by weight of zinc oxide was homogenized in a rubber kneader at 1350C and fed, in the form of a mill hide which is still plastic, into an extruder having an L/D ratio of 15. The material was extruded, with external cooling to 800C, through a die with 16 holes each of 2.8 mm diameter, the throughput being about 50 kg/h. The extruded strands, whilst still plastic, were severed, by means of a twin-arm high speed knife rotating on the ground-flat face of the die plate, to give cylindrical pellets having a diameter of 4.25 mm and a length of 7.3 mm.

To achieve this, the cutter had to rotate at 250 rpm. The pellets produced were quenched in water and discharged.

To crosslink the particles, they are spread on a sheet of tungsten-plated steel which is firmly coupled to a mechanically operated vibration generator, and are passed, whilst continuously altering their position, under a source of electrons of 1 MeV under conditions corresponding to the absorption of a total radiation dose of 4 Mrad. The foarnable particles obtained after this treatment have an insoluble gel content, in boiling toluene, of from 43 to 46% by weight.

To foam the particles, they are first coated with 0.5% by weight of a hydrophobic silica gel having a mean particle size of 5 ssm and then passed, on a coarse-mesh conveyor belt of PTFE-coated glass fibers, through an infrared radiation field of 0.5 W/cm2. The pressure with which the particles rest on the conveyor belt can be reduced by blowing hot air onto the particles through the perforated belt. During this treatment, the particles foam uniformly to give approximately spherical foam particles. However, in order to measure the degree of foaming accurately, individual primary particles having the above dimensions were spiked on a fine needle and foamed in an infrared radiation field of 0.50 W/cm2 whilst continuously rapidly rotating the needle about its vertical axis.The particles foamed in this way and then cooled had a mean length L of 14.7 mm and a mean diameter D of 14.7 mm, and therefore an L/D ratio of 1.0:1. The density of the foam is 42 grams per liter.

COMPARATIVE EXAMPLE 1 a.

If, in the above extrusion experiment, the speed of rotation of the chopper is altered to 430 rpm, primary particles with D = 4.25 mm and L = 4.3 mm, i.e. with an L/D ratio of 1.01:1, are obtained. If these primary particles are exposed to the infrared radiation field on the rotating needle under strictly identical conditions, foam particles with L = 8.6 mm, D = 14.4 mm and an L/D ratio of 0.6:1 are obtained. The foam density is 42 grams per liter.

Manufacture of moldings.

The moldings were manufactured in a rectangular metal mold of dimensions length X width X height = 400 X 300 X 120 mm.

The bottom consisted of a PTFE-coated coarse-mesh glass fiber fabric. Foam particles produced as described in Example 1 or in Comparative Example la were loosely into this mold to a height of 100 mm. The particles were then surface-heated for 3 minutes by blowing air, heated to 145"C, through the coarse-mesh glass fiber fabric which formed the bottom of the mold; thereafter the mold, containing the particles, was transferred into a hydraulic press and the particles were compressed to a height of about 40 mm and the mold cooled. After 15 minutes it was possible to take the molding out of the press, the ejection being facilitated if the inner walls of the mold and the fabric which formed the bottom of the mold were coated beforehand with a silicone oil release agent.

The molding thus produced substantially has the imposed contours and dimensions of the mold. If the molding process has been carried out with foam particles manufactured as described in Example 1, i.e. in which the nen-foamed primary particles have an L/D ratio of 1.72:1, the molding has a pleasing uniform structure with negligibly small void space.

The use of the foam particles obtained in Comparative Example la, and having an L/D ratio of 1.01:1, on the other hand re sulted in a very inhomogeneous foam molding with many unfilled inter-particle voids and low mechanical strength.

EXAMPLE 2.

A mixture of 88 parts by weight of high pressure polyethylene (MFI = 2 g/10 min), 12 parts by weight of azodicarboxamide and 3 parts by weight of zinc stearate was homo- genized and extruded by means of an extruder cooled to 80"C and having an L/D ratio of 15, through a die plate with 56 holes each of 1.5 mm diameter. The extruded strands, of circular cross-section, were chopped, whilst plastic, by a rotating twin-arm chopper so as to give particles with D = 2.4 and L = 3.7 mm, an L/D ratio of 1.54: 1. For this purpose, the speed of rotation of the chopper was set to 46 rpm. The particles were quenched rapidly.The particles were passed, on a conveyor chute of chromium-nickel steel, the chute having an U-shaped cross-section and being caused to undergo periodic vibrations, under a 1 MeV electron source at a speed such that their mean residence time under the output window of the electron source was about 8 minutes; because of the forced vibrations the particles altered their position in rapid sequence; the beam current was 110 gA and the particles received a mean radiation dose of 4 Mrad.

Foaming the particles in an infrared radiation field of 0.5 W/cm2 gave foam particles with L = 8.0 mm and D = 8.0 mm, a foam density of 40 g/l and a gel content, in boiling toluene, of 42% by weight.

EXAMPLE 3.

If a homogeneous mixture of 88 parts by weight of - high pressure polyethylene (MFI = 2 g/10 min), f2 parts by weight of azodicarboxamide and 1 part by weight of zinc oxide is extruded, in the manner described above, through a perforated plate with 56 holes each of 1.5 mm diameter, and the frequency of the chopper is set to 460 min-1, particles having a length L = 4.0 and a diameter D = 2.3 mm are obtained, so that L/D = 1.54:1. A layer, one particle deep, of these particles on a flat conveyor belt was passed at a speed of V = 90 cm/min under an electron source of 1 MeV with a beam current of 110,aA. After the first pass, the particles were shaken manually on the conveyor belt in order to alter their position.

Thereafter they were passed under the radiation source 7 more times, their position being altered, by brief shaking, after each pass.

Foaming gave almost round particles with L = 8.0 mm and D = 7.7 mm. The average gel content was 41.5% by weight and the foam density 43 g/l.

Comparative Example 3 a.

If, in the above experiment, the speed of rotation of the chopper is altered to 800 rpm, primary particles having about the same length and diameter, each being about 2.3 mm (L/D = 1) are obtained. After crosslinking and foaming under the conventional conditions, these primary particles give cylindrical foam particles having a length of 4.6 mm and a diameter of 7.7 mm.

In spite ob the repeated change of position of the primary particles, this experiment gives a number of foam particles which, because of inadequate change of position, foam asymmetrically and exhibit an inhomogeneous foam structure.

EXAMPLE 4.

A kneaded mass of 92 parts of high pressure polyethylene, 8 parts of azodicarboxamide and 1 part of zinc oxide was forced, in the manner described above, through a perforated plate with 56 holes each of 1.5 mm diameter, and the strand which issues was severed by a chopper rotating at 390 revolutions per minute to give pellets of 4.1 mm length and 2.4 mm diameter, which were cooled with water.

In this experiment, crosslinking was carried out under an industrial linear accelerator with a voltage of 500 kV, by passing the pellets continuously under the 1.5 m wide accelerator window, on several parallel flat chromium-nickel steel sheets caused to undergo a characteristic vibration of 50 c/s by means of electromagnetic vibrators. The throughput was about 80 kg per hour, the beam current being 40 mA. The treated pellets had an insoluble gel content, in boiling toluene, of 47% by weight.

The crosslinked granules were then coated with 1% by weight, based on the weight of the pellets, of a mixture consisting ob equal parts of talc and fine PVC powder. The coating could best be applied in a high-speed mixer in the presence of 1% by weight of paraffin oil as an adhesive. Foaming ob the material, treated as described above to prevent premature sticking-together, was carried out continuously on an endless conveyor belt of PTFE-coated glass fiber fabric, having a mesh size of 2 X 2 mm. The conveyor belt, loosely laden with the foamable particles, passed through a heating zone of total length 1 merer, comprising infrared radiators mounted above and below the conveyor belt.

The surface temperature of the radiators was chosen so as to achieve a radiation density of 0.6 watt/cm2 in each direction of radiation at the location of the foamable particles. With a mean residence time of 2.8 minutes, the particles foamed substantially uniformly in all directionis and adhered only slightly to the conveyor mesh. On leaving the heating zone, they could easily be lifted off the belt by means of an air jet directed against the belt at an acute angle, and be collected.

The measurement of a substantial number of foam particles gave a mean length of 7.5 mm and a mean diameter of 7.4 mm, with a foam density of 56 grams per liter.

The approximately spherical foam particles manufactured in this way were molded, by the method already described above, to give a foam sheet of size about 400 X 300 X 45 mm with a final density of 65 grams per liter.

The void space observed on splitting the molding into several layers was less than 3% by volume.

Comparative Experiment 4 a.

If, in the above experimental method, the speed of the chopper is altered to 660 rpm, primary particles having a length to diameter ratio of about 1.0 are obtained (L = D = 2.4).

After processing under the conditions described in the above Example, lenril-shaped foam particles with a mean diameter of 7.4 mm and a length of 4.4 mm are obtained.

In order to be able, when manufacturing the molding, to accommodate the same weight of foam particles as a loose mass in the available mold, the mold would have to be filled with this material to a height of 114 mm.

The foam sheet, after being released from the mold and split into several layers, showed a large number of channels and voids, especially in its interior. Some of the particles were deformed into an asymmetrical shape.

WHAT WE CLAIM IS: - 1. A process for the manufacture of foamable particles of an olefin polymer, which includes mixing the polymer with a blowing agent to form a mixture, heating the mixture below the decomposition point of the blowing agent and extruding it, and comminuting the resulting continuous foamable strand to give cylindrical foamable particles, which are then irradiated with ionizing radiation, wherein the extruded continuous foamable strand is chopped, whilst still hot, perpendicular to the extrusion direction so as to form cylindrical foamable particles of from 0.5 to 5 mm diameter and having a length to diameter ratio of from 1.3:1 to 1.8:1 and these particles are irradiated substantially uniformly on all ,sides by means of ionizing radiation.

2. A process as claimed in claim 1, wherein the irradiation treatment, from all sides, of the particles is carried out with ionizing radiation whilst the particles are on a vibrating conveyor, the particles being subjected to a continual change of position in rapid time sequence.

3. A process as claimed in claim 2, wherein the ionizing radiation is derived from an electron beam generator.

4. A process as claimed in claim 2 or 3, wherein the conveying surface of the vibrating conveyor is made of a metal of high reflectance for electrons.

5. A process as claimed in any one of the preceding claims, wherein the olefin polymer is polyethylene, polypropylene or a copolymer of ethylene with a minor amount of vinyl acetate, butadiene, propylene or an acrylic ester, and the blowing agent is an organic, normally solid compound.

6. A process as claimed in any one of the preceding claims, wherein the blowing agent is azodicarboxamide.

7. A process as claimed in any one of claims 1 to 5, wherein the polymer-containing mixture contains an activator to lower the decomposition temperature of the blowing agent.

8. A process as claimed in any one of the preceding claims, wherein the irradiated par

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (14)

**WARNING** start of CLMS field may overlap end of DESC **. EXAMPLE 4. A kneaded mass of 92 parts of high pressure polyethylene, 8 parts of azodicarboxamide and 1 part of zinc oxide was forced, in the manner described above, through a perforated plate with 56 holes each of 1.5 mm diameter, and the strand which issues was severed by a chopper rotating at 390 revolutions per minute to give pellets of 4.1 mm length and 2.4 mm diameter, which were cooled with water. In this experiment, crosslinking was carried out under an industrial linear accelerator with a voltage of 500 kV, by passing the pellets continuously under the 1.5 m wide accelerator window, on several parallel flat chromium-nickel steel sheets caused to undergo a characteristic vibration of 50 c/s by means of electromagnetic vibrators. The throughput was about 80 kg per hour, the beam current being 40 mA. The treated pellets had an insoluble gel content, in boiling toluene, of 47% by weight. The crosslinked granules were then coated with 1% by weight, based on the weight of the pellets, of a mixture consisting ob equal parts of talc and fine PVC powder. The coating could best be applied in a high-speed mixer in the presence of 1% by weight of paraffin oil as an adhesive. Foaming ob the material, treated as described above to prevent premature sticking-together, was carried out continuously on an endless conveyor belt of PTFE-coated glass fiber fabric, having a mesh size of 2 X 2 mm. The conveyor belt, loosely laden with the foamable particles, passed through a heating zone of total length 1 merer, comprising infrared radiators mounted above and below the conveyor belt. The surface temperature of the radiators was chosen so as to achieve a radiation density of 0.6 watt/cm2 in each direction of radiation at the location of the foamable particles. With a mean residence time of 2.8 minutes, the particles foamed substantially uniformly in all directionis and adhered only slightly to the conveyor mesh. On leaving the heating zone, they could easily be lifted off the belt by means of an air jet directed against the belt at an acute angle, and be collected. The measurement of a substantial number of foam particles gave a mean length of 7.5 mm and a mean diameter of 7.4 mm, with a foam density of 56 grams per liter. The approximately spherical foam particles manufactured in this way were molded, by the method already described above, to give a foam sheet of size about 400 X 300 X 45 mm with a final density of 65 grams per liter. The void space observed on splitting the molding into several layers was less than 3% by volume. Comparative Experiment 4 a. If, in the above experimental method, the speed of the chopper is altered to 660 rpm, primary particles having a length to diameter ratio of about 1.0 are obtained (L = D = 2.4). After processing under the conditions described in the above Example, lenril-shaped foam particles with a mean diameter of 7.4 mm and a length of 4.4 mm are obtained. In order to be able, when manufacturing the molding, to accommodate the same weight of foam particles as a loose mass in the available mold, the mold would have to be filled with this material to a height of 114 mm. The foam sheet, after being released from the mold and split into several layers, showed a large number of channels and voids, especially in its interior. Some of the particles were deformed into an asymmetrical shape. WHAT WE CLAIM IS: -

1. A process for the manufacture of foamable particles of an olefin polymer, which includes mixing the polymer with a blowing agent to form a mixture, heating the mixture below the decomposition point of the blowing agent and extruding it, and comminuting the resulting continuous foamable strand to give cylindrical foamable particles, which are then irradiated with ionizing radiation, wherein the extruded continuous foamable strand is chopped, whilst still hot, perpendicular to the extrusion direction so as to form cylindrical foamable particles of from 0.5 to 5 mm diameter and having a length to diameter ratio of from 1.3:1 to 1.8:1 and these particles are irradiated substantially uniformly on all ,sides by means of ionizing radiation.

2. A process as claimed in claim 1, wherein the irradiation treatment, from all sides, of the particles is carried out with ionizing radiation whilst the particles are on a vibrating conveyor, the particles being subjected to a continual change of position in rapid time sequence.

3. A process as claimed in claim 2, wherein the ionizing radiation is derived from an electron beam generator.

4. A process as claimed in claim 2 or 3, wherein the conveying surface of the vibrating conveyor is made of a metal of high reflectance for electrons.

5. A process as claimed in any one of the preceding claims, wherein the olefin polymer is polyethylene, polypropylene or a copolymer of ethylene with a minor amount of vinyl acetate, butadiene, propylene or an acrylic ester, and the blowing agent is an organic, normally solid compound.

6. A process as claimed in any one of the preceding claims, wherein the blowing agent is azodicarboxamide.

7. A process as claimed in any one of claims 1 to 5, wherein the polymer-containing mixture contains an activator to lower the decomposition temperature of the blowing agent.

8. A process as claimed in any one of the preceding claims, wherein the irradiated par

ticles are subsequently surface-coated with a coating material which prevents adhesion of the particles.

9. A process as claimed in claim 8, wherein the foamable particles are surface-coated with finely divided silica gel, zinc oxide, kaolin or talc to prevent adhesion during foaming.

10. A process as claimed in any one of the preceding claims, wherein from 0.25 to 1% of the agent to prevent adhesion during foaming is employed.

11. A process as claimed in any one of the preceding claims, wherein the irradiation is suffident to provide a gel content of from 25 to 60% by weight in the irradiated particles.

12. A process for the manufacture of foamable particles of an olefin polymer carried out substantially as described in any of the fore going Examples 1, 2, 3 and 4.

13. Foamable olefin polymer particles when manufactured by a process as claimed in any one od the preceding claims.

14. Foam moldings made from foamable olefin polymer particles claimed in claim 13.