GB1581252A

GB1581252A - Expandable olefin polymer compositions

Info

Publication number: GB1581252A
Application number: GB49238/77A
Authority: GB
Original assignee: Dynamit Nobel AG
Current assignee: Dynamit Nobel AG
Priority date: 1976-11-26
Filing date: 1977-11-25
Publication date: 1980-12-10
Also published as: DE2653748B1; IT1090687B; FR2372193A1; JPS5369273A

Description

(54) EXPANDABLE OLEFIN POLYMER COMPOSITIONS (71) We, DYNAMIT NOBEL AKTIENGESELLSCHAFT, a German Company of 521Troisdorf, bez Koln; Postfach 1209, 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: This invention relates to olefin polymer compositions particularly but not exclusively those compositions suitable for forming into fine-celled foam materials.

From United States patent specification 3 098 831, it is known that a foamed material is obtained from the combination of an organic peroxide and a blowing agent with a polyolefin. The components are mixed together and the mixture obtained is first of all heated to the decomposition temperature of the peroxide so that crosslinking of the polyolefin takes place, with an increase in melt viscosity but without decomposition of the blowing agent. Subsequently, the temperature is raised to the decomposition zone of the blowing agent, so that the polyolefin is foamed by the gases evolved by the agent. The blowing agent preferably used for the foaming is azodicarbonamide. In accordance with this process a coarse-celled polyolefin foam is formed.

Furthermore, it is known from German Offenlegungsschrift 2 351 515 that by employing a similar process a fine-celled foam material is obtained by virtue of the addition to the combination of a very small quantity (from 0.1 to 1.5% by weight, relative to the first blowing agent) of a second blowing agent. This second blowing agent is different from the first blowing agent, which has a decomposition zone above 1900C, in that it has a decomposition zone from 1300C to 1900C. As second or additional blowing agents, various sulphohydrazides are preferably used.The disadvantage of this procedure is that the generation of gas by the blowing agent can still take place before crosslinking occurs, so a very exact adherence to the process and dosage requirements is necessary in order to carry out the process successfully.

From German Auslegeschrift No. 1 936 098 it is known to add to the azodicarbonamide blowing agent, as activating agent, from 0.05 to 10 weight % of a compound containing chromium in order to lower the decomposition temperature of the blowing agent and to increase the gas output therefrom. This combination then serves as a blowing agent for producing foamed thermoplastic synthetic plastics materials. It must be made clear that increasing the gas output has no effect on the number or size of the pores formed in the foamed material.As may be seen from Figures 1 and 2, as well as from the description of German Auslegeschrift No. 1 936 098, only larger proportions, from about 0.2 weight %, of the chromiumcontaining compound, relative to the weight of the azocarbonamide, are sufficient to reduce the decomposition temperature noticeably by a few degrees and at the same time to increase the gas output. The disadvantage in using the blowing agent known from this German Auslegeschrift for producing foamed polyolefin materials is that the foamed material which is formed has a relatively high gross density in comparison with a foamed material formed without using a blowing agent in combination with a chromium containing activating agent; further, the material shows an irregular foam structure and uneven surface, and is, in addition, changed in colour by the high proportion of chromium-containing compound.It is also disclosed that quantities of chromiumcontaining compound in excess of 0.1 weight % chromium content, relative to the azodicarbonamide, do not lead to an even foam and can disturb the foaming process.

There is thus a need to provide a composi tion suitable for producing fine-celled olefin polymer foam materials having a great number of pores which does not lead to many of the disadvantages associated with known processes for the production of foam materi als..

Surprisingly it has now been found that although the known compounds containing chromium as activating agent for the azodicarbonamide blowing agent in the quantities specified in German Auslegeschrift No. 1 936 098 actually lower the decomposition temperature of the blowing agent and increase the gas output, they do not have the previously mentioned deleterious effect when used in particular, lower amounts, but rather, generally lead to a fine-celled foam with an increased number of pores in the case of foam materials based on olefin polymers.

Accordingly, the present invention provides a composition suitable for forming a foamed material, comprising (a) an olefin polymer, (b) a blowing agent comprising azodicarbonamide, (c) a crosslinking agent for the olefin polymer, the crosslinking agent having a decomposition temperature lower than that of azodicarbonamide, and (d) as an activating agent for azodicarbonamide either (i) elemental chromium in an amount of from 1 x 10-4 to less than 5 x 10-2 parts by weight, calculated as elemental chromium, per 100 parts by weight of azodicarbonamide or (ii) a compound of chromium in an amount of from 1 x 10-4 parts by weight, calculated as elemental chromium, to less than 5 x 10-2 parts by weight, calculated as compound of chromium, per 100 parts by weight of azodicarbonamide.

In a preferred embodiment the compositions contains from 2 x 10-4 to 1.5 x 10-2 parts by weight of elemental chromium or compound of chromium, calculated as elemental chromium, per 100 parts by weight of azodicarbonabide.

Another aspect of the invention provides a process for producing a foamed material, which comprises heating a composition as defined above to a first temperature which is at least equal to the decomposition temperature of the crosslinking agent but less than the decomposition temperature of the blowing agent to crosslink the olefin polymer, and thereafter heating the crosslinked composition to a second temperature which is at least equal to the decomposition temperature of the blowing agent to release gas therefrom and form the desired foamed material.

In one embodiment the activating agent is added to the other components of the composition to be foamed in accordance with the invention, the mixture, without foaming, is worked thermoplastically into the form of a mould body at a temperature below the decomposition temperature of the blowing agent but above the crosslinking temperature, and is finally foamed at temperatures over 1900C.

The extremely small quantities (chromium contents) proposed in accordance with the invention, a chromium or compounds containing chromium as activating agents in compositions suitable for producing foam materials based on olefin polymers with the use of azodicarbonamide and crosslinking agents, have been found generally to lead to the desired increase in the number of pores, and at the same time to a fine-celled foam structure, without the gross density of the foam material being considerably increased, in relation to a similar material foamed without the inclusion of a chromium based activating agent in the specified proportions.

The olefin polymer may be, for example, a high pressure or low pressure polyethylene, or a copolymer which contains ethylene, or a mixture thereof. Such copolymers are, for example, ethylene-propylene copolymers ethylene-butylene copolymers, copolymers of ethylene and vinyl acetate or its derivatives, copolymers of ethylene and acrylates or their derivatives, and copolymers of ethylene and methacrylic acid or their derivatives. Also, mixutres of olefin polymers with rubber and/or synthetic plastics materials can be present in the compositions in accordance with the invention. The mixtures may contain up to 100 parts of the olefin polymer. Rubbers which may be mixed with the olefin polymer are, for example, natural rubber, ethylene-propylene rubber, butylene rubber, polyisobutylene, styrene-butadiene rubber, polybutadiene, polybutene and polyisoprene.Plastics materials which may be mixed with the olefin polymers are, for example, polystyrene, polypropylene, chlorinated polyethylene and sulphochlorinated polyethylene.

Preferably, polyethylenes are used as the olefin polymer, and, depending on the formulation of the composition, there is used low pressure or high pressure polyethylene.

Preferably high pressure polyethylene with a density of 0.91 to 0.94 d/cm3 is employed.

As crosslinking agent for the olefin polymer there is preferably used an organic peroxide. Depending on the olefin polymer, the peroxide may be for example, 2,5dimethyl-2,5-di- (ter.-butylperoxy)-hexane ter.-butyl-hydroperoxide, cumyl-tert.butylperoxide, or di.tert.-butylperoxide; however dicumylperoxide is preferred. The peroxides are preferably used in quantities of ca. 1%.

The azodicarbonamise blowing agent included in the composition according to the invention has a decomposition temperature which is higher than that of the crosslinking agent. The concentration of the blowing agent depends on the gross density of the composition to be foamed, and is preferably from 0.5 to 25% by weight, more preferably from 1 to 15% by weight of the total weight of the composition which is subsequently to be formed into a mould body.

Conventional additives which are generally used with synthetic materials based on olefin polymers, for example antioxidation agents, light resistant agents, pigments, fillers such as chalk, flame resistant agents, anti-static agents and lubricants may also be included in the composition to be crosslinked and foamed.

Of the chromium-containing compounds employed as activating agents in accordance with the invention chromium oxide and the chromates and bichromates of mono- and di-valent metals have been particularly considered. Of these, in particular sodium chromate and sodium bichromate have proved to be very suitable, both in terms of their effectiveness and also for cost reasons.

In a preferred embodiment of the invention the composition is formed by first forming an admixture of the activating agent and the azodicarbonamide, and then combining this admixture with the other components of the composition.

Preparation of the composition according to the invention may take place by mixing the olefin polymers with the crosslinking and blowing agents, the usual additives and the activating agents on a set of mixing rollers or on an extruder for the continuous production of mould bodies, such as sheets or continuous belts. The mixing takes place below the decomposition temperature of the crosslinking agent. In accordance with the process, crosslinking of the olefin polymer takes place below the decomposition point of the blowing agent by the decomposition of the crosslinking agent; when the temperature increases to over 1900C (the decomposition temperature of azodicarbonamide) the composition, for example in the form of a sheet or belt, foams.Depending on the quantity of blowing agent, crosslinking agent, and the requirements of the process, there is preferably formed a fine-celled foamed material with a smooth surface, having a gross density of 20 kg/m3 to 300 kg/m3, more preferably 20 to 150 kg/m3.

The addition of the compound containing chromium can be carried out by mixing according to the dry method and by subsequent compounding of the component parts of the composition in the thermoplastic zone for example on a set of rollers or in an extruder. Thereupon the further production of continuous foam material belts may ensue for example according to the process described in German Auslegeschrift 1 694 130.

The fine-celled foam materials produced in accordance with the process of the invention as exemplified hereinafter can be used as insulating material in the building trade since they have improved heat insulation as a result of the fine cells, in the packing industry since they have for example improved behaviour with regard to imprinting by applied compression, and frequently as lagging for floors, roofs and walls, where the fine cell structure in conjunction with the smooth surface is of advantage.

The following Examples illustrate the invention; in the absence of indication to the contrary parts means parts by weight.

Example 1 (Comparison) A mixture of 84 parts by weight high pressure polyethylene (melting index ca. 3) measured according to DIN 53 735 (190/2), and a density 0.92 g/cm3), 1 part by weight dicumyl peroxide and 15 parts by weight azodicarbonamide were compounded for 20 minutes on a set of rollers having a surface temperature of 110 C. The the material was withdrawn as a roller skin and was moulded into a smooth 4 mm sheet in a press at 1 300C for 8 minutes. Samples 7 x 7 cm in size were cut out of the sheet and were foamed in a drying chamber at 210by into foamed bodies about 160 x 160 mm in size, 15 mm thick and having a gross density of 30 kg/m3. The total time from the beginning of the test until completion of the foaming was about 10 mins.The gross density of the foam was dependent on the formulation of the initial mixture. The pores of the foam were on average 2 mm in diameter, so that about 2.4 x were were present in a cubic metre.

Example 2 The processing of Example 1 was performed on a mixture of 84 parts by weight high pressure polyethylene (melting index ca. 3 according to DIN 53 735 (190/2), gross density 0.92 g/cm3), 1 part by weight dicumylperoxide 15 parts by weight azodicarbonamide and 0.006 parts by weight of sodium bichromate (corresponding to about 0.002 parts by weight or Cr). A foam with an average pore size of 1 mm diameter was produced, corresponding to, on average, 2 x 10+9 pores per m3 of foam material Example 3 (Comparison) A mixture as in Example 2, but, instead of 0.006 parts by weight sodium bichromate, having 2 parts by weight sodium chromate (corresponding to about 0.304 parts by weight Cr), was processed and foamed under the same conditions as in Example 1. The samples did not foam up properly and were coloured brown.The gross density of the foam was measurably over 30 kg/m3.

Example 4 (Comparison) A mixture as in Example 2, but, instead of 0.006 parts by weight sodium bichromate.

containing 1.0 parts by weight sodium bichromate (corresponding to about 0.35 parts by weight Cr), was processed and foamcd under the same conditions as in Example 1.

Here also only a poorly foamed product was obtained, which was coloured dark brown.

Example 5 84 parts by weight high pressure polyethylene (as in Example 2) and 1 part by weight dicumylperoxide were combined with a mixture of 15 parts by weight azodicarbonamide and 0.0009 parts by weight Cr, the chromium being already contained in the azodicarbonamide. This formulation was moulded on an extruder at temperatures under 140"C into the form of a continuous belt with a thickness of about 5 mm. The melt was foamed on line in a channel by means of hot air at temperatures of ca. 190 to 2500C, forming a foam with a thickness of 20 mm and a gross density of 30 kg/m3.. The foam had a smooth surface, a thickness of 20 mm, and an average pore size of 0.9 mm diameter, which corresponds to about 2.5 x 10+9 pores per m3.

Example 6 Using a formulation and processing as an Example 5 there was produced a foam with a thickness of 10 mm. The average pore size was about 0.8 mm diameter, which corresponds to a pore count of 3.6 x 10+9/m3 of foam material.

Example 7 (Comparison) Using a formulation and processing as in Example 6, but with no chromium content, there was produced a foam material having a thickness of 10 mm. The average pore diameter was 1.2 mm, corresponding to a pore count of ca. 1 x 109/m3 of foam material.

Example 8 92.5 parts by weight high pressure polyethylene (melting index 3 measured according to DIN 53 735, 190/2 gross density 0.92 g/cm3), 1 part by weight dicumylperoxide, 7.5 parts by weight azodicarbonamide and 0.0005 parts by weight Cr wer processed as in Example 5. A foam material was produced with a thickness of 15 mm and a gross density of 70 kg/m3.

The average pore diameter was 0.8 mm, which corresponds to a pore count of 3.6 x 10+9/m3 of foam material.

Example 9 83.5 parts by weight of a copolymer of ethylene and vinyl acetate in ratio 92:8, having a melting index of 5 according to DIN 53 735 (190/2), 16.6 parts by weight azodicar bonamide, 0.0009 parts by weight Cr and 0.9 parts by weight dicumylperoxide were processed as in Example 1. The gross density of the foam material obtained was 32 kg/m3, the average pore size being 1 mm diameter; this corresponds to a pore count of about 2 x 10+9/m3 of foam material.

Example 10 (Comparison) Using a formulation and processing as in Example 9, but with no chromium addition, a foam material was obtained which had a gross density 32 kg/m3 and an average pore diamter of 2 mm, which corresponds to a pore count of 2 x 10-8/m3 The examples processed in accordance with Example 1 were bench tests, carried out in batch form using a drying chamber. The examples processed in accordance with Example 5 were carried out on a large scale continuous working apparatus. It was found that for the same formulation a finer foam is produced using the continuous large scale technical apparatus, than in the bench test.

This is made clear by the data specified in the Examples with reference to the pores obtained.

As may be seen from the Examples according to the invention, the desired increase in the pore count (smaller pore dimeters) is produced by the addition of compounds containing chromium or respectively by the presence of a corresponding chromium content to the usual components used in the production of foamed materials. Both the lower limit (chromium content equal to zero) and the practicable upper limit (too much chromium content) are made clear therein.

WHAT WE CLAIM IS: 1. A composition suitable for forming a foamed material, comprising (a) an olefin polymer, (b) a blowing agent comprising azodicarbonamide, (c) a crosslinking agent for the olefin polymer, the crosslinking agent having a decomposition temperature lower than that of azodicarbonamide, and (d) as an activating agent for azodicarbonamide, either (i) elemental chromium in an amount of from 1 x 10-4 to less than 5 x 10-2 parts by weight, calculated as elemental chromium, per 100 parts by weight of azodicarbonamide or (ii) a compound of chromium in an amount of from 1 x 10-4 parts by weight, calculated as elemental chromium, to less than 5 x 10-2 parts by weight, calculated as compound of chromium, per 100 parts by weight of azodicarbonamide.

2. A composition as claimed in claim 1, additionally comprising a rubber and/or a synthetic plastics material.

3. A composition according to claim 1 or 2, which contains from 2 x 10 to 1.5 x 10-2 parts by weight of elemental chromium or compound of chromium, calculated as elemental chromium, per 100 parts by weight of azodicarbonamide.

4. A composition according to any of claims 1 to 3, wherein the crosslinking agent is an organic peroxide.

5. A composition according to claim 4 wherein the organcic peroxide is dicumyl peroxide.

6. A composition according to any of the preceding claims, wherein the compound of chromium comprises a chromate or bichromate of a mono- or 'di-valent metal.

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Claims

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under the same conditions as in Example 1.

Here also only a poorly foamed product was obtained, which was coloured dark brown.

Example 5

84 parts by weight high pressure polyethylene (as in Example 2) and 1 part by weight dicumylperoxide were combined with a mixture of 15 parts by weight azodicarbonamide and 0.0009 parts by weight Cr, the chromium being already contained in the azodicarbonamide. This formulation was moulded on an extruder at temperatures under 140"C into the form of a continuous belt with a thickness of about 5 mm. The melt was foamed on line in a channel by means of hot air at temperatures of ca. 190 to 2500C, forming a foam with a thickness of 20 mm and a gross density of 30 kg/m3.. The foam had a smooth surface, a thickness of 20 mm, and an average pore size of 0.9 mm diameter, which corresponds to about 2.5 x 10+9 pores per m3.

Example 6 Using a formulation and processing as an Example 5 there was produced a foam with a thickness of 10 mm. The average pore size was about 0.8 mm diameter, which corresponds to a pore count of 3.6 x 10+9/m3 of foam material.

Example 7 (Comparison) Using a formulation and processing as in Example 6, but with no chromium content, there was produced a foam material having a thickness of 10 mm. The average pore diameter was 1.2 mm, corresponding to a pore count of ca. 1 x 109/m3 of foam material.

Example 8 92.5 parts by weight high pressure polyethylene (melting index 3 measured according to DIN 53 735, 190/2 gross density 0.92 g/cm3), 1 part by weight dicumylperoxide, 7.5 parts by weight azodicarbonamide and 0.0005 parts by weight Cr wer processed as in Example 5. A foam material was produced with a thickness of 15 mm and a gross density of 70 kg/m3.

The average pore diameter was 0.8 mm, which corresponds to a pore count of 3.6 x 10+9/m3 of foam material.

Example 9 83.5 parts by weight of a copolymer of ethylene and vinyl acetate in ratio 92:8, having a melting index of 5 according to DIN 53 735 (190/2), 16.6 parts by weight azodicar bonamide, 0.0009 parts by weight Cr and 0.9 parts by weight dicumylperoxide were processed as in Example 1. The gross density of the foam material obtained was 32 kg/m3, the average pore size being 1 mm diameter; this corresponds to a pore count of about 2 x 10+9/m3 of foam material.

Example 10 (Comparison) Using a formulation and processing as in Example 9, but with no chromium addition, a foam material was obtained which had a gross density 32 kg/m3 and an average pore diamter of 2 mm, which corresponds to a pore count of 2 x 10-8/m3 The examples processed in accordance with Example 1 were bench tests, carried out in batch form using a drying chamber. The examples processed in accordance with Example 5 were carried out on a large scale continuous working apparatus. It was found that for the same formulation a finer foam is produced using the continuous large scale technical apparatus, than in the bench test.

This is made clear by the data specified in the Examples with reference to the pores obtained.

As may be seen from the Examples according to the invention, the desired increase in the pore count (smaller pore dimeters) is produced by the addition of compounds containing chromium or respectively by the presence of a corresponding chromium content to the usual components used in the production of foamed materials. Both the lower limit (chromium content equal to zero) and the practicable upper limit (too much chromium content) are made clear therein.

WHAT WE CLAIM IS: 1. A composition suitable for forming a foamed material, comprising (a) an olefin polymer, (b) a blowing agent comprising azodicarbonamide, (c) a crosslinking agent for the olefin polymer, the crosslinking agent having a decomposition temperature lower than that of azodicarbonamide, and (d) as an activating agent for azodicarbonamide, either (i) elemental chromium in an amount of from 1 x 10-4 to less than 5 x 10-2 parts by weight, calculated as elemental chromium, per 100 parts by weight of azodicarbonamide or (ii) a compound of chromium in an amount of from 1 x 10-4 parts by weight, calculated as elemental chromium, to less than 5 x 10-2 parts by weight, calculated as compound of chromium, per 100 parts by weight of azodicarbonamide.
2. A composition as claimed in claim 1, additionally comprising a rubber and/or a synthetic plastics material.
3. A composition according to claim 1 or 2, which contains from 2 x 10 to 1.5 x 10-2 parts by weight of elemental chromium or compound of chromium, calculated as elemental chromium, per 100 parts by weight of azodicarbonamide.
4. A composition according to any of claims 1 to 3, wherein the crosslinking agent is an organic peroxide.
5. A composition according to claim 4 wherein the organcic peroxide is dicumyl peroxide.
6. A composition according to any of the preceding claims, wherein the compound of chromium comprises a chromate or bichromate of a mono- or 'di-valent metal.
7. A composition according to claim 6,

wherein the compound of chromium is sodium chromate or sodium bichromate.
8. A composition according to any claims 1 to 5, wherein the compound of chromium is chromium oxide.
9. A composition according to any of the preceding claims, which contains from 0.5 to 25% by weight of azodicarbonamide.
10. A composition according to claim 9, which contains from 1 to 15% by weight of azodicarbonamide.
11. A composition according to any of the preceding claims, wherein the olefin polymer comprises a polyethylene.
12. A composition according to claim 11, wherein the olefin polymer comprises a high pressure polyethylene having a density of from 0.91 to 0.94 g/cm3.
13. A composition to any of the preceding claims formed by forming an admixture of azodicarbonamide and the activating agent, and thereafter combining the admixture and the remaining components of the composition.
14. A composition according to claim 1, substantially as described in any one of Examples 2, 5, 6, 8 and 9.
15. A process for producing a foamed material, which comprises heating a composition according to any of the preceding claims to a first temperature which is at least equal to the decomposition temperature of the cross-linking agent but less than the decomposition temperature of the blowing agent to crosslink the olefin polymer, and thereafter heating the crosslinked composition to a second temperature which is at least equal to the decomposition temperature of the blowing agent to release gas therefrom and to form the desired foamed material.
16. A process according to claim 15, wherein the crosslinked composition is heated to a temperature above 1900C.
17. A process according to claim 15, substantially as described in any one of Examples 2, 5, 6, 8 and 9.
18. A foamed material whenever produced by the process according to any one of claims 15 to 17.
19. A foamed material according to claim 18 and having a density of from 20 to 300 kg/m3.
20. A foamed material according to claim 19 and having a density of from 20 to

150 kg/m3.