CA1054298A - Process for the production of foam plastics with a fine cellular structure - Google Patents

Abstract

Abstract of the Disclosure An improved process for the production of a foam pla-stics with fine cells from a polymer based on an ?,.beta.-monoole-finicially unsaturated monomer by peroxidic cross-linking and foaming with a chemical blowing agent, said polymer, the im-provement comprises adding in addition to said peroxide and said blowing agent a highly disperse amphoteric oxide to said polymer subjecting the mixture thus obtained to a shaping process and cross-linking and foaming by heating it to a temperature of at least 190°C.

Description

This invention relates to a process for the production of foam plastics with a fine cellular structure from polymers of ~ monoolefinically unsaturated monomers by cross-linking and foaming the polymer in the presence of a highly disperse ampho-teric oxide.
It is known to add organic peroxides and chemical blowing agents to polyethylene and extrude the mixture to form a plate under such conditions that neither cross-linking takes place to any significant extent nor the blowing agent is decomposed. The plate may then be heated in a hot air oven sufficiently to start the cross-linking reaction and the temperatury may then be rai-sed to decompose the blowing agent whereby the gas liberated foams up the cross-linked polyethylene (see German Offenlegungs-schrift No. 1,694,130 and US-Patent Specification No. 3,098,831).
The blowing agent preferably used for this process is azo dicarbonamide because it has no deleterious effect on ths cross-linking of polyethylene by organic peroxides.
But azodicarbonamide causes the formation of a foam with relatively large cells. Therefore it was tried to lower the de-composition point of azodicarbonamide by adding so-called "kik-kers" and thus to influence the size of the cells of the fini-shed foam product. Substances which have been used as kickers includeJ eOgO compounds of magnesium, zinc, cadmium, barium or leadO
These kickers, however, cannot be used in the process for the production of polyethylene foam wherein the foam is cross-linked by peroxides9 because the kicker destroy the func-tion of the peroxide and hence partly or completely prevent cross-linking and thus inhibit the subse~uent foaming process.
Accordingly foams with a density too high are produced. More-Le A 15 433 - 1 -over, the cells are not smaller but, if anything, larger because decomposit-ion of the blowing agent starts earlier, at a stage ~hen cross-linking has not yet sufficiently progressed.
If was therefore an object of this invention to reduce or sub-stantially to obviate the disadvantages of the known blowing agents and pro-duce thermoplastic foams with particularly fine cells uniformly distributed.
It has surprisingly been found that this object could be achiev-ed by adding a small quantity of an amphoteric oxide to the foamable mixture, optionally together with blowing agents.
This invention relates to an improved process for the production of a foam plastics with fine cells from a polymer based on an a,~-monoole-f;n;cally unsaturated monomer by peroxidic cross-linking and foaming with a chemical blowing agent selected from azodicarbonamide or dinitrosopentame-thylene-tetramine, said polymer, the improvement comprises adding in addition to of from 0.3 to 1.5% by weight, based on the total mixture of said perox-ide and of from 2 to 30%, based on the total mixture of said blowing agent, from 0.01 to 10% by weight, based on the blowing agent, of pyrogenic orpre-cipitated silica to said polymer subjecting the mixture thus obtained to a shaping process and cross-linking and foaming by heating it to a temperature of at least 190& .
This invention also relates to foam plastic- material prepared in accordance with the above process. The polymers based on a,~-monoolefini-cally unsaturated monomers may be polyolefins with 2 to 4 carbon atoms, pre-ferably polyethylene, including both low pressure polyethylene (d~ 0.94 -0.97 g/cm3) and high pressure polyethylenes (d~ 0.01 to about 0.94 g/cm3) or polypropylenes, polymers or optionally halogenated monovinyl monomers with 2 to 8 carbon atoms, e.g. polyvinyl chloride, polystyrene or ethylene vinyl ester copolymers, preferably polyethylene vinyl acetate containing up to 30%
by weight of vinyl acetate.

. - .

The peroxides used for cross-linking are organic peroxi-des such as dicumyl peroxide, 2,5-dimethyl-2,5-di-(tert.-butyl-p~roxy)-hexane, 2,5-dimethyl-2,5-di-(tert.-butylperoxy)-hexyne, tert.-butylhydroperoxide, cumyl-tert.-butylperaxide, di-tert.-butylperoxide or bis-(tert.-butyl-peroxi-isopropyl)-benzene, di-cumyl peroxide being preferred.
The peroxides are used in quantities of 0.3 to 1.5 Yo by weight, based on the total mixture. They have a decomposition point or range of about 150 to 1~0C.
The chemical blowing agents with a deco~position point or range of about 190C used are azodicarbonamide and dinitroso-pentaméthylenetetramine, preferably azodicarbonamide in quanti-ties of 2 to 30 %, based on the total mixture -~ The highly disperse, amphoteric oxides are added to thepolymer to be foamed in quantities of 0.01 to 10 % by weight, based on the blowing agent, preferably 0.01 to 2 % by weight.
The oxides preferably used are alumina, stannic oxide, zinc oxide, pyrogenic or precipitated silica and the corresponding commercial products such as Aerosilen@ or Vulkasilen~ (very pure, highly disperse silica with amorphous structure). Silica is particularly preferred.
The mixing of the polymers with the cross-linking agents, blowing agents and the oxide may be carried out in any mixing -apparatus, preferably in an extruder, followed by a shaping step at temperatures below the decomposition point of the per-oxide. The a~erage residence time Ln the extruder is about 5 minutes in order to pre~ent any substantially cross-linking or foaming of the foamable mixture at this stage.
Cross-linking takes place at temperatures upwards of 160C and foaming at temperatures of 190 to 250C. According Le A 15 433 -3 -to a preferred embodiment, the highly disperse oxide in the ~orm of a solid, solution or gel is first mixed with the blowing agent, e.g. by grinding in a ball mill, and then added to the peroxide-containing polymer. Alternatively, the oxide may be added to the blowing agent in a preiiminary stage durLng the preparation of the blowing agent.
The foam products produced by the process according to the invention are distinguished by their uniform, fine cellu-lar pore structure and soft, velvety surface and may be used in the building and packaging industry as insulating material.
The densities of the foams produced according to the invention lîes between 20 and 200 kglm3. The foams preferably have 18 or more cells per cm, which corresponds to a pore number of 194,400 per g in a foam which has a density of 30 kg/m3.

Le A 15 433 - 4 -1054'~g8 Comparison Example 1 A paste consisting of 50 parts of polyvinylchloride paste, K-value 70, 37.5 parts of dioctylphthalate, 12.5 parts of ben yl butyl phthalate, 1.0 part of azodicarbonamide, 1.5 parts of dibasic lead phthalate is painted on a siliconised cotton fabric and foamed up in a gelling channel at a temperature of 190C for 3.5 minutes.
The pure white foam obtained, which has a thickness of 1.1 mm and a density of 0.35 g/cm3, has a soft, smooth surface with a velvety handle and uniform cell structure with an average pore diameter of 0.25 mm.

Example 1 990 g of azodicarbonamide and 10 g of disperse silicic acid are homogenised in a ball mill for 5 hou~s.
A polyvinylchloride paste of the same composition as in comparison Example 1, except that silica has been added to the azodicarbonamide, is painted on a siliconised cotton fabrlc and foamed in a gelling channel at 190C for 3.5 minutes.
The resulting foam has a thickness of 1.19 mm and a density of 0.31 g/cm3 and a completely flat, matt surface, soft handle and absolutely uniform cell structure. The cell diameter is 0.18 mm.

Comparison ExamPle 2 A mixture of 70 part~ of a high pressure polyethylene having a density of 0.921 g/cm3 and a melt index of 3.5 g per 10' at 190C and 2.16 kp loading and 30 parts of a Le A 15 433 ~ 5 ~

commercial azodicarbonamide is homogenised and granulated in a double screw extruder of the kind conventionally u~ed for producing thermoplast granulates.
The resulting granulate, mixed in proportions of 1 : 1 with a granulate which has been obtained in the same manner from 98 parts of the polyethylene described and 2 parts of dicumyl peroxide, is used to produce a band 30 mm in width, ~.5 mm in thickness and any desired length. This band is cross-linked and ~oamed up by passing it through a continuous heatLng furnace in which it is heated to 215C.
A cross-linked polyethylene foam 11 mm in thickness, 850 mm in width and weighing 33 g/litre is obtained. ~he interior of the foam was pure white but the surface was yellowish and slightly uneven. It had a uniform cell structure and an average pore diameter of 1 mm.

Example 2 ~he azodicarbonamide concentrate described in ; comparison Example 2 is prepared not from commercial azo-dicarbonamide but from the mixture of azodicarbonamide and silica homogenised in a ball mill as described in Example 2.
This concentrate is processed to produce a cross-linked polyethylene foam as described in comparison Example 2.
The resulting foam is 910 mm in width, 13 mm in thickness and has a density of 28.5 g/litre. It has almost pure white, even surfaces and a very uniform cell structure with an average pore diameter of 0.35 mm~

Example 3 50 kg of azodicarbonamide, either moist or dry, are eA.15433 105~'~98 suspended in 50 1 of water. 4 kg of sodium silicate (d24 : 1.35, approximately 25~ by weight silica) are added to the stirred dispersion which is then adjusted to pH 1-2 with approximately 2.5 l of hydrochloric acid (dilution 1 : 1). ~he reaction mixture is stirred for 30 minutes, filtered and washed with water until neutral and the moist blowing agent is dried under vacuum. When the agglomerates have been broken up mechanically, a free flowing powder is obtained which is used for foaming high pressure polyethylene as described in comparison ~xample 2.
A foam 12 mm in thickness and 900 mm wide and weighing 32 g/l is obtained. It has a pure white, uniform cell structure with an average pore diameter of 0.45 mm.

Example 4 A12 (S04)3 . 18 H20 is dissolved in water, precipitated with sodium hydroxide solution and redissolved in an excess of water and added to an aqueous suspenslon of azodicarbon-amide in a quantity of 2 % calculated as the hydroxide and based on the quantity of azodicarbonamide. The suspension is stirred for 30 minutes and then adausted to pH 1-2 with half concentrated hydrochloric acid. After stirring for 30 minutes, the reaction mixture is filtered and washed with water until neutral and the moist blowing agent obtained is dried under vacuum. After the agglomerate has been broken up mechanically, a free flowing powder is obtained which is used for foaming high pressure polyethylene as described in comparison Example 2.
A foam which is only slightly yellow on the surface is obtained. It has a thickness of 11 mm, a width of 890 mm, a density of 35 g/l and a pore diameter of 0.45 mm, ~eA.15433

Claims (8)

1. THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
   PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
   
   l. An improved process for the production of a foam plastics with fine cells from a polymer based on an a,.beta.-monoolefinically unsaturated monomer by poroxidic cross-linking and foaming with a chemical blowing agent selected from azodicarbonamide or dinitrosopentamethylene-tetramine, said polymer, the improvement comprises adding in addition to of from 0.3 to 1.5%
   by weight, based on the total mixture of said peroxide and of from 2 to 30%, based on the total mixture of said blowing agent, from 0.01 to 10% by weight, based on the blowing agent, of pyrogenic or precipitated silical to said polymer subjecting the mixture thus obtained to a shaping process and cross-linking and foaming by heating it to a temperature of at least l90°C.
2. 2. A process as claimed in claim l, wherein the mixture is subjected to the shaping process without causing substantial foaming or cross-linking.
3. 3. A process as claimed in claim l, wherein the polymer based on an a,.beta.-monoolefinically unsaturated monomer is a polyolefin, a polymer of an optionally halogenated monovinyl monomer, or an ethylene/vinyl ester copolymer.
4. 4. A process as claimed in claim 3, wherein the polymer is a polye-thylene, polypropylene, polyvinyl chloride, polystyrene or a polyethylene vinyl acetate containing up to 30% by weight of vinyl acetate.
5. 5. A process as claimed in claim 1, wherein the peroxide is dicumyl peroxide, 2,5-dimethyl-2,5-di-(tert.-butlperoxy)-hexane, 2-5-dimethyl-2,5-di-(tert.-butylperoxy)-hexyne, tert.-butylhydro-peroxide, cumyl-tert.-butyl-peroxide, di-tert.-butylperoxide or bis-(tert.-butylperoxi-isopropyl)-benzene.
6. 6. A process as claimed in Claim 1, wherein the oxide is mixed with the blowing agent and added to the peroxide-containing polymer.
7. 7. A foam plastics material produced by a process as claimed in Claim 1.
8. 8. A foam plastics material as claimed in Claim 7, which has a density of from 20 to 200 kg/m3.