Blowing Agents

Abstract: Coagent compositions for use with azodicarbonamide in the peroxide cross-linking of polyolefin foams, which compositions comprise one or more compounds containing two or more maleimide groups, and a free-radical scavenging compound.

Description: This invention concerns blowing agent compositions for use in the production of peroxide cross-linked polyethylene foams having improved properties, and a process for the production of such foams. A widely used process for the manufacture of peroxide cross-linked polyethylene foams comprises heating the polymer together with a cross-linking agent, eg dicumyl peroxide, and a chemical blowing agent, this being a compound which decomposes on heating to liberate the gases which cause the foaming. Such gases will only be retained in the polymer, and thus give good quality foams, if the rate of decomposition of the blowing agent and the crosslinking reaction itself are synchronised at the temperature employed. If the blowing agent decomposes too soon, the gases evolved escape from the polymer and, if the cross-linking proceeds too quickly, insufficient expansion is possible and poor cell structure can result. Considerable advantages exist in the use of relatively low reaction temperatures, since these avoid the decomposition of the polymer, leading to whiter foamed products. There are also savings in production costs. However, at the desired temperatures, it has been found that the blowing agent must usually be used in conjunction with an activator which causes it to decompose sufficiently. The use of azodicarbonamide as a chemical blowing agent in the production of polyethylene foams is well known, as also is the use of this compound in combination with an activator, eg a zinc compound, which modifies the temperature and rate at which the compound liberates the foam-forming gases. It is also well known to use a coagent in conjunction with the cross-linking agent, eg a polyfunctional unsaturated compound, which increases the rate of peroxide cross-linking so as to balance the rate of cross-linking with the temperature and rate at which the activated blowing agent decomposes. Such coagents are usually polymerisable polyfunctional unsaturated compounds such as acrylates or methacrylates of polyhydric alcohols or allyl compounds, a specific example of such a compound being trimethylolpropanetri-methacrylate. Unfortunately, most coagents are liquids at normal temperatures or are low-melting, waxy solids, which are difficult to blend with other ingredients, giving formulations which are not free-flowing and which are generally difficult to disperse in the polymer. An attempt to overcome this problem by adsorbing the coagent on silica has not been wholly successful since it is found that this reduces the efficacy of the formulation. In addition, the use of less active coagents such as methacrylates leads to inadequate cross-linking at the gas liberation temperature, and consequent sticking of the foam to the production surfaces, pin-holed surfaces, coarse cell structure, and inadequate gas retention in the foam. The compound m-phenylenebismaleimide is known as a peroxide cross-linking agent in the production of non-foamed rubbers and also as a coagent in the production of non-foamed peroxide cross-linked chlorosulphonated polyethylene, but this compound is far too reactive to be employed as a coagent in the preparation of polyethylene foams, giving premature cross-linking relative to the evolution of gas, and leading to foams having undesirable properties. We have now found that it is surprisingly possible to employ compounds containing two or more maleimide (1H-pyrrole-2,5-dione) groups as coagents in the production of polyethylene foams, provided that certain other compounds are also present. Accordingly, in one aspect, this invention provides a method for the production of peroxide cross-linked polyethylene foam in which polyethylene is heated with a peroxide cross-linking agent in the presence of a suitable amount of a blowing agent composition which comprises azodicarbonamide, one or more compounds containing two or more maleimide groups, and a free-radical scavenging compound. In another aspect, the invention provides a coagent composition for use in the production of peroxide cross-linked polyethylene foams, which comprises a free-flowing blend of one or more compounds containing two or more maleimide groups, and a free-radical scavenging compound. In a further aspect, the invention provides a blowing agent composition for use in the production of peroxide cross-linked polyethylene foams, which comprises a free-flowing blend of azodicarbonamide, one or more compounds containing two or more maleimide groups, and a free-radical scavenging compound. The free-radical scavenging compound slows the activity of the maleimide compound sufficiently and reproducibly so that by suitable adjustment of the amounts of each employed, peroxide cross-linked polyethylene foams may be produced having a fine, uniform cell structure and a smooth, pinhole-free, non-sticky surface. Moreover, being a high melting point solid, the formulations employed may be free-flowing. The blowing agent composition preferably contains from 80 to 99 % by weight of azodicarbonamide, especially from 90 to 98 % by weight thereof. The compound containing two or more maleimide groups is preferably N,N′-p,p′-diphenylmethanebismaleimide, N,N′-m-phenylenebismaleimide, N,N′-p-phenylenebismaleimide, N,N′-m-tolylenebismaleimide, 1,1′-(oxydi-4,1-phenylene)bis[1H-pyrrole-2,5-dione], 1,1′-(2,2,4-trimethyl-1,6-hexanediyl)bis[1H-pyrrole-2,5-dione], 1,1′,1″-(1,3,5-benzenetriyl)tris[1H-pyrrole-2,5-dione] or 1,1′-(1,3-phenylene)bis[3-methyl-1H-pyrrole-2,5-dione]. A particularly preferred such compound is N,N′-m-phenylenebismaleimide. The compound containing two or more maleimide groups is preferably present in the composition in an amount of from 0.1-10% by weight, especially from 0.2-5% by weight. Preferably, the blowing agent composition also contains an activator for the azodicarbonamide, for example a zinc compound, eg zinc oxide, zinc carbonate, zinc laurate or diamminozinc diisocyanate, or a mixture of such compounds. Such an activator is preferably present in the composition in an amount of from 0.1 to 10% by weight, especially from 1 to 5% by weight. The free-radical scavenging compound employed is preferably an antioxidant compound, particularly a phenol having 1 to 4 phenolically connected OH groups (eg hydroquinone or 4-methoxyphenol), especially a sterically-hindered phenol (eg bisphenol A, ie 2,2-bis(4-hydroxyphenyl)propane, Irganox® 1010, ie 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, Permanax® WSP, ie 2,2′-methylenebis[6-(1-methylcyclohexyl)-p-cresol or Topanol® CA, ie 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane), a phenylenediamine (eg N-isopropyl-N′-phenyl-p-phenylenediamine), a trimethylquinoline derivative (eg polymeric dihydrotrimethylquinoline), a sulphide (eg dilaurylthiodipropionate), or a phosphite (eg tris(nonylphenyl)phosphite). The free-radical scavenging compound may alternatively be, for example, triallylcyanurate or triallylisocyanurate. It is preferably present in the composition in an amount of from 0.02 to 3 % by weight. The invention is illustrated by the following Examples: EXAMPLES 1-5 The following ingredients were blended together in a Papenmeier blender (all figures being wt%): The blends were all free-flowing. Master batches were prepared by mixing 200 parts by weight of low-density polyethylene with 16 parts by weight of the above compositions and 2.5 parts by weight of dicumyl peroxide (40% active). These were then granulated, blended with 400 parts by weight of low density polyethylene, and extruded on a 32mm screw extruder, die setting 115°C, to obtain approximately 3mm thick sheet. Sheet samples were then expanded in an air-circulated oven at 195°C for 6 minutes. The foams were all found to have fine, uniform cell structures, with pinhole-free, non-sticky surfaces. Example 6 The following ingredients were blended together in a Papenmeier blender (all figures being wt%): This blend was also free-flowing, and when blended into master batches and treated as described in Examples 1-5 above, again gave high quality foams with fine, uniform cell structures, with pinhole-free, non-sticky surfaces.

Claims: A method for the production of peroxide cross-linked polyethylene foam in which polyethylene is heated with a peroxide cross-linking agent in the presence of a suitable amount of a blowing agent composition which comprises azodicarbonamide, one or more compounds containing two or more maleimide groups, and a free-radical scavenging compound. A method according to claim 1 in which the compound containing two or more maleimide groups is N,N′-m-phenylenebismaleimide, N,N′-p,p′-diphenylmethanebismaleimide, N,N′-m-phenylene-bismaleimide, N,N′-p-phenylenebismaleimide, N,N′-m-tolylenebismaleimide, 1,1′-(oxydi-4,1-phenylene)-bis[1H-pyrrole-2,5-dione], 1,1′-(2,2,4-trimethyl-1,6-hexanediyl)bis[1H-pyrrole-2,5-dione], 1,1′,1″-(1,3,5-benzenetriyl)tris[1H-pyrrole-2,5-dione] or 1,1′-(1,3-phenylene)bis[3-methyl-1H-pyrrole-2,5-dione]. A method according to claim 1 or claim 2 in which the azodicarbonamide comprises from 80 to 99 % by weight of the blowing agent composition. A method according to any of claims 1 to 3 in which the compound containing two or more maleimide groups comprises from 0.1 to 10 % by weight of the blowing agent composition. A method according to claim 4 in which the compound containing two or more maleimide groups comprises from 0.2 to 5 % by weight of the blowing agent composition. A method according to any of claims 2 to 5 in which the free-radical scavenging compound is a phenol having 1 to 4 phenolically connected OH groups. A method according to any of claims 2 to 5, in which the free-radical scavenging compound is hydroquinone, 4-methoxyphenol, 2,2-bis(4-hydroxyphenyl)propane, 3(3,5-di-t-butyl-4-hydroxyphenyl)-propionate, 2,2′-methylenebis[6-(1-methylcyclohexyl)-p-cresol, 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, N-isopropyl-N′-phenyl-p-phenylenediamine, polymeric dihydrotrimethyl-quinoline, dilaurylthiodipropionate, tris(nonylphenyl)phosphite, triallyl isocyanurate or triallyl cyanurate. A method according to any of claims 1 to 7 in which the free-radical scavenging compound comprises from 0.02 to 3 % by weight of the blowing agent composition. A composition for use in the production of peroxide cross-linked polyethylene foams, which comprises polyethylene, azodicarbonamide, one or more compounds containing two or more maleimide groups, and a free-radical scavenging compound as defined in claim 6 or claim 7.

Inventors: CONNELL ANDREW MARK, KRUTKI PETER, CONNELL, ANDREW MARK, KRUTKI, PETER
Applicants: SCHERING IND PROD LTD, SCHERING INDUSTRIAL PRODUCTS LIMITED
Application year: 1990
Main classification: C08J9
IPC codes: C08J9, C09K3