Influence of the additives on polyethylene crosslinking initiated by peroxides

Abstract

The crosslinking of polyethylene was investigated in the prcsencc of three different additives. The peroxide concentration a t the gel point as well as thc corsslinking efficiency were calculated. The additives act always negatively when peroxide concentration a t the gel point is regarded. When the crosslinking efficiency is considered, hydroquinone and p-benzoquinone act as retarders while diallyl phthalate shows a promoting effect. The mechanism of additive action during crosslinking is discussed.EinfluB von Additiven auf die durch Peroxide initiierte Vernetzung des Polyethylens Die Vernetzung des Polyethylens wurde in Gegenwart von drei verschiedenen Additivcn untersucht. Die Peroxidkonzentration a m Gelpunkt und die Vernetzungseffektivitat wurden berechnet. Die Additive wirken immer negativ bezogen auf die Peroxidkonzentration am Gelpunkt. Hinsichtlich der Vernetzungseffektivitiit wirken Hydrochinon und p-Benzochinon als Retarder, wahrend Diallylphthalat einen Beschleunigungseffekt zeigt. Der Mcchanismus der Wirkung der Additive hei der Vernetzung wird diskutiert. IntrodtictionCrosslinking is one possibility of effective modification of polymers. Several properties of the original polymer can he altered substantially by crosslinking, e.g. solubility, creep behaviour, mechanical properties [l] without changing the chemical nature of the material significantly. Crosslinking of polyolefins can he initiated effectively by free radicals. Thermal decomposition of peroxides and high-energy irradiation is used for this purpose. If the macroradicals formed are sufficiently stable and the termination proceeds preferentially by recombination, the crosslinking efficiency reaches the theoretical value of 1.0 [2].Various side reactions are observed in the presence of various additives. Decrease of the crosslinking efficiency is the result of such reactions in most cases, crosslinking in the presence of antioxidants being a typical example [3]. On the other hand, the enhancement of crosslinking efficiency is known to occur in the presence of other additives. Polyfunctional monomers are the most frequently used compounds having such effect in polyolefins [ 4 ] .In this paper crosslinking efficiency of polyethylene was studied when initiated b y thermal decomposition of peroxide a t presence of three different additives: bifunctional monomer diallyl phthalate, a compound with stabilizing properties, namely, hydroquinone, and benzoquinone. The last one acts as an initiator of radical processes in some reactions, on the other hand it could react similar to a polyfunctional monomer because of the presence of reactive double bonds. All additives mentioned act as efficient coagents in polypropylene cross-linking [5-71. Therefore, the comparison of the efficiency in polyethylene crosslinking contributes to explaining the mechanism of the process and can he significant also from the point of view of application of crosslinked polyolefins. Experimental Materials and proceduresPowdered unstahilized high-pressure polyethylene (Bralcn,...

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