Role of the source of free radicals in crosslinking of polyethylene

Abstract Crosslinked polyethylene, owing to changes in structure, is a product with technically important properties. The crosslinking may be brought about with the aid of various free radical sources, such as ultraviolet light, ionizing irradiation of high intensities, and organic peroxides. The kinetics and mechanism of the crosslinking process depend predominantly upon the free radical source employed for crosslinking of polyethylene. When peroxides are used, the kinetics and mechanism depend on both the chemical mechanism of the peroxide decomposition and the reaction of primary radicals with the polyethylene macromolecules. When radiation is used, the excited molecules dissociate after absorption of energy, hydrogen is evolved, and radicals from the polymer form crosslinks. The hydrogen atom eliminated is quite mobile and easily abstracts another hydrogen from the polymer. Hence, during a single primary reaction two polymer radicals or one crosslink and one hydrogen molecule are formed. The number of crosslinks as well as the amount of hydrogen produced are then simply proportional to the absorbed radiation dose. Crosslinking of polyethylene by the peroxide method is a more complicated process, the polymer radicals not forming directly, but by transfer to the polyethylene macromolecules of radicals produced by peroxide decomposition. In the termination reaction, in addition to crosslinkages, products of the reaction between primary and polymer radicals may form. When the peroxide decomposes by a chain mechanism the dependence of the crosslinks upon the initial peroxide concentration becomes more involved. In the induced decomposition, the free radical sources are inefficiently consumed and. owing to the difference in reaction orders of the simultaneous ineffective induced and effective self decomposition, of peroxide, the ratio between the two reactions varies in the course of the process. From a kinetic analysis of the experimental results quantitative relations between the formation of crosslinks in polyethylene and the free radicals produced from the various sources were established. Both the simplest cases of direct production of radicals capable of crosslinkage as well as the cases in which the formation of the primary radicals is determined by the initiation process and rate of radical transfer and the nature of the termination reactions are of major importance have been considered. Finally a still more complicated case, in which crosslinking of polyethylene takes place during chain decomposition of the peroxide, has been treated. With the aid of the derived equations and the experimentally determined rate constants the crosslink concentration was calculated and the results have been compared with experimental values.