Cross-linking of polythene by pile radiation

Abstract

Polythene subjected to irradiation in the Harwell B.E.P.O. pile becomes cross-linked, and a new type of plastic is produced which does not melt at about 115° C, nor dissolve in hot organic compounds. The mechanical properties are also altered, especially above 115° C, when the plastic shows rubber-like elasticity. The paper is mainly confined to a study of the relationship between the degree of cross-linking and the amount of incident radiation causing cross-linking. Possible mechanisms of cross-linking are briefly considered. Cross-linking is shown to arise primarily from the fracture of C ̶ H bonds, and the liberation of hydrogen. The weight changes Δ M of a specimen of weight M , and surface area A , subjected to radiation R is found to be represented by the equation Δ M = —α 1 MR +α 2 MR 2 +β 1 AR 2 —β 2 AR 2 . These terms are considered to arise from hydrogen evolution from the bulk of the polymer, methane, ethane, etc., evolution from near the surface, and surface oxidation. From the hydrogen-loss term α 1 the efficiency of cross-linking is deduced as 1 % of carbons cross-linked per unit radiation defined as 10 17 thermal neutrons/cm 2 , and the associated fast neutrons and gammas. Microchemical analysis reveals a reduction in H/C ratio with radiation. The cross-linking ratio deduced is 1.1 % of carbon cross-linked per unit radiation. The corresponding figure deduced from the amount of radiation required to render cross-linked polythene insoluble is 1.1 to 1.4 %. For paraffin wax the figure is 0.9 %. From the volume of gas evolved a value of 1.16% is obtained. The effects of different forms of radiation are considered. It is concluded that γ-radiation as well as fast and thermal neutrons are responsible. The energy required to break a C—H bond is found to be of the order of 25 eV. The value of this physical method of producing cross-linking in polythene and in other long-chain polymers under accurately controllable conditions, without the incorporation of other chemical compounds and without heat treatment, is discussed. Since the mechanism of polymerization is different, a range of new polymers can be envisaged, of which the physical properties can be studied as a function of the degree of cross-linking.