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 degrees C, nor dissolve in hot organic compounds. The mechanical properties are also altered, especially above 115 degrees 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$\chembond{1,0} $H bonds, and the liberation of hydrogen. The weight changes $\Delta $M of a specimen of weight M, and surface area A, subjected to radiation R is found to be represented by the equation $\Delta $M = - $\alpha _{1}$MR + $\alpha _{2}$MR$^{2}$ + $\beta _{1}$ AR$^{2}$-$\beta _{2}$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 $\alpha _{1}$ 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$\cdot $1% of carbon cross-linked per unit radiation. The corresponding figure deduced from the amount of radiation required to render cross-linked polythene insoluble is 1$\cdot $1 to 1$\cdot $4%. For paraffin wax the figure is 0$\cdot $9%. From the volume of gas evolved a value of 1$\cdot $16% is obtained. The effects of different forms of radiation are considered. It is concluded that $\gamma $-radiation as well as fast and thermal neutrons are responsible. The energy required to break a C$\chembond{1,0} $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.