Diffusion of additives in polyolefins

A simple radio‐tracer method based on 14 C is described for determining the diffusion coefficient of an additive in a polymer. Two theoretical models representing extreme cases for the diffusion system are discussed and shown to yield the same solution to within 10%. The diffusion coefficients of didodecyl 3,3′‐thiodipropionate, N ‐octadecyldiethanolamine, and 1,1,3‐tri(2‐methyl‐4‐hydroxy‐5‐ tert ‐butylphenyl)butane migrating in polyethylene, polypropylene, and poly‐4‐methylpentene‐1 have been measured over temperature ranges of 50–80°C., within the range 20–200°C. In each case the variation of the diffusion coefficient D with temperature T can be represented by an Arrhenius equation D = D 0 exp {‐ E/RT }, where the activation energy E is virtually independent of the size and shape of the diffusing molecule. The activation energy depends upon the polymer and is about 12.5, 21, and 14.5 kcal./mole for polyethylene, polypropylene, and poly‐4‐methylpentene‐1, respectively.