Phr Level Research Articles

The Migration of Extender Oil in Natural and Synthetic Rubber. VI. The Use of Oil Type Molecules as Penetrants to Elucidate Elastomeric Molecular Structure

Abstract Previous work in diffusion of whole extender oil and extender oil fractions in practical rubber compounds has shown the nature of the rubber matrix to be the major controlling factor. Therefore, a sensitive diffusivity approach, using radiotracer labeled hydrocarbon penetrants of sizes comparable to the size of a rotating segment, was used to study various aspects of the matrix structure on penetrant diffusion for polybutadiene, styrene—butadiene, and Butyl rubbers. These results were compared with fractional free volume measurements made by a thermal expansion technique. Neither fractional free volume nor diffusivity of phenyldodecane, benz-a-anthracene, or n-octadecane was affected by 5-fold variations of crosslink density or by variations in loading of 10 to 80 parts of carbon black (HAF-LS); nor were they affected by changes from FEF to SAF black at a 50 phr level. Glass transition temperature and fractional free volume are both affected by increasing percentages of benzene or phenyldodecane, While diffusivity rises with increasing dilution, states of equal free volume in different elastomers are not states of equal diffusivity; for example, at a fractional free volume of 0.10, corresponding to 20% benzene in both polybutadiene and Butyl at 70° C, the diffusivity of benzene in Butyl is lower by a factor of 15. Comparison of diffusivity of large penetrants with their cross-sectional area gives two basic curves, one for flexible penetrants and another for molecularly rigid species. These results are explained in terms of a distribution of free volumes, wherein only holes above a critical size are effective in diffusion. Swelling the system with aromatic diluent greatly increases the total fractional free volume, but only slightly the number of holes above this critical size. To understand the diffusivity of rigid molecules in elastomers, a probability factor must be applied in estimating their effective cross-sectional area.