City Research Online

Abstract

Autoadhesion (autohesion) is the spontaneous adherence between two unvulcanised surfaces of anelastomer. Autohesive tack can occur between elastomeric compounds. For hydrocarbon elastomers, the accepted autohesion mechanism is the interdiffusion of chain portions across the interface, so that chain packing and molecular weight are significant parameters.

Deficiencies in existing tack tests led to the development of the Dunlop Rotary Tackmeter, which uses a "sun and planet" motion for the continuous peel measurement of the tack between the peripheral surfaces of two squat cylindrical rubber testpieces. The system is in dynamic equilibrium during testing and gives high contact pressures and realistically short contact times. Consequently, tack and bulk strengths can be largely differentiated.

Optimum autohesive strengths have been measured ranging from high magnitudes for natural rubber through intermediate levels for synthetic elastomers to low values for ethylene propylene terpolymer. The use of specially-isomerised natural rubber has shown that stress crystallization is supplementary only. Theories of diffusion suggest that the extreme diffusion coefficients differ by an order of ten and that this difference arises from the influence of chain structure. Space regions "contained" in cavities associated with permanent chain structural features have been defined collectively as "intrachain' free volume. The coincidence of several cavities plus the interchain space causes easier formation of holes. These holes may attain sufficient size during normal chain thermal fluctuations to facilitate forward motion of an incoming chain portion.

The proposed model has been compared with a simple alternative involving interchain free volume only, and has shown better agreement between distributed free volume and autohesive characteristics. Chain structure affects both chain flexibility and the provision of intrachain cavities and therefore provides an explanation of the different autohesive behaviour of various elastomers. The proposed concept is supported by an experiment involving unilateral interfacial diffusion, and its apparent correlation with the diffusion through elastomers of a range of gases and solvents has been considered in detail.

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science > QD Chemistry
Departments:	School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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