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University of Bath: School of Materials Science

Grant GR/J 33012

D.E. Packham

    The purpose of this project was to examine some of the fundamentals of the industrially important vulcanisation bonding process, under conditions sufficiently close to industrial practice that the insights obtained would be of value the industry and to our collaborator, Avon Clevite Ltd.

    The basic system investigated comprised a phosphated mild steel substrate, to which a commercial bonding system was applied (Chemlok 211 primer and Chemlok 220 adhesive). A simple black-filled natural rubber compound was moulded against this surface and cured in situ.

    The peel energy increased by a factor of about five as the peel angle increased from 30° to 90°. Throughout this range the locus of failure was cohesive within the rubber, but it moved more deeply within the rubber as the peel angle approached 90°. At 90° a severely torn fracture surface was produced and the trace showed marked slip-stick behaviour. This behaviour was modelled on the basis of two critical energies, one for peel front propagation an other for crack arrest.

    The steadier peel trace associated with 45° peeling and smoother fracture surface with failure closer to the substrate suggests the suitability of this peel angle as a 'standard' in this type of work.

    Adhesion to differently phosphated steel surfaces was compared with that to acid-etched steel. In all cases good adhesion was found with failure mostly cohesive within the rubber. This indicated that the rough phosphated surface was not a prerequisite to effective bonding in the circumstances. Differences were seen which were associated with morphological and thickness differences in the phosphate coating. These gave additional insights into the interaction between surface topography and adhesion.

    In order to get an understanding of the mechanisms by which the proprietary adhesive interacts with the rubber to give a strong bond, studies were performed using a chlorinated natural rubber to represent the adhesive.

    A mixture of natural rubber and chlorinated rubber cast from xylene was found to be essentially immiscible. However heating at 150^oC results in a morphological change with apparent homogenisation: the resulting material no longer contained rubber-like polyisoprene units. Tests using the homogenised mixture as a model adhesive suggested that the interaction between the two components of the blend is significance during formation of effective bonds.