The Relation Between Chemical Structure and Mechanical Behaviour of Phenolic Resins:High-Speed Shearing of Paper-Based Phenolic Laminates

Abstract

A wide range of electrical and electronic components are produced by punching from paper-based phenolic laminates. The inherent brittleness of these materials often leads to various defects around the punched edges. Plasticisation of the resin may be used to overcome this problem but industrial methods are largely based on empirical observations and there is a need for a greater understanding of the fundamental principles involved. There is also a need for information about which phenolic starting materials, readily capable of commercial synthesis, may be used in manufacturing laminates which are easily punched. An investigation into these problems has begun by instrumenting a power-press, such as might be used in commercial practice. A technique has been developed to enable the stress/strain characteristics of a laminate to be observed over a wide range of temperatures during the punching process. This technique has been used with a series of commercial materials to provide background information about industrial practice against which information from experimental materials may be examined. The effect of processing variables introduced in preparing laminates from a given resin/paper system has been investigated to preclude the unanticipated influence of such variables in comparing different resin compositions. This preliminary work has shown that punching temperature is of primary importance but that the type of base-paper used is also important. Processing variables show surprisingly little effect provided that resin content and laminate thickness are held constant. A comparison of laminates based on different papers, but otherwise similar, has shown that, of the papers in common use, bleached kraft is the most suitable for demonstrating resin effects. Work with different resins has suggested that, apart from cross-link frequency, the presence of hydrogen bonds may influence their brittle behaviour. Further work has been suggested to investigate this observation.