Environmental stress cracking behaviour of cross-linked acrylic based resins

• Main Author: Li, J. J.
• Published: Swansea University 1995
• Subjects: 620.110287
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.637899

The environmental stress cracking behaviour of several cross-linked acrylic based resins has been investigated under various testing conditions (bending, creep and constant strain rate tension.). Urethane methacrylate resins were found to undergo crazing in organic liquids only, and the critical strains and critical stresses for craze initiation can be related to the solubility of the liquid environments. PMMA resins with different EGDMA contents (and thus different cross-link densities) underwent crazing in all the environments tested. Small amounts of EGDMA, up to 3% were found to have little effect on critical strains, although in general, materials with higher cross-link densities had higher resistance to crazing. This capability tended to be reduced in aggressive agents. Environmental diffusion was found to play a very important role in determining the ESC behaviour of these resins. There was always competition between diffusion enhanced crack initiation and growth, and diffusion promoted crack blunting. Under most testing conditions, conventional ESC was the main failure mechanism for these materials since such an attack could come into effect over a wide range of liquid uptakes. However, strong environmental diffusion was also found to lead to crack blunting by the mechanisms of surface softening and compression. This occurred when a plasticised layer developed either on the specimen surface before crack nucleation, or ahead of the crack tip. The effects of cross-linking the chains were to restrict environmental liquid diffusion, to suppress craze initiation and to increase crack growth rate, although these effects were only significant at high levels of cross-linking A combination of these effects determined the overall ESC behaviour of cross-linked polymers Amongst them, restriction to environmental diffusion by cross-linking, appeared to be the most important factors.