Chemical stress cracking of acrylic fibres

The generation of periodic microscopic transverse cracks in oriented acrylic fibres immersed in hot alkaline hypochlorite solution is described in detail and shown to be a variety of chemical stress cracking. It is greatly accelerated by external tensile stress, high fibre permeability, moderate fibre orientation, and water-plasticization. The proposed mechanism for bond cleavage involves cyclization of nitrile groups (similar to the prefatory reaction in pyrolysis of acrylic fibres), followed immediately by N-chlorination and chain scission. Mechanical retractile forces (internal or external) then cause chain retraction and crack growth. Despite the remarkable regularity of the crack pattern, which typically resembles a series of stacked lamellae, the process is independent of any such underlying fibre morphology. The cracking process does, however, appear to be a sensitive indicator of residual latent strain in the fibre, which may persist even after high-temperature annealing.