Abstract
An investigation was made into the nature of permanent non-annealable mottle in the epoxy-resin system CT200/HT901 (C.I.B.A. Ltd.) as used for photoelastic analysis. Tensile and shear fracture specimens showed fibrilar formations, termed spicules attached to the fracture surface. Investigation of these spicules showed them to have been formed shortly prior to gel. They consisted of heavily cross-linked, fully cured resin having a higher ester content than the surrounding matrix. The spicules were more dense than the matrix, the difference being consistent with the strain necessary to produce 0.1 stress fringes; this irregular unannealable birefringence is termed mottle. Extraction of the insoluble fraction prior to gel showed fully cured spheres which could be the initiation points for spicular growth. The self-diffusion constant of the phthalic anhydride hardener was determined at various temperatures and cure times. Extent and rates of cure were determined using infra-red analysis and these were used to calculate the value of the self-diffusion constant necessary to sustain the spicule growth throughout cure. Comparison of the calculated and experimentally determined diffusion constants showed spicule growth to be possible for about two thirds of the gel-time. Density measurements at elevated temperatures enabled calculation of displacements in castings; differential displacements showed the post-gel stresses to be insufficient to cause microfracture. As spicule growth is apparently a diffusion controlled process, methods suggested for the elimination of spicules include the use of less diffusive hardeners of low exotherm coupled with low oven temperatures.