Cure induced property changes and warpage in thermoset resins and composites
The aim of the present work was to investigate the evolution of thermal and mechanical properties during the polymerisation of a thermosetting resin that is typical those used as the matrix in advanced composites. The mechanism of the cure reaction was studied using differential scanning calorimetry (DSC) in both dynamic (thermal scanning) and isothermal modes, and procedures for correlating the two types of calorimetric data were developed. The model finally chosen encapsulates the diffusion- controlled mechanism of reaction by establishing a one-to-one relationship between the degree of cure and the glass transition temperature, which is assumed to be a structural parameter during the polymerisation. A detailed experimental investigation of specific heat capacity, thermal conductivity, secondary transformations (gelation and vitrification), thermal and chemical volume changes and stress relaxation moduli was carried out to establish a suitable database for the resin. Where possible, a closed analytical model was employed; alternatively, an interpolation procedure was developed evaluate the changes in a selected property during a more complex temperature profile. Experimental equipment was developed to perform shrinkage measurements on the neat resin system; the results obtained were later compared with experimental data from standard liquid dilatometry tests. A simulation of the curing of a bi-material cantilever beam is presented as a test case highlight the influence of property changes on the final curvature. Sample curvature during the experiment was recorded using a digital camera and then analysed using graphical software. The correlation between the observed values of curvature and the results of a finite element based simulation was used to validate the kinetics model and property modelling for the chosen thermosetting resin.