Thermomechanical Properties during Cure of Glass-Polyester RTM Composites: Elastic and Viscoelastic Modeling

Abstract

Resin transfer molding (RTM) is a widely used technique for the manufacturing of composite parts. A proper selection of process parameters is the key to yield successful molding results and obtain a good part. During composite consolidation, resin cure, also called chemical conversion, plays a decisive role on the final mechanical properties of the part. The modeling of resin kinetics and the evolution of composite properties during cure are crucial for process optimization. In this paper, the curing of a thermosetting polyester resin is studied by differential scanning calorimetry (DSC). A semiempirical autocatalytic model is developed to describe the kinetics of the chemical reaction. The model accounts for the maximum degree of polymerization as a function of cure temperature and induction time, i.e., the time required to attain total inhibitor degradation. The evolution of mechanical properties during resin cure for two glass-polyester composites is also studied with a dynamical mechanical thermal analyzer (DMTA) and a thermomechanical analyzer (TMA). Given that for a low chemical conversion, the elastic properties of the resin remain low, an initial degree of polymerization called after gel point (AGP) is introduced in the analysis of the mechanical properties during cure. A normalized elastic modulus is defined from the value at AGP, taken as a reference. The normalized elastic modulus is then compared to the polymerization degree. For pure resin samples, the logarithm of chemical conversion is found to be almost linearly related to the logarithm of the elastic modulus. Based on this comparison, a thermochemical model is proposed to describe the evolution of mechanical properties during the cure of composite samples with different fiber volume fractions. The viscoelastic behavior is also determined by performing stress relaxation tests with the DMTA. Resin specimens are tested for different cure states below the glass transition temperature, and master curves of stress relaxation during cure are constructed by applying the time-temperature superposition principle. The measurements depict the relaxation modulus of polyester resins as sharply affected by the degree of polymerization. Based on the experimental data, a relaxation modulus is modeled in a thermorheologically simple manner using exponential and power laws. Finally, a linear volume change model is constructed based on the TMA measurements of thermal expansion and resin shrinkage. The volume changes resulting from composite expansion-contraction and resin polymerization shrinkage are modeled as a function of temperature and degree of polymerization. The purpose of this work is to develop appropriate models of chemo- and thermomechanical behaviors of glass-polyester composites during cure. A resin cure kinetics model is developed by adding the glass transition effects to the J.L.B. model. For the mechanical properties, two new models are presented to account for the elastic and viscoelastic behaviors of the resin and the composite. Finally, the coefficients of the volume changes model are measured to account for the composite thermal expansion-contraction and resin chemical shrinkage. These models will be used in future investigations for thermal and curing optimization of composites processed by resin transfer molding.