Curing Behavior of Thick-Sectioned RTM Composites

Abstract

This study provides direct guidance for the process design of thick-walled RTM parts in China, particularly filling knowledge gaps in engineering practice regarding thermal management and cure degree control. The core challenge in thick-walled RTM parts lies in the conflict between the strong exothermicity of resin curing and the mold's heat dissipation capacity. This research systematically reveals a long-overlooked phenomenon through DSC experiments: there is an upper limit to the final conversion rate of resin during low-temperature curing. Taking vinyl ester resin as an example, the final conversion rate is only 70% during isothermal curing at 55°C, meaning that even with extended curing time, the remaining 30% of the resin is difficult to react completely. This is completely different from the curing assumptions for traditional thin-walled parts, which usually assume near-complete curing with sufficient time. This finding explains why thick-walled parts require the curing temperature to be lowered to below 50% of the recommended temperature for thin-walled parts. Simply lowering the temperature without considering the conversion rate limit can lead to internal residual stress and performance degradation. On the methodological level, the study established an improved autocatalytic kinetic model, incorporating temperature-dependent maximum conversion rate parameters into the prediction framework. More innovative is the zero-order kinetic description of the inhibitor (shelf-life extender)—at low temperatures of 55°C, the inhibitor can double the curing time, but it is rapidly consumed at high temperatures. This provides inspiration for formulation design: the amount of inhibitor used for thin-walled parts cannot be simply carried over; it needs to be re-evaluated based on the actual curing temperature of thick-walled parts. The finite difference model successfully predicted the temperature gradient distribution of a 2.54cm thick part, verifying the engineering applicability of the theory. The difference from mainstream practices is that traditional RTM processes often rely on empirical heating curves or single-point temperature control, whereas this study emphasizes the temperature dependence of the cure degree limit—a quantitatively controllable parameter. In domestic pultrusion and HP-RTM processes, similar thick-walled parts (such as the root of wind turbine blades, large-diameter pipes) often suffer from delamination and creep due to insufficient curing, the root cause of which is often the neglect of this conversion rate bottleneck. Engineering implementation recommendations: For RTM parts with a thickness exceeding 20mm, the maximum conversion rate of the resin system at the target curing temperature should first be measured by DSC, rather than blindly referring to data sheets. If the conversion rate at 55°C is below 85%, it is necessary to evaluate whether to adjust the resin system or adopt a staged heating strategy. The amount of inhibitor should be re-optimized based on the actual curing temperature to avoid excessive cycle time extension. The establishment of the finite difference model requires data on the thermal conductivity and specific heat capacity of the material, parameters that are often overlooked but crucial in domestic composite enterprises. For companies already experiencing quality issues with thick-walled parts, it is advisable to first check if the curing temperature is too low, leading to insufficient conversion, which is a more targeted approach than blindly extending the curing time.