Curing cycle modification for RTM6 to reduce hydrostatic residual tensile stress in 3D woven composites

Abstract

Triaxial residual tensile stresses resulting after cooling a 3D woven composite from the curing temperature cause cracking in the resin pockets for weave architectures that have high through‐the‐thickness constraint. We show how curing cycle modifications can reduce the hydrostatic tensile stress generated by thermal mismatch during cooling of Hexcel RTM6 epoxy resin constrained in a quartz tube which simulates extreme constraint in a composite. The modified curing schedule consists of a high temperature cure to just before the glass transition, a lower temperature hold that takes the resin through the glass transition thereby freezing in the zero stress state, followed by high temperature cure to bring the resin to full conversion. We show that this process is sensitive to heating rates and can reduce the zero stress state of non‐toughened RTM6 resin to a temperature similar to a commercial rubber‐toughened resin, Cycom PR520. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43373.

Kinetics of Thermal Polymerization Can Be Studied during Continuous Cooling

“…Despite the obvious practical importance of the cooling stage there appears to have been no systematic studies of the thermal polymerization kinetics during continuous cooling. Instead, when the experiment with the cooling segment is designed, it is assumed that either the polymerization rate is negligibly small during cooling or the kinetics on cooling remains the same as on heating . However, it has been demonstrated that polymerization may continue for significant period of time after the heating source is removed from the system and it undergoes cooling.…”

Aspects of reproducibility and stability for partial cure of epoxy matrix resin

“…Very small heating rates allow a constant curing below T g , which leads to highly controlled partial cure. This approach is of little practical relevance as it requires long periods of time . Isothermal cure at moderate temperatures always leads to vitrification and therefore to a transition to diffusion‐controlled reaction.…”

“…This approach is of little practical relevance as it requires long periods of time. 34,35 Isothermal cure at moderate temperatures always leads to vitrification and therefore to a transition to diffusion-controlled reaction. Thus, this method is favorable for fast and reproducible adjustment of partial cure states.…”

All You Need to Know about the Kinetics of Thermally Stimulated Reactions Occurring on Cooling

“…The issue has been discussed in connection with decomposition of amino acids [6], pyrolysis of hydrocarbons [7], cracking of heavy oils [8], pyrolysis of heterogeneous materials such as almond shells, municipal solid waste, lignin, and polyethylene [9]. It has also been brought up in regard to epoxy materials crosslinking [10,11] and vulcanization of rubber [12,13,14,15]. That is, there is no doubt that cooling is an integral part of many manufacturing processes and, as such, it affects the properties of the final product.…”

“…In the absence of such studies, it is often assumed that the reaction rate is negligibly small during cooling or that kinetics determined on heating can be applied to the cooling conditions [10,16,17]. However, as demonstrated in the previous section, the similarity of the kinetics on heating and cooling should be expected only for single-step reactions.…”