Thermosetting reactive resin systems such as epoxy, bismaleimide, and polyimide classes of material are brittle. The origin of brittleness is attributed to the high crosslinking density that exists in the fully cured forms of these materials. Traditionally, the toughness of these resins is enhanced by adding toughening agents such as rubber particles to the initial unreacted mixture of monomers and solvents. For the toughening of resin matrix fiber-reinforced composites, an interleafing approach is adopted. The interleafing approach is accomplished by inserting thermoplastic films (with inherent high toughness) alternately between the stacked prepreg plies, and then co-curing to form an integrated structural part.
The innovation described herein utilizes a thermal pretreatment technique — the green approach — to alter the degree of toughness/brittleness in the thermosetting reactive resins. Unlike the prevailing methods for toughening resins, the green approach is purely a (physical) thermal pre-treatment technique, without alternating chemical compositions of the underlying resin system. This innovative approach is practical and cost-effective in material and labor. It requires no additional capital investments for equipment. Furthermore, it does not impose weight penalty to the composite structural component, and is applicable to a broad range of thermosetting reactive high-performance resin/composite systems.
This innovation is both novel and unique because it toughens the thermosetting reactive resins without altering the original chemical compositions. Unlike traditional toughening techniques, the green approach provides flexibility in tailoring the degree of toughness and brittleness in the final fully cured part. This flexibility in tailoring properties is achieved by varying the thermal pre-treatment conditions and consequently, the resulting molecular weights.
This innovation can be applied to all thermosetting reactive resins. In these resin systems, there are two distinct reactions, namely linear chain extension and crosslinking reactions. Activation energy for the initial chain extension reaction is low, while it is high for the crosslinking reaction at the later curing stage under elevated temperatures. The key to this innovation is finding a thermal pre-treatment condition (temperature and dwell time) that provides intermediate activation energy for the given resin. This intermediate activation energy is sufficient for continuous chain extension to occur for a long period of dwell time without triggering the crosslinking reaction. Fully cured, toughened resin matrix is obtained through crosslinking reaction at elevated temperatures, which starts with higher MW polymer chains, and results in a network that possesses low-crosslinking density.