As advanced composites continue to be needed as primary and secondary structures on aircraft, in recent years there has been an emphasis on damage tolerant composites that will increase the allowable strain level as well as improve low velocity impact performance. A unique approach to solving these problems has been the use of a semi-interpenetrating polymer network (SIPN) as a composite matrix. An example of a semi-interpenetrating polymer network is one that combines a thermoplastic with a thermoset. Both types of materials have their inherent advantages and disadvantages. Thermoplastics offer substantial improvements in toughness and potential processing, but have limits in upper temperature use and often have unacceptable solvent resistance. Thermosets, on the other hand, often have high use temperatures, but are inherently brittle and often are susceptible because of moisture sensitivity.

The concept behind SIPNs is to combine the best of both materials, toughness and high end use temperature. It was found that when a dicyanate is combined with a copolyestercarbonate a SIPN is formed that is very useful as a matrix material. The prepregging was extremely simple and the low boiling solvent was easily removed. The cure cycle was essentially state-of-the-art epoxy processing except that a higher final temperature was used with epoxy-like pressure. No volatiles were given off and void-free composites were easily obtained. Composite properties were excellent with a glass transition temperature of 222°C (430°F). Interlaminar fracture toughness determined from double cantilever beam tests indicated at least a fivefold increase in critical energy release rate for stable crack growth for Mode I loading (G_Ic) over the epoxy composite.

Mechanical spectrometry was used to determine the glass transition temperatures and to aid in cure cycle determination. Transmission electron microscopy (TEM) was used to determine the nature of the SIPN and to measure the size of the individual components.