Control of Adhesion in Carbon Fiber Reinforced Polymeric Composites

Abstract

Fiber-reinforced polymeric composites have grown in popularity for use in structural materials due to their low weight, high specific strength, high toughness, customizability, and versatility. A critical element to the performance of composite materials is how well fiber reinforcements adhere to the surrounding polymer matrix. Different applications can benefit from different levels of adhesion, so its control is a crucial element in the design of a composite. One general drawback to the widespread use of composites is that their production with the desired adhesion level is tedious and time-consuming. It was therefore useful to investigate which steps of production might be shortened and what effects this might have on composite properties. It was found that when controlling for the degree of cure, higher temperature and faster curing schedules lead to an increase in fiber-matrix adhesion due to a freezing of internal squeezing stresses. However, the superficial increases of interfacial adhesion would return to baseline levels, but not below, if the polymer matrix was given time at elevated temperatures to anneal, decreasing internal stresses. Additionally, it was found that tertiary cure-acceleratingcompounds increased curing speed while having no deleterious effect on adhesion. Lastly, it was found that fiber handling agents, or sizings, had no noticeable effect on adhesion. Polyolefins have garnered interest for use in fiber-reinforced composites because of their low cost, high toughness, high impact strength, and excellent corrosion resistance. However, they adhere poorly to most substrates, including carbon fibers, leading to poor composite properties. Thus, different techniques to increase polyolefin-carbon fiber adhesion were explored. While most produced only modest improvements, the most promising were the addition of maleic anhydride block co-polymers to the polyolefin (polypropylene) together with the use of 6-azidosulfonylhexyl triethoxysilane to treat carbon fiber surfaces. While increases in interfacial adhesion generally improve composite performance, there are instances where excessive adhesion can result in undesirable properties such as brittleness. Therefore, methods were investigated to tailor interfacial adhesion in carbon fiber-reinforced thermoset composites in an inexpensive, scalable manner. Treatment of the carbon fibers by a room-temperature vulcanizing (RTV) silicone dispersion in a paraffinic solvent (IsoparTM L) produced reductions in adhesion and corresponding increases in toughness in a controlled manner depending on the amount of silicone added. “Charpy” impact toughness for 12k carbon fiber composite tows was approximately doubled with the addition of silicone to the interface, with only limited losses in modulus.