A new "tunable" polyurethane could revolutionize composites

Most of us in the composites industry know why composites have not yet been widely adopted in high-volume segments, such as the auto industry:  Part production is not scalable to high volumes as a result of constraints in cycle time and processing methods that make composites cost-prohibitive. Common aerospace-grade epoxy resins require cure cycle times of 30 minutes or more, well above the transportation industry’s goal of five-minute-or-less cycle times. But a novel polyurethane-based resin system with tailorable pot life and cure, from Huntsman Polyurethanes (Auburn Hills, Mich. and Everberg, Belgium), aims to change that paradigm. Huntsman Polyurethanes’ Michael Connolly and Dan Heberer recently discussed their company’s trademarked VITROX isocyanate resin system at CompositesWorld’s High-Performance Resins conference, held in Schaumburg, Ill., Sept. 23-24, 2010.

Two-part polyurethanes (PURs) traditionally have been limited to small parts or to continuous processes, such as pultrusion, because of fast reaction time and rapid increase in viscosity after PUR’s two components are mixed. But Connolly reported that the VITROX resin combines isocyanates, polyols and a unique, proprietary catalyst system that permits processors to “dial in” a desirable gel time and viscosity profile, yielding previously unachievable processing benefits together with mechanical properties that exceed those achieved with some epoxies. 

The company conducted a study in which the new resin was compared to conventional amine-cured epoxy resin and traditional PURs, via samples of neat resin, cast resin coupons and carbon/resin composite coupons. Viscosity profiles over time, thermal performance, and mechanical properties were measured, as was fire, smoke and toxicity (FST) performance, for the various samples. In one test, 100g of resin was mixed and placed in an insulated container, and monitored for temperature. The results showed that VITROX can remain at low viscosity at room temperature for more than two hours, compared to traditional PUR pot life of 20 to 25 minutes, making it possible to infuse large parts where long pot life is a requirement. In a second test, viscosity profiles measured during heating show that the VITROX remains at a consistently low and stable viscosity until a trigger or “kick off” temperature, dictated by the catalyst blend and specific formulation, is reached. At that point, a snap cure occurs, in contrast to a traditional PUR and epoxies, which steadily increase in viscosity as cure progresses: “We can adjust gel times so that cure is faster for short demold times, or longer for large part production,” said Connolly. Indeed, gel time tests, in which a small amount of mixed resin was applied to a preheated mold and monitored until the resin solidified, confirmed that cure time can be precisely tailored to a customer’s application, from less than five minutes to up to several hours.

Neat resin coupons of the new resin and epoxy were tested for Tg by heating the samples while measuring storage modulus, or the stored elastic energy, during dynamic mechanical analysis (DMA). Results showed a significantly higher Tg for the VITROX resin, of about 240°C/464°F, compared to the Tg of the epoxy at 120°C/250°F. Additional mechanical tests showed the novel polyurethane with tensile modulus numbers as good as or better than the neat epoxy. Results also showed a slightly higher strain to failure, with comparable tensile strength. The real difference was that fracture toughness and fracture energy (G1c) were 1.5 to 2.5 times higher with VITROX for both neat resin and carbon-fiber composite coupons, which means that the new resin has a high service temperature without sacrificing toughness.

FST performance was equally impressive, because the samples were prepared without the addition of any external flame retardants. Under NF P92-501: Exposure to Radiant Heat, the French fire test for rail interior applications, the VITROX samples achieved the highest classification, M1. Likewise, in tests for smoke and toxicity, the samples were classified as F1, the second highest classification, with zero emission of bromides, Connolly reported.

So what does this mean for composites? It means polyurethanes now have the potential to be used in resin transfer molding (RTM), vacuum-assisted resin infusion, filament winding and other processes, to create tough, impact-resistant high-performance parts, with significantly shorter cycle times compared to competitive advanced resins. Connolly admits there may be questions about the creation of voids due to the generation of carbon dioxide during cure of urethanes, but adds, “We tested numerous samples in glass and carbon, and believe that proper processing techniques will create composite laminates with near-perfect impregnation.” Keep an eye on this technology, which might lead to wider adoption of composites in high-volume applications.