Pultrusion processes, which create structural components by pulling reinforcing fibers through a resin bath and die, have traditionally relied on polyester or vinyl ester as the primary material choices. But thermoset polyurethanes may soon provide a tough, cost-effective alternative.
Engineers at Bayer Polymers (Pittsburgh) and the University of Mississippi recently demonstrated the feasibility of pultruding two-component polyurethane. In doing so, they've had to overcome a couple of technical hurdles. As Jim Lambach, Bayer's manager of composite urethane technology, explains, the urethane chemistries used in reaction injection molding (RIM) gel too fast to work on a pultrusion line. So Bayer tinkered with its urethane chemistry and came up with something with relatively slow gel times and a low viscosity to ensure fiber wetting. "Something like a RIM chemistry but slower," is how Lambach describes it.
Pultruded urethane also won't work on standard pultrusion equipment without a few modifications. Whereas pultrusion has until now relied on open resin bath, two-component urethanes use a proprietary "injecting box," or enclosed resin metering and injecting system, to wet out the fibers. "It pumps the two components of the resin at a rate that matches the line's rate of resin consumption and maintains sufficient pressure in the injector box to ensure adequate wet out," Lambach says.
So why go through all the trouble? Thermoset urethanes do promise a couple of mechanical property advantages. According to Lambach, urethane pultrusions material is significantly stronger than polyester materials. "It's about 50 percent stiffer in short shear beam tests," he says. In tests conducted at the University of Mississippi, for example, the average flexural strength of one high-modulus polyurethane resin came to 232 ksi with a short-beam strength of 11.5 ksi. The upshot of the extra stiffness and strength: Thinner pultrusions with the same stiffness, less glass, or a bit of both.
Lambach says polyurethane also has a toughness advantage over other thermosets, allowing the use of cheaper types of glass reinforcements without sacrificing cross-web properties. "Parts currently made using expensive glass mats could be made using less expensive roving," Lambach says. He adds that the material shows its toughness in secondary operations. It can accommodate self-tapping screws, while polyester pultrusions usually need pilot holes to prevent splitting.
The only drawback to the new material right now is line speed. Lambach reports that developmental pultrusion runs of the polyurethane system achieved a maximum line speed of 5 ft/min, while commercial processes often hit speeds up to 8 ft/min. But Bayer is working. "The five feet per minute came without us optimizing the chemistry or equipment," Lambach says. What's more, if urethanes favor roving, which can be faster to process than other types of reinforcements, line speeds would speed up as well. "We believe we'll ultimately have a productivity advantage," Lambach says.