Polyurethane vs. Polyester in Composites
Once a novelty in the composites industry, polyurethane has gained a significant share of the composites market. This is because of the unique advantages of polyurethane resin compared with unsaturated polyester resin and vinyl ester resin.
Polyurethane composites first gained a foothold in composites in foamed structural reaction injection molding (SRIM) automotive applications. These were interior and exterior parts such as pickup truck boxes, load floors, package shelves and inside door panels. Automakers and their suppliers continue to use polyurethane SRIM today for many of these parts.
Meanwhile, new, emerging polyurethane composite developments have included polyurethane pultrusion, filament winding, vacuum infusion and long-fiber spraying technologies. Through its European and North American polyurethane R&D centers, Bayer MaterialScience has been involved in the development of most of these technologies with a number of non-foamed, full-density composite polyurethane systems and commercial applications.
There are many reasons why polyurethane resins are expanding farther into the composites market. An initial attraction was the fact that polyurethane resins do not produce styrene emissions during processing.
Polyurethane composites also have significantly faster cure times than polyester spray-up Ð 20 minutes for polyurethane and 2 to 4 hours for polyester in non-automotive parts, and 30 seconds to 2 minutes for polyurethane SRIM in automotive parts compared with 2 to 10 minutes for polyester and vinyl ester SMC. Polyurethane spray processes also are less labor intensive than polyester spray-up, where applicators must roll out the glass reinforcement to remove air and ensure full wet-out. Because SRIM requires lower molding pressures than SMC, tooling costs can be lower, too.
The reaction speed of polyurethane resins used to limit their use in large, glass-filled composite parts. But advances in polyurethane chemistry and advances in processing equipment and technology have resolved that issue.
More recently, expansion into long-fiber technology and pultrusion are demonstrating the substance behind polyurethane composites. Polyurethane composites use rigid thermoset resins to produce parts with superior tensile strength, impact resistance and abrasion resistance compared with unsaturated polyester and vinyl ester resins. Tensile modulus of approximately 430,000 psi, tensile strength of about 12,500 psi and elongation to failure of more than 7.5 percent are typical physical properties of these polyurethane composites. And when weight savings are needed, reinforced polyurethane can be foamed to cut weight up to 20 percent.
Polyurethane composites also excel in such secondary operations as drilling, machining and assembly. Machined and punched edges show little or no micro-cracking that can occur in traditional thermoset composites.
Bayer encourages potential customers to evaluate the total cost of a part using polyurethane or polyester resin because the results can be surprising.