In Part 1 of our series, Engineering Breakthrough Speeds Up Manufacturing Process, we learned how fourth-generation structural polyurethane adhesives have a much shorter cure time than previous generations of urethane adhesives, while still offering good open times. Lab testing confirms the improved open times of fourth-generation polyurethanes, as well as their high shear strength performance on various substrates.
In addition to lab testing, real-world applications show the benefits OEMs are reaping when they adopt the new polyurethanes.
Heavy-Duty Truck Application
A heavy-duty truck OEM was using a two-component, heat-cured adhesive for assembly operations of sheet molding compound (SMC) truck cabs and fiber-reinforced plastic (FRP) truck hoods. Assembly methods required that the SMC be cured in a heated fixture for six minutes at 143 °C (290 °F). The FRP hood assembly method used a lower temperature of 102 °C (215 °F) for nine minutes. (The lower temperature and longer time in the fixture prevented degradation of the gelcoat surface.) The fourth-generation structural polyurethane adhesives showed equivalent or better lap shear performance than the heat-cured polyurethane adhesive, and cycle time was improved by two minutes. Furthermore, the SMC processing temperature was lowered to 104 °C (220 °F). This allowed for better in-process quality as the tooling may have cold spots during start-up operations. The heated tooling process was kept in place, though not required by the fourth-generation adhesive, in order to avoid changes in the manufacturing cell.
Another customer case involved vehicle lightweighting. An OEM wished to use glass-filled polypropylene thermoplastic composite as a lightweight alternative to SMC. The cycle time to assemble a rear hatch back using SMC was a mere 90 seconds within a heated fixture. But, LORD 7800 A/C, one of our fourth-generation polyurethane adhesive family, has a two- to four-minute open time and was able to achieve sufficient handling strength within the 90 second cycle time, while providing enough open time that the adhesive did not cure in the static mixer. This balance of open time and rapid strength development kept the robotic assembly operation moving without having to shut down the line while static mixers are changed out. Although glass-filled polypropylene thermoplastic is a low surface energy composite, LORD 7800 offers excellent adhesion once the surface has been treated either with a flame or plasma surface treatment.
Automotive Headlamp Assembly
Automotive headlamp assembly is another process that requires use of an adhesive with the right balance of open and cure times. Headlamps are typically a polycarbonate lens bonded to a rear glass- or talc-filled housing and tier 1 (tier one) suppliers have long used either a one-component reactive, hot-melt polyurethane (PUR) or a two-component polyurethane adhesive for their assembly. Critical to the assembly operation is a 100 percent leak check on all headlamp assemblies. The leak check is performed using between 0.007 and 0.034 MPa (1 and 5 psi) air pressure to ensure moisture won’t penetrate the headlamp and lead to fogging of the lamps after assembly. When manufacturers use one-component PURs, they typically build into their assembly lines a time or waiting queue so that the PURs can cool and re-crystallize. This time queue may be from 10 to 60 minutes and it delays the next critical quality check. Lord 7800 A/D, which has a five- to seven-minute open time, affords sufficient time to reduce waste from purging the static mixers when the robot is at the home station during the manufacturing cycle. Furthermore, the critical pressure test for quality can be accomplished after 20 minutes of room temperature curing (or on a shortened timeline of as little as five minutes if using IR heaters to accelerate cure).
Fast-curing structural polyurethane adhesives increase manufacturing throughput—not only in theory but, increasingly, in practice. OEMs are reaping the benefits of a cure profile similar to that of an acrylic, achieving cures at room temperature and avoiding cold spots in tooling.