Hot Melt Polyurethane Used in Adhesives
Although hot melt polyurethane reactive adhesives have been available for 25 years, their novel properties and the development of diverse products have accelerated their recent growth. They are now found in many applications as products are developed to fit specific needs. In general, hot melt polyurethane reactive adhesives have the following characteristics.
1. They are solid at room temperature, similar to other hot melt adhesives that are thermoplastic polymers, such as ethylene vinylacetate (EVA), polyalphaolefin (PAO), polyester and polyamide.
2. The temperature for application as a melt ranges from 85°C to 140°C (185°F to 284°F), which is less than that for typical thermoplastic hot melt products. The viscosity at the application temperature can be designed in a range from 2,000 to 60,000 cPs depending on the specific requirements.
4. Hot melt polyurethane reactive adhesives are being produced with open times varying from 10 seconds to 10 minutes to match application requirements.
5. The unique property that gives hot melt polyurethane reactive adhesives their performance advantage compared to thermoplastic hot melts is that they cure to a thermoset material that resists melting. This curing process is the reaction with moisture found in the air or in typical substrates to produce a strong, tough, temperature-resistant adhesive.
6. The advantages of hot melt polyurethane reactive adhesives result from the properties that were described above. Some of these advantages are:
7. The cured adhesive has excellent temperature and environmental resistance. Many of the hot melt polyurethane reactive adhesives can withstand exposure to temperatures from -40°F to +200°F while maintaining strong bonds.
8. Minimal fixturing is needed due to the controlled set time and rapid development of green strength that can be designed into these products. They are typically applied by roll coating, slot-die coating, gravure printing, screen printing or spray coating (swirl or fiberized) and then briefly nipped or pressed to provide a bond with handling strength. Final cure and ultimate strength are achieved in a few hours to a few days depending on the adhesive, substrate and conditions in the plant.
9. Hot melt polyurethane reactive adhesives can be designed and developed with widely varying properties for specific applications. How this is done and some of these applications are described later in this article.
10. Hot melt polyurethane reactive adhesives are 100%-solids and therefore have no volatile organic compounds (VOCs) and do not require drying ovens. This eliminates many of the environmental issues associated with solvent-based adhesives and the energy requirements for drying water-based and solvent-based products.
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