EVA, PA and polyester hot-melt adhesives have been successfully used for many years; each has advantages for specific applications and markets. EVA-based hot melts are general-purpose products whose applications are limited by a relatively low upper use temperature (60-80°C). Polyamides and polyesters are higher performance products with superior elevated temperature properties, but the application temperatures required (~ 200°C) are quite high, which can be an issue for substrates that are temperature sensitive.
Polyurethane hot-melt adhesives fall into two categories: thermoplastic or thermoset. Both have strong adhesion to a many substrates, are used at lower temperatures than polyamides and polyesters, and exceed EVA performance. The thermoplastic polyurethanes are linear high-molecular-weight polymers. They are supplied as either pellets or powders, and are mainly used for textile applications. The pellets can be extruded into non-tacky free-standing films or can be coated directly onto a substrate. The pellets are supplied with a minimal gel content to prevent film defects. The bonding process involves briefly exposing the films to mild temperatures of ~ 60-75°C, which exceeds the polymer’s crystalline melting temperature (Tm). A tacky film results and the substrates are bonded.
Thermoplastic powders are used in a scatter coating process. These polymers are composed of crystalline polyester polyols and most often 4,4’-diphenylmethane diisocyanate (MDI). The temperature to activate the films, the open time between activation and use, and heat resistance are primarily determined by the polyester polyol used.