Polyurethane hot melts are different from traditional hot melt adhesives. They are typically applied from a heated cartridge at a rather low temperature, which makes them more suitable for heat sensitive materials than other hot melts. This type of adhesive offers good adhesion to many substrates due to presence of polar groups. Typical urethane hor melts have very low glass transition temperatures and are very elastic with a wide range of softening points.
Polyurethanes consist of long linear chains with flexible, soft segments (polyether / polyester) that alternate with short, rigid diurethane bridges resulting from diisocyanate reacting with polyol chain extenders. The rigid segments form hydrogen bonds with other rigid segments. A higher ratio of soft to hard segments improves flexibility and lowers the glass transition temperature, and therefore, improves the low temperature performance, but it also lowers hardness, modulus, and abrasion resistance. The choice of diisocyanate and polyols allows for tailoring the polyurethane properties for various applications. They can be formulated with and without a plasticizer and resin and are compatible with many of them.
A major drawback of urethanes is there susceptibility to UV light, which causes discoloration, and degradation of mechanical properties. The stability can be improved by using aliphatic isocyanates and by adding UV stabilizers and antioxidants.
PUR hot melts are either thermoplastic or thermosetting:
Thermosetting urethane hot melts (PURs) have rather low initial bond strength that increases over time as the adhesive reacts with moisture in the air or is further cured with UV light. The chemical bond of reactive polyurethane hot melts is usually stronger and more resistant to heat and chemicals than typical hot melt adhesives. This makes PUR hot melt adhesives an excellent choice for more demanding applications.
PURs initially set like a conventional hot melt adhesives by solidifying when cooled below the softening point. In the case of moisture cure, the adhesive absorbs a small amount of moisture from the surrounding materials or from the atmosphere and subsequently crosslinks. The setting can take several days. Since a small amount of moisture is required for the crosslink reaction, at least one substrate has to be porous; otherwise the cure will be rather slow. The uncured solidified "green" strength tends to be lower than that of non-reactive TPU’s, but mechanical strength develops with curing and the green strength can be improved by blending the prepolymers with other polymers of higher softening point. The open time of PUR adhesives can vary from 10 seconds to about 10 minutes to match application requirements. The application temperature is in the range of 85°C to 140°C (185°F to 284°F), which is less than that for traditional thermoplastic hot melt adhesives. The viscosity at the application temperature is often in the range of 2,000 to 60,000 cPs depending on composition.
Thermoplastic polyurethanes (TPUs) have similar properties as PUR adhesives, but due to the absence of crosslinks are less resistant to heat and chemicals and have usually lower bond performance but have better green strength.
The adhesives market is expected to slowly shift from traditional solvent based adhesives towards the use of high-performance hot melts like reactive PURs and pressure sensitive hot melt adhesives.