5. Reactive polyurethane hot melt adhesives

These adhesives have a mixed setting:

• They are first melted at around 130-160°C, then bonded and allowed to cool rapidly as with conventional hot-melt adhesives;

• And because the adhesive reacts slowly with ambient humidity and the moisture in the bonded materials, and hardens through chemical "cross-linking", it offers excellent resistance to water, heat and creep.

Moisture content is obviously of great importance for these adhesives.

Figure 14 shows that when humidity rises above 16%, bonding strength develops very rapidly during the first 15 minutes.