PUR reactive hot melts combine the speed of a hot melt with the strength of a reactive adhesive. The secret is a two-stage setting mechanism.
Stage 1: Physical setting
After application, molten PUR cools and gains initial green strength within seconds, so parts can move straight to the next operation.
Stage 2: Chemical curing
Free isocyanate (NCO) groups in the adhesive react with moisture in the air and in the substrate. This reaction turns the thermoplastic into a crosslinked structure that no longer melts. The result: high heat, water and chemical resistance.
Factors affecting curing
| Factor | Effect |
|---|---|
| Relative humidity | Higher humidity speeds curing |
| Substrate moisture | 8–12% ideal for wood |
| Film thickness | Thicker films cure more slowly |
| Temperature | Cold slows the reaction |
What it means in practice
- Full strength usually develops within 1–7 days; run final tests after that.
- Molten PUR also cures in the tank when exposed to air, so closed systems and regular cleaning matter.
- Flushing the system with a suitable cleaning agent at the end of a run prevents cured adhesive building up in the machine.
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