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Paul Waites: Paul Waites is in Technical Services, Evode Limited, Stafford UK
ACKNOWLEDGMENT: [copyright] Evode Ltd, 1997
Introduction
Hot-melt polyurethane reactive adhesives (subsequently abbreviated to HMPUR) have only been available for a few years, but have already captured a significant market share. This trend has been accelerated by the market demand for environmentally-friendly, high performance adhesive systems with fast setting times.
Formulation of a moisture-curing reactive polyurethane hot-melt adhesive
The basic components in HMPUR production are polyols and diisocyanates. Their reaction results in a polyurethane prepolymer with reactive isocyanate groups which cure under the influence of moisture. This produces an adhesive which combines the advantages of two adhesive systems: the conventional hot-melt and the reactive system.
For a better understanding of the working mechanism of the HMPUR, a short introduction to these different adhesive systems is required.
Conventional hot melts
Conventional hot-melt adhesives are based on high molecular weight organic compounds, e.g. polyamide, polyester, thermoplastic rubber polyolefins or EVA. During processing, the hot-melt is applied in molten form and, on cooling, it solidifies and the bond gains its strength (physical hardening). This step is reversible, i.e. through application of heat, the adhesive can be remelted.
The advantages of hot melt are:
- immediate bond;
- wide field of application;
- adhesion to a wide range of materials;
- environmentally-friendly, since they have 100 per cent solids content;
- no drying time.
Reactive system
The initial bond is formed in the same way as a conventional hot melt, purely by cooling the molten adhesive to a solid. The second stage of bond formation takes place as the adhesive reacts with moisture from the atmosphere and/or the substrate. This produces an irreversible chemical reaction which produces a tough polymer with high temperature resistance, up to 120[degrees]C, and short-term exposure to 200[degrees]C. The low temperature performance is impressive, showing good elasticity at -40[degrees]C.
From these brief comparisons it can be seen that PUR offers a combination of the best properties for the two technologies it combines.
Properties of the moisture-curing reactive PUR hot melts
All-round adhesion with high final strength
The HMPURs achieve good adhesion to many plastics, metals, wood and wood fibreboard. In comparison with other hot melts the bond strength is very high.
Green strength
HMPURs...