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Hot Melt Adhesive Innovations I


The chemistry of composite materials is changing at a breakneck pace. The fast-changing world has put more complex requirements on newly introduced products. Each new material advancement and development must not only be better and more favorably priced, but also meet sustainability requirements. So-called composite materials, produced by combining different materials, have played a major role in this development. Among the thermal, mechanical and bonding techniques used to manufacture composite materials, it is anticipated that bonding technology will assume an ever more important role in industry. The bonding technique involves the joining of two substrates using an adhesive.

An adhesive is defined as a non-metal, a binder that acts via adhesion and cohesion. Adhesives can be organic or inorganic compounds. Organic and silicone adhesives can be physically hardening (hot melts, wet solvent-containing adhesives, contact, dispersion, water-based, pressure-sensitive adhesives, and plastisols) or chemically curing (polymerization, polycondensation, and polyaddition).

In 1968, development work began on the moisture-curing polyurethane adhesives for sealing/bonding the front and rear windscreens on cars. And 1970 witnessed the rapid development of polyurethane chemistry with a wide range of adhesive formulations, including reactive hot melts.

Reactive hot melts are systems that are solid at room temperature and become fairly highly viscous liquids at application temperature of 120 °C. Typical formulations consist of an isocyanate, crystalline, amorphous, liquid polyols, moisture-curing catalysts as well as non-reactive additives. Each of these components can be appropriately adjusted to obtain the required pot life, open time and green strength. The initial bond strength of reactive hot melts is triggered on cooling by the solidification of non-reactive additives and prepolymers, with the final strength reached via chemical secondary cross-linking by heat, humidity or both.

Although synthetic hot melt adhesive technology is almost a century old, researchers continue to advance the technology with an eye on market needs and new regulatory requirements. This article highlights recent advances in adhesive research with special emphasis on polyurethane hot melt adhesives, in particular, polyurethane hot melt adhesives with very fast set rates, resulting in high green bond strength. These new polyurethane hot melt adhesives are designed from conventional polyisocyanates and a mix of proprietary polyols.