Polyurethane bonding mechanism

A, bonding of metal, glass, ceramics, etc.

Metals, glass and other materials have high surface tension and belong to high-energy surfaces. In the cured products of PU adhesives, they contain urethane bonds and urea bonds with high cohesive energy. Under certain conditions, they can aggregate on the bonding surface and form high surface tension. Adhesive layer. In general, the higher the percentage content of isocyanate or derivative thereof in the adhesive, the greater the surface tension of the adhesive layer, the tougher the adhesive, the better match with the substrate such as metal, and the higher adhesive strength.

The bonding mechanism of an adhesive containing an NCO group to a metal is as follows:

Adsorption water is generally present on metal surfaces (even traces of adsorbed water or metal oxide hydrates are present on polished metal surfaces), and the urea bond formed by the reaction of NCO with water is chelated with metal oxides due to hydrogen bonding. A ureide-metal oxide complex is formed, and an NCO group can also form a covalent bond with a metal hydrate.

In the absence of an NCO, van der Waals forces and hydrogen bonds are generated between metal surface hydrates and metal atoms and urethane bonds and urea bonds, and TDI, MDI-based polyurethane adhesives contain benzene rings, with redundant electronic systems, and The metal forms a valence bond. The composition of the metal surface is relatively complex, and the types of chemical bonds or secondary bonds (such as hydrogen bonds) formed between the PU and the PU adhesive are also complicated.

Inorganic materials such as glass, slate, and ceramics are generally composed of Ah09, S02, CaO, and Na20. The surface also contains adsorbed water and hydroxyl groups. The adhesion mechanism is roughly the same as that of metals.

B, plastic, rubber bonding

The rubber adhesive is generally a polyisocyanate adhesive modified with a polyisocyanate adhesive or a rubber adhesive. The organic solvent contained in the adhesive can swell the surface of the rubber. The polyisocyanate adhesive has a small molecular weight and can penetrate into the interior of the rubber surface and exist in the rubber. The active hydrogen reacts to form a covalent bond. The polyisocyanate also reacts with moisture to form urea or biuret, and the isocyanate undergoes self-polymerization upon heating and curing to form a crosslinked structure and crosslink network with rubber molecules to form a polymer crosslinked interpenetrating network (IPI). The adhesive layer has good physical properties. When the rubber is bonded with a common polyurethane adhesive, good adhesion can also be produced due to the chemical and physical interactions between the various material groups.
The polar groups of PVC, PET, FRP and other plastic surfaces can form hydrogen bonds with urethane bonds, ester bonds, ether bonds and other groups in adhesives to form joints with certain adhesive strength. Some people think that glass fiber reinforced plastic (FRP) contains an OH group, in which an OH on the surface reacts with an NCO in the PU adhesive to form a chemical bonding force.

Non-polar plastics such as PE and PP, whose surfaces are very low, may encounter difficulties when bonded with polar polyurethane adhesives. This can be accomplished by surface treatment of polyolefin plastics in a variety of ways. One approach is to use corona treatment to oxidize the surface and increase the polarity: Another approach is to use a polyisocyanate adhesive or the like as a tackifying coating agent (primer, primer) on the surface of the adhered plastic. For example, when the melted and extruded film is extruded on a plastic film such as PET, the bonding strength is not satisfactory due to the presence of a weak interface layer with a low degree of polymerization on the Xing surface. When the primer is used, the polyisocyanate is on the surface of the hot polyethylene. Diffusion enhances the weak interface layer and the composite film has very good peel strength.

C, bonding of fabrics, wood, etc.

Fabrics, wood and other substrates are composed of fibers, and fibers have a certain moisture absorption rate, and often contain polar bonds such as ether bonds, ester bonds, and amide bonds, and carboxyl groups, hydroxyl groups, and the like. Water and hydroxyl groups easily react with an NCO group in the PU adhesive to form strong bonds such as urethane bonds and urea bonds, while hydrogen bonds are formed between the polar groups in the fiber and the polar groups in the adhesive, and the adhesive Molecules also easily penetrate between fibers. PUs generally form strong bonds to these materials.

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