A, metal, glass, ceramics, etc.
Metal, glass and other substances have high surface tension and are high-energy surfaces. The cured PU adhesive contains urethane bonds and urea bonds with high cohesive energy, which can aggregate on the bonding surface under certain conditions to form high surface tension. Adhesive layer. Generally speaking, the higher the percentage of isocyanate or its derivatives in the adhesive, the greater the surface tension of the adhesive layer, the tougher the adhesive, and the better match with the metal and other substrates, and the bonding strength is generally higher. high.
The bonding mechanism of the adhesive containing an NCO group to the metal is as follows:
There is generally adsorbed water on the metal surface (even the polished metal surface also has a small amount of adsorbed water or metal oxide hydrate), and the urea bond generated by the reaction of NCO with water and the metal oxide are chelated due to hydrogen bonds. The formation of ureide-metal oxide complexes, an NCO group can also form covalent bonds with metal hydrates, etc.
In the absence of 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 and MDI-based polyurethane adhesives contain benzene rings, which have a redundant electron system and can interact with Metals form coordinative bonds. The composition of the metal surface is complex, and the types of various chemical bonds or secondary bonds (such as hydrogen bonds) formed with the PU glue are also complex.
Inorganic materials such as glass, slate and ceramics are generally composed of Ah09, S02, CaO and Na20, and the surface also contains adsorbed water and hydroxyl groups, and the bonding mechanism is roughly the same as that of metals.
B, plastic, rubber bonding
The bonding of rubber is generally made of polyisocyanate adhesives or polyisocyanate adhesives modified by rubber-based adhesives. The organic solvent contained in the adhesive can swell the surface of the rubber. The molecular weight of the polyisocyanate adhesive is small and can penetrate into the interior of the rubber surface. The active hydrogen reacts to form a covalent bond. The polyisocyanate will also react with moisture to form a urea group or a biuret, and when heated and cured, the isocyanate will self-polymerize to form a cross-linked structure and form a polymer with a cross-linked network of rubber molecules. Cross-linked interpenetrating network (IPI), so the adhesive layer has good physical properties. When using ordinary polyurethane adhesives to bond rubber, due to the chemical and physical interactions between the material groups, good bonding can also be produced.
The polar groups on the surface of plastics such as PVC, PET, and FRP can form hydrogen bonds with groups such as urethane bonds, ester bonds, and ether bonds in the adhesive to form joints with a certain bonding strength. Some people think that glass fiber reinforced plastics ( FRP) contains an OH group, in which a OH on the surface reacts with a NCO in the PU adhesive to form a chemical bond.
Non-polar plastics such as PE and PP have a very low surface and may encounter difficulties in bonding with polar polyurethane adhesives. This can be solved by surface treatment of polyolefin plastics in a variety of ways. One way is to use corona Treatment to oxidize the surface and increase the polarity: Another method is to use polyisocyanate adhesives as tackifying coating agents (primers, primers) on the surface of the plastic to be adhered. For example, melt concave extrusion films in When extruding and compounding on plastic films such as PET, due to the existence of a weak interface layer with a low degree of polymerization on the surface, the bonding strength is not ideal. When primer is used, the polyisocyanate diffuses on the hot polyethylene surface to strengthen the weak interface layer. , the composite film has very good peel strength.
C, fabric, 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, amide bonds, as well as carboxyl groups, hydroxyl groups, etc. Water and hydroxyl groups are easily combined with NCO in PU adhesives. Groups react to form strong chemical bonds such as urethane bonds and urea bonds, while hydrogen bonds are formed between polar groups in the fibers and polar groups in the glue, and the adhesive molecules can easily penetrate between the fibers. These materials generally form strong bonds.