The bonding mechanism of polyurethane adhesives

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Polyurethane adhesives represent an important part of the rapid development in polyurethane resins today. They possess excellent properties and are widely used in various applications; they are one of the eight major types of synthetic adhesives, suitable for use in a wide range of structural bonding applications. One might wonder: given that polyurethane adhesives have such strong bonding strength and can bond a wide variety of materials, how exactly do they manage to bind these different materials together? Below, Luoyang Tianjiang Chemical New Materials Co., Ltd. has categorized the bonding mechanisms of polyurethane adhesives based on the types of materials they bond to. The categories are as follows: 1. Bonding of metals, glass, ceramics, etc. The surfaces of materials such as metals and glass have very high surface tension; thus, they are considered high-energy surfaces. The cured polyurethane adhesive contains urethane and urea bonds with relatively high cohesive energy. Under certain conditions, these bonds can accumulate on the bonding surface, thereby forming an adhesive layer with high surface tension. Generally speaking, the higher the percentage of isocyanates or their derivatives in an adhesive, the greater the surface tension of the adhesive layer; the adhesive becomes more resilient, can bond well with substrates such as metal, and typically exhibits high adhesion strength. 1. The adhesion mechanism of adhesives containing –NCO groups to metals is as follows: Adsorbed water is generally present on metal surfaces (even on polished metal surfaces, there is still a small amount of adsorbed water or metal oxide hydrates). The urea bonds formed by the reaction between –NCO and water chelate with metal oxides via hydrogen bonds, thereby forming a ureido–metal oxide complex. Additionally, –NCO groups can also form covalent bonds with metal hydrates. 2. In the absence of –NCO groups, hydrates on the metal surface as well as metal atoms form van der Waals forces and hydrogen bonds with urethane and urea bonds; moreover, polyurethane adhesives based on TDI and MDI contain benzene rings and possess an electron-rich system that enables them to form coordinate bonds with metals. The composition of metal surfaces is relatively complex, and the types of various chemical bonds or secondary bonds (such as hydrogen bonds) formed with polyurethane adhesives are also complex. 3. Inorganic materials such as glass slates and ceramics are generally composed of components such as SO2, CaO, and Na2O; their surfaces also contain adsorbed water and hydroxyl groups, and their bonding mechanism is roughly the same as that of metals. II. Adhesion of plastics and rubber
For bonding rubber, polyisocyanate adhesives or polyisocyanate adhesives modified with rubber-based adhesives are generally used. The organic solvents contained in these adhesives can cause the rubber surface to swell. Polyisocyanate adhesives have a relatively low molecular weight, enabling them to penetrate into the inner layers of the rubber surface and react with the active hydrogen present in the rubber, thereby forming covalent bonds. Furthermore, polyisocyanates react with moisture to form urea or biuret groups, and during heating and curing, the isocyanates undergo self-polymerization to create cross-linked structures. These structures interact with the rubber molecules to form a polymer cross-linked interpenetrating network (IPI), thereby endowing the adhesive layer with excellent physical properties. When bonding rubber using ordinary polyurethane adhesives, good adhesion can also be achieved due to the chemical and physical interactions between the functional groups of the respective materials. The polar groups on the surfaces of plastics such as PVC, PET, and FRP can form hydrogen bonds with groups like urethane bonds, ester bonds, and ether bonds in adhesives, thereby creating joints with a certain bonding strength. Some believe that fiber-reinforced plastics (FRP) contain –OH groups, and that the surface –OH groups react with –NCO in polyurethane adhesives to form chemical adhesion. Non-polar plastics such as PE and PP have very low surface polarity; therefore, using polar polyurethane adhesives for bonding them can present difficulties. This issue can be resolved by subjecting the polyolefin plastics to various surface treatment methods. There are two common treatment methods: one is corona treatment, which causes oxidation on the surface, thereby increasing its polarity ； Another method is to apply an adhesive such as a polyisocyanate adhesive as an adhesion-promoting coating agent (primer) on the surface of the plastic to be bonded. In the case of melt concave extrusion films, when performing extrusion lamination on plastic films such as PET, an interface layer with low polymerization degree exists on the surface, resulting in unsatisfactory adhesion strength. When an adhesive is used, polyisocyanate diffuses across the surface of the hot polyethylene, thereby strengthening this weak interface layer and giving the laminated film very good peel strength. III. Bonding of fabrics, wood, etc. Substrates such as fabrics and wood are composed of fibers, which have a certain moisture absorption rate. They often contain polar bonds such as ether bonds, ester bonds, and amide bonds, as well as functional groups like carboxyl groups and hydroxyl groups. Water and hydroxyl groups readily react with the –NCO groups in polyurethane adhesives, forming chemical bonds such as strong urethane bonds and urea bonds ； Hydrogen bonds form between the polar groups in the fibers and those in the adhesive, and the adhesive molecules can also easily penetrate between the fibers. Polyurethane generally can form a strong bond with such materials.