The synthesis of polyurethane adhesives is based on the unique chemical properties of isocyanates. Isocyanate is a compound containing an isocyanate group (-NCO) in the molecule. This group has a highly unsaturated bond structure with overlapping double bonds and can react with various compounds containing active hydrogen. In the field of polyurethane adhesives, isocyanates containing 2 or more -NCO characteristic groups are mainly used. According to whether the product is yellowing under light, polyurethane adhesives are divided into general isocyanate polyurethane adhesives and yellowing resistant isocyanate polyurethane adhesives.
General Purpose PU Adhesive
General-purpose isocyanates, that is, aromatic isocyanates, are currently the most widely used isocyanates in the polyurethane industry. The methylene group connected to the benzene ring in the structure is easily oxidized to form quinone chromophores, which leads to yellowing of the material. Commonly used general-purpose isocyanates include TDI, MDI and polymethyl polyphenyl isocyanate (PAPI). TDI is liquid at room temperature and easy to use. It is the first isocyanate used in the polyurethane industry.
Anti-yellowing PU Adhesive
In order to improve the yellowing of polyurethane materials caused by universal isocyanates, in addition to the use of related additives, the generation of chromophores with benzene ring conjugated quinone structure should be avoided. For this reason, researchers have developed many yellowing-resistant isocyanates: such as xylylene diisocyanate (XDI), hexamethylene diisocyanate (HDI), isophorone isocyanate (IPDI) and so on.
PU adhesive modification
Although polyurethane adhesives have excellent properties, they are easily affected by external environments such as light, heat, oxygen, and water, reducing their use value. With the development of society, the single performance of polyurethane adhesives can no longer meet the application requirements, and the modification of polyurethane adhesives has become a hot area, in which physical modification and chemical modification are the main modification methods.
(1) Physical modification
Physical modification is mainly a method to improve the performance of the adhesive by doping some fillers and additives under certain conditions during the preparation process of the polyurethane adhesive. Quartz powder has good compatibility with the polyurethane adhesive system, which can significantly improve the tensile strength, elongation at break, and tear strength of polyurethane adhesive products. Nano boron nitride (BN) is ultrasonically dispersed in polyol, and then reacted with MDI to prepare polyurethane adhesive film for food packaging. Compared with the adhesive without nano-BN, the water vapor transmission rate of the film is reduced by 50%, the bonding strength is increased by 37%, and the peeling strength is increased by 7.14%. Adding SiO 2 nanofibers to the polyurethane matrix, it is found that the hydroxyl groups on the surface of the SiO 2 nanofibers and polyurethane form a close cross-linked structure, which improves the adhesion of the adhesive, the hardness and tensile strength of the film, but also increases the colloid. Viscosity.
(2) Chemical modification
Chemical modification is a modification method that changes the types of atoms or atomic groups on the molecular chain and their combination methods through polymer chemical reactions. Among them, block , graft, etc. are several commonly used chemical modification methods for polyurethane adhesives. Among them, high-performance epoxy resin-modified polyurethane adhesives, acrylate-modified polyurethane adhesives, and silicone resin- modified polyurethane adhesives are the targets of competition in the industry. Epoxy resin has many advantages such as good adhesion, corrosion resistance, high strength, etc., but its toughness is poor. The introduction of epoxy groups into the polyurethane system can obtain products with better performance. Silane-modified polyurethane adhesives can not only improve flexibility, but also avoid the shortcomings of traditional polyurethane adhesives that are easy to foam after curing and have poor adhesion to smooth substrates.
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