How to reduce the shrinkage of UV glue? What are the methods?
In the curing process of UV adhesive there is a gelation process. Before gelation, the system is still fluid despite the shrinkage, while after gelation, the molecular movement is hindered and further curing reactions are the main cause of the shrinkage stress. Therefore, the shrinkage stress is not proportional to the volumetric shrinkage of the entire curing process. Rather, it depends on the volumetric shrinkage after gelation. The higher the functionalities of the reactants, the lower the degree of reaction of the functional groups when gelation occurs. Therefore, a reaction system with a high degree of functionality will produce a higher shrinkage stress after curing.
Ways to reduce the shrinkage of UV adhesive
1. UV glue formulation in the use of low-shrinkage monomers and oligomers
In the UV glue formula, the use of low viscosity acrylate oligomer, less dilution monomer, is also an effective way to reduce the shrinkage rate.
2. UV glue added polymer or inorganic filler
In the method of adding polymers, the newly generated system and pre-added polymers between the phase separation sometimes occurs during the curing process, can also offset part of the volume shrinkage. However, the disadvantage of this phase separation is that it reduces the transparency of the cured product. It has been reported that the addition of polyvinyl acetate, polyvinyl acetal, polyester and other thermoplastic polymers to the free radical polymerization system can result in a significant reduction in volume shrinkage.
3. The use of cationic or hybrid curing method
The cationic UV glue uses monomers of epoxy resin and vinyl ether, with ium salts as photoinitiators. The shrinkage rate of cationic curing reaction of epoxy resin is much lower than that of free radical reaction, so the curing shrinkage rate can be reduced by using cationic formula.
Hybrid UV glue is a mixed system of free radical and cationic type, usually in the form of mixed acrylate and epoxy compounds. This hybrid UV curing system can improve the initiation efficiency when photoinitiated, give full play to the advantages of both, reduce volume shrinkage, and have good synergistic effects in other properties.
In addition, according to the process requirements, UV and other curing methods can also be used to compound, such as UV and heat curing or anaerobic curing compound method, can also reduce the curing shrinkage.
4. Adding swelling monomer
From the epoxy resin cationic polymerization reaction already know, open-ring polymerization reaction than the addition of polymerization reaction volume shrinkage rate is smaller. For monocyclic monomers, for every van der Waals distance converted to a covalent bond distance, there is a covalent distance converted to a van der Waals distance. The resulting shrinkage and swelling can be partially offset. Further studies revealed that the size of the volume contraction of the ring-opening polymerization reaction is related to the size of the ring. As the ring increases in size, the distance between the carbon and oxygen atoms that open covalent bonds gets closer to the van der Waals distance, and the volume contraction thus decreases. At present, the cost of spiro ring type monomer is very high, so its practical application is still very little, but it is a promising method to eliminate the shrinkage of UV glue.