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UV adhesive involves two bonded materials during use, making it more difficult to eliminate the stress and volume defects caused by shrinkage during the curing process compared to coatings such as UV ink. Therefore, UV adhesive has higher requirements for reducing shrinkage rate.

The volume shrinkage of UV adhesive not only affects the dimensional accuracy of bonding, but also leads to a decrease in bonding force. For the bonding of rigid materials, it is easy to produce grain like voids or even cracks, and for flexible materials, it is easy to cause warping deformation. So reducing the volume shrinkage of UV adhesive during the curing process is of great significance for improving the dimensional accuracy and bonding strength of the adhesive.

Production mechanism

The volume shrinkage generated during the UV adhesive curing process is due to the change in the tightness of the atomic arrangement during the curing process. The main reason for this is the change in atomic distance caused by polymerization reactions during the curing process, followed by the change in entropy generated during the transition from monomers to polymers, i.e. the change in free volume. Due to the thermal expansion coefficient of UV glue being on the order of 10-4, the volume change caused by thermal expansion and contraction is negligible.

The curing reaction of UV adhesive can be divided into free radical type and cationic type, both of which are accompanied by changes in atomic distance.

The shrinkage rate of free radical type UV adhesive is relatively large, generally between 5% and 10%. UV adhesive improved by cationic or other methods can reach nearly 2%. The curing shrinkage rate of epoxy resin adhesives is usually between 2% and 3%. The main resin (oligomer) and diluent monomer used in the free radical UV adhesive are both acrylic esters. During the polymerization process, the acrylic monomer molecules, which were originally subjected to van der Waals forces, became covalently bonded, reducing the distance between the corresponding atoms from 0.3-0.5nm to 0.154nm, by about half. In this way, the arrangement of atoms in the polymer is much tighter than in the monomer, leading to volume contraction during the polymerization process. So the shrinkage rate of free radical reactions is relatively high, such as the volume shrinkage of commonly used styrene and methyl methacrylate homopolymers, which are 14.5% and 21.3%, respectively. The volume shrinkage rate of acrylic monomers used for dilution in UV glue is generally above 8%.

The monomers used in cationic UV adhesives are generally epoxy compounds or vinyl ethers. Vinyl ethers have a slower curing rate and are not as widely used as epoxides. The curing mechanism of epoxy compounds is ring opening polymerization reaction in the presence of cationic photoinitiators. When epoxy compounds undergo ring opening polymerization, on the one hand, the distance between epoxy monomers changes from the van der Waals distance before curing to the covalent bond distance after curing, causing volume shrinkage; On the other hand, during the polymerization of epoxy monomers, the ring on the monomer opens, and the covalent bond distance within the molecule becomes similar to the van der Waals distance between molecules, leading to volume expansion and partially offsetting the volume contraction caused by bonding. The overall result of both is a slight volume shrinkage of the epoxy compound after curing, but its shrinkage rate is much lower than that of the acrylic radical polymerization reaction.

Another volume change factor in the polymerization process is the change in entropy from monomers to polymers, which is the change in free volume from monomers to corresponding polymers, i.e. the change in molecular packing density between monomers and polymers. Due to the incomplete and tight stacking of molecules in polymers, there is a free volume between molecules. Before UV curing, the liquid monomer molecules are in a loose and free state, with a large free volume. After UV curing, acrylic monomers form a three-dimensional network cross-linked polymer with multiple intermolecular crosslinking points, which significantly limits the movement of chain segments and reduces the free volume, resulting in volume shrinkage.

influence factor

1. Content of functional groups

The main reason for the volume shrinkage of UV adhesive is the change in the distance between atoms caused by polymerization reactions during the curing process. Therefore, the size of the shrinkage rate of UV adhesive mainly depends on the crosslinking density, and the higher the crosslinking density, the higher the shrinkage rate. For free radical UV adhesives, the content of acrylic groups is the determining factor. As the relative molecular weight increases and the content of acrylic ester groups decreases, most of them show a trend of decreasing shrinkage rate. Among them, isobornyl acrylate has the lowest shrinkage rate, which is 8.2%.

Acrylate oligomers generally have lower shrinkage rates due to their higher relative molecular weight and lower functional group content. The shrinkage rate of polyurethane acrylate is generally lower than that of epoxy acrylate, and the molecular flexibility is easier to design and adjust, so polyurethane acrylate is more commonly used than epoxy acrylate.

2. The regularity of the molecular structure

The regularity of molecular structure also has a significant impact on the shrinkage rate of the polymerization process. For linear molecular structures, volume contraction is approximately proportional to the number of monomer molecules per unit volume. Branched or irregular monomer molecules do not follow this pattern, and their shrinkage rate after polymerization is relatively small.