Study on shrinkage of UV-cured adhesives (1)

Foreword

Ultraviolet light-curing adhesives (UV-curable adhesives, referred to as UV-adhesives) have received increasing attention due to their advantages such as fast curing speed, solvent-free, high production efficiency, and energy saving, and thus have developed rapidly in the past 10 years. In recent years, the use of UV-curing adhesives has grown at a rate of 20% per year in the world. Due to the rapid development of the electronics industry and other manufacturing industries, the use of UV-curable adhesives has grown faster.

UV-adhesives are divided into free radicals and cationics according to the curing reaction. At present, more than 95% of them are acrylate free radicals, which are characterized by a relatively large volume shrinkage during curing. Since the adhesive involves two adhered interfaces, it is more difficult to eliminate stress and volume defects caused by shrinkage during curing than UV-materials such as paints and inks. Therefore, the requirement for reducing shrinkage is even greater. high. The occurrence of volumetric shrinkage not only affects the dimensional accuracy of the bond, but also directly leads to a decrease in the bond strength. For the bonding of rigid materials, striated voids and even cracks are easily generated, and warpage is easily generated for flexible materials. Therefore, reducing or even eliminating the volume shrinkage in the curing process has important significance for improving the dimensional accuracy and bonding strength of the bonding and extending the application range of the UV-adhesive. This article analyzes and discusses the causes of UV-adhesive volume shrinkage, influencing factors, research methods, and the relationship between shrinkage and shrinkage stress, in order to find effective ways to reduce the shrinkage rate for different use requirements.

1 Shrinkage mechanism

The volume shrinkage that occurs during UV-curing of adhesives is due to the tightness of the arrangement of the atoms during the curing process. The main reason is the change in the distance between atoms brought about by the polymerization during the curing process, followed by the change in the entropy generated from the monomer to the polymer, that is, the change in the free volume. Since the thermal expansion coefficient of the UV adhesive is on the order of 10-4, the volume change due to thermal expansion and contraction is very small and will not be discussed here.

The UV adhesive curing reaction is divided into free radical type and cationic type, both of which are accompanied by changes in the distance between atoms.

The shrinkage rate of free radical UV adhesive is relatively large, generally 5% to 10%, and the UV adhesive modified by cationic or other methods can reach nearly 2%, and the curing shrinkage of epoxy adhesive is usually 2 %～3% (Fig. 1) Radical type UV adhesive, the main resin (oligomer) and diluent monomer used are all acrylic esters. During the polymerization process, the acrylate monomer molecules originally acting as van der Waals forces became covalently bonded, and the distance between the corresponding atoms was shortened from 0.3 to 0.5 nm to 0.154 nm (Fig. 2(a)), shortening half. In this way, the arrangement of atoms in the polymer is much tighter than in the monomer, resulting in volumetric shrinkage during the polymerization. Therefore, the shrinkage rate of free radical reaction is relatively large, such as the volume shrinkage of commonly used styrene and methyl methacrylate homopolymers are respectively 14.5% and 21.3%. The volumetric shrinkage of acrylate monomers used in UV adhesives is generally above 8%.

Monomers used for cationic UV adhesives are generally epoxy compounds or vinyl ethers (Figure 3). Vinyl ethers cure slowly and are not as widely used as epoxies. The curing mechanism of an epoxy compound is a ring-opening polymerization reaction in the presence of a cationic photoinitiator. As can be seen from Figure 2 (b), epoxy ring-opening polymerization, on the one hand, the distance between epoxy monomers from the van der Waals distance before curing to the covalent bond distance after curing, this process caused by volume shrinkage; On the other hand, when the epoxy monomer is polymerized, the ring on the monomer is opened, and the intramolecular covalent bond distance becomes similar to the inter-molecular van der Waals distance, resulting in volume expansion, partially canceling out the volume contraction caused by bonding. The overall result of both is to shrink the volume of the epoxy compound after curing, but the shrinkage is much less than the acrylate radical polymerization.

Another volume change factor in the polymerization process is the change in the entropy from the monomer to the polymer, that is, the change in the free volume of the monomer to the corresponding polymer, that is, the change in the bulk density of the molecules in the monomer and the polymer. Because the molecules are not completely packed in the polymer, there is a free volume between the molecules. Before the photocuring, the liquid monomer molecules are loosely free and have a large free volume. After photocuring, the acrylate monomer generates a three-dimensional network cross-linked polymer, which has many intermolecular crosslinking points, which obviously limits the movement of the segments, and the free volume becomes smaller, which also brings volume shrinkage.