Why does UV glue and MMA glue shrink after curing?

At present, most of the MMA glues and more than 95% of UV glues on the market are free-radical curing acrylic glues, which are characterized by relatively large volume shrinkage during curing.

The generation of volume shrinkage not only affects the dimensional accuracy of bonding, but also directly leads to a decrease in bonding strength. For the bonding of rigid materials, it is easy to produce striated voids or even cracks, and for flexible materials, it is easy to warp and deform.

Therefore, reducing or even eliminating the volume shrinkage during the curing process is of great significance for improving the dimensional accuracy and bonding strength of the bonding, and expanding the scope of application of UV and MMA adhesives.

So why does this type of glue have such a large shrinkage rate?

1. Changes in the distance between atoms caused by polymerization

Free radical polymerization of acrylic adhesives (including 2K MMA and UV glue), the main resin (oligomer) and diluent monomers used are all acrylic esters.

During the polymerization process, the acrylate monomer molecules that originally acted on Van der Waals forces became covalent bonds, and the distance between the corresponding atoms was shortened from 0.3 to 0.5 nm to 0.154 nm (as shown in the figure below), which was about half shortened .

In this way, the arrangement of atoms in the polymer is much tighter than in the monomer, resulting in volume shrinkage during the polymerization process. Therefore, the free radical reaction shrinkage rate is relatively large, such as the volume shrinkage of commonly used styrene and methyl methacrylate homopolymers at 14.5% and 21.3%, respectively. The volume shrinkage of acrylate monomers used for dilution in UV adhesives is generally above 8%.

2. The change in entropy produced during the process of monomer to polymer

Another volume change factor in the polymerization process is the change in 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 polymer.

Since the molecules are not completely packed tightly in the polymer, there is a free volume between the molecules. Before light curing, the liquid monomer molecules are in a loose and freely movable state, with a large free volume. After light curing, the acrylate monomer generates a three-dimensional network cross-linked polymer, which has many intermolecular cross-linking points, which obviously limits the movement of the chain segment, and the free volume becomes smaller, which also brings volume shrinkage.

So, what are the factors that affect the shrinkage rate? How to reduce the volume shrinkage rate of this type of glue? Please see the next decomposition.

references:

Wu Jianwei, Study on the shrinkage of UV-curable adhesives, Chemistry and Adhesion, 2005, 96-100

statement:

The above information is excerpted from the plastic industry public information for reference and communication only. If there is any infringement, please contact us and delete it as soon as possible.