1. Mobility and toxicity of photoinitiators
The FDA of the United States clearly stipulates that UV ink cannot be used for printing materials in direct contact with food. The European Radiation Curing Association proposed that some kinds of photoinitiators in UV ink will pollute the packaging after the migration of residues after the ink is cured. At the same time, the odor problem of photoinitiator also limits the application of UV ink in food and sanitary products. These problems have attracted extensive attention in the industry, and further reducing or eliminating the mobility, toxicity and odor of residual photoinitiators in UV inks has become a new research topic in the industry.
2. Poor adhesion to special plastic substrates
When UV ink is cured on a non absorbable substrate, the intermolecular force between monomers and prepolymers becomes covalent bond, the spacing is reduced, and the volume shrinks. In addition, the curing time is very short, and there is a large residual stress in the ink layer, which leads to low adhesion of the ink on PP, PE and other non-polar substrate substrates. The surface polarity of such materials is low, the monomer cannot be swelled, and the effective force between the cured ink film and the substrate cannot be formed. By selecting high functional monomers with low surface tension (such as ethoxy or propoxy modified acrylate) in the ink system, the wettability of the ink on the substrate is improved, and the stress and volume shrinkage of the ink are reduced; The selection of prepolymers with high molecular weight and low glass transition temperature, the use of prepolymers modified by special functional groups (such as chlorinated polyester acrylate), or the addition of adhesion promoters can improve the ink adhesion to varying degrees. However, such improvement measures often lead to poor curing performance and printability of ink. From the practical application situation, it is very difficult to make UV ink have good adhesion fastness to all substrates. In some special cases, in addition to improving the ink adhesion, the substrate must also be surface pretreated (such as corona treatment) or tackifying primer to improve the surface performance of the substrate.