Adhesion of UV Ink

Adhesion refers to the binding force between the ink layer and the substrate, which has a significant impact on the performance of the ink. The wear resistance is not exactly the same. If the adhesion of the ink is not good, other properties will be difficult to achieve. Therefore, adhesion is one of the most basic indicators for evaluating the performance of printing ink. The adhesion is generally evaluated by the crosshatch method. Draw 10*10 small squares on the ink layer, adhere it to the ink layer with 600# adhesive tape, peel off the tape at 90° or 180°, and inspect. Whether the ink layer peels off the substrate, the adhesion is measured by the peeling off ratio of the ink layer in small squares.

There are many factors that affect ink adhesion, including pretreatment of the substrate, surface tension, ink viscosity and ink formula.

a. The cleanliness of the substrate surface seriously affects the adhesion of the printing ink layer. Especially when there are oil stains, paraffin, silicone oil and other release agents and silicone additives on the surface of the substrate, the surface polarity is very weak and hinders the ink from connecting with the substrate. Direct contact with the surface of the material will seriously reduce the adhesion. Surface treatments such as grinding and cleaning of this substrate can greatly improve adhesion. For porous or polar substrates, adhesion problems are easy to solve, and most conventional formulations can meet basic adhesion requirements. Certain commercial adhesion promoters are also very effective when added to formulations. Chlorinated resin can improve the adhesion to polypropylene materials; resins or small molecule compounds containing base, phosphorus (phosphine) acid group, play the role of "molecular rivets" in the printing ink layer in enhancing adhesion through chemical effects applicable to UV ink for metal substrates. However, polyethylene materials generally need to undergo flame spray treatment or corona discharge treatment to enhance surface polarity to ensure the adhesion of ink.

b. The adhesion of UV curing ink on the substrate has an important relationship with the wetting of the UV ink on the substrate, and the wetting of the UV ink on the substrate is mainly determined by the surface tension of the UV ink and the substrate, that is It is said that only when the surface tension of the UV ink is smaller than the surface tension of the substrate, good wetting can be achieved. If the wetting is not good, many surface defects may appear, and there will be no good adhesion. Therefore, if you want UV ink to have good adhesion on the substrate, you must reduce the surface tension of the UV ink as much as possible and increase the surface tension of the substrate. The surface tension of V oil can be reduced by adding surfactants. Different substrates have different surface tensions, and substrates are divided into polar and non-polar substrates. For substrates with low surface tension and difficulty in adhesion, chemical oxidation treatment, corona treatment, and light treatment can be used. Increase the polarity of the substrate, thereby increasing the surface tension of the substrate, to achieve the purpose of improving the adhesion between the substrate and the ink.

c. Adding a tackifier to the UV ink formula can improve the adhesion of the ink. However, the viscosity of the ink is too high, resulting in poor leveling of the ink and an excessive amount of ink, resulting in streaks and orange peel. On the other hand, temperature has an important impact on the viscosity of ink. The viscosity of UV ink changes with the ambient temperature. When the temperature is high, the viscosity of the ink decreases. When the temperature is low, the viscosity of the ink increases. Therefore, the production environment using UV ink Keep the temperature as constant as possible. Therefore, to improve the adhesion of ink, it is necessary to control the appropriate amount of added tackifier and to control the production environment.

d. To improve the adhesion of UV ink, it is also important to adjust the formula of UV ink. It is necessary to choose oligomers and reactive diluents with small volume shrinkage. The smaller the volume shrinkage, the smaller the internal stress generated during the photopolymerization process, which is more conducive to adhesion. The larger the molecular weight and lower the functionality of the oligomer, the smaller the volume shrinkage and the easier it is to produce good adhesion. The main function of reactive diluents in most formulations is dilution, but since reactive diluents also participate in photo-crosslinking reactions, they can also have an impact on the performance of the ink. The molecular weight of reactive diluents is generally small, and the double bond density per unit volume is greater than that of oligomers. The volume shrinkage during curing is often greater than that of oligomers, and the higher the functionality, the greater the volume shrinkage, which is important for adhesion. Adverse. However, the viscosity of reactive diluents is low, and some reactive diluents have low surface tension, which is beneficial to improving the wetting and spreading ability of the ink on the substrate, increasing the contact area between the two, and forming a strong interlayer force. , which helps to improve adhesion. Therefore, due to its small molecular weight, high double bond density, and large volume shrinkage, reactive diluents will have an adverse effect on adhesion; precisely because of their small molecular weight, their molecular mobility is higher than that of oligomers, and they can form a good effect on the substrate. Swelling, penetration and other effects can form a cross-linked network structure connected to the ink body in the shallow layer of the substrate during photopolymerization, thereby improving adhesion.

e. The lower glass transition temperature (Tg) of oligomers and reactive diluents is also beneficial to improving adhesion. The glass transition temperature is low and the main chain is softer, which is conducive to the release of internal stress. The cured ink layer will not deform or warp due to the accumulation of internal stress, which is conducive to improving adhesion.