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Properties of UV paint materials, an introduction to coating, and defect analysis 1. Preparation of UV paint: Depending on the different types and models, UV paint uses various prepolymers such as epoxy acrylate, acrylate polyurethane, unsaturated polyester, acrylate polyester, and acrylate ether. All of these contain double bonds. During the preparation process, despite differences in the origin of the raw materials and their proportions, as well as variations in process parameters such as temperature and time, a very common issue is that an exothermic reaction occurs as the molecular weight increases during the synthesis of the prepolymer; in other words, polymerization easily takes place via free-radical polymerization, leading to gelation. Therefore, during mass production, it is essential to control exothermic reactions. a. The diluents are usually active monomers that participate in the photopolymerization cross-linking reaction during the curing of UV paint; some also contain a small amount of non-reactive diluents used to adjust viscosity. The density of the coating is related to the number of double bonds and the amount used, while different types are associated with the shrinkage rate of the coating film. b. Initiators such as dibenzoyl BP and benzoin butyl ether generate free radicals under light exposure to initiate the cross-linking reaction. c. If the additive is required to be colored, some require dyes. The preparation of UV paint is simpler than that of ordinary paint; it does not require three-roll grinding, and it mainly involves a mixing process, similar to that used in the production of varnishes, even for colored UV paint. It is also convenient because transparent dyes are used instead of pigments. That is, mix the prepolymer, thinner, initiator, and additives in a mixing tank by adding them while stirring until a homogeneous mixture is obtained. It should be noted that the mixture must not be exposed to direct sunlight, as UV paint is extremely sensitive to ultraviolet rays. Meanwhile, during storage, the temperature is generally maintained between 16 and 28°C; high temperatures cause self-polymerization, while low temperatures lead to separation of the various components. 2. Coating: Pre-treatment. Any coated product requires surface treatment before painting; the method of surface treatment varies depending on the type of substrate. After plastic parts are injection-molded, burrs must be removed first, the surface is cleaned with a cleaning agent, rinsed with water, dried, and then sent to the coating workshop. During this period, it is particularly important to avoid direct contact with the product surface using hands, as sweat and dirt on the hands are difficult to remove completely; therefore, gloves must be worn throughout the process from injection molding to spraying. Painted plastic products have a very high volume resistivity, which makes them prone to absorbing dust from the air. Only by eliminating or reducing static electricity can the dust on their surface be blown away, thereby improving the quality of the coating. a. For spraying, select the appropriate type of UV paint as required, and adjust it to the right viscosity for spraying using thinner. It is usually 10-14"/15℃. The IR drying tunnel evaporates the inactive solvents in the uncurated paint film through preheating; infrared lamps are generally most effective for this purpose. The pre-drying parameters of 60°C for 2–3 minutes can be adjusted as appropriate. b. UV curing is the critical stage during which the cross-linked resin changes from a fluid state to a solid state; it is the process of free radical polymerization and inter-chain bridging. UV lamps are currently available in China by companies such as Blue Sky Company. UV rays are high-frequency radiation with wavelengths ranging from 200nm to 400nm. In this range, not only light but also heat energy is present; therefore, cooling devices such as ventilation systems, heat-insulating filter glass, and mirrors are necessary as auxiliary measures, which ensures great safety for plastic products. Most plastics have low hardness; their surfaces are prone to wear and scratches, and they cannot withstand high temperatures. Therefore, the use of UV paint is highly ideal. The most successful application of this technology is in the UV-cured, wear-resistant hard coatings for polycarbonate (PC) front lamp lenses, which were first developed in Japan before being introduced to the United States and Europe. Many lighting manufacturers in China now use this technique on PC materials, which significantly reduces the need for heating-based curing processes and greatly improves resistance to scratches. The hardness of UV paint exceeds 3H. Similarly, UV paint is also used for coating decorative elements and protective panels, thereby enhancing the quality of the products. To protect the polished surfaces of aluminum-plated products, UV painting is employed as a protective layer, **extending their service life. 3. Defect analysis: Generally, UV paint has good adhesion to ABS, but its adhesion to PC and BMC materials is relatively poor, which presents some tricky problems. (1) The edges of the irregular-shaped products have cracks and white spots. It is likely that the UV paint does not have enough energy and time during the IR pre-baking stage to allow the solvents contained in it to evaporate; if these solvents remain undissipated in the paint film, white spots will appear. If they evaporate during the photopolymerization stage, it will cause cracks. (2) The surface of the product’s UV paint is not dry and lacks sufficient hardness. This indicates that the UV light energy is insufficient, preventing the polymer chains from fully cross-linking. It is necessary to check the illumination level; the power of the UV lamps should be at least 80 W/cm. Additionally, attention should be paid to adjusting the angle of irradiation so that all areas coated with UV paint can reach a solid state. In the case of products that require an aluminum coating, if the UV paint does not cure properly, the surface of the aluminum coating will appear dull, reducing the quality of the product. (3) On the contrary, if the UV paint film becomes brittle, it is often due to excessive light intensity, which causes aging of the polymer chain structure; this can be resolved by adjusting the printing speed or reducing the light intensity. Another factor is that the UV paint film is too thick. (4) Low adhesion. One factor is that the UV paint used is not compatible with the substrate, and another is that the UV paint applied to the substrate is too thick, as the adhesion test is related to the thickness of the paint film. (5) Local scorching and pitting. UV paint contains solvents that cause swelling in thermoplastic materials; it is necessary to quickly move into the IR and UV ranges after spraying, unlike other paints which require a period of cooling before drying. (6) Small bubbles are generated. Visually, the painted surface features large transparent spots that float on the top of the paint film. This phenomenon is more noticeable during cold and humid winter conditions. We believe that low temperatures may cause the components within the UV paint to separate; raising the temperature or stirring can help eliminate some of these spots. Replacing the paint generally resolves the issue as well. It seems that the causes are complex, including the presence of water vapor. (7) The product after exiting the UV section emits an unpleasant odor, clearly indicating that the UV paint has not cured completely; this can be felt by pressing it with the hand. One way to reduce this odor is to decrease the spraying thickness; if that doesn’t work, then the formulation of the prepolymer, initiator, reactive monomers, and additives needs to be adjusted. Although UV paint has developed rapidly, its widespread use is not as extensive as that of solvent-based paints; as a result, the understanding of and solutions to the problems associated with its use in coating applications are not as advanced as those for traditional materials. In particular, once UV paint has cured, defects on the coated surface are difficult to rectify by sanding. Moreover, unlike traditional primers, intermediate coats, and topcoats, the adhesion between layers in UV paint coatings cannot be improved through sanding. UV paint is usually applied in a single coat, while some applications use a composite coating method that involves UV paint + vacuum coating + UV paint. At this point, UV paint often acts like a magnifying glass. This further exacerbates the coating defects, making them difficult to overcome. 4. Conclusion: To achieve good results in coating, it is necessary to understand the composition of UV paint and the properties of the materials used, as well as to be aware of environmental factors. Only by ensuring effective control over each step of the process can UV painting be carried out quickly and effectively. For a wide range of products, UV paint has proven to be highly effective.