1. Radiation curing paint was developed in the 1960s. Because it has the characteristics that traditional solvent/water-based paints do not have, and is effective and low in pollution, it is favored by developed countries in Europe and the United States. my country pays great attention to the manufacture and application of this type of paint. There are more than 10 domestic production units to meet the application market of UV paint. Radiation curing is divided into ultraviolet curing and electron beam EB curing, the mechanism of which is to cause the coating film to form free radicals (or ions) to initiate polymer prepolymers and cross-link to form films through radiation. The two have both similarities and differences. In UV paint, the photoinitiator absorbs a certain amount of energy to generate free radicals to initiate polymerization, while EB curing uses electron beams of sufficient energy to make accelerated electrons and unsaturated radicals in the resin Group action generates free radicals (or ions) for polymerization. From the current application cost, the penetration rate of UV curing is much higher than that of EB.
UV paint light curing, with high production efficiency, small coating area, UV paint contains very little volatile solvent, so less VOC emissions, low pollution, low film forming temperature, can adapt to plastic, wood, paper and other substrates, The surface hardness of the coating film is >3H, but it also has its limitations. It is difficult to achieve consistency in complex products such as irregular shapes and concave cavities (caused by uneven luminous flux), and it is difficult to rework after the coating waste is produced. It is suitable for the surface coating of regular rotating body-shaped products.
2. Preparation
The preparation of UV paint, UV paint has various prepolymers due to different types and models, such as epoxy acrylic acid, acrylic polyurethane, unsaturated polyester, acrylic polyester, acrylic ether, etc. These all contain double bonds. In the preparation process, although the origin and proportion of the raw materials are different, and the process parameters, temperature, and time are different, a very common problem is: when the molecular weight increases during the preparation of the prepolymer Both are exothermic, i.e. unsaturated bond-by-radical polymerization occurs from gelation which readily produces the polymerization process. Therefore, it is important to control the exothermic reaction during large-scale production.
a The diluent is usually an active monomer, which participates in the photocuring crosslinking reaction when the UV paint is cured, and some have a small amount of non-reactive diluent, which is used to adjust the viscosity. The coating density is related to double bonds and consumption. Different varieties It is related to the shrinkage of the coating film.
b Initiators such as benzophenone BP, benzoin butyl ether, etc., generate free radicals under the action of light to initiate crosslinking reactions.
If the c additives are required to be colored, some must have dyes.
The preparation of UV paint is easier than that of ordinary paint. It does not require three-roller grinding. It is mainly a mixing process, similar to varnish manufacturing, even for colored UV paint. It is also convenient because of the transparent dyes used instead of pigments. That is to say, according to the prepolymer, diluent, initiator and auxiliary agent, in the mixing tank, stir and mix evenly while adding. The North and South Chao reminds you: Do not have an environment with direct sunlight, because UV paint is the most ultraviolet. At the same time, during the storage process, it is generally at 16-28 ℃. High temperature will cause self-polymerization, and low temperature will cause separation of components.
3. Coating
Pre-treatment Any painted product must be surface-treated before painting, but the base material is different, and the surface treatment method will be different. After the plastic part is injection-molded, it must be deburred first, and the surface is cleaned by cleaning agent and water. Dry and send to the paint shop. During this period, special attention should be paid to not directly touching the surface of the product with hands, because it is difficult to clean the sweat and dirt on the hands, so it is required to wear gloves from injection molding to spraying.
Due to the high volume resistivity of coated plastic products, it is easy to absorb dust in the air. Only by eliminating or reducing static electricity can the dust on the surface be blown away and the spraying quality will be improved.
A. Spraying Select the paint type of UV paint according to the requirements, and adjust to the appropriate paint viscosity with thinner. Typically 10-14"/15°C.
The IR drying tunnel evaporates the non-active solvent in the uncured paint film by preheating. Usually, infrared lamps are used for better results. The pre-baking process can be adjusted appropriately at 60°C/2-3min.
B. UV light curing is the key stage of crosslinking resin from flow to solid, that is, the process of free radical polymerization and chain bridging. UV tubes are currently provided by domestic blue sky companies. UV rays are high-frequency radiation rays with a wavelength of 200nm to 400nm. In this section, there is not only light but also heat energy, so cooling devices are required, such as ventilation, heat-insulating filter glass, reflectors and other auxiliary facilities. It is very safe for plastic products.
Most plastics have low hardness, the surface is easy to wear and scratch, and it is not resistant to heat baking. Therefore, it is very satisfactory to use UV paint. The most successful one is developed first by Japan, and then spread to the United States and Europe. Carbonate (PC) headlight lens UV curing wear-resistant hard coating, now many domestic lighting factories have adopted this process on PC, which greatly shortened the process of heating and curing in the past, and greatly improved the scratch resistance Improve the hardness of UV paint > 3H. Similarly, some decorative parts and guards are also covered with UV paint, which improves the grade of the product. service life.
4. Defect Analysis
Generally, the adhesion of UV paint on ABS is relatively satisfactory, while PC and BMC materials are relatively poor, and some thorny problems have also been exposed.
(1) There are cracks and white spots at the corners of special-shaped products. I am afraid that the UV paint does not have enough energy and time to volatilize the solvent in the IR pre-baking. When the remaining paint film does not evaporate, there will be white spots. If it evaporates in the light curing section, it will cause cracks.
(2) The surface of the UV paint of the product is not dry and the hardness is insufficient. This shows that the UV light energy is insufficient, and the polymer chains have not been completely cross-linked. The illumination should be checked, and the power of the UV tube should not be lower than 80w/cm, and attention should be paid to how to adjust the irradiation angle so that each UV paint surface can reach a solid state. It is a product that requires aluminized film. If there is insufficient curing of the UV paint, the surface of the aluminum film will be dark and the quality will be reduced.
(3) On the contrary, if the UV paint film is brittle, it is often caused by too strong light energy, which causes the aging of the polymer chain structure. It can be solved by adjusting the travel speed or reducing the amount of light. There is another factor that is caused by the UV paint film being too thick. Sincerely.
(4) Low adhesion. One factor is that the UV paint made does not match the substrate, and the other is that the UV paint sprayed on the substrate is too thick, because the adhesion test is related to the thickness of the paint film.
(5) Partial scorching and pitting. There is a solvent in UV paint, which has a swelling effect on thermoplastic materials. It is required to enter the IR and UV sections quickly after spraying, unlike other paints that have a proper drying section after spraying.
(6) Small bubbles are generated. From the appearance, the painted surface has large transparent dots floating on the surface of the paint film, especially during the low temperature and humidity period in winter. We analyze that the temperature of some paints may be low, which causes the components to separate in the UV paint. Or the way of stirring can eliminate some, and the replacement of paint can basically eliminate it. It seems that the reasons are more complicated, including moisture.
(7) There is an unpleasant smell coming out of the product after the UV section. It is clear that the UV paint is not fully cured, and it can be felt by hand. One method is to reduce the spray thickness to reduce the smell. If it still does not work, adjust it. Prepolymers, initiators, reactive monomers, additives and other formulations.
Although UV paint is developing rapidly, because it is less popular than solvent-based paint, the understanding and solution of its problems in coating are not as good as traditional materials. Sandpaper is used to overcome the problem. At the same time, the coating of UV paint is not like the traditional primer, middle coat and top coat. The adhesion between layers is improved by sanding. And UV paint is often finished by one coating, and some use the composite coating method of UV paint + vacuum coating + UV paint. At this time, UV paint often acts as a magnifying glass. It is difficult to overcome the coating defects.
V. Conclusion
To do a good job in painting, we must not only understand the composition of UV paint, the performance of product materials, but also understand environmental factors, so that each process can be effectively controlled, so that the UV paint can be painted quickly and well. Coating, UV paint has shown strong vitality.
