Vacuum coating UV primer UV paint thinner photoinitiator curing degree

In recent years, due to the environmental protection, quick drying and energy-saving advantages of UV-curable coatings, their application fields have become increasingly extensive. UV coatings have excellent surface smoothness and are very suitable as primers for vacuum metal coating technology. They can achieve a very shiny metal appearance on plastic substrates. With the development of the automotive industry, many metal substitution processes have been applied. However, only by clearly understanding the influencing factors of vacuum coating UV primer can one better utilize its value.
In recent years, due to the environmental protection, quick drying and energy-saving advantages of UV-curable coatings, their application fields have become increasingly extensive. UV coatings have excellent surface smoothness and are very suitable as primers for vacuum metal coating technology. They can achieve a very shiny metal appearance on plastic substrates. With the development of the automotive industry, many metal substitution processes have been applied. Currently, in the application field of automotive lamp reflectors, vacuum aluminum coating films have been fully adopted on the surface of PC and BMC plastics to improve the reflection effect. The UV coatings for this field require good leveling property and high-temperature resistance (for PC substrates, the required temperature is above 120℃, and for BMC substrates, it is above 180℃).

Then, why is UV coating applied to the surface of plastic substrates before vacuum coating? The main reasons are as follows: First, through UV coating to seal the substrate, preventing volatile impurities in the substrate from escaping during UV vacuum electroplating or during the use of the workpiece, which will affect the coating quality. For example, in the application of automotive lamp reflectors, since the temperature during use will rise to above 100℃, if there are volatile impurities in the substrate, the aluminum coating layer will discolor, change color, and affect the reflection effect. For example, at a temperature of about 90℃, there is a significant thermal flow change in PC substrates, which can be considered as the release of substances, and its glass transition temperature Tg is approximately 140℃. The usage temperature of this material should be lower than this temperature. Second, to improve the surface flatness of the substrate, ensuring the achievement of a mirror-like coating effect. Generally, the plastic surface has a roughness of about 0.51nm, the thickness of vacuum coating does not exceed 0.2μm, and it cannot fill the unevenness of the substrate surface, resulting in an unsatisfactory mirror-like effect. UV coatings with a thickness of 10-20μm have a surface flatness of less than 0.1μm, which can fill the defects of the substrate and achieve the desired coating effect.

The UV coatings applied to automotive lamp reflectors must have the following basic characteristics: (1) Good leveling property, thick and shiny paint film, which can ensure a complete reflection film after vacuum coating. (2) The coating has a sealing effect, which can ensure that the escaping substances in the substrate do not affect the coating at the limit high temperature. (3) The coating itself has certain heat resistance, and there will be no substance decomposition and impact on the coating in the required high-temperature environment.

Through relevant experiments, it has been proved that the type of oligomers has a significant impact on the leveling property of the coating. Some low-viscosity and low-gamma oligomers have particularly good leveling property. Their high functionality makes their photopolymerization speed much faster than other two-functional aliphatic polyurethane acrylic oligomers, such as Desmolux2265. The experiments also proved that the type of oligomers has little effect on the heat resistance of UV coatings.

Changing the type and ratio of active diluents will affect the adhesion of UV coatings to plastics. The effect of active diluents mainly lies in their erosion ability on the substrate. The active diluents slightly swell or soften the plastic substrate, forming a cross-linked network between the interfaces, which can improve adhesion. With the decrease in the amount of multi-functional active diluents, the adhesion between the coating and the coating is greatly affected. During the vacuum coating process, the evaporated aluminum easily forms high-energy particles, which can easily penetrate the superficial layer of the coating, blur the phase boundary, and make the coating tightly adhere to the substrate. When the amount of bifunctional and monofunctional diluents in the UV coating increases, the crosslinking density of the coating decreases. During the vacuum coating process, some released substances and unreacted components of the coating in the substrate will reach the coating surface under the vacuum effect, weakening the effect of the high-energy particles, and the coating will not have good adhesion on these uncoated substances, thus showing a decrease in coating adhesion.

In addition, the effect of active diluents on the thermal resistance of the coating is determined by the degree of functionality of the active diluents. The main reason is that when other components are fixed, the higher the degree of functionality of the active diluents, the greater the crosslinking density of the coating, and the better the sealing effect on the substrate. When heated, the released substances in the substrate are difficult to react with the coating through the coating and the coating, resulting in changes in the appearance of the coating. From this perspective, while ensuring the other properties of the coating, we should try to increase the crosslinking density of the coating to obtain a better sealing effect.

Furthermore, the curing degree of the coating will also affect the performance of the coating. The conversion rate of double bonds during the UV coating process cannot reach 100%. Through infrared spectrum analysis before and after the coating curing, the changes in the content of double bonds can be observed. The stretching vibration absorption peaks of carbon-carbon double bonds are very obvious at 40875px-1 and 20230px-1. In the spectrum after curing, the absorption peaks of carbon-carbon double bonds at these two positions are greatly weakened, but do not disappear. When we increase the exposure of the coating from 350mJ/cm2 to 2100mJ/cm2, this absorption peak still exists. Therefore, we must know at what degree of coating curing the impact on the coating performance is minimized.
With the continuous improvement of UV coating curing, the hardness of the coating will become better and better. As the crosslinking continues during the UV coating curing, the hardness of the coating has a significant change under certain curing conditions. After this, increasing the irradiation energy does not significantly increase the hardness of the coating. Therefore, we must find out at what degree of coating curing the impact on the coating performance is minimized.

Another point is that: when using a mixed light initiator system to obtain the coating, the coating obtained after vacuum coating has better thermal resistance.

In summary: When designing the UV primer for the reflector of the vehicle lamp, attention should be paid to increasing the crosslinking density of the coating to ensure good sealing of the coating. By adjusting the ratio of active diluents in the formulation to make the adhesion between the coating and the substrate, between the coating and the coating, and the adhesion between the workpiece structure and the coating reach the best, special attention should be paid to ensuring that the parts with insufficient UV irradiation can also be well cured.