1 Preface

In the 1940s, powder coatings, as an environmentally friendly product, brought new opportunities for the development of coatings due to their advantages of solvent-free evaporation, easy recycling, and low pollution during the coating process. With the continuous improvement of process conditions and the increasing market requirements, powder coatings have made significant progress in recent decades due to their unique advantages of high automation and production efficiency.

Transfer printing technology began in the 1970s and was introduced to China in the late 1990s, mainly including two processes: water transfer printing and heat transfer printing. Thermal transfer printing technology is a process in which patterns and colors are realistically transferred onto materials such as metal, ceramics, glass, and plastic through sublimation ink under heating conditions. In recent years, it has been widely used in decorative powder coatings.

Pearlescent pigment refers to a decorative pigment with a pearl luster, which is composed of mica, flake quartz, flake alumina or flake glass powder as the matrix material, coated with a high refractive index transparent film of metal oxide on its surface. When light shines on the smooth interface of the pearl pigment, it produces reflection and refraction phenomena, and they interfere with each other to produce a three-dimensional pearl color. Pearlescent pigments are widely used in various fields, including automotive industry, coatings, plastics, leather, ink printing, ceramics, cosmetics, toys, rubber, decorations, etc., due to their excellent characteristics such as strong luster, good decorative effect, non toxicity, light resistance, heat resistance, and good chemical stability. 3D pearl pigments have good dispersibility and stability. Adding an appropriate amount of pearl pigment to powder coatings can form a coating with pearl effect. If the pattern is penetrated into the layer 40-60μ through transfer printing technology; Internally, while ensuring that the transferred image has bright colors, clear and rich layers, and the effect can be comparable to printing, it also guarantees a more abundant flash effect of the product.

Early heat transfer powder coatings were mainly based on polyurethane/isocyanate systems, but the high cost of curing agents limited the application of transfer technology in powder coatings. The polyester/TGIC system has excellent weather resistance and comprehensive performance, which not only meets the technical requirements of heat transfer printing, but also has been widely used in transfer powder coatings due to its low cost.

2 Experimental section

Weigh various raw materials such as resin, pigments, fillers, and additives according to the formula design requirements, and obtain powder coating samples through pre mixing, melt extrusion, crushing, and screening. Then spray prepare the coating sample. Using flat plate heat transfer technology to transfer the pattern onto the coating, the heat transfer conditions are as follows: 180s@200 ℃.

In addition, various raw materials such as resin, pigments, fillers, and additives are weighed according to the formula design requirements, and the powder coating base powder is obtained through pre mixing, melt extrusion, crushing, and screening. Adjust the temperature and speed of the bonding machine, mix the pearl pigment and base powder using the bonding method, and then transfer the pattern onto the pearl coating film through the heat transfer printing process. The heat transfer conditions are as follows: 180s@200 ℃.

The resin used in the formula test is provided by Guangzhou Qingtian Material Technology Co., Ltd., with polyester material characteristics

3 Results and Discussion 3.1 Influence of Resin Acid Value on Transfer Printing Effect

The crosslinking density, curing degree, and resin Tg are important factors affecting the transfer effect during the transfer printing process. This experiment selected two high acid value polyester resins for testing, among which NH-8081 polyester has a higher Tg (about 67 ℃) and NH-8001 polyester has a lower Tg (about 62 ℃), and their reaction activities are different. The experimental results are shown in Table 2:

The influence of polyester resin characteristics on transfer printing effect

Experiments have shown that a formula system with high acid value and high resin cross-linking density can effectively solve the problem of heat transfer printing; Tear off paper; The problem is that the transferred texture is clear and the residual imprint on the layer is greatly reduced. If high acid value resin is used alone, the leveling effect of the coating is not good. By adding an appropriate amount of low acid value resin with moderate reactivity, the leveling effect can be adjusted appropriately, and the transfer effect can also be improved. If the reaction activity is low, there may be a problem of incomplete curing, and thermal transfer printing often results in serious paper sticking. The experiment found that while ensuring a high cross-linking density of the coating, it is necessary to ensure that the coating is fully cured in order to obtain satisfactory transfer printing effect.

3.2 The influence of wax powder on wood grain transfer printing

Adding a certain amount of low molecular weight wax powder to the formula of aluminum profile powder coating can improve the coating's scratch resistance, anti caking and extinction properties. This experiment combines the surface tension analysis of wax powder to study the influence of wax powder on transfer printing effect. The experiment is shown in Table 3.

The experimental results of the influence of wax powder dosage on the transfer printing effect in Table 3 show that the interfacial tension plays a significant role in the transfer printing effect of the coating during the thermal transfer printing process. The smaller the surface tension of wax powder, the better the transfer printing process; Tear off paper; The better the effect, the more hydrophilic the substance that produces adhesive paper from transfer paper or transfer film. With the increase of wax powder dosage, the phenomenon of transfer paper sticking is significantly improved, and the texture clarity of the transfer pattern decreases. Therefore, selecting the appropriate wax powder and controlling its dosage can effectively improve the transfer paper adhesion and texture effect.

3.3 The influence of additives on wood grain transfer printing effect

The function of leveling agent is to reduce the melt viscosity of the coating, improve the fluidity of the melt, and migrate to the coating when heated, resulting in a coating with better leveling. The amount of leveling agent not only affects the leveling effect, but also has a significant impact on the transfer printing effect. The experiment found that reducing the amount of leveling agent can reduce the phenomenon of paper sticking during the transfer printing process; Excessive increase in leveling agent, as it migrates to the surface during the melting and solidification process, resulting in excessive aggregation of low molecular weight polymers on the surface, significantly exacerbating the transfer effect and paper sticking phenomenon. Wetting accelerators have little effect on the surface of the coating and have little impact on improving the transfer printing effect.

3.4 The influence of bonding temperature on pearl effect

Using the superior resin transfer printing formula mentioned above, transparent powder coatings were prepared through a melt extrusion process without adding fillers, and bonded according to the conditions in Table 4.

Table 4 shows the effect of bonding temperature on coating heat transfer printing. It can be concluded from Table 4 that bonding temperature has a significant impact on the pearl effect. When the bonding temperature is lower than the powder glass transition temperature, the powder particles do not soften enough, the surface adsorption force is small, and the amount of pearl pigment adsorbed on the surface is also low; When the bonding temperature is too high, due to the temperature reaching above the glass transition temperature, some powder will gel, affecting the spraying of powder coating; When the temperature is slightly lower than the glass transition temperature, the powder softens well, with more pearl pigments adhering to the surface, and the bonding effect is correspondingly improved

The influence of 3.5 wax powder on pearl effect

Usually, a certain amount of wax powder is added to the powder, which not only provides excellent surface properties such as wear resistance, scratch resistance, anti sticking, and increased slip, but also improves technical issues such as paper tearing during the transfer printing process.

Figure 1a shows the pearl effect of a coating film with a certain amount of wax powder added. It can be seen that wax powder, as a low molecular weight polymer, migrates to the surface of the powder during the curing process, affecting the pearl effect of the bonded powder coating. Figure 1b shows that the absence of wax powder has a significant impact on transparency and pearl effect. Therefore, based on Table 2, wax powder has a good effect on transfer printing, and a small amount of wax powder is beneficial for transfer powder coatings.

Figure 2 Transfer printing effect board after adding pearl

(a: ordinary pearl; b: 3D pearl; c: Ordinary pearl transfer printing; d: 3D pearl transfer printing)

From Figures 2c and 2d, it can be seen that transfer printing technology can allow patterns to penetrate into the interior of the coating, while pearl pigments float on the surface of the transfer coating, resulting in a transfer image with bright colors and clear and rich layers. Figures 2a and 2b illustrate that the scintillation effect of ordinary pearl pigment is not as good as that of 3D pearl pigment. In Figure 2a, ordinary pearl pigment is in a flat layer without scintillation effect, while 3D pearl pigment has better scintillation effect.

4 Conclusion

With the continuous demand for the development of heat transfer printing technology in the market, especially the application of pearl materials in powder coatings, new opportunities have been brought to powder coatings, enriching the application diversity of powder coatings. By improving the formula of transfer powder coatings and enhancing 3D pearl bonding technology, we can meet the increasingly changing and growing needs of the market.