1、 The influence of heat transfer resin on transfer printing effect

There are many types of commercially available polyester/TGIC heat transfer resins for powder manufacturers to choose from (as shown in Table 2), each with its own advantages and disadvantages.

Improving the adhesion of paper and surface oil printing during the heat transfer printing process, as well as the excellent leveling, color saturation, and aging resistance of the coating, have become the main factors in selecting suitable resins. According to experiments, there is a strong correlation between the quality of heat transfer coating transfer and the reactivity of the resin, because choosing low acid value resin can easily leave some uncured small molecules in the coating, which can stick to the paper during transfer and cause difficulty in tearing off the paper and oil printing.

High acid value resin is selected, and the resin and curing agent react fully. The coating can be fully cured, and the transfer paper tearing effect is good. Oil printing can be significantly reduced, as shown in Figure 1. For resins with the same acid value, choose resins with high flow and low viscosity. The transfer coating has excellent leveling properties.

2、 The influence of resin curing behavior on transfer printing effect

Polyester resin designed by ordinary manufacturers can fully cure at 200 ℃/10min, but there may still be paper sticking and oil printing during the transfer printing process, which limits the transfer printing effect.

It can be seen that when selecting resin, the curing impact of the resin does not mean that the resin reaction is sufficient and the crosslinking density is good. When choosing heat transfer resin, it is necessary to use resin with high cross-linking density, strong reactivity, and good coating density after reaction, such as Qingtian resin A.

As shown in Figure 2, we compared the curing behavior of Qingtian resin A, resin B, and resin C, which have good transfer printing effects.

It is known that Qingtian Resin A has a conversion rate of 64.64% at 180 ℃ and 87.85% at 200 ℃; Resin B has a conversion rate of 39% at 180 ℃ and 72% at 200 ℃; Resin C has a conversion rate of 34.33% at 180 ℃ and 65.56% at 200 ℃.

By comparison, it can be seen that the curing degree and cross-linking density of resin A are significantly better than the other two products under the same conditions. Under the curing condition of 200 ℃/10min, the coating can be fully cured, reducing the occurrence of secondary short curing caused by heat during high-temperature transfer printing of the coating, and the damage to the coating surface such as paper sticking and oil printing.

3、 The influence of the dosage of additives and pigments in the formula on the heat transfer printing effect

In the design of heat transfer powder coating formulations, leveling agents, brighteners, benzoins, and wax powders are usually added to improve the performance of the coating.

When the surface leveling of the coating is slightly poor during formula debugging, the experimenters usually increase the amount of leveling agent and wetting agent added to improve the leveling effect.

The addition of leveling agents, brighteners, and benzoin within an appropriate range through experiments has little effect on the transfer printing effect of powder coatings. Increasing the brightener will improve the clarity of transfer printing to a certain extent.

High end heat transfer powder coatings are generally semi transparent systems with full and bright colors. In the semi transparent state of the coating, the selection of pigments in the formula is very important and is easily overlooked by experimental personnel.

Firstly, pigments with uniform particle size, good dispersibility, rapid dispersion when heated in the coating, easy dispersion during curing, and strong coloring power should be selected. Secondly, the selected pigments should also have good heat resistance, light resistance, and weather resistance.

Due to poor heat resistance and uneven dispersion of pigments in the formula of heat transfer powder, pigment molecules usually migrate to the surface of the coating during transfer printing, resulting in sticky paper and oil marks, causing defects in the transfer effect.

The experimental results in Table 3 indicate that under the same conditions, the tear off and oil printing effects of the weather resistant and well dispersed Yellow 3950 and Yellow 10c112 yellow pigments are significantly better than those of the 420 yellow pigment.

The experiment shows that the fullness and clarity of coating transfer printing are closely related to the selection of pigments, and the influence of pigments must be considered.

Therefore, in the heat transfer printing system, the selection of pigments should first be based on the same type of pigments in formulas 1 and 3, which can greatly improve the transfer effect of the coating.

4、 The influence of wax powder and accelerator in the formula on the transfer printing effect

There have been discussions and studies on the formulation of heat transfer polyester powder coatings that are not prone to excessive addition of wax additives. There are literature references available, and the reaction principles are basically the same. Therefore, we will not elaborate further here.

However, it should be pointed out that the selection of matting wax additives is best done by adding wax powder that reacts with other components in the formula to dull the coating. In research, it has been found that improper selection can easily cause adhesion between transfer paper and coating paper. Therefore, careful selection should be made in the actual process.

Sometimes, in the actual formulation, a certain amount of accelerator is added to improve the reaction performance of the resin in the formulation, promote the full curing of the coating, and achieve the improvement of defects such as film adhesion and paper adhesion.

According to specific requirements, adding can effectively improve the transfer effect, but it cannot be excessive. A slight excess will seriously affect the leveling effect of the coating and affect the transfer pattern, as shown in Figure 3.

5、 Study on Resin Tg and Storage Properties

The curing behavior of TGIC in heat transfer polyester/TGIC powder coating will gradually decay over time under high temperature environment, resulting in orange peel and insufficient reaction of the coating, which affects the transfer effect. Therefore, its storage stability must be considered. The better the storage stability of the powder, the better the quality guarantee for the performance of the transfer product.

Figure 4 shows the Tg comparison of three resins, Qingtian Resin A (1), Resin B (2), and Resin C (3). It can be seen that Qingtian Resin A has a higher Tg than the other two resins and better storage performance.

Figure 4 shows a comparison of the anti agglomeration performance of three types of powders at 40 ℃/8h. Figure 5 shows the anti agglomeration performance of Qingtian Resin A. The powder is fluffy and has almost no agglomeration, which is significantly better than the other two types. Therefore, when selecting resin, it is preferred to choose resin with good anti agglomeration performance and high Tg to ensure the thermal transfer effect of the powder.

6、 Conclusion

In the past two years, the performance of resins that meet the requirements of heat transfer printing in the resin market has greatly improved, bringing unlimited opportunities to the transfer printing market and providing more possibilities for powder formula design. However, when designing heat transfer powder coatings, the following elements should be noted:

(1) Choose a suitable heat transfer resin, preferably one with high acid value, low viscosity, good leveling properties, and can ensure good transfer effect, such as Qingtian resin A, which can almost achieve the transfer effect of polyurethane.

(2) The amount of wax powder and accelerator added in the formula should be controlled within an appropriate ratio range to prevent insufficient curing from affecting the leveling of the coating, resulting in defects such as oiliness and paper sticking.

(3) Pay attention to the selection of pigments in the formula, choose pigments that are easily dispersible, heat-resistant, light resistant, and weather resistant, which can effectively prevent secondary migration of pigments due to heat and avoid coating defects.