Heat transfer printing technology has been applied to fabric transfer printing production for a long time. With the rapid development of technology, the application of heat transfer printing technology is becoming increasingly widespread. Thermal transfer printing can be divided into hot press transfer printing and sublimation transfer printing. This article explores the popular sublimation transfer printing in the powder industry. Transferring exquisite patterns onto powder coating to change the wood grain effect or other patterns on the metal surface has become a popular trend nowadays. The application of this green and environmentally friendly technology has changed people's dependence on solid wood and reduced excessive deforestation. This technology was jointly developed in the 1990s based on the market's pursuit of popular trends and successfully applied to profiles and various substrates.

1、 Fundamentals of Heat Transfer Printing Technology

1.1 Wood grain heat transfer printing process

Print sublimation dye ink onto transfer paper or transfer film, and then transfer the pattern of the transfer film/paper onto the powder coating through the combined action of vacuum, heat, and pressure.

1.2 Sublimation/Condensation

Sublimation is the process by which a substance changes from a solid phase to a gas phase, while condensation is the process by which a substance changes from a gas phase to a solid phase. High temperature causes solid dyes to sublime and detach from transfer paper or film, penetrating into the powder coating structure. After sublimation, the dye condenses from the gas and becomes a solid state through cooling, thus becoming a part of the powder coating.

1.3 Dye Ink

Solid ink sublimates into gas under heating conditions and can be transformed into a solid again under cooling.

1.4 Transfer paper or transfer film

A carrier for pattern design, printing, and transfer printing reproduction.

1.5 Substrate

Metal or other substrates that can withstand 200 ℃, including ceramics, glass, and high and low gloss powder coated decorative substrates.

2、 Application of heat transfer printing

Wherever there is a need for changes in appearance, there is transfer printing. According to market segmentation, powder heat transfer printing is widely used in the fields of construction, transportation, general industry, and home appliances, such as doors and windows, garages, decorative ceilings, store fixtures, shelves, display cabinets, countertops, sunshades, benches, tables and chairs, decorative platforms, handrails, home appliances, kitchenware, refrigerators, barbecue equipment, clocks, sunrooms, and luxury yacht interiors.

3、 Discussion on the mechanism and influencing factors of sublimation heat transfer printing of powder coatings

3.1 Transfer printing mechanism and process flow

Usually, substances are divided into four states, namely solid, liquid, gas, and plasma states. Figure 3 describes the transition between phases. Sublimation heat transfer printing uses the sublimation and condensation of dye ink to achieve the penetration and migration of dye molecules from transfer paper or transfer film to the powder coating of the substrate, thereby achieving pattern transfer and forming wood grain or other desired effects.

3.2 Factors affecting transfer printing

The effectiveness of transfer printing depends on various factors, with a focus on the pre-treatment of the substrate, powder coating and its curing degree, the degree of coating of the transfer film on the substrate, transfer temperature, pattern design, and ink quality. The following discussion focuses on the influence of resin, ink, and formulation.

3.3 The influence of resin on heat transfer printing

Resin, as an important chemical factor behind applications, largely determines the final performance. Specially designed high-performance functional resins have the following characteristics. Good resin made heat transfer powder coating should have easy to tear paper, precise reproduction, high clarity, highly stable gloss control, excellent chemical resistance and weather resistance, comprehensive stability performance to make the process simpler, non-volatile technology, extremely excellent flow smoothness, high cross-linking density, and meet the strict requirements of Qualicoat and GSB standards during transfer printing. In terms of resin parameters, it should have a high acid value or hydroxyl value.

3.4 The influence of dye ink on transfer printing effect

The depth of dye ink entering the coating determines the quality and stability of the pattern. At the same time, dye ink also greatly affects the color saturation, color fastness, and color retention time of wood grain or other transfer printing effects. Through analysis of the cross-section of the transfer printing template, it was found that the ink entered the powder coating to a depth of about 50%; The fullness of the transfer pattern reaches its optimal level, as shown in Figure 5. Of course, many other factors also affect the quality of transfer printing patterns.

3.5 Heat transfer powder coating formula

In the initial research and development stage of heat transfer printing, European powder coating engineers preferred hydroxyl terminated polyester resin and isocyanate curing agent for heat transfer printing, while the high and low hydroxyl one-step extrusion method became a classic low gloss solution with low gloss, high fullness, and smooth hand feel. With the learning and use of transfer printing technology by Chinese powder engineers, thermal transfer printing technology has sprung up like mushrooms in Guangdong, China, and formed Chinese powder transfer printing technology. Among them, high acid value outdoor polyester and medium hydroxyl terminated polyester, polyurethane curing agent and TGIC curing are the most popular

Albester 3310 (110 hydroxyl value) combines the advantages of TGIC system and polyurethane system, with high cross-linking density, easy tearing paper, high plumpness, and lifelike wood grain after transfer printing. In addition, highly innovative Chinese powder engineers have developed skin wood grain, GMA system, and super weather resistant wood grain.

3.6 Super weather resistant heat transfer printing

With the development of wood grain transfer printing, more and more wood grain profiles are being used outdoors. Intense ultraviolet radiation constantly tests the weather resistance quality of wood grain. At the same time, Qualideco in Europe has also introduced certification for wood grain powder coatings and wood grain transfer printing, requiring powder coatings to reach level 2 super weather resistance. Several transfer film (paper) manufacturers in Italy have also increased their research and development of super weather resistant transfer films. We have launched a series of super weather resistant polyester solutions, such as Albester 6140 (93:7 TGIC super weather resistant), Albester SilkyMat6520/6580 (TGIC/HAA dual system low gloss super weather resistant), and Albester SilkyMat6615/6620 (polyurethane high and low hydroxyl low gloss super weather resistant), all of which can meet the requirements of Qualideco/Qualicoat powder coating level 2. Figure 6 shows some of the products' Florida exposure results. At present, in the field of transfer film, only some color transfer films can meet the requirements of super weather resistance level 2, and upstream sublimation transfer ink manufacturers still need to overcome the level 2 weather resistance requirements of all popular color series.

The above is all the content shared by Yongji Industrial, a heat transfer powder coating company.