Abstract: This article introduces sublimation heat transfer printing technology and the application of polyurethane powder coating in sublimation heat transfer printing, and analyzes some factors that affect paper tearing in the transfer printing process.

1 Preface

Powder coatings are environmentally friendly, economical, efficient, and have excellent performance; Four E” advantage. With the increasing awareness of environmental protection among people, the application of powder coating is becoming more and more popular. Especially for art textured powder coatings, their unique decorative effects and beautiful appearance are highly favored by people. In recent years, transfer printing technology has been applied in powder coating, allowing us to freely obtain various complex texture patterns. The emergence of transfer printing technology has a milestone significance in the research history of art texture powder coatings. Polyurethane powder coatings have high cross-linking density, excellent pattern transfer performance, and better flatness. The combination of polyurethane powder coatings and heat transfer printing technology has become mainstream in Europe, and can be seen everywhere in buildings and home decoration. However, in Asia, due to patent protection and the monopoly of certain materials, polyester/TGIC systems are the main choice for heat transfer powder. However, in certain special applications, polyurethane powder must still be selected.

2 Transfer printing technology

Transfer powder is another breakthrough in texture powder. It is easier to control the texture than traditional texture powders (colored powder, honeysuckle, dot pollen) and can obtain various complex patterns. Transfer printing can be divided into two types: heat transfer printing and water transfer printing. Water transfer printing is a transfer printing method that uses water as a medium to transfer colored patterns already printed on paper or special PVA film to the surface of a substrate. It is divided into two types: water stickers and water coating. Water transfer printing equipment is more expensive than heat transfer printing equipment and is rarely used in the powder coating industry; Thermal transfer printing is the process of first printing digital patterns onto special transfer paper using printing equipment, and then pressing the transfer paper onto the substrate to accurately transfer the patterns onto the surface of the substrate using high-temperature and high-pressure sublimation transfer technology. The methods of pasting transfer paper in heat transfer printing are divided into two types: adhesive stickers and vacuum stickers.

2.1 Heat transfer printing

Thermal transfer printing works based on the principle of sublimation. The substrate is coated with thermosetting material, and after complete curing, it tightly contacts the transfer paper. The printing ink is transferred to the substrate through sublimation at 150-250 ℃. Heat transfer printing initially used heat transfer technology to print the patterns on the transfer film onto the polyurethane powder coating surface of aluminum profiles, in order to achieve a decorative effect similar to wood grain or stone. It can quickly transfer the required wood grain or marble pattern to the powder coating surface of aluminum profiles within 3-5 minutes and can penetrate into the interior of the coating by 40-60 μ m; m。 Due to its excellent corrosion resistance, impact resistance, wear resistance, and aging resistance, it can maintain its color for 15 years when used outdoors; Excellent physical and chemical properties such as light weight and high strength; Clear and realistic patterns; Due to the false and realistic visual effect, aluminum profiles coated with heat transfer powder coating are widely used in industries such as doors and windows, furniture, indoor and outdoor decoration, and building materials. However, the curing agent of polyurethane powder coating is affected by technological monopoly and expensive, so China has applied transfer printing technology to polyester/TGIC and epoxy/polyester systems.

2.2 Heat transfer printing process

Now let's take the vacuum transfer printing paper process as an example to introduce the process of aluminum profile heat transfer printing.

(1) Cut the heat transfer film roll (purchased heat transfer film is 1000m/roll) into the required size using a film cutting machine.

(2) According to the size of the workpiece, the cut transfer film is welded into a workpiece wrapping bag using an ultrasonic packaging machine.

(3) Insert the workpiece that has been sprayed with powder and fully cured into a workpiece wrapping bag made of transfer film on the packaging platform.

(4) Place the workpiece wrapped with transfer film on the trolley platform of the heat transfer machine, and sequentially place 18 aluminum materials. Connect the two ends of the aluminum wrapping bag to the vacuum tap and lock it, turn on the vacuum switch, and make the transfer film tightly adhere to the aluminum material. The car automatically sends the aluminum material into the baking oven, which heats up to 220 ℃ and keeps it warm for 3 minutes. The car then automatically exits the baking oven.

(5) Release the vacuum tap and remove the workpiece. Inflate to separate the transfer paper from the workpiece, remove the transfer film, and check the quality of the workpiece.

2.3 Precautions for formula design of transfer powder

(1) The ratio of resin to curing agent is based on equivalent reactions to ensure complete curing of the coating.

(2) Do not add irrelevant wax powder to avoid blurring the transfer pattern.

(3) Add appropriate functional fillers to increase the hardness of the coating.

(4) Ensure that the coating surface is full and has good clarity.

2.4 Transfer Paper Tearing Technology

The polyurethane powder formula requires the use of two resins with different hydroxyl values to create micro unevenness on the coating surface, making it easier to tear off paper. Secondly, the coating cannot produce back adhesion, and a Tg of 80-110 ℃ is better. When the Tg is below 50 ℃, the coating softens during the transfer printing process (180-220 ℃), making it more difficult to tear off the paper; When Tg is higher than 130 ℃, the coating is too brittle and easily damaged. In addition, the hardness of the coating is also very important. Paper with low hardness is difficult to tear, and a hardness of at least 3N is required to tear the paper; If the hardness of the coating is too high, the pattern cannot be completely transferred to the surface of the coating (or it takes a long time for complete transfer), and the adhesion and penetration of the transferred pattern are also poor. Based on experience, the scratch resistance of the coating should be measured at a transfer temperature (such as 200 ℃) using the Oesterle 435 scratch resistance tester. At this point, the scratch resistance value of the coating should be higher than 8N, preferably above 11N. The upper limit of scratch resistance of the coating depends on the time of heat transfer printing, and the longest transfer time depends on the thermal stability of the coating. Generally speaking, the maximum scratch resistance value should be less than 30N.

Application of 3 Polyurethane Powder Coatings in Heat Transfer Printing

Polyurethane powder coating is a thermosetting powder coating composed of hydroxyl polyester, closed isocyanate, flow control agent, pigment filler, and additives. The reaction between hydroxyl groups (R-OH) and isocyanates (R-N=C=O) to form urethane bonds is considered a true polyurethane powder coating. Polyurethane powder coating has good physical properties and high cross-linking density, providing excellent water resistance, solvent resistance, weather resistance, strong oil resistance, and graffiti resistance, making it particularly suitable for pattern heat transfer printing.

Looking at the powder coating market in China, due to the unique application of polyurethane powder in the fields of heat transfer printing and outdoor low gloss powder, there is a trend of increasing application volume. Polyurethane powder can be co extruded with high and low hydroxyl value resins to obtain low gloss powder in one step, without fogging like polyester/TGIC wax low gloss powder, and without poor gloss reproducibility and hardness like polyester/GMA acrylic low gloss powder. Now Degussa Company has also developed special crystalline resin products, specifically designed for low gloss ultra weather resistant powders, specifically for heat transfer printing.

3.1 Polyurethane High gloss Powder Coating

Relatively speaking, polyurethane high gloss powder coating is difficult to tear off paper in heat transfer printing, and the tearing effect of this thermosetting coating composed of a single polyester is not ideal. The reference formula for polyurethane high gloss powder (curing conditions 200 ℃/10min) is as follows:

Material name, quality percentage

Hydroxypolyester (HV=30) 56.6

1530 (NCO content 15%) 8.5

Dibutyltin laurate 0.1

Leveling agent 1

Titanium dioxide (rutile) 22

Barium sulfate 14

Benzoin 0.4

Appropriate amount of pigment

This powder has a 60 ° C; The gloss is about 90%, and the coating is not easily separated from the transfer paper. So we often choose polyester with high and low hydroxyl values when designing formulas. If the difference in hydroxyl values between the two resins is not significant, the difference in gelation time will not be significant, and the separation between the coating and transfer paper will still be difficult. Through extensive experiments, we set the hydroxyl value of 75mgKOH/g for the hydroxyl polyester resin as the reference point, with>75mgKOH/g indicating a high hydroxyl value and<75mgKOH/g indicating a low hydroxyl value. We used these two types of hydroxyl value polyester resins to obtain coatings that are easy to separate from transfer paper.

3.2 Polyurethane semi gloss powder coating

Polyurethane semi gloss powder coating is relatively easy to tear off paper in heat transfer printing, mainly due to the non-uniform reaction caused by the high and low hydroxyl value resins during the curing reaction, resulting in a rough microstructure coating and extinction. Due to the micro unevenness on the surface of the coating, the transfer paper cannot fully adhere to the coating, making it easy to tear off. The particle size of the powder particles is controlled at 35 μ m; About m. During the baking process, the highly reactive powder particles react first, but they cannot achieve moderate agglomeration, forming a partially raised network on the surface of the melted coating. When less active powder particles crosslink, they contract on the formed network, resulting in network interpenetration extinction.

Polyurethane semi gloss powder coating is formulated with two different resins and one curing agent; Or formulated from one resin and two different curing agents; Or it can be formulated using two types of resins and two types of curing agents. Of course, polyurethane semi gloss powder coatings can also use the extinction filler method for extinction like other powder systems; Or add incompatible materials such as physical extinction agents for extinction; Or use powders with two different activities to mix and blend extinction, etc. But there is a fundamental difference between this co extruded blend and dry blend, the former has better repeatability. Polyester with a hydroxyl value of 30 and a hydroxyl value of 300 are usually mixed together, with polyester with a hydroxyl value of 300 accounting for 25% to 40% of the total polyester, resulting in lower gloss levels; When polyester with a hydroxyl value of 300 accounts for less than 10% of the total polyester, it only has slight extinction. Increasing the amount of polyester with hydroxyl value of 300 will shorten the gel time and lead to an increase in chemical resistance. At the lowest gloss level, there is a noticeable phase separation in the extruded mixture, which requires the addition of additives for optimization. These two types of polyesters, when combined with a polydiazobutanone curing agent, can produce a coating with a gloss of 10% to 15%, good leveling properties, excellent mechanical properties, and good solvent resistance.

The above is all the content shared by Yongji Industrial, a manufacturer of heat transfer powder coatings.