Abstract: The article focuses on the film formation mechanism, coating system, and coating process technology of heat sublimation transfer printing wood grain powder and three-dimensional tactile wood grain powder. It is pointed out that these two types of outdoor low gloss wood grain powder coatings have broad application prospects in China.

1. Preface

Powder coating has the characteristics of; 5E” The advantages of non terminal application products have become very mature after more than half a century of development. As an important component of decorative powder coatings, heat transfer powder coatings are widely used in the construction industry, such as aluminum profiles, curtain walls, anti-theft doors, etc.

Currently, heat transfer powder coatings have also been successfully applied in decorative fields such as general industry and household appliances. As a three-dimensional wood grain powder coating that emerged after heat transfer printing powder coating, it has quietly developed in China. Wood grain powder coating was initially the most widely used type of heat transfer powder coating, but has now become a common name for heat transfer powder coating. The current wood grain powder coating is no longer simply a synonym for sublimation transfer powder coating, but also includes three-dimensional hand feel wood grain powder coating, and the connotation of wood grain powder coating has been extended.

Stereoscopic wood grain powder coating is a special type of multi coating powder coating.

The technology of heat sublimation transfer powder coating for indoor applications has become very mature in China, especially in the development and application of anti-theft door market, which has created a large number of indoor (epoxy/polyester type) heat transfer powder coating manufacturers.

However, the heat transfer powder coating technology for outdoor applications is still controlled by foreign-funded enterprises, especially for outdoor low light heat transfer powder coatings. Domestic enterprises have little research and development on three-dimensional wood grain powder coatings, let alone mass production.

Therefore, so far, the development of outdoor low gloss weather resistant powder coatings still troubles a large number of domestic powder manufacturing enterprises. This article will conduct in-depth discussions on outdoor low gloss wood grain powder coatings.

2. Comparison and Selection of Outdoor Low Gloss Thermal Sublimation Wood Grain Powder Coatings

In China, the main types of powder coatings used for outdoor applications are polyester/TGIC and polyester/beta- HAA type and polyester/acrylic type, as well as polyester/isocyanate type (commonly known as "polyurethane type") in foreign-funded enterprises.

In fact, there are far more products that can be used for outdoor applications. Based on the domestic market situation, this article only explores the application of the four chemical systems mentioned above in thermal sublimation transfer wood grain powder coatings. What is mentioned here; Low Gloss” It refers to 60º The gloss of the coating measured by the gloss meter is below 40%. The polyester resin used in this study was sourced from companies such as Cytec, DSM, and Anwei Shenjian; Acrylic resin originated from companies such as Dongguan Zhongtian and Ningbo Zhihua; The curing agent is sourced from companies such as Degusa, Bayer, Ciba, and Ningbo Nanhai; The additives are provided by Clariant, ESTRON, Ningbo Nanhai, Zhejiang Zhihua, Guangzhou Zehe, etc; The pigments are selected from companies such as Clariant, Ciba, and KLORMAX, and the fillers are provided by domestic enterprises. The extruder selected for the experiment is a domestically produced twin-screw extruder, the electrostatic spray gun is domestically produced, the small grinder is a domestically produced coffee or Chinese medicine grinder, the baking oven is a domestically produced electric heating oven, and the heat transfer machine is a domestically produced specialized hot stamping machine. The physical graphics and operation of the hot stamping machine are shown in Figure 1 and Figure 2.

2.1 Polyester/Isocyanate Powder Coatings

Commonly known as polyurethane powder coating, it mainly appears in the Chinese market as outdoor matte to high gloss sublimation transfer powder coating. The earliest resin base materials used in the formula were DSM's P-6504 (low to medium hydroxyl value) and P-1444 (high hydroxyl value) resins; The isocyanate curing agent used is Degusa's BF1530 or BF1400.

Now companies such as Cytec and Guangzhou Production Association Polymer can provide corresponding resins, and Bayer also has corresponding isocyanate curing agent products.

High gloss powder coatings can be obtained by combining hydroxyl functional polyester resin with isocyanates, and low gloss powder coatings can also be obtained by combining hydroxyl resin with isocyanates. The formula for low gloss polyurethane powder coatings by thermal sublimation is shown in Table 1. From Table 1, it can be seen that the formula for low gloss polyurethane powder coatings usually requires two types of polyester resins with high and low hydroxyl values. Adjusting the ratio of the two resins under a certain total amount of film-forming substances can reduce the gloss of the coating film, with a minimum gloss of about 5%.

This formula structure is significantly different from common powder coating formulas, and the system composition is relatively complex. The hydroxyl value of commonly used low hydroxyl value resins is 25-35mgKOH/g;

The hydroxyl value of low to medium hydroxyl value resins is 40-50 mg KOH/g, and the hydroxyl value of high hydroxyl value polyester resins is 280-320 mg KOH/g. Their formulation ratios with isocyanate crosslinking agents (such as BF1400) are 85:15, 80:20, and 40:60, respectively. Obviously, the higher the hydroxyl value, the more curing agents (blocked isocyanates or urea diketones) are used.

Low hydroxyl value resin is produced by Cytec's CRYLCOAT® Represented by 2890-0, 2891-0, 2845-0, 2920-0, 4833-0, the resin has a hydroxyl value of 30-35mgKOH/g.

Low to medium hydroxyl value resin with CRYLCOAT® E04060 is representative, with a resin hydroxyl value of around 50mg KOH/g. High hydroxyl value resin with CRYLCOAT® Represented by 2814-0 and E04076, the resin has a hydroxyl value of around 280mgKOH/g. Under the same dosage of high hydroxyl value polyester resin, formulations A and B require less curing agent due to the use of low hydroxyl value polyester resin, while formulations C and D use medium low hydroxyl value polyester resin, resulting in a significant increase in curing agent dosage.

The experiment showed that the 60 ° C ratio between formula A and formula C; The gloss is 15% to 25%, while formula B and formula D have a 60% gloss; Gloss is below 10% (no light).

Practical applications have also shown that the gloss of powder coating products prepared in the absence of high hydroxyl polyester resin is at least 60% or higher;

With the increase of high hydroxyl value resin dosage, the coating gloss rapidly decreases. When it reaches a certain level (bottom), the coating gloss actually increases, until the formula uses all high hydroxyl value resins and the coating gloss reaches another peak.

The mechanical properties of the coating will change with the amount of high hydroxyl value polyester resin used. Generally, the impact resistance and bending performance of the coating decrease with the increase of high hydroxyl value polyester resin used, while the hardness of the coating increases and the chemical resistance of the coating increases. In recent years, with the development of polyester/isocyanurate powder coatings, blocked isocyanates and internally blocked isocyanates curing agents have emerged in the market.

And a bifunctional curing agent with carboxyl and isocyanate groups coexisting (Crelan VP LS 2181/1 from Bayer, the ideal structure of the product is shown in Figure 3).

Powder coatings with a gloss of less than 10% can also be obtained by combining with crosslinking agents containing epoxy groups, such as TGIC or PT910/912. As there is currently no corresponding application of powder coating products in China, it will not be further elaborated here. However, the mechanism by which such systems achieve extinction or matte powder coating effects is very complex, and their applications are not as widespread as single systems. The basic requirements for application, such as transportation, production reproducibility, good mechanical properties, economy, etc., cannot fully meet the requirements of traditional processes.

A newly developed extinction method for polyurethane powder coatings has been successfully developed, which can adjust the gloss of the coating and has excellent leveling properties. This is a chemical extinction system that can obtain polyurethane powder coatings with different levels of gloss, and can improve the mechanical properties of the coating, with excellent leveling and weather resistance.

This new polyurethane extinction system uses amorphous hydroxyl functional polyester resin combined with special hydroxyl functional crystalline polyester resin, and the extinction effect is independent of the type of crosslinking agent (blocked isocyanate or urea ketone). Degusa Company has launched a special crystalline resin product EP-R4030, which has achieved industrial production and been launched on the market. This product is designed specifically for low gloss, ultra weather resistant powder coatings, with a resin hydroxyl value of 30 and a melting point of 110 ℃. When the addition amount accounts for 35% to 45% of the total resin in the formula, it can provide low gloss effect and flexibility (see Figure 4).

The low gloss (5%~25%) coating formulated with it has super weather resistance and good flexibility, and the coating gloss has excellent batch stability. As is well known, the use of a mixture of amorphous polyester and crystalline polyester can achieve excellent leveling and mechanical properties in powder coatings. However, the typical coating appearance of these mixtures is high gloss.

By using special crystalline resins, such as Degusa's crystalline polyester resin, it is possible to obtain dull to matte polyurethane powder coatings, effectively improving the mechanical properties of the coating and maintaining good storage stability. Figure 5 shows the use of crystalline polyester resin VESTAGON® The relationship between curing conditions and mechanical properties of polyurethane powder coatings formulated with EP R-4030 as a substitute for amorphous polyester resin. The designed super weather resistant polyurethane has a high impact resistance of 160 lb. in and a gloss variation range of 10% to 90%.

It has been found that the curing temperature and extrusion conditions have little effect on the gloss of the coating. The super weather resistant low gloss powder coating still maintained the majority of the coating gloss after 3000 hours of exposure test. The polyester/isocyanate powder coating extinction system has a very high cross-linking density and does not stick to paper during thermal sublimation transfer printing. Has good absorption capacity for sublimation ink and clear transfer patterns.

Therefore, from the perspective of transfer printing technology and effect, polyurethane powder coating is very suitable for sublimation transfer printing.

However, due to the exorbitant prices of isocyanate curing agents and high hydroxyl value polyester resins in the market, the price of low gloss polyester powder coatings has exceeded the acceptable level of the domestic market, making promotion very difficult and market share not high. However, many application areas of powder coating heat transfer printing still require polyurethane powder coating.

2.2 Polyester/Acrylic Powder Coatings

The technology of using carboxyl terminated polyester resin and GMA acrylic resin (with epoxy group) to prepare matte and weather resistant outdoor powder coatings is already very mature in China.

Dongguan Zhongtian's T-308 and Ningbo Zhihua's AR607 belong to this type of acrylic resin (see Table 2). The carboxyl polyester resin used to prepare matte powder coatings in combination with this type of GMA acrylic resin is mainly composed of resins with acid values of 33-38mgKOH/g (medium low acid value) and 24-26mgKOH/g (low acid value);

Polyester resins with an acid value of 16-24mgKOH/g are generally not recommended for use. The ratio of acrylic resin to carboxyl polyester resin in the formula is 1:3 to 1:4. GMA acrylic resin is a copolymer of acrylic esters containing glycidyl groups, with epoxy groups as the reactive groups. Its chemical structure is shown in Figure 6. The compatibility between this GMA acrylic resin and polyester resin is seriously insufficient. Although there is a reaction between epoxy and carboxyl groups in the product system, the final coating has a very low gloss (less than 2%), forming an outdoor extremely matte powder coating system.

In practical applications, not all products require such low gloss. People adjust the coating gloss by adding carboxyl or alkyl amides (i.e. DDDA, TGIC, or HAA). The coating formula is shown in Table 3.