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 resins with isocyanates, and low gloss powder coatings can also be obtained by combining hydroxyl resins with isocyanates. As shown in Table 1, 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. The experimental and application results show that the optimal ratio of carboxyl terminated polyester to acrylic resin is 3:1-4:1, with a small amount of beta added- HAA, TGIC, DDDA (dodecanedioic acid), B31 and other reaction regulators are used to control the gloss and physical and mechanical properties of coatings.

By adjusting the formula structure and ratio, a 3% to 30% change in coating gloss can be achieved, resulting in a smooth and delicate appearance of the coating. It cannot be denied that there are still many problems with this system, such as unstable gloss in the range of 10% to 30% gloss, whether it is powder coating or painting construction.

Acrylic resin has a certain selectivity towards polyester resin, and if the variety is not selected properly, a smooth and greasy surface cannot be achieved.

In addition, the surface of the modified extinction system has poor scratch resistance and is prone to scratches. Therefore, this type of product is currently only recommended for general industrial product coating and is not recommended for powder coating products with high comprehensive performance requirements such as building aluminum. Given the aforementioned issues with polyester/acrylic extinction systems, foreign powder raw material markets have introduced dual reactive functional group polyester resins that simultaneously contain hydroxyl and carboxyl functional groups. These resins react with isocyanates and extinction acrylic resins, respectively, and adjust the coating gloss by adding acid reaction adjusters (such as TGIC, PT910/912, etc.).

Other improvement methods include using two resins, high gloss acrylic and matte acrylic, to adjust the gloss of the coating; Alternatively, a multiple curing system consisting of polyester resin with dual active functional groups and acrylic resin with dual active functional groups (epoxy and hydroxyl) can be used;

Alternatively, isocyanates can be used to crosslink the hydroxyl groups in polyester resins and acrylic resins, achieving extinction through multiple crosslinking curing reactions that coexist in the system. The optimization and improvement of the polyester/acrylic extinction system undoubtedly promote the expansion of the system from the matte system to the extinction system, improve the stability of powder and coating production, and significantly enhance certain properties of powder coatings. The negative impact of this is that the product formula design becomes more complex, and the performance of certain coatings or films may decrease or be lost. This type of powder coating has good bending resistance, impact resistance, weather resistance, heat resistance, and resistance to yellowing. But this type of powder coating can only be used as a low gloss coating, and the adjustable range of coating gloss is relatively narrow, usually not greater than 10% (60 ° gloss meter).

2.3 Polyester/TGIC Powder Coatings

The polyester/acrylic extinction system can undoubtedly replace polyester/isocyanate low gloss (below 10%) heat transfer powder coatings, significantly reducing the cost of low gloss sublimation transfer powder coatings below 10%, and effectively promoting the application of heat transfer powder coatings in China.

However, for heat transfer powder coatings with a gloss range of 10% to 40%, it has almost become a blind spot, and formula designers are helpless in this regard.

We conducted experimental research by drawing on the dry blend technology in composite powder coatings, and ultimately established a formulation design and application plan. The experimental formula for this type of product is shown in Table 4. Table 4 lists the optimal powder coating formula for coating gloss in the range of 10% to 30%. The key to the wax powder extinction formula lies in the selection of resin and the amount of wax powder added. The gloss of the coating can be adjusted within a wide range of 10% to 60%, but the extinction wax powder has a certain selectivity for polyester resin.

There are certain requirements for the extrusion process and equipment of powder coatings. As the amount of matting wax powder increases, the dry powder flowability and storage stability of the powder decrease. The advantage of wax powder extinction formula is that the coating appearance is smooth and flat, and fillers with high oil absorption, such as extinction barium sulfate, should be selected. Scheme I formula is a traditional TGIC dry mix system formula. The resin acid values in formulas IA and IB are relatively close, and the difference in crosslinking curing speed is small. Therefore, if no wax based matting agent is added, the coating gloss can reach about 30%;

Adding 1% to 2% mass fraction of extinction wax powder synergistically reduces the gloss of the coating (10% to 30%), although the amount of wax powder added is significantly lower than that of a single component extinction system.

The IA and IB formula systems require the selection of fillers with high oil absorption, such as matte barium sulfate. The high acid value resin used in Scheme I is Uralac P 2220 and P5500 (acid value 46-53mgKOH/g) from DSM company and CRYLCOAT&reg from Cytec company; 2414-0, 2490-2, 4420-0 (acid value 47-51 mgKOH/g) are representative. The characteristic of formula II is that the difference in acid value between the fast and slow components of polyester resin increases, and the difference in crosslinking curing speed increases exponentially. The acid value of polyester resin with low acid value in formula IIA has decreased to 25mgKOH/g, and the surface effect of the coating is very rough.

The advantage of Formula IIA is that it frees powder coatings from the troubles of wax powder and can be designed as a wax free formula, with no negative impact on surface coating. The decrease in acid value of low acid value polyester resin reduces the requirement for fillers in the dry mixing system, and the use of any type of filler has almost no effect on the gloss of the coating film.

So how to adjust the gloss of the coating of this type of extinction system? Experiments have shown that there is a significant difference in coating gloss between the IIB or IIC formula and the IIA formula when dry mixed, with the latter having a coating gloss that is more than 10% lower than the former.

This means that by adjusting the fast reaction group resin (i.e. mixing high and low acid value polyester resins), the goal of adjusting the coating gloss can be achieved. However, the curing of high acid value resin and TGIC in Scheme II is not an easy task, and a too fast reaction rate can easily lead to material gelation during the extrusion process, ultimately resulting in sandblasted products rather than matte products.

Therefore, the polyester resin with an acid value of 72-78mgKOH/g and used for TGIC curing in Scheme II is very rare. We hope that polyester resin manufacturers can carry out relevant research work. For systems that use extinction wax powder alone, both Scheme I and Scheme II can achieve a coating gloss of 10% to 30%, but each scheme's powder coating has certain advantages and disadvantages:

(1) The curing speed of all formulation systems is relatively slow, and the complete curing of the coating mainly depends on the slow reaction components. Among the extinction schemes in Table 4, Scheme I has a fast curing speed, Scheme II has a slow curing speed, and the extinction system using extinction wax powder alone has a medium curing speed. The curing speed is very important for heat transfer printing (especially for Formula A in the dry mix system, resins with acid values of 16-24mgKOH/g and ultra-low acid values should not be selected);

(2) The use of wax powder, Scheme II can be designed as a formula system without wax powder. For products that require spray coating, Scheme II is the best choice;

(3) The surface effect is very smooth and delicate for both Scheme I and the formula coating using only matte wax powder, but the formula coating surface for Scheme II is relatively rough.

3. Outdoor extinction system and thermal sublimation transfer printing

We have conducted in-depth discussions on the formulation, design ideas, and design schemes of extinction and matte systems for polyurethane (polyester/isocyanate) powder coatings, polyester/acrylic powder coatings, and polyester/TGIC powder coatings. So the application of outdoor low gloss powder coatings in sublimation transfer printing still needs to consider the following issues. 3.1 Powder coating cost

The cost and selling price of powder coatings are the main factors affecting the marketing of powder coatings. The maturity of the raw material market, supply situation, and the maturity of the powder coating system all affect the cost and selling price of powder coatings.

Under the same conditions of gloss and film-forming substance content, the cost of various systems is as follows:; Polyester/blocked isocyanate system>Polyester/acrylic system>Simple matting wax powder system>Scheme I>Scheme II.

The cost of polyurethane powder coating extinction system is 10000 to 20000 yuan/t higher than that of polyester/TGIC (Scheme II) extinction system, and the cost of polyester/acrylic extinction system is 2000 to 4000 yuan/t higher than that of polyester/TGIC (Scheme II) extinction system.

Obviously, the cost of powder coatings with different extinction systems varies greatly. When designing the formula, customer and market acceptance should be considered, but of course, resources are wasted due to the inability to pursue high performance. The cost of the polyester/TGIC (Scheme II) extinction system is the lowest.

3.2 Curing speed and detachment of paper (film) or adhesive paper

The most common problem in sublimation transfer printing is paper sticking, which has led to the emergence of the so-called 'powder raw material market'; Release agent& mdash; Powder coating additives.

The experiment shows that; Adhesive paper” Mainly related to the product system and the degree of coating curing. Polyester/&beta- The HAA system is the easiest product system to stick paper, and no matter what method is used, it cannot solve the problem of paper sticking.

Polyurethane system, polyester/TGIC system, and epoxy/polyester hybrid system all have; Adhesive paper” If the phenomenon occurs, as long as the coating is fully cured, the problem of paper sticking can be solved.

That is to say, "; Adhesive paper” It is a manifestation of incomplete curing of heat transfer powder coating, and the choice of wax powder is also related to the adhesive paper. As mentioned earlier, most outdoor extinction systems use low hydroxyl or low acid value resins with slow reaction rates, which have a very slow curing reaction with crosslinking agents.

The curing reaction rate between hydroxyl polyester resin with a hydroxyl value of about 30mgKOH/g and isocyanate is significantly lower than that of hydroxyl polyester resin with a hydroxyl value of 45mgKOH/g. Therefore, it is not recommended to use low hydroxyl value resins in polyurethane extinction heat sublimation transfer powder coatings.

The reaction rate of GMA acrylic resin, a polyester resin with an acid value of 24mgKOH/g, is slow, and the reaction with TGIC is very slow. This polyester/TGIC extinction powder coating must be cured at 200 ℃ for more than 15 minutes, otherwise serious problems will occur; Adhesive paper” Phenomenon.

The resin with ultra-low acid value (16-24mgKOH/g) has good extinction effect in dry mixed systems, but the curing speed is too slow (the gelation time is extremely long), and the powder coating cannot be completely cured under normal curing conditions, resulting in poor solvent resistance of the coating and some problems; Sticky” Phenomenon.

No matter how good the mechanical properties of the coating are, as long as the slow reacting components in the system cannot be completely cured, any“ Release agent; It's also useless, complete curing is a prerequisite for sublimation transfer printing.