1. Introduction

Powder coatings, often referred to as dry paints, are environmentally friendly products offering a wider variety of types and colors. We see powder coatings being used in more and more industries and fields. With the rapid development of the aluminum profile industry, powder coatings applied to aluminum profiles have become a specialized branch, gradually replacing electrophoretic paints and fluorocarbon paints. The emergence of transfer printing technology has further enriched the variety of powder coatings specifically for aluminum profiles.

Aluminum profiles require high weather resistance and decorative properties; therefore, surface treatment is essential. The main surface treatment for aluminum profiles is organic coating treatment, which includes powder coating spraying, fluorocarbon paint spraying, and wood grain finishing. Wood grain finishing is also done on top of powder coatings. Wood grain finishing technology is further divided into water transfer printing and heat transfer printing. This technology entered my country in the late 1990s and has seen rapid development in recent years. Its main characteristics are vibrant colors and rich detail in the transferred images, achieving an effect comparable to printing. Unlike printing, heat transfer involves sublimating the ink at high temperatures, allowing for multi-color patterns to be created in a single pass without the need for color matching. After sublimation, the ink forms a brightly colored pattern that can penetrate 40-60 micrometers into the powder coating. Therefore, heat transfer products are durable, and the patterns will not peel, crack, or fade. The original gloss, abrasion resistance, weather resistance, and acid/alkali resistance of the product are not altered by heat transfer; heat transfer only serves a surface decorative effect. The entire heat transfer industry has now developed into a complete supporting system, including the manufacture of powder coatings, spraying processes, heat transfer machines, transfer paper, and transfer film. The emergence of heat transfer technology has also presented new challenges to powder coatings, requiring that the powder coatings used be compatible with the heat transfer paper and film to achieve good transfer results. Therefore, exploring the heat transfer process of powder coating films and researching the formulation design of heat transfer powder coatings is essential.

2. Heat Transfer Technology

Heat transfer, also known as sublimation, is a special process that transfers any graphic, such as portraits, landscapes, wood grain, marble patterns, or 3D reliefs, onto high-precision paper using heat transfer ink. The paper is then heated to a specific temperature within a short time using appropriate heat transfer equipment, realistically transferring the image and colors from the paper onto materials such as aluminum profiles.

2.1 Transfer Paper

There are two main types of transfer paper: wood grain heat transfer paper and PET heat transfer film.

Both wood grain heat transfer paper and PET heat transfer film utilize the principles of heat sublimation and penetration to achieve decorative transfer. However, because the operating processes and transfer equipment differ, using PET heat transfer film offers higher production efficiency and is particularly suitable for transferring irregularly shaped or complex aluminum profiles.

2.2 Transfer Printing Process

The thermal transfer printing process for aluminum profiles is a simple procedure requiring less equipment and fewer operators, and is more efficient and lower in cost than water transfer printing.

The transfer printing process is as follows:

Selecting the substrate → Tightly wrapping the substrate with wood grain thermal transfer paper or PET thermal transfer film → Heating and transferring → Surface cleaning → Inspection → Packaging.

2.2.1 Application of Thermal Transfer Paper Process

⑴ Process 1 (Suitable for large-area and irregularly shaped substrates): Take the pre-coated aluminum profile, apply thermal transfer adhesive to the front of the thermal transfer paper against the front of the aluminum profile, bake in an oven (approximately 180℃, about 6 minutes), then peel off the thermal transfer paper, clean the surface, and the desired wood grain or stone grain will be transferred onto the aluminum profile.

(2) Process 2 (Suitable for large-area and irregularly shaped substrates): Take a pre-coated aluminum profile, place the front side of the heat transfer paper against the front side of the aluminum profile, and use a small amount of high-temperature resistant tape to seal the paper joint (cover with a high-temperature resistant PET bag and secure both ends of the bag; after vacuuming from both ends, place in an oven to bake (approximately 180℃, about 6 minutes). Remove the PET bag, peel off the heat transfer paper, clean the surface, and the desired wood grain or stone texture can be transferred onto the aluminum profile.

(3) Process 3 (Suitable for various flat aluminum profiles): Take a pre-coated aluminum profile. Place the front side of the heat transfer paper against the front side of the aluminum profile, heat with a dedicated flatbed heat transfer machine (approximately 180℃, about 20 seconds), peel off the heat transfer paper, and the desired wood grain or stone texture can be transferred onto the aluminum plate.

2.2.2 Application of PET Heat Transfer Film Process Process: (Suitable for large-area and irregularly shaped aluminum profiles) Take a pre-coated aluminum profile, place the PET heat transfer film face-to-face with the aluminum profile, wrapping the profile with a bag. Seal the PET heat transfer film into a bag using an ultrasonic sealing machine, leaving the ends of the bag unsealed. Secure the ends of the PET heat transfer film bag, vacuum-dry from both ends, and then bake in an oven (approximately 180℃, about 6 minutes). Peel off the PET heat transfer film; the desired wood grain or stone pattern will then be transferred onto the aluminum profile.

3. Special Powder Coatings for Aluminum Profile Heat Transfer

Both polyester/TGIC and polyurethane powder coatings used in the aluminum profile heat transfer process exhibit excellent pattern transfer performance.

3.1 Formulation Design of Special Powder Coatings for Aluminum Profile Heat Transfer

3.1.1 Choose a polyester resin with a lower softening point, but a glass transition temperature (Tg) of 80-110℃ is preferable. Based on the characteristics of heat transfer printing, some polyester manufacturers currently offer polyester resins specifically designed for heat transfer powder coatings.

3.1.2 The amount of curing agent added must be sufficient. A suitable amount of catalyst can also be added to ensure complete curing in one pass, preventing the coating from becoming sticky during the second pass and making it difficult to peel off the paper.

3.1.3 The leveling agent should not be added excessively, but good leveling is essential.

3.1.4 Heat transfer powder coatings do not require a base powder with high hiding power, so titanium dioxide should be added sparingly to ensure good fullness and brightness of the coating. 3.1.5 When comparing the colors of heat transfer paper or film, it is essential to select a comparison sample with the same texture and location, using the same transfer conditions, transferring it onto a substrate with the same base color, and comparing it under the same light source. Directly comparing the surface color of the untransfer-prepared paper or film sample will be inaccurate.

3.2 Heat Transfer Powder Coating Technical Formulation

Due to the cost of polyurethane powder coatings, despite their many advantages, they are not commonly found on the market. Most manufacturers produce polyester/TGIC and polyester/HAA powder coatings specifically for heat transfer on aluminum profiles. Below are the basic formulations for these three types of powder coatings.

3.2.1 Outdoor Matte Wood Grain Powder Coating Formulation

Polyester 600. XG665 matte curing agent 140. TGIC curing agent 1. PV88 10. Benzoin 4. Precipitated Barium 300. R-902 100. Polyethylene wax 7. Pigment as needed;

Thermosetting conditions: 200℃/10min Heat transfer conditions: 180℃/5min Gloss (60°): 3%

3.2.2 Outdoor High-Gloss Wood Grain Powder Coating Formulation

Polyester 600. 333 Curing Agent 32. PV88 9. 701 11. Benzoin 4. Precipitated Barium 300. R-902 100. Polyethylene Wax 7. Pigment as needed.

Thermosetting conditions: 200℃/10min Heat transfer conditions: 180℃/5min Gloss (60°): 90%

3.2.3 Polyurethane Matte Wood Grain Powder Coating Formulation

Hydroxyl Resin (High Hydroxyl Value) 200. Hydroxyl Resin (Low Hydroxyl Value) 460. B1530 310. Leveling Agent 20. Barium Sulfate 270. R902 240. Other Additives 30. Pigment as needed.

Thermosetting conditions: 200℃/10min Heat transfer conditions: 180℃/5min Gloss (60°): 6%

4 Factors Affecting Texture Clarity

4.1 Effects of Physical Matting Agents KI1099. SA206. and 3329 on Wood Grain Clarity

Physical matting agents KI1099. SA206. and 3329 have a certain impact on wood grain texture, especially the amount added; the higher the amount added, the less clear the wood grain texture becomes.

4.2 Effects of 6600A Polyester and Catalyst Combination on Wood Grain Clarity

The type and amount of catalyst added have a significant impact on wood grain clarity.

4.3 Effects of Filler Addition on Wood Grain Clarity

The amount of filler added also affects wood grain clarity, but more is not necessarily better.

4.4 Effects of Polyester Resins from Different Manufacturers on Wood Grain Clarity

Polyesters from different manufacturers have an impact on wood grain clarity.

4.5 Effect of Hydroxyalkylamide (HAA) Hardener on Wood Grain Clarity

The amount of HAA hardener added has a slight effect on wood grain clarity.

4.6 Effect of Wax Powder on Wood Grain Clarity

Different types of wax powder have little effect on wood grain clarity.

4.7 Effect of Filler Type on Wood Grain Clarity

Choosing different types of barium sulfate or calcium carbonate has some effect on wood grain clarity.

4.8 Effect of Degassing Agent on Wood Grain Clarity

The type and amount of degassing agent have little effect on wood grain clarity.

4.9 The Influence of Titanium Dioxide Addition on Wood Grain Clarity

The amount of titanium dioxide added has a significant impact on the clarity of wood grain; it should be added in smaller amounts.

The conclusions above regarding the influence of the ten materials on the clarity of wood grain in heat transfer powder coatings are derived from experiments and production practice. However, due to limitations in production processes and experimental conditions, as well as variations in the quality of the selected materials, the conclusions presented may not be entirely accurate and are for reference only.