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introduction
During the forming process of the coating of skin-feel furniture panels, it is necessary to use an excimer lamp or other pre-curing methods to make it produce an elastic wrinkled texture, that is, to produce a smooth and delicate touch similar to that of touching the skin [1]. Conventional skin-feel coatings currently on the market usually add a large amount of additives to achieve a physical matting effect, which results in a high viscosity and rich granularity of skin-feel ultraviolet (UV) curing (hereinafter referred to as UV) coatings, making them difficult to spray. In addition, a certain amount of chemical matting agent is also added to the skin-feel coating. During the spraying process, the atomized coating comes into contact with the air, which affects its chemical matting performance. Therefore, traditional skin-feel coatings are not suitable for spraying. The main coating method is roller coating, which can only produce large flat panels [2]. Therefore, the current production of skin-feel panels has problems such as complex process, low production efficiency, insufficient smoothness and fineness of the skin-feel effect, and environmental risks, which affect the promotion and application of skin-feel coatings in furniture panel finishing.
This study attempts to overcome the problem that skin-feel UV coatings cannot be sprayed due to the addition of additives, matting agents, etc. by adjusting the viscosity and spraying air pressure of skin-feel UV coatings. By limiting the spraying air pressure, the length of time that skin-feel UV coatings are in contact with air is controlled, thereby reducing the impact of the environment on the chemical matting properties of skin-feel coatings. In addition, ultra-matte skin-feel decorative panels for furniture manufacturing are developed to provide a reference for the production of ultra-matte skin-feel furniture panels.
1. Materials and Methods
1.1
Material
The physical and chemical properties of the ultra-matte, skin-feel finish are based on the functional properties of the finish coating. By applying an electron beam (EB)-cured topcoat over a UV primer, the coating can be enhanced with multiple functionalities. Testing of commonly used electron beam (EB)-cured (hereinafter referred to as EB) skin-feel coatings on the market revealed the following raw material composition and weight percentages for the EB skin-feel topcoat: 78% polyurethane acrylic resin, 16% reactive monomer, 3% additive, and 3% functional powder.
The UV resin system determines the gloss, surface effect, and physical properties of UV super-matte skin-feel coatings [3]. During the preliminary test, the performance of various UV resins was evaluated, and the UV resin with better performance was determined. Then, a defoamer, photoinitiator, thixotropic agent, wetting dispersant, and matting powder were used to prepare UV super-matte skin-feel coatings. A total of two UV super-matte skin-feel coating formulas were screened (see Table 1 for details). These two coating formulas were combined with excimer curing equipment to prepare UV super-matte skin-feel coatings with better performance and surface effects than conventional low-gloss system UV coatings.
Table 1 UV super matte skin-feel coating formula
The following is the selection method of UV super matte skin-feel coating:
1) UV resin screening. Monofunctional resins offer good dilution but slow curing speeds. Trifunctional resins offer poor dilution and yellowing resistance, but also exhibit high shrinkage. Considering the impact of UV resins on the viscosity, curing speed, wettability, and shrinkage of the formulated system, this test combined the performance advantages of monofunctional and trifunctional resins.
2) Defoamer Screening. Defoamers (organic silicone and polymer types) were screened and their effects on the defoaming properties and compatibility of UV coatings were analyzed. It was found that defoamers with good defoaming properties had high defoaming efficiency but poor compatibility, making them more susceptible to cratering. Therefore, this experiment selected an organosilicone defoamer with moderate defoaming efficiency and compatibility.
3) Photoinitiator screening. The performance of three photoinitiators (1173, 2-hydroxy-2-methyl-1-phenyl-1-propanone; the long-wavelength photoinitiator TPO-L, ethyl 2,4,6-trimethylbenzoylphosphonate; and MBF, methyl benzoylformate) was compared. Their effects on the UV coating's curing speed, base cure, surface cure, and yellowing resistance were analyzed, as well as their compatibility with the equipment within the overall curing system. Ultimately, a combination of MBF and a small amount of TPO-L was selected as the photoinitiator.
4) Thixotropic Agent Screening. The performance of three thixotropic agents (inorganic fumed silica, bentonite, and polyurea-type polymers) was compared to analyze their anti-settling effects and their impact on the viscosity and leveling properties of UV coatings. The UV coating system used in the experiment used excimer equipment for matting, resulting in a low proportion of fillers and matting agents in the system, making sedimentation easier to address. However, to achieve a better surface finish, high leveling properties were required. Therefore, a polyurea-type polymer with a low thixotropic index was selected as the thixotropic agent.
5) Wetting and dispersant screening. Three wetting and dispersant agents (polyether-modified polydimethylsiloxane solution, organosilicon gemini surfactant, and nonionic organic surfactant) were compared and their effects on the wettability, cratering, and foam stabilization of UV coatings were analyzed. We found that wetting agents with low surface tension had good wetting effects, but large additions were prone to causing cratering and foam stabilization issues. Therefore, a high-molecular-weight organosilicon gemini surfactant was selected as the dispersant in this experiment.
6) Matting agent screening. Screen matting agents and analyze their effects on the viscosity, gloss, and feel of UV coatings. Matting agents with a rough feel have high matting efficiency but also result in higher surface roughness. Surface-treated matting agents have better dispersibility. In this system, matting agents are primarily used to adjust the feel of the coating surface, so the tactile feel, defoaming properties, and oil absorption of the matting agent are of particular importance. Therefore, silica was selected as the matting agent for this experiment.
1.2
method
1.2.1 Coating process
EB coating curing is a process that uses electron beams as a radiation source to initiate resin polymerization or cross-linking, converting liquid into solid. Compared with UV curing, EB curing has better yellowing resistance and weather resistance, is not limited by color or coating thickness, has less energy loss, high energy conversion rate, no attenuation, and has the advantages of high cross-linking structure, high hardness, chemical resistance, wear resistance, and aging resistance [4]. Therefore, the skin-feel product finishing process adopts the technology of primer UV curing and topcoat EB curing, combining the advantages of both for production. The specific process flow is as follows:
1) Clean the substrate. Use a white cloth dipped in UV thinner (ZUX2014) to wipe the surface of the impregnated film paper veneer wood-based panel to remove stains. For any remaining stains, use a sponge to lightly sand them.
2) Spray UV Skin-Feel Primer 1. Apply UV Skin-Feel Primer 1 to the substrate obtained in step 1) using a spray method. A coating weight of 90-100 g/m² was used. The spray pressure was 0.5 MPa, and the nozzle was approximately 20 cm from the substrate. Spray in a zigzag pattern, with the reciprocating speed set at 3.0 m/min.
3) Curing of UV Skin-Feel Primer 1. Use a UV lamp for semi-curing, with a UVA lamp (wavelength range: 320-390 nm) providing a curing energy of 350-600 mJ/cm² and a UVV lamp (wavelength range: 395-445 nm) providing a curing energy of 1000-1800 mJ/cm². Alternatively, use a mercury lamp for semi-curing, with a UVA lamp providing a curing energy of 110-180 mJ/cm² and a UVV lamp providing a curing energy of 150-160 mJ/cm². The line speed should be 2.0-5.0 m/min. Primer 1 is semi-cured to produce the first primer coating.
4) Spraying UV Skin-Tone Primer 2. Apply UV Skin-Tone Primer 2 to the semi-cured UV Skin-Tone Primer 1 using a spraying method. The coating weight is 94 g/㎡. The spraying pressure is 0.5 MPa. The nozzle is 15-40 cm away from the substrate. The angle between the nozzle and the substrate is 30°-90°. Spray in a zigzag pattern. The reciprocating speed is set at 3.0 m/min.
5) Curing of UV Skin-Feel Primer 2. Use UV curing with a UVA curing energy of 600 mJ/cm² and a UVV curing energy of 1700 mJ/cm², or use a mercury lamp curing method with a UVA curing energy of 580 mJ/cm² and a UVV curing energy of 1650 mJ/cm² at a line speed of 5.0 m/min. Complete curing of UV Skin-Feel Primer 2 yields a primer coating.
6) Grind the primer coating and use a sander equipped with 400# and 600# sanding belts to sand the surface coating of the panel.
7) Spraying EB Skin-Feel Topcoat. Apply the EB Skin-Feel Topcoat to the polished surface of UV Skin-Feel Primer 2 using a spraying method. The coating weight is 80-90 g/m2, the spraying pressure is 0.5 MPa, the nozzle is 15-40 cm away from the substrate, the angle between the nozzle and the substrate is 30°-90°, and the spraying is performed in a zigzag pattern. The reciprocating speed is set at 3.0 m/min.
8) Semi-curing of EB skin-feel topcoat: Using an excimer lamp with a wavelength of approximately 172 nm, the curing energy in the UVD (wavelength range of 100-200 nm) range is 180-200 mJ/cm², and the line speed is 2.0-5.0 m/min.
9) EB skin-feel topcoat fully cured. Use EB curing with a curing energy of 0.15-0.5 MeV and a line speed of 15-25 m/min.
In this experiment, 200 samples of double-faced particleboard with impregnated film paper were sprayed and produced with the specifications of 1 220 mm × 2 800 mm × 18 mm.
1.2.2 Performance Evaluation Method
Skin feel is mainly based on the special structure and properties of the paint film, such as surface morphology (three-dimensional texture, concave and convex), surface friction, elasticity, flexibility, compressibility, density and thermal properties, which make people feel soft, full, warm and comfortable when in contact with the skin. Based on the above characteristics, in addition to the conventional paint film evaluation index requirements, the surface touch (roughness, contact angle, flatness), anti-fingerprint performance and glossiness of the paint film are used as key evaluation indicators for the surface quality of super matte skin-feel decorative panels [5]. According to the standards shown in Table 2, the conventional paint film properties and glossiness of the decorative panel surface were tested, and the surface touch (roughness, contact angle, flatness) and anti-fingerprint performance were tested using the methods specified in the corporate standard Q/SFYJJ 35-2022 "Super Matt Skin-Feel Paint Decorative Panels" of Sofia Home Furnishing Co., Ltd. [5].
The surface feel is assessed using the back-of-the-hand contact method, which involves gently rubbing the surface of the test piece with the back of your hand. The result is determined based on the tactile sensation of the back of your hand. The rating is divided into five levels: Level 1 (very rough, with a noticeable graininess); Level 2 (relatively rough, with a slight graininess); Level 3 (average, between Levels 2 and 4); Level 4 (relatively fine, smooth, with no noticeable graininess); and Level 5 (very fine, smooth, with no graininess).
The surface anti-fingerprint evaluation method is to wash your hands with clean water and then dry them with a towel. Keep your hands in a natural state without touching anything for 10 minutes, then press the surface of the coated particleboard hard. Then remove your hands, observe the disappearance of the fingerprints and record the time.
Table 2 Conventional paint film testing items and testing standards
2. Results and Discussion
From 200 super-matte skin-feel impregnated film-paper double-faced particleboard samples produced in a small trial batch, three panels were randomly selected for surface performance testing. The average of the test results was taken as the application result of this experiment. The specific results are shown in Table 3. As shown in Table 3, the veneer panels produced using the super-matte skin-feel coating process have a paint film hardness of 2 H, a gloss of 6 GU, a surface feel of level 5, and a fingerprint resistance of disappearing within 25 seconds. The paint film adhesion is level 1, the abrasion resistance is level 2, the moisture and heat resistance is level 1, the dry heat resistance is level 1, the liquid resistance is level 1, the impact resistance is level 1, the yellowing resistance is 1.2, the formaldehyde emission is 0.011 mg/m³, and the TVOC emission is 0.04 mg/(m²·h). The heavy metal resistance and temperature difference resistance also meet or exceed the requirements of the enterprise standard Q/SFYJJ 35-2022, meeting the surface performance requirements of super-matte skin-feel products for custom furniture.
Table 3 Performance test results of super matte skin-feel finish wood-based panels
3. Conclusions and Recommendations
This study used a coating process that used UV-curing paint as the primer and EB-curing paint as the topcoat to produce an ultra-matte skin-feel impregnated film-paper double-faced particleboard with a smooth and delicate skin feel. A quantifiable surface performance evaluation method for ultra-matte skin-feel veneer panels was developed based on relevant standards, and the surface properties of the prepared ultra-matte skin-feel impregnated film-paper double-faced particleboard were tested. The results showed that the ultra-matte skin-feel veneer coating process proposed in this study can achieve a skin-feel effect on the surface of special-shaped panels. The overall preparation process is simple, and the UV curing process does not require nitrogen filling or oxygen exhaust. This not only meets the diverse processing needs of customized home furnishing components, but also conforms to the national green manufacturing development direction and is of great significance for promoting the high-quality and healthy development of the customized home furnishing industry. Currently, no similar technologies have been found in the same industry at home or abroad. The ultra-matte skin-feel veneer coating technology proposed in this study is pioneering research and lays the foundation for the market promotion and application of ultra-matte skin-feel veneer panels. In addition, the product performance evaluation method for ultra-matte skin-feel decorative panels developed in this study solves the problem of lack of unified quantitative standards in the market and difficulty in evaluating the surface quality of skin-feel panels, and provides a reference for the production, sales and promotion of skin-feel decorative panels.
Wang Haidong, Zheng Zhihua, Yang Linhui, Hu Xinyue, Li Lei
Sofia Home Furnishing Co., Ltd., Guangzhou 511300
References: Omitted
For details, please refer to "China Wood-Based Panels" Issue 12, 2023