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introduction
During the molding process, the coating of skin-feel furniture boards needs to be produced by using excimer lamps or other pre-curing methods to create elastic wrinkled textures, resulting in a smooth and delicate feel similar to touching skin [1]. Currently, conventional skin-feel coatings on the market usually add a large amount of additives to achieve a physical matting effect, resulting in skin-feel UV curing (hereinafter referred to as UV) coatings with high viscosity and rich particle texture, making them difficult to spray. In addition, skin-feel coatings also contain a certain amount of chemical matting agents. During the spraying process, the atomized coating comes into contact with air, which affects its chemical matting performance. Therefore, traditional skin-feel coatings are not suitable for spraying, and the coating method is mainly roller coating, which can only produce large flat panels [2]. Therefore, the current production of skin-feel boards has problems such as complex processes, low production efficiency, insufficient smoothness and delicacy of the skin-feel effect, and environmental hazards, which affect the promotion and application of skin-feel coatings in furniture board finishing.
This study attempts to overcome the problem of skin-feel UV coatings being unable to be sprayed due to the addition of additives and matting agents by adjusting the viscosity and spraying air pressure. By limiting the spraying air pressure, the duration of contact between the skin-feel UV coating and the air is controlled, thereby reducing the impact of the environment on the chemical matting properties of the skin-feel coating. This leads to the development of ultra-matte skin-feel decorative panels for furniture manufacturing, providing a reference for the production of ultra-matte skin-feel furniture panels.
1. Materials and Methods
1.1
Material
The various physical and chemical properties of the ultra-matte finish decorative panel surface are based on the function of the decorative coating. By applying an electron beam (EB) cured topcoat over a UV primer, the coating can acquire multiple functions. Tests were conducted on commonly used electron beam (EB) cured (hereinafter referred to as EB) skin-feel coatings on the market, and the raw material composition and mass percentage of EB skin-feel topcoats were determined to be: 78% polyurethane acrylic resin, 16% reactive monomers, 3% additives, and 3% functional powder.
The system of UV resin determines the gloss, surface effect, and physical properties of UV ultra-matte skin-feel coatings [3]. During the preliminary test, the performance of various UV resins was evaluated, and a UV resin with better performance was determined. This resin was then combined with defoamer, photoinitiator, thixotropic agent, wetting and dispersing agent, and matting agent to prepare UV ultra-matte skin-feel coatings. Two UV ultra-matte skin-feel coating formulations were screened (see Table 1 for details). These two formulations, combined with excimer curing equipment, produced UV ultra-matte skin-feel coatings with superior performance and surface effect compared to conventional low-gloss UV coatings.
Table 1 UV Ultra-Matte Skin Feel Coating Formulation
The following are the selection methods for UV ultra-matte skin-feel coatings:
1) UV Resin Screening. When screening UV resins, monofunctional resins showed good dilution ability but slow curing speed. Trifunctional resins had poor dilution ability and poor resistance to yellowing, but high shrinkage. Considering the influence of UV resins on the viscosity, curing speed, wettability, and shrinkage of the formulation system, this experiment combined the performance advantages of both monofunctional and trifunctional resins for use in combination.
2) Screening of defoamers. Defoamers (organosilicon type and polymer type) were screened, and their effects on the defoaming and compatibility of UV coatings were analyzed. It was found that defoamers with good defoaming properties had high defoaming efficiency, but poor compatibility and were prone to pinholes. Therefore, this experiment selected an organosilicon type defoamer with moderate defoaming efficiency and compatibility.
3) Screening of photoinitiators. The performance of three photoinitiators (1173, i.e., 2-hydroxy-2-methyl-1-phenyl-1-propanone; long-wavelength photoinitiator TPO-L, i.e., ethyl 2,4,6-trimethylbenzoylphosphonate; MBF, i.e., methyl benzoylformate) was compared. The effects of the three photoinitiators on the curing speed, bottom drying, surface drying, and yellowing resistance of UV coatings, as well as their matching effect with the equipment in the whole curing system, were analyzed. Finally, "MBF + 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 polymer) was compared, and their anti-settling effects and influence on the viscosity and leveling properties of UV coatings were analyzed. The UV coating system used in the experiment employed excimer lasers for matting, resulting in a low proportion of fillers and matting agents, thus making the sedimentation problem easier to solve. However, to achieve better surface finishes, higher leveling performance was required. Therefore, a polyurea polymer with a lower thixotropic index was selected as the thixotropic agent.
5) Screening of wetting and dispersing agents. Three wetting and dispersing agents (polyether-modified polydimethylsiloxane solution, organosilicon twin-structure surfactant, and nonionic organic surfactant) were compared, and their effects on the wetting properties, cratering, and foam stabilization of UV coatings were analyzed. It was found that wetting agents with low surface tension had good wetting effects, but large addition amounts could easily cause cratering and foam stabilization problems. Therefore, this experiment selected a high molecular weight organosilicon twin-structure surfactant as the dispersant.
6) Screening of matting agents. The effects of matting agents on the viscosity, gloss, and feel of UV coatings were analyzed. Matting agents with a rougher feel have higher matting efficiency but also higher surface roughness. Surface-treated matting agents have good dispersibility. In this system, the matting agent is mainly used to adjust the feel of the coating surface; therefore, the feel, defoaming properties, and oil absorption of the matting agent are more important. Thus, silica was selected as the matting agent for this experiment.
1.2
method
1.2.1 Coating Process Flow
EB coating curing is a process that uses an electron beam as a radiation source to initiate resin polymerization or cross-linking, transforming a liquid into a solid. Compared with UV curing, EB curing offers better resistance to yellowing and weathering, is not limited by color or coating thickness, has less energy loss, higher energy conversion rate, no attenuation, and advantages such as high cross-linking structure, high hardness, chemical resistance, wear resistance, and aging resistance [4]. Therefore, the coating process for skin-feel products adopts the technology of UV curing of primer and EB curing of topcoat, combining the advantages of both for production. The specific process flow is as follows:
1) Clean the substrate. Wipe the surface of the impregnated paper-faced engineered wood panel with a white cloth dampened with UV thinner (ZUX2014) to remove stains; for any remaining stains, use a sponge to lightly sand them.
2) UV Skin-Feel Primer 1 Spraying. The UV skin-feel primer 1 is sprayed onto the substrate surface obtained in step 1) using a spraying method. The coating amount is 90-100 g/㎡, the spraying air pressure is 0.5 MPa, the nozzle distance from the substrate is about 20 cm, the spraying is done in a zigzag pattern, and the reciprocating speed is set to 3.0 m/min.
3) Curing of UV Skin-Feel Primer 1. A UV lamp is used for semi-curing, with a curing energy of 350–600 mJ/cm² for the UVA band (wavelength range 320–390 nm) and 1000–1800 mJ/cm² for the UVV band (wavelength range 395–445 nm). Alternatively, a mercury lamp can be used for semi-curing, with a curing energy of 110–180 mJ/cm² for the UVA band and 150–160 mJ/cm² for the UVV band; the linear velocity is 2.0–5.0 m/min. The semi-cured primer 1 yields the first primer coating.
4) UV Skin-Feel Primer 2 Spraying. The UV Skin-Feel Primer 2 is sprayed onto the semi-cured UV Skin-Feel Primer 1 surface using a spraying method. The coating amount is 94 g/㎡, the spraying air pressure is 0.5 MPa, the nozzle distance from the substrate is 15-40 cm, the included angle between the nozzle and the substrate is 30°-90°, the spraying is done in a Z-shape, and the reciprocating speed is set to 3.0 m/min.
5) Curing of UV Skin-Feel Primer 2. UV curing is used, with a curing energy of 600 mJ/cm² for the UVA band and 1700 mJ/cm² for the UVV band. Alternatively, full curing with a mercury lamp is used, with a curing energy of 580 mJ/cm² for the UVA band and 1650 mJ/cm² for the UVV band, at a linear velocity of 5.0 m/min. The primer coating is obtained after complete curing of UV Skin-Feel Primer 2.
6) Sand the primer coating. Use a sander equipped with 400# and 600# sanding belts in sequence to sand the coating on the surface of the board.
7) EB Skin-like Topcoat Spraying. The EB skin-like topcoat is sprayed onto the surface of the sanded UV skin-like primer 2 using a spraying method. The coating amount is 80-90 g/㎡, the spraying air pressure is 0.5 MPa, the nozzle distance from the substrate is 15-40 cm, the included angle between the nozzle and the substrate is 30°-90°, the spraying is done in a Z-shape, and the reciprocating speed is set to 3.0 m/min.
8) Semi-cured EB skin-feel coating. An excimer lamp with a wavelength of approximately 172 nm is used, with a curing energy of 180–200 mJ/cm2 in the UVD (wavelength range of 100–200 nm) band and a linear velocity of 2.0–5.0 m/min.
9) EB skin-feel topcoat fully cured. EB curing is used, with a curing energy of 0.15–0.5 MeV and a linear velocity of 15–25 m/min.
This experiment produced 200 samples of double-sided particleboard with impregnated paper and dimensions of 1220 mm × 2800 mm × 18 mm.
1.2.2 Performance Evaluation Methods
Skin feel is mainly based on the special structure and properties of the paint film, such as surface morphology (three-dimensional texture, unevenness), surface friction, elasticity, flexibility, compressibility, density, and thermal properties, which give people a soft, full, warm, and comfortable feeling when in contact with the skin. Based on the above characteristics, in addition to the conventional paint film evaluation index requirements, the surface feel (roughness, contact angle, flatness), anti-fingerprint performance, and gloss of the paint film are used as the key evaluation indexes for the surface quality of ultra-matte skin feel decorative panels [5]. According to the standard test of the surface conventional paint film performance and gloss of the decorative panel shown in Table 2, the surface feel (roughness, contact angle, flatness) and anti-fingerprint performance are tested according to the method specified in the enterprise standard Q/SFYJJ 35-2022 "Ultra-matte Skin Feel Paint Decorative Panels" of Sophia Home Furnishings Co., Ltd. [5].
The surface tactile sensation is assessed using the back-of-hand contact method. This involves gently touching the surface of the specimen with the back of the hand and rubbing it back and forth, then judging the result based on the tactile sensation. There are five levels: Level 1, very rough and coarse with a noticeable grainy texture; Level 2, somewhat rough and coarse with a slight grainy texture; Level 3, average, between Level 2 and Level 4; Level 4, relatively smooth and fine with no noticeable grainy texture; and Level 5, very smooth and fine with no grainy texture.
The surface fingerprint resistance evaluation method involves washing hands with clean water and drying them with a towel, keeping them in a natural state and not touching any objects for 10 minutes, then pressing firmly on the surface of the coated particleboard, removing hands afterward, observing the state of fingerprint disappearance, and recording the time.
Table 2. Routine paint film testing items and standards
2. Results and Discussion
From 200 samples of ultra-matte skin-feel impregnated paper double-sided particleboard produced in a small-batch trial production, three boards were randomly selected for surface performance testing. The average value of the test results was taken as the application result of this experiment. The specific results are shown in Table 3. As can be seen from the results in Table 3, the paint film hardness of the decorative panel produced using the ultra-matte skin-feel coating process is 2H, gloss is 6 GU, surface touch is level 5, fingerprint resistance is 25s fingerprint disappearance, paint film adhesion is level 1, abrasion resistance is level 2, damp heat resistance is level 1, dry heat resistance is level 1, liquid resistance is level 1, impact resistance is level 1, yellowing resistance is 1.2, formaldehyde emission is 0.011 mg/m³, TVOC emission is 0.04 mg/(m2·h), and heavy metals and resistance to cold and heat differences also meet or exceed the requirements of the enterprise standard Q/SFYJJ 35-2022, satisfying the surface performance requirements of ultra-matte skin-feel products for customized furniture.
Table 3 Performance test results of ultra-matte finish engineered wood panels
3. Conclusions and Recommendations
This study developed a coating process using UV-cured primer and EB-cured topcoat to produce ultra-matte, skin-like impregnated paper double-sided particleboard with a smooth and delicate skin-like effect. A quantifiable evaluation method for the surface performance of the ultra-matte, skin-like decorative panel was established based on relevant standards, and the surface performance of the prepared ultra-matte, skin-like impregnated paper double-sided particleboard was tested. The results show that the proposed ultra-matte, skin-like decorative panel coating process can achieve a skin-like surface effect on irregularly shaped panels. The overall preparation process is simple, and the UV curing process does not require nitrogen purging or oxygen removal. This not only meets the diverse processing needs of customized home furnishing components but also aligns with the national green manufacturing development direction, which 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 industry both domestically and internationally. The ultra-matte, skin-like decorative panel coating technology proposed in this study is pioneering research, laying the foundation for the market promotion and application of ultra-matte, skin-like decorative panels. Furthermore, the performance evaluation method for ultra-matte skin-feel decorative panels developed in this study solves the problem of the lack of unified quantitative standards in the market and the difficulty in evaluating the surface quality of skin-feel panels, providing 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 Furnishings Co., Ltd., Guangzhou 511300
References: Omitted
For details, please refer to the December 2023 issue of "China Wood-based Panels".