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Article

Effect of Photoinitiator Concentration and Film Thickness on the Properties of UV-Curable Self-Matting Coating for Wood-Based Panels

by
Haiqiao Zhang
1,2,
Xinhao Feng
1,2,
Yan Wu
1,2 and
Zhihui Wu
1,2,*
1
College of Furnishings and Industrial Design, Nanjing Forestry University, Nanjing 210037, China
2
Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing 210037, China
*
Author to whom correspondence should be addressed.
Forests 2023, 14(6), 1189; https://doi.org/10.3390/f14061189
Submission received: 17 May 2023 / Revised: 3 June 2023 / Accepted: 6 June 2023 / Published: 8 June 2023
(This article belongs to the Section Wood Science and Forest Products)
Browse Figures
Versions Notes

Abstract

:
Matte coatings have found wide-ranging applications across diverse industries. In this study, self-matting films with surface wrinkles were produced by exposing UV-curable polyurethane acrylate (UV-WPUA) resin to 172 nm Xe2* excimer and medium-pressure mercury lamps. The gloss values, micromorphologies, water contact angles (WCAs), roughness values, and friction behaviors of UV-WPUA films with different photoinitiator (PI) concentrations and thickness were investigated for the first time. The results indicate that the gloss values of the films at the same thickness enhance with the increase of PI concentration, while the amplitude of wrinkles, roughness, and WCAs decrease; however, the friction coefficient shows insignificant variations. While the PI concentration is unchanged, an increase in film thickness results in a decrease in gloss value and an increase in roughness and friction coefficient. Nevertheless, the WCA is relatively constant. The PI concentration of 0.5 wt% (lowest gloss value of cured film) was utilized to prepare the UV-WPUA wood coating. The cured coating film exhibited low gloss (4.9 GU at 60° and 5.2 GU at 85°) and outstanding mechanical properties, including 3H pencil hardness, grade 0 adhesion, excellent wear resistance, and tensile property. These findings can be utilized to guide the development of self-matting wood coatings and the production of wood-based panels used in industrial finishing.

1. Introduction

Wood coatings are thin films that are applied to the surface of wooden products to provide both protection and aesthetic enhancement. Wooden furniture, floors, doors, and stairs are some of the common household wooden products that require coating. Due to the reduction of forest resources and the inherent properties of wood that may affect the quality of wood products, such as anisotropy, shrinkage, and swelling [1], engineered wood-based panels, such as plywood [2], fiberboard, laminated veneer lumber [3], and particleboard were invented and manufactured to meet the demands of a wider range of applications conditions. Due to the advancements in intelligent manufacturing technology, wood-based panels are being utilized in household wooden products [4]. As a result, there is a growing need for improved finishing requirements for these wood-based panel products. One such requirement is the “soft-touch” coating or “soft-feel” surface [5], which are matte coatings with micro-wrinkles on the surface of the cured film. These coatings not only fulfill the protection and decoration requirements but also have low gloss and micro-wrinkles that reduce the contact area with human skin and provide a pleasant tactile experience.
Matte coating is a type of surface decoration coating that has been extensively used in leather finishing [6], vehicle interior topcoats [7], paper [8], textiles [9], and wood finishing [10]. Typically, three strategies are employed for preparing matte coatings, including adding matting agents to the coatings, developing self-matting resins or phase-separated hybrid systems [11,12], and the physical extinction of the coating surface by different curing techniques and equipment (such as electron beam curing and vacuum UV curing), with the ultimate goal of achieving a rough surface and low gloss [13,14]. In the previous study, we found that the third method mainly utilizes the instability of the coating film during the curing process, resulting in wrinkles on the surface of the coating film [15]. Based on existing research, there are two typical methods for this approach. The first method involves curing the self-matting coating using a VUV or UV-C excimer lamp (which provides short-wavelength UV, typically a 172 nm Xe2* excimer lamp) and a UV mercury lamp or UV-LED (which provides long-wavelength UV) [16]. The short-wavelength UV emitted by the Xe2* excimer lamp has poor penetration but high energy, resulting in the production of biradicals, ∙C−C∙ and ∙C−O∙ of C=C and C=O, in the top thin layer of the wet coating film surface, which participate in the UV curing reaction [17]. Under the influence of the photoinitiator (PI), the surface layer is the first to complete curing and forms a rigid skin layer that floats on the uncured liquid coating below [18]. The rigid skin layer develops wrinkles due to the transfer of interlayer pressure. The second method utilizes the equilibrium conditions of the oxygen inhibition rate and polymerization rate during UV curing in the ambient air [15]. During this UV curing process, the surface of the liquid coating is significantly affected by oxygen inhibition, while the lower part of the surface is less affected. Under equilibrium conditions, the surface forms a very thin liquid layer that swells the cured part of the lower layer, resulting in wrinkles [19,20,21].
According to the review of Rodríguez-Hernández, three typical film wrinkling systems are capable of fabricating wrinkles: layered film structure composed of an elastic substrate and a rigid skin, homogeneous films, and gradient film with variable mechanical properties [22]. The method using a 172 nm Xe2* excimer lamp and a UV mercury lamp irradiation belongs to the gradient film system, but it can also belong to the layered film system after forming the surface skin layer. The surface skin layer and the lower elastic material have different elastic modulus and Poisson’s ratio, which generates internal stress induced by depth crosslinking gradient to make the surface wrinkle [23]. Upon further crosslinking of the film, the stress produced by the bulk developed and stabilized the wrinkle formation. The characteristics of matte and skin-tactile feeling are related to the wrinkles on the surface of the coating. Bauer et al. [16,24] carried out a series of studies on this project from 2007 to 2014, but more detailed studies have not continued. This method has been industrialized and has started to gain popularity in China in recent years, known as “skin-tactile” coating [25]. Sun et al. [26] controlled the width and height of wrinkles on the surface of polyurethane acrylate coating by changing the UV intensity and adjusting the coating performance. The gloss values of self-wrinkled polyurethane acrylate coating were less than 3 GU at 20° and 60° and 6.5 GU at 85°, which satisfied the demand for matting coating.
Herein, we investigated the impact of PI concentration and film thickness on the properties of UV-curable self-matting polyurethane acrylate (UV-WPUA) resin film using dual-UV curing method, irradiation with a 172 nm Xe2* excimer lamp and a medium-pressure UV mercury lamp. Specifically, we investigated its effect on gloss value and prepared the coating for application to wood-based panels. We conducted tests on the physical and mechanical properties of the coating on the surface of medium-density fiberboard. Our study aimed to provide fundamental research on reducing the cost of self-matting coating and lowering the gloss value of the coating film.

2. Materials and Methods

2.1. Materials

The initial materials employed in this study were procured as follows: the UV-WPUA oligomer (JZ-4234) was obtained from Nanjing Jiazhong Chemical Technology Co., Ltd. located in Nanjing, China. The PI (819-DW, a dispersion of bis-acylphosphine oxide in water) was supplied by Shanghai Yinchang New Material Co., Ltd. based in Shanghai, China. The defoamer (SRE-2020), dispersant (SRE-4190), and leveling additive (SRE-3251) utilized in the study were purchased from Guangzhou Dong Prosperous Chemical Raw Materials Co., Ltd. situated in Guangzhou, China. The filler silicon dioxide (SiO2, the particle size is about 1 μm) was obtained from Hebei Yonghua Wear Resistant Materials Co., Ltd. located in Xingtai, China. Ultrapure water utilized in the research was prepared using the Plus-E3-10th ultrapure water machine (EPED, Nanjing, China).

2.2. Preparation of UV-WPUA Resins

The formulations and viscosities of the UV-WPUA resins were provided in Table 1. The UV-WPUA oligomer was initially mixed with the defoamer at 2000 rpm for 20 min using a disperser (SDF-600, Qiwei, Hangzhou, China). The viscosities of the UV-WPUA resins was measured using an RTS CPS rheometer (Ametek Brookfield, Middleboro, MA 02346, USA).

2.3. Preparation of Cured Films by UV-WPUA Resins

Three Mayer bars were used to coat pre-coated medium-density fiberboard (MDF, supplied by Superus Smart Home Co., Ltd. Coated with white water-based primer and ground with 240 mesh sandpaper, it provides a flat substrate and prevents cracks on the MDF surface from affecting the quality of self-matting coatings) with UV-WPUA resins. The thicknesses of wet films were about 60 μm, 100 μm, and 120 μm after leveling. The UV curing was conducted using a 172 nm Xe2* excimer lamp (UV intensity: 17.35 mW/cm2) for 1 min, followed by a medium-pressure UV mercury lamp (UV intensity: 6.6 mW/cm2) for 3 min. The cured films were designated F0.5–60, F0.5–100, F0.5–120, F2.0–60, F2.0–100, F2.0–120, F4.0–60, F4.0–100, and F4.0–120, as shown in Table 2. The UV-WPUA resin films obtained after UV irradiation were confirmed to have completely cured by employing the method of GB/T 1728-2020 [27].

2.4. Preparation of UV-WPUA Coating and Cured Film

The objective of this study was to produce a self-matting wood coating. Therefore, the PI concentration of UV-WPUA resin film with the lowest gloss value was determined. The UV-WPUA coating was then prepared by adding water as a diluent and other additives. Specifically, 100 g of UV-WPUA oligomer was combined with 0.5 wt% PI, 10.0 wt% water, 3.0 wt% defoamer, 2.0 wt% dispersant, 2.0 wt% leveling additive, and 2.0 wt% SiO2 (as the filler), and the mixture was homogenized at 2000 rpm for 20 min using the disperser to produce a uniform UV-WPUA coating. The method for preparing the wet and cured UV-WPUA coating film was identical to that for the UV-WPUA resins. The wet UV-WPUA coating film had a thickness of 120 μm.

2.5. Characterization

2.5.1. Chemical Structure and Composition

The attenuated total refraction (ATR) mode on a Vertex 80 Fourier transform infrared spectrometer (FTIR, Bruker, Germany) was used to characterize the chemical structures and compositions of the UV-WPUA resins, coating, and cured films. The FTIR-ATR spectra were collected between wavenumbers of 4000 cm−1 and 400 cm−1 at a resolution of 0.5 cm−1. As the biradical ∙C=O∙ formatted by C=O after 172 nm Xe2* excimer lamp, the band at 1720 cm−1 (C=O stretching vibration) was not employed as an internal standard band, the acrylate conversion of UV-WPUA films was only determined by analyzing the CH2=CH twisting band area around 808 cm−1 [24]. The FTIR-ATR spectra of the resins, coating, and cured films were analyzed using Thermo’s Omnic 9.3 software, without the application of any smoothing process. The acrylate conversion was calculated using Equation (1).
% conversion = 1 − A 808 t A 808 0 × 100
where (A808)0 and (A808)t refer to the relative absorption of the liquid resin and cured film after dual-UV irradiation, respectively.

2.5.2. Microscopy Investigation

Surface and cross-sectional morphologies of the cured films were analyzed using a Quanta 200 scanning electron microscope (SEM, FEI, Hillsboro, OR, USA). The cured film was attached to a copper sample table and sprayed with an ultrathin gold layer under vacuum condition. Observations were performed with an acceleration voltage of 10 kV. The wrinkles’ periodicity (λ) and amplitude (A) were measured based on the SEM images.

2.5.3. Physical and Mechanical Properties Characterization

Gloss. Gloss values were conducted using an HG268 gloss meter (3nh, Shenzhen, China) following the method supplied by GB/T 9754-2007 [28]. Measurements were taken at eight different points at three different reflectometer geometries: 20°, 60°, and 85°.
Roughness. The arithmetical mean roughness (Ra) values of the cured films were evaluated using a JB-4C precision roughness meter (Shanghai Precision Instruments Co., Ltd., Shanghai, China) with a measured length of 0.8 mm. Each cured film was tested at five different positions.
Water contact angle (WCA). The WCAs of cured films were determined using a DSA100S (KRÜSS, Hamburg, Germany) drop-shape analyzer, and the volume of water droplets was 2 μL. Each cured film was tested at five different positions and recorded WCAs at different elapsed times (0 s, 5 s, 10 s, and 20 s).
Frictional behavior. The frictional properties were evaluated to assess the haptic characteristics of the cured films. The method by Fu et al. [29] was employed to measure the friction coefficients of the films. In this test, the MDF block coated with the film was used as the upper specimen, while A4 paper served as the lower specimen. The evaluation friction coefficient was obtained by performing three tests for each sample. The static and dynamic friction coefficients (μs and μd) of the films’ surface were calculated according to Coulomb’s friction law, as represented by Equations (2) and (3).
μ s = F s G w + G s
μ d = F d G w + G s
where Fs and Fd refer to the static and dynamic frictional force, respectively, and Gw and Gs refer to the gravity of the counterweight and specimen (MDF block covered with film), respectively.
Tensile strength. The tensile strengths of samples were measured with an AGS-X electronic universal testing machine (Shimadzu, Kyoto, Japan). The samples were made into a dumbbell shape by a silica gel mold with the type of 1BA according to GB/T 1040.2-2022 [30]. Each sample was tested three times.
The hardness (measured with BY-500g, Pushen, Shanghai, China), cross-cut adhesion (determined with QFH-A, Airuipu, Quzhou, China), and abrasion resistance (measured with BGD-532 with CS-10 abrading rubber wheels, 1000 g, 500 r, Biuged, Guangzhou, China) of the cured films were evaluated according to GB/T 6739-2006 [31], GB/T 4893.4-2013 [32], and GB/T 1768-2006 [33], respectively.

3. Results and Discussion

3.1. Investigation of UV-WPUA Resins and Cured Films

3.1.1. Chemical Structure and Composition

This study employed a Xe2* excimer lamp with a wavelength of 172 nm for initial irradiation. The research conducted by Bauer et al. [17,24] demonstrated that the high-energy UV radiation at 172 nm can create free biradicals, (∙C−C∙ and ∙C−O∙) from the C=C and C=O in acrylate, thereby promoting the UV curing reaction, as shown in Figure 1A. Nevertheless, the penetration depth of the 172 nm UV light is limited (<1 μm), and the reaction range of ∙C−C∙ and ∙C−O∙ is restricted to a thin layer on the surface. It is worth noting that the process of free-radical polymerization can extend beyond the near-surface layers and lead to the photopolymerization of an acrylate film that exceeds a depth (d172) of 10 μm [17]. In this study, the PI utilized was 819-DW, type I PI, which produces free radicals (R∙) after UV irradiation and initiates the free-radical polymerization of UV-WPUA resin [34]. The C=C bond in acrylate undergoes a reaction with R∙, experiencing processes such as chain initiation, chain growth, chain transfer, and chain termination to complete free-radical polymerization and crosslinking curing [35].
The FTIR-ATR spectra can provide information on the chemical structure changes of the liquid UV-WPUA resins and the cured films after UV irradiation. Figure 1B depicts the changes in chemical groups observed in the FTIR-ATR spectra of UV-WPUA resin and film. The results demonstrate that the characteristic bands of C=C vibrations at 1636, 1407, and 808 cm−1 disappeared or became less distinct after UV irradiation [16], indicating that the C=C bond in UV-WPUA resin underwent a free-radical polymerization reaction with the R∙ or formed ∙C−C∙. The absorption bands near 1720 and 1105 cm−1 correspond to −C=O and C−O−C vibrations, respectively [36,37,38,39]. The bands at 2959 and 2871 cm−1 are attributed to C−H stretching vibrations in CH2 and CH3, while the band at 1458 cm−1 corresponds to C−H deformation vibrations in CH2 and CH3 [40]. The FTIR-ATR spectra changes observed in R0.5, R2.0, R4.0, and their corresponding films were consistent. The acrylate conversions of the nine films were also obtained from the FTIR-ATR spectra (as shown in Figure 1C). The results indicate that all acrylate conversions exceeded 93% and that the acrylate conversions of the film increased with the increase the PI concentration. The acrylate conversions of the films obtained by R4.0 (96.2%, 96.5%, and 96.6%) were higher than those of the film obtained by R2.0 (95.9%, 95.4%, and 96.0%), and also higher than that of the film obtained by R0.5 (94.7%, 94.1%, and 93.9%). This phenomenon can be attributed to the fact that an increase in the concentration of PI results in a higher acrylate conversion [16,41].

3.1.2. Gloss and Micromorphology

The gloss values of the nine cured films by UV-WPUA resins are presented in Figure 2. The gloss values of the films decrease with an increase in thickness when PI concentration is unchanged, regardless of the reflectometer geometry, i.e., 20°, 60°, and 85°. Commonly, the 60° geometry is utilized for assessing the gloss of all types of coating films, whereas the 85° geometry is specifically designed to enhance the differentiation capability of low-gloss coating films, which refers to films having a 60° specular gloss value lower than about 10 GU. Based on the gloss value obtained at 60° geometry, all nine films can be classified as low-gloss films, satisfying the ISO 2813 standards for the low-gloss film; however, the gloss value varies considerably at 85° geometry. Out of the nine films, F0.5–120 and F4.0–60 display the lowest and highest gloss values at 85° geometry, measuring 7.5 GU and 25.5 GU, respectively. The findings of this study indicate that the gloss values of the films increase with the rise in PI concentration, irrespective of the reflectometer geometry used. It is noteworthy that the cost of PI is relatively high in the coating industry; therefore, this study’s results could be a useful reference for the development of cost-effective formulations of low-gloss coatings by considering the optimal amount of PI required to achieve the desired level of gloss. Furthermore, the results of this study could also be beneficial in the context of wood coatings, which usually have a thickness of around 100 μm. Thus, these results could offer valuable insights into the practical applications of wood coatings.
According to previous research, we also found that the gloss value of films at 85° geometry increased with increasing PI concentration, which was related to the morphology of surface wrinkles of the film [15]. The mechanism of preparing the self-matting coating using dual-UV curing technology is illustrated in Figure 3A. The 172 nm UV penetration is not strong enough to penetrate the wet film completely, only a gradient curing film is generated on the surface, as shown in Figure 3(Aa,Ab). As the curing process progresses, pressure stress builds up inside the system due to the varying degrees of curing [23], causing the surface layer to wrinkle and release the pressure to maintain system stability (as shown in Figure 3(Ac)). It is important to note that the wrinkles on the film’s surface are formed during the curing of the Xe2* excimer lamp [16], and the uncured liquid resin in the lower layer is deep-cured using a medium-pressure UV mercury lamp with strong penetration (as shown in Figure 3(Ad)). After dual-UV curing, the wrinkles on the coating surface are retained, resulting in a self-matting coating (Figure 3(Ae)). In Figure 3B, SEM images show that the film surfaces are covered with disorderly arranged wrinkles, which appear as a continuous curve on the cross-section. Figure 3C shows the A and λ values of the wrinkles on the film surface. As the PI concentration increases, the A of wrinkles on the film surface decreases. The A of the films’ wrinkles obtained by R0.5 (3.71 μm, 3.92 μm, and 3.93 μm) are higher than those of the films obtained by R2.0 (3.23 μm, 3.29 μm, and 3.47 μm) and also higher than those of the films obtained by R0.5 (3.18 μm, 2.77 μm, and 2.90 μm); however, λ seems random, and only the λ of wrinkles on F4.0–60 is significantly smaller than of the wrinkles on other films. The A of wrinkles on the surface of film cured by R0.5 are higher than those in R2.0 and R4.0, which causes incident light to be reflected or “captured” by the interspace of wrinkles; therefore, the light received by the gloss meter acceptor is smaller, resulting in a low-gloss value [42].

3.1.3. WCA and Roughness

The WCA of the cured film is positively correlated with the roughness of the film surface. The larger the roughness value, the higher the WCA [43,44]. The roughness of the film surface is directly related to its microstructure [45]. Figure 4 shows the WCAs and Ra values of the films. Figure 4A demonstrates the WCAs of the six films prepared by R0.5 and R2.0 are between 101° and 104°, indicating hydrophobic surfaces [46,47], while the WCAs of the three films prepared by R4.0 are all below 80°, indicating hydrophilic surfaces. The WCAs of films decrease with increasing contact time; however, F4.0–60, F4.0–100, and F4.0–120 exhibit a faster decrease (decrease ~ 10° after 20 s), while the WCAs of the films prepared by R0.5 and R2.0 decrease more slowly (decrease ~ 4° after 20 s). It is well known that the WCA is typically correlated with the micro-nano structures [48,49] and the roughness [45] of the film surface. Figure 4B shows that the Ra value reduces as the PI concentration increases, and when the concentration of PI is unchanged, the Ra value increases with the increasing film thickness. The Ra values of the films with varying thickness obtained by R0.5 (0.88, 0.94, and 1.07 μm) are higher than those of the films obtained by R2.0 (0.70, 0.79, and 1.06 μm), and also higher than those of the films obtained by R4.0 (0.44, 0.54, and 0.64 μm). The change of the Ra values is positively associated with the A of wrinkles on the surface of the film, which is related to the calculation of the Ra value. According to the Wenzel wetting model, the WCA increases with increasing surface roughness when the WCA exceeds 90° [50]; however, the hydrophilic or hydrophobic surface’s WCAs did not exhibit significant changes in this study. One possible reason for this is that the surface wrinkles on the film are randomly oriented, and the surface roughness cannot be fully represented by the Ra value. In addition, the roughness of the film surface is also related to its gloss value [51]. According to Figure 2 and Figure 4B, the two are negatively correlated in this study, that is, the higher the Ra, the lower the gloss value.

3.1.4. Haptics

In general, smooth and glossy polymer coatings are often described as “cold and sticky”, and tend to be highly sensitive to fingerprints. In contrast, micro-structured coatings are typically insensitive to fingerprints and can produce a “soft-feel” surface [5]. The friction forces of nine films are shown in Figure 5A, and the curve type is similar to that obtained by Schubert et al. [5]. The results demonstrate that the friction force increases with increasing film thickness when the PI concentration is unchanged. Since the variation in the mass of the samples, represented by Gs in Equations (2) and (3), the obtained friction coefficients were used for the comparative and analytical of the different films, as depicted in Figure 5B. In agreement with the results reported by Xu et al. [52] and Fu et al. [29], the friction coefficient of the films, both μs or μd, was observed to increase with an increase in film thickness, while the PI concentration is unchanged. The μs values of the films with different thicknesses obtained by R0.5 (0.27, 0.35, and 0.41) are similar to those of the films obtained by R2.0 (0.31, 0.33, and 0.41) and also similar to those of the films obtained by R0.5 (0.33, 0.34, and 0.42). Similarly, the μd values of the films with different thicknesses obtained by R0.5 (0.20, 0.22, and 0.23) are similar to those of the films obtained by R2.0 (0.17, 0.19, and 0.26) and also similar to those of the films obtained by R0.5 (0.18, 0.21, and 0.34). The results for friction coefficients of the films prepared by R0.5, R2.0, and R4.0 showed no correlation with the PI concentration, and the friction coefficients of the films with the same thickness but different PI concentrations were not much different. We speculate that the friction coefficient is only the contact between the planes during measurement and is not influenced by the A of the wrinkles.

3.2. Investigation of UV-WPUA Coatings and Cured Films

3.2.1. Morphology, Gloss, and WCA of the Cured Film by UV-WPUA Coating

The surface and cross-sectional SEM images of the UV-WPUA coating film are presented in Figure 6. It can be seen from Figure 6A that the presence of disordered wrinkles on the film’s surface. The wrinkles on the film have a λ of 22.46 μm and an A of 3.83 μm, which are lower than those on the surface of F0.5–120 (λ: 25.71 μm, A: 3.93 uμm). We speculate that the slight influence of additives and water used during the coating preparation process may affect the wrinkles’ dimension. The gloss value is presented in Figure 6B, which demonstrates the gloss values at 20°, 60°, and 85° geometries are 1.7 GU, 4.9 GU, and 5.3 GU, respectively. The gloss values obtained at 60° geometry indicate that the cured coating film is the low-gloss film, which meets the criteria of extinction classification [53]. The gloss value is lower than that of F0.5–120, possibly due to the SiO2 added as a filler in the coating preparation process, which remains on the film’s surface after UV curing, thereby decreasing the gloss value (as shown by the white arrow) [54]. Figure 6C shows the WCA-elapsed time curve of UV-WPUA coating film, indicating an initial contact angle of 98.1°, followed by 96.8°, 96.4°, and 96.0° at 5 s, 10 s, and 20 s, respectively. These results suggest that the cured coating film exhibits a hydrophobic surface, which is related to its surface wrinkles structure [50,55].

3.2.2. Basic Properties

The coating film’s mechanical properties, including pencil hardness, resistance to abrasion, cross-cut adhesion, and tensile strength, were tested and recorded in Table 3. The results indicate that the film has outstanding mechanical properties that are suitable for finishing wood-based panels. As noted in section “3.1.1 Chemical structure and composition”, the film prepared using the method in this study exhibits high acrylate conversion, exceeding 93%, leading to a higher crosslinking density and robust mechanical strength [56,57]. For purposes of completeness, it should be mentioned that the pencil hardness test was conducted on the wrinkled surface of the coating film, requiring an eraser to remove pencil scratches. The test results were determined by assessing the hardness of the pencil when no significant difference in the reflected light between the area crossed by the pencil and the surrounding area was observed. The film’s abrasion resistance is better than that of previous research [15], mainly due to the dual-UV curing process used in this study, which significantly improved the acrylate conversion from 75.1% to 93%. Additionally, high-hardness SiO2 was added as a filler, enhancing the wear resistance of the film [58]. The cross-cut adhesion of the film was graded using a magnifying glass due to the white primer present on the MDF’s surface, which made naked-eye grading challenging. The cross-cut adhesion of the wood coating film prepared in this study was observed using a magnifying glass (magnification: 3×). According to the picture in Table 3, The cross-cut adhesion test showed no detachment of cross-cut patterns [59], indicating the best grade of adhesion (grade 0). Furthermore, the film’s tensile strength (18.32 MPa) also indicated its excellent mechanical properties.

4. Conclusions

A series of self-matting films with wrinkles on the surface were obtained by irradiating UV-WPUA resin with a 172 nm Xe2* excimer lamp and a medium-pressure UV mercury lamp. Following the irradiation process, the acrylate conversions of cured films exceeded 93%. The films were prepared using resins with different PI concentrations and thicknesses, and their gloss values, micromorphologies, WCAs, Ra values, and friction behaviors were investigated. The results indicate that the gloss value of the film increases with an increase in PI concentration, whereas the A of surface wrinkles, Ra value, and WCA decrease; however, there is little change in the friction coefficient. When the concentration of PI is unchanged, it was observed that an increase in film thickness leads to a decrease in the gloss value, while the Ra value and friction coefficient exhibit an increase; however, the WCA remains relatively constant under these conditions. Furthermore, a UV-WPUA wood coating was formulated using the resin with the lowest gloss value of PI concentration (0.5 wt%). The cured coating film exhibit 3H pencil hardness, grade 0 adhesion, excellent abrasion resistance, and high tensile properties. The findings of this study pave the way for the development of self-matting wood coatings and the production of wood-based panel used in industrial finishing.

Author Contributions

H.Z.: conceptualization, methodology, data curation, investigation, writing—original draft, writing—review and editing, funding acquisition. X.F.: measurement, investigation, formal analysis. Y.W.: methodology, conceptualization, validation. Z.W.: resources, project administration, funding acquisition, validation, supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Key Research & Development Program of China (Nos. 2016YFD0600704, 2018YFD0600304) and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (No. KYCX22_1095).

Data Availability Statement

Data will be made available upon request.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (A) Illustration of the fabrication of the self-matting coating film using dual-UV curing; (B) FTIR-ATR spectra of liquid UV-WPUA resins and cured films; (C) the acrylate conversions of cured films.
Figure 1. (A) Illustration of the fabrication of the self-matting coating film using dual-UV curing; (B) FTIR-ATR spectra of liquid UV-WPUA resins and cured films; (C) the acrylate conversions of cured films.
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Figure 2. The gloss values of cured films with three measurement geometries.
Figure 2. The gloss values of cured films with three measurement geometries.
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Figure 3. (A) Illustration of the surface wrinkles formation mechanism during dual-UV curing; (B) SEM images of cured films with surface and cross-section; (C) the A and λ values of the wrinkles on the film surface.
Figure 3. (A) Illustration of the surface wrinkles formation mechanism during dual-UV curing; (B) SEM images of cured films with surface and cross-section; (C) the A and λ values of the wrinkles on the film surface.
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Figure 4. (A): WCAs of cured films with different elapsed time; (B): Ra values of cured films.
Figure 4. (A): WCAs of cured films with different elapsed time; (B): Ra values of cured films.
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Figure 5. The friction forces and coefficients of cured films.
Figure 5. The friction forces and coefficients of cured films.
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Figure 6. (A) The SEM images of the surface and cross-section of cured UV-WPUA coating film; (B) The gloss values of UV-WPUA coating film at different geometries; (C) the WCAs of cured UV-WPUA coating film with different elapsed time.
Figure 6. (A) The SEM images of the surface and cross-section of cured UV-WPUA coating film; (B) The gloss values of UV-WPUA coating film at different geometries; (C) the WCAs of cured UV-WPUA coating film with different elapsed time.
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Table
Table 1. Formulations and viscosity of UV-WPUA resins.
Table 1. Formulations and viscosity of UV-WPUA resins.
UV-WPUA ResinPI (g)UV-WPUA Oligomer (g)Defoamer (g)Viscosity (Pa∙s)
R0.50.2 (0.5 wt%)40.00.816.0 ± 2.0
R2.00.8 (2.0 wt%)40.00.817.0 ± 1.2
R4.01.6 (4.0 wt%)40.00.815.3 ± 0.8
Table
Table 2. Orthogonal experiments to explore the parameters of UV-WPUA films.
Table 2. Orthogonal experiments to explore the parameters of UV-WPUA films.
Thicknesses of the Wet Film (μm)Used UV-WPUA Resin
R0.5R2.0R4.0
60F0.5–60F2.0–60F4.0–60
100F0.5–100F2.0–100F4.0–100
120F0.5–120F2.0–120F4.0–120
Table
Table 3. Basic properties of UV-WPUA coating film.
Table 3. Basic properties of UV-WPUA coating film.
Parameter
Resistance to Abrasion (g)Cross-Cut AdhesionTensile Strength (MPa)
0.0148 ± 0.0035Grade 018.32 ± 2.14
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Zhang, H.; Feng, X.; Wu, Y.; Wu, Z. Effect of Photoinitiator Concentration and Film Thickness on the Properties of UV-Curable Self-Matting Coating for Wood-Based Panels. Forests 2023, 14, 1189. https://doi.org/10.3390/f14061189

AMA Style

Zhang H, Feng X, Wu Y, Wu Z. Effect of Photoinitiator Concentration and Film Thickness on the Properties of UV-Curable Self-Matting Coating for Wood-Based Panels. Forests. 2023; 14(6):1189. https://doi.org/10.3390/f14061189

Chicago/Turabian Style

Zhang, Haiqiao, Xinhao Feng, Yan Wu, and Zhihui Wu. 2023. "Effect of Photoinitiator Concentration and Film Thickness on the Properties of UV-Curable Self-Matting Coating for Wood-Based Panels" Forests 14, no. 6: 1189. https://doi.org/10.3390/f14061189

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