Abstract
To develop a waterborne wooden flooring coating, the existing problems of the current waterborne coated wooden floorings were analyzed, and their corresponding solutions were considered. In this paper, based on the final effects of a novel waterborne UV wooden floor coating, the authors used a waterborne UV coating with different solids contents, as well as finishing and drying technology to treat the substrate with different tree species. According to the LY/T standard 1859-2009 (2009) and Q/YFL standard 0035-2018 (2018), the performance of the varnish film of the waterborne UV wooden floor coating was determined. The waterborne coating wooden flooring processes and effects of the varnish film were optimized to provide excellent performance. The performance of the varnish film and the developed waterborne coated wooden flooring met the standard requirements: the surface wear-resistance was less than or equal to 0.15 g/100r, the varnish film hardness was greater than or equal to H, the level of varnish film adhesion was less than or equal to 2, and the total volatile organic compounds was less than or equal to 20 µg/m3. This study provided a demonstration and basis for wooden flooring companies to develop a waterborne wooden floor coating.
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Development and Finishing Technology of Waterborne UV Lacquer-Coated Wooden Flooring
Zinan Zhao,a Yiting Niu,b and Fengyi Chen a,*
To develop a waterborne wooden flooring coating, the existing problems of the current waterborne coated wooden floorings were analyzed, and their corresponding solutions were considered. In this paper, based on the final effects of a novel waterborne UV wooden floor coating, the authors used a waterborne UV coating with different solids contents, as well as finishing and drying technology to treat the substrate with different tree species. According to the LY/T standard 1859-2009 (2009) and Q/YFL standard 0035-2018 (2018), the performance of the varnish film of the waterborne UV wooden floor coating was determined. The waterborne coating wooden flooring processes and effects of the varnish film were optimized to provide excellent performance. The performance of the varnish film and the developed waterborne coated wooden flooring met the standard requirements: the surface wear-resistance was less than or equal to 0.15 g/100r, the varnish film hardness was greater than or equal to H, the level of varnish film adhesion was less than or equal to 2, and the total volatile organic compounds was less than or equal to 20 µg/m3. This study provided a demonstration and basis for wooden flooring companies to develop a waterborne wooden floor coating.
Keywords: Waterborne UV coatings; Wooden flooring; Finishing technology; Drying technology; Varnish film performance
Contact information: a: Langfang Huari Furniture Co., Ltd., Langfang 065000 China; b: Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, China;
* Corresponding author: chenfengyi423@163.com
INTRODUCTION
In recent years, the Chinese and local governments have been advocating for advances in environmental protection, issuing multiple environmental protection policies and improving the standards for volatile organic compounds (VOC) limits for coating materials and promoting pollution treatments to reduce the environmental pollution and improve the living quality (Xiong et al. 2018b; Eva et al. 2020; Xiong et al. 2020). In 2017, the Chinese national standard GB/T 35601-2017 (2017), issued by the General Administration of Quality Supervision and Standardization Administration of the People’s Republic of China, clearly stipulated that the use of waterborne coatings was a necessary condition for wooden flooring products to become green products (Feng et al. 2020; Challener 2015). Meanwhile, with an increase in consumption levels of the individual, high-quality and environmentally friendly wooden floorings will become the primary consumption product. The development of waterborne coated wooden flooring and the application of waterborne coatings on wooden flooring are imminent (Tang et al. 2003; Zhu et al. 2016; Xiong and Wu 2018).
Waterborne varnish refers to a coating that uses water as the dispersion medium or solvent, and the film-forming materials are dissolved or dispersed in the water (Yan et al. 2020a). Different from the traditional solvent-borne paint, the biggest advantage of using a waterborne coating is that more than 80% of VOC and other harmful substances, e.g., formaldehyde, benzene, and xylene, found in the coating can be eliminated. Currently, the application of waterborne coatings on furniture and wooden doors is relatively established (Lin et al. 2019; Panek et al. 2019). However, because the surface performance (hardness and wear resistance) of wooden flooring is greater than that of wooden furniture, waterborne coating technology presently cannot meet the requirements needed for manufacturing wooden flooring. Therefore, the application of waterborne coatings to flooring still needs to overcome many problems (Spilman et al. 2018; Cai et al. 2020; Yan et al. 2020b). These problems include: how to promote the transformation of the flooring industry from being resource and energy intensive to resource saving and environment friendly, and to promote green production and green consumption as an inevitable choice for the flooring industry (Xiong et al. 2017; Tao et al. 2019). The use of waterborne coatings to manufacture wooden floors can eliminate the VOC emissions from wooden floor coating materials at the source and provide a new option for flooring companies to transform and upgrade, improving product quality and increasing profitability. The application of waterborne coating materials can meet the requirements needed for green and sustainable development at home and abroad (Xiong et al. 2018a; Xu et al. 2020).
Several past studies have considered the waterborne coating of wooden flooring. Hwang et al. (2009) studied the effect of the drying temperature and time of waterborne UV-curable coating in flash-off step on the surface morphology of varnish film. The results showed that insufficient drying can cause defects, e.g., bubbles and peeling, on the surface of varnish film. Sufficient drying not only can ensure a high-quality appearance, but also it can enhance the performance of the varnish film. Bongiovanni et al. (2002) used UV-curable coating with additives containing fluorine to coat the surface of wood materials, and the properties of varnish film, e.g., glossiness, adhesion, hardness, etc., were tested. The results showed that the varnish film formed from the coating with additives containing fluorine had a greater performance in terms of surface glossiness and chemical resistance. Wu et al. (2019) coated the wood materials with novel a waterborne UV coating and studied the effects of the coating weight on the drying time and performance of the varnish film. The results showed that with an increase in the weight of the waterborne coating, the longer was the drying time of the varnish film and the longer was the forming time of the varnish film. When the thickness of the varnish film was 60 m, the drying time was 4 min, the hardness was 2 H, and the level of adhesion was 2, which met the requirements of Chinese national standard GB/T 18103-2013 (2013). Li and Shu (2007) coated a wooden flooring substrate with a waterborne UV coating with different compositions and ratios, and the transmittance index of the varnish film was compared and analyzed. The results showed that there was a greater effect of coating with different ratios on the grain and color distinctness of image (DOI). By consulting and analyzing the existing literature, it is not difficult to find that the studies on waterborne coating wooden flooring focuses on the influence of waterborne coating in terms of appearance and performance. However, there have been few studies on the effects of finishing technology and process, or the function of the waterborne coating on the hardness, wear resistance, etc.
During the research on waterborne coated wooden flooring, it was found that the physical and chemical properties of the surface, especially the hardness and surface wear resistance, were strongly affected by the application of waterborne UV coating on wooden flooring. First, due to the low solids content of water-based coating, the varnish film is relatively thin, and the filling effect of the low-density wood used for floor production is poor, resulting in low hardness and poor wear resistance of the floor varnish film (Shen et al. 2010; Huang et al. 2018; Li et al. 2018). Second, due to the need for dewatering treatment in the production process of waterborne coated wooden flooring, the production efficiency of waterborne coating for wooden flooring is extremely low (Yan et al. 2019). Third, due to the high moisture content of the blanks used for the production floor, the poor dimensional stability of the tree species, and the poor sealing properties of the waterborne coating, the stability of the floor in the width direction of the waterborne coating is poor, and it is prone to reduce the size of the width direction and increase the gap between adjacent wooden flooring pieces (Li et al. 2019; Sun et al. 2020).
In this study, the authors developed and applied a novel waterborne coating to wooden flooring products in combination with the finishing technology of wooden flooring and the performance requirements of varnish film. Through the optimization of tree species, waterborne coating formulations, and finishing technology adjustments, this study focused on solving the physical and chemical performance problems of the surface varnish film hardness and wear resistance of waterborne UV coated wooden flooring. This achievement not only expands the range of wooden floor coatings, but also provides a basis and data support for the research and development of waterborne coated wooden flooring.
EXPERIMENTAL
Materials
Wooden flooring substrate
The various wood species used were oak, toothed oak, Robinia, walnut, elm, pinnate pometia, and Newtonia spp. The size of the samples was 910 mm × 122 mm × 18 mm, and the materials were purchased from Xifeng County Senhai Wood Industry Co., Ltd. (Tieling, China).
Waterborne UV coatings
A: The waterborne UV coating was purchased from Shandong Penglai Luyuan Paint Industry Co., Ltd. (Penglai, China), which included primers and a topcoat, and the primers and topcoat were classified as varnish. The primers include a waterborne UV colored primer, a waterborne UV transparent primer, and a waterborne UV functional primer, with solid contents of 6.4%, 42.6%, and 40.0%, respectively, which were all white emulsions and dispersible in water. The topcoat was a waterborne UV matte topcoat with a solid content 41.1%, which was white emulsion and mixed in water, with a glossiness of level 2.
B: The waterborne UV coating was purchased from Shandong Penglai Luyuan Paint Industry Co., Ltd. (Penglai, China), which included primers and a topcoat, and the primers and topcoat were types of varnish. The primers included a waterborne UV colored primer, a waterborne UV filled primer, and a waterborne UV transparent primer, with solid contents of 6.9%, 60.7%, and 60.4%, respectively, which were all white emulsions. The topcoat was a waterborne matte topcoat with a solid content of 48.7%, which was white emulsion and dispersible in water, with a glossiness of level 2.
C: The waterborne UV coating was purchased from Guangdong Hongfang Paint Co., Ltd. (Huizhou, China), which included primers and a topcoat, and the primers and topcoat were varnish products. The primers include a waterborne UV colored primer, a waterborne UV high-definition primer, and a waterborne UV sanding primer, with solid contents of 7.3%, 98%, and 98%, respectively, which were all white emulsion. The topcoat was a waterborne UV matte topcoat with a solid content of 98%, which was a light yellow liquid and dispersible in water.
Experimental Instruments
The following instruments were used: Coating machine (Xiamen Maosen Automation Equipment Co. Ltd., Xiamen, China); humidity and temperature control (Shenzhen Changxu Machinery Equipment Co. Ltd., Shenzhen, China); electric thermostat blast oven (Chengdu Shengjie Technology Co. Ltd., Chengdu, China); infrared drying equipment (Suzhou Green Painting Technology Co. Ltd., Suzhou, China); UV-curing equipment (Dongguan Jingyu Environmental Testing Equipment Co., Ltd., Dongguan, China); varnish film multi-purpose detector (Shanghai Meiyu Instrument Equipment Co. Ltd., Shanghai, China); rolling wear teat machine (Shandong Zhongyi Instrument Co. Ltd., Jinan, China); and electronic balance (Shanghai Sunny Hengping Scientific Instrument Co. Ltd., Shanghai, China), which had a measuring accuracy of 0.1 mg and a measuring range of 0 g to 220 g.
Experimental Principles and Methods
Selection of tree species and optimization of water content
Tree species screening: First, as the raw materials for wooden flooring, tree species with a high density and dimensional stability not only can make up for poor waterborne coating performance, but also highlight the grain and visual characteristics of wood itself. In addition, it can achieve the aesthetic effect of “seeing the wood but not the coating” (Wilfried et al. 2004). Second, it is necessary to select different species as the substrate for the experiment since the different finishing technologies and effects have different requirements.
Optimization of water content: After finishing the waterborne coating process, the moisture content of the treated wood will increase by 1% to 2%, and the increase of moisture content will cause the reduced scale and gap of the waterborne coated wooden flooring after a period of time. Therefore, before finishing, the moisture content of the blank of waterborne coated wooden flooring should be balanced and cured to ensure product quality. The moisture content of walnut, oak, Robinia, and toothed oak was controlled at 8% to 12%. The moisture content of elm, pinnate pometia, and Newtonia spp. was controlled at 9% to 13%.
Process of Finishing Technology
Opening effect of waterborne coating A
Comprised of a waterborne UV primer and a waterborne UV topcoat, the waterborne UV coating A was used for experimental research and analysis with three different finishing technological processes.
Process 1: The finishing process included 5 primer and 2 topcoat layers, and the UV-curing was carried out directly after each coating was finished, the time of UV-curing was 6 s, and the irradiation energy of UV-curing was 230 to 260 mJ/cm2. The wet coating weight of each primer was 8 g/m2 to 10 g/m2, and the wet coating weight of each topcoat was 5 g/m2. The process flow is shown in Fig. 1.