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    Home > Coatings News > Paints and Coatings Market > Study on the surface treatment method of the substrate for the application of electrostectrelectrative powder of birch wood

    Study on the surface treatment method of the substrate for the application of electrostectrelectrative powder of birch wood

    • Last Update: 2021-01-22
    • Source: Internet
    • Author: User
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    Li Xuetao 1, Zhang Wenzhu 1, Li Wenzhu 1, Chen Tianjun 2, Chen Junli 2
    1. Zhejiang Agricultural and Forglin University School of Engineering
    2.
    Summary
    of Green Sea Craft Factory Co., Ltd., Pan'an County, Zhejiang Province, this article preheats the wood, applys polyurethane primer, water-based primer and preheats and sprays powder coatings in four different surface treatments. After the treatment of the plate electrostation powder spraying, coating film curing and other processes to make powder decorative birch board, and sample coating performance in accordance with the requirements of the relevant national standards for testing, analysis and comparison of surface coating film adhesion, pencil hardness and other chemical properties of the cause, to determine the birch sheet in a certain moisture content conditions, through sand, preheated treatment, and then 2 static powder spraying the best surface treatment method.
    beech, Chinese fagus sylvatica, widely distributed in Asia, Europe and North America, is also one of the main tree species of temperant broad-leaved deciduous

    . It is often called "beech" in the course of Trade Use in China. In China furniture materials, interior decoration and art frame and other fields play a unique role. However, birch is a bio-organic material. Will be affected by the climate and biological factors in the environment and prone to decay, mold, moisture absorption, cracking and deformation defects, in order to overcome and improve its defects, the surface coating treatment, the common methods of painting are scratching, painting, brushing, spraying, roller coating, immersion, ionizing and coating and other 8 kinds. At present, the birch decorative materials and birch products, the general use of coating, brushing, roller coating and immersion and other coating methods, and most of the use of organic solvents as the main coating. The volatility of solvents will cause environmental pollution and endanger human health, while the water-based coatings developed in recent years have greatly reduced the toxic effect of paint. However, in the process of coating, there are complex processes, long drying time, low production efficiency, poor product coating adhesion and poor resistance to dry and wet heat and other shortcomings. In addition, there are transport safety issues.
    Powder Coating, a new solvent-free 100% solid powder coating with environmentally friendly, safe, recyclable and wide range of applications, has become a hot topic in recent years in wood materials, especially in Europe MDF (Medium Density Fiberboard, medium density fiberboard) powder coating research and application. The pre-treatment of birch surface is an indispensable and important part of electrostation powder coating, which is related to the spray effect and the quality of the product coating film. Wood electrostitive powder spraying requires the surface of the wood to be smooth and conductive, so that it can quickly absorb the accompanying electrical powder, and finally the powder coating is cured by the hot melt level.
    This paper, through the study of several surface treatment methods of birch static powder spraying, obtains the appropriate surface treatment method for birch electrostitive powder spraying, and provides reference for other solid wood surface powder electrostectrical spraying.
    1 Materials and Methods
    1.1 Test Materials and Equipment
    Test Materials: 18 pieces of birch wood (150 mm x 150 mm x 18 mm) with a moisture content of 9% to 11%, provided by Green Sea Process Factory Co., Ltd., Pan'an County, Zhejiang Province; epoxy polyester powder coating, particle size 200, resin TsI>50 degrees C, theoretical curing conditions under infrared lamps of 150 degrees C/d 3 min, Tiger Powder Coating Manufacturing (Taicang) Co., Ltd.; Wood primer (Gentleman's Orchid PU primer, Carrie's water-based varnish), Jiangshan OpaiMen Co., Ltd.; 180, 240, 32 0 sandpaper, Wuxi Shunda grinding wheel factory; 10% sodium carbonate solution, Changzhou Hengguang Reagent Co., Ltd.; 10% ethanol solution, Changzhou Hengguang Reagent Co., Ltd.; Coffee, Dongguan Nestle Co., Ltd.; etc.
    test equipment: GUYI rotary-wing electrostature powder spray room, Suzhou Solid Art Coating Equipment Co., Ltd.; DK2008 wood moisture rate tester, Hangzhou Taik drying equipment Co., Ltd.; 101-5A type electric thermostat thermostat blowing drying box, Shanghai Espek Environmental Equipment Co., Ltd.; WJD 12000 static high-voltage generator, Hangzhou Wanjia Coating Equipment Co., Ltd.; DR60 A gloss meter, Guangzhou Dongru Electronic Technology Co., Ltd.; QDX paint film multi-use detector, Tianjin fine materials test machine factory; QHQ-A-type portable pencil scratch tester, Taizhou City Ai test instrument Co., Ltd.; QFH-HV600 paint film scratching instrument, Huizhou Yupson Instrument Co., Ltd.; JA5003 electronic balance; ZNLD 1800W homemade infrared lamp solidification device.
    1.2 test method
    1.2.1 surface treatment process
    the surface of the birch plank is rough, there are many small pit pores. The powder coating penetrates and fills these pores during the melting leveling process, and only when these pore defects are filled can the coating form a continuous coating film on the surface level. This test is carried out using 4 different surface treatment methods, the specific process steps can be seen in Table 1.
    method (1) in Table 1 can be called a one-time coating. The substrate of birch wood in turn uses 180, 240, 320, sandpaper sanding, after each sanding with compressed air for dust removal treatment, after sanding to improve the flatness of the surface, (1) in the use of warm-up of birch wood Treatment, the purpose is to regulate its moisture content and surface resistance to improve the amount of powder on the surface of the birch board, (2) and (3) are using polyurethane (Pu) primer and a water-based primer spray surface treatment, the primer plays the role of filling the surface gap of the substrate. Polyurethane primers and water-based primers are two different primers, water-based primers are more environmentally friendly than polyurethane primers, and the amount of primer application is approximately 15 to 20 m2/kg. The primer sander uses 240 , 320 , sandpaper in turn, and (4) is another spray, called secondary coating, on the basis of (1) of the process steps. (4) The thickness of the first spray is about 50 m, cured after sanding and then (1) of the step, the purpose of the first spray is mainly to provide a more flat and smooth surface for the secondary spray, so that the powder coating can melt flat. The effects of different substrate surface treatment methods on coating performance were analyzed to determine the best surface treatment method.
    1.2.2 Electrostectrative Powder Spraying
    The electrostectrectrecturic spray effect of powder coatings is directly related to the surface resistance of the birch plank, which is associated with its moisture content and warm-up temperature. The moisture content of the surface of the birch plank should be regulated to a certain water content range, and it is too high or too low to have an effect on the spraying and coating performance. After many tests, it can be seen that the moisture content of the birch board is controlled at 4% to 8%, the water content of the surface at room temperature is about 7%, the surface resistance is in the order of magnitude 109, there is a better powder rate; With the increase of water content, the moisture volatility is too large when the coating film is heated, which can easily cause the coating film to produce pinhole bubbles and cracking. Therefore, wood water content is preferably not greater than 10%. (1) and (4) warm-up temperature is 80 to 125 degrees C, warm-up 5 to 10min. The wood products to take warm-up measures, so that the water inside the wood to the surface, and reduce the surface resistance, so that the surface resistance value of less than 109 s, the surface moisture content of 4% to 8%, is conducive to improve the surface of the product electrostature powder absorption capacity. Coating thickness after coating curing is generally good at 160 to 180 m, too little powder may lead to leakage and melt flow is not equal, spray too much film orange pattern is serious. The curing conditions set by the test are 150 degrees C/3 min.
    this experiment set the static voltage of 60 kV, the voltage is too high, easy to occur anti-ion flow, resulting in the appearance of the coating ills. The powder pressure is 0.04 MPa and the distance between the nozzle and the sheet is 15 to 20 cm. The preheated wood sample is suspended on the electrostectoral generator shaft of the spray room for spraying test.
    1.3 Coating Performance Detection
    This test is based on the substrate treatment and coating performance related conditions, coating gloss, adhesion, pencil hardness, water-resistant vapor detection method by reference to GB/T 17657-20134 artificial plate and finish artificial plate physicization performance test method" and GB/T 13323-2008 (wood furniture general technical conditions).
    2 Experimental Results and Analysis
    2.1 Experimental Results
    Table 2 shows the effect of 4 different substrate surface treatment methods on coating performance.
    2.2 Results Analysis
    2.2.1 Effects of different substrate surface treatment methods on coating film quality
    Membrane pinhole refers to the defect formed by the failure of the powder coating to escape from the melting to curing process by passing the gas in the powder coating through a highly viscous, near-closed elastic resin layer to the coating surface. A shrink hole is a vortex formed by small particles that are not fully moisturized with a resin that is incompatible with the surrounding area, i.e. a protruding dot in a larger sinking vortex. Birch wood is tight and hard, the surface is rough, the pores are not easy to polish, after the powder-coated birch wood is heated, the air and moisture inside the board will evaporate to the plate to affect the curing of the coating film. The roughness of the surface also prevents the powder from melting and curing from flowing well and does not form an even coating. The water content of the plate is too high, there will also be a bubble phenomenon. It is very important for the process of method (1) to control the moisture content of the substrate and the flatness of the substrate surface effectively by means of reasonable warm-up. Method (2) the substrate has been treated with polyurethane primer, through spraying will make the surface of the plate closed and uniformity to be better, the substrate surface coating quality is good without pinholes and shrink holes and other phenomena, mainly due to polyurethane bottom The paint fills the pores on the surface of the substrate, prevents the water inside the substrate from moving toward the surface, and the sand-glossed primer coating surface is more flat, which helps the flow of the melting and curing phase of the powder coating, while improving the gloss of the coating film. Methods (3) in the substrate using environmentally friendly water-based primer treatment, coating film surface has a slight shrink hole, water-based primer compared to polyurethane primer more environmentally friendly, water-based primer is water-based solvent, in the substrate surface treatment process, the substrate will absorb the water content of the substrate surface so that the water content of the substrate surface increased. After curing the water-based primer film is hard, compared to polyurethane primer, grinding will be more difficult, will lead to the substrate surface is not smooth enough, resulting in slightly worse coating quality (2). Method (4) treatment of the substrate coating quality is good, the main reason is that a coating can well fill the pores on the substrate surface, a spray coating film thickness of about 50 m, after grinding the film surface more flat is conducive to secondary spraying.
    2.2.2 Effect of different substrate surface treatment methods on coating adhesion
    From Table 2, it can be seen that the coating adhesion of methods (1) and (4) is superior to that of methods (2) and (3).
    the adhesion of the paint film refers to the degree to which the lacquer film is firmly combined by physical and chemical forces between the paint film and the surface of the attached object. The adhesion of paint film is one of the important indicators to assess the performance of paint film, and neither method (1) nor (4) is treated with other primers. Powder coating in the melting process by heat on the substrate surface flow and seep into the wood surface gap, curing to form a "paint nail", increased the adhesion of the paint film. The size of the adhesion depends to a large extent on the degree of penetration of the coating into the wood surface and the number of "paint nails". The coating used in the test is epoxy polyester powder coating, its official energy group is polyester resin - COOH group, and birch in cellulose, hexaculose cellulose contains a large number of - OH group, two groups under certain conditions of reaction binding, so as to get good adhesion. Method (1) of the plate surface by sand treatment, the substrate surface is more flat and smooth, at the same time may cause the wood surface polymer molecular chain break and produce some new function groups, enhance the activity of the substrate surface, and then with the powder coating to produce a series of cross-link chemical reactions to improve the adhesion of the paint film surface. Methods (4) The powder coating of the central oxygen resin and polyester resin had an interlinking reaction during the curing process, and the crosslinking reaction between the pyridine base in the polyester resin and the epoxy base in the epoxy resin was an addition polymerization reaction, which was produced without water. Curing coating by sand treatment to smooth the surface, powder spray again, curing powder coating continues to cross-link chemical reaction with the original coating, so its binding force is very good, so the use of method (4) the resulting coating adhesion is good.
    method (2) is the substrate treated by the primer and then powder coating, from the test results, the coating shedding mainly lies between the powder coating and the upper primer layer. This shows that the adhesion between the primer and the powder coating is not as good as the adhesion between the primer and the substrate surface, and also less than the adhesion between the powder coating and the substrate surface. (2) The primer used in the test is a common polyurethane primer on the market, and its film-forming method is natural film-forming, without special process. Polyurethane primer is a two-part chemical structure: isocyanate-based (-NCO) of the A-group (curing agent) and hydroxyl (-OH) of the ethyl (-OH) of the ethyl (-OH) of the aerate resin. The substrate and polyurethane primer have a physical and chemical form of binding, which is mainly chemical binding, adhesion enhancement. A chemical reaction occurs between hydroxyl, which contains a small amount of aolate resin, and the carboxyl in the powder coating to enhance the binding force between the two coatings. Because the amount of aliclicylic resin is small, the binding force between the upper primer and the powder coating is mainly physical. The binding of chemical bonds is the key to adhesion. Therefore, the adhesion of the upper coating film is lower than that of the lower layer. (3) The main component of the water-based primer used in the test is the unsaturated acrylic resin, the functional energy group is mainly polar group -OH, unsaturated polyester resin and birch surface has the same polar group -OH, between them can form a molecular van der Walol, enhance the binding force between the water-based primer and the substrate surface. The molten powder coating is fully in contact with the polished substrate surface and has a chemical reaction with the COOH substrate, which increases the binding force. The increase in water content of the substrate surface may reduce the adhesion of its powder coating.
    2.2.3 Effects of different substrate surface treatment methods on coating gloss and pencil hardness
    Glossyness refers to the ratio of the amount of positive reflected light on the coating surface to the positive reflected light on the ideal standard plate surface under certain human angle conditions (IV). As can be known from Table 2, the gloss of method (2) is 33.8, the highest of the four treatment methods. Method (4) the surface coating film gloss is slightly lower than the method (3), but the surface coating gloss is much smaller than other methods, mainly because the coating surface is rough, the coating film quality is relatively poor. Therefore, the results of this test are mainly due to the different roughness of the substrate surface. Methods (2) and (3) are sprayed primer, substrate surface gloss is higher than powder curing coating sanding treatment (4), because the primer better closed the substrate surface pores, reducing the roughness of the substrate surface, that is, the smoother the surface, the more reflective capacity
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