Waterborne Coating Process and Formula Design for Solid Wood Furniture

WANG De-hua, LIU Jian-ping, ZHANG Jun, WANG Chen (Beijing Zhanchen New Materials Group Co., Ltd., Beijing 101106, China)

Abstract: Timber characteristics for solid wood furniture were introduced. The water content of wood should be suitable controlled. Clear coatings instead of solid colour coatings were suitable for the coating of solid wood furniture to match the natural texture of the wood. The coating process and formula design for solid wood furniture were discussed.

0 Preface

Wood is a blessing given to human beings by nature. Among them, the lines, wood tumors, holes, and insect bites all perfectly show the laws of nature. In a sense, the finishing of paint on the surface of solid wood furniture is just to protect the natural perfection of the wood, or it has a little deepening and contrast that embodies the texture as a supplement, and its main body has always been the wood itself. At present, most of the clear varnish systems used for solid wood coatings, especially primers, have extremely high requirements for transparency, because this can express the natural beauty of the wood grain, thereby enhancing the value of the wood. At present, most commonly used coatings are solvent-based coatings, such as PU (polyurethane) coatings, NC (nitrocellulose) coatings, etc. With strict national environmental protection requirements, due to VOC (volatile organic compound) emissions Too high, it has been gradually phased out and replaced by green environmentally friendly waterborne wood coatings. However, ordinary water-based wood coatings are difficult to achieve the effect of solvent-based coatings in preventing wood fiber bulging, transparency, substrate tinting, fullness and porosity. In particular, ordinary water-based wood coatings have obvious defects in preventing wood fiber bulging and substrate polishing effects. Compared with traditional solvent-based coatings, the color and texture of the wood itself cannot be displayed well. This article introduces the water-based coating technology design and formula design of solid wood furniture.

1 Requirements for moisture content of solid wood furniture

Wood is a naturally growing organic material that consists of high-molecular and low-molecular substances. The main substances that make up the wood cell wall are cellulose, hemicellulose and lignin, which generally account for more than 90% of the wood. In the wood cell wall, cellulose plays the role of skeleton, hemicellulose plays the role of cohesion, and lignin plays the role of hardening. They are crisscrossed in the cell wall, the arrangement and combination are complicated, and their distribution is uneven. Wood has a strong ability to adsorb water, because lignocellulose has a large number of hydroxyl groups (-OH) and is a hydrophilic substance. The moisture contained in the wood can be divided into the following three forms:
(1) Free water. Refers to the water present in the cell cavity and in the interstices of cells. Free water affects the apparent density, preservation, corrosion resistance and flammability of wood.(2) Adsorb water. Refers to the water absorbed between the fine fibers in the cell wall. It directly affects the strength and volume expansion of the wood.(3) Combined water. Refers to the bound water in the chemical composition of wood. Has no major impact on the properties of the wood.The moisture content of freshly cut wood is usually more than 35%, the moisture content of air-dried wood is 15% to 25%, and the moisture content of indoor dry wood is often 8% to 15%. In order to prevent the wood from being deformed or cracked due to the large change in moisture content during use, the wood must be dried to the average equilibrium moisture content of the use environment for many years before use. Because if the wood is not properly dried and the moisture content has not reached the equilibrium point suitable for the local environment, the coating will be applied, which will affect the color and gloss of the final coating film, and cause coating film defects such as bubbles, pinholes, whitening, and even the coating Poor adhesion, paint peeling problems, and thermal expansion and contraction of the wood after coating, which easily cause problems such as cracking and peeling of the coating film.

 2 Common coating types for solid wood furniture

In order to express the natural texture of wood, solid wood furniture mostly adopts varnish coating process. If the solid color coating process is used, it will cover the natural beauty of the wood texture and cannot reflect the value of the wood itself. Board as substrate. Different types of wood have different colors. Due to the deposition of various pigments, resins, tannins and other oxidizing substances in wood cells, the wood will show different colors. In order to reflect the unique color of wood, varnish is the most suitable coating process. Especially the mahogany furniture, the color itself is very colorful, and the color will change with the angle and the light. If you use real color coating, it will cover up the natural color of mahogany, which will not reflect the value of mahogany. Even apparently white wood such as Fraxinus mandshurica and Elm wood, in order to reflect the texture of the wood itself, is only painted in transparent color, so that customers can intuitively feel the natural texture of the wood.

3 lacquer painting process design of solid wood furniture

In order to reflect the real texture and three-dimensional sense of solid wood materials, and the true value of solid wood furniture, solid wood furniture coatings generally make the background color relatively heavy, increase the three-dimensional sense of color, and enhance the appreciation and use value of solid wood furniture.

3.1 Material sanding

The material sanding process is a very important link in the production process of solid wood furniture. It directly affects whether the subsequent production can be carried out smoothly and the appearance quality of the product, which has an important impact on the final sales of the furniture. According to the final coating effect, the sanding process adopted is different. The wood that needs to be open-painted needs to be pulled and pulled to make the pipe more prominent. When sanding, the softer wood should not be sanded with larger pressure, because when the softer wood receives greater damage, the wood fibers will be loosened, which will cause water absorption in the later construction of water-based wood coatings. Too much will cause the wood to swell and affect the final coating effect of the furniture; and the softer wood needs more detailed sanding to ensure that the burr is sanded as clean as possible to ensure the subsequent construction effect.

3.2 Anti-swelling transparent primer

The solid-water-based anti-swelling transparent primer is very important. The anti-swelling primer has two functions. One is to prevent the moisture brought by the subsequent coating process from penetrating into the wood, causing the wood to swell and affect The flatness of the coating film; the other is that a colorant can be added for under-coloring to increase the three-dimensional feeling of the color. Anti-stretch primer is generally not too high in viscosity, and the -2 cup viscosity (the same later) can be controlled at 8 ~ 12 s (30 ℃, the same later), so that it can better penetrate into the wood and act as anchor Determine the role of glitches. In addition, the coating amount can be controlled at (50 ± 10) g / m2, which guarantees that Gris can color better. Anti-stretch primer generally needs only 30 to 60 minutes to dry (30 ℃, the same hereinafter).

3.3 Grace

Grace’s role is to play a better role in expressing color gradation, especially rough wood such as Fraxinus mandshurica and elm. Grace also has a role in that there will be a sanding process after the previous background color is painted. Because the burr will stand up after the anti-stretch primer is applied and then it will be polished away, resulting in some of the background color will be Abrasive. Through Grice’s wiping construction, the ground color can be polished and the white areas can be recolored to ensure the uniformity of the color. The Gris coating amount generally only needs (30 ± 5) g / m2, and the drying time generally only needs 30 to 60 minutes.

3.4 clear primer

Gris can apply a clear primer once the drying time has elapsed. The choice of clear primer varies depending on the effect of the open or closed paint. The open primer needs better wettability to the substrate, to ensure that the catheter is uniform without small round eyes, and the construction viscosity and solid content are relatively low. The construction viscosity is generally 20-50 s, and the coating amount is (60 ± 10) g / m2 can be used, and the drying time is about 2 h; while the closed coating must ensure a certain filling, the wood should be as shallow as possible, such as red cherry, black walnut and so on. In order to ensure that a certain coating thickness can be achieved without sagging, the construction viscosity should be a little higher. Generally, it requires 60 to 100 s, the coating amount is (150 ± 10) g / m2, and the drying time is more than 4 h. In solid wood coating, a transparent primer is very important, because to reflect the texture of the wood, the transparent primer is required to have excellent transparency, can not be white, hazy, and thick coating can not foam. And in order to reflect the color of the wood itself, the transparent primer needs to have a certain dissolving power and color rendering effect, so as to dissolve the pigment in the wood to the surface, which has the effect of flashing color with different angles and light.

3.5 Finishing paint

The solid color coating surface should be controlled within 20% of the overall color, so as to better reflect the coating film and color layering, and make the coating film more transparent. The color correction topcoat is required to have excellent leveling and transparency to ensure the permeability of the coating film. Generally, the open coating coating amount (30 ± 10) g / m2 is sufficient, and the drying time is about 1 h; while the closed coating coating amount can reach (60 ± 10) g / m2, and the drying time is more than 2 h. It can better cover the sanding marks and improve the fullness of the coating film.3.6 Clear finishIn order to ensure the anti-scratch performance of the coating film, the two-component or water-based UV is the best. Although the overall cost of one component and the ease of construction are good, the scratch resistance, hardness, and chemical resistance of one component are generally inadequate, which cannot meet the requirements of the above aspects of solid wood furniture, especially dining tables, office furniture, and coffee tables. Water-based UV varnish can be used for flat furniture. Water-based UV has the characteristics of fast drying, good pressure resistance, excellent chemical resistance, and excellent scratch resistance. However, it requires light curing, so it is only suitable for the coating of flat parts. Water-based two-component varnish can be used. The two-component system is a cross-linked coating. Through crosslinking, the denseness of the coating film is greatly improved. The final physical and chemical properties of the coating film have even reached the effect of solvent-based PU coatings. It has excellent chemical resistance, hardness, fullness and scratch resistance, but water-based two-component varnishes will dry relatively slowly and require a large drying room to dry.
Similarly, if it is open coating, the coating amount is (60 ± 10) g / m2, and the water-based UV varnish can be packaged after UV curing and drying after 1 hour; the water-based two-component varnish needs to be dried for more than 24 h. . The coating amount of water-based UV varnish for closed coating can be controlled at (100 ± 10) g / m2. After 2 hours of drying time, it can be packed after UV curing and cooling; the coating amount of water-based two-component varnish requires (120 ± 10) g / m2, drying time more than 48 h.

 4 Importance of drying conditions

Due to the nature of water-based wood coatings, wood will swell when exposed to water, especially at the ducts of wood. Therefore, how to control wood tendons is the key point of water-based wood coatings in solid wood coating. If the drying temperature is high, the water volatilizes quickly, and the coating film surface is baked before the water has penetrated into the wood, so that the wood tendons will not rise. In order to prevent bulging, it can be solved well by fast high temperature baking. But temperature has a direct effect on wood strength. After the wood is heated, the cement fibers in the wood fibers and cell walls will soften, which will cause the strength of the wood to decrease. Experiments show that when the temperature is increased from 25 ℃ to 50 ℃, the compressive strength of wood along the grain will decrease by 20% to 40%, and the tensile strength and shear strength will decrease by 12% to 20%. When the temperature is above 100 ℃, part of the tissue in the wood will be decomposed, volatilized, the wood will become black, and the strength will be significantly reduced. Therefore, if the drying time exceeds 1 h, the drying temperature should not exceed 50 ℃.

5 Product formula design

5.1 Anti-swelling transparent primer

In any waterborne wood coating formulation, the film-forming substance is the most important, so the choice of anti-swelling transparent primer resin is very important. The resin for preparing the anti-swelling transparent primer can be selected from acrylic resins with fine particle size (good permeability), high hardness (high Tg, good anchor burr), fast drying, and good film formation.

5.2 Greski materials

Greski materials play a pivotal role in the above coating system, which requires excellent wiping performance (slow drying, good color collection), and excellent tinting power, color development power and adhesion. The formulation design is mainly improved from three aspects: one is the choice of resin, which can choose the resin with fine particle size, and it must have good film-forming, flexibility and adhesion; the other is to choose the appropriate rheological additive, The main function is to control the tinting strength and prevent splashing. The third is to add a suitable co-solvent to extend the opening time of Tigris and improve the construction performance. If it cannot be dried immediately after construction, the color will not be cleaned and the layering will be poor.

5.3 Clear primer

Transparent primer is the key point in solid wood coating, which requires excellent transparency and hardness, so as to reflect the beauty of wood. The resin system can choose special modified polyurethane dispersion, because the polyurethane dispersion has a particularly fine particle size, excellent fullness and transparency; in order to make the transparent primer have excellent color rendering effect, a small amount of solvent can be added to the formula In order to achieve the role of dissolving wood pigments.

5.4 Retouching finish

Due to national restrictions on atmospheric emissions, alcohol-based color correction cannot meet the current national requirements for VOC emissions, so the process of color correction water cannot be used, and only the method of color correction topcoat can be used to solve the problem. The resin can be selected from polyurethane dispersions with fine particle diameters, good film-forming properties, and good leveling properties, and requires good compatibility with aqueous color concentrates.

5.5 Clear finish

The transparent topcoat is a direct reflection of the advantages and disadvantages of the entire furniture. A good transparent topcoat has excellent transparency, hardness, chemical resistance and scratch resistance. In order to meet the special requirements of the coating film, there are two schemes: one is to use water-based two-component resin, you can choose a resin with a medium hydroxyl value (hydroxyl value 80 ~ 100), and choose a water-based curing agent mixed with IPDI + HDI. Meet the requirements of scratch resistance, chemical resistance and hardness of the coating film, and can achieve fast drying speed and transparency; the other is the use of water-based UV resin as a transparent clear paint, mainly used for the coating of flat parts It has the advantage of high efficiency when mounted. However, it is necessary to choose products with good transparency, because solid wood furniture needs higher permeability to achieve the expression of wood beauty. Water-based UV resins can be selected from DSM, Covestro, Zhanxin and Opeldi.

6 Conclusion

With the continuous expansion of the coating field, the types of objects to be painted have increased, and the requirements for the effects of coating and the performance of the coating film have also varied, which has prompted the coating industry to divide the types of coatings more and more fine . In the development of water-based wood coatings, according to its characteristics, choose a suitable coating field, and then choose an optimal coating method, which can promote strengths and avoid weaknesses, so that water-based wood coatings can find a good breakthrough in the market breakthrough process , Thereby accelerating the industrialization process and technical development of waterborne wood coatings. Through reasonable process and formula design, waterborne wood coatings can solve the problems of solid wood tendons, transparency, surface scratch resistance, chemical resistance and hardness, and accelerate the technical improvement of solid wood furniture waterborne coatings.

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