Wood bonding technology plays an important role in the wood industry. The bonding performance of wood bonding products is largely affected by the properties of wood. The factors affecting the bonding performance of wood are not only inherent in wood, but also produced during wood processing. The former includes wood density, tree species, extracts, pH, etc., while the latter includes wood moisture content and surface roughness.

　　1 Wood density The strength of wood is usually proportional to its density, because the amount of wood cell wall substances contained in a unit volume is the material basis for determining the strength of wood, and the strength of wood increases with the increase of wood density. In an ideal bonding system, bonding failure should occur in the xylem.

 

Many hardwood bonding tests have proved that when the wood is bonded with urea-formaldehyde resin adhesive, when the wood density is less than 0.8 g/cm3, due to the cohesive strength of the urea-formaldehyde resin adhesive itself and the interfacial strength of the urea-formaldehyde resin adhesive and wood Greater than the strength of the wood itself, the bonding failure often occurs in the weakest link in the bonding system, the wood. Therefore, the bonding strength is closely related to the strength of the wood. Generally speaking, the bonding strength increases with the increase in wood density. When the wood density is greater than 0.8 g/cm3, because the cohesive strength of the urea-formaldehyde resin adhesive itself and the interface strength between the urea-formaldehyde resin adhesive and the wood are less than the strength of the wood itself, the bonding failure does not occur in the wood part, but in the wood. Glue layer or interface, therefore, in this case, the bonding strength is almost independent of wood density. When resorcinol-formaldehyde resin adhesive is used to bond all needles and hardwoods, the cohesive force of resorcinol-formaldehyde resin adhesive is very large, so the bonding strength of wood glued products bonded with resorcinol-formaldehyde resin adhesive As the density of the bonded wood increases.

 

　　Although the high-density wood-glued products have high bonding strength, the stress due to changes in their moisture content is also large. When the adhesive is determined, the bonding durability of wood with high density is often worse than that of wood with low density. For example, oak wood laminated lumber made of water-based polymer isocyanate adhesive, although its compressive shear strength is relatively large, when the humidity of the use environment changes greatly, the adhesive layer is more likely to crack. In addition, in order to achieve the ideal bonding effect when bonding wood with high density, the cohesive force of the adhesive used should also be high, so that the strength of the wood itself can be fully utilized. For wood with very high density, such as ebony, iron pigeon, etc., the adhesive is not easily absorbed, so that the drying speed of the adhesive is very slow and the bonding strength is low. Therefore, for wood with a particularly high density, in order to allow the adhesive to penetrate into the cells, it is required to perform double-sided sanding and double-sided gluing, and to wait for sufficient time for the adhesive to penetrate before applying pressure. Sometimes, it is necessary to use polyurethane adhesive, because the adhesion of this type of adhesive does not depend on its penetration, mainly chemical bonding.

　　2. Wood fiber direction and cutting plane direction Wood is an anisotropic material. The wood fiber directions in the bonding system can be perpendicular to each other, parallel to each other or at a certain angle to each other. When two pieces of wood are bonded in the same fiber direction, the bonding strength is the largest. Such products are laminated lumber and laminated veneer lumber (LVL). Commonly used wood products with fiber directions bonded perpendicular to each other include plywood and three-layer Parquet and so on. As the fiber angle between the two pieces of wood to be bonded increases, the bonding strength of the bonding material gradually decreases. When the fiber directions are bonded perpendicular to each other, the bonding strength is the lowest. Compared with wood fibers bonding parallel to each other, the bonding strength is only 1/4 to 1/3 of that when the fibers are parallel to each other. The cutting direction of the bonding surface also affects the bonding strength. Martin et al. studied the influence of wood fiber direction on the penetration of liquid adhesive to the wood surface and showed that: when the adhesive is applied to the end face of the wood, since the wood ducts and tracheids are open on the end cut surface, the adhesive will infiltrate a large amount of the cell cavity of these cells. It is easy to make the end face lack of glue, resulting in a decrease in the bonding strength of the wood. Therefore, under the same bonding conditions, the bonding strength between the end surface and the end surface is lower than the bonding strength between the radial surface and the radial surface, the chord surface and the chord surface.

In actual work, if end-cut surface bonding must be used, in order to prevent the lack of glue, the viscosity and solid content of the adhesive must be increased; use the glue twice, or apply glue on both sides of the substrate, or increase the thickness of the glue layer; use moderate pressure And prevent the glue from squeezing out. For example, when the wood is extended longitudinally by the butt joint, the above measures can be used to avoid lack of glue at the end. Under the condition that the fibers are bonded parallel to each other, the bonding surface can also be divided into a radial section and a chord section. In some cases, there are also differences in the bonding strength of the radial-cut and chord-cut bonded products. When the cohesive force of the adhesive and the interface strength between the adhesive and the wood are greater than the strength of the wood itself, wood damage is likely to occur. If there is a difference in the strength of the wood's own chord section and the diameter section, the bonding strength of the chord section and the chord section bonding product and the diameter section and the diameter section bonding product are also different.

 

　　 For example, the radial shear strength of larch wood along the grain is greater than its chord direction. When using water-based polymer isocyanate adhesive to bond larch boards, the normal compressive shear strength of larch radial cut boards is higher than that of string cut boards. The wood failure rate of larch string cutting board and string cutting board glued products is relatively high. It is basically wood damage, and most of the wood damage occurs at the junction or wheel boundary of its sharp change in the morning and evening (therefore, in API When the adhesive is used to produce larch glulam, it should be bonded as far as possible. 3 Wood moisture content Wood moisture content has a great influence on the bonding performance. When the wood moisture content is high, the adhesive applied on the bonding surface When it is diluted, its viscosity will decrease, excessively soaking into the wood tissue, causing lack of glue, resulting in a decrease in bonding strength. Wood drying at high temperatures for a long time, its moisture content is too low, it will also weaken the wetting effect of the adhesive and reduce the bonding strength 。 Generally, the bonding strength of wood is higher when the moisture content is 5% to 16%.

　　 In addition, wood has the properties of shrinkage and wet swelling. If the moisture content of the wood is too low, its products will be damp and swell; if the moisture content is too high, it will desorb and shrink. Generally speaking, the adhesive itself cannot resist the very high stress caused by the expansion and drying of wood, which is one of the main reasons for cracks and deformation of wood products. This situation occurs from time to time in the production and use of furniture. The equilibrium moisture content of the wood in the production area of ​​the furniture is often inconsistent with the place where it is used. This requires that the moisture content of the wood used in the production of furniture is as close as possible to the humidity of the use environment. For example, furniture purchased in Guangzhou is prone to cracking when shipped to Xinjiang for use. This requires that the moisture content of the wood used by the furniture manufacturer should be close to the equilibrium moisture content of the wood where the customer purchasing the furniture is located. Taking into account the hygroscopic hygroscopicity of wood drying, generally speaking, the final moisture content of wood drying and the wood equilibrium moisture content of the use area differ by about 2%.

　　4 The shrinkage and expansion rate of wood Due to the natural shrinkage of the adhesive when curing and the difference in properties between the adhesive and wood, internal stresses exist in the bonded joints. The concentration of internal stress will reduce the bonding strength. In order to reduce the stress generated by the alternating heat or high temperature solidification and cooling, the thermal expansion coefficient of the adhesive and wood should be as close as possible.

 

　　 There are two main ways to reduce internal stress: one is to add fillers, and the other is to use adhesives with good elasticity. When wood is bonded to other materials, if the shrinkage rate difference between the materials is too large, measures should be taken in advance to reduce the dimensional change of wood, or use an adhesive with excellent elasticity for bonding. For example, during metal-wood bonding, the hydroxyl components in the wood cause the wood to shrink and swell through moisture absorption or desorption, which is the main reason for the poor dimensional stability of wood. By blocking the wood hydroxyl groups and forming chemical crosslinks between the hydroxyl groups, the dimensional stability of the wood can be improved, and the strength of the modified wood can be improved. Therefore, the wood is treated with phenolic resin in advance, and then bonded with the metal after fixing its size, which can reduce the stress caused by the difference in shrinkage between the two. For another example, when veneer or plywood is combined with foam board to make partition board, it will be better to fix the size of veneer or plywood and then compound with foam board.

 

　　 5. Surface processing method The bonding plane of the wood must be smooth and straight. If it is uneven (curved or warped), the two planes cannot be in close contact, which will affect the strength and quality of the bonding. Therefore, the wood must be mechanically processed before the bonding operation, which may cause mechanical damage to the wood cells, and the degree of damage varies with the type of mechanical processing and the degree of processing. During the planing process, the surface wood tissue is less damaged, and the lumen of the wood cells is in an open state. After the adhesive is immersed, it is easy to form an effective glue nail. In order to reduce the damage to the wood structure, the planer must be sharp, otherwise, the damage of the wood structure will be more serious. A. P. Singh's research found that the surface of P. radiate wood planed with a sharp planer, except for slight damage to the tracheids adjacent to the glue layer, the axial tracheids and rays have normal shapes. The glue layer between the wood surface is thin and has a normal width (see Figure 3). In contrast, the surface of radiata pine wood planed with a blunt planer has severely damaged axial tracheids and rays. Due to the blunt planer, the cells are seriously crushed, especially the crushing of the cells next to the glue layer is likely to block the cell cavity and prevent the penetration of the adhesive into the cells, resulting in a thicker glue layer and uneven thickness. The fine planing of the wood surface helps to ensure the uniformity of the adhesive layer on the surface of the bonding material. Planing the surface of the wood can expose the S2 layer of cell wall with strong polarity, which is very beneficial for bonding. Although sanding process will make the wood surface very smooth, the lumen of the wood tissue is damaged. At the same time, some cells are very smooth, and the cavity is easy to be blocked by sanding powder. Therefore, the bonding strength is lower than that of planing. .

6 Surface characteristics of technical lumber. Because wood is a porous material, when two pieces of wood are bonded together, only a small part of the apparent area is actually in contact, and this area is affected by factors such as wood structure, surface roughness and applied pressure. Different but different. That is to say, even if the pressure applied during bonding is not very large, the bonding material may endure a relatively high pressure on a small part of the contact area, or even be crushed. Generally speaking, a certain roughness of the surface, its unevenness, pores and grooves are conducive to the penetration of the adhesive, which can produce mechanical bonding. However, if the roughness is too large and the cavities and grooves on the surface are too deep, the remaining air or adsorbed moisture will hinder the penetration of the adhesive and not be conducive to bonding.

　　7 Wood surface passivation Keeping the wood surface clean and preventing pollution are the basic conditions for good bonding. In addition to air pollution (such as pollution by dust particles in the air), the surface of wood is prone to experience a kind of "self-pollution" over time, which changes the properties of the wood surface and affects the bonding properties of the wood.

 

Within a few hours after cutting, the lipophilic, low-molecular-weight substances move to the surface of the wood to form a "weak interface layer" with low surface energy. The change experienced by the wood surface is often called "surface passivation". The rate and degree of chemical conversion vary with wood species and storage temperature. The lipophilic substances in wood are mainly composed of resins, fatty acids and their esters, paraffin wax and terpene compounds. Acidic and neutral adhesives cannot easily penetrate this lipophilic layer, but alkaline adhesives can saponify fatty acids to a certain extent, so that the lipophilic layer disappears, so that the adhesive can penetrate the wood well. R. M. Russbaum found that the surface wettability of European spruce (Picea abies) wood was significantly reduced after 3 days of cutting. Therefore, he pointed out that the maximum storage time of European spruce wood used for coating and bonding to avoid surface passivation is 3 days, and it should be used within 2 to 3 days after its processing to avoid natural passivation of the surface and affect its bonding strength .

 

　　In the production of wood glued products, it should be emphasized that the wood must be bonded within 24 hours after planing or sanding. For example, in the production of glulam, wood should be bonded and pressurized as soon as possible after planing or milling the fingers, and the pressure should not exceed 24 hours from planing or milling to the bonding. In order to eliminate the "self-contamination" of the wood surface and the impact of environmental pollution on the wetting of the wood surface, the wood surface must be effectively pretreated before bonding. Wood surface treatment can use mechanical and chemical methods: cleaning with solvents to clean the bonding surface of the wood to make the bonding surface as large as possible; mechanical methods (planing or sanding) to remove surface contaminants and improve surface roughness To increase the surface area of ​​bonding.

　　8 Wood extracts Wood extracts have a certain effect on the wetting, penetration and curing process of the adhesive. Generally speaking, wood with a large amount of extracted components is difficult to be fully wetted by the adhesive and has poor bonding strength. The wood that is difficult to glue, such as Dipterocarpaceae wood, is boiled in hot water at 100°C for 24 hours, and 5% to 8% of the weight of the wood will be extracted. , Boiled under the same conditions, the extracted components only accounted for 0.1% to 0.2%. Surfactants can also be added to hot water or alkaline water for cooking, but this requires a lot of costs, so there are certain difficulties in practical applications.

　　 For resin-containing wood such as larch and masson pine, it is first dried at a high temperature to make the resin seep out from the wood, then the surface is washed with a solvent to remove the resin, and then the surface is planed. Bond the wood immediately after finishing the surface treatment to prevent the resin from overflowing again. Phenolic adhesives are recommended for bonding.

　　9 The pH value of wood The pH value of wood affects the curing time of the adhesive. For example, poplar, especially Daqing poplar, has alkaline pH value, which can prolong the curing time of urea-formaldehyde resin adhesive. Because urea-formaldehyde resin is cured under acidic conditions, and poplar is alkaline will reduce the acidity of the adhesive. Adding more curing agent when adjusting the glue and adjusting the pH value to 4.0~5.0 is the simplest way to solve the problem of poplar alkalinity affecting the curing time of urea-formaldehyde resin. In actual production, NH4C1 or mixed curing agent can be used according to actual conditions, such as adding some kind of acid to ammonium salt. Acidic wood, such as oak (pH 3.5), high acidity will hinder the gel time of some adhesives that cure under alkaline conditions. On the contrary, it helps to shorten the curing time of urea-formaldehyde resin adhesives.

Adhesive formula for bamboo products 5

1. Purpose

 

　　Bamboo is an indispensable material for industrial and agricultural production, construction, and furniture manufacturing. my country’s bamboo crafts enjoy a high reputation in the world. However, because bamboo is difficult to bond, the quantity and quality of bamboo products are seriously affected.

 

This agent is prepared by mixing formaldehyde, urotropine, sodium hydroxide, ammonia, ammonium chloride, urea, etc. as raw materials, and is specially used for bonding bamboo products.

 

2. Raw materials

 

(1) Formaldehyde: colorless gas. It has a special irritating odor, which is irritating to human eyes and nose. Easily soluble in water and ethanol. The concentration of the aqueous solution can reach up to 55%, usually 44%, and is called formalin, commonly known as formalin. It has a strong reducing effect, especially in alkaline solutions. In this agent, it is used as an adhesive in combination with other chemicals. Use industrial products.

 

(2) Urotropine: the scientific name hexamethylenetetramine. White crystalline powder or colorless shiny crystals. Almost odorless, soluble in water, acetone, ethanol and chloroform, insoluble in ether. Used as a curing agent for resins and plastics, a vulcanization accelerator for rubber, an anti-shrinking agent for textiles, and used to make drugs, fungicides, explosives, etc. Used as an adhesion aid in this agent. Use industrial products.

 

(3) Sodium hydroxide (5% solution): also known as caustic soda and caustic soda. White solid, in the form of granules, flakes, rods or blocks. It is a strong alkali, which is highly corrosive to skin, fabrics, paper, etc. It has strong hygroscopicity and easily absorbs moisture and carbon dioxide in the air and gradually turns into sodium carbonate. It is easily soluble in water and emits heat strongly. It is widely used in papermaking, rayon, dyeing, soap, petroleum and other chemical industries. Used as a pH regulator in this agent. Use industrial products.

 

(4) Ammonia: Aqueous solution of gaseous ammonia dissolved in water. Ammonia is easily volatile and escapes, so it has a strong ammonia irritating smell. Weakly alkaline. It is generally called ammonium hydroxide (NH4OH) solution. The most concentrated ammonia contains 35.28% ammonia. Mainly used as liquid fertilizer. This agent is used as a raw material for curing agent. Use 25% ammonia water.

 

(5) Ammonium chloride: commonly known as Naosha. White crystals, easily deliquescent, soluble in water and glycerin, slightly soluble in ethanol, used as an expectorant in medical treatment. This agent is used as a raw material for curing agent. Use industrial products.

 

(6) Urea: also known as urea and carbonamide. Colorless crystals. Exist in large amounts in the urine of humans and mammals. Soluble in water, ethanol and benzene, almost insoluble in ether and chloroform. The aqueous solution is almost neutral. Used in fertilizer, explosives and other industries. Used as an adhesion aid in this agent. Use industrial products.

 

　　3. Formula (parts by weight)

 

　　 Formaldehyde (37%) 372

 

　　 urotropine 4.1～5.2

 

　　 Sodium hydroxide (5% solution) appropriate amount

 

　　 Urea 145

 

　　 Ammonia (25% solution) 40

 

　　 Ammonium chloride 8

 

　　 water amount

 

　

 

4. Preparation method

Put the formaldehyde in an enamel reaction pot, add urotropine, stir to dissolve it, add 5% sodium hydroxide, and adjust the pH to 7-7.2. Finally, add 100 parts of urea, heat directly with fire to 95°C, keep stirring for 1 hour, then lower the temperature to 75°C, and adjust the pH of the solution to 7.0-7.2 with 5% sodium hydroxide. Incubate at 75°C and stir for dehydration for 3 hours, heat up to 90-95°C, concentrate until the material liquid becomes silky, cool down to 50°C, adjust the pH to 7.0-7.2 with 5% sodium hydroxide, reduce to room temperature, stop Stir and discharge is the winner. It can be stored at room temperature for 1 to 2 years.

 

　　 Take another 40 parts of 25% ammonia water and 8 parts of ammonium chloride to mix, stir to dissolve, then add the remaining 45 parts of urea and about 50 parts of water, fully stir to get the curing agent.

 

5. How to use

 

      When using, first clean, level and dry the bamboo products to be bonded. According to the weight ratio of bonding main agent: curing agent=9:1, stir the two thoroughly to obtain the adhesive. When in use, apply the agent evenly on the surface of clean and dry bamboo products that need to be bonded. After bonding, they must be tied or pressed with a rope, cured after 2 days, and can be used after planing.

The good compounding effect of two-component polyurethane adhesive is related to various conditions, among which the change of working environment is also a very important factor. That is to say, as the seasons and climate change, in order to obtain the ideal composite effect, it is necessary to make some fine adjustments to the glue application process.

 

Simply put, the seasonal climate change that affects the compound is the change in the two major indicators of environmental humidity and temperature: specifically, in spring and summer, especially during the rainy season, the relative humidity of the air is relatively large, and it can even reach saturation while in autumn and winter. The air is dry and the humidity is low; in terms of temperature, summer is much higher than winter, and the maximum difference between the two can be close to 30-40°C (here is the southern region without indoor heating as an example for comparison). If you don’t pay attention to these differences, the following problems are likely to occur during compounding: When the air is humid, the glue is often incompletely cured, that is, it will not dry completely, and the residual viscosity will be large. In serious cases, it can even be peeled off the composite film. It is observed that there is a phenomenon of wire drawing, especially when the film itself is hygroscopic, such as when it is laminated with a nylon film. This phenomenon is more likely to occur; secondly, the humid air will condense on the gluing anilox roller, thereby removing moisture When it is brought into the glue basin, as time goes by, the glue gradually changes from transparent to chaotic, white, and even loses its bonding effect. Humidity and high temperature also make it difficult to preserve the glue. If the working glue is not used on the same day After leaving it overnight, it will often turn white and agglomerate, forming a gel (jelly-like). In contrast, when the weather is cold in winter, the working glue that has not been used up will still maintain good transparent fluidity overnight, and it can be taken directly without even being mixed into the newly prepared working glue in batches. Come on the machine to use. On the other hand, in winter when the temperature is low, the glue will become viscous and the leveling properties will decrease. When the laminating machine is running at high speed, a large number of bubbles are likely to accumulate on the surface of the glue and the edge of the gluing anilox roller in the glue basin. At this time, it may cause the transfer of cavitation, and the insufficient glue quantity will affect the composite fastness; at the same time, due to flow The flat dispersion performance is not good, and the appearance effect of the composite film will also become worse. For example, the flow of glue is more obvious, sometimes in the shape of orange peel. When used in composite aluminum foil or aluminized film, if the printing surface has a large area of ​​white ink or When the light-colored ink is used, it is easier to form small white spots and spots; in addition, due to the low temperature in winter, the temperature of the curing room is very different from the temperature of the outside environment. If the insulation measures are not adequately done, the heat loss rate is much faster than that of the high temperature. Summer is fast, which often makes the temperature in the curing room not reach the set temperature (usually 50°C). Therefore, the curing effect is also affected to a certain extent. Under the same curing time, the composite fastness may be lower than in summer.

 

　　Analyzing the reasons for the above phenomenon, it is necessary to understand the influence of moisture and temperature on the adhesive. First of all, for two-component polyurethane adhesives, moisture can react with the NCO groups in the curing agent, just like the main agent, namely polyester/polyether polyol. According to calculations, 1g of water can consume 26-32g of curing agent. Of course, this is purely in terms of reaction weight ratio. In practice, the water mixed into the working glue will react with the curing agent when it reacts with the curing agent. Competing. But no matter how much water is involved in the reaction, this is undoubtedly consuming the curing agent, making the amount of its reaction with the main agent not reach the originally set working ratio, thus resulting in incomplete curing and residual viscosity . The viscosity and reactivity of the adhesive are greatly related to temperature. The viscosity value given by the adhesive manufacturer is measured with a rotational viscometer at 25°C as the standard temperature. This means that in actual use, the temperature of the working environment can be more than 10 degrees higher than that in summer, and it may be higher in winter when it is cold. It is as low as 20°C. The viscosity of the glue is just the opposite of the temperature, that is, the same glue exhibits a low viscosity value at high temperature, good flow and dispersion performance, low temperature, high viscosity and poor leveling. In addition, the crosslinking and curing reaction between the two components of the adhesive is slow when the temperature is low, and fast when the temperature is high. This is precisely why there is a curing room (to speed up the curing reaction speed and improve production effectiveness).

 

In view of this situation, when using two-component polyurethane adhesives for compounding, the following adjustments can be made according to environmental changes: If the air is humid and the temperature is high, the amount of curing agent should be appropriately increased by 10% to 20% to compensate for the moisture. Consumption; often use dry cotton yarn or cloth scraps to absorb the condensed water droplets on the laminating machine to prevent them from falling into the rubber basin; the unused working glue can be diluted with a small amount of solvent, and then sealed for storage. If conditions permit, it can be placed Keep it in a small refrigerator for better results. When you reuse it next time, it should be thawed in a closed condition and mixed with the newly prepared working glue. When the temperature in winter is low, the working glue can be prepared by adding more solvent to reduce the viscosity of the system, improve the leveling and dispersion, and also reduce the generation of bubbles in the rubber basin during work. However, the concentration of the working glue will be reduced in this way. If you do not want to change the working concentration, you can replace part of the ethyl acetate with a small amount of acetone, that is, use a mixed solvent of acetone and ethyl acetate as the diluent. The ratio can be 2:8 or 3:7. In addition, when the temperature is low in winter, the temperature setting of the curing room can be adjusted slightly higher to ensure that the actual temperature can meet the requirements, so as not to affect the curing effect.

(1). Purpose of wood surface coating: 1. Beautify the surface-enhance the three-dimensional sense of wood texture and the touch of the surface while imparting color, luster and smoothness; 2. 2. Material protection-moisture resistance, water resistance, oil resistance, chemical resistance, insect resistance, corrosion resistance, moth resistance, etc.; 3. Special functions-temperature indication, electrical insulation, sound insulation, heat insulation, etc.

 

　　 (2). Requirements for coating on the surface of wood There are two main difficulties in coating the surface of wood: 1. Porosity of water materials: The pore area on the surface of wood accounts for about 40% of the surface area on average, less than 30%, and as many as 80%. Therefore, the wettability and adhesion of wood water materials is a big problem in wood coating; 2. The hydrophilic expansion and contraction of wood, the wood swells when exposed to water, dehydration and shrinks, all causing the coating film to crack and fall off, so the long-lasting stability of the coating film is another major problem in wood coating; in addition, some wood contains acid and grease Similar substances, easy to seep out of the surface. In addition, the softness and hardness of wood texture/material structure/uniformity of surface hue are also difficulties in wood coating because of the characteristics of wood itself and the difficulties in coating. The characteristics of the wood coating and the special coating system

 

　　(3). Wood coating requirements for coatings: 1． Because the wood furnishing is a multi-layer supporting system, the primer is required to have good permeability, wettability and superior adhesion to the wood. The layer should also have good adhesion between layers to form a whole. 2. The coating must have a good strain force to ensure the durability of the coating; 3． The coating should have good decorative properties to ensure the clarity of the wood grain and the obvious three-dimensional effect; 4． The coating should have good mechanical properties, water resistance, chemical resistance, and pollution resistance; The coating film has high hardness, strong friction resistance and good hand feeling; 6． The coating should have good workability, recoatability, and can be changed according to the requirements of use.