FAQ

(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.

The good composite effect of two-component polyurethane adhesive is related to many conditions, and the change of working environment is also an important factor. That is to say, with the change of seasonal climate, in order to obtain the ideal composite effect, it is necessary to make some fine adjustments to the glue use process.

To put it simply, the seasonal climate change that affects the compound is the change of the two major indicators of environmental humidity and temperature: specifically, in spring and summer, especially during the plum rains season, the relative humidity of the air is relatively high and 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, the maximum difference between the two can be nearly 30~40 ℃ (here is to compare the southern area without indoor heating as an example). If these differences are not paid attention to, the following problems are likely to occur during compounding: when the air is humid, the glue is often not cured completely, that is, it is not dry and has large residual viscosity. In serious cases, wire drawing can even be observed during peeling of the composite film, especially when the film itself has large moisture absorption, such as when nylon film is used for compounding, this phenomenon is more likely to occur. Secondly, moist air will condense on the gluing anilox roller, thus bringing water into the glue basin. With the passage of time, the glue gradually changes from transparent to chaotic and whitish, and even loses its bonding effect. Damp, heat and high temperature also make it difficult to preserve glue. If the prepared working glue solution is not used up on the same day, it will often turn white and agglomerate to form gel (jelly gel-like) after being left overnight. In contrast, when it is cold in winter, the previously unused working glue solution still maintains good transparent fluidity overnight, and even can be directly used on the machine without being mixed into the newly prepared working glue solution in batches. On the other hand, in the winter when the temperature is low, the glue will become sticky and the leveling will decrease. When the laminating machine is running at high speed, a large number of bubbles are easy to accumulate on the glue surface and the edge of the gluing anilox roller in the glue basin, which may cause cavitation transfer, insufficient gluing amount and affect the compound fastness. At the same time, due to poor leveling dispersion performance, the appearance effect of the composite film will also deteriorate. For example, the flow pattern of glue is obvious, sometimes orange peel. When used for composite aluminum foil or aluminized film, if the printing surface has a large area of white ink or light ink, 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 external environment temperature. If the heat preservation measures are not in place, the heat loss rate is much faster than that in high temperature summer, which often makes the temperature in the curing room not reach the set temperature (generally 50 ℃). Therefore, the effect of curing is also affected to a certain extent. Under the same curing time, the compound fastness may be lower than that in summer.

To analyze the causes of the above phenomenon, it is necessary to understand the influence of moisture and temperature on the adhesive. First of all, for the two-component polyurethane adhesive, the water can react with the NCO group in the curing agent as the main agent, I .e. polyester/polyether polyol. It is estimated that 1g of water can consume 26~32g of curing agent. Of course, this is the pure reaction weight ratio. In practice, the water mixed into the working glue is competitive with the main agent when reacting with the curing agent. However, no matter how much water is involved in the reaction, it undoubtedly consumes the curing agent, making the amount of reaction with the main agent less than the original set of working ratio, thus causing incomplete curing and residual viscosity. The viscosity and reactivity of the adhesive are closely related to temperature. The viscosity value given by the adhesive manufacturer is measured by a rotational viscometer at a standard temperature of 25 ℃, which means that the temperature of the working environment can be more than 10 degrees higher in summer and more than 20 degrees lower in winter when it is cold. The viscosity of the glue is just the opposite of the temperature, that is, the same glue at high temperature shows a lower viscosity value, good flow dispersion performance, low temperature is high viscosity, poor flow. In addition, the crosslinking and curing reaction between the two components of the adhesive, the reaction speed is slow when the temperature is low, and the reaction speed is fast when the temperature is high, which is precisely why there is a curing room (accelerate the curing reaction speed and improve production efficiency).

In view of this situation, the following adjustments can be made according to environmental changes when using two-component polyurethane adhesive for compounding: if the air is humid and the temperature is too high, the dosage of curing agent should be appropriately increased by 10% ~ 20% to make up for the consumption of water; Dry cotton yarn or cloth is often used to absorb the condensed water droplets on the laminating machine to prevent them from falling into the glue basin. A small amount of solvent can be added to dilute the unused working glue, then sealed storage, if conditions permit, can be placed in a small freezer refrigerated storage, so that the effect is better, the next time again, in the case of closed thawing, and mixed with the newly equipped working glue. When the temperature is low in winter, the preparation of working glue can be appropriately added with more solvents to reduce the viscosity of the system, improve the leveling and dispersion, and also reduce the production of bubbles in the glue basin during work. However, the concentration of the working glue solution will be reduced. If the working concentration is not changed, a small amount of acetone can be used to replace part of ethyl acetate, that is, a mixed solvent of acetone and ethyl acetate is used as a diluent, and the ratio of the two can be 2:8 or 3:7. In addition, the temperature of the curing room can be set slightly higher in winter to ensure that the actual temperature can meet the requirements, so as not to affect the curing effect.

Before plugging in the hot melt glue gun, please check whether the power cord is intact, whether the bracket is ready; whether the glue gun that has been used is pouring, etc. Please preheat the glue gun for 3-5 minutes before use. Please stand the glue gun upright on the table when not in use. Please keep the surface of the hot melt adhesive strip clean to prevent impurities from blocking the nozzle. If you find that the glue gun cannot produce glue during use, please check whether the glue gun is hot; if the glue gun does not heat up normally, the reasons may be: 1 The glue gun power supply is not plugged in; 2 The glue gun burns out due to a short circuit. When the glue gun is normally heated, the reasons may be: 1 The nozzle of the gun is blocking the glue outlet due to impurities, so please ask a professional to deal with it; 2 When the glue gun is poured and the glue strip becomes thick, you only need to gently apply the glue strip. Rotate once and carefully pull out a small part back, peel off the thickened part of the rubber strip, and then continue to use it. When the glue stick in the glue gun is not used up, please try not to pull the glue stick out of the glue gun. If the glue strip in the glue gun flows back, please stop using it immediately and wait until the professional has cleaned the backflowing hot melt glue before using it. If the glue gun is not used for heating for more than 15 minutes, please cut off the power.

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 commonly used gluing methods in production include glue brushing, glue scraping, glue spraying, glue dipping, glue injection, glue leakage and glue rolling, etc. The glue film can be pasted and rolled before the solvent is completely evaporated, and the glue powder can be sprinkled on the heated bonded surface.
Brush glue: use a brush (glass rod can also be used) to apply the glue solution to the entire bonding surface from the center to the four sides, or follow one direction, do not reciprocate, the speed should be slow to prevent bubbles, and try to apply evenly and consistently.
Scraping glue: is to use a scraper to apply a high viscosity adhesive or paste-like glue to the surface of the glue, and it should be scraped evenly.
Spray glue: with a special spray gun, with the help of dry compressed air, the glue is sprayed onto the bonding surface, the glue layer is uniform, the efficiency is also high, suitable for large-area bonding and mass production.
Dipping: immerse the bonded parts into the glue solution and hang the glue solution for screw fixing and bonding the ends of bars or plates.
Glue injection: inject glue liquid into the bonding gap with a syringe, which is suitable for spot welding before glue injection.
Glue leakage: the glue liquid is evenly and continuously leaked into the bonding surface by the small mouth of the receptacle, with high efficiency and good quality, which is suitable for continuous production. Glue rolling: when gluing on the surface of wide flat objects, it is more convenient to operate with rubber rollers. The glue quality is good, the operation is simple and the efficiency is high. Rubber rollers are usually made of porous adsorption materials such as wool, foam plastic and sponge rubber. This kind of roller long-term contact with solvent-based adhesive, easy to corrosion, deformation, so more suitable for rolling latex water-based adhesive. During operation, first roll the glue on a flat plate, then apply a slight pressure, and then cover the surface of the adherend.
The rolled adhesive film is relatively uniform and has no sagging phenomenon, but the corners are not easy to roll and need to be brushed with a brush.
The precautions in the gluing process are as follows:
① The amount of glue and the number of times of coating vary from adhesive to adhesive. It should be carried out according to the regulations. For example, epoxy glue is only applied once, while most solution adhesives are applied twice or even three times. In addition, for the bonding of porous materials, the amount of glue and the number of times of coating should be appropriately increased. When applying glue many times, be sure to apply it again after the solvent has basically volatilized before, and the first layer should be as thin as possible.
② Control the thickness of the adhesive layer The amount of glue applied can control the thickness of the adhesive layer, and the thickness of the adhesive layer has a density relationship with the bonding strength. The general rule is that the bonding strength increases with the decrease of the thickness of the adhesive layer. The thinner the adhesive layer, the smaller the possibility of glue shortage, and therefore the smaller the internal stress, the higher the bonding strength. However, after the thickness of the adhesive layer is less than a certain value, the adhesive strength decreases because a continuous adhesive layer cannot be formed. When subjected to peeling force, the thickness is larger and the peeling strength is higher. Different types of adhesives suitable for the thickness of the adhesive layer is different, the general inorganic adhesive is 0.1~0.2mm, organic adhesive is 0.3~0.5mm.
The uniform layer of the adhesive layer contains bubbles or lack of glue to make the adhesive joint produce weak links, which seriously affects the bonding strength of the adhesive joint. When coating the adhesive layer, pay attention to the uniformity of the adhesive layer, especially when coating the paste adhesive with high viscosity, to prevent the phenomenon of bubbles in the adhesive layer due to unevenness.
④ The solvent in the adhesive layer should be fully volatilized. The residual solvent in the adhesive layer will seriously damage the performance of the adhesive joint. It is particularly important for metal bonding joints. In order to make the solvent in the adhesive layer fully volatile, when coating the adhesive containing solvent, it should be carried out in stages. Do not dry too much, especially for the last time, otherwise the viscosity is too high to glue, and the glue surface should be prevented from being polluted by dust in the air during the drying process. ⑤ Timely bonding For solvent-free adhesives, gluing can be carried out immediately after gluing; for solvent-containing adhesives, the bonding surfaces after drying are tightly attached together again, and pay attention to the alignment position. For liquid solvent-free adhesive, it is best to move back and forth several times to increase contact and exclude air.

Wood bonding technology plays an important role in the wood industry, and the bonding performance of wood bonding products is largely affected by the performance of the wood itself. The factors that affect the bonding performance of wood are both inherent in wood and produced in the process of wood processing. The former includes wood density, tree species, extract, pH, etc., and the latter includes wood moisture content, surface roughness, etc.
The strength of wood is usually proportional to its density, because the amount of wood cell wall material contained in the unit volume is the material basis for determining the strength of wood, and the strength of wood increases with the increase of wood density. For an ideal bonding system, bonding failure should occur in the xylem.

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

Although the bonding strength of the high-density wood-bonded product is high, the stress generated due to the change of the moisture content is also large. In the case of adhesive determination, the bonding durability of wood with high density is often worse than that of wood with low density. For example, with water-based polymer isocyanate adhesive manufacturing of oak wood, although the compression shear strength is relatively large, but when the use of environmental humidity changes, more prone to cracking of the adhesive layer. In addition, the density of wood bonding, in order to achieve the ideal bonding effect, the requirements of the adhesive itself should also be very high, so as to give full play to the strength of the wood itself. For particularly high-density wood, such as ebony, wood bean, etc., the adhesive is not easy to be absorbed, so that the adhesive drying speed is very slow, the bonding strength is low. Therefore, for wood with a particularly high density, in order to make the adhesive penetrate into the cells, it is required to sand and glue on both sides, and to wait for sufficient time for the adhesive to penetrate before applying pressure. Sometimes, it is necessary to use polyurethane adhesives, because the bonding 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 directions of wood fibers in the bonding system can be perpendicular to each other, parallel to each other, or at an angle to each other. When two pieces of wood are bonded in the same fiber direction, the bonding strength is the largest, such products as laminated wood and laminated veneer lumber (LVL), etc.; commonly used wood products that are bonded vertically to each other in the fiber direction include plywood, three-layer solid wood composite flooring, etc. With the increase of the fiber angle between the two pieces of bonded wood, the bonding strength of the bonding material gradually decreased. When the fiber direction is perpendicular to each other, the bonding strength is the lowest, and compared with the wood fiber parallel to each other, the bonding strength is only 1/4 to 1/3 of the fiber parallel to each other. The sectioning 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 into the wood surface, which showed that when the adhesive was applied to the wood end face, since the wood ducts and tracheids on the end cut face were open, the adhesive would penetrate into the cell cavity of these cells in large quantities, which would easily lead to the lack of adhesive on the end face, resulting in a decrease in the bonding strength of the wood. Therefore, under the same bonding conditions, the bonding strength of the end face and the end face is lower than that of the diameter face and the diameter face, the chord face and the chord face.
In actual work, if it is necessary to use the end cut surface bonding, in order to prevent its lack of glue, the viscosity and solid content of the adhesive must be increased; The use of two glue, or in the substrate on both sides of the glue, or increase the thickness of the glue layer; Use moderate pressure and prevent glue extrusion. For example, when the wood is longitudinally connected by butt joint, the above measures can be taken to avoid the lack of glue at its end. Under the condition that the fibers are bonded parallel to each other, the bonding surface can also be divided into radial and chord sections. In some cases, there is also a difference in the bond strength between the diameter section and the chord section. When the cohesive force of the adhesive and the interfacial strength between the adhesive and the wood are greater than the strength of the wood itself, the wood is prone to failure. If there is a difference in the strength of the wood's own chord and diameter sections, there is also a difference in the bond strength between the chord and chord sections and the diameter and diameter sections.

For example, the radial shear strength of larch wood is greater than its chordwise shear strength. When the larch plate is bonded with water-based polymer isocyanate adhesive, the normal compressive shear strength of the larch diameter cutting plate is higher than that of its string cutting plate. However, the wood breakage rate of larch string cutting board and string cutting board glued products is relatively high, which is basically wood damage, and most of the wood damage occurs at the junction or wheel boundary of the sudden change of wood in the morning and evening (therefore, when using API adhesive to produce larch glulam, diameter cutting board should be used for bonding as much as possible. 3 Wood moisture content Wood moisture content has a great influence on bonding performance. When the moisture content of the wood is high, the adhesive coated on the bonding surface is diluted, its viscosity decreases, and it is excessively soaked into the wood tissue, causing the lack of glue, resulting in a decrease in the bonding strength. Wood is dried at high temperature for a long time, and its moisture content is too low, which will weaken the wetting effect of the adhesive and reduce the bonding strength. Usually wood moisture content of 5% to 16% when the bonding strength is higher.
In addition, the wood has dry shrinkage and wet properties, if the wood moisture content is too low, its products will be affected with moisture and expansion; moisture content is too high, and will be desorbed dry shrinkage. Generally speaking, the adhesive itself cannot resist the very high stress caused by the expansion and drying of wood, which is a major cause of cracks and deformation in wood products. This happens when furniture is produced and used. The equilibrium moisture content of the wood where the furniture is produced and where it is used is often inconsistent, which requires the moisture content of the wood used in the production of furniture to be as close as possible to the humidity of the environment. For example, furniture purchased in Guangzhou is easy to crack when transported to Xinjiang, which requires that the moisture content of wood used by furniture manufacturers should be close to the equilibrium moisture content of wood in the location of the customer who purchased the furniture. Considering the lag of wood drying moisture absorption, in general, the final moisture content of wood drying and the equilibrium moisture content of wood in the use area are about 2%.
4 The shrinkage expansion rate of wood due to the natural shrinkage of the adhesive during curing and the difference in the properties of the adhesive and the wood, resulting in the existence of internal stress in the bonding joint, the concentration of internal stress will reduce the bonding strength. In order to reduce the stress caused by heat alternation or high temperature curing and cooling, the thermal expansion coefficient of the adhesive and the 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 select adhesives with good elasticity. When bonding wood with 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 the wood, or use an adhesive with excellent elasticity for bonding. For example, when metal-wood bonding, the hydroxyl component in wood causes wood dry shrinkage and swelling through moisture absorption or desorption, which is the main reason for the poor dimensional stability of wood. The dimensional stability of wood can be improved and the strength of modified wood can be improved by blocking the hydroxyl groups of wood and forming chemical crosslinking between hydroxyl groups. Therefore, the wood is treated with phenolic resin in advance, and the metal is bonded after its size is fixed, which can reduce the stress caused by the difference in shrinkage between the two. Another example, the use of veneer or plywood and foam board composite made of partition board, the first veneer or plywood fixed size, and then with the foam board composite effect will be better.
5. Surface processing method The bonding plane of the wood must be smooth and straight. If it is uneven (bending or warping), the two planes cannot be in close contact, which will affect the strength and quality of the bonding. Therefore, the wood is mechanically processed before the bonding operation, which may cause mechanical damage to the wood cells, and the degree of damage varies with the type and degree of machining. During planing processing, the surface wood tissue is less damaged, the inner cavity of wood cells is open, and the adhesive is easy to form effective glue nails after immersion. In order to reduce the damage to the wood tissue, the planer must be kept sharp, otherwise the damage to the wood tissue will be more serious. A.P.Singh found that the surface of radiata pine (P.radiate) wood planed with a sharp planer has a normal shape, except for a slight damage to the tracheids adjacent to the glue layer. The glue layer between the wood surfaces is thin and has a normal width (see Figure 3). In contrast, the surface of radiata pine wood planed with a blunt planer was severely damaged by axial tracheids and rays. Because the planer knife is blunt, the cells are severely crushed, especially the crushing of the cells next to the adhesive layer is likely to occlude the cell cavity and prevent the adhesive from penetrating into the cells, resulting in a thicker and uneven thickness of the adhesive layer. The fine planing of the wood surface helps to ensure the uniformity of the adhesive layer on the surface of the adhesive. The planing of the surface of the wood can expose the S2 layer of the cell wall with strong polarity, which is very beneficial to bonding. Although sanding will make the surface of the wood very smooth, the inner cavity of the wood tissue is damaged more, at the same time, some large surfaces 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 wood surface characteristics due to the wood is porous material, when two pieces of wood surface bonding, the actual only a small part of the apparent area of contact, and the area with the wood structure, surface roughness and the applied pressure and other factors vary. In other words, even if the pressure applied during bonding is not very large, the bonding material may withstand a relatively high pressure or even be crushed in a small part of the contact area. Generally speaking, a certain roughness of the surface, its concave and convex, pores and grooves are conducive to the penetration of the adhesive, can produce a mechanical bonding effect. However, when the roughness is too large and the concave holes and grooves on the surface are too deep, the residual air or adsorbed moisture will hinder the penetration of the adhesive and is not conducive to bonding.
7 wood surface passivation to keep the wood surface clean, to prevent pollution is the basic condition of its good bonding. In addition to air pollution (such as dust particle pollution in the air), the wood surface is also prone to a "self-pollution" over time, which changes the surface properties of the wood and affects the bonding properties of the wood.
 
Wood in the cutting process within a few hours, the oleophilic, low molecular weight substances will move to its surface to form a low surface energy of the "weak interface layer" wood surface experienced this change is often referred to as "surface passivation", the rate and degree of passivation according to wood species and storage temperature varies. The lipophilic substances in wood are mainly composed of resin, fatty acid and its esters, paraffin and terpene compounds. Acidic and neutral adhesives cannot easily penetrate this lipophilic layer, but alkaline adhesives can saponify fatty acids to some extent, so that the lipophilic layer disappears, so that the adhesive can penetrate well into the wood. R.M. Russbaum found that the surface wettability of European spruce (Picea abies) wood decreased significantly after 3 days of cutting. Therefore, he pointed out that the maximum storage time for 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 processing to avoid natural surface passivation and affect its bonding strength.

In the production of wood glued products, it should be emphasized that the wood should be planed or sanded within 24h after the completion of the bonding operation. For example, in the production of laminated timber, the wood shall be bonded and pressurized as soon as possible after planing or milling the finger tenon, and the bonding and pressurizing shall not exceed 24h from planing or milling the finger tenon. In order to eliminate the "self-pollution" of the wood surface and the influence of the surrounding environmental pollution on the wetting of the wood surface, the wood surface should be effectively pretreated before bonding. Wood surface treatment can be used mechanical and chemical methods: cleaning with solvent, cleaning the bonding surface of the wood, so that the bonding surface as large as possible; mechanical methods (planing or sanding) to remove surface contaminants, while improving surface roughness to increase the bonding surface area.
8 Extracts of wood Extracts of wood have a certain influence on the wetting, penetration and curing process of the adhesive. In general, the extraction of more components of the wood, it is difficult to be fully wet by the adhesive, and the bonding strength is poor. When wood that is difficult to glue, such as Dipterocaceae wood, is boiled in hot water at 100 ℃ for 24 hours, a water-soluble gum composition accounting for 5% ~ 8% of the weight of the wood will be proposed. However, for Liu 'an and other woods with good gluing performance, when boiled under the same conditions, the extracted components only account for 0.1-0.2. It is also possible to add a surfactant to hot water or alkaline water for cooking, but this requires a lot of cost, so there is a certain difficulty in practical application.
For larch, Pinus massoniana and other resin-containing wood, it is first dried at high temperature to make the resin seep from the wood, then the surface is cleaned with a solvent to remove the resin, and then the surface is planed. Wood surface treatment immediately after the completion of the bonding, to prevent re-overflow resin. Phenolic adhesives are recommended for bonding.
9 the pH value of wood affects the curing time of the adhesive. For example, poplar, especially the pH value is alkaline, can prolong the curing time of urea formaldehyde resin adhesive. Because the urea-formaldehyde resin is cured under acidic conditions, and the alkaline poplar wood will reduce the acidity of the adhesive. Adding more curing agent and adjusting the pH value to 4.0~5.0 is the simplest way to solve the effect of poplar alkalinity on the curing time of urea-formaldehyde resin. In actual production, NH4C1 or mixed curing agent can be used according to the actual situation, such as adding some acid to the ammonium salt. Acidic wood, such as oak (pH 3.5), high acidity will hinder the gel time of some adhesives cured under alkaline conditions. On the contrary, it is beneficial to shorten the curing time of urea-formaldehyde resin adhesives.