1. Adhesive and its modulation
(1) Types and performance requirements of adhesives used for composite plates
There are two main types of glue for plywood: protein glue and synthetic resin glue. Because the strength and waterproof properties of protein adhesive are lower than those of synthetic gum resin adhesive, the adhesive commonly used in production is synthetic resin adhesive, which mainly consists of synthetic resin, curing agent, filler and modifier.
Synthetic resin is the main component of adhesive. The production of plywood is mainly made of urea-formaldehyde resin and phenolic resin. Urea formaldehyde resin is easy to manufacture, low price, good bonding strength and fast curing speed, with a certain water resistance, is the main adhesive for plywood. Because of the poor aging resistance of urea-formaldehyde resin, it is mainly used for indoor products. Phenolic resin adhesive has high bonding strength and anti-aging properties, but the price is higher, and it is mainly used in the production of outdoor plywood.
Curing agent is the catalyst that accelerates the curing of adhesive. The most commonly used curing agent for urea-formaldehyde adhesives is ammonium chloride. In addition, some acids and strong acids and weak base salts can also be used as hardeners of urea-formaldehyde adhesives, which can be used alone or in combination. Sometimes some buffering agents are added to the curing agent, such as urea, hexamethylene tetramine, ammonia water, etc., to extend the application period of the glue. In recent years, the study of low toxicity urea-formaldehyde resin adhesive shows that due to the low content of free formaldehyde in the adhesive, only ammonium chloride is used as curing agent, and the curing time is longer, so multi-component curing agent is used to accelerate the curing of the adhesive.
Phenolic resin adhesive due to the large resin activity, it will be cured quickly after heating, so the hot pressing glue generally does not add curing agent, but the cold pressing glue to add benzene sulfonate, petroleum sulfonic acid and other curing agent.
The glue used for plywood is generally filled, the purpose is to reduce the amount of resin, reduce the cost of the glue and improve the performance of the glue. After adding an appropriate amount of filler; It can increase the solid content of glue, improve the viscosity, prevent the penetration of glue and thin veneer through glue, and improve the precompression performance of glue; Reduce the shrinkage stress of the adhesive layer, improve the adhesive strength and improve the aging resistance of the adhesive layer; Reduce the brittleness of glue layer and the difference of expansion coefficient between glue and wood caused by temperature change; Reduce the content of free formaldehyde, reduce environmental pollution; It can extend the application period and improve the working conditions.
Fillers can be divided into organic fillers and inorganic fillers according to chemical composition. Organic fillers can improve resin bonding, strengthen adhesive layer, improve elastic modulus and change its properties, such as wheat flour, wood flour, bean flour, starch, chestnut cocoa, walnut shell powder, alpha hydroxymethylcellulose and bark powder. However, excessive use of these fillers will reduce the performance of the glue. Inorganic fillers are kaolin, chalk soil, asbestos, gypsum and glass powder, inorganic fillers will not change the performance of the glue, mainly increase the volume of the glue.
The commonly used urea-formaldehyde tree glue and phenolic tree glue in the production of plywood are ideal kinds of glue, but there are some shortcomings in the performance, usually in these resins or their synthesis process to add some modified materials to improve their properties.
Urea-formaldehyde resin is commonly modified with rubber emulsions, polyvinyl acetate emulsions, phenol, resorcinol, phenylguanosamine, melamine, polyvinyl alcohol and other materials. Adding an appropriate amount of rubber emulsion to urea-formaldehyde adhesive can greatly improve the strength of the adhesive layer, reduce the amount of free formaldehyde in the adhesive by 1/2, decrease the viscosity of the adhesive and increase the activity period. This adhesive is used for cold pressing or hot pressing. Adding polyvinyl acetate emulsion to the glue can greatly improve the elasticity and bonding strength of the glue layer, and speed up the curing speed at room temperature. The adhesive strength and water resistance of the resin were improved greatly by adding phenol, resorcinol and phenylguanosamine during the reaction. The water resistance and bonding strength of urea-formaldehyde adhesive can be improved and the content of free formaldehyde can be reduced by using 30% amino epoxy resin as modifier. This adhesive can be used for wood and bonding with high moisture content at low temperature. Adding an appropriate amount of polyvinyl alcohol (0.5% ~ 1%), it can be applied to precompression molding, and has been widely used in domestic plywood production.
The curing temperature of phenolic gum is high, and the hot pressing time is long. After adding 1.5% and tannin extract, the phenolic gum can be cured at a lower temperature of 120 ~ 130℃, and the hot pressing time can also be shortened. The same effect can be obtained by adding 25% to 30% resorcinol. The addition of rubber materials to phenolic resin has higher heat resistance and viscoelasticity.
(2) adhesive preparation process
When mixing, attention should be paid to controlling the number of ingredients that should be added in the glue, the order of adding materials, and there should be enough mixing time to make various materials mixed evenly, so that multi-component materials do not precipitate and precipitate in a short time. Glue quality can be controlled by "viscosity after glue adjustment". The following introduces several kinds of glue mixing process.
The formulation (by mass) of the foam urea-formaldehyde resin adhesive is: 100 parts resin, 0.5 to 1 part blood powder, 0.2 to 1 part ammonium chloride, and 2 to 4 parts water. Blood powder is a foaming agent. It is soaked in water with 4 times its mass for 1h before use. When mixing, the resin is added to the foaming machine and stirred. The speed of the foaming machine is 250-300r /min. The appearance of this adhesive is non-flowing thick paste, the density is 0.3 ~ 0.4g/cm3, and the activity period is not less than 3h.
The preparation formula (by mass) of the urea formaldehyde resin adhesive for prepressing is: 100 parts resin, 0.2 ~ 1 part ammonium chloride, 0 ~ 0.4 parts ammonia water, 3 ~ 6 parts flour, and 6 ~ 9 parts peanut shell powder. When mixing, first add the resin to the mixing machine, then add flour and stir for 10 ~ 15min, until there is no dough in the glue (you can also mix the dough with other resin first until it is paste, and then add all the resin to stir well). Then add peanut shell powder and stir for 5 minutes, and finally add 20% ammonium chloride solution and 25% chlorine water, and stir together for 5 minutes before use.
The preparation formula of phenolic resin glue (by mass) is: phenolic resin 100 parts, chalk soil 7 ~ 12 parts, wood powder 3 parts, triformaldehyde 1 ~ 3 parts, water 2.5 ~ 5.0 parts. Mixer speed is 140 ~ 150r/min. Add each component in turn when mixing. Stir for 5 ~ 10min each time, and then stir for 20min after adding each group.
2.sizing amount and sizing quality
(1) Sizing amount
The sizing amount refers to the weight of the adhesive on the unit area of the veneer after sizing, expressed in /㎡. There are two kinds of single-side coating and double-side coating. Plywood raw and mostly use core plate double-sided coating, so the amount of glue is double-sided coating.
The amount of sizing is one of the factors affecting the bonding strength. Too much glue thickens the glue layer, increases the stress, causes the bonding strength to decrease, and wastes the glue liquid, which is not economical. The sizing amount is too small to form a continuous adhesive layer, and is not conducive to the transfer of glue to another glued surface.
The size of the sizing is determined by the rubber species, tree species and veneer thickness. If the thickness of 1.25 ~ 1.50mm veneer sizing, using phenolic resin glue (solid content 45% ~ 50%), birch veneer gluing amount of 220 ~ 250g/㎡, basswood 240 ~ 260g/㎡, ash 280 ~ 300g/㎡; Using urea formaldehyde resin adhesive (solid content 60% ~ 65%), birch veneer 240 ~ 260g/㎡, basswood 260 ~ 300g/㎡, manchu 300 ~ 350g/㎡.
(2) Factors affecting sizing quality
Sizing quality can be measured from the thickness of the glue layer and the uniformity of sizing. The thickness of the adhesive layer is thin, and a continuous and uniform adhesive film can be formed, indicating that the sizing quality is good.
The main factors that affect the sizing quality are: the quality of adhesive itself and the conditions of sizing. The quality of the adhesive, especially the viscosity of the adhesive, should meet the process requirements. If the viscosity is too large, it is not easy to glue evenly; If the viscosity is too small, it is easy to produce glue penetration during sizing, so that the surface of the veneer can not form a continuous film. When sizing, in addition to the adhesive meets the requirements, other process operations should also meet the requirements, such as roller coating, should avoid the uneven wear of the roller and affect the uniformity of sizing, and to ensure the uniform coating and a certain amount of glue.
3. Sizing method and equipment
Veneer sizing is a process in which a certain amount of adhesive is evenly coated on the surface of the veneer. For a class of thin plywood used in aviation, in order to ensure quality and convenient construction, the adhesive film is directly used for gluing, which is very expensive. At present, the adhesive used for flexible plywood is mostly liquid adhesive, which can be divided into roller coating, spraying, extrusion and pressure spraying according to different equipment used.
(1) Roller coating method
The roller coating method is to apply the adhesive liquid attached to the roller to the board. This method is mostly double-sided gluing of veneer, which can be used as a two-roll gluing machine or a four-roll gluing machine.
The glue liquid in the upper roller of the two-roll coater is transferred by the lower roller. When applying glue, the veneer passes through the middle of the two rollers, and the glue on the two rollers is applied to the veneer by contacting each other. The size of the sizing amount is mainly controlled by adjusting the distance between the upper and lower rollers and the pressure of the upper rollers. The shape and number of grooves on the roller also affect the sizing amount. The height of the liquid surface of the glue should reach 1/3 of the diameter of the lower roller.
Two-roll gluing machine is a relatively backward equipment, the amount of gluing is not easy to control, the board is not usually easy to be crushed, the circumference length of the roll must be greater than the length of the board, so the roll diameter is large. However, the structure is simple and easy to maintain, so it is still used in our production.
The four-roll coater overcomes the shortcomings of the two-roll coater to a certain extent, and the four-roll coater has two extruding rollers in addition to the two coater rollers, which are a pair of steel rollers coated with chrome or hard rubber. The linear speed of the extrusion roll is 15% ~ 20% lower than that of the coating roll, which plays a scraping role. The distance between it and the roller is adjustable to control the amount of sizing. There is a cleaning tank under the rubber roll to discharge sewage for washing the rubber roll. Because the four-roller coater and the lower part of the coater supply glue at the same time, the problem of uneven coating of the two-roller coater is solved.
In order to maintain the good process performance of the gluing machine, attention should be paid to its maintenance, regular cleaning with warm water, such as local curing can be used brush dip 3% ~ 5% caustic soda solution cleaning. If the urea-formaldehyde resin is coated, it is necessary to neutralize it with acetic acid and then wash it with water. When the groove of the roller has a certain wear, it should be repaired again to ensure that the glue is evenly applied and the amount of glue is certain.
(2) Gluing method
Glue is a high-efficiency single-sided sizing method, which is introduced from the paint industry. The adhesive enters the glue head and flows out from the bottom seam under a certain pressure to form a uniform thickness of the glue screen. When the veneer passes through the glue screen, a layer of glue is left on the surface of the board. The thickness of the adhesive layer sprayed on the veneer is related to the flow rate, viscosity, surface tension of the material and the feed speed of the veneer. Increasing the flow rate of glue, improving the viscosity of glue, and reducing the feed speed of veneer can thicken the glue layer, and the temperature of the glue should be slightly higher than 20℃, otherwise the thickness of the glue layer is not uniform. This method is suitable for continuous and automatic production line, but the effect on uneven veneer is poor.
(3) extrusion method
The glue extrusion device is composed of a glue storage tank and a row of cylindrical flow glue holes arranged in the lower part of the tank. The glue extrusion method is to apply the high viscosity or foam glue to the board through the small hole of the glue squeezer. The board passes under the glue squeezer, and the glue flows down in strips and falls onto the board. The direction of the rubber strip should be perpendicular to the direction of the board fiber. About half of the area of the coated veneer is free of glue, and the glue liquid can be extended into a complete rubber layer during prepressing, or the rubber strip on the veneer can be flattened by a roller covered with a layer of silicone rubber. When using foam glue, the diameter of the extrusion hole can be increased to avoid the blockage of the hole.
The main advantage of the extrusion method is to save adhesive, but the glue hole should be prevented from plugging when used.
(4) Spray glue method
Spray glue method is to apply high pressure to the glue (3 ~ 6MPa), so that it is sprayed from the glue nozzle at high speed, and the sprayed glue is rotating forward, so that the dispersion is good. In order to uniform sizing, the spray nozzle should be as small as possible (diameter 0.3 ~ 0.5mm), but it is easy to block, which requires the cleaning of the glue, but also pay attention to the viscosity of the glue. The spraying method has higher efficiency, but it is difficult to control the amount of glue. The operation of the spray glue method is similar to that of the spray glue method, which is also applied in the advance of the board.
The three sizing methods of spraying, squeezing and spraying are the new sizing methods in recent years. Their common characteristics are high production efficiency, good sizing quality, and easy to realize the continuity of sizing and billet. At present, the production scale of plywood in China is small, the product specifications are changeable, and the roller type glue machine is still widely used. Roller coating is suitable for hand operation, but is not conducive to the realization of mechanization and continuity.
