Plywood veneer gluing

1. Adhesive and its preparation

(1) Types and performance requirements of adhesives for plywood

There are two main types of adhesives used in plywood: protein glue and synthetic resin glue. Since protein glue is weaker in strength and water resistance than synthetic resin glue, synthetic resin adhesives are currently the most commonly used adhesives in plywood production. These adhesives are primarily composed of synthetic resin, curing agent, filler, and modifier.

Synthetic resins are the primary adhesive components in glue. Urea-formaldehyde and phenol-formaldehyde resins are primarily used in plywood production. Urea-formaldehyde resin is easy to manufacture, inexpensive, and offers excellent bond strength, a fast cure rate, and a certain degree of water resistance, making it the primary glue used for plywood. However, due to its poor aging resistance, urea-formaldehyde resin is primarily used in interior applications. Phenolic resin glue offers high bond strength and aging resistance, but is more expensive and is primarily used in the production of exterior plywood.

A curing agent is a catalyst that accelerates the curing of adhesive. The most commonly used curing agent for urea-formaldehyde adhesive is ammonium chloride. In addition, some acids and salts of strong acids and weak bases can also be used as curing agents for urea-formaldehyde adhesive, either alone or in combination. Buffers such as urea, hexamethylenetetramine, and ammonia water are sometimes added to the curing agent to extend the adhesive's shelf life. Recent research on low-toxic urea-formaldehyde resin adhesives has shown that due to the low free formaldehyde content in the adhesive, using only ammonium chloride as a curing agent results in a longer curing time. Therefore, multi-component curing agents are used to accelerate the curing of adhesives.

Phenolic resin glue has high activity and will solidify quickly after heating, so hot pressing glue generally does not require curing agent. Only cold pressing glue requires the addition of curing agents such as benzene sulfonate and petroleum sulfonic acid.

Fillers are commonly added to plywood glue to reduce resin usage, lower glue costs, and improve performance. Adding the right amount of filler can increase the glue's solids content and viscosity, prevent glue penetration and bleed-through of thin veneers, and improve the glue's pre-compression properties. It can also reduce shrinkage stress in the glue layer, increase bond strength, and enhance its aging resistance. It can also reduce the brittleness of the glue layer and minimize the difference in expansion coefficient between the glue and wood due to temperature fluctuations. It can also lower free formaldehyde content, minimizing environmental pollution, extending the pot life, and improving working conditions.

Fillers can be divided into organic and inorganic fillers based on their chemical composition. Organic fillers, such as wheat flour, wood flour, soy flour, starch, chestnut, cocoa, walnut shell powder, α-hydroxymethyl cellulose, and bark powder, can improve resin bonding, strengthen the adhesive layer, increase the elastic modulus, and modify its properties. However, excessive use of these fillers can actually reduce the performance of the adhesive. Inorganic fillers include kaolin, chalk, asbestos, gypsum, and glass powder. Inorganic fillers do not alter the properties of the adhesive; they primarily increase its volume.

Modifiers Urea-formaldehyde resin and phenolic resin commonly used in plywood production are both ideal types of glue, but they also have certain shortcomings in performance. Usually, certain modifying materials are added to these resins or their synthesis process to improve their performance.

Urea-formaldehyde resin is commonly modified with materials such as rubber emulsions, polyvinyl acetate emulsions, phenol, resorcinol, phenylguanamine, melamine, and polyvinyl alcohol. Adding an appropriate amount of rubber emulsion to urea-formaldehyde glue can significantly increase the strength of the bondline, reduce the free formaldehyde content by half, decrease the viscosity, and increase the active period. This glue is suitable for cold or hot press bonding. Adding polyvinyl acetate emulsion to the glue significantly improves the elasticity and bond strength of the bondline and accelerates room temperature curing. Adding phenol, resorcinol, and phenylguanamine during the resin reaction significantly enhances its bond strength and water resistance. Using an amino epoxy resin with a depth of 30% as a modifier can improve the water resistance and bond strength of urea-formaldehyde glue and reduce the free formaldehyde content. This glue can be used for gluing high-moisture-content wood at low temperatures. Adding an appropriate amount of polyvinyl alcohol (0.5% to 1%) makes it suitable for pre-press molding and has been widely used in domestic plywood production.

Phenolic resin glue has a high curing temperature and a long hot pressing time. However, adding 1.5% tannin extract during the glue preparation allows it to cure at a lower temperature of 120-130°C, shortening the hot pressing time. Adding 25-30% resorcinol can achieve the same effect. Adding rubber-like materials to phenolic resin glue can enhance its heat resistance and viscoelasticity.

(2) Adhesive preparation process

When mixing adhesive, pay attention to controlling the amount of ingredients added, the order of addition, and sufficient stirring time to ensure uniform mixing of the various materials and prevent precipitation or settling of the multi-component materials within a short period of time. The quality of the mixed adhesive can be controlled by its "post-mix viscosity." The following describes the mixing techniques for several popular adhesive types.

The formula for preparing foamed urea-formaldehyde resin glue (by mass) is: 100 parts resin, 0.5-1 part blood meal, 0.2-1 part ammonium chloride, and 2-4 parts water. Blood meal acts as a foaming agent and should be soaked in four times its mass of water for one hour before use. To prepare the glue, first add the resin to a foaming machine and stir at 250-300 rpm. Then add the blood meal and let it foam for 5 minutes. Finally, add a 20% ammonium chloride solution and stir for another 5 minutes. Once the volume has increased by two to three times, it is ready for use. This glue appears as a non-flowing, thick paste with a density of 0.3-0.4 g/cm³ and a working life of at least 3 hours.

The formula for preparing urea-formaldehyde resin glue for pre-pressing (by weight) is: 100 parts resin, 0.2-1 part ammonium chloride, 0-0.4 part ammonia water, 3-6 parts flour, and 6-9 parts peanut shell powder. To prepare the glue, first add the resin to the glue mixer, then add the flour and stir for 10-15 minutes until there is no dough in the glue. (You can also first use some of the resin to mix the flour until it becomes a paste, then add all of the resin and stir thoroughly.) Then add the peanut shell powder and stir for 5 minutes. Finally, add a 20% ammonium chloride solution and a 25% chlorine solution, stir for 5 minutes, and then use.

The formula (by weight) for phenolic resin adhesive is: 100 parts phenolic resin, 7-12 parts chalk, 3 parts wood flour, 1-3 parts trioxymethylene, and 2.5-5.0 parts water. Stir at 140-150 rpm. Add each component sequentially, stirring for 5-10 minutes. After adding each component, stir for another 20 minutes before use.

Film Preparation: To ensure product quality, thermosetting phenolic resin films are used in the manufacture of aviation plywood. Therefore, a phenolic resin mold must be pre-fabricated. Impregnation films typically use alcohol- or water-soluble phenolic resins. See 4-1 for technical specifications.

If using water-soluble phenolic resin, dilute it with water and alcohol to reduce the film's brittleness and minimize breakage. The formula is as follows: 50kg water-soluble phenolic resin (45% solids content), 15kg water, and 15kg alcohol.

Horizontal or multiple impregnation machines can be used for impregnation and drying of adhesive-coated paper. See Table 4-2 for the impregnation process conditions. The temperature, air volume, and moving speed should be maintained stable during drying. For adhesive-coated paper, the film thickness, amount of adhesive impregnation, volatile matter content, and resin solubility should be determined. Adhesive films used for aviation plywood should also be tested for quality, soluble resin content, and bonding strength.

2. Glue application amount and quality

(1) Glue application amount

Glue application refers to the weight of adhesive applied per unit area of ​​veneer, expressed in square meters. There are two types of adhesive application: single-sided and double-sided. Plywood is often produced with double-sided glue applied to the core board, so the double-sided glue application is used.

The amount of glue applied is one of the factors that influences bond strength. Excessive glue application leads to thicker glue layers, increased stress, and decreased bond strength. It also wastes glue and is uneconomical. Too little glue application prevents a continuous bond line and hinders transfer of glue to the next bonding surface.

The amount of glue applied is determined by the type of glue, wood species, and veneer thickness. For example, for veneers 1.25-1.50mm thick, using phenolic resin glue (45%-50% solids content), the amount applied to birch veneer is 220-250g/m2, basswood 240-260g/m2, and ash 280-300g/m2. Urea-formaldehyde resin glue (60%-65% solids content) is 240-260g/m2 for birch veneer, 260-300g/m2 for basswood, and 300-350g/m2 for ash.

(2) Factors affecting sizing quality

The quality of glue application can be measured by the thickness of the glue layer and the uniformity of the glue application. If the thickness of the glue layer is thin and it can form a continuous and uniform glue film, it means that the glue application quality is good.

Factors that primarily impact gluing quality include the quality of the adhesive itself and the process conditions during gluing. The adhesive's quality, particularly its viscosity, must meet process requirements. Excessive viscosity can make even application difficult; too low a viscosity can easily lead to bleed-through, resulting in a lack of adhesive on the veneer surface and preventing the formation of a continuous film. During gluing, not only must the adhesive meet requirements, but other process operations must also meet them. For example, during roller gluing, uneven wear of the roller, which could affect gluing uniformity, must be avoided. Furthermore, a consistent and consistent amount of glue must be applied.

3. Gluing methods and equipment

Veneer gluing is the process of evenly applying a certain amount of adhesive to the surface of a veneer. For thin plywood used in aviation, direct film gluing is often used to ensure quality and ease of construction, but this method is very costly. Currently, the adhesive used for flexible plywood is mostly liquid glue, which can be categorized by the equipment used, including roller coating, sprinkling, extrusion, and pressure spraying.

(1) Roller coating method

Roller coating is to apply glue attached to the rubber roller to the veneer. This method is mostly for double-sided gluing of veneers, and can use a double-roller gluing machine or a four-roller gluing machine.

In a twin-roller gluing machine, glue is applied from the upper roller to the lower roller. During gluing, the veneer passes between the two rollers, where contact spreads the glue from both rollers onto the veneer. The amount of glue applied is primarily controlled by adjusting the gap between the upper and lower rollers and the pressure of the upper roller. The shape and number of grooves on the rollers also affect the amount of glue applied. The glue level should ideally reach 1/3 of the diameter of the lower roller.

The twin-roller gluing machine is a relatively outdated piece of equipment. The glue application rate is difficult to control, and uneven veneers are easily crushed. The circumference of the rollers must be greater than the length of the veneer, resulting in a large roller diameter. However, its simple structure and ease of maintenance make it still widely used in Chinese production.

Four-roller gluing machines overcome the shortcomings of two-roller gluing machines to a certain extent. In addition to the two gluing rollers, four-roller gluing machines also feature two squeeze rollers-a pair of chrome-plated or hard rubber-coated steel rollers. The squeeze rollers operate at a speed 15% to 20% slower than the gluing rollers, acting as scrapers. The distance between the squeeze rollers and the gluing rollers is adjustable to control the amount of glue applied. A cleaning tank is located below the gluing rollers to drain wastewater from the rollers. Because the four-roller gluing machine supplies glue simultaneously from the top and bottom, it eliminates the uneven glue application problem associated with two-roller systems.

To maintain good performance, the glue applicator requires careful maintenance. Regularly clean it with warm water. If any areas of solidification occur, use a brush dipped in a 3% to 5% caustic soda solution to clean. If applying urea-formaldehyde resin glue, neutralize it with acetic acid and then rinse with clean water. If the roller grooves show any wear, they should be repaired to ensure uniform glue application and a consistent amount of glue.

(2) Glue spraying method

Adopted from the paint industry, gluing is a highly efficient, single-sided gluing method. Adhesive enters the gluing head and flows out through the bottom seam under pressure, forming a uniformly thick curtain. As the veneer passes through the curtain, a layer of adhesive is left on the veneer surface. The thickness of the adhesive layer applied to the veneer depends on the adhesive flow rate, viscosity, surface tension, and feed rate. Increasing the adhesive flow rate, increasing the adhesive viscosity, or reducing the feed rate can all thicken the layer. The adhesive temperature should be slightly above 20°C, otherwise uniformity in the layer will be difficult to achieve. This method is suitable for continuous and automated production lines, but is less effective on uneven veneers.

(3) Glue extrusion method

The glue extrusion device consists of a glue storage tank and a row of cylindrical glue flow holes mounted below it. The glue extrusion method involves applying high-viscosity or foamed glue to the veneer through the small holes in the glue extruder. The veneer passes under the glue extruder, and the glue flows down in strips onto the veneer. The direction of the glue strips should be perpendicular to the fiber direction of the veneer. Approximately half of the surface of the glued veneer is left uncovered. During pre-pressing, the glue liquid can be expanded into a complete glue layer. Alternatively, a roller coated with silicone rubber can be used to flatten the glue strips on the veneer. When using foamed glue, the diameter of the glue extrusion holes can be increased to prevent clogging.

The main advantage of the glue squeezing method is that it saves adhesive, but the glue holes should be prevented from being clogged when used.

(4) Spraying method

The spray glue method applies high pressure (3-6 MPa) to the glue, causing it to be ejected from the nozzle at high speed. The glue is sprayed forward in a rotating motion, which improves dispersion. To ensure uniform glue application, the nozzle should be as small as possible (0.3-0.5 mm in diameter). However, this method is prone to clogging, which requires clean glue and careful attention to glue viscosity. The spray glue method is highly efficient, but it is more difficult to control the glue volume. The operation of the spray glue method is similar to that of the pour glue method, with glue applied as the veneer moves forward.

Three new gluing methods-sprinkling, squeezing, and spraying-have emerged in recent years. Their common characteristics are high production efficiency, high-quality glue application, and the ease of continuous gluing and assembly. Currently, my country's plywood production is relatively small, with varying product specifications, and roller gluing machines are still widely used. While roller gluing is suitable for manual operation, it is not conducive to mechanized and continuous production.