Plywood veneer gluing

1. Adhesives and their preparation

(1) Types and performance requirements of adhesives for plywood

There are two main types of adhesives used for plywood: protein adhesives and synthetic resin adhesives. Because protein adhesives have lower strength and water resistance than synthetic resin adhesives, synthetic resin adhesives are currently the most commonly used adhesives in production. These adhesives mainly consist of synthetic resins, curing agents, fillers, and modifiers.

Synthetic resins are the main adhesive component in plywood. Urea-formaldehyde resin and phenolic resin are primarily used in plywood production. Urea-formaldehyde resin is easy to manufacture, inexpensive, has good bonding strength and a relatively fast curing speed, and possesses some water resistance, making it the main type of adhesive used for plywood. However, due to its poor aging resistance, urea-formaldehyde resin is mainly used for indoor products. Phenolic resin adhesives have very high bonding strength and aging resistance, but are more expensive and are mainly used in the production of outdoor plywood.

Curing agents are catalysts that accelerate the curing of adhesives. Ammonium chloride is the most commonly used curing agent for urea-formaldehyde adhesives. In addition, some acids and salts of strong acids and weak bases can also be used as curing agents for urea-formaldehyde adhesives, either alone or in combination. Sometimes, buffers such as urea, hexamethylenetetramine, and ammonia are added to the curing agent to extend the adhesive's pot life. Recent research on low-toxicity urea-formaldehyde resin adhesives has shown that because the free formaldehyde content in the adhesive is very low, using only ammonium chloride as a curing agent results in a long curing time. Therefore, multi-component curing agents are used to accelerate the curing process.

Phenolic resin adhesives are highly reactive and cure quickly when heated. Therefore, hot-press adhesives generally do not contain curing agents, while cold-press adhesives require the addition of curing agents such as benzene sulfonate and petroleum sulfonic acid.

Filler is generally added to the adhesive used in plywood. The purpose is to reduce resin usage, lower adhesive costs, and improve adhesive performance. Adding appropriate amounts of filler can increase the adhesive's solids content, improve viscosity, prevent adhesive penetration and seepage through thin veneers, and improve pre-compression performance; reduce shrinkage stress in the adhesive layer, increase bond strength, and enhance the aging resistance of the adhesive layer; reduce brittleness of the adhesive layer and minimize the difference in expansion coefficients between the adhesive and wood caused by temperature changes; reduce free formaldehyde content, thus reducing environmental pollution; extend the service life; and improve working conditions.

Fillers can be classified into organic fillers and inorganic fillers according to their chemical composition. Organic fillers can improve resin bonding, strengthen the adhesive layer, increase the elastic modulus, and modify its properties. Examples include wheat flour, wood flour, soybean flour, starch, chestnut cocoa, walnut shell powder, α-hydroxymethyl cellulose, and bark powder. However, excessive use of these fillers can actually reduce the adhesive's performance. Inorganic fillers include kaolin, chalk, asbestos, gypsum, and glass powder. Inorganic fillers do not change the adhesive's properties; they mainly increase the adhesive's volume.

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

Urea-formaldehyde resin is commonly modified with rubber emulsions, polyvinyl acetate emulsions, phenol, resorcinol, phenylguanidine, melamine, and polyvinyl alcohol. Adding an appropriate amount of rubber emulsion to urea-formaldehyde resin significantly increases the strength of the adhesive layer, reduces the free formaldehyde content by half, decreases viscosity, and increases the active period. This type of resin is used for cold or hot pressing. Adding polyvinyl acetate emulsion greatly improves the elasticity and bonding strength of the adhesive layer and accelerates curing at room temperature. Adding phenol, resorcinol, and phenylguanidine during the resin reaction significantly improves its bonding strength and water resistance. Using amino epoxy resin with a 30% depth as a modifier can improve the water resistance and bonding strength of urea-formaldehyde resin and reduce the free formaldehyde content. This type of resin can be used for bonding wood with high moisture content at low temperatures. Adding an appropriate amount of polyvinyl alcohol (0.5%–1%) makes it suitable for pre-pressing and is widely used in plywood production in China.

Phenolic resin adhesives have a high curing temperature and a long hot-pressing time. Adding 1.5% tannin during preparation allows for curing at a lower temperature of 120–130°C, and also shortens the hot-pressing time. Adding 25%–30% resorcinol achieves the same effect. Adding rubber-like materials to phenolic resin adhesives can improve their heat resistance and viscoelasticity.

(2) Adhesive preparation process

When preparing adhesives, it is important to control the amount of each component added and the order of addition. Sufficient mixing time is crucial to ensure thorough mixing and prevent precipitation or sedimentation of the multi-component materials within a short period. The quality of the prepared adhesive can be controlled by its post-preparation viscosity. Below are the preparation processes for several types of aesthetically pleasing adhesives.

The formulation (by weight) for preparing foamed urea-formaldehyde resin adhesive is as follows: 100 parts resin, 0.5-1 part blood powder, 0.2-1 part ammonium chloride, and 2-4 parts water. Blood powder is a foaming agent; before use, soak it in 4 times its weight of water for 1 hour. When preparing the adhesive, first add the resin to a foaming machine and stir at 250-300 rpm. Then add the blood powder and foam for 5 minutes. Next, add a 20% ammonium chloride solution and stir for another 5 minutes. The adhesive is ready for use when the volume increases 2-3 times. This adhesive has a non-flowing, thick paste appearance, a density of 0.3-0.4 g/cm³, and an activity period of not less than 3 hours.

The formula for preparing urea-formaldehyde resin adhesive for pre-compression (by weight) is as follows: 100 parts resin, 0.2-1 parts ammonium chloride, 0-0.4 parts ammonia, 3-6 parts flour, and 6-9 parts peanut shell powder. When preparing the adhesive, first add the resin to the mixing machine, then add the flour and stir for 10-15 minutes until there is no dough in the adhesive (alternatively, you can first use some of the resin to stir the flour into a paste, then add all the resin and stir well). Next, add the peanut shell powder and stir for 5 minutes. Finally, add a 20% ammonium chloride aqueous solution and a 25% chlorine solution, stir together for 5 minutes, and it is ready for use.

The formulation for preparing phenolic resin adhesive (by weight) is as follows: 100 parts phenolic resin, 7-12 parts chalk, 3 parts wood flour, 1-3 parts paraformaldehyde, and 2.5-5.0 parts water. The stirrer speed should be 140-150 rpm. Add each component sequentially during preparation. Stir for 5-10 minutes each time, and stir for another 20 minutes after adding all components before use.

Thermosetting phenolic resin adhesive films are used in the manufacture of aerospace plywood to ensure product quality. Therefore, phenolic resin molds must be prepared in advance. Alcohol-soluble or water-soluble phenolic resins are typically used for impregnation of the adhesive film; their technical specifications are detailed in 4-1.

If water-soluble phenolic resin is used, it needs to be diluted with water and alcohol to reduce the brittleness of the film and minimize breakage. The formula is as follows: 50 kg of water-soluble phenolic resin (45% solids content), 15 kg of water, and 15 kg of alcohol.

Impregnation and drying of the adhesive film paper can be carried out using horizontal or double-stage impregnation machines. The impregnation process conditions are shown in Table 4-2. During drying, stable temperature, airflow, and moving speed should be maintained. The thickness of the adhesive film, the amount of impregnation, the content of active ingredients, and the resin solubility should be specified for the adhesive film paper. For adhesive films used in aviation plywood, quality testing, soluble resin content testing, and adhesive film bonding strength testing should also be conducted.

2. Application amount and quality of adhesive

(1) Amount of adhesive applied

Adhesive application rate refers to the weight of adhesive per unit area after veneer application, expressed in square meters (/m²). There are two types: single-sided adhesive application rate and double-sided adhesive application rate. Plywood production often uses double-sided adhesive application on the core veneer, therefore the adhesive application rate is the double-sided adhesive application rate.

The amount of adhesive applied is one of the factors affecting bond strength. Too much adhesive results in a thicker adhesive layer, increased stress, and decreased bond strength; it also wastes adhesive and is uneconomical. Too little adhesive fails to form a continuous adhesive layer and hinders adhesive transfer to other bonding surfaces.

The amount of glue applied depends on the type of glue, tree species, and veneer thickness. For example, when applying glue to veneers with a thickness of 1.25–1.50 mm, using phenolic resin glue (45%–50% solids content), the application rate is 220–250 g/m² for birch veneer, 240–260 g/m² for linden, and 280–300 g/m² for ash. Using urea-formaldehyde resin glue (60%–65% solids content), the application rate is 240–260 g/m² for birch veneer, 260–300 g/m² for linden, and 300–350 g/m² for ash.

(2) Factors affecting the quality of adhesive application

The quality of adhesive application can be measured from two aspects: the thickness of the adhesive layer and the uniformity of the application. A thin adhesive layer that forms a continuous and uniform film indicates good adhesive application quality.

The main factors affecting the quality of adhesive application are the quality of the adhesive itself and the process conditions during application. The quality of the adhesive, especially its viscosity, must meet the process requirements. If the viscosity is too high, it is difficult to apply the adhesive evenly; if the viscosity is too low, it is easy for adhesive to seep through, resulting in insufficient adhesive on the veneer surface and preventing the formation of a continuous adhesive film. During application, in addition to the adhesive meeting the requirements, other process operations must also meet the requirements. For example, with roller coating, uneven wear of the coating roller should be avoided to prevent affecting the uniformity of application, and the uniformity and consistent amount of adhesive must be ensured.

3. Application methods and equipment

Veneer gluing is a process of evenly applying a certain amount of adhesive to the surface of a veneer. For thin plywood used in aerospace applications, direct film gluing is used to ensure quality and ease of construction, but this method is very costly. Currently, most adhesives used for conventional plywood are liquid adhesives, and depending on the equipment used, they can be categorized into roller coating, spraying, extrusion, and pressure spraying methods.

(1) Roller coating method

Roller coating involves applying adhesive, which is applied to the veneer using a glue roller. This method is often used for double-sided coating of veneers and can be achieved using a two-roller or four-roller glue coater.

In a twin-roller glue applicator, the glue on the upper roller is transferred to the lower roller. During application, the veneer passes between the two rollers, and the glue on the rollers is applied to the veneer through mutual contact. The amount of glue applied is mainly controlled by adjusting the distance 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.

Twin-roller glue applicators are relatively outdated equipment. The amount of glue applied 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, their simple structure and ease of maintenance mean they are still widely used in my country's production.

The four-roller glue applicator overcomes some of the shortcomings of the two-roller glue applicator. In addition to two applicator rollers, it also has two extrusion rollers, which are a pair of steel rollers with chrome plating or hard rubber coating. The extrusion rollers have a linear speed 15%–20% lower than the applicator rollers, acting as a scraper. The distance between them and the applicator rollers is adjustable to control the amount of glue applied. A washing tank is installed below the applicator rollers for washing and draining wastewater. Because the four-roller glue applicator supplies glue simultaneously from both the top and bottom, it solves the problem of uneven glue application found in two-roller machines.

To maintain the good performance of the glue coating machine, proper maintenance is essential. Regularly clean it with warm water. If localized hardening occurs, clean it with a brush dipped in a 3%–5% caustic soda solution. If applying urea-formaldehyde resin glue, neutralize it with acetic acid and then rinse with clean water. Repair any wear on the roller grooves to ensure even glue application and consistent glue volume.

(2) Spray coating method

Applying adhesive by spraying is a highly efficient single-sided adhesive application method introduced from the paint industry. The adhesive enters the spraying head and, under pressure, flows out from the bottom seam, forming a uniform adhesive curtain. When the veneer passes through this curtain, a layer of adhesive is left on the veneer surface. The thickness of the adhesive layer on the veneer is related to the adhesive flow rate, viscosity, material surface tension, and veneer feed speed. Increasing the adhesive flow rate, improving the viscosity, and decreasing the veneer feed speed all result in a thicker adhesive layer. The adhesive temperature should be slightly above 20°C; otherwise, the adhesive layer thickness will not be uniform. This method is suitable for continuous and automated production lines, but its effectiveness is poor for uneven veneers.

(3) Extrusion method

The extrusion device consists of a glue reservoir and a row of cylindrical glue outlets at its lower part. The extrusion method involves applying high-viscosity or foamed glue through the small orifices of an extruder onto the veneer. The veneer passes under the extruder, and the glue flows down in strips onto the veneer. The direction of the glue strip should be perpendicular to the fiber direction of the veneer. Approximately half of the veneer area is initially uncoated; during pre-pressing, the glue can be expanded into a complete glue layer. Alternatively, a roller with a silicone rubber coating can be used to flatten the glue strip on the veneer. When using foamed glue, the diameter of the extrusion orifices can be increased to prevent clogging.

The main advantage of the extrusion method is that it saves adhesive, but care should be taken to prevent the glue holes from becoming clogged.

(4) Spray adhesive method

The spray glue method involves applying high pressure (3-6 MPa) to the glue, causing it to be sprayed at high speed from the nozzle. The sprayed glue travels in a rotating motion, resulting in good dispersion. To ensure even application, the nozzle should be as small as possible (0.3-0.5 mm in diameter), but it is prone to clogging, requiring clean glue and careful attention to the glue's viscosity. The spray glue method is highly efficient, but controlling the glue volume is more difficult. The working principle of the spray glue method is similar to that of the drip glue method, where glue is applied while the veneer is moving forward.

Applying glue through dripping, extrusion, and spraying are three relatively new glue application methods that have emerged in recent years. Their common characteristics include high production efficiency, good glue application quality, and ease of achieving continuous glue application and assembly. Currently, my country's plywood production scale is relatively small, and product specifications vary widely, so roller-type glue applicators are still widely used. Roller glue application is suitable for manual operation, but it is not conducive to achieving mechanized and continuous production.