Poplar wood, due to its inherent characteristics such as loose material and high pH value (8-9), In plywood manufacturing is prone to process problems such as interlaminar delamination, warping, insufficient bonding strength and decreased mechanical properties. In order to break through the limitations of raw material defects on product performance, this study focuses on the systematic optimization of poplar plywood production process, explores the transformation path from economical raw materials to cost-effective products through adhesive adaptability improvement, veneer pretreatment technology innovation and structural enhancement design, and maximizes the performance of poplar resources in industrial applications.
1.Raw material pretreatment
Alkaline neutralization
By spraying pH buffer (such as oxalic acid solution), the pH value of the veneer surface is adjusted to 5-6 to improve the curing efficiency of urea-formaldehyde resin adhesive (experiments show that the bonding strength can be increased by more than 30%).
Moisture content control
Using microwave-hot air combined drying technology, the moisture content of the veneer is accurately controlled at 8-12%, reducing the risk of internal stress cracking during the hot pressing stage.
2.Veneer preparation
Softening treatment (optional): Sometimes, in order to improve the efficiency and quality of peeling, hydrothermal treatment or room temperature impregnation is performed to soften the wood. The process of heat treatment and forging of poplar wood is very similar, except that the cooking temperature is slightly lower than that of basswood and the cooking time is less than that of basswood. Since poplar has a smaller specific gravity, is a diffuse-porous wood, has a light and soft texture, and belongs to a medium-coarse grain structure, the peeling temperature cannot be too high. When peeling the veneer at high temperature, small pits are prone to appear and it is easy to fluff. Therefore, under normal circumstances, we use low-temperature steaming for poplar wood. Its process conditions are as follows: After thawing in winter, steam it in warm water at 25-30℃ for 24 hours to allow the temperature in the wood core to reach 15 degrees Celsius. Steaming is not required in spring and autumn. However, in order to make the core temperature of the wood reach 15℃, the wood should also be soaked in warm water at 20-25℃ for about 16 hours. Since the outdoor temperature is high in summer, it can be soaked in warm water at room temperature for about 12 hours. If it is a newly cut wood, it can be directly peeled without soaking under summer conditions.
Peeling: Use a peeling machine to cut the softened wood into continuous thin slices. The veneer of modern plywood is usually made by a peeling machine.
3.Drying
Drying: The veneer needs to be dried to achieve a moisture content suitable for glue coating (generally controlled at about 8-10%). This step can be completed by an efficient jet veneer dryer. Most of the poplars used to produce plywood now are divided into Populus simonii Carr and Populus ussuriensis Kom. The moisture content of Populus simonii Carr and Populus ussuriensis Kom is different at the beginning. The one with higher moisture content is Populus ussuriensis Kom. When drying Populus simonii Carr and Populus ussuriensis Kom together, when the moisture content of Populus simonii Carr reaches the requirement of producing plywood, the moisture content of Populus ussuriensis Kom must be greater than 10%; this means that when the moisture content of Populus ussuriensis Kom reaches the production requirement of plywood after drying, Populus simonii Carr is in an over-dried state. Therefore, we must pay attention to separating Populus simonii Carr and Populus ussuriensis Kom when drying, and cannot stack them together for drying. Dry until the final moisture content of the veneer is between 6% and 8%.
Adhesive adaptation and glue application optimization plan
I. Adhesive modification plan
Toughened urea-formaldehyde resin
Add an appropriate amount of PVA to the urea-formaldehyde resin. As a water-soluble polymer, PVA has good film-forming properties and flexibility, which can effectively compensate for the low hardness defects of poplar wood itself and improve the durability of the final product.
Low-alkali phenolic resin
Suitable for scenes that require good weather resistance, such as building formwork, by adjusting the pH value of phenolic resin to better adapt to the alkaline environment of the poplar surface, thereby improving the bonding strength and stability.
Method: Reducing the alkalinity of phenolic resin can be achieved by selecting a suitable acid catalyst or using a partial neutralization method. This can not only reduce corrosion to the poplar base, but also improve the interface compatibility between the adhesive and the wood.
II. Precise control of glue application
Double-roller coating machine glue application control
Range setting: The recommended glue application amount is controlled between 180-220g/m². This range can ensure sufficient bonding strength without causing unnecessary cost increases and extended hot pressing time due to excessively thick adhesive films.
Cause analysis:
The adhesive film is too thin: If the amount of adhesive applied is lower than the recommended range, the adhesive layer may not be strong enough and may easily delaminate, especially when subjected to external forces.
The adhesive film is too thick: On the contrary, too much adhesive not only increases production costs, but also may prolong the hot pressing time and affect production efficiency. In addition, an overly thick adhesive layer may generate internal stress and affect the overall performance of the board.
The core role of hot pressing process
Hot pressing is the core process of poplar plywood production. Through the synergistic effect of the three elements of temperature, pressure and time, the following goals are achieved:
Adhesive curing: Activate the cross-linking reaction of the resin to form a stable adhesive layer;
Structural shaping: Eliminate the gap between the single board and ensure the density between the layers;
Performance regulation: Improve the mechanical strength and dimensional stability of the board.
Hot pressing three-stage control
| Stage | Target | Key Operations |
| Preheating | Even heating to avoid thermal shock | Preheat the platen to 80-90℃ and pre-adjust the moisture content of the veneer to 8-10% |
| Pressurization Heating | The adhesive layer is completely cured and the structure is dense | Set the temperature/pressure according to the adhesive type (urea-formaldehyde resin: 110-120℃/1.0-1.2MPa; phenol-formaldehyde resin: 130-140℃/1.2-1.5MPa) |
| Cooling Anti-hypertension | Prevent springback deformation | Step-by-step pressure reduction (0.2MPa reduction every 30 seconds), synchronous air cooling to below 50℃ before unloading |
Plywood processing
The sawn veneer is smooth on both sides, has no burrs, has good grinding effect, and has a relatively flat and smooth surface. The wood texture and color are very similar to the pole wood, but there are also differences. For example, the plywood produced from Populus simonii Carr has certain medullary spots on the surface, but this will not affect the decoration and use value of the plywood.