New process for producing environmentally friendly and moisture-proof engineered wood panels

Introduction

This paper introduces three methods for producing environmentally friendly and moisture-proof engineered wood products, including adhesive modification, the use of formaldehyde-reducing additives, and the selection of waterproof materials. Using modified urea-formaldehyde resin in combination with formaldehyde-reducing additives, along with a waterproof material made from a mixture of paraffin wax and nano-waterproofing agents, resolves the contradiction between the two important indicators of formaldehyde release and water absorption thickness expansion rate, while also demonstrating certain advantages in mechanical properties.

In recent years, the demand for fiberboard and particleboard products with certain moisture-proof functions has been increasing in the board market. Moisture-proof medium-density fiberboard and particleboard are generally used in humid indoor environments such as cabinets and bathroom vanities, while moisture-proof high-density fiberboard is mainly used to make engineered wood flooring. Generally speaking, the production of moisture-proof fiberboard and particleboard mainly involves adding waterproof materials, such as molten paraffin or paraffin emulsion. In recent years, national environmental protection requirements have become increasingly stringent. The promulgation of the national mandatory standard GB 18580-2017 "Formaldehyde Emission Limits in Wood-based Panels and Their Products for Interior Decoration and Renovation" has set further requirements for formaldehyde emission limits in board materials. However, as the formaldehyde emission of board materials decreases, the difficulty of controlling waterproof performance increases. The main reason is that the most direct and effective way to achieve the low-formaldehyde target is to reduce the amount of adhesive, but a lower amount of adhesive leads to poorer waterproof performance. Only by increasing the amount of adhesive, extending the hot-pressing cycle, and increasing the amount of paraffin used can the various performance standards be met.

Through extensive laboratory trials and online testing on several medium and large production lines, the author has summarized a more effective and lower-cost production process for environmentally friendly and moisture-proof fiberboard. This process also has some reference value for environmentally friendly and moisture-proof particleboard.

1. Several methods to improve the environmental protection and moisture resistance of engineered wood products

1.1 Melamine-modified urea-formaldehyde resin

The increase of melamine in the reaction system reduces the number of hydrophilic groups -OH and -CONH, resulting in more water- and heat-resistant triazine heterocycles in the resin structure, thereby improving the heat and water resistance of the cured adhesive layer. The proportion of melamine is generally between 5% and 20%, with the specific application ratio varying depending on the performance requirements of the board material. However, as the proportion of melamine increases, the polycondensation reaction rate accelerates, easily leading to gelation. Therefore, mild reaction conditions need to be adopted in the adhesive preparation process, or additives that can moderate the reaction rate and stabilize the resin structure need to be introduced. Controlling the molar ratio and pH before the polycondensation reaction, as well as using a method of multiple additions of melamine, can effectively control the polycondensation reaction rate.

1.2 Use formaldehyde-reducing additives to increase the molar ratio of adhesives.

With the implementation date of GB 18580-2017 approaching, many companies have begun preparing for the environmental upgrade of their products. Currently, most formaldehyde-reducing products on the market are ammonium salts, urea, ammonia, or similar substances, with limited formaldehyde-reducing effects and a significant impact on the water absorption thickness expansion rate of boards, making large-scale application difficult. Since 2014, our company has established a special project to develop a fast and efficient formaldehyde-reducing product, the HC series environmentally friendly accelerator. Drawing on the reaction principle of urea-formaldehyde resin, it captures and grafts formaldehyde into the adhesive-wood fiber network structure in the shortest possible time. Because this reaction is irreversible and the molecular structure is highly stable, it avoids the re-release of formaldehyde molecules due to reverse reactions and hydrolysis.

Table 1 shows the performance test results of 12 mm high-density fiberboard using environmentally friendly accelerators under identical process conditions. Three samples each of high-density fiberboard were selected from those without environmentally friendly accelerators (control group) and those with environmentally friendly accelerators (experimental group). Performance tests were conducted according to GB/T 17657-2013 "Test Methods for Physical and Chemical Properties of Wood-based Panels and Decorative Wood-based Panels". The results showed that under the same process conditions, the formaldehyde reduction rate reached 50%–60% after using environmentally friendly accelerators, while other performance indicators remained stable.

1.3 Nano-waterproofing agent replaces part of paraffin wax

Another crucial component in the production of moisture-resistant engineered wood panels is paraffin wax. To enhance waterproofing performance, the amount of paraffin wax used often exceeds 10 kg per cubic meter. Molten paraffin wax comes into contact with fibers in a hot mill, penetrating into the fiber gaps under high temperature and pressure through the grinding action of the grinding discs, physically blocking the channels for water molecules to enter the board. The main interaction between paraffin wax molecules, fibers, and adhesives is van der Waals force. When the amount of paraffin wax increases to a certain level, the presence of paraffin wax molecules affects the hydrogen bonding force between fibers and between fibers and adhesives, leading to a decrease in board strength. In the production of moisture-resistant engineered wood panels, our company's patented nano-waterproofing agent, used in conjunction with paraffin wax, can largely compensate for the shortcomings of using paraffin wax. Microstructurally, the microcapsule structure of the nano-waterproofing agent is similar to that of urea-formaldehyde resin adhesive, with the core consisting of small paraffin wax molecules sheared to the nanoscale. The nano-waterproofing agent is mixed evenly with the fibers in the spray pipe before application and drying. Because of the microcapsule structure that acts like a coupling agent, the amount of nano-waterproofing agent applied is no longer limited and can be adapted to different fiber sizes. The large molecules of paraffin wax and the small molecules of nano-waterproofing agent work together to create numerous "wax nail" structures in the gaps between the fibers and adhesives, which greatly improves the waterproof performance of the fiberboard.

Table 2 compares the performance of medium-density fiberboard (MDF) with a thickness of 15 mm and a density of 700 kg/m³ (raw materials: 30% poplar branches and 70% rotary-cut veneer waste) produced using paraffin wax and a mixture of paraffin wax and nano-waterproofing agent. Three MDF samples each were selected for the experiment: one using only paraffin wax (control group) and the other using a mixture of paraffin wax and nano-waterproofing agent (experimental group). Performance tests were conducted according to GB/T 11718-2009 "Medium-Density Fiberboard". The results, as shown in Table 2, indicate that the water absorption thickness swelling rate of MDF using only paraffin wax fails to meet the requirements of GB 11718-2009 for MDF used under humid conditions. However, the method of using a mixture of paraffin wax and nano-waterproofing agent completely solves this problem, and the product performance meets the requirements of GB 11718-2009.

2. New production process for environmentally friendly and moisture-proof engineered wood panels

To address the challenge of simultaneously achieving low formaldehyde release and waterproof performance in environmentally friendly and moisture-proof engineered wood products, our company collaborated with a large-scale continuous production line to develop a new type of engineered wood product. This product utilizes 8%–12% melamine to modify urea-formaldehyde resin (mass ratio 1.03–1.05), employs 10 kg/m³ of environmentally friendly accelerator HC, and utilizes a process combining paraffin wax and a waterproofing agent to produce a low-formaldehyde, high-moisture-resistant flooring substrate. The performance test results are shown in Table 3. The results indicate that the formaldehyde release of the 12 mm high-density fiberboard produced by this process is consistently controlled at approximately 5.0 mg/100 g using the perforation extraction method. The product's formaldehyde release meets the US CARB II export standard, while other performance indicators exceed those of LY/T 1611-2011 "Fiberboard for Flooring Substrates".

For the same formaldehyde emission, if environmentally friendly accelerators and nano-waterproofing agents are not used, but traditional methods are employed, the molar ratio of the adhesive needs to be reduced to 0.95–0.98, and 13 kg/m³ of molten paraffin wax needs to be used. Other properties will be affected by the decrease in the molar ratio of the adhesive, especially the internal bond strength after 2 hours of boiling in water, which cannot meet the requirements of LY/T 1611-2011. The performance indicators are shown in Table 4.

3. Conclusion

Based on the results of individual and comprehensive tests, the environmentally friendly and moisture-proof engineered wood products produced by this new composite process, which uses melamine-modified urea-formaldehyde resin, applies environmentally friendly accelerators in the later stages, and combines them with nano-waterproofing agents and paraffin wax, can largely compensate for the adverse effects of the reduced adhesive content in low-formaldehyde engineered wood products, and resolve the contradiction between the two important indicators of formaldehyde release and water absorption thickness expansion rate.