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1. Causes and control measures for excessive formaldehyde emissions
Causes:
1) High formaldehyde content in adhesives. The higher the free formaldehyde content in the urea-formaldehyde resin adhesive used in thin sheets, the higher the formaldehyde content in the sheet blank, and the more formaldehyde is released from the finished product.
2) Excessive use of adhesive for wood shavings. Thin boards have a higher density than thick boards, meaning there are more wood shavings per unit volume, which also increases the amount of adhesive used per unit volume. As the amount of adhesive used increases, the formaldehyde release from the board also increases.
3) Inappropriate hot-pressing process. The amount of free formaldehyde released during the hot pressing of thin sheets generally depends on the hot-pressing temperature and time. The higher the hot-pressing temperature, the less free formaldehyde in the finished product; the shorter the hot-pressing time, the more free formaldehyde in the finished product. If the moisture content of the sheet is high, the amount of free formaldehyde in the finished product will also be higher.
Control measures:
1) Improve the adhesive synthesis process. Reduce the release of free formaldehyde in urea-formaldehyde resin adhesives by lowering the mass ratio of formaldehyde to urea, increasing the number of times urea is added during adhesive preparation, and adding formaldehyde scavenging agents.
2) Adjust the amount of adhesive applied to the wood shavings. The setting of the adhesive application amount should take into account the relationship between formaldehyde release, mechanical properties, and production costs.
3) Use formaldehyde-reducing additives. Additives with formaldehyde-capturing functions, such as urea and ammonia, are added during the adhesive preparation process to react with free formaldehyde molecules during hot pressing.
4) Adjust hot pressing process parameters. In order to reduce free formaldehyde in the thin plate, the moisture content of the blank can be appropriately reduced and the hot pressing temperature increased, while extending the hot pressing time.
5) Post-curing and stacking. During the cooling and stacking of thin boards, the urea-formaldehyde resin inside the board will continue to react, which will reduce the free formaldehyde content in the board. Generally, a curing time of more than 48 hours is more suitable.
2. Reasons for unqualified static bending strength and control measures
Causes:
Static bending strength represents the maximum ability of particleboard to resist external forces without breaking. The low static bending strength of thin boards is mainly due to improper control of raw material quality and production processes.
1) Poor wood shaving shape. Generally speaking, thinner wood shavings produce particleboard with higher static bending strength than thicker wood shavings. However, excessively thin wood shavings are prone to cracking, affecting properties such as static bending strength. Other factors affecting wood shaving shape include: excessively low moisture content leading to increased cutting resistance and excessive chipping; improper blade extension and blade clearance settings in the wood chipper; excessively low moisture content after drying; and breakage during transportation.
2) The structure of the board is unreasonable. The static bending strength is linearly related to the highest density of the surface layer of the particleboard. The higher the surface layer density, the higher the static bending strength [2]. The surface layer of the thin board has a large proportion of particleboard, and the fibers in the surface particleboard have been destroyed, so its strength is poor. Therefore, if the proportion of surface particleboard is set too high, it will affect the static bending strength of the board.
3) Poor adhesive application effect. Due to the large specific surface area and strong moisture absorption capacity of the surface material of thin sheet, sufficient adhesive application is required during production, and the adhesive must be applied evenly. Otherwise, the static bending strength will be affected.
Improvement measures:
1) Control the shaving morphology. Wood raw materials should be air-dried in the raw material yard for a period of time, controlling the moisture content of the wood chips to within the range of 40%–60% before use to prepare shavings. During the shaving process, the blade extension and blade clearance of the shaving machine should be adjusted according to the shaving morphology. The drying process should ensure that the moisture content of the shavings at the drying outlet remains stable within the process requirements.
2) Adjust the installation process. Increase the bulk density of wood shavings in the thinner board so that more shavings per unit volume can bear the force, and appropriately reduce the proportion of fine shavings in the surface layer and increase the proportion of coarse shavings in the core layer, thereby increasing the bending stress on the board and improving its static bending strength.
3) Improve adhesive application. Appropriately increase the amount of adhesive applied to the surface wood chips and improve the atomization of the adhesive spray to enhance the uniformity of adhesive application. At the same time, adjust the amount of water added to the surface adhesive through the adhesive mixing system or add a humidifying spray system to increase the moisture content of the surface wood chips, making the surface wood chips easier to plasticize and increasing the surface density of the thin board.
4) Adjust the hot-pressing process. Increasing the hot-pressing temperature allows the adhesive to cure rapidly, increasing its fluidity and distribution uniformity, thereby improving production efficiency and product quality. Increasing the hot-pressing pressure, especially the initial maximum pressure, can increase the density gradient between the surface and core layers of the board and improve its static bending strength.