Causes and control measures for excessive formaldehyde emission from thin particleboard

1. Causes and control measures for formaldehyde emission exceeding the standard

Cause:

1) High formaldehyde content in the adhesive. The higher the free formaldehyde content in the urea-formaldehyde resin adhesive used in the sheet, the higher the formaldehyde content in the slab, and the more formaldehyde released in the finished product.

2) Excessive wood particle glue application. Thin boards have a higher density than thick boards, meaning more wood particles are packed into a unit volume, which in turn increases the amount of adhesive required. As the amount of glue applied increases, formaldehyde emissions from the board also increase.

3) Improper hot pressing process. The amount of free formaldehyde released during hot pressing of thin sheets generally depends on the temperature and time of the press. The higher the temperature, the less free formaldehyde in the finished product; the shorter the press time, the more free formaldehyde in the finished product. High moisture content in the slab also increases free formaldehyde in the finished product.

Control measures:

1) Improve the adhesive synthesis process. Reduce the release of free formaldehyde in urea-formaldehyde resin adhesives by lowering the formaldehyde-to-urea mass ratio, increasing the number of urea additions during adhesive production, and adding formaldehyde scavengers.

2) Adjust the amount of shavings glue. The setting of the glue amount parameter should comprehensively consider the relationship between formaldehyde emission, mechanical properties and production costs.

3) Use formaldehyde-reducing additives. During the adhesive preparation process, some additives with formaldehyde-capturing functions, such as urea and ammonia, are added to react with free formaldehyde molecules during the hot pressing process.

4) Adjust the hot pressing process parameters. In order to reduce the free formaldehyde in the thin plate, the moisture content of the slab can be appropriately reduced, the hot pressing temperature can be increased, and the hot pressing time can be extended.

5) Post-curing stacking. When the thin boards are cooled and stacked, the urea-formaldehyde resin in the boards will continue to react, reducing the free formaldehyde content in the boards. Generally, a stacking curing time of more than 48 hours is more appropriate.

2. Causes and control measures for unqualified static bending strength

Cause:

Static flexural strength is the maximum ability of particleboard to resist external forces without breaking. The low static flexural strength of thin boards is mainly caused by improper raw material quality and production process control.

1) Poor particle shape. Generally speaking, particleboard produced from thinner particles has greater static bending strength than particleboard produced from thicker particles. However, excessively thin particles are prone to cracking, affecting static bending strength and other properties. Too low a moisture content in the wood chips increases cutting resistance and produces excessive debris. Improper settings for the flaker blade extension and blade clearance, too low a moisture content after drying, and breakage during transportation can also affect particle shape.

2) Irrational slab structure. Static bending strength is linearly correlated with the maximum density of the particleboard surface layer. The higher the surface density, the higher the static bending strength [2]. Thin boards use a large proportion of surface particles, but the fibers in the surface particles have been destroyed and their strength is poor. Therefore, setting the surface particle ratio too high will affect the static bending strength of the board.

3) Poor gluing effect. Since the surface material of the thin plate has a large specific surface area and strong moisture absorption capacity, sufficient gluing amount is required during production, and the adhesive must be applied evenly. Otherwise, the static bending strength will be affected.

Improvement measures:

1) Controlling particle shape. Wood raw materials should be air-dried in the raw material yard for a period of time to maintain a moisture content of 40% to 60% before being used to produce particles. During the flaking process, the flaker blade extension and blade clearance should be adjusted based on particle shape. The drying process should ensure that the particle moisture content at the drying outlet remains within the process requirements.

2) Adjust the paving process. Increase the packing density of the thin board shavings so that more shavings per unit volume can withstand the applied force. Appropriately reduce the proportion of fine shavings in the surface layer and increase the proportion of coarse shavings in the core layer. This will increase the bending stress on the board and improve the static bending strength of the board.

3) Improve sizing results. Appropriately increase the amount of glue applied to the surface wood particles and improve the atomization of the glue spray to improve sizing uniformity. Also, increase the moisture content of the surface wood particles by adjusting the amount of water added to the surface glue through the glue adjustment system or by adding a surface moisturizing spray system. This will facilitate plasticization of the surface wood particles and increase the surface density of the sheet.

4) Adjust the hot pressing process. Increasing the hot pressing temperature can accelerate the curing of the adhesive, increase its fluidity and distribution uniformity, and improve 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 sheet and improve its static bending strength.