The Source and Mechanism of Formaldehyde Emission from Wood-Based Panel
A considerable amount of research has been conducted on the issue of formaldehyde emission from wood-based panels during the last decade. The formaldehyde emissions are mainly from three sources: (1) formaldehyde compound in wood material, (2) residual free formaldehyde of the formaldehyde-based resin that is not involved in the reaction, and (3) formaldehyde released by the structural degradation of the wood-based panel used. Among them, the residual free formaldehyde in the panel is the main source of indoor air pollution.
Formaldehyde of wood-based panels can be divided into two parts, free formaldehyde easier to be released and more difficult to release with bonded formaldehyde. Formaldehyde emission rates can be in two stages. In the first stage, the emission is mainly free formaldehyde. The emission rate depends on the diffusion rate of free formaldehyde in the panel, which is affected by the formaldehyde concentration gradient in the panel. Ventilation can speed up the release of formaldehyde. This stage can be 1-2 weeks or 1–3 months, depending on the amount of free formaldehyde in the board. In the second stage, the emission is from the bonded formaldehyde, which the release rate depends on the bonding force, and ventilation would have little effect. Stage 2 can be up to several years.
Liu found that the necessary and sufficient conditions of formaldehyde release from wood-based panels were when the air pressure inside the wood-based panel is greater than the environment and the channel for air circulation is available so that the formaldehyde can release. Xing et al. concluded that the formaldehyde emission was mainly from the edges of panel, which was more than 2 times than that from the panel surface. Therefore, the thinner the board, the more formaldehyde emission. Meng and Hong separated the formaldehyde emission process from wood-based panels into three stages: rapid release, slow release, and full release.