Wood-based panel is widely used in furniture manufacturing and interior decoration projects due to the advantages of easy processing and dimensional stability. However, wood-based panel will release formaldehyde, if a formaldehyde-based resin is used, and other harmful gases, causing indoor air pollution. Formaldehyde has been classified as a potentially dangerous carcinogen and an important environmental pollutant by the World Health Organization and the United States Environmental Protection Agency.
1. Residual free formaldehyde that is not polymerized into the network and which will emit during or quickly after panel production;
2. Formaldehyde can be released due to adhesive hydrolysis, which will emit over the life time of the panel depending on moisture and temperature;
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. It has been 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. From the study and literature, 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 would be there.
The mechanism is the formaldehyde emitted during the hot cure can partly be traced back to the substances containing methylene ether linkages. When subjected to heat, these ether bridges will decompose through hydrolysis and form formaldehyde. The free formaldehyde emitted from aging UF resins is believed to be a result of that molecule containing for example methylol gropus and di-methyl ether bridges will decompose through hydrolysis, and thus form free formaldehyde. However, if free urea is added to the resin, the urea will capture the free formaldehyde produced in all the different stages, and bind it to the resin thus decreasing the formaldehyde emissions. This is usually done by adding free urea at the end of the production. By adding more urea to the resin in the end, the formaldehyde/urea ratio is changed, and this will affect the reaction and thus also which types of different species of UF molecules that are formed. The free urea will not only react with the free formaldehyde but also with the methylol groups of the resin itself. As it is the methylol groups that form cross links during the curing process, the addition of urea will decrease the adhesive property of the resin. This means that the manufacturer have to sacrifice some of the adhesive properties of the adhesive in order to keep down the emissions of formaldehyde.
Formaldehyde is emitted from solid wood under very high heat and is not expected to be a significant source of the emissions from composite wood products during normal service. Formaldehyde is also detectable even if wood has never been heated as well as under more or less ambient conditions. The presence of formaldehyde in the emissions from wood that does not contain adhesive resin has been explained by thermal degradation of polysaccharides in the wood. The emission levels of formaldehyde depend on factors such as wood species, moisture content, outside temperature, and time of storage. Additionally, the pyrolysis of milled wood lignin at 450 °C yields benzaldehyde, and the pyrolysis of spruce and pinewood at 450 °C generate formaldehyde, acetaldehyde, 2–propenal, butanal, and butanone, which can be attributed to the breakdown of the polysaccharide fraction of the wood.
FORMALDEHYDE EMISSION FROM WOOD-BASED PANEL CAN BE A COMPLICATED PROCESS, WHICH CAN BE AFFECTED BY:
1. Factors related to the materials, such as type of panel, wood species, adhesive, and Overlay used for the panels;
2. Factors related to the environment, such as temperature, humidity, air velocity, and air exchange rate.
3. Factors related to treatment.
4. Factors related to panel fabrication process, such as resin content, moisture content of the panel, and others.
The standard and limit values of the formaldehyde emission from different wood composite products are followed in different counties are as follows.
AUTHOR: S. C. Sahoo, Scientist, Indian Plywood Industries Research &Training Institute, (IPIRTI)