Formaldehyde emission processes, board

However, it has to be considered that it is neither the content of free formaldehyde itself nor the molar ratio which eventually should be taken as the decisive and the only criterion for the classification of a resin concerning the subsequent formaldehyde emission from the finished board. In reality, the composition of the glue mix as well as the various process parameters during the board production also determine both performance and formaldehyde emission. Depending on the type of board and the manufacturing process, it is sometimes recommended to use a UF-resin with a low molar ratio F/U (e.g. F/U = 1.03), hence low content of free formaldehyde, while sometimes the use of a resin with a higher molar ratio (e.g. F/U = 1.10) and the addition of a formaldehyde catcher/depressant will give better results [17]. Which of these two, or other possible approaches, is the better one in practice can only be decided in each case by trial and error. 

The overall objective of this and a companion paper (22) is to define the extent to which board formaldehyde emission is controlled by resin hydrolysis or other processes. In the companion paper I have critically reviewed the literature and presented original Forest Products Laboratory (FPL) data in three related aspects of the formaldehyde emission phenomenon the chemistry of and formaldehyde liberation from formaldehyde-urea and formaldehyde-phenol states the chemistry of and formaldehyde liberation from formaldehyde-cellulose and resin-cellulose states and our knowledge of the board emission mechanism derived from actual board and wood systems. Whereas my oral presentation at the American Chemical Society (ACS) Symposium made use of information from all three of those parts, this written paper, in the interest of saving space, is limited to literature and FPL data dealing with actual wood-containing systems. The Conclusions section of this paper, however, makes use of the results from all three parts of the companion paper. Experimental details of the... 

It appears, therefore, that formaldehyde emission rate from a given large panel may be controlled by chemical processes within the board or by diffusion either in the board-air interface or within the board. Which of these predominates depends upon the board s age, composition, physical structure, and exposure conditions. 

Board formaldehyde emission, effect of resin hydrolysis or other processes, 88... 

A considerable amount of effort has gone into overcoming the sticking problems. One approach has been to manufacture a three-ply board in which the two outside layers utilize a conventional wood binder, such as a phenol-formaldehyde resin, and the internal layer is bound with an isocyanate adhesive.Deutsch Novopan GmbH of West Germany has commercialized such a process to manufacture exterior structural grade particleboard. A board prepared in this manner does not completely circumvent the potential problem of formaldehyde emissions, although they may be reduced relative to a board made solely with a formaldehyde adhesive. 

Melamine-fortified resins with a melamine content of up to approximately 10% based on liquid resin are used for various applications where straight UF resins cannot provide the desired combination of processing tolerance, formaldehyde emission, and specific board properties such as a low thickness swelling. MUF resins with higher content of melamine (up to 30% based on liquid resin) find applications in enhanced performance grade boards for use in hiunid conditions (moisture-resistant application). 

---