Panel large, formaldehyde emission

Product tests. Clearly, the best product test is full-scale testing of finished panels under actual use conditions. This has been done (27,38) but is expensive, because several full-sized panels of each product must be pre-conditioned at constant temperature and humidity for at least a week. The next best approach is to test product samples in air chambers under standardized conditions. A summary of such methods is contained in Table I. A very large effort has been made over the last three decades world-wide to develop quick, reliable and meaningful product tests. Wittmann (16), Zartl (20), Plath (17), Verbestel (1, Neusser (21,22), Roffael (25), HUD, the U.S. Forest Products Industry (39,40), many standaraization organizations (41-43) and others have published many viable methods, but the testing involves a combination of complex factors and there is simply no single test that fulfills everybody s specific needs. Table I list some of the currently accepted test methods for formaldehyde emission from particleboard, plywood and medium density fiberboard. 

Finishes in some cases also appear to reduce emissions from wall paneling products. The effectiveness of a vinyl film overlay was evaluated using high emitting hardwood plywood wall panels (O. Formaldehyde emissions from the vinyl surface of plywood were compared with the back or unexposed plywood surface using both the large chamber and the two hour desiccator. This comparison indicated that a 2-mil vinyl was about 90% effective in reducing emissions. 

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. 

In this section, I offer an analysis of these experimental results and speculate about their implications for large panel formaldehyde emission. 

The U.S.Department for Housing and Urban Development s rule 3280.308 established formaldehyde emission standards for particleboard and hardwood plywood paneling used in mobile homes. These standards took effect February 11, 1985. The certification program under this rule requires each manufacturer to develop a quality control in-plant testing program that relates to tests conducted in a large scale environmental chamber. 

The main parameter for the application of tannins as adhesives for wood-based panels is the content of reactive polyphenols and the reactivity of these components towards formaldehyde. Tannins can be used as adhesives alone (with a formaldehyde component as crosslinker) or in combination with aminoplastic or phenolic resins. These resins can react chemically with the tannin component in a polycondensation reaction, form only two interpenetrating networks, or both. The simplest adhesive mix formulation consists of the tannin solution and powdered paraformaldehyde as crosslinker [283]. The addition of paraformaldehyde can cause in the short term a relatively high level of formaldehyde emission. Glue mixes using paraformaldehyde for the production of particleboards with low formaldehyde emission are described and used industrially [284]. In the literature a large number of papers describe the combinations of tannins with synthetic resins (Table 14). 

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