Board formaldehyde emission, effect

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

At this point of the study it is not possible to improve our knowledge of the emission trends with this method. However, given that the formaldehyde emission from a particleboard must decrease with time, we decided to measure this effect. Two sets of experiments were carried out parallel to each other for one year at 23 L and 65% RH, 80% RH, or 30% RH. The boards were tested at regular intervals by both the perforator method and the gas flow... 

More interesting are the cases in which the tannin extract was added to the chips rather than to the glue mix. The effect here is also a depressed formaldehyde emission. The effect appears also to last much longer due to the higher amount of tannin added. (It must be borne in mind that pure tannin-formaldehyde commercial boards... 

The question of the contribution of UF resin hydrolysis to board emission is not a trivial one. If resin hydrolysis contributes significantly to emission, then, in principle, the board would retain the potential to emit during its useful life, in contrast to the situation if all the emission results from free formaldehyde. In the former case, efforts to minimize emission must be directed toward resin stabilization and/or to ensuring that incorporated formaldehyde scavengers retain their effectiveness at low formaldehyde activities for the board s entire useful life. Another consequence of continued resin hydrolysis is possible limits on the durability of UF bonded products in this case improvement may be expected from more stable resins. 

Overall, therefore, the available literature supports the generally held view that the durability of UF-bonded wood products is governed by the susceptibility of cured UF resin bonds to scission by both hydrolysis and swell/shrink stresses. Note, moreover, that in either case, the most likely product of scission will ultimately be formaldehyde and further that mechanical stress enhances the rates of many chemical reactions (37). In fact, simplistic calculations based on formaldehyde liberated from bond ruptures at least indicate the possibility that formaldehyde from swell/shrink stress rupture could contribute significantly to total emission. Assume, for example, that board failure occurs due to rupture of one chemical bond type which liberates one molecule of formaldehyde and consider two cases (a) a conservative one in which only 5 percent of those bonds rupture in 50 years, i.e., probable board durability greater that 50 years, and (b) a much less conservative case in which 30 percent of those bonds rupture in 20 years, i.e., probably failure in 20 years or less. Case (a) leads to a first order scission rate constant of 3.3 x 10 s and a hypothetical board emission rate (see Appendix 3a) that is below the maximum liberation rate permitted by the German E-1 standard (7). However, Case (b) leads to a first order scission rate constant of 5.7 x 10 s and a hypothetical board emission rate above that allowed by the HUD standard (8). (FormaIdehyde-wood interactions and diffusion effects would... 

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