Resins, hydrolysis

This free formaldehyde is presumed to derive from excess formaldehyde present in the UF resin. It exists in ill-defined, relatively loosely bound states within the board, states whose stabilities are sensitive to temperature and humidity. At high resin F/U ratios, the free formaldehyde content and board emission rate fall rapidly after pressing and later decrease more slowly. The free formaldehyde content and board emission rate are lower after pressing when using resins with F/U ratios approaching 1.0, and they decrease more slowly with time. What has never been clear, however, is whether actual UF resin hydrolysis, with attendant formaldehyde production, is responsible for a significant amount of the board s emission, and if so, at what point in the board s life that occurs. 

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. 

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... 

Factors to be considered in this paper include (a) the degree to which formaldehyde emission rate from wood systems is controlled by diffusion processes, (b) the contribution of resin hydrolysis to emission rate, and (c) the contribution of formaldehyde-wood states to emission rate. In the following, therefore, I first summarize briefly the reported evidence regarding diffusion control and resin hydrolysis in actual bonded wood products. Thereafter, I present and discuss some of my own recent experiments on wood systems that attempted to shed additional light on the questions of resin hydrolysis and the emission mechanism more generally. 

Literature Evidence for Resin Hydrolysis in Actual Boards... 

Despite the rather massive literature on formaldehyde emission from UF-bonded wood products, evidence for a direct causal relationship between resin hydrolysis and formaldehyde emission from bonded products is almost nonexistent. Indeed, evidence in the literature that UF resin hydrolysis actually does occur in a board arises primarily from studies into the question of whether the limited durability of UF-bonded wood products is caused by resin hydrolysis or by a particular susceptibility of UF resin-wood bonds to rupture from swelling/shrinkage stresses. 

Evidence for Resin Hydrolysis. That UF resin hydrolysis can occur in boards is strongly indicated by the following ... 

To shed additional light on the emission mechanism and the contribution of resin hydrolysis to formaldehyde emission, my recent experiments have examined the liberation or extraction of formaldehyde from particleboards, from wood containing sorbed formaldehyde, and from cured resins. Here, I present results from particleboard and formaldehyde-sorbed wood experiments in which rates of formaldehyde removal were measured by three different procedures (see Appendix 1 for experimental details). 

Some authors claim that subsequent hydrolysis of the resin itself also contributes to the formaldehyde emission. This is not likely, among other things because the formaldehyde emission is not accompanied by the bond deterioration and strength loss that would be the result of resin hydrolysis. 

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

Silicone Resins. Hydrolysis of (CH3)2SiCl2 produces linear flexible molecules for rubber. Hydrolysis of CH3SiCl3 produces highly cross-linked molecules for thermoset plastics. These are too cross-linked and brittle for most purposes. Useful thermoset plastics are prepared by copolymerizing difunctional and trifunctional monomers. In commercial practice, the ratio of difunctional to trifunctional is generally 80/20 to 40/60. For some products, methyl silicon may be partly replaced by phenyl silicon. 

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