Formaldehyde concentration wood products

In dynamic (ventilated) chambers, release rate coefficients were increased by a factor of 4.4 for particle board and 2.2 for plywood at loadings of 1.4-1.6 m /m over values at loadings of 9-11 m2/m3 (Table IV). Increased pressure of formaldehyde in the chamber was associated with reduced release of formaldehyde from wood products, as indicated by comparing equilibrium concentrations of formaldehyde (H). 

Groah, W. J. (2006). Predicting indoor-air formaldehyde concentrations from emissions of urea-formaldehyde-bonded wood products using the Versar model. Forest Products J 55, 97-100. 

While discussing ethers we should mention that the presence of unreacted anisoles or methyl anisoles is highly undesirable in the manufacture of phenol-formaldehyde resoles. These materials tend to be unreactive relative to phenol under normal resole conditions. They are also volatile and have odors detectable at very low concentrations. They have been the source of worker complaints and costly claims in the wood products industry. Benzophenones and methyl phenyl ketones are also common phenol contaminants that are problematic in this regard. 

The so-called El-emission class describes a wood panel presenting formaldehyde emission which is low enough to prevent any danger, irritation or inflammation of the eyes, nose and mouth mucous membranes. However, it is important that not only the boards themselves, but also the veneering and carpenter s adhesive resins, laquers, varnishes and other sources of formaldehyde are under control, since they also might contribute to the mixture steady state formaldehyde concentration [9]. Table 3 gives an overview on some European regulations. However, it is necessary here to introduce the principal types of composite wood products, especially panels, that are produced in this industry ... 

Formaldehyde release from pressed wood products is due to latent formaldehyde. During the pressing process, hot steam from moist wood particles transfers heat, formaldehyde, and other volatiles from the surface of the mat to the core of the board where un reacted urea-formaldehyde resin components accumulate. The resulting formaldehyde concentration in the core is approximately twice that of the surface. Release of formaldehyde is diffusion-controlled and gradually decreases over time (Meyer and Hermanns 1985). Formaldehyde can also be produced by hydrolytic cleavage of unreacted hydroxymethyl groups in the formaldehyde resins. Melamine formaldehyde resins generally are more stable, and the amounts of formaldehyde emitted from them are much lower (WHO 1989). 

Formaldehyde is released from many pressed wood products used in the construction of furniture (Konopinski 1983). When placed in a new crib manufactured from these materials, infants may be exposed because of tlieir proximity to the furniture s structural components. Also, small rooms that have new furniture manufactured from pressed wood products installed may have localized, elevated concentrations of fonnaldehyde because of their low total volume. 

The curing conditions are equally important for reducing formaldehyde emission. The curing process is not yet fully understood. In fact, there is even still some question about the nature of the reactive resin. The latter subject is described in a later chapter by Johns. Appropriate resin cure conditions must take into account the wood moisture content and wood acidity, as well as resin concentration, temperature gradients, and press duration. In excessively cured UF bonded wood products, and in products that are stacked while still hot from the press, UFR can hydrolyse so strongly that particleboard loses internal bond strength. 

Progress in quality control and in basic understanding of the physical and chemical factors affecting formaldehyde emission processes have made it possible to predict formaldehyde indoor air levels for most use conditions. Progress in manufacturing techniques and implementation of new technology have reduced formaldehyde emission so much that UF-bonded wood products can now be used in almost all applications without indoor air concentrations exceeding 0.I pp. 

If we consider as an example a relative air humidity of 50% and a temperature of 25 l, the wood moisture content would be 9.2 wt% (3. If we further consider that the product manufacturing process leaves about 1 wt% of the formaldehyde content of the UF resin as unreacted formaldehyde, we obtain for particleboard or medium density fiberboard (MDF), where UF-resin makes up 6-10 wt%, an approximate formaldehyde concentration of 0.2 M in the S-2 cell of the wood. 

This vapor acts as a driving force for formaldehyde diffusion from the wood cel I towards the product surface, and for emission from the finished wood product. An internal vapor pressure of 20 Torr would approximately correspond to a formaj ehyde air concentration of about 1 ppm at 25 t, a load factor of I m and a ventilation rate of 1 ach. However, as emission continues and depletes the methylene glycol concentration in the wood moisture, the dissociation of hemiacetals will set in and add to the formaldehyde source. The bottleneck in the formaldehyde transport will be diffusion through the product towards the product surface. This process depends on the permeability of the product which, in turn, depends on diffusion... 

Due to its affinity for water, formaldehyde will concentrate in wood products in their water reservoirs. Since wood collects water in Its S-2 secondary wall on the surface of wood cellulose, formaldehyde will come into contact with wood cellulose. This work shows that formaldehyde can be expected to react with wood cellulose forming hemiacetals. Since this reaction is reversible, these hemiacetaIs constitute a temporary reservoir for formaldehyde within wood. This fact may explain the complex formaldehyde release and absorption properties of UF-bonded wood products. 

Two sensitive fluorometric enzymatic methods for the determination of formaldehyde release from wood products were described. These methods were developed using the enzyme formaldehyde dehydrogenase to catalyze the oxidation of formaldehyde to form formic acid and NADH in the presenc of oxidized nicotinamide adenine dinucleotide (NAD ). The increase in NADH, which is directly proportional to the concentration of formaldehyde, is measured fluorometrically at em ... 

Formaldehdye generation and recovery studies 3.) Air exchange measurement techniques 4.) Preconditioning of test boards 5.) Temperature effect on chamber formaldehyde concentrations 6.) Relationship of popular quality control test methods to the large chamber 7.) Loading, air exchange rate, and wood product combination effects on chamber formaldehyde concentrations 8.) Chamber Round Robin studies between Georgia-Pacific s chamber and other outside lab chambers 9.) Chamber concentrations and its relationship to actual field measurements. 

The first was the Clayton Study (29) sponsored by H.U.D. in which four mobile home units were constructed with wood products of known formaldehyde emission characteristics as determined in the large scale chamber. The other three studies were from an association and two industrial laboratory chambers working independently of each other. Essentially, all four studies came to the same conclusion -it is possible to predict chamber concentrations from a combination of two formaldehyde emitting products. 

Figure 10. Effect of two dissimilar wood products on chamber formaldehyde concentration.

Actual formaldehyde measurements made while performing field investigations using the CEA 555 Air Monitor were corrected to 25 C. Wood samples removed from the investigation site were returned to the laboratory, and the corresponding quality control test method was used to determine formaldehyde content of the specific wood product. The formaldehyde value obtained from the quality control test method was then used to determine the chamber concentration from the established correlations (Figures 13 14). 

The need for control of formaldehyde emission from UF-bonded wood products has been recognized since Wittmann (4) reported in 1962 that extensive use of particleboard in furniture and building envelopes can cause indoor formaldehyde concentrations exceeding occupational threshold levels. However, it proved to be difficult to define the problem because formaldehyde emission from finished products was not regularly measured, and the correlation between emission rate and the environmental factors were not yet well established. 

Formaldehyde is directly emitted into the air from vehicles. It is released in trace amounts from pressed wood products such as particleboard and plywood paneling, from old sick bnildings, and from cotton and cotton-polyester fabrics with selected crosslink finishes. Formation of formaldehyde has been observed in some frozen gadoid fish due to enzymic decomposition of the additive trimethylamine oxide (Rehbein 1985). Its concentration can build up during frozen storage of fish (Leblanc and Leblanc 1988 Reece 1985). It occurs in the upper atmosphere, cloud, and fog it also forms in photochemical smog processes. 

Solvent eontaining formaldehyde resin paints and laequers have been used extensively in Nordie furniture and wood product industry. In flie early 1980 s, eombined solvent concentration and especially formaldehyde levels often exeeeded the OELs. The OEL violations became rare in the late 1980 s. The recent concentrations of other solvents than ethanol (even its mean eoneentration was only 17 ppm) have been below 10 ppm in Finland. Nowadays, solventless acrylics are mainly used for industrial wood coatings. This substitution has, however, ereated a new occupational health problem. The new products have eaused many eases of dermal sensitization among exposed workers. 

Formaldehyde is the smallest and simplest aldehyde. It can be found in substantial concentrations in new buildings as commonly used wood products age. 

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