Formaldehyde emission panels

The primary adhesive used ia hardwood plywood is urea—formaldehyde (UF) mixed with wheat flour as an extender to improve spreadabiUty, reduce penetration, and provide dry-out resistance. A catalyst may also be added to UF resias to speed the cure or to cause the UF to cure. Scavengers also may be added to reduce formaldehyde emissions from finished panels. If more water-resistance is requited using a UF bond, small amounts of melamine maybe added, producing a melamine—urea—formaldehyde (MUF) adhesive. 

Formaldehyde emission from finished panels is due to the residual formaldehyde present as a gas in the wood and recesses of UF-bonded and MUF-bonded boards as well as dissolved in the boards residual moisture content. On the other... 

Actual regulations cone, the subsequent formaldehyde emission from wood-based panels (Germany) according to the German Regulation of Prohibition of Chemicals (former Regulation of Hazardous Substances)... 

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 ... 

This problem, namely the formaldehyde emission of the wood panel in service, after its manufacture, can be fortunately regarded to day as more or less solved, due to unequivocal and stringent regulations in many European countries, and successful long-term joint R D in the adhesives and wood-working industries. 

Only a small amount of work has been done up to now concerning the prediction of bond strengths and other properties based on the results of the analysis of the resin. Ferg et al. [59] worked out correlation equations evaluating the chemical structures in various UF-resins with different F/U molar ratios and different types of preparation on the one hand and the achievable internal bond as well as the subsequent formaldehyde emission on the other hand. These equations are valid only for well defined series of resins. The basic aim of such experiments is the prediction of the properties of the wood-based panels based on the composition and the properties of the resins used. For this purpose various structural components are determined by means of - C NMR and their ratios related to board results. Various papers in the chemical literature describe examples of such correlations, in particular for UF, MF, MUF and PF resins [59-62]. For example one type of equation correlating the dry internal bond (IB) strength (tensile strength perpendicular to the plane of the panel) of a particleboard bonded with PF adhesive resins is as follows [17]... 

European Norm EN 717-2, Wood-based panels. Determination of formaldehyde emission. 

It is important to realize that a thermosetting polymer reacted to high (but not full) conversion contains a small fraction of free monomers if the monomers are volatile, their emissions may produce forbidden contamination levels, particularly for indoor applications. The decline in the use of urea-formaldehyde resins in agglomerated wood panels resulted from contamination problems associated with formaldehyde emission. 

Standard practice for the determination of VOCs (excluding formaldehyde) emissions from wood-based panels using small environmental chambers under defined test conditions... 

Brown (1999b) reported formaldehyde and VOC emissions from new, unfinished particleboard and MDF (both using urea formaldehyde resins) in Ausbalia. Formaldehyde emissions over the first three weeks exhibited first-order decay behavior that predicted little to no formaldehyde emission after 6 months. However, further emission measurements at 8 months showed the products sbll emitted formaldehyde at approximately one-half the new product rate (also further unpublished measurement at 2 years showed the same emission rate as at 8 months). It was concluded that the wood-based panels emitted formaldehyde by a double-exponen-ttal model, the early- to late-term emissions including the free formaldehyde in the products but the long-term emissions consisbng of only the formaldehyde... 

Over the last 20 years, environmental concerns have become an important consideration in adhesive formulation and use. Firstly, in the plant the adhesive may require particular handling or the use of protective equipment. Secondly, volatile emissions arising from adhesive reactions both in the hot press and subsequently when the panels are in service are subject to tight regulatory control. In particular the reduction formaldehyde emissions from wood-based panels has been a major objective in adhesive development over the last few years, both because formaldehyde-based adhesives are the major type used and because these have been implicated in environmental and health concerns. 

The earliest wood adhesive for mass produetion of panels was phenol formaldehyde (PF). This is widely used for products designed to perform in severe weathering eonditions and dominates the exterior plywood market. Resorcinol formaldehyde is a somewhat similar resin system that cures at ambient temperatures, but the cost is much higher, limiting its use in high-volume applications. These systems, although formaldehyde-based, do not have the continuing formaldehyde emission problem that is associated with other formaldehyde-based systems. 

This has been made possible by the development of suitable adhesive systems that are able to bond the partieles together. The synthetie adhesives offer a consistency of performance that is diffieult to aehieve with natural produets sueh as tannins, and at a cost that has enabled rapid growth to be aehieved. At the same time the adhesive systems have shown a toleranee to a range of wood properties that has enabled most wood residues sources to be used. The environmental effects of formaldehyde emissions from panels made using UF adhesives have been successfully addressed. Initially these required higher resin addition rates to offset a loss in physieal property levels, but further development has reclaimed much of this additional cost. 

In China, formaldehyde emission of the pressed-wood products sold is generally high. A test showed that the average formaldehyde release rates of four kinds of typical wood-based panels, i.e., particle board, medium-density... 

Limit of formaldehyde emission of wood-based panels and finishing products... 

Particleboard and other products made with isocyanates emit only little formaldehyde (IJ, but these adhesives are expensive and require expensive manufacturing procedures. In contrast, phenolic soft wood plywood is a well established product that is predominantly used for exter i or appIi cat i ons. It conta i ns forma Idehyde i n chemically strongly bonded form and also emits little formaldehyde, as shown in a later chapter in this book. In fact, under almost all common use conditions this type of board contributes not much more formaldehyde than is already present in ambient air in many urban areas. The same is true for waferboard, which has recently become popular for replacing plywood. Likewise, phenolic particleboard emits little forma Idehyde, unIess the phenoIi c res in is bIended w i th UFR. Normally, the products with highest potential for formaldehyde emission are those bonded with UFR. During the past year, approximately 300,000 metric tons of UFR have been used for panel manufacturing in the U.S. 

When particleboard was first introduced, the risk of consumer exposure to formaldehyde emission was comparatively small as long as only moderate quantities of products were used in consumer applications. This situation changed when particleboard became popular and when its production reached millions of tons per year. This popularity caused different types of formaIdehydic products, such a wall panelling, flooring, tables, cabinet work and furniture to accumulate in homes and offices, yielding load ratios of I m of product surface area per 1 m indoor air space. 

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. 

Formaldehyde Emissions Hardwood Plywood and Certain Wood-Based Panel Products... 

Lines of demarcation between hardwood plywood, softwood plywood and certain other wood based panel products have become less distinct in recent years. One of the most important distinctions in respect to formaldehyde emission potential is that softwood plywood is typically bonded with phenol-formaldehyde while hardwood plywood is typically bonded with urea-formaldehyde. Phenol-formaldehyde adhesives are more stable and have less tendency to emit formaldehyde than do urea-formaldehyde adhesives. Some important features of hardwood plywood ... 

Surface finishes can be an important factor in either increasing or decreasing emissions. This became apparent as formaldehyde emissions decreased as a result of changes in UF adhesives. Wall panel products can be segmented by the type of decorative surface finish in order of commercial importance. 

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. 

While the initial concern for formaldehyde emissions in the hardwood plywood industry was in the wall paneling sector there is a strong and decided movement by many manufacturers to apply new low emitting adhesive technology to other hardwood plywood products. Low emitting UF products are nearing the emission characteristics of certain other resin systems assumed to be likely substitutes. 

It would appear that the wall paneling industry, on average, has probably been able to achieve a 70% to 95% reduction in formaldehyde emissions and still maintain the use of urea-formaldehyde adhesives. 

G. Gramp, W. Groah. "Evaluation of the relationship between formaldehyde emission from particleboard mobile home decking and hardwood plywood wall paneling as determined by product test methods and formaldehyde levels in experimental mobile homes." U.S. Dept. of Housing and Urban Development, 1982. 

Because phenolic panels have not presented formaldehyde-related problems in the marketplace, there has not been much need to develop information on formaldehyde emissions from these products. 

Any formaldehyde that might be present initially in fresh phenolic panels, would be expected to diminish through time, since additional formaldehyde is not released from a breakdown of the resin. Thus, barring contamination from other sources, formaldehyde emissions associated with thoroughly aged phenolic panels should be nil. 

Nestler ( ) thoroughly reviewed the worldwide literature on formaldehyde emissions from wood products published through January, 1977. According to Blomquist ( .)> Nestler s literature review includes only three citations which even mention phenolic adhesives, and none of these citations made specific mention of any problems associated with the use of phenolic panels. 

Since Nestler s review was published, some additional information on formaldehyde emissions from phenolic panels has appeared in the literature. Information obtained using dynamic test chambers is summarized in Table I. Perforator and two-hour desiccator data are summarized in Table II. 

Much of the information pertaining to formaldehyde emissions from phenolic panels has been obtained by manufacturers of these products but has not been published previously in the open literature. This information has been obtained primarily to form a basis for answering consumer inquiries. 

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