Panel products, formaldehyde

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 ... 

Developments in glued laminated structures and panel products such as plywood and chipboard raises the question of the durability of adhesives as well as wood. Urea-formaldehyde adhesives are most commonly used for indoor components. For exterior use, resorcinol adhesives are used for assembly work, whilst phenolic, tannin and melamine/urea adhesives are used for manufactured wood products. Urea and casein adhesives can give good outdoor service if protected with well-maintained surface finishes. Assembly failures of adhesives caused by exudates from some timber species can be avoided by freshly sanding the surfaces before glue application. 

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at 8 months showed the products sbll emitted formaldehyde at approximately one-half the new product rate (also further unpublished measurement at 2 years showed the same emission rate as at 8 months).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... 

It is worthwhile to review the U.S. market size for the four principal resins currently used in wood-panel products today (4 )- These are phenol-formaldehyde (PF), urea-formaldehyde (UF), melamine-formaldehyde (MF), and resorcinol-formaldehyde (RF) (Table III). When these production figures are compared to the quantities of lignin potentially available (Table II), it is immediately obvious that all wood adhesives could be replaced by only a very small fraction of the lignin produced annually during chemical woodpulping processes. 

Wood-based panel products are usually bonded with synthetic adhesives based on condensates of phenol, resorcinol, urea, or melamine with formaldehyde. Particleboards and fiberboards can also be bonded with mineral binders like cement or gypsum. Wood adhesives derived from natural products have more... 

The most widely used wood panel products are particleboard, softwood plywood, hardwood plywood, medium density fiberboard (MDF) and waferboard. The most common adhesive is urea-formaldehyde resin (UFR). Phenol-formaldehyde resins (PFR) are second in volume and melamine-formaldehyde resins (MFR) are a distant third. Recently,... 

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 ... 

The single largest end use for hardwood plywood is interior wall panels, generally 3-ply and 1/4 and thinner, and frequently machined with decorative v-grooves. Furniture, cabinets, door skins and a number of specialties complete an array of end use products. Many of the non wall panel products can be characterized as being industrial panels and are of 5 or more ply veneer core, 3-ply particleboard core, or 3-ply medium density fiberboard (MDF) core construction. Broad end use patterns indicate that interior wall panels represent approximately 55% of total hardwood plywood consumption. Furniture, cabinets, and fixtures represent about 30%, and door skins and specialty products about 15% (2), Potential sources of formaldehyde in two of the more typical hardwood plywood constructions are displayed in Figure 1. 

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. 

The domestic hardwood plywood industry has been trending towards the use of water based topcoats for some paper overlay, printed, and natural hardwood paneling products to reduce volatile organic compound emissions. To achieve desired surface product properties formaldehyde is often a component of the topcoat. 

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. 

Formaldehyde Release from Wood Panel Products Bonded with Phenol Formaldehyde Adhesives... 

Both the published literature and previously unpublished information obtained by the structural panel industry indicate that formaldehyde levels associated with panel products glued with phenol formaldehyde adhesives are extremely low. Large dynamic chamber tests which simulate conditions that might be found in tightly sealed residences indicate consistently that formaldehyde levels associated with freshly manufactured phenolic panel products are less than 0.1 parts per million. The data, as well as theoretical considerations, also indicate that the amount of formaldehyde contributed to the environment by phenolic panel products should rapidly approach zero as the small quantity of formaldehyde initially present in the products is released. 

Virtually all wood panel products such as plywood and particleboard are manufactured using either urea formaldehyde or phenol formaldehyde adhesives. Urea formaldehyde adhesives are used in hardwood plywood and in certain types of particleboards. These adhesives are not waterproof, and products made with them are normally used indoors for paneling, furniture, shelving and floor underlayment. 

Phenol formaldehyde, on the the other hand, is used to make the waterproof adhesives that are used in structural wood panel products such as softwood plywood, oriented strand board, waferboard and exterior (phenolic) particleboard. These products are commonly used for roof, floor and wall sheathings, exterior sidings, concrete forms and in pallets and numerous other products. 

A third reason for predicting very low emissions of formaldehyde from phenolic panels is that the cured resin is extremely stable and does not break down to release additional formaldehyde, even under extremely harsh environmental conditions ( ). The high resistance of phenolic resins to deterioration under severe service conditions is, of course, a principal reason they are used so widely in making exterior types of wood panel products. Because of their chemical stability the U.S. Environmental Protection Agency has declared that phenol formaldehyde resins represent a consumptive use of formaldehyde, meaning that formaldehyde is irreversibly consumed in its reaction with phenol so that the formaldehyde loses its chemical identity (3). 

The formaldehyde emitting potential of wood panel products can be evaluated in numerous ways, including the use of dynamic chamber tests (tests involving chambers which are ventilated and simulate real-world conditions) static (unventilated) tests, such as... 

The higher formaldehyde levels associated with the first two studies summarized in Table I can probably be attributed primarily to the relatively high temperatures employed. Numerous investigations have shown that formaldehyde levels increase exponentially with temperature ( 2 Several studies have shown that formaldehyde levels associated with wood panel products can increase by more than a factor of 3 as the temperature increases from 25°C to 40 C ( ). 

EMERY Wood Panel Products Bonded w ith Phenol Formaldehyde Adhesives 31... 

The two-hour desiccator and Perforator test results shown in Table II are also indicative of very low formaldehyde levels for phenolic panels. As with most of the results obtained in dynamic chamber tests, the uniformity of these test results, both within and between studies, indicates that the various phenolic panel products are quite similar with respect to their emitting potential. 

Table III. Summary of Formaldehyde Test Data for Various Phenolic-Bonded Panel Products...

The first three chapters deal with particleboard, medium density fiberboard, hardwood plywood, and softwood plywood, the four most widely used wood panel products. Chapter four compares these products with other consumer products. Chapters five through seven explain the basic chemistry of formaldehyde with cellulose and wood components and provide a current understanding of the nature of liquid urea-formaldehyde adhesive resins. The next two chapters present new analytical methods that might become useful in the future. Chapters eight and eleven through sixteen explain the complex nature of the latent formaldehyde present in the products and its correlation to formaldehyde emission from wood products. Chapters fifteen and sixteen describe currently popular formaldehyde reduction methods. The last two chapters discuss the problems involved in reducing formaldehyde emission by regulating air levels or source emissions. 

Exactly the same type of approach can be used for the formulation of other polycondensation adhesives. For example, the formulation of melamine urea formaldehyde adhesive resins for wood panel products can also be successfully approached in the same way as has been shown above for the cold-setting PRF adhesives. 

The so-called El-emission class regulations shown in Table 4 for different panel products describe the level of formaldehyde emission which is low enough to prevent... 

The most common resin for lignocellulosic composites is urea formaldehyde. About 90% of all lignocellulosic composite panel products are bonded with UF [12]. UF is inexpensive, reacts quickly when the composite is hot-pressed, and is easy to use. UF is water-resistant, but not waterproof. As such, its use is limited to interior applications unless special treatments or coatings are applied. UF resins are typically used in the manufacture of products where dimensional uniformity and surface smoothness are of primary concern, for example, particleboard and medium density fibreboard (MDF). Products manufactured with UF resins are designed for interior applications. They can be formulated to cure anywhere from room temperature to 150 °C press times and temperatures can be moderated accordingly. UF resins (often referred to as urea resins) are more economical than PF resins and are the most widely used adhesive for composite wood products. The inherently light colour of UF resins make them quite suitable for the manufacture of decorative products. 

Then investigations were carried out to examine the cure time requirements of various copolymer resins and to determine whether the amount of phenol replacement could or should be increased. The copolymer resins synthesized from extracts, phenol and formaldehyde, were made with 40% by weight (vs. 20% in previous experiments) of the phenol replaced with extracts. The resin synthesis process as well as the veneer types, method of panel production and glueline evaluation were the same as outlined in preceding paragraphs. However, press times of 2, 3 and 4 minutes were used. 

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