Hardwood plywood formaldehyde

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. 

For exterior appHcations, where water exposure is expected, phenol—formaldehyde (PF) or phenol—resorcinol—formaldehyde (PRF) adhesives are used. Only small quantities of this type of hardwood plywood are made, primarily for marine use. 

One type of thick hardwood plywood stiU available is imported from the northern Scandinavian countries and is generally known as Finnish birch. Characteristically, these plywoods are manufactured using multiple layers of veneer of the same thickness, about 1.5 mm (1 /16 in.), and bonded with a urea—formaldehyde or melamine—urea—formaldehyde adhesive. 

Approximately one million metric tons of urea-formaldehyde resin are produced annually all over the world. More than 70% of this urea-formaldehyde resin is consumed by the forest products industry. The resin is used in the production of an adhesive for bonding particleboard (61% of the urea-formaldehyde used in the industry), medium-density fiberboard (27%), hardwood plywood (5%), and as a laminating adhesive (7%) for bonding furniture case goods, overlays to panels, and interior flush doors, for example. 

In the April, 1975, issue of Plywood Panel magazine, we find reported square feet, quarter inch basis, domestic hardwood plywood manufactured in this country was approximately 3.5 billion and softwood plywood was 27.5 billion. This means 89 percent of the total plywood produced was softwood plywood, and 97 percent of this production was glued with phenol-formaldehyde resin adhesives, according to the American Plywood Association. This leaves 11 percent domestic hardwood plywood which was 95 percent glued with urea-formaldehyde resin adhesives,... 

In 1973, over 500 million solid pounds of phenol-formaldehyde resin were used in the manufacture of softwood plywood in the U. S. A. About 60 million solid pounds of urea-melamine-formaldehyde resin were consumed in the hardwood plywood industry. 

Some report that over 50 percent of the urea-formaldehyde resins consumed went into particleboard. This is brought out because there may be a shift away from urea resin for certain types of oriented particleboard used in structural plywood constructions. Historically, particleboard has been used for inner plies as previously mentioned in some hardwood plywood. There is now one plant in production in Idaho which produces mechanically oriented strand particleboard for use specifically as core for softwood plywood production. It is anticipated that this trend to some degree will increase in the future, and phenolic resins appear to be the mechanism with which this particleboard will be bonded. 

In hardwood plywood, the glue mix is typically formed by adding water and soft wheat flour at rather high proportions with a minor addition of filler, such as nut shell flour to the urea-formaldehyde resin. Urea-formaldehyde resins are usually shipped in 60 - 65 percent non-volatile solids form. Acid salts such as ammonium sulfate are added to increase the rate of cure of the urea-formaldehyde resin when under pressure and subsequently heated in hot presses. Some acid salts cure the urea-formaldehyde sufficiently well under ambient "cold" press conditions. It is evident this type of plywood would be oriented toward interior use such as furniture. If water-proof type bonds are required of hardwood plywood, then melamine-formaldehyde resins are used in similar mix form but with higher resin solids... 

Well over 95 percent of the hardwood plywood production in the United States uses urea-formaldehyde as the adhesive bonding agent between veneers for reasons of quality, intended use and economics. In this industry, formaldehyde evolution is a subject of concern because normally all of the adhesive formaldehyde released under elevated temperatures is not tied up in the set glue line. This is an area that is of real concern to the particleboard industry which uses urea-formaldehyde as its binder. 

The types of adhesives suitable for laminating beams are restricted by the conditions of application and by their end-use requirements. A wider choice of adhesives for plywood depends on whether softwoods or hardwoods are used, whether they are required for internal or external exposures, or whether they are to be used for ornamental or structural purposes. Thus phenol-formaldehyde types would be used for marine or exterior construction uses urea-formaldehyde types would be advantageous for cold pressing, or melamine-urea adhesives might be preferred for hardwood plywood, or lumber-core panels used in furniture production. 

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,... 

Sma11 Sea Ie Test Method for Determ i n i ng Forma Idehyde Em i ss i on from Wood Products, Two Hour Dessicator Test, FTM-1," National Par id eboard Association, Hardwood Plywood Manufacturers Association, Formaldehyde Institute and U.S. Department of Housing and Urban Development, Federal Register, 1982, 48, 37169. 

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

Hardwood plywood products are decorative in nature and are designed for interior use. Over 95X of all hardwood plywood is made with urea-formaldehyde adhesives. Responding to concerns about formaldehyde and certain wood products, test methods for measuring surface emissions were developed in the early 1980 s. Emissions from most hardwood plywood and particleboard products have decreased 65% to 95% in recent years primarily by use of low emitting UF adhesives and/or scavengers. Good correlation has been demonstrated between product test methods and indoor levels of formaldehyde in experimental manufactured homes. Decorative surface finishes can act to either increase or decrease surface emissions, depending on the nature of the finish and the substrate. 

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 ... 

Because face veneers are decorative and thin, a colorless glue line is desired to prevent discoloration on the face. Urea-formaldehyde adhesives are predominate in the manufacture of hardwood plywood. Well over 95% of all hardwood plywood consumed in the U.S. is made from UF adhesives. 

Both particleboard and MDF core are characteristically 3-ply and have two potential sources of formaldehyde the adhesive used to adhere the hardwood face and back to the core, and the adhesive binder used in the manufacture of the particleboard or MDF. Hardwood plywood manufacturers are typically not vertically integrated and do not produce composition board cores, thus are dependent on other companies or plants for particleboard and MDF. 

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. 

Figure 1. Potential sources of formaldehyde in two typical hardwood plywood constructions.

Technical considerations, resin cost, and resin availability have and are dictating low emitting UF systems as the primary substitute for standard UF adhesives. Relatively small quantities of hardwood plywood have been made with polyvinyl acetate and phenol formaldehyde, the two most likely substitutes. Cost is a primary disadvantage of PVA types and certain use parameters and the dark color of phenol limit that adhesive to certain hardwood plywood factories and for certain applications. 

Reduction in the emission characteristics of unfinished hardwood plywood is currently being achieved primarily by the use of low formaldehyde to urea molar ratio formulations. For the manufacture of hardwood plywood and particleboard, formaldehyde to urea molar ratios have been reduced to a range of 1.15/1 to 1.3/1. An important caveat low F/U ratios perhaps should be considered a proxy for the potential to reduce emissions through improved urea-formaldehyde adhesive technology rather than the exclusive means for improvement. Reducing the F/U ratio is not always the most effective way of reducing emissions in consideration of the variety of hardwood plywood constructions, products, and thicknesses. ... 

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. 

Particleboard decking and hardwood plywood wall panels can represent 80% to 90% of the total exposed surface of formaldehyde containing wood based products in new mobile homes. Kitchen cabinets, vanities, shelving and other built-ins are primarily made from industrial particleboard, MDF or hardwood plywood panels. 

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. 

Hardwood Plywood Manufacturers Association, Comments before the Environmental Protection Agency, regulatory investigation of formaldehyde exposures determined to be within section 4(f) of the toxic substances control act, 1984. 

National Particleboard Association, Hardwood Plywood Manufacturers Association, Formaldehyde Institute. October 10,... 

National Particleboard Association, Hardwood Plywood Manufacturers Association, October 10, 1983. Large scale test method for determining formaldehyde emissions from wood products, large chamber method, FTM 2, Reston, VA. 

Myers, G. E., Formaldehyde dynamic air contamination by hardwood plywood effects of several variables and board treatments. Forest Products Journal, 1982 32(4) 20-25. 

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. 

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. 

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