Pressed Wood Products

Pressed wood products originated in Europe in the 1960s as an alternative to wood furnishings, and entered the U.S. market in the 1970s. Pressed wood products (Fig. 20-2) include particleboard, medium-density fiberboard (MDF), hardwood plywood, chipboard, and hardboard such as pegboard. These materials emit VOCs including formaldehyde, a-pinene, xylenes, butanol, butyl acetate, hexanal, and acetone.

Chemicals that emit VOCs are used in pressed wood products to provide strength and moisture resistance. Phenol-formaldehyde (PF) resins resist moisture degradation, and are used in products destined for exterior

Particle board

Oriented strand board

Figure 20-2 Plywood, particle board, and oriented strand board (OSB).

Plywood:

High-density overlay (HDO) plywood is exterior plywood with resin-fiber overlay on both sides.

Medium-density overlay (MDO) plywood has phenolic or melamine resin overlay on one or both sides.

Particle board

Oriented strand board

Figure 20-2 Plywood, particle board, and oriented strand board (OSB).

applications, as well as interior plywood and as bonding for laminates on wood and steel surfaces. Urea-formaldehyde (UF) resins are less expensive, but can only be used for interior applications. Urea-formaldehyde resins offgas 10 to 20 times as much as PF resins. They are present in particleboard and in MDF, which has the highest VOC content of the pressed wood products.

Pressed wood products are used extensively in residential and commercial interiors projects. Worksurfaces in offices account for 15 to 35 percent of the floor space. Shelving adds another 10 to 20 percent, is usually located near workers' faces, and is exposed to air on both upper and lower sides. In mobile homes, where pressed wood products cover virtually every surface within a confined space, formaldehyde is concentrated and poses an increased threat to the health of occupants. Newly constructed and furnished buildings present a greater threat than older buildings, where the VOCs have had time to dissipate. High temperatures and humidity increase the decomposition of VOCs, releasing more formaldehyde during summer months.

Particle board, also called industrial board, is made of chips and shavings of soft woods such as pine held together with UF resins and glues, which constitute 6 to 10 percent of the product's weight. Medium-density fiberboard (MDF) combines wood pieces and chips with UF adhesives and other chemicals comprising 8 to 14 percent of its weight. These are pressed together in a hot hydraulic press. Medium-density fiberboard is used for drawer fronts, cabinet doors, and furniture tops.

Hardwood plywood consists of thin sheets and veneers of hardwoods like oak and maple, held together by PF resins and glues that make up 2.5 percent of its weight. Hardwood plywoods are used for cabinets and furniture.

Chipboard is made of untreated wood fiber and paper by-products pressed together with small amounts of formaldehyde resins. Chipboard is used for the innermost layer of many modular office partitions. Hard-board is used for pegboard and other inexpensive functions. Wood fibers are pressed into a dense sheet while applying heat to allow the natural resins to hold the sheet together without glue. Relatively small amounts of formaldehyde resins are then added along with other chemicals to improve strength and moisture resistance.

Other pressed wood products, such as softwood plywood and flake strand board or OSB, are produced for exterior construction use and contain the dark, or red/ black-colored PF resin. Although formaldehyde is present in both types of resins, pressed woods that contain PF resin generally emit formaldehyde at considerably lower rates than those containing UF resin. Where you are using extensive amounts of pressed wood products in an interior, investigate whether PF resin products are an option.

Since 1985, HUD has permitted only the use of plywood and particleboard that conform to specified formaldehyde emission limits in the construction of prefabricated and mobile homes. In the past, some of these homes had elevated levels of formaldehyde because of the large amount of high-emitting pressed wood products used in their construction and because of their relatively small interior space. We should note here that some natural wood products can also emit VOCs.

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