Hazardous Emissions from Wood Products
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Acidity and outgassing of formaldehyde make wood a problematic material from a conservation viewpoint.
Why is wood, used in exhibit construction, a conservation concern?[edit | edit source]
Acidity, naturally occurring in wood, is the primary conservation concern when using solid wood products. This acidity can migrate into collection objects that come in contact with the wood, and can release emissions thus creating an acidic environment. Studies have shown that hardwoods are higher in acidity; however; soft woods are more permeable, and therefore release volatile substances more readily.
- Tropical hardwoods are relatively impermeable and do not emit large quantities of acetic acid, makingthem good choices for museum applications.
Maximum evolution of organic acids occurs when wood is rendered into sawdust. Knot holes, end grain and open-grained woods are also sites of accelerated emissions.
- Protect collection objects from redeposition of acidic deposits by containing dust generated during exhibit production.
- Exclude knots, seal end grain and avoid exposed wood joints inside an exhibit case. (Paraffin wax applied thickly is an effect end grain sealant)
Why are wood-bonded boards of concern?[edit | edit source]
Both acidity from the wood and chemical outgassing from adhesive used to produce wood boards pose dangers to collections. Formaldehyde adhesive systems are the major source of formaldehyde emissions from bonded boards, although some formaldehyde may result from thermal processing of the wood. Within sealed enclosures, concentrated levels of volatile formaldehyde and acid initiate or accelerate the deterioration of some artifacts.
Outgassing formaldehyde has been shown to:
- crosslink proteins and cellulose;
- cause color change in some pigments;
- corrode metals;
- cause crystal formation on glass.
Two types of formaldehyde-based adhesive systems, urea and phenol formaldehyde, are used to manufacture plywoods, particle boards, and medium-density fiberboard (MDF).
- Urea formaldehyde: This system outgases both volatile formaldehyde and residual ammonia used during the manufacturing process as a scavenger of formaldehyde. Most interior hardwood plywoods are made with urea formaldehyde. Wood composite boards— particleboard and medium-density fiberboard (MDF)—are made with the urea formaldehyde adhesive systems and are acidic (typical pH is below 7).
- Phenol formaldehyde: Although unreacted or free formaldehyde is always present in small amounts, once cured, phenol formaldehyde is stable. Plywood manufactured with phenol formaldehyde has extremely low formaldehyde emissions. Industry tests typically record emissions of 0.1 ppm from fresh panels; these emissions decrease over time (four to six months after manufacture) to 0.03 to 0.04 ppm.
What are the alternatives?[edit | edit source]
For obvious practical and financial reasons plywood and composite panels remain primary construction materials. Whenever wood products are used for exhibit case construction, use a solid wood board, plywood or composite panel that has minimal levels of acid and outgassing. Other materials that can be substituted for wood in the display chamber of an exhibit case are listed below. Due to cost restrictions, several of these options are usually reserved for critical situations.
- aluminum, steel or sheet metal;
- commercially available metal and paper honeycomb panels; some have similar strength and rigidity to wood products, and are machined in the same way. Visible areas are covered with a fabric lining, attached mechanically to avoid the introduction of volatile adhesive emissions;
- dry wall board; due to its porosity, sheet rock cannot be used in climate control cases.