Using Plywood for Exhibit Case Construction
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The least problematic plywood for use in exhibit cases are softwood plywood with an exterior adhesive system and hardwood plywood which meets the ANSI/HPVA HP-1-1994 standard.
What types of plywood are used to construct exhibit cases?[edit | edit source]
There are four basic types of plywood panels:
- Veneer core: several thin sheets of wood called veneers bound together with an adhesive. The grains of the adjacent veneers are arranged at right angles with the face grain parallel to the panel length. After the adhesive is applied to the core veneers, it is cured in a hot press under high temperature and pressure.
- Particleboard core: particleboard between face and back sheets of wood.
- Lumber core: face and back sheets of wood with a solid lumber core between.
- Medium-density core: face and back sheets of wood with a medium-density fiberboard (MDF) core.
Plywood can be made from either softwood or hardwood panels; both are used in museum exhibits.
- Softwood plywood: generally used for structural applications such as roof sheathing, floor sheathing and single floor systems. Because it must withstand temporary exposure to the elements, softwood plywood is often made with exterior grade phenol formaldehyde adhesive.
- Hardwood plywood: designed for interior applications such as furniture, wall paneling and cabinets. Since the exterior requirements for adhesion do not apply, and because a clearer adhesive is desirable, most interior plywood contain urea formaldehyde. Hardwood panels bonded with phenol formaldehyde to meet HUD (Housing and Urban Development) standards and state regulations can be recommended; however, they are not always available.
How do I select a conservation appropriate plywood?[edit | edit source]
Exterior softwood plywood are considered among the safest plywood because they are bonded with phenol-formaldehyde resin. Softwood plywood is generally made from pine, fir, spruce and often consists of the same species of wood throughout the board. Sitka spruce is the preferred wood due to its lower resin content; Douglas fir may be more readily obtained and is preferable to pine.
- The only softwood plywood considered suitable for exhibit case fabrication are those with exterior adhesive systems of phenol formaldehyde.
The outer veneers of hardwood plywood are often birch, oak, cherry, maple, etc. The veneer core can be made from particle board, MDF, or a hardwood lumber core. Plywood with solid wood core are hard to find and expensive; they usually must be specified from a softwood core and ordered ahead of time. They are marketed for cabinet construction and have therefore been used to construct exhibit cases. Birch is preferred due to its lower acid content. Oak and cedar are not used because of its high acidity.
- A phenol formaldehyde bonded hardwood with a hardwood veneer core should be requested when choosing hardwood plywood.
Approximately 25 years ago hardwood plywood emitted very high levels of formaldehyde. HUD (Housing and Urban Development) standards were created in response to health problems associated with use of hardwood plywood in residential construction. These interior hardwood plywood now have emissions of 0.13 to 0.14 ppm. To assure that you are purchasing this type of hardwood plywood with the lowest emission rate, check for the code ANSI/HPVA HP-1-2016 on the back. Only plywood panels with this code have passed federal and state regulations which allow a volatile emission rate of up to 0.3 ppm.
- Purchase plywood that are stamped as meeting ANSI/HPVA HP-1-2016 standards.
Although such a low-emitting product eliminates most of the formaldehyde, plywood remains a problem for case construction because the wooden components release acid emissions. When possible, use ventilated display case designs to assure a higher rate of air exchange and thus avoid the buildup of unwanted pollutants. If a case is to be tightly sealed, plywood must be isolated with a barrier film, a conservation appropriate sealant, or plastic laminate.
For more information about plywood products contact:[edit | edit source]
Engineered Wood Association (APA)
P.O. Box 11700
Tacoma, WA 98411
The Hardwood Plywood and Veneer Manufacturers Association
P.O. Box 2789
1825 Michael Faraday Drive
Reston, VA 22090
United States Forest Service
Department of Agriculture