Barriers and Sealants for Wood Products
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Volatile emissions from construction materials can be blocked from entering the display chamber by a layer of impermeable foil or composite film (e.g. aluminum/polyethylene), or by a number of coats of a conservation appropriate sealant.
When should I use a barrier film or sealant?
Reducing emission levels inside a case to the lowest possible level is particularly important when exhibit cases are sealed and if the display objects are especially vulnerable to chemical deterioration. Isolating the surfaces of problem materials is a practical and cost effective measure to reduce emissions. Numerous advantages are gained from the application of barrier foils, films and sealants:
- blocking of volatile construction material outgassing from entering into the display chamber;
- physical isolation of collections from the migration of acid from wood products used in construction;
- blocking of water vapor transmission through construction material for cases designed as microclimates;
- extending of the useful life of a pollution absorbent.
What are barrier films and how are they used in exhibit case construction?
Barrier films are thin sheets of metal, and/or plastic, that are impermeable to moisture vapor and most gases. These films were originally designed to meet military specifications for the storage of equipment. Moisture permeability ratings are published, but specifications on gas permeability are more difficult to find in product literature. Barrier films have been used by some museums since 1980 as a way to lower emissions from wood products.1
Most vapor barrier films used in exhibit cases are multi-layered, flexible sheets that include a film that limits water vapor transfer. This film is often aluminum, or in transparent films, polychlorotrifluoroethylene (PCTFE for example in Aclar). This layer is sandwiched between two layers of polyethylene with an outside surface layer that provides specific handling or application properties such as strength, printability or heat-sealability. The outer layer also protects the laminate. Barrier film is relatively easy to work with as it can be heat-sealed to itself allowing case panels to be completely wrapped in the film. Points to consider when using a film barrier, such as Marvelseal 360 and Aclar, include:
- film can be sealed with a hot iron;
- Aclar requires higher seal temperatures than the aluminum foil films;
- seal dull sides together;
- joints in the film should be overlapped by at least 1/4 inch;
- the films can also be heat-sealed directly to wood and other materials;
- acrylic and polyvinyl acetate adhesives can be used to laminate most films to wooden surfaces;
- the film must remain puncture-free to provide an effective seal.
What coating systems can be used as a sealant inside an exhibit case?
Choosing a coating system that forms an impermeable layer to volatile contaminates is difficult, largely due to
- the permeability of most dried paint films and coating, and
- the tendency for films to develop micro-cracks from dimensional changes in wooden substrates.
Over the years museum exhibitors have tried numerous sealants without great success. Polyurethane resin coatings have been recommended in the past; however, the common type of oil-modified polyurethane contains alkyds. Research and testing performed at the Canadian Conservation Institute has shown that moisture borne (emulsion or latex) urethanes are a better choice. Also, two-part resin coatings such as 100% solid epoxy are relatively successful in providing a vapor barrier. Most paint systems, however, do not form effective barriers, even if the paint itself has relatively low emissions, such as acrylic latex paint.
To increase the effectiveness of coatings as vapor barriers, apply material liberally and in several applications and follow the manufacturer ’s directions. As a rule of thumb, coatings should be at least 1 mm. in thickness. This translates into approximately 3 coats applied with a roller or paint brush, or 6-8 coats of spray application. Sealing joints with caulking material prior to painting is recommended.
Products, Manufacturers, and Suppliers
Mention of a product, manufacturer, or supplier by name in this publication is for information only and does not constitute an endorsement of that product or supplier by the National Park Service. Listed materials have been used successfully in past applications. It is suggested that readers also seek alternate product and vendor information to assess the full range of available supplies and
- Marvelseal 360 (aluminum foil)
- Marvelseal 1177 (transparent)
- FoilORap FR 2176 (aluminum foil)
- Rap FR 7750 (transparent)
- FR 3300 (flame resistant and transparent)
- Nomex®Crate Liner (Dupont aramid fiber)
- Sancure 878
- Dexter Brothers Interprotect 1000 (Interlux 1000-1001)
- Aqua-Coat (water-soluble polyurethane)
- Polyglaze 1-146 Spray Grade
- Polyglaze 1-175 Brush Grade
- Interprotect 1000 (Interlux 1000-1001)
Tétreault, J. Low-Cost Plastic/Aluminum Barrier Foil, CCI Notes 1/9, Canadian Conservation Institute. Ottawa (2010).