Barriers and Sealants for Wood Products

Category:Exhibition
Category: Exhibit Technical Notes
Category: Exhibit Case Design
Category: Exhibit Fabrication
Category: Standards & Guidelines
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Contributors: Members of the MWG's Materials Fact Sheet Working group, Jenifer Bosworth, Anne Ennes, Lisa Goldberg, Jamie Gleason, Jennifer Herrmann, Nancy Lev-Alexander, Patricia Silence, Catherine H. Stephens, and Theresa Voellinger
=== How are barriers and sealants/coating systems used in exhibits? ===
Barrier films and sealants/coating systems are often added to cover construction materials used for cases, plinths, and pedestals. The barrier film, a layer of impermeable foil or a composite film, can block volatile organic compounds (VOCs) from entering a display chamber or gallery. A number of coats of an appropriate sealant/coating may slow VOCs from entering a display case, but so far no coating has been found that acts as an impervious seal to prevent VOC migration. Barrier film options are typically the best preservation choice. The choice of a barrier film or appropriate sealant/coating depends upon many factors, such as object vulnerability, choice of construction materials, and length of display. For more information see: additional resources. Choosing Materials, CAMEO, CCI Tech Bulletin 32
=== Why are barrier films and sealants/coating systems used for preservation? ===
Reducing emission levels inside a case is important when exhibit cases are sealed and if display objects are vulnerable to chemical deterioration. Isolating the surfaces of problem materials is a practical and cost-effective measure to reduce emissions. Advantages gained from the application of barrier films and sealants are: 
* Blocking of volatile construction material off-gassing from entering into a 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 that are designed as microclimates
* Extending the useful life of a pollution absorbent
=== What types of barriers and sealants can be recommended for use inside exhibit cases? ===
While sealants can be easier to apply and may slow transmission, only a properly applied barrier film can block the migration of VOCs from construction materials into an exhibit chamber/case.
==== Barrier Films ====
Thin laminates of polymers and/or metal are suitable for use as barrier films because they are highly impermeable to moisture vapor and most VOCs. Most vapor barrier films used in exhibit cases are multi-layered, flexible sheets that include a film which limits water vapor transfer. These films are often constructed from sandwiched materials, including aluminum, transparent plastics, and polychlorotrifluoroethylene. See the Barrier film comparison and specific manufacturer product literature for permeability data. 
The layers provide specific handling or application properties such as strength, printability, or heat-sealability. The outer layer also protects the laminate. Considerations when using a barrier film:
* 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. Product specifications include specific information about the required temperature needed to create a heat seal.
* Some films are adhesive backed and some must be secured directly to a wood substrate with a separately applied adhesive, such as an acrylic based formulation.
* Joints in the film should be overlapped by at least 1/4 inch.
* The film must remain puncture-free to provide an effective seal.
Polymer films, such as Mylar (a brand name polyester) can also slow transmission and are generally used as a barrier to prevent direct contact rather than as a vapor barrier. These films are also sometimes used to create sealed encapsulation packages to protect individual objects from the surrounding environment.
==== Sealants ====
Most sealants/coating systems are not effective in blocking the migration of volatile organic compounds, especially over long time periods. Failure to block VOCs involves complex issues involving permeability, aging, and dimensional changes. Liquid applications of sealants/coating systems do not form effective barriers, even if the paint itself has relatively low emissions, such as acrylic latex paint.
A wide range of sealants are available. Specific information about sealant/coating products is available on CAMEO, and many of these products have been tested to determine their suitability for use in display environments (see: AIC Material Testing Results). A good summary of sealants/coating systems can be found at: Caulks and Sustainable Use of Coatings in Museums and Archives – Some Critical Observations (Table 5). 
Considerations when applying sealants/coating systems: 
* Follow all manufacturer directions and recommendations for application and health and safety considerations.
* When caulks or taped joints are required, be sure to verify compatibility between materials in determining when sealant/coating systems should be applied.
* Many institutions recommend a four week offgassing period prior to installation, to allow sealant/coating systems to fully cure and off-gas properly.
* Although they do not provide a barrier to volatile emissions from the wood, shellac and other air-drying lacquers are acceptable clear coatings. Some plastics and resins or gasket and caulk products may react with the alcohol in shellac.
=== Preservation strategies ===
* Consider the vulnerability of the collection items (in contact or enclosed in a case).
* Consider the duration of the exhibit; sealants/coating systems can be useful for short-term exhibits, while intact barrier films can provide a longer lasting protection against migration of VOCs.
* Use a metal/polymer laminate as a vapor barrier, making sure there are no punctures and the substrate is completely sealed.
* Choose only an inert and impermeable material such as polyester film to serve as a barrier film to prevent direct contact.
* Consult Choosing Materials, or Sheet/Film, Composite if choosing a barrier film.
* If using a sealant/coating system, be sure to build a 4 week drying/curing time into the production schedule to allow the sealant/coating system to offgas before installation of objects.
* Consult Choosing Materials, Coatings,  and CCI Tech Bulletin 32 to help determine which sealant/coating system would be best suited for the specific application.
=== Additional Resources ===
* Choosing materials for storage, exhibition, & transport
* Products used in Preventive Conservation - Technical Bulletin 32 by Jean Tétreault
* Conservation Case Building | Field Museum
* Sustainable Use of Coatings in Museums and Archives – Some Critical Observations by Jean Tétreault, e-preservation science, 2011, 39-48 (see table 5)
* Low-Cost Plastic/Aluminum Barrier Foil, CCI Notes 1/9, Canadian Conservation Institute by Jean Tétreault. Ottawa (2010).
Archived Barriers and Sealants for Wood Products page