Caulk Sealants to Seal Exhibit Cases

Category:Exhibition
Category: Exhibit Technical Notes
Category: Exhibit Case Design
Category: Exhibit Fabrication
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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
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=== How are caulks used in exhibits? ===
Caulk sealants are viscous elastomers that harden either by exposure to air or by chemical reaction. These products are rubbery polymers when cured, consisting of silicone, acrylic latex, polyurethane, polyvinyl chloride, and polyvinyl acetate. Caulk sealants are available in a variety of colored and transparent forms; careful selection and application can make the caulk nearly invisible.
Caulk is used to: 
* Seal construction seams or fill unwanted gaps and holes 
* Improve the seal between joints in exhibit cases in order to inhibit dust and insect entry, reduce air exchange, and assist in climate control
* Seal joints in the display chamber to reduce offgassing from problematic materials, such as adhesives, wood products, etc, used outside the display chamber, etc
* Seal around electrical wiring or other utility penetrations into the case
Caulks are easy to apply and are used in hard-to-access and difficult-to-seal areas, such as between fixed glazing panels, around wall/floor/ceiling joints, at screw and bolt holes, and for gaps around door openings and moveable panels. Caulks form durable seals; e.g., fixed glazed panels, corner seams.  Their elastic quality creates a tight seal that is retained over time. Final caulking is applied after the exhibit cabinetry is in location and when disassembly and transport of the case is unlikely.
=== Why are caulks a preservation concern? ===
Caulks based on silicone and acrylic latex cure through either an acidic or a neutral chemical reaction. Both are problematic when fresh; neutral cure caulks are more inert after cure. 
* Acidic curing: Emissions of acetic acid may continue indefinitely; exposure to acetic curing caulks is dangerous to collections. Avoid acid curing caulks.
Acetic acid is often used as a tube-release agent; always ask the manufacturer if any acetic acid is present in the caulk product.
* Neutral curing: Even neutral cure sealants may release volatiles of concern.
Check the Technical Data Sheet to confirm that the product is neutral cure. Check the Safety Data Sheet for chemicals that may be problematic.
Sealants based on polyurethane, polyvinyl chloride, polyvinyl acetate, rubber, butyl, kraton, or oil are not acceptable because of high initial emissions of VOCs during cure and continued out-gassing as they degrade. Commercial caulks can also contain a variety of fillers and additives so must be selected carefully to assure that they are safe to be used around collections.
=== What types of caulks can be recommended for use inside exhibit cases? ===
Choose caulk sealants that are composed of stable materials and have a low volatile organic compound (VOC) emission level; neutral-curing silicone or acrylic based caulks are recommended. See the Materials Testing Results wiki pages for products that have been tested. 
After selecting a number of potential caulks that are neutral cure silicone or acrylic latex products, choice of the right caulk depends on several practical factors:
* Usable lifetime (acrylic latex and silicone-based sealants have a shelf-life of three to five times longer than polyvinyl acetate and oil based sealants)
* Durability
* Flexibility
* Extrusion profile
* Adhesion characteristics; (suitability for use with wood, glass or metal is identified on the label)
* Appearance, including color match
* Ease of clean up (water-based are less toxic)
Manufacturers can supply test data relevant to the American Society for Testing and Materials ASTM standards. To ensure an effective case seal, choose a caulk that meets ASTM C834 requirements for flexibility, extrusion and adhesion. ASTM C920 addresses durability and weathering characteristics.
=== Preservation Strategies ===
Take care to examine and monitor a seam to see if there has been shrinkage or some change in the seal quality.
Allowing proper cure time prior to installation of the object will minimize volatiles interacting with objects. Full cure from a preservation standpoint may be longer than manufacturer specifications. Emission levels drop off dramatically after the cure time recommended by the manufacturer; a safety margin of more time further decreases levels of harmful VOCs in the case. Be especially careful if caulking is applied after installation or to seal wiring once the cases are in place.
=== Additional Resources ===
Silicone cure systems
CCI Bulletin 32: Products used in Preventive Conservation
Products Used in Preventive Conservation - Technical Bulletin 32 by Jean Tétreault
Archived Using Caulk Sealant to Seal Exhibit Cases page