Archived Using Caulk Sealant to Seal Exhibit Cases page

n.b. - This is an outdated version of the "Using Caulk sealant to seal exhibit cases" page. For the most up-to-date version of this information, please go here. (posted January 5, 2023)
Neutral curing silicone or acrylic caulk can be used to seal gaps along joints in display cases.
=== What are caulk sealants and how are they used in exhibit case construction? ===
Caulk sealants are viscous fluid elastomers that harden either by exposure to air or by chemical reaction. These products are based on rubbery polymers such as silicone, acrylic latex, polyurethane, polyvinyl chloride, and polyvinyl acetate. Commercial caulks also contain a variety of fillers and additives. Caulk sealants come in a variety of colored and transparent forms; careful selection can make the caulk nearly invisible.
Caulk is used to form a seal along construction seams or to fill unwanted gaps and holes. Caulk sealants:
·      Improve the seal between joints in exhibit cases in order to inhibit dust and insect entry, reduce air exchange, and to assist in climate control
·      Seal joints in the display chamber to reduce outgassing from problematic materials, such as exterior adhesives, end/edge grain of plywood, etc
Due to the ease of their application, caulks 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 at gaps around door openings and moveable panels. The elastic quality of caulking materials allows for a tight seal to be retained over time even when slight shifting of the exhibit case elements occurs.
Unlike gaskets, which are removable, caulk sealants form permanent seals; e.g., fixed glazed panels, corner seams. (In rare applications caulk has been used with saran type sheeting to create removable seams.) Caulk sealants should be used after the exhibit cabinetry is in location and when disassembly and transport of the case is unlikely. Applications include:
·      To further seal joints in stationary elements of the case
·      When gaskets are not sufficient to reduce air exchange within a case
·      When a tight surface-to-surface seal is required
·      To conceal or level a surface
=== What types of commercially available caulk sealants are appropriate for exhibit use? ===
It is important that caulk sealants selected for use in exhibit cases be composed of stable materials and have a low volatile organic compound (VOC) emission level. 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. More stable silicone or acrylic latex based caulks should be used.
Caulks based on silicone and acrylic latex set through either an acidic or a neutral curing process. 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.
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: The best silicone or acrylic latex sealant from an emission standpoint is neutral curing.
Many neutral cure sealants release methanol or ammonia during cure; these emissions can be harmful to artifacts, but less so than acetic acid.
=== How do I choose a caulk for my application? ===
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 require- ments for flexibility, extrusion and adhesion. ASTM C920 addresses durability and weathering characteristics.
=== How much curing time must be allowed for caulk sealants? ===
One of the most important guidelines for using a caulk sealant is to allow at least three weeks curing time before exposing objects to the case. 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.
Some caulk sealants require a primer coating. If such a product must be used, apply a low VOC primer which meets exhibit case paint standards; consult the technical data provided by the manufacturer.
=== Release of Volatile Organic Compounds ===
The following neutral cure, silicone or acrylic caulks emit low amounts of VOCs as measured in grams per liter (g/l). (Low g/l numbers are desirable; when a range of VOCs is given, the higher numbers are for colored caulking materials.)
Silicone:
* ·       Dow Corning; Dow Corning 832 Silicone (39 g/l)
* ·       Darworth; Polyflex, 15-28 g/l
* ·       Red Devil; Lifetime, 34 g/l
Acrylic:
* ·       Darworth; Acrylic Caulk, 14-42 g/l
* ·       Macco; Painter's Caulk, 70 g/l
* ·       OSI; Acrylic Latex, 41 g/l
* ·       Red Devil; 25-Year Acrylic, 34 g/l
* ·       Wal-Mart; House Beautiful, 41.6 g/l
Acrylic/Silicone:
* ·       DAP; Energy Saver and Alex Plus, 25 g/l
* ·       Macco; Super Caulk LC, 20-40 g/l
* ·       OSI; Siliconized AL, 41 g/l
* ·       Percora; AC-20+, 31 g/l
NOTE: Mention of a product, manufacturer, or supplier by name here is for information only and does not constitute an endorsement of that product or supplier. The figures supplied above were reported in the original National Park Service publication. It is suggested that readers also seek alternate product and vendor information to assess the full range of available supplies and equipment.