Using Gaskets to Seal Exhibit Cases
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Gaskets are used to seal gaps between two surfaces that are not permanently closed, such as case access doors and removable panels.
What are gaskets and how are they used in exhibit case construction?
Gaskets are rubbery substances used at the interface of adjoining materials to reduce gaps and create tighter fitting joints. In exhibit cases, gasketry materials are frequently used at the junction of the exhibit cabinet and moveable access panels and doors. Gaskets are not only necessary at the entry- ways to the display chamber but also for the maintenance and lighting chambers. Using a gasket to seal gaps in exhibit case construction provides several conservation benefits:
- Controls air exchange between the case and the room
- Provides a sealed case that supports environmental control
- Inhibits the entry of unwanted pests, airborne soils, and atmospheric pollutants
- Safeguards against the accidental entry of water
Over the years, gasketry products have been made from a wide range of natural and synthetic materials. Today the manufacturing industry produces gaskets primarily from synthetic, rubber like polymers called elastomers.
What types of gaskets are to be avoided?
Not all gaskets available in the marketplace can be recommended for use in museum cases; volatile by-products are a major problem with many commercial gaskets as they can react with objects held within the enclosure.
- Avoid unstable elastomers: Polyurethane foam gaskets used in exhibit cases in the past have deteriorated within a few years. Oxidation and exposure to ultraviolet radiation break down the polyurethane, causing the foam to lose resiliency and structural integrity; the gasket crumbles and falls away. Neoprene gasketry has been found to out-gas sulfur which can be harmful to sensitive objects.
- Avoid unknown adhesive backings: Commercial gasket products often include a pressure- sensitive adhesive. These adhesive systems frequently lose their bonding ability, so that the gasket detaches or moves out of position. The adhesive itself can also be the source of harmful, volatile emission.
Are there guidelines for selecting conservation appropriate gaskets?
Five important characteristics guide the selection of a gasket for use in an exhibit case: (1) elas- tomer type; (2) shape (3) size and thickness; (4) density; and (5) adhesive backing system. An appropriate gasket for an application is both chemically and physically stable, and fulfills the required function.
- To ensure that the gasket will not contaminate the case interior, select an elastomer proven to be stable through testing. The following are some examples of stable elastomers.
- Silicone rubber (dimethyl silicone): most stable of all elastomers, silicone is resistant to high and low temperature ranges, fatigue, most chemicals, oils, oxygen, and ozone.
- Cellular silicone sponge (poly-dimethyl siloxane also referred to as PDMS): silicone rubber that has been foamed into a uniform unicellular structure. It has the same characteristics as silicone rubber and is shaped by either extrusion or cutting.
- Ethylene propylene diene monomer (EPDM): resistant to ultraviolet light, ozone, oxidants, heat, compression set and resists the swelling effects of polar chemicals.
- Ethylene vinyl acetate closed-cell elastomeric seal (EVA): similar to other elastomers in flexibility and softness, EVA has relatively low resistance to heat and solvents but excellent ozone resistance.
Size and thickness:
- An effective seal depends on an appropriately sized gasket which allows the moveable surface to close and seat uniformly against the gasket. The objective is to fill the space between the closing surfaces without holding them apart: thickness should be roughly equiva- lent to the gap measurement plus 25% for solid gaskets and 40% for hollow gaskets. Excessive gasket thickness exerts stress on door and panel fasteners, causes hinges to bind, and deforms flat panels. Undersized gaskets leave gaps at corners and between irregularities in the surfaces.
- Commercial gaskets are formed by extrusion or by cutting. Extruded sponge, which ispushed or forced through a shaped die, is identifiable by its smooth, slightly shiny surface. Blocks of formed foam are cut into a shape, having two open sides revealing the interior cellular structure. Whether extruded or cut, the most useful shapes are rectangular or rounded pads and hollow "O" or "P" shapes. Different applications require different shapes; trial and error, includ- ing an examination of the products resilience, are usually required to choose a good gasket shape. Consult the gasket engineering literature as well.
- The gasket must be soft enough to conform fully to the opposing surface but have a "memory" enabling it to spring back into shape when compression is removed. The softer foam densities are generally more appropriate; product literature specifies the elastomer density.
Adhesive Backing System:
- Avoid using gaskets with adhesive backings whenever possible; existing adhesives can often be removed using organic solvents. If a pressure sensitive adhesive backing is desirable, select an acrylic adhesive system or apply an archival quality double-sided tape.
How do I use the gasket?
The following are a few practical tips for gasket design:
- Size the gasket for the largest gap, but do not allow over-compression to occur at narrower gaps. Design the gasket system such that closure compresses the gasket by approximately
- 25% for solid gaskets and 40% for hollow gaskets.
- Practice extreme care when preparing gaskets to meet at corners. Cut gasket runs longer than necessary, carefully mitering ends to form a tight fit. Do not apply an adhesive to bond gasket joints because it can stiffen the elastomer. 25 - 50% of the thickness of the gasket.
- Test the effectiveness of gasket seams used to obtain a sealed exhibit case for climate control. Physical and pressurized testing procedures exist and leak detection equipment is available
Products, Manufacturers, and Suppliers
Mention of a product, manufacturer, or supplier by name here for information only and does not constitute an endorsement of that product or supplier. 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 equipment.
Gasket Sources for Silicon, EPDM and EVA
- Bisco Products (Cellular Silicone Sponge HT 800 Series), Elk Grove Village, IL, 60007-6120
- Clean Seal, Inc, Southbend, IN, 46614
- Delta Designs LTD, Topeka KA, 66618
- Netherlands Rubber, Cinninnati, OH, 45241
- Resource Conservation Technology Inc, Baltimore, MD, 21218