Building Design and Construction Handbook - Merritt - Ventech!

BUILDING MATERIALS 4.101
Characteristics of the preceding formulations vary. Hence, the proper choice of
materials depends upon the application. A sealant with the appropriate hardness,
extensibility, useful temperature ranges, expected life, dirt pickup, staining, colorability,
rate of cure to tack-free condition, toxicity, resistance to ultraviolet light,
and other attributes should be chosen for the specific end use.
In many joints, such as those between building panels, it is necessary to provide
backup; that is, a foundation against which the compound can be applied. This
serves to limit the thickness of the joint, to provide the proper ratio of thickness to
width, and to force the compound into intimate contact with the substrate, thereby
promoting adhesion. For the purpose, any of various compressible materials, such
as polyethylene or polyurethane rope, or oakum, may be employed.
To promote adhesion to the substrate, various primers may be needed. (To prevent
adhesion of the compound to parts of the substrate where adhesion is not
wanted, any of various liquid and tape bond-breakers may be employed.) Generally,
good adhesion requires dry, clean surfaces free of grease and other deleterious
materials.
4.88 GASKETS
Joint seals described in Arts. 4.86 and 4.87 are formed in place; that is, soft masses
are put into the joints and conform to their geometry. A gasket, on the other hand,
is preformed and placed into a joint whose geometry must conform with the gasket
in such a way as to seal the joint by compression of the gasket. Gaskets, however,
are cured under shop-controlled conditions, whereas sealants cure under variable
and not always favorable field conditions.
Rubbery materials most commonly employed for gaskets are cellular or noncellular
(dense) neoprene, EPDM (ethylene-propylene polymers and terpolymers),
and polyvinylchloride polymers.
Gaskets are generally compression types or lock-strip (zipper) types. The former
are forced into the joint and remain tight by being kept under compression. With
lock-strip gaskets, a groove in the gasket permits a lip to be opened and admit glass
or other panel, after which a strip is forced into the groove, tightening the gasket
in place. If the strip is separable from the gasket, its composition is often harder
than the gasket itself.
For setting large sheets of glass and similar units, setting or supporting spacer
blocks of rubber are often combined with gaskets of materials such as vulcanized
synthetic rubber and are finally sealed with the elastomeric rubber-based sealants
or glazing compounds.
4.89 JOINT SEALS BIBLIOGRAPHY
‘‘Building Seals and Sealants,’’ STP 606, ASTM, Philadelphia, Pa.
J. P. Cook, ‘‘Construction Sealants and Adhesives,’’ John Wiley & Sons, Inc.,
New York.
A. Damusis, ‘‘Sealants,’’ Van Nostrand Reinhold Company, New York.