Handbook of Adhesives and Sealants

Selecting and Using Sealants 523accurate dimensions. They are generally supplied on a paper backingwith a release lining that is removed prior to application. Usually, asuitable length of the tape sealant is cut from a roll and applied toone of the joining surfaces with the backing paper still in place. Thesealant is pressed into place by hand, and then the paper is removed.Butt splices and corner splices are easily formed by pushing the tapeinto place. Extruded tapes of polybutene, polyisobutylene, and butylare available in many states of cure, and in various combinations withother resins. These tapes are available in continuous lengths and ina variety of forms such as bead, half-bead, ribbon, and sheet.Heat-applied preformed sealants are also available. These sealantsare also supplied with backing paper. Usually the backing paper isremoved and the solid, relatively nontacky sealant is fitted into thejoint. The sealant is then heated to the manufacturer’s recommendedtemperature with a hot air gun or oven. Caution must be taken thatthe heat softened material achieves continuous contact with the sidesof the joining surfaces and that wetting and adhesion occur. Thesepreformed sealants can also be applied through a heated gun.14.4.3 Preformed gasketsPreformed seals are supplied specifically for the application in usuallya multitude of available shapes and materials. An example of such agasket would be a continuous length of fully cured silicone tape withpressure sensitive adhesive on both sides. Other materials and formsare commonly available.Common to all these shapes is the fact that they are installed undercompression. In this application, adhesion is not a problem since sealintegrity is dependent on the gasket remaining under compression.Thus, joint cleaning consists of simply blowing or brushing loose dirtand contamination out of the joint. Preformed gaskets can have a pressuresensitive adhesive on one side of the sealant material to hold thegasket in place until it is mated with its other substrate. In certainapplications, it may be desirable to have adhesive on both sides of thegasket.The critical factors to be considered in gasket installation arestretching and splicing. The gasket must be designed for the joint sothat it is under compression at all times. Most gaskets can leak atcorners unless properly spliced or molded. Molded 90 degree pictureframegaskets prevent leakage at the corners.14.4.4 Foam sealantsFoam sealants are available as: (1) solid, preformed closed cell gaskets(a type of preformed gasket) and as (2) extrudable hot melt or multi-