Page 521 - Handbook of Adhesives and Sealants

452 Chapter Twelve

sealant materials and formulations are identified in the next chapter.
Chapter 14 will cover the selection of sealants for specific applications.

12.2 Basic Sealant Classifications
The large number and diversity of sealant formulations make their
classification difficult. Often sealants are divided into the same prod-
uct classes, chemical families, and industrial applications as are ad-
hesives. However, there are also particular methods of classification
that have been developed specifically for sealants. These unique meth-
ods of classifying sealants are by their:

Hardening ability
Cure
End-use
Performance

12.2.1 Hardening and non-hardening types
Sealants are often classified as either hardening or non-hardening.
Hardening sealants are applied as a heavily bodied fluid, and they
‘‘set’’ to form either a rigid or a flexible solid seal. Non-hardening seal-
ants do not ‘‘set’’. They stay ‘‘wet’’ or flowable after application and
have physical properties similar to before they were applied. It should
be noted, however, that although non-hardening sealants remain soft
after application, they might still go through a chemical conversion,
solvent evaporation, or some other ‘‘curing’’ mechanism.
Hardening sealants can be further sub-divided into rigid and flexi-
ble classifications. Rigid sealants have little or no resiliency and are
characterized by their inability to flex. These types of sealants are
commonly used when both sealing and joining functions are needed.
Sealants that are representative of this group include the epoxies and
acrylics. Table 12.1 lists the common sealants that are classified as
rigid systems along with their more significant characteristics.
Flexible sealants remain flexible after curing and are generally elas-
tomer-based products. Their range of flexibility and hardness varies
considerably. Hardness values from Shore 10A to 85A durometer are
common. Flexible sealants are either true elastomers and return to
their original dimensions after compression or elongation (e.g., sili-
cones and nitriles), or else they are made from materials that per-
manently deform on stress without tearing (e.g., asphalt and various
gum stocks). Typical flexible sealants are described in Table 12.2.
Non-hardening sealants generally cannot be depended upon to per-
form a joining function although they may provide adequate sealing