Page 547 - Handbook of Adhesives and Sealants

472 Chapter Twelve

12.4.2 Performance properties
In some applications, strength may be more important than elasticity.
Low strength or more precisely low tensile modulus may be the most
important factor in a situation where a sealant joins one or more weak
surfaces. Tensile strength is needed primarily to avoid cohesive failure
under stress and not to transfer stress between substrates as the case
with most adhesives.
Modulus sometimes can predict the extension or compression char-
acteristics of a sealant. In general, low to medium modulus sealants
are able to take significant movement without putting much stress on
the sealant or the substrate materials. Some high performance seal-
ants are formulated for a higher movement capability than a joint
actually was designed to accommodate. The fact is that joints designed
for about 25% extension/compression often must accommodate move-
ment of 50% or more. Thus, higher performance sealants provide an
added safety factor. A change in elasticity or hardness on aging may
be an indication that further curing or degradation is taking place.
Compressive strength is the maximum compressive stress that a
sealant can withstand without breaking down or experiencing exces-
sive extrusion from the joint. Compression set is the inability of a
sealant to return to its original dimension after being compressed.
High compression set is usually caused by further curing or degra-
dative cross-linking of the material while under compression. Com-
pression set is undesirable in a joint that needs to expand and con-
tract. Stress relaxation is a condition in which the stress decays as
the strain remains constant. Some very low modulus sealants literally
become pulled apart when held at low elongation.
Sealants may be exposed to scuffing and mechanical wear. Examples
include the sealant used as an expansion joint in the highway, and
the sealant used in preparing stone walkways. Thus, they must offer
good abrasion, puncture, and tear resistance. Flexible sealants, which
are available in either chemical curing or non-curing types, exhibit
varying degrees of tear resistance. Urethanes have the highest tear
resistance. Dynamic loads, shock, and rapid variations in stress also
can cause seals to fail. Thus, tough, flexible elastomeric sealants that
can stretch and then return to their original length in a short time
are often selected for occillating mechanical loads.
Weatherability is the degree of resistance of a sealant to heat, mois-
ture, cold, solar radiation, etc. The degree of weatherability is deter-
mined by the base polymer and the nature of the additives in the
sealant formulation. Generally, sealants are formulated for maximum
resistance to a single element such as moisture. Often, this chemistry
will give it resistance to other elements as well. However, if a sealant’s