23 / 150
17
Polyurethanes:
Various polymers produced by chemical
reactions formed by mixing di-icocymate with
hydroxyl, used to make flexible and rigid foams,
elastomers and resins.
Many urethanes are moisture-cured materials.
Other two-component urethanes are chemically
curing mixtures. Their compatibility with most
substrates and waterproofing capability has made
them a commonly specified sealant for most
waterproofing projects. Their formulations range
from one-component and two-component self-
leveling grade for horizontal joints, to one-
component and two-component non-sagging
grade for vertical expansion joints. Some
urethanes are manufactured to meet the USDA
requirements for food processing plants.
Urethanes have excellent recovery capabilities,
90% or more, and have very good weathering
characteristics.
As urethanes are extremely moisture-sensitive
during curing, closed-cell backer rod should be
used, except for the one-component materials
where open-cell is acceptable.
In most applications, priming is not required,
however, manufacturers differ in their
specifications especially for very smooth
substrate surfaces. So it is important to follow
the manufacturer’s specification.
Do not use urethanes in joints containing
polysulfide or asphalt base sealants or residue
unless they can be completely removed. They
should not be used in glazing applications or
high performance glass, plastics or acrylics.
Most sealants, except latex, exceed the
movement capabilities of paint, so they should