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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