Page 444 - Handbook of Adhesives and Sealants

384 Chapter Ten

Component A: O = C − N − R − N = O
Diisocyanate or Isocyanate Prepolymer

+
Component B: HO~~~~OH + H 2N − R’ − NH 2 + Catalyst
Polyol Amine (Hydroxyl)
Chain Extender
↓

Cured Adhesive:

~~~~O − CO − NH − R − NH − CO − NH − R’ − NH = CO − NH − R − NH − CO − O~~~~
Polyurea Polyurethane
Figure 10.7 Typical two component polyurethane adhesives reaction. 4

reverts back to its original components. The isocyanate is regenerated
and can react with the polyol and/or chain extender to form a polymer
network.
The chemistry of a typical two component polyurethane is summa-
rized in Fig. 10.7. The pot lives of these two component urethanes can
vary from as little as 15 secs to as long as 16 hrs, depending on the
type of reactant and catalysts. Many structural urethane adhesive for-
mulations set quickly and require no primer or other surface prepa-
ration.
Polyurethane-based adhesives form tough bonds with high peel
strength. They have better low temperature strength than other ad-
hesives. They have exceptionally high strength at cryogenic temper-
atures. Only silicone adhesives have better performance at lower tem-
perature, but their tensile strengths are much lower than the
polyurethanes. Polyurethane adhesives also have good flexibility, ab-
rasion resistance, and toughness. They have good chemical resistance,
although not generally as good as epoxies or acrylics. Urethanes do
not have high temperature resistance. Maximum upper use limits are
about 300 F, and most formulations are used below 250 F.
Some polyurethane adhesives degrade substantially when exposed
to high-humidity environments. This moisture sensitivity occurs with
both the cured adhesive and the uncured components. Once a poly-
urethane component is open to the air it should be used immediately.
To prevent moisture contamination, many large volume polyurethane
users resort to keeping dry nitrogen blankets on the adhesive com-
ponents before they are metered and mixed.
Because of their wetting ability and good flexibility, polyurethane
adhesives bond well to many substrates including hard-to-bond plas-