Page 443 - Handbook of Adhesives and Sealants

Adhesives Families 383

(TDI) or methylene diphenyl isocyanate (MDI). Most newer urethane
adhesive formulations use MDI, as a result of toxicological concerns
associated with TDI. The aliphatic isocyanates, being more expensive,
are less commonly used than MDI. Isocyanates can react with active
hydrogen compounds. Some of the major reactions of single component
urethane adhesives are shown in Fig. 10.6. Reaction mechanism of a
two component urethane adhesive is shown in Fig. 10.7.
One-component systems can be formulated to cure when exposed
to moisture at room temperature, or by heating. Most one compo-
nent polyurethanes are based on an isocyanate terminated pre-
polymer that is catalyzed with a tin salt or with a tertiary amine. The
moisture in the air is sufficient to cure the adhesive, but the sub-
strates must have some permeability to allow the moisture to get to
the adhesive. A second type of single component polyurethane adhe-
sive is based on ‘‘blocked isocyanates’’. Diisocyanates or isocyanate
prepolymers can be reacted with certain active hydrogen containing
compounds called blocking agents. Common blocking agents are nylon
phenol, caprolactam, and methylethyl ketoxime. 21 After the blocking
reaction is complete, the blocked isocyanate is stable and can be for-
mulated with polyols and certain chain extenders without additional
reaction. Once heated during cure, the blocked isocyanate thermally

One Component Moisture Curing Urethane Reaction
2 O = C = N~~~~N = C = O + H 2O → ~~~~[− NH − CO − OH] →
Catalyst
Prepolymer Carbanic Acid

~~~~NH2 + CO 2 → ~~~~NCO + ~~~~NH − CO − NH~~~~
Polyurea

One Component Blocked Urethane Reaction

B − CO − NH − R − NH − CO − B + HO~~~~OH →
Heat
Blocked Isocyanate

~~NH − CO − O~~~~O − CO − NH − R~~

Polyurethane
Figure 10.6 Reaction mechanisms of single component polyurethane adhe-
sives. 4