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The main monomer unit used in the syntheis of polyurethanes is the isocyanate unit. Once reacted with a polyfunctional alcohol can form the urethane linkage. The chemistry is shown below:
alcohol
isocyanate
polyureathane
Some of the common monomers used
in polyurethanes for coatings are:
4,4'-diphenylmethane diisocyanate (MDI)
hexamethylene diisocyante(HDI)
2,4
toluene diisocyanate (80%)
2,6 toluene diisocyanate (20%)
(TDI)
The different polyeurethane systems are:
One Package Pre-Reacted:
Characterized by the absence of any significant quantity of free isocyanate
groups. Formed from a
polyhydric alcohol and polyisocyanate.
One Package Moisture Cure:
Contain free isocyanate groups which react with ambient moisture.
One Package Heat Cured:
Cure by the thermal release of monohydric alcohol blocking agents and regeneration
of isocyanate
groups.
Two Package (Polyol):
One package contains a prepolymer with free isocyanate groups and the second
package contains a
substantial quantity of polyol with active hydrogens.
Two Package (Catalyst):
One package contains a prepolymer with free isocyanate groups and the second
package contains a
catalyst and a polyol or polyamine
to
return to the Main Polymers Page