It may also be a polyurethane prepolymer that is NCO terminated but this functionality has also been chemically reacted with a blocking agent.
A formulation containing a blocked isocyanate is a single component material (and thus usually considered more convenient) but reacts like a two-component product but will not react until heated to the temperature required for activation which varies depending on the blocking agent.
[7] On heating up to the activation temperature, cure can be as short as 20 minutes depending on the blocking agent and other formulation parameters.
A number of blocking agents maybe employed but a common one is Methylethyl ketone oxime (MEKO).
[8] When blocked, there is no isocyanate (NCO) functionality, so it is much easier to disperse the species in water if the desire is to produce waterborne resins.
[15] As with regular isocyanates organometallic compounds and tertiary amines may lower the unblocking temperature.
[18] The use of sodium bisulfite as a blocking agent has also allowed them to be used in waterborne resins such as PUDs.