No. 3

No chemistry required (it would not hurt to know some)

Some chemistry or physics required

Isocyanates are extremely reactive and also toxic compounds. The are mainly toxic by inhalation. Extreme care must be used in handling these materials and a MSDS from the manufacturer has to be consulted. Special protective clothing and air supply for breathing might be required. Repeated exposure can lead to sensitization.

Aromatic isocyanates, like TDI (toluenediisocyanate) and MDI (methylenediphenyl diisocyanate) represent the largest market for diisocyanate monomer. These isocyanates are used in polyurethane foams, elastomers, adhesives and coatings. Aromatic isocyanates are under catalyzed conditions extremely fast in reacting with polyols. 

Aliphatic isocyanate are used predominately in coating applications because they produce polyurethanes with excellent UV resistance and exterior durability. In comparison to aromatic isocyanates the aliphatic isocyanates are slower in their reaction with polyols. For crosslinking of coatings polymers of diisocyanates, such as the trimers are used. Hexamethylene diisocyanate is used in the manufacture of polyurethane polymers and emulsions. IPDI is a less reactive aliphatic isocyanate and is used in the preparation of elastomers.