Isocyanate monomers, physical properties

Trimerization to isocyanurates (Scheme 4.14) is commonly used as a method for modifying the physical properties of both raw materials and polymeric products. For example, trimerization of aliphatic isocyanates is used to increase monomer functionality and reduce volatility (Section 4.2.2). This is especially important in raw materials for coatings applications where higher functionality is needed for crosslinking and decreased volatility is essential to reduce VOCs. Another application is rigid isocyanurate foams for insulation and structural support (Section 4.1.1) where trimerization is utilized to increase thermal stability and reduce combustibility and smoke formation. Effective trimer catalysts include potassium salts of carboxylic acids and quaternary ammonium salts for aliphatic isocyanates and Mannich bases for aromatic isocyanates. 

Attention in this section will be focussed on the thermoplastic polyurethane elastomers. These polymers are based on three monomers (1) an isocyanate source, (2) a macroglycol or carbonate, and (3) a chain extender, or curing agent. The isocyanates can be either aromatic or aliphatic. Although the aliphatic based polyurethanes are more expensive, and inferior in physical properties they do not show the embrittlement, weakening, and progressive darkening of the aromatic equivalents. 

Other isocyanates can be used, but they may impart different physical properties to the polyurethane application. Also, isocyanates in general can pose a health hazard to workers. MDl monomer is less volatile and might be less of a health risk to workers than the more volatile TDl monomer alternative. 

Polyacrylates as binders consist of copolymers of acrylate and methacrylate esters. Other unsaturated monomers (e.g., styrene and vinyltoluene) may also be incorporated, but usually to a lesser extent. Copolymers formed exclusively from acrylates and/or methacrylates are termed straight acrylics. The comonomers differ as regards the alcohol residues of the ester group, which also allow incorporation of additional functional groups. Choice of suitable monomers allows wide variation of the physical and chemical properties of the resulting polymer. Hydrophilicity, hydrophobic-ity, acid base properties as well as can be adjusted resins containing hydroxyl, amine, epoxy, or isocyanate groups can also be produced. 

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