Blocked diisocyanate

Developments in aliphatic isocyanates include the synthesis of polymeric aliphatic isocyanates and masked or blocked diisocyanates for appflcafions in which volatility or reactivity ate of concern. Polymeric aliphatic isocyanates ate made by copolymerizing methacrylic acid derivatives, such as 2-isocyanatoethyl methacrylate, and styrene [100-42-5] (100). Blocked isocyanates ate prepared via the reaction of the isocyanate with an active hydrogen compound, such as S-caprolactam, phenol [108-95-2] or acetone oxime. 

Figure 5.27 Polyimide synthesis with imidazole blocked diisocyanate and dianhydride.

Half-blocked Dllsocyanates. The half-blocked diisocyanates were prepared using conventional methods (2) by adding dropwlse, over a period of one hour, 1 mole of alcohol to 1 mole of diisocyanate and 100 mg dibutyl tin dilaurate in methyl amyl ketone under an inert atmosphere. After the addition of the alcohol, the reaction was heated at 60-80°C for 2 hours. For half-blocked diisocyanates prepared from tertiary alcohols, the heating period was replaced with room temperature stirring for 24-36 hours to prevent undesirable side reactions. 

Chain Extendable Urethane Modified Epoxy Oligomer. The chain extendable urethane modified oligomers were prepared by combining equimolar amounts of epoxy-diol adduct and half-blocked diisocyanates, and heating the resulting mixture at 80°C for 4-6 hours until the isocyanate Infrared band disappeared. 

Masked or blocked diisocyanates are used in coatings applications. The blocked diisocyanates are storage-stable, nonvolatile, and easy to use in powder coatings. Blocked isocyanates are produced by reaction of the diisocyanate with blocking agents such as caprolactam, 3,5-dimethylpyrazole, phenols, oximes, acetoacetates, or malonates. 

Welsh, E.R. and Price, R.R. (2003) Chitosan cross-linking with a water-soluble, blocked diisocyanate.2. Solvates and hydrogels. Biomacromolecules, 4, 1357-1361. 

Choudhari, S.K., Kittur, A.A., Kulkarni, S.S. and Kariduraganavar, M.Y. 2007. Development of novel blocked diisocyanate crosslinked chitosan membranes for pervaporation separation of water-isopropanol mixtures. 302 197-206. 

Gibson, S.L., Walls, H.J., Kennedy, S.B., Welsh, E.R. Reaction kinetics and gel properties of blocked diisocyanate crosslinked chitosan hydrogels. Carbohydr. Polym. 54, 193-199 (2003)... 

Thermoplastic linear polyurethanes which are usually chain-terminated so that no unreacted free NCO groups remain available. Environmental considerations direct growing attention to these newer non-polluting urethane adhesive forms, e.g. powders, films, aqueous dispersions and 100% solids reactive systems. Some systems do possess blocked diisocyanates which are activated on heating to produce chemically reactive solid systems. 

Examples of the use of blocked diisocyanates for rubber-fabric adhesion are as follows vulcanized polychloroprene and SBR can be adhered strongly to nylon and polyester fibre fabric by means of aqueous adhesive systems (Table 8.4). This combination is spread or roller coated on to the fabric which is then allowed to dry. Bonds to sheet rubber stock can be made immediately after the treated fabric is dried or at any time thereafter. When the sheet rubber is applied it should be held under moderate pressure to provide intimate contact with the treated fabric and to prevent lifting if any gases are emitted during cure. Press cures of 20-40 min at 140°C are sufficient to cure the adhesive and most elastomer compositions being adhered. If a latex film is applied to the treated fabric, the assembly can be cured in a hot-air oven at 120"C. A chemical bond results between fabric and the diphenylmethane-/7,/ -diisocyanate generated on the thermal cleavage of the blocked diisocyanate. 

Diethyl malonate blocked diisocyanates cross-fink polyols at 120°C for 30 min. The reaction with alcohols does not srield urethanes, rather transesterification occurs (134), and reaction with amines srields amides, not ureas. Storage-stable coatings can be formulated by using monofimctional alcohol in the solvent (135). Clear coats for automobiles that have both excellent environmental etch and abrasion resistance have been formulated with a combination of a hydroxy-functional acrylic resin, malonic ester blocked HDI and IPDI trimers, and an ME resin (136). 

Because of its relatively low boiling point HDI is currently only supplied in the form of derivatives (trimer, allophanate, biuret, etc) (18). Polyrmeric aliphatic isocyanates are obtained by copolymerization of 2-isocyanatoethyl methacrylate and styrene (19). Hydroxyalkyl acrylates and methacrylates are used as monomers to form hydroxyl group containing polymers, which are cross-hnked with diisocyanates or blocked diisocyanates (20). The elusive parent dusocyanate, 0=C=N—N=C=0, is only stable at —75°C, and therefore it is not suitable as a monomer or comonomer in polymerization reactions (21). 

The manufacture of TMXDI, developed by American Cyanamid, is based on the reaction of m-isopropyhdenebenzene (19) with ethyl carbamate to give the blocked diisocyanate (20). Thermolysis affords a mixture of TMXDI (21) and the monoisocyanate (TMI) (22) (30). 

Welsh, ER Schauser, CL Quadri, SB Price, RR. Chitosan Cross-Linking with a Water-Soluble, Blocked Diisocyanate. 1. Solid State. Biomacromolecules, 2002, 3, 1370-1374. 

Chen-Chi M. Ma, Han-Thing Tseng and Hew-Der Wu, Blocked diisocyanate polyester-toughened Novolac-type phenolic resin Synthesis, characterization, and properties of composites . Journal of Polymer Science, Part B Polymer Physics, 1998, 36,10. 

---