Prepolymer amine curatives

Amine curatives tend to give too fast a cure rate when used with MDI-based prepolymers and give the best results when used with castable systems based on TDI. 

The prepolymer as received from the manufacturer has a simple chain that has been terminated with an isocyanate. The isocyanate ends with this magical NCO group. The NCO is the reactive part of it. The higher the percentage of NCO in the prepolymer, the harder the material will be. An 80 Shore A will have an NCO of approximately 3.1 to 3.2%, whereas a 75 Shore D will have an NCO content of about 11.2%. To obtain the chain extension, one must add an appropriate amount of an amine or diol curative. For every curative, there is a different amount that must be added. The manufacturers of the prepolymers and curatives will give the appropriate factors for mixing the polyurethane. The prepolymer must be heated before use. This is to reduce the viscosity of the material as well as to obtain the correct cure rate and complete cure time. 

Amine-based curatives are a popular choice when working with TDI-terminated prepolymers. The two most used amine-based curatives are MOCA and Ethacure 300. Other amine curatives used include Versalink 740M, MCEDA, and Cyanacure. 

Polyurethanes are made by extending chains of a prepolymer made from a macro diol and a diisocyanate. The prepolymer is further extended with a diol or an amine curative. The long chains form a solid which is relatively weak. When the part is given a longer heat treatment, the molecules align themselves and intermolecular bonds (hydrogen bonding) are formed. At this point the full mechanical properties are established and the material, if suitably formulated, has excellent mechanical and chemical properties. 

The major problem is moisture that is absorbed into the polyurethane system or into the curative and auxiliary materials. Free water will liberate carbon dioxide when the chain extension is carried out. It is important to keep the reactants dry, as any moisture that may have come in contact with the prepolymer will react to give an amine and carbon dioxide. This amine reacts with more isocyanates to form a disubstituted diamine. The reaction is outlined in Figure 2.9. 

If a diamine curative is used, the amine groups (NH2) will react with an isocyanate group of the prepolymer to form a urea bond. The remaining amine group will react with a further isocyanate group to extend the chain, as shown in Figure 2.30. 

Amine stoichiometry was kept at 95% based on prepolymer content. Except where noted, prepolymers were preheated at 93°C and mixed with curatives at desired temperature. The prepolymer, curative and prepolymer-curative mixture were each vacuum degassed prior to pouring the mixture into a preheated mold. Degassing before and after mixing is especially important to achieve optimum cures. The mold was closed when gelation started. Demolding time was one hour or less. The sample was then post-cured for 16 hours at 120°C. The elastomer was conditioned for one week at 50% relative humidity at room temperature prior to physical properties measurement. 

More recent efforts have focused on developments that create true hybrids. For example, blocked isocyanate prepolymers have been mixed with epoxy resins and cured with amines [68-70]. These blocked prepolymers will react initially with the amines to form amine-terminated prepolymers that cross-link the epoxy resin. Several blocked isocyanates are commercially available. The DESMOCAP (Bayer) llA and 12A products are isocyanates (believed to be blocked with nonylphenol) used as flexibilizing agents for epoxy resins. ANCAREZ (trademark. Pacific Anchor, Inc.) 2150 is a blocked isocyanate epoxy blend used as an adhesion promoter for vinyl plastisols. A one-package, heat-cured hybrid adhesive was reported consisting of isophorone diisocyanate, epoxy resin, and a dispersed solid curative based on the salt of ethylenediamine and bisphenol A [71]. Urethane amines are offered commercially that can be used with epoxy resins to develop hybrid adhesive systems [72]. 

Curatives for polyols, isocyanates or isocyanurates play a role of curatives prepolymers are cured either by polyols (frequently multifunctional to obtain tridimensional networks) or by amines considering that amines are very reactive with isocyanate groups, amines are frequently used in a blocked form of ketimines and aldomines which require moisture to hydrolyze them to free reactive amines and this slows curing process up to the extent that frequently catalytic systems have to be used to bring reaction to a required rate ... 

The most useful curatives of CTPB are the aziridines and epoxides, whereas HTPB may be cured with isocyanates. HTPB has also been capped with isocyanates and the resulting prepolymer cured with low molecular weight diols or amines... 

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