Stable Prepolymer Systems

Finished product Fig. 5.2. Prepolymer process for cast systems. 

---

In the stable prepolymer systems the isocyanate used to tip the polyol is more stable on storage in dry cool conditions. In addition a trace of an acid or acid chloride may be added to neutralize trace catalytic impurities that might promote undesired cross-linking reactions on storage. Such systems are offered by several suppliers, with polyesters used in some formulations and polyethers in others. Toly-... 

The ketimine of isophorone diamine is formed by reacting it with methyl isobutylketone, splitting off water in the process. When said ketimine is added to an isocyanate-terminated prepolymer based on IPDI, a semi-stable system is established with a pot life of several hours. The ketimine is a Schiff base and thus can react even in the absence of water. The complexities and advantages of this system are reviewed by Bock and Halpaap [75] ... 

In order to bring about crosslinking of polyesters with styrene one of two types of initiator systems is used, which differ in the temperature at which they are effective. For curing at elevated temperatures, peroxides are used which decompose thermally to yield free radicals. Among those peroxides employed are benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, di-t-butyl peroxide, and dodecyl peroxide. Mixtures of polyester prepolymer, styrene, and such initiators are reasonably stable at room temperatures but undergo fairly rapid crosslinking at temperatures between 70 °C and 150 °C, depending on which particular peroxide is used. 

The acid-crosslinkable prepolymers are polyester-alkyd, urea-formaldehyde, or melamine-formaldehyde resin adducts combined with a "blocked" acid catalyst that becomes "unblocked" at the dryer temperatures f7, 11, 12). e. g., a melamine-polyester resin adduct with "blocked" -toluenesulphonic acid catalyst (12). These acid-crosslinkable prepolymers are stable on the press distribution system but are converted to tiiree-dimensional polymer networks by heating to 275-325° F. 

Reaction-setting adhesives usually consist of two individually stable parts that react when mixed, in the cold or when heated, to form a polymer. The products are usually rigid, but some systems produce flexible products, as in the case of some polyurethanes (see Polyurethane). One part of a system is a monomer or a prepolymer and the other is... 

Another approach to formulating one-part polyurethane systems has been to render the curing agent inactive towards isocyanates at room temperature. Exposure to heat activates these materials, whereupon curing of the isocyanate components takes place. Adhesives based on this type of technology have been described in the patent literature. A sodium chloride complex of 4,4 -methylenedianiline (MDA) is reported to form room temperature stable mixtures with isocyanate prepolymers, which cure upon heating. - " ... 

---