Structoset prepolymers

New Prepolymers. Interesting among the newer structoset prepolymers are the Butons (Figure 7), now under development by Esso Research (5). Monomers are styrene and butadiene. These structopendant prepolymers are cross-linked with peroxides, styrene, and diallyl phthalate. 

Epoxides Structoset prepolymers are designed to have controlled and defined structures with functional groups located either at the chain-ends (structo-terminal) or located along the chain (structopendant). Epoxy resins make use of the epoxy group for the former, and hydroxyl groups for the latter. One of the major resin prepolymers formed by the reaction of bisphenol A and epichlorohydrin can be treated as either. 

This sudden gel formation is usually interpreted as the formation of a small per cent of infinite space network. The formation and control of the gel phase are vital in the processing of random thermosets and structosets. For example, too rapid gelation can cause poor bond strength in laminated and bonded structures. Too slow gelation could cause the collapse of a foam. The amount of reaction required for gelation can be controlled by the functionality of prepolymer units, /pp. 

When considering the gelation of structosets, the molecular weight of the prepolymer segments is relatively high and the functionality of the prepolymers can be very high as compared to the functionality of the cross linker, which often is onlv 2 or 3. 

Prepolymers for structosets fall into two major groups. 

Stnictoterminal Prepolymers. The reactive sites are located at the end of a major polymer chain. Maximum control of the length and type of chain in finished structoset polymers is obtained with this type of prepolymer. 

The excellent properties which can be designed into the final molecule, as well as low application temperatures, make epoxies extremely useful in many electrical applications. Structoset coatings, based on a great variety of prepolymers, are widely used because one is able to tailor coatings for a variety of end uses and to obtain satisfactory properties combined with good economics. 

The first stage is the production of an incompletely reacted prepolymer this is either solid or liquid and of moderately low-molar mass. The second stage involves conversion of this into the final cross-linked product in situ (i.e., a mold or form of some description). The prepolymers are either random or structoset and are discussed separately ... 

---