Processing of Thermoplastic Polymers Using Reactive Solvents

Abstract

The use of reactive solvents offers an interesting and flexible route to extend the processing characteristics of thermoplastic polymers beyond their existing limits. This holds for both intractable and tractable polymers. The first mainly applies for amorphous high-T _g polymers where processing may be limited due to the high temperatures required which can cause problems related to degradation and where the solvent helps to decrease the processing temperature considerably. A prime example here can be found in the system poly(phenelyne–ether) PPE/epoxy. The second mainly holds for semicrystalline polymers and the attention here is focused on obtaining low viscosities in order to be able to apply alternative processing routes, like pouring or casting, for those polymers which are generally easy to process by more conventional techniques like injection moulding or extrusion. In this paper the model system poly(ethylene) PE/styrene is investigated. In both cases, based on intractable and tractable polymers, the solvent is polymerized after moulding, thus converting into a non-solvent, and becomes, after the concurrent phase separation and phase inversion, an integral and often structural part of the final product. Interestingly, specific morphologies, in terms of the size of the dispersed (previous solvent) phase formed or the position and thickness of in situ formed interlayers when polymerization occurs in the presence of a polar surface, can be obtained which can otherwise extremely difficult, or not at all, be realized. Moreover, flexibility of the choice of the reactive solvent creates tuneable mechanical (and, if requested, other) properties of those phases, varying from glassy, with a T _g of typically 200°C, to complete rubbery, with a T _g far below room temperature. Of course a disadvantage of the technique is that a polymerization step must occur after the shaping process. Compared to more standard reactive processing techniques, however, clear advantages can exist with respect to the occurrence of early vitrification—yielding a fast demoulding possibility—induced by the reaction induced phase separation, and the fact that the continuous thermoplast phase ultimately determines the main product properties, including the possibility of second-stage deformability and reprocessability.