Reactive Processing of Thermoplastic Polymers

Scientific and industrial interest in the chemical modification of polymers has obviously increased since the middle of the last century. Some important discoveries include the vulcanization of natural rubbers (by Goodyear in 1839) and cellulose nitration (by Braconnot in 1833). This latter modified natural polymer was used as a substitute for ivory, by Hyatt in 1870, after plasticizing by camphor and lead to the first easily processable material: celluloid. Besides these examples based on chemically modified natural macromolecules, it is worthwhile noticing how many possibilities exist for chemically modifying synthetic polymers, as indicated in the exhaustive review published by Fettes.1 Generally, the use of solvents or dispersed media facilitates the control and adjustment of reactivity between polymers and other components of the system. The rate of conversion as well as the final structure of the modified polymer depend to a large extent on the reaction scheme and on the process parameters, including the nature of the continuous phase (solvent or dispersing agent) and such processing parameters as temperature and pressure. It is important to point out that the low concentration of polymer (around 10%) and the related separation and purification processes, which have a great influence on the final costs of modified polymers, are among the main disadvantages of reactions conducted in solvent media. Another approach is to perform these modifications of thermoplastic polymers, leading mainly to grafting and crosslinking reactions, by reactive processing in a discontinuous or continuous mixing equipment, which will be the chemical reactor, involving problems of high temperature, viscosity, risks of corrosion, etc. The main medium is the molten polymer, with an associated polarity related to its chemical composition, and the corresponding reaction and processing parameters are very different from those in solution. Adjustment of the reactivity requires specific basic research on the kinetic behaviour under these conditions, even if the main reaction is well known in classical organic or polymer chemistry.2