Synthesis, microstructure, morphology and properties of ABS polymers: rosin soap vs polymerizable surfactants

Abstract

ABS polymer is considered an engineering plastic due to its broad balance of properties between the elastic behavior and durability of polybutadiene (PB), the rigidity and good processability of polystyrene (PS) and the excellent thermal and chemical resistance of polyacrylonitrile (PAN). ABS polymer is composed by a multiphase structure that consists of a dispersion of PB particles in a styrene-acrylonitrile (SAN) matrix. At the same time, these PB particles are grafted with SAN chains and this grafting of the SAN onto the PB is critical to achieve effective dispersion and compatibility of both phases. Therefore, the grafting of acrylonitrile (AN) and styrene (S) monomers onto a PB substrate is the essence of the ABS process.Emulsion polymerization process for the manufacturing of ABS has been practiced for more than 60 years and is considered as the standard method for the synthesis of grafted-ABS since about 85 % of the grafted polymer is produced by this method. The industrial manufacturing processes comprise polymerization, where the grafted-ABS latex (raw material) is yield by emulsion polymerization, followed by coagulation and compounding, which is mainly necessary for preparing the final ABS product.So far, in the grafting emulsion polymerization, rosin soaps and fatty acid soaps have been the most widely used emulsifiers. However, the main goal of this thesis was to implement the use of polymerizable surfactants in the grafting reaction of ABS latex in order to overcome the negative effects caused by the use of conventional rosin soap surfactants. Specifically for the ABS system, the use of non-pH dependent surfactants could help to limit the production of volatile organic compounds (VOC) since the reactions could be run at lower pH values and thus limit acrylonitrile secondary reactions. In addition, the replacement of rosin soap by other surfactants may reduce the yellowness of the final polymer and therefore less amount of pigments would be use during the processing stage. Furthermore, when polymerizable surfactants are used, a smaller amount of them is desorbed into the aqueous phase during the coagulation since they are bonded to the particle. This reduction in free surfactant could help as well to decrease the amount of residual surfactant in the injections molds, slightly reducing the economic cost and time consumed during the posterior cleaning stage.For that purpose, the ABS latexes obtained with the polymerizable surfactants were assessed and compared with the latexes synthesized with the conventional rosin soap, in terms of their behaviour during the emulsion polymerization process (i.e. kinetics, microstructure and grating properties as well as colloidal and morphology of the polymer particles), during the coagulation and compounding of the ABS latexes, and in the mechanical properties.