Developing a continuous emulsion pBD-graft-SAN polymerization process: Factors impacting morphology control

An experimental program was conducted to determine the feasibility of graft polymerizing styrene and acrylonitrile onto polybutadiene seeds in a conti nuous emulsion polymerization system. These particles, used for toughening acrylonitrile-butadienestyrene (ABS) polyblends, are usually manufactured i n batch or semi-batch emulsion systems and continuous solution polymerizati on processes. Translating the system to a continuous emulsion polymerizatio n may afford economic and environmental advantages over the alternative met hods. A reactor configuration of two continuous stirred tank reactors (CSTR 's) in series was chosen to study the system. Graft morphology is critical to the particle's ability to efficiently tough en the ABS while maintaining acceptable surface properties. The inputs for morphology control studied in this system were potassium persulfate initiat or level, overall residence time, and the proportion of the monomer charged to the first reactor (feed split). The experimental responses observed inc luded individual reactor conversion, total conversion, degree of grafting a nd graft efficiency by percent gel analysis, free SAN molecular weight by G PC, and graft structure by TEM. A wide variety of graft morphologies, ranging from perfect core/shell to pa rticle separation, is achievable by varying the reaction conditions. The gr aft structures observed were strongly affected by potassium persulfate init iator level. The anchor effect caused by the affinity of polar end groups f or the particle surface was evident in that higher initiator levels resulte d in more core/shell behavior. Decreasing monomer concentration also contri buted to core/shell behavior. Higher monomer levels generally resulted in g reater polymer separation due to the higher chain mobility afforded by lowe r particle viscosity.