Network Formation in Free-Radical Emulsion Polymerization

Abstract

The share of thermoplastic polymer produced by radical polymerization in water dispersion is increasing steadily. This move towards water borne systems is obviously driven by the environmental needs to reduce the use of hydrocarbons in the manufacturing or polymer processing sites, and for the final customer to avoid exposure to solvents. Besides, emulsion polymerization technology enjoys a number of advantages: it is usually a kinetically fast reaction leading to high molecular weight polymeric materials in high solids content water dispersion; the viscosity of which remains in the low to medium range, making temperature control during process relatively easy. Compared with homogeneous radical polymerization, the mechanisms of emulsion polymerization are still unclear as far as the reaction proceeds, and the level of predictability remains poor in real systems when the number of monomers, chain transfer agents and the range of operational variables is high. This is particularly true for the development of molecular weight distribution (MWD) in emulsion polymers produced at high conversions. The kinetic models for linear polymerization become increasingly complicated as one approaches typical industrial recipes, requiring a large number of kinetic parameters which cannot be determined independently: thus in practice these unknown rate constants are estimated by fitting measurable characteristics such as MWD to a given model. In non linear polymerization, i.e. systems where the average functionality of the monomers exceeds 2, additional events like pendant or terminal double bond polymerization and long chain branching due to chain stoppage by radical transfer to polymer, make the kinetic scheme even more difficult to handle and the mathematics so sophisticated that we may lose the actual physics which lies behind it. Non-linear polymerizations however are most encountered in commercial polymers produced in emulsions: e.g. vinyl-divinyl systems such as butadiene homo and copolymers, polychloroprene and branched polymer such as polyvinylacetate and certain polyalkylacrylates and ethylene copolymers. In a recent work, Gilbert [1] pointed out that relatively little properly characterized experimental data on MWD that enables thorough quantitative analysis, were available. They are even fewer in the field of nonlinear polymers of commercial interest such as those mentioned above. Moreover important features of non-linear polymers such as crosslinks or branching densities are not obtained through direct titration of chain connection points, but rather from model dependant methods like swelling or dynamic mechanical properties. However, the measurement of the fraction of insoluble polymer, or gel content, is a powerful but simple method to assess the level of connectivity of the polymer. In the first part of this review, we will comment on some of the available techniques on gel fraction determination, as well as on its role on latex film formation and the mechanical properties that are derived therefrom. In the second part, the main kinetic models applied to non-linear emulsion polymerization will be introduced.