Hadi K. Mahabadi

Interfacial/free radical polymerization microencapsulation: kinetics of particle formation

Microcapsules containing pigment and polymer were prepared by dispersing a viscous mixture of pigment, core monomers, initiators and oil-soluble shell monomer in an aqueous solution of surfactants, forming oil-in-water droplets. Subsequently, a water-soluble shell monomer was added to these droplets, encapsulating them via interfacial (IF) polycondensation. These microcapsules were then heated for free radical (FR) polymerization of the core monomers. Effects of primary variables, such as the shearing time during particle formation, surfactant concentration, organic phase concentration, and mode of water-soluble shell monomer addition, were studied. The results indicated that polyvinylalcohol (PVOH), used as the surfactant/stabilizer, reacted with the oil-soluble shell monomers. The depletion of PVOH, especially when PVOH concentration was low, resulted in rapid growth of particle size and, eventually, suspension failure. The kinetic data revealed a particle formation mechanism which consists of two processes. The first process is the formation of an equilibrium particle size by the equilibrium process of particle breakage due to the mechanical shearing force and coalescence due to collisions among particles and surface tension forces. The second process is the reaction between PVOH and oil-soluble shell monomer which leads to the depletion of PVOH and consequently causes more coalescence of particles and a significant increase in the equilibrium particle size. The net effect of these two processes shows an optimum shearing time where the smallest particle size can be attained, and this optimum time is a function of several primary variables. Methods to prevent the reaction and therefore the depletion of PVOH are proposed.

Interfacial polymerization encapsulation of a viscous pigment mix: emulsification conditions and particle size distribution

A viscous organic phase, containing up to 65 per cent solid pigment, was dispersed into water with an emulsifier by a rotor-stator homogenizer and the droplets formed were encapsulated by interfacial polymerization. Microcapsules with volume median diameters d50 ranging from 10 to 25 microns and geometric standard deviation (GSD), from 1.25 to 1.65, were obtained depending on emulsification conditions. Larger impellers gave smaller d50 and slightly narrowed GSD; d50 decreased and GSD increased as volume fraction of dispersed phase is decreased. Higher homogenizer speed and emulsifier concentration decreased d50 but slightly increased GSD. Increasing pigment content in dispersed phase decreased d50 but had little effect on GSD. These effects were assessed quantitatively by fitting an empirical model to the data.

Kinetics of Controlled ‘Living’ Free Radical Polymerization: I. Ideal Case

ABSTRACTThe kinetics of an ideal controlled ‘living’ free radical polymerization (LFRP) are studied using a combination of analytic and numeric methods. It is shown that the ideal LFRP kinetics are characterized by the fast capping/uncapping reactions and the slow propagation of polymer radicals. The polymerization rate is determined by the rate of initiation, monomer and free capping agent concentrations, as well as, the rate of capping/uncapping. The molecular weight is shown to be a linear function of conversion, and the ultimate molecular weight is determined by the initial monomer to initiator concentration ratio. The polydispersity is determined by the ultimate molecular weight and polymerization rate.

Effects of various design parameters on the properties of interfacial/free-radical microencapsulated particles

Dyed or pigmented synthetic microcapsules are employed in a variety of printing, electronic imaging, and non-silver imaging processes. At the Xerox Research Centre of Canada, we have studied the preparation of synthetic microcapsules, with high pigment content. Various options, useful for the microencapsulation process will be discussed with focus on a new hybrid interfacial polycondensation/free radical polymerization process to produce uniform thin polymeric shells surrounding discrete pigmented core particles. The factors which control the formation and the particle size ofcapsules in this process will be outlined. The influence ofpigment loading on the bulk density and flow properties of the microcapsules will be discussed. The effect ofvarious reaction parameters on the molecular and rheological properties ofthe core and shell polymer will be presented.

Bulk Polymerization in Tubular Reactors II. Production of Low Molecular Weight Poly(Methylmethacrylate)

ABSTRACTTubular reactors can potentially provide a simple and economic route for the polymerization of many monomers. However, because they are prone to fouling (accumulation of polymer on the reactor walls), they have not been widely exploited. In this study a pilot scale tubular reactor was constructed and used to investigate the production of low molecular weight poly(methyl methacrylate) by bulk polymerization. Conversion was monitored on-line using a vibrating U-tube densitometer. Low molecular weights were produced using the catalytic chain transfer agent cobaloxime boron fluoride. Earlier work conducted in our laboratory (1) determined that the optimum configuration for a tubular polymerization reactor was vertical orientation with the feed flowing downwards and (2) determined a feasible operating region as a function of molecular weight and conversion. Using this optimum configuration, the pilot scale reactor has been used to verify the operability of a tubular reactor for bulk methyl methacrylate...

Bulk polymerization in tubular reactors. III: Modelling fouling behaviour

ABSTRACTMathematical modelling of the fluid dynamics and polymerization kinetics in a tubular reactor has been conducted. The configuration modelled was a pilot scale reactor oriented vertically with the feed flowing downward. We have previously demonstrated that, because of gravitational effects, reactors with differing spatial orientations or flow directions cannot be considered mathematically equivalent. The configuration modelled is least prone to fouling (accumulation of polymer on the reactor walls). Results from the modelling study showed the assumption of the traditional no-slip boundary condition at the tube wall yields a model incapable of predicting experimentally observed behaviour. In particular the model cannot predict the onset of reactor fouling, nor observed conversions and molecular weight distributions. However, introduction of an empirical expression for wall slip yields a model capable of predicting the behaviour observed experimentally.