Radiation polymerization of methacrylic acid nominally 2.5% crosslinked with divinylbenzene

A process was devised using 60Co irradiation to initiate polymerization of methacrylic acid/divinylbenzene in an acetone-monochlorobenzene solvent from which the required small spheres of crosslinked polymer precipitated as the polymerization proceeded. In the initial small-scale experiments at ICI Australia a very uniform dose rate distribution was used, and factors investigated included the effects of dose rate and total absorbed dose, temperature, stirring rate, initial monomer concentration, and the composition of the solvent. Sub-micron particles and a very low polymerization rate were obtained in acetone alone, whereas a fast uncontrollable polymerization occurred in monochlorobenzene. An optimum solvent composition was established and by adjustment of the above parameters crosslinked polymer spheres within the desired size range were obtained in up to 2-litre batches. A 200-fole scale up was carried out using the AAEC Gatri facilities where problems were encountered with the effects of stirring, uneven dose rate distribution, and temperature control. These difficulties were successfully overcome and micronsized crosslinked polymethacrylic acid beads meeting the Sirotherm specification were produced in good yield on the 100-gallon scale. Both the laboratory investigations and the large scale operation of this process will be outlined and the nature of the product discussed. (author)