Abstract
Polymerization of acrylic acid (AA) with native and hydrolyzed maize starches was carried out via a potassium bromate-thiourea dioxide redox initiation system. The factors affecting the efficiency of the redox system and, in turn, the polymerization process were studied. These factors included the concentrations of AA, KBrO 3, thiourea dioxide (TUD) (and equimolar ratio of the latter two), polymerization temperature and the extent of starch hydrolysis. The behaviour of the apparent viscosity of the cooked poly(AA)-starch composite paste, obtained under different polymerization conditions, was also studied. The polymerization reaction was monitored via the determination of the total percentage conversion (% TC) of AA. The poly(AA)-starch composite was evaluated by calculating the polymer yield, namely the graft yield (% GY), the grafting efficiency (% GE), the percentage homopolymer (% HP) and the total conversion. Results obtained indicated that the optimum conditions, expressed as percent of total conversion are: (a) with native starch (NS): (KBrO 3), 6 mmol/100 g NS; (TUD), 7.4 mmol/100 g NS; (AA), 30% of weight of starch; polymerization temperature, 50 dgC; and polymerization time 30 min. (b) With hydrolyzed starch (HS): (KBrO 3), 4 mmol/100 g HS; (TUD), 4 mmol/100 g HS; (AA), 30%; polymerization temperature, 40 dgC; and polymerization time, 30 min. The results also indicated that, for a given rate of shear, while the apparent paste viscosity of the poly(AA)-NS decreases with increasing concentration of KBrO 3, TUD, and AA (within the studied range), it increases with increasing polymerization temperature from 30 to 50 dgC. Regardless of the polymerization conditions used, the apparent viscosity of NS is higher than that of the poly(AA)-NS composite. On the other hand, the apparent viscosity of the poly(AA)-HS (hydrolyzed starch) composite depends upon the extent of hydrolysis of the starch used as well as the polymerization conditions.
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