Regeneration technique optimization of aging bleached shellac by alkaline hydrolysis method

Abstract: Bleached shellac, an important product modality of shellac, is a discolored resin which is bleached from seedlac by decolorant (sodium hypochlorite). It is widely used in food, pharmaceutical and coating industry etc, for example, excipient and moisture resisting agent of food and tablet, coating agents for food freshness, sustained release coating of capsule, priming paint of high-quality furniture and environmental protection coating of artware. In fact, bleached shellac accounted for half of consumption of shellac in the whole world every year. However, bleached shellac was not extremely stable due to the introduction of combined chlorine in the sodium hypochlorite bleaching process. It easily induced aging reaction of agglomeration, color deepening and thermal life reducing when bleached shellac was stored for more than 6 months. When this reaction occurred, bleached shellac would become ageing bleached shellac which was insoluble in ethanol and could not melt even under heating conditions. Apparently, ageing bleached shellac lost useful value for it was difficult to use under normal condition. Structurally, aging reactions of bleached shellac were initiated by active groups such as hydroxyl, carboxyl, aldehyde in molecule of bleached shellac. Some cross-linking reactions, esterification, etherification and condensation, happened under long-term storage or heated condition. Therefore, it could be supposed that as long as cross-linking bonds were destroyed at a certain extent and the original ester functions were retained simultaneously, the regeneration of ageing bleached shellac could be realized in theory. The objective of this investigation was to obtain the regeneration technique of ageing bleached shellac by hydrolysis method. Alkaline hydrolysis was successfully employed in this study to regenerate ageing bleached shellac. Firstly, the experiments of the influences of some single factors, NaOH concentration, liquid-material ratio, hydrolysis reaction temperature and time on the hydrolysis of ageing bleached shellac were performed. Then, the regression model of dual response value, regeneration yield and hot ethanol insoluble substance, was established. Based on this, the optimum conditions for the regeneration of ageing bleached shellac were determined as hydrolysis temperature of 90 ℃, 0.3 mol/L NaOH concentration, liquid-material ratio of 14:1 mL/g, and hydrolysis time of 1.84 h. The regeneration yield of ageing bleached shellac was 76.1%±0.5% and the hot alcohol insolubles reached 0.360%±0.030%. In terms of the regenerated bleached shellac, some physicochemical indices such as hot ethanol insoluble substance, cold ethanol soluble substance, and thermal lifetime, met the requirement of Chinese national standard and their values were 0.360%, 92.6% and 9.80 min, respectively. The color index was 1.68 and met the second level requirement of ordinary standard of bleached shellac. But the acid value rose to 178 mg/g and the softening point reduced to 44.5 ℃, simultaneously. From the actual point of view, the traditional beltline of bleaching shellac completely satisfied the requirement of hydrolysis process of ageing bleached shellac and there was no need to extra added equipment. The final cost of this hydrolysis process was much cheaper and even ignorable compared with the marketable selling price of bleached shellac. In summary, the regeneration method by alkaline hydrolysis is very effective to ageing bleached shellac. It is very useful for recycling and reuse of ageing bleached shellac in commercial production.