| Lac was a natural mixture which was secreted by lac insect living in the various types of host plants. Lac was also called gum lac or red gum. Lac resin was the main component of lac. In addition, lac also contained lac dye, lac wax, dead insect, protein, sediment and other substances. Lac resin was a macromolecular and polyester resin which was gaven consideration to thermoplastic and thermoset. As the only natural animal resin has been developed and utilized, shellac resin was widely used in military, electronics, paint, food, medicine, cosmetics and other industries because of its good abrasion, oil, salt fog and saline resistance ability and its good adhesion, film properties. Since the new century, with the rise of synthetic industry, in spite of lac resin was impacted by synthetic resin, its application in food, pharmaceutical, cosmetics and other industries was still irreplaceable because of its green, non-toxic, natural and other excellent attributes. In fact, there were more than a half of shellac was bleached shellac in the consumption of lac every year. However, the study on bleaching technique of shellac and bleaching mechanism did not see more both at home and abroad. At present, domestic method of bleaching was still the traditional and extensive sodium hypochlorite bleaching. But, combined chlorine of shellac limited its application in food, medicine, cosmetics and many other industries greatly and the storage cycle was shortened in six months at the same time.On the basis of previous studies, a complete totally chlorine-free(TCF) bleaching process was developed in this paper. Not only the existence of combined chlorine of shellac was avoided in the new process but also TCF bleaching of shellac and green, non-toxic in the bleaching process was realized. More important, a train of thought was provided for shellac bleaching especially chlorine free bleaching.In the lac as raw material, the effect of salting precipitation process like the factors as salt precipitation temperature, concentration of sodium chloride, p H, concentration of shellacsolution, salt precipitation time were studied in this paper. The regression equation was Y=5.8793-0.05408A+0.02017B+0.005917C+0.07167D+0.1468A2-0.08463B2-0.1098C2-0.03863D2+0.047AB-0.0425AC+0.08425AD+0.3138 BC. The optimum process conditions was salt precipitation temperature 10℃, concentration of sodium chloride 0.2501g/m L, salt precipitation time 4h,p H 10.0, concentration of shellac solution 0.0533g/m L. On this basis, the washing series of shellac was determined as 4. The color index from 5.90 to 31.43 and the anthraquinone content from 1.445% to 0.459% after washing. FTIR and DSC was used to charactered shellac sodium salt. The results found that there was little difference between shellac sodium salt and shellac resin. But thermal properties of shellac sodium salt was better than shellac resin. Starting point, peak point and termination point of melting peak was 62.2℃,76.5 and 87.5℃ ℃.Response surface method was used to study effect of factors as concentration of shellac bleaching liquor, use level of hydrogen peroxide, bleaching temperature and bleaching time and use level of stabilizer to shellac bleaching. The regression equation was Y=0.5613-0.2866A-0.895B-0.2851C+0.05783A2+0.7321B2+0.1718C2+0.3198AB+0.117AC+ 0.5603 BC. The optimum process conditions was concentration of shellac bleaching liquor 0.08g/m L, use level of hydrogen peroxide 2m L/g, bleaching temperature 85℃, bleaching time 6h, use level of stabilizer 0.5%. On this basis, H2O2 bleaching shellac was charactered by FTIR, DSC, test of physical and chemical indexes and other methods.The results found that functional types and main molecular structure of H2O2 bleachingshellac had little change after chlorine free bleaching. The starting point and peak point of melting peak on the DSC curve decreased to 65.1 ℃and 29.6℃.The other physical and chemical indexes were color index 0.979<1, thermal life 12.07 min, cold alcohol soluble substance 93.22%, hot alcohol-insoluble substances 1.10%, acid value 156.57 mg/g, softening point 36.5℃.In order to improve the product quality and productive efficiency of modified shellac, the method of ultrasonic was used to intensify the washing process of modified shellac. It was defined to the ultrasonic power, ultrasonic time, pulse time and material/liquid ratio by single factor experiment. The optimal factor of technique about ultrasonic intensify the washingprocess of modified shellac was obtained by experimental design with response surface method(RSM)and optimization to the kinds of factors. The test results show that the best conditions of technique about washing process of modified shellac was ultrasonic power 1200 W, ultrasonic time 8 min, pulse time 5.6 s, material/liquid ratio 1:9. The ash content of modified shellac achieved national grade 1 after washing 3 times.In a one-month cycle, storage stability was studied by determination of color index, hot alcohol-insoluble substances, cold alcohol soluble substances, thermal life, softening point, acid value and chlorine content at diffrent conditions as 4 ℃refrigerator, indoor exposure, indoor seal, outdoor sunshine and indoor dark in 150 days. The results showed that: the color index slightly rised in 5 conditions after 5 months but the range was small and the color index was still less than 1. Except the refrigerator, alcohol soluble declined at all the other conditions. Especially outdoor sunshine, hot alcohol-insoluble and cold alcohol soluble substances dropped to 7.69% and 80.60%. Acid value declined at all conditions and it was most obvious at outdoor sunshine from 163.3mg/g dropped to 142.4mg/g. Iodine value slightly increased firstly and then decreased. Negative value of iodine value showed that anthraquinone pigments damaged in the process of bleaching reacted rearrangement very likely and oxidation product was created at the same time. Softening point rised at all conditions and it was most obvious at outdoor sunshine from 36.5℃ to 59.1℃. Thermal life declined greatly at all conditions and it was from 12.07 min to 5.98 min at sunshine condition.Two bleaching lines were evaluated by comparing the differences of hydrogen peroxide bleaching shellac and sodium hypochlorite bleaching shellac in physical and chemical indexes, the distinction of two bleaching conditions, the costs of two bleaching line. Combined with the results of detection methods which including gel permeation chromatography(GPC), inductively coupled plasma mass spectrometer(ICP), ultraviolet spectrophotometry(UV), the contents of anthraquinone before and after bleaching, saponification separation and detection, the mechanism of hydrogen peroxide bleaching was preliminary inferred: shellac resin was colorless, the bleaching reaction of shellac was the destabilization response of bleaching agenthydrogen peroxide on anthraquinone pigment in shellac resin, the color of shellac resin comes mainly from anthraquinone pigment, which was divided into two types, free pigment and bonding pigment. Free pigment could be get rid through extraction and elution, while bonding pigment and shellac resin were joined by chemical bond connection, which was the reason that shellac resin could not be separated and decolored through extraction. |
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