A novel approach to the fabrication of bleached shellac by a totally chlorine-free (TCF) bleaching method

Abstract

This study made use of a novel totally chlorine-free (TCF) bleaching approach to prepare bleached shellac. Some single factor experiments were employed to investigate the effects of the main factors in the H₂O₂ bleaching method. A new chlorine-free bleached shellac resin was obtained in the Box–Behnken design responsive surface optimal experimental conditions (pH, 9.0–11.0; 1.33 mL g⁻¹, concentration of 30% hydrogen peroxide; 2.0 h, dropping time of hydrogen peroxide; 7.2 h, bleaching time; 90 °C, temperature). The yield and color index of the bleached shellac obtained under the optimal experimental conditions were 80.84% and 0.5675 respectively. The result of the physicochemical property test, IR spectrum and DSC characterization indicate that the properties of H₂O₂ bleached shellac are similar to commercially refined shellac resin. Moreover, it is noteworthy that the H₂O₂ bleached shellac was remarkable in alcohol solubility and thermal lifetime tests. It had a higher acid value and a lower softening point compared with commercially refined shellac resin, which can be applied safely in many fields including food and medical industries without chlorine residues.

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Introduction

Bleached shellac is a commercial light-colored resin product which can be obtained from bleached sticklac or seedlac.^1,2 The annual worldwide output of the bleached shellac could reach several thousand tons.^3,4 It retains the excellent properties of natural shellac and its light-colored characteristic,^5,6 and is widely used in the food,^7–11 pharmaceutical^12–16 and many other industries.^17–22 With the increasing concern about food and medicine safety in recent times, bleached shellac, a natural product, could be used as a seal, waterproof agent, green paint etc.^23–26 Sodium hypochlorite is used as decolourant to prepare bleached shellac in the conventional method after which combined chlorine is introduced to the shellac molecule in the bleaching process.^27–29 Combined chlorine is an organochlorine and its potential harms have been gradually recognized.^30,31 The bleached shellac containing chlorine is not suitable in some applications, particularly in food and pharmaceutical industries.^32,33

Therefore, the fabrication of bleached shellac by the TCF bleaching method is very important and provides opportunities for the shellac industry. Hydrogen peroxide, a green (environmentally friendly) bleaching agent, has high reactivity and meets the requirement^34–37 of being totally free from chlorine. However, there is a problem: the bleaching reaction that occurs under alkaline condition and acidulous hydrogen peroxide is easily neutralized by alkali in the bleaching solution. So, with the occurrence of neutralization reaction, the bleaching reaction may easily fail because the bleaching efficiency of hydrogen peroxide is reduced quickly and shellac is separated out from the bleaching solution. In view of this, the pH of the bleaching liquor may be the key factor. It needs to be controlled during the bleaching process to make sure that the bleaching reaction is performed under optimal pH conditions.

Therefore, the TCF bleaching technique of making shellac resin can be processed and optimized by single factor experiments and the Box–Behnken design responsive surface optimal experiment. Meanwhile, the physical and chemical properties of the obtained chlorine-free bleached shellac have been characterized to provide data for the application of bleached shellac.

Experiment

Materials and chemicals

Seedlac, the commercially refined shellac resin and bleached shellac product (standard substance with color index 1.0) obtained by used sodium hypochlorite as the bleaching agent, was purchased from Kunming Shellac Bio-technology Co., Ltd. (Kunming, China). Other chemical agents were of analytical grade and were purchased from Aladdin Industrial Co. Ltd. (Shanghai, China).

Fabrication of bleached shellac by totally chlorine-free (TCF) bleached method

The individual factor experiment. As shown in Fig. 1, 30 g of commercially refined shellac was added to 200 mL of Na₂CO₃ solution (0.15 mol L⁻¹) and dissolved in an oil bath at 85 °C. The pH of the shellac solution was adjusted to different values (7.0, 8.0, 9.0, 10.0, and 11.0) by mixing the 10% sulfuric acid with NaOH solution (6 mol L⁻¹). A colorimeter (NH310, 3nh, China) was employed to measure the change of bleaching efficiency of bleaching liquor with the changing pH value of solution and determine the suitable range of pH for conducting shellac bleaching. The determining method was used in the procedure as follows: sodium hypochlorite bleached shellac with color index of 1.0 was applied as the standard to prepare the bleached shellac sodium carbonate solution which had the same concentration with the bleaching liquor as the standard sample to measure the color differences of the shellac bleaching liquor every 20 min during the bleaching process for 2 h.

Fig. 1 Schematic illustration of bleaching process.

The stabilizers made from the mixture of Na₂SiO₃ and MgSO₄ in the ratio of 4 : 1 (w : w), namely 0.6 g and 0.15 g, were added to the solvent, and 30% H₂O₂ solution (volume 20, 30, 40, 50 and 60 mL; equivalent concentration 0.667, 1.00, 1.33, 1.67 and 2.00 mL g⁻¹) was injected into the solvent at different times (2, 3, 4, 5 and 6 h). Finally, the bleaching procedure was carried out at varying times (3, 4, 5, 6, 7, 8, and 9 h) and temperatures (50 °C, 60 °C, 70 °C, 80 °C and 90 °C).

After the bleaching procedure was completed, the liquor was cooled down and filtered by adding diatomite. After acidification with 5% sulfuric acid, washing and freeze drying, the bleached shellac was weighed to measure the moisture content. The yield and color index of the bleached shellac was calculated by using the method in the section above.

The yield of the bleached shellac is calculated as:

Y represents the yield of the bleached shellac; M₀ stands for the mass of the weighed commercially refined shellac; M₁ is the mass of the bleached shellac after bleaching; W₀ is the moisture content of the alkaline extracted shellac; W₁ is the moisture content of the bleached shellac.

The color index of the bleached shellac was determined using the ultraviolet spectrophotometric procedure (UV, DU800, Beckman, United States) as the method provided by Chinese national GB standard (GB/T8143-2008).³⁸

The Box–behnken design responsive surface method (RSM) experiment. The centre and range of optimization were determined through single factor experiments. Based on the Box–Behnken composite theory, an optimization experiment design was carried out by using Design Expert 8.0.6 Software to establish regression equations and surface plots. The optimum technological conditions were calculated according to the fitting equation. Based on the results of these single factor experiments, RSM was employed to perform the optimization with three factors and three levels by considering the concentration of H₂O₂, bleaching time and bleaching temperature as independent variables as well as the yield of bleached shellac and the color index as response values. The data ranges of experimental factors are presented in Table 1. The verification test was conducted based on the optimum technological conditions obtained by the optimization experiment. By comparing the experimental results with the results predicted by the fitting equation, the errors were computed to evaluate the gap between the fitting equation and the actual operation.

Table 1 The experiment factor level and coding

Independent variable		Coding level
		−1	0	1
A	H2O2	1.00 mL g^−1	1.33 mL g^−1	1.67 mL g^−1
B	Bleaching time	4 h	6 h	8 h
C	Bleaching temperature	70 °C	80 °C	90 °C

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Analysis of totally chlorine-free (TCF) bleached shellac. The other physicochemical properties, including hot ethanol insoluble substance, cold ethanol soluble substance, acid value, thermal lifetime, of bleached shellac were determined according to the Chinese national GB standard (GB/T8143-2008).³⁸ It must be emphasized that the acid value was determined by the roboticized titrator (904, Metrohm, Switzerland) which replaced the manual titration in the GB standard. Fourier transform infrared spectroscopy (FTIR, Tenson 27, Bruker, Leipzig, Germany) was utilized to analyze the shellac prepared under the optimum technological conditions. The test conditions include scanning range of 4000–400 cm⁻¹, resolution with 4 cm⁻¹ and cumulative scanning for 32 times. The determination of thermal properties was conducted by differential scanning calorimeter (DSC, 200F3, Nestch, Germany). Glass transition temperature was obtained using this method.³⁹ The test conditions are as follows: to eliminate the effect of the thermal history, first, the temperature was increased to 35 °C from 0 °C (the initial temperature) at a heating rate of 7 °C min⁻¹ and kept at 35 °C for 2 min. Next, it was reduced to −20 °C at a cooling rate of 7 °C min⁻¹ and remained at this temperature for 2 min. Finally, it was increased again to 400 °C at a heating rate of 10 °C min⁻¹.

Results and discussion

As shown in Fig. 2, the pH had a significant impact on the bleaching effect. Around the pH range of 7.0–11.0, the bleaching efficiency of H₂O₂ improved significantly as the pH value increased. When the pH was >9.0 and the bleaching time was over 80 min, the color index of the bleaching liquor decreased rapidly. This suggests that when the pH was >9.0, the bleaching effect of the H₂O₂ improved greatly.⁴⁰ But when the pH was >11.0, the bleaching effect was not too high. This indicates that shellac is an alternating copolyester which consists of aleuritic and sesquiterpene acids, and its molecule monomers connected by ester bonds⁴¹ (Fig. 3) are easily hydrolyzed at high pH. Therefore, the pH value of the shellac bleaching liquor should be kept in the range of 9.0 to 11.0.

Fig. 2 Influence of pH to bleaching effect.

Fig. 3 Molecular structure of shellac resin.

As shown in Fig. 4a, within a certain range, as the H₂O₂ increased, the color index of the bleached shellac decreased and the bleaching effect tended to be preferable, while the bleaching yield reduced at the same time. When the concentration of H₂O₂ was 1.33 mL g⁻¹, the bleaching yield curve tended to be balanced and the color index was close to 1.0. However, when the concentration of H₂O₂ increased to 2.00 mL g⁻¹, the color index of the bleached shellac continuously decreased to 0.48 (less than 1.0). To optimize the concentration of H₂O₂, the maximum bleaching yield was expected to be found on the premise that the color index of bleached shellac could meet the standard. Thus, the range of 1.00–1.67 mL g⁻¹ could be selected as the optimization range of the variable for the next optimal experiment.

Fig. 4 Influence of single factors on the yield and color index of shellac TCF bleaching.

In Fig. 4b, when the dropping time of H₂O₂ was longer than 2.0 h, the bleaching yield and color index showed little change under the conditions proposed in this paper. In the range of 2 to 6 h, all the corresponding color indexes were less than 1.0, meeting the standard of refined bleached shellac Grade I in GB/T8140-2009. Besides, the change in the bleaching yield was only by ±4%. The influence of the dropping time was insignificant. Hence, there was no need to optimize the factor except in choosing the dropping time of H₂O₂ as 2 h. This indicates that the adsorption of H₂O₂ by the stabilizer was very likely to be saturated because of high dropping speed of H₂O₂. This made the bleaching reaction difficult to control and unsafe. Therefore, the dropping time of 2 h was proved to be more suitable.

Fig. 4c showed that within the given range of the bleaching time, the yield of shellac bleaching changed slightly and peaked at 5–7 h of bleaching reaction. Meanwhile, the color index kept decreasing with the bleaching reaction and tended to be generally stable after 7 h. This shows that it met the prescribed requirements for refined bleached shellac Grade I according to GB/T8140-2009. After 3 h of reaction, prolonging the bleaching time had no significant influence on the bleaching effect. On the condition that the color index meets the standard, maximizing the yield of shellac bleaching can expand the optimization range of the bleaching time. Thus, taking the change of color index and yield into account, the optimization of the variable ranging from 4 to 8 h was the best choice.

In the range of 50 °C to 90 °C, the color index of the bleached shellac decreased with rising temperature under the same condition (see Fig. 4d). It was obvious that the higher temperature was conducive for shellac bleaching. However, the yield of the bleached shellac declined with increasing temperature, which demonstrates that it is essential to find the balance between the yield and color index of the bleached shellac. Therefore, the bleaching temperature needs to be optimized. Owing to the high color (>2.0) index at higher temperatures (>70 °C), it is better to set the optimization range in the range of 70 °C to 90 °C for the next optimal experiment.

Dual response modeling for TCF bleaching process of shellac

After the single factor experiments were completed, the concentrations of H₂O₂ (1.00–1.67 mL g⁻¹), bleaching times (4–8 h) and bleaching temperatures (70–90 °C) were selected as three key factors in the TCF bleaching process of shellac for the next optimal experiment. Taking bleaching yield and color index as response values, an optimization test with three factors and three levels was conducted based on the Box–Behnken composite theory using RSM. The optimal experiment was designed according to Box–Behnken theory and the results are listed in Table 2. Design Export 8.0.6 Software was used to conduct quadratic polynomial fitting of nonlinear regression. The equation of the bleaching yield is: Y = 57.18 − 5.77A − 0.94B − 8.27C − 0.67AB − 0.79AC + 1.82BC + 5.62A² + 8.45B² + 16.25C². Similarly, the obtained equation for color index I is: I = 0.99 − 1.04A + 0.018B − 0.95C − 0.002AB + 0.84AC − 0.12BC + 0.48A² − 0.083B² + 0.70C² − 0.54A²C + 0.61AB².

Table 2 Box–Behnken design and experiment results

Run order	A, H2O2, mL	B, bleaching time, h	C, bleaching temperature, °C	Bleaching yield/%	Color index
1	1	−1	0	68.26	0.8576
2	1	1	0	63.51	0.8953
3	1	0	−1	82.14	1.6975
4	0	−1	−1	91.93	2.4222
5	0	−1	1	72.15	0.7474
6	−1	1	0	75.56	1.7709
7	1	0	1	63.61	0.3998
8	−1	0	1	77.54	0.8002
9	0	0	0	54.99	0.9692
10	−1	−1	0	77.65	1.7254
11	0	1	1	75.46	0.5432
12	0	0	0	55.86	1.0017
13	0	1	−1	87.96	2.6854
14	−1	0	−1	92.9	5.4639
15	0	0	0	60.68	0.9913

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The acquired coefficients of the predictive equation and the variance are presented in Table 3. The influence of each factor was judged by P value, i.e., when the P value was less than or equal to 0.05, the influence of the factor was significant.⁴² As indicated in Table 3a, for the linear terms, the influences of bleaching temperature and the concentration of H₂O₂ reached significant levels. The degree of influence is as follows: bleaching temperature > the concentration of H₂O₂ > bleaching time. For the interaction terms, only the interactive influence of the concentration of H₂O₂ and the bleaching time was significant, and the degree of influence indicate a descending order as follows: AB > BC > AC. The bleaching yield was significantly impacted by all quadratic terms in the order of C² >B² > A². By analyzing the degree of influence of the optimizing factors selected in Table 3 on the response values, it can be shown that the independent variables and the ranges were able to reflect the variation of the bleaching yield with the change in the factors mentioned using this model. Among the parameters, the lack of fit was 0.8715 (much higher than 0.05), which suggests that the lack of fit was insignificant. Meanwhile, the regression coefficient of the model was R²_Adj = 0.9662, which indicates that the fitting degree of the regression equation was favorable.^43,44

Table 3 The variance analysis of bleaching yield and color index

(a) Bleaching yield as response value		(b) Color index as response value
Source	P value	Source	P value
Model	0.0003	Model	<0.0001
A	0.0008	A	<0.0001
B	0.2907	B	0.0399
C	0.0001	C	<0.0001
AB	0.0001	AB	0.8044
AC	0.5118	AC	<0.0001
BC	0.1659	BC	0.0005
A^2	0.0048	A^2	<0.0001
B^2	0.0008	B^2	0.0016
C^2	<0.0001	C^2	<0.0001
—	—	A^2C	<0.0001
—	—	AB^2	<0.0001
Lack of fit	0.8715	Lack of fit	0.6575
R^2 = 0.9880 R^2Adj = 0.9662		R^2 = 0.9999 R^2Adj = 0.9999

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Table 3b shows that the influence of the bleaching kinetics model, with the concentration of H₂O₂, bleaching time and bleaching temperature as influencing factors on the color index of bleached shellac, achieved an extremely significant level. For the linear terms, the influences of the concentration of H₂O₂ and bleaching temperature were extremely significant, while that of bleaching time was significant. Among the interaction terms, the interactive influence of the concentration of H₂O₂ and bleaching temperature (AC), and the bleaching time and bleaching temperature (BC) were extremely significant, while that of the concentration of H₂O₂ and bleaching time (AB) was below the significant level. Besides, all quadratic terms showed extremely significant levels including the correction terms A²C and AB². Taking all the model parameters into account, the lack of fit was 0.6575, which was greater than 0.05, and the fitting coefficient of the model equation was R²_Adj = 0.9999, suggesting that the regression equation exhibited preferable fitting degree.⁴³

3D surface and contour analysis diagrams for the influence of each group of interactive parameters on the yield and color index of bleached shellac were generated according to the regression equation, as shown in Fig. 5.

Fig. 5 3D surfaces and contours of RSM for the interactive influences of each factor on bleaching yield and color index.

From the 3D response surface plots of the yield of bleached shellac and each factor in Fig. 5a, it can be seen that the concentration of H₂O₂, bleaching temperature and bleaching time were the key factors influencing the yield of bleached shellac. In addition, all the yield bleaching curves revealed a tendency to form an inverse parabola under the interactive influence of each factor, which means that the influences of the three factors should be comprehensively considered to find the optimum balance point for the yield of the bleached shellac. However, since the yield of the bleached shellac was not the only indicator to be considered in the model, it was essential to ensure that the color index of the bleached shellac could meet China's national standard while maximizing the yield of the bleached shellac. Therefore, the model needed to be further optimized with color index as the response value.

Fig. 5b revealed that the influence of the interaction factors on the color index of the bleached shellac was in a monotonous trend. This was attributed to the mode of influence of each factor on the bleaching effect. For example, the concentration of H₂O₂ and the bleaching temperature were directly proportional to the color index of the bleached shellac, while the influence of the bleaching time was closely associated with the concentration of H₂O₂ and the bleaching temperature.

The optimum conditions for TCF bleaching process of shellac

Under the pH control model, the final optimum technological conditions for H₂O₂ bleaching of shellac were acquired as follows: the dropping time of H₂O₂ was 2 h, the concentration of H₂O₂ was 1.33 mL g⁻¹, the bleaching time was 7.19 h and the bleaching temperature was 90 °C. The prediction results of the model include: the color index, which is 0.5, and the bleaching yield, which is 81.23%.

Verification experiment under optimum conditions

The verification test was performed under specified optimum technological conditions: the dropping time of H₂O₂, the concentration of H₂O₂ and the bleaching time were 2.0 h, 1.33 mL g⁻¹, and 7.2 h respectively at 90 °C. The bleaching process and the TCF bleached shellac are shown in Fig. 6. We found that the yield and color index of the bleached shellac were 80.84% and 0.5675 respectively, while the expected values were 81.23% and 0.5, with errors of 0.48% and 13.5% (less than 20%) respectively.⁴⁵ It was obvious that the error of the bleaching yield between the measured and the expected values of the model was very small and the color index was able to meet the most stringent requirement, less than 1.0, for color index of GB/T8140-2009.

Fig. 6 Bleaching process of shellac in optimum condition.

Characterization of H₂O₂ bleached shellac

As indicated in Fig. 7, the hot ethanol insoluble substance of H₂O₂ bleached shellac was only 0.35%, while the cold ethanol soluble substance was 98.4%. This shows that the alcohol solubility of H₂O₂ bleached shellac is better than that of commercially refined shellac resin. Also, the thermal life was 4.45 min, meeting the requirements of Chinese national standard GB/T8140-2009. Further, the new bleached shellac has some novel features. For instance, the acid value of H₂O₂ bleached shellac was 180 mg KOH per g which is higher than the commercially refined shellac resin with a value of 72.5 mg KOH per g. This suggests that shellac resin was oxidized apparently under the optimum condition during the bleaching of the chromophoric groups. Since the softening point of the new shellac resin reduced from 68.1 °C to 47.9 °C, H₂O₂ bleached shellac had its own distinctions derived from the novel bleaching process. It can be applied in fields which demand stronger hydrophilicity and easier softening than shellac resin, and in industries which strictly implement totally chlorine free environment such as food, medicine and cosmetics.

Fig. 7 Physicochemical properties of H₂O₂ bleached shellac and commercially refined shellac resin.

As shown in Fig. 8, based on infrared spectrogram, the main molecular framework of H₂O₂ bleached shellac is consistent with commercially refined shellac resin. This demonstrates that H₂O₂ bleached shellac is one of the new species of shellac resin. However, there are differences in the main absorption peaks between the two shellac resins: 1565 cm⁻¹ in commercially refined shellac and 1415 cm⁻¹ in H₂O₂ bleached shellac.⁴⁶ Firstly, the peaks at 1565 cm⁻¹ are C=C stretching vibration absorption peaks of aromatic hydrocarbons carbon framework. Also, the aromatic hydrocarbon in alkaline extracted shellac resin is anthraquinone chromogenic material. Evidently, the disappearance of this peak in H₂O₂ bleached shellac shows that a bleaching reaction assuredly occurred in the process. To an extent, it supports the theory that the main colored constituents of shellac are laccaic acids^47,48 (pigments in shellac resin, as shown in Fig. 9). Secondly, a new absorption peak, 1415 cm⁻¹, was found in H₂O₂ bleached shellac. It is O–H bond deformation vibration absorption peak of carboxylic acid. Therefore, a new carboxylic acid was generated in the bleaching process of shellac. The hydroxyl or aldehyde of shellac resin was simultaneously oxidized when hydrogen peroxide attacked the chromophore of anthraquinones. Other absorption peaks were assigned as follows:⁴⁹ stretching vibration absorption peak of hydroxyl at 3425 cm⁻¹, stretching vibration absorption peak of the hydroxyl C–O bond at 1042 cm⁻¹, C–H stretching vibration absorption peaks of methyl and methylene at 2932 cm⁻¹ and 2857 cm⁻¹, deformation vibration absorption peak of olefin in C–H bond surface at 1042 cm⁻¹, stretching vibration absorption peak of carbonyl at 1718 cm⁻¹, O–H bond deformation vibration absorption peak of carboxylic acid at 1411 cm⁻¹ and stretching vibration absorption peak of C–O bond of aromatic oxide bond at 1146 cm⁻¹ and 1254 cm⁻¹.

Fig. 8 FTIR of H₂O₂ bleached shellac and commercially refined shellac resin.

Fig. 9 Laccaic acids in the commercially refined shellac resin.

DSC curves of H₂O₂ bleached shellac and commercially refined shellac are shown in Fig. 10. By analyzing these curves, it was discovered that the glass transition of these two kinds of shellac occurred at the temperature range of −20 °C to 50 °C. The glass transition temperature of commercially refined shellac was 32.23 °C, which is higher than that of bleached shellac, with a glass transition temperature of 26.50 °C. Thus, stress relaxation of intermolecular rearrangement was more likely to happen in H₂O₂ bleached shellac than in commercially refined shellac.³⁹ It also suggests that H₂O₂ bleached shellac has higher requirements regarding storage conditions. As for the melting process, there were significant differences between H₂O₂ bleached shellac and bleaching materials. There were two relatively obvious melting and endothermic processes, at 0–100 °C and 150–350 °C within the test range of 0–400 °C. The first endothermic peak of commercially refined shellac was obvious and sharp with initial, final and peak temperatures of 60.1 °C, 74.7 °C and 68.3 °C respectively, while the temperature range of the second one was greater in a smooth peak shape with initial and final temperatures of 182.8 °C and 380.2 °C respectively, but without a clear peak value. The thermal absorption characteristics of H₂O₂ bleached shellac were opposite those of commercially refined shellac. In other words, the first endothermic peak was smooth with initial and final temperatures of 19.1 °C and 119.5 °C respectively. Its peak value temperature was 68.4 °C, which is close to that of commercially refined shellac. However, the second peak was sharper than the first one, with initial, final and peak temperatures of 121.4 °C, 224.3 °C and 222.5 °C respectively. Based on the results, it can be said that the components of high melting point in H₂O₂ bleached shellac increased, and at the same time, the thermal response temperature of the components of low melting point decreased greatly (with initial melting temperature of 19.1 °C) compared with that of commercially refined shellac. The result is consistent with that of IR spectrum, which shows that the amount of carboxyl increased. This suggests that shellac resin was oxidized during the H₂O₂ bleaching process.

Fig. 10 DSC analysis of H₂O₂ bleached shellac and commercially refined shellac resin.

Conclusions

After the single factor and response surface optimal experiments, the TCF bleaching approach under controlled pH mode was developed and the optimum technological conditions were obtained as follows: pH ranged from 9.0 to 11.0, dropping time of H₂O₂, concentration of H₂O₂ and bleaching time were 2.0 h, 1.33 mL g⁻¹, and 7.2 h respectively at 90 °C. Also, the acquired color index of the H₂O₂ bleached shellac and the bleaching yield were 0.5675 and 80.84% respectively. The IR spectrum showed the difference between H₂O₂ bleached shellac and commercially refined shellac, which indicate that the main molecular structure of the two resins are basically consistent. As the DSC curves showed, the single melting peak temperature (68.3 °C) was translated into double peaks of H₂O₂ bleached shellac (68.4 °C and 223 °C). The acid value of the shellac changed from 72.5 mg KOH per g (before the bleaching process) to 180 mg KOH per g (after the bleaching process). Thus, the alcohol solubility of the bleached shellac was better than that before the process. Above all, the H₂O₂ bleached shellac resin without combined chlorine almost reserved the properties of commercially refined shellac, and could be safely applied in the food and medical industries.

Acknowledgements

This investigation was supported by the National High Technology Research and Development Program of China (863 Program, 2014AA021801) and the Fundamental Research Funds for the Central Non-profit Research Institution of CAF (riricaf 2014005M). We express our sincere gratitude to everyone who joined in this research.

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