A Value Added Finish from Silk Degumming Waste Liquor

Introduction

Sericin is a hot water-soluble macromolecular globular protein. It represents a family of proteins having molecular mass of 10 to 310 kDa. Sericin envelops the fibroin fibre with successive sticky layers that help in the formation of cocoon. Sericin contributes about 20-30% of the total cocoon weight. The sericin protein is made of 18 amino acids most of which have strongly polar side groups such as hydroxyl, carboxyl and amino groups ^1-3.

Figure 1: Composition of Silk Filament

During last decade, sericin has emerged as a biomaterial of commercial value having diverse physiological properties such as antioxidant, UV protection, moisture adsorption, antibacterial activity etc ⁴. The cosmetics industry is using sericin in its moisturizing and skin care products since it inhibits the tyrosinase activity and promotes biosynthesis⁵. Moreover, its high hydroxyl-amino acid content has high water-binding capacity that keeps the skin moist. Masahiro et al.⁶ have reported that consumption of sericin enhances bioavailability of Zn, Fe, Mg and Ca in rats therefore it can be a valuable natural ingredient for food industry.

Some studies have indicated that sericin, exerts inhibitory activity on ultra-violet radiation induced acute damage, posses the biological activity of preventing cell death and promoting cellular growth after acute serum deprivation ⁷. Moreover, sericin has also been found to be useful as a degradable biomaterial, biomedical material and polymers for forming articles, functional membranes, fibres and fabrics ⁴.

Use of sericin as a finishing agent for natural or man made textiles enhances moisture absorption, antistatic properties, softness and comfort ⁸. Filters made of polyamide or polyester fibres coated with sericin have antioxidation and antimicrobial activity, suggesting their potential use as indoor air filters to reduce free radicals and fungi or bacteria contamination⁹. Sericin can be cross linked, copolymerized or blended with other polymers to produce a new range of biodegradable materials with improved properties ⁴.

Thus because of its varied properties, sericin can be used as an additive in food, cosmetics, textiles and pharmaceutical products as indicated in Fig. 2.

Figure 2: Application of sericin in various industries

In the silk processing industry, sericin is removed through a process known as degumming prior to dyeing. This is a fundamental step in the silk processing cycle, which allows silk to gain the characteristic shiny aspect and soft handle that are highly appreciated by consumer. The processing of raw silk produces about 50,000 tons of sericin worlds wide each year that is discarded in the degumming waste liquor ⁴.

Various methods have been proposed ^10-15 to recover this valuable protein material from degumming liquor. Most of them are based on adsorption, precipitation, coagulation, evaporation, chromatography and ultrafiltration for recovering this protein. The recovered sericin can be converted in to the powder form by using freeze drying, tray drying and spray drying techniques. A dry powder product is highly desirable since it not only possesses long shelf life but also requires relatively low transportation cost and storage space.

India is the second largest producer of silk. In general, in India, approximately every year 3,000 tons of sericin is removed through soap and alkaline degumming and discharged in the silk processing waste water⁴. The recovery of the sericin from the degumming waste liquor can give two fold economical benefits to the Indian silk processing industry. At one end it will generate revenue by selling the sericin powder as moistening /antioxidant/ finishing agent to food/cosmetic/textile industry. On the other hand it will reduce the processing cost of effluent treatment.

The aim of this study was to recover the sericin, a valuable protein material from the industrial degumming liquor and utilize it as value added finishing agent for textiles.

Recovery of Sericin from Industrial Degumming Liquor

The industrial waste degumming liquor (alkaline degumming liquor) generated by Bombyx mori silk processing was procured from Nath Brother Exim Ltd, Noida, India. It was centrifuged at 9000 rpm for 60 min. After centrifugation the suspended soap and large particle were removed. The supernatant liquor was taken out and filter through Wattmann filter paper grade 1 (~11mm pore size). This pre-filtered degumming liquor was used as feed solution for cross flow micro-filtration. Micro-filtration membrane of 0.22 pore size was used to separate the soluble soap that had not been removed earlier. Permeate of micro-filtration containing sericin and salt, fed to membrane based ultrafiltration unit. The molecular weight cut off of the membrane was 10 kDa. The ultrafiltration of the partially refined liquor could remove the sodium salts and concentrate the sericin solution. The retained solution was converted into the powder on a spray drier (Model - LU - 227 Advanced, Labultima) keeping inlet temperature at 180C and the atomization pressure of 3.00 kg/cm². A schematic over view of the process is shown in Fig. 3.

Figure 3: Overview of the process

The advantage of spray drying over the other method is that it gives free flowing powder with minimum denaturation. The salient characteristics of the recovered sericin powder are summarized in the Table 1.

Table 1: Characterization of the Sericin Powder

The sericin powder was cream in colour. To check the purity of the sericin powder, nitrogen and protein content of the powder were determined using Kjeldahl method¹⁶. The powder had 9-10% nitrogen content and 58-62% protein content. However, the pure sericin has 15-16% nitrogen content. The ash content of the powder was high that may be attributed to sodium ions bound to the carboxylic groups of sericin i.e., during alkaline degumming process the R-COOH groups present in the sericin are converted to R-COONa and get solubilized. The molecular weight distribution of recovered sericin was estimated with sodium dodecycle sulphate Polyacrylamide gel electrophoresis (SDS-PAGE) ¹⁷ and it was found to be in the range of 14 kDa to 44 kDa.

In order to know the reduction in the pollution load in the discharged liquor due to this method, the COD and BOD of the degumming liquor were estimated according to APHA method 5220C and 5210A¹⁸. The results are summarized in the Table 2. The concentration of sericin in permeate (or final discharge liquor) is reduced after ultra filtration. There is substantial reduction in COD and BOD value (> 85%). These findings indicate that the recovery of sericin from the degumming liquor using above technology also helps in reducing the effluent load generated in the processing of silk.

Table 2: Characteristic of discharge liquor

Application of Sericin on Polyester

The polyester fabric was pre-treated with 15% NaOH (o.w.f) keeping material to liquor ratio of 1:40 at 60⁰C for 30 min to get a weight loss of 5%. This weight loss was expected to give sufficient number of (hydroxyl and carboxyl) end groups so that further treatment can be carried out¹⁹. The pre-treated fabrics were padded (80% expression) with the sericin solution (20 g/l) along with glutaraldehyde (1% v/v) magnesium chloride (1% w/v), acetic acid (0.1% v/v) in a laboratory padding mangle by 2-dip 2-nip process.

The padded fabric was dried and cured. The cured samples were washed (at 60⁰C) and dried. The samples were conditioned for 48 hours prior to performance testing. Antistatic property of the fabric was measured with Static Honestmeter as per Japanese Standard JIS L 1094: 1997. Results were interpreted in the form of half-decay time in seconds. It is half of the time the fabric needs to discharge the static charges imposed by the test machine. The moisture content of the finished samples was measured using Sartorius Moisture Analyzer (MA 51, 98648-002-76). The instrument automatically calculates the moisture content based on the initial and final weight.

The performance properties of the sericin treated samples in terms of moisture content and antistatic property are given in Table 3. It has been found that the original polyester fabric has a moisture content of 0.82%. On pre-treatment with sodium hydroxide the moisture content increases up to 1.47%. At 20 g/l of sericin concentration in padding liquor gave a moisture content of 2.09%.

Table 3: Performance properties of Sericin Treated Fabrics

Absence of polar groups in polyester causes static charge generation. It is estimated that sericin having polar groups can interact with the air moisture and bind the water molecules reducing static build up. The half-decay time of the treated fabric is lower than that of the untreated one (Table 3). This may be due to the increase in moisture content of the fabric in the presence of sericin on the fabric surface. Figure 4 shows the sericin finished polyester garments with improved moisture absorption and antistatic properties.

Figure 4: Sericin finished polyester garments with moisture absorption and antistatic properties

Conclusions

The process for the recovery and production of sericin powder from industrial degumming liquor has been developed at laboratory scale. The recovered sericin powder has molecular weight range from 14 kDa to 44 kDa and 9-10% nitrogen content. The sericin concentration of 20 g/l in padding liquid is sufficient to improve the comfort properties i.e. moisture content (2.09%) and antistatic property (half decay time 2.8 sec) on the polyester fabric without affecting the feel of the fabric. The recovery of sericin from the degumming waste liquor and its utilization on textiles gives a potential socioeconomic advantage to the textile industry.

Acknowledgement

The authors are grateful to Department of Biotechnology, Government of India, for financial support.

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About the Authors:

The authors are associated with the Department of Textile Technology, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi.