silk degumming

4th INTERNATIONAL TEXTILE, CLOTHING & DESIGN CONFERENCE – Magic World of TextilesOctober 05th to 08th 2008, DUBROVNIK, CROATIA

DYEING BEHAVIOUR OF DIFFERENTLY DEGUMMED SILK FIBERS

Vedran ĐURAŠEVIĆ; Waldemar MACHNOWSKI & Anna KOTLINSKA

Abstract: About 100000 tons/year or less than 0,2% of globally produced fibres is silk and the production is likely to continue falling. Nevertheless, all of the production processes involved in manufacture of silk are carefully and scientifically reviewed and improved to enhance economical and ecological viability. This paper reviews two of the most important processes in the production of silk, degumming and dyeing. Silk fibres were degummed by boiling off in soap solution and by using proteolytic enzymes. In order of determining efficiency and differences among these commonly used degumming processes, fibres were subsequently dyed by acid dyes C.I. Acid Red 18 and C.I. Acid Yellow 11 and metal complex dyes C.I. Acid Blue 193 and C.I. Acid Red 211:1. Obtained results showed significantly lower dye exhaustion for soap degummed fibres regardless of dye used or pH value of the fibres as a result of alkaline degumming conditions.Keywords: silk, degumming, dyeing, exhaustion, acid dyes, metal complex dyes.

1. Introduction

Legend of silk tells of a princess Xi Lin Shi drinking a tea in mulberry garden, when a cocoon dropped into her cup. Outer layer of the cocoon was partially dissolved by hot tea, while princess, trying to remove the cocoon with her fingernail, discovered thin long thread. Modern knowledge fairly exceeds the legend. Core of the silk fiber – fibroin is shielded by the protective coating called sericin. Depending on the type, raw silk may contain 15-25% of sericin. Although the sericin is responsible for silk’s harshness, it is useful to manufacturer during the production process acting as a protective layer. The removal of sericin occurs in a process called degumming, which can be entitled as a key process in which all sericin is removed, revealing softness and gloss of silk. Being water soluble, sericin is nowadays removed by boiling off in aqueous soap containing solutions, alkali, detergents or organic acids. Batch degumming of silk is carried out under alkaline conditions in baths containing soap or detergents, which have lately replaced soap because of it’s inability to compensate the acidity of sericin hydrolysis products, which tend to accumulate in the bath and limit the use of degumming baths for weekly cycles. Mechanism of sericin removal can be described as a combination of dispersion/solubilization and hydrolysis, which is a prevailing process caused by strong alkaline compounds added to the bath. Therefore, on-line monitoring of degumming process parameters, such as time, pH, temperature and alkalinity must be carried out on an industrial scale, resulting only in sericin removal without damaging fibroin by setting off hydrolytic degradation. Undesirable fibre degradation appears as loss of aesthetic and physical properties, such as drop of tensile strength, surface fibrillation, poor handle, dull appearance, as well as uneven dyestuff absorption during dyeing and printing processes [1-3]. Along with these conventional methods, over the last twenty years much of the studies dealt with removal of sericin using protease i.e. proteolytic enzymes, among which alkaline proteases performed better than acidic and neutral ones, resulting in complete sericin removal and retaining tensile properties, while improving surface smoothness and lustre. Although displaying higher whiteness, enzyme degummed silk posses higher shear and bending rigidity, lower fullness and softness of handle, when compared to soap and alkali degummed. Higher cost of enzymes compared to chemicals as well as concerns regarding their continuous use in degumming plants have so far limited the development of industrial use, however, a development of an effective degumming process based on enzymes would enable savings in water, energy, chemicals, effluent treatment and would satisfy awareness of legislators and citizens regarding ecological awareness and sustainability of implemented industrial processes [3-6].

This paper reviews enzymatic degumming process, comparing it to standard, conventional method of soap degumming, suggesting differences and improvements gained. Both soap and enzyme degummed fibres were dyed by acid dyestuffs C.I. Acid Red 18, C.I. Acid Yellow 11 and following metal complex dyestuffs C.I. Acid Blue 193 and C.I. Acid Red 211:1, paying special attention to pH of fibres and dye-baths throughout the dyeing process, respectively discussing it’s influence. Dye-bath extinction was monitored during the entire dyeing process over five process points. All conclusions were made after observing the dyeing behaviour of silk fibres and quantifying the amount of dye absorbed.

2. Materials and Methods

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For the comparison purposes of soap degumming to enzymatic treatment, as well as proving the advantages and disadvantages of these methods, 100% pure silk fibres were used in all investigations. SEM of raw silk fibres is shown in Figure 1a. Silk hanks of approximately 15g were prepared and weighed on an analytical scale in order of subsequently determining the efficiency of degumming process. All degumming and dyeing processes were carried out in Ugolini s. r.l.’s Redkrome 400V – DATEX TOUCH sampling equipment. A process scheme of degumming and dyeing process parameters is shown in Figure 1b.

a. b.

Figure 1: SEM image of raw silk – magnification 1000x (a) and scheme of degumming and dyeing of silk fibres (b)

2.1 Soap degumming

Silk hanks were prepared and weighed accurately after drying in an oven for 1h at 110 °C. They were than boiled for 30 min. in a solution consisting of Na-stearat (6g) soap flakes and water (600ml). Solution was then poured off and fibres were boiled for another 30 min in fresh soap solution. Solution was then poured off and remaining fibroin fibres were washed with five separate quantities (200 ml each) of distilled water to remove soap and sericin. Excess water was removed by filter paper and fibres were dryed for 1h at 110 °C prior to weighing. Quantity of removed sericin, expressed in % was obtained using following equation:

(1)

where, S - amount of removed sericin [%], B - mass of fibres before degumming [g], and A - mass of fibres after degumming [g].

2.2. Enzymatic degumming

Silk fibres were prepared and weighed according to the procedure described in 2.1. Subsequent degumming process was carried out in solution consisting Alcalase 2,5 L DX and other auxiliary agents. Contents of enzyme degumming bath and process parameters are shown in Table 1. After degumming samples were washed with five separate quantities (200 ml each) of distilled water. Excess water was removed by filter paper and fibres were dryed for 1h at 110 °C prior to weighing.

Table 1: Contents of enzyme degumming bath and process parameters

Enzymatic degummingAlcalase 2,5 L DX 1,0 g/lNaHCO3 5 g/lNon-ionic surfactant 1,0 g/lTemperature 60 °CTime 60 minpH 9,0Liquor ratio 1:30

InactivationTemperature 80 °CTime 20 min

2.3. Dyeing

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Samples of silk fibres obtained after previously described degumming processes were weighed and cut to exactly 0,5 g after drying in an oven for 1 h at 110 °C. Dyes of different chemical constitution, molecule size and solubility were used, while max values were determined spectrophotometrically over preparation of calibration diagrams based on Beer-Lambert’s law, deriving mathematical functional dependence. Figure 2 shows dyes used in the tests. Matrix solutions were prepared for each dye in concentration of 10 g/l, while each sample was dyed by 1% of dyestuff (on weight of fibres), with liquor ratio 1:50. Kinetics of dyeing was followed over five process points and extinction measured to calculate amount of absorbed dye. Dye-bath samples for spectrophotometric measurement were prepared by sampling 15 ml of dye-bath, then 30 ml of distilled water were added to the pots in order of rinsing dyed silk fibres. Ending by sampling 10 ml and adding to the priory sampled 25 ml. pH of acid dye-baths with C.I. Acid Yellow 11 and C.I. Acid Red 18 was set to 4,5 using CH3COOH, while pH value of metal complex dye-baths was not modified.

C.I. Acid Yellow 11 C.I. Acid Red 18

C.I. Acid Blue 193 C.I. Acid Red 211:1

Figure 2: Equations of investigated dyestuffs

2.4. Scanning Electron Microscopy (SEM)

The surface of silk fibres before and after degumming, as well as after dyeing (C.I. Acid Red 211:1) was examined by the scanning electron microscope JSM – 5200 LV (JEOL, Japan). Observation was carried out with magnifications of 1000x and 5000x, with the accelerating voltage of 20 kV. Images were registered by digital image recording system Semafore. Obtained images are shown in Figures 5 and 6.

3. Results

Tables 2 and 3 show the results of soap and enzymatic degumming processes. Comparing the pH of soap and enzymatic degumming baths, significantly higher mean value of 10,3 is noticed for soap degumming bath, compared to mild 8,6 for enzyme degumming bath. pH values of baths measured before and after degumming processes, regardless of process type or stage did not exhibit significant change. Enzymatic degumming may also be considered as a milder process regarding temperature, while being carried out at mere 60 °C, with a short enzyme inactivation period of 20 minutes at 80 °C. Furthermore, soap degumming requires high process temperature of 100 °C (Figure 1b), which in combination with alkaline treatment conditions caused undesirable pH of silk fibres, shifting the aqueous extract well into alkaline range with a mean pH value of 8,7. Regarding time needed, enzymatic degumming was more time consuming, though

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soap degumming was carried out as a two stage process (Figure 1b). Removal of sericin as a measure of degumming efficiency was almost 1% higher in case of enzyme degummed silk fibres. Table 2: Parameters of soap degumming bath process

Silk hank no. 1 2 3 4 5 6Mass (g) 7,414 7,80 6,881 7,425 6,654 7,558 7,29Mass after degumming (g) 5,650 5,992 5,315 5,641 5,130 5,784 5,59Sericin content (%) 23,79 23,20 22,83 24,02 22,89 23,47 23,37pH of initial degumming bath 10,3 10,5 11 9,8 10,1 9,9 10,3pH of degumming bath after I. step 9,8 10,2 10,8 9,7 9,9 8,7 9,8pH of degumming bath after II. step 10,0 10,0 10,6 9,9 10,2 9,3 10,0pH of aqueous extract; ISO 3071:2005 8,7 8,6 8,6 8,9 8,5 8,9 8,7

Table 3: Parameters of enzymatic degumming process

Silk hank no. 1 2 3Mass (g) 17,32 19,1 23,8 20,07Mass after degumming (g) 13,5 15 18,2 15,57Sericin content (%) 22,1 21,5 23,54 22,38pH of initial degumming bath 8,2 8,6 8,7 8,6pH of degumming bath after process 7,9 8,2 8,3 8,2pH of aqueous extract; ISO 3071:2005 7,3 7,4 7,4 7,4

Figures 3 and 4 show kinetics of dyeing i.e. absorption of dyestuffs in dependence of process time. Results shown refer only to enzymatically degummed silk fibres.

Higher absorption of C.I. Acid Yellow 11, exhibited in Figure 3, as opposed to C.I. Acid Red 18, was contributed by molecule size and number of functional groups (three sulphonic in Acid Red 18, opposed to one in Acid Yellow 11) and more importantly by the fact of Acid Yellow 11 being a more linear molecule (Figure 2).

Comparing Figures 3 and 4 in general, higher dye absorption may be noticed for both of the investigated metal complex dyestuffs, reason of which is to be sought in more appropriate pH of silk fibres to dyeing by this dyestuff group.

Figure 3: Silk fiber dye absorption vs. time for acid dyestuffs

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Figure 4: Silk fiber dye absorption vs. time for metal complex dyestuffs

In Figure 4 dyeing kinetics for group of metal complex dyestuff is shown and the results obtained indicate high dye absorption of these with chromium complex dyes, opposed to acid dyestuffs investigated. More dye absorbed may be noticed for the silk fibers dyed by C.I. Acid Red 211:1, which is the result of molecule size and type and number of functional groups.The reason of exhibiting only the results for enzyme degummed samples, was the high turbidity of dye-baths after dyeing soap degummed samples, as a result of fibre fibrillation and dusting i.e. damage caused by high process temperature and alkaline conditions under which the fibres were degummed [7]. Damage done to the soap degummed fibres can be seen in SEM image in Figure 5a, as uneven fibre surface, with thin visible fibrils on fibre surface.

a. b.

Figure 5: SEM images of degummed silk fibres – magnification 1000x: (a) soap degummed silk fibres (b) enzyme degummed silk fibre

a. b.

Figure 6: SEM images of degummed and dyed silk fibres (C.I. Acid Red 211:1) – magnification 1000x: (a) soap degummed silk fibres (b) enzyme degummed silk fibre

Damage of soap degummed fibres was enhanced in a subsequent dyeing process (C.I. Acid Red 211:1). Submitting fibres to relative high dyeing temperature of 85 °C for 60 minutes caused further visible fibrillation and surface rupture, as seen in Figure 6a. Topography of enzymatically degummed silk fibres shows no changes after dyeing process, as seen in Figure 6b.

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4. Conclusion

In this paper certain advantages of enzymatic over soap degumming were established. These include greater degumming efficiency, expressed over removal of sericin and energy save, achieved through lower process temperature. Milder treatment conditions under which the fibres were processed during enzymatic degumming prevented fibrillation and dusting i.e. fiber damage. Damage to the soap degummed fibres was enhanced in subsequent dyeing process, which was the reason of inability to spectrophotometrically measure dyestuff concentration in dye-bath. Constitution, as well as type of functional groups in dyestuff molecules influenced absorption significantly. Exhaustion of metal complex dyestuffs from the dye-bath was higher then of acid dyes. Regarding acid dyes, smaller and more linear molecules exhibited greater absorption. When dyeing silk fibres, method of degumming should be considered.

5. Acknowledgements

Research presented in this paper has been made within CEEPUS, CII-SI-0217-01-0708 - Intelligent Textile Products of New Generation – Design & Development, program. The authors would also like to express their gratitude to Henryk Wrzosek of the Department of Fibre Physics and Textile Metrology, Technical University of Lodz for the preparing of samples and taking SEM images of silk fibres.

References

[1] Franck, R. R.: Silk, Mohair, Cashmere and Other Luxury Fibres, 0-8493-1311-2, Cambridge, (2001) [2] Shukla, S. R. et al.: Efficiencies of silk degumming process, Colorage, 39 (1992) 4, 31-33, ISSN 0010-

1826[3] Giuliano, F. et al.: Degumming of silk fabric with several proteases, Journal of Biotechnology, 106 (2003)

1, 101-112, ISSN 0168-1656[4] Sharma, H.S.S.: Textile Biotechnology in Europe – Hydrolases and Oxidoreductases in Processing,

AATCC Review, 5 (2005) 10, 44-48, ISSN 1532-8813 [5] Arami, M. et al.: Degumming of Persian Silk with Mixed Proteolytic Enzymes, Journal of Applied Polymer

Science, 106 (2007) 1, 267-275, ISSN 00218995 [6] Sonwalkar, T. N. & Prabhu, J.: Single stage degumming of silk with enzymes, Colourage, 39 (1992) 7,

37-40, ISSN 0010-1826[7] Rouette, H. K.: Silk, Encyclopedia of Textile Finishing, Woodhead Publishing, ISBN 978-3-540-65031-7,

Cambridge, (2001), 2061-2066

Author(s):

Vedran ĐURAŠEVIĆ, B.Sc.University of Zagreb, Faculty of Textile Technology, Department for Technologies of Textile Chemistry and EcologyPrilaz baruna Filipovića 30, HR-10000 Zagreb, CroatiaPhone: +(385) (1) 4877 365 Fax: +(385) (1) 4877 355 E-mail: [email protected]

Waldemar MACHNOWSKI, Ph. D.Technical University of Lodz, Department of Fibre Physics and Textile MetrologyZeromskiego 116, 90-924 Lodz, PolandPhone: +(48) (42) 6313 375 Fax: +(48) (42) 6313 318 E-mail: [email protected]

Anna KOTLINSKA, M. Sc.Technical University of Lodz, Department of Fibre Physics and Textile MetrologyZeromskiego 116, 90-924 Lodz, PolandPhone: +(48) (42) 6313 308 Fax: +(48) (42) 6313 318 E-mail: [email protected]

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