Comparison of Degumming Techniques of Silk

D. Saravanan & G. NalankilliDepartment of Textile Technology

Bannari Amman Institute of TechnologySathyamangalam Erode Dist

Introduction

The various processes involved in the raw silk manufacture

include stifling, cooking, reeling and re-reeling. The sericin

in the silk forms a cementing layer on the surface of the twin

filaments facilitating good adhesion to keep the filament intact.

The gum present in the silk filament is, chemically, a non-

filamentous protein because of which raw silk possesses neither

luster nor the softness that are commonly associated with silk.

Besides fibroin and sericin, raw silk also contains other natural

impurities, which includes small amount of silk wax (0.4 – 0.8

percent), inorganic matter (0.7 percent), carbohydrates (1.2 –

1.6 percent) and coloured pigment (0.2 percent). The nature of

pigment varies with the type of silk [1]. Colour / pigments in

the domesticated silk appear prior to cocoon formation (yellow

pigments, green pigments) or one or two days after the cocoon

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formation (brown pigments) which is mainly decided by dietary

nature of the worms

Sericin and Fibroin

Sericin plays a major role in the structure of cocoon, binds the

twin filaments together and constitutes about 25% weight of the

cocoon. It is soluble in hot water and gels on cooling. Sericin

has white, greenish or golden-yellow colour depending upon the

breed of the silk spinner. Sericin is a kind of globular protein

with mainly amorphous structure without any crystallization

taking place in it eventhough it is subjected to a similar stress

levels while spinning. Sericin is rich with amino acids such as

serine, glycine and aspartic acid totaling to the extent of 2/3.

These proteins are synthesized, secreted and stored in the middle

gland, form a sheath around the fibroin core during spinning of

the silk filament. The main function of the sericin is to lower

the shear stress and to absorb the water squeezed from the

structured fibroin during the process of fibre formation and

extrusion [2, 3].

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In the fibre, fibroin chains are aligned along the fibre axis,

held together by a close network of inter-chain hydrogen bonds

amount the –(Ala-Gly)n – sequences leading to formation of beta

sheet crystals, besides the salt linkages / electrovalent bonds

formed between the amino acids of adjacent chains such as

aspartic, glutamic acid and lysine, arginine.

The crystallites of mulberry silk contain both alpha and beta

forms to the extent of 63 – 67 %. Swelling and reactivity are

the lowest in the isoelectric region of pH 4 – 5 [4].

Isoelectric point of sericin is pH 4.0 - 4.1 whereas it is 4.9

for fibroin [5, 6], which is in resonance with higher proportion

of ionizable acidic groups. Compared to fibroin, sericin is

richer in cystine, which varies in gland and cocoon as well as

among the species of silk worms. The sericin content of reeled

filament has been found to vary from the outer to inner layers.

The density of the sericin lies around 1.41 g/cc with a degree of

crystallinity of approximately 15 % and the moisture regain of,

around, 16%. In contrast to fibroin, more polar groups are

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available in sericin of which 60% are hydroxyl groups, 30% are

acidic groups and 10% are basic groups [5, 7].

Sericin Fraction

The sericin content varies among the various types of the natural

silk with the highest content existing in the mulberry silk [8].

Silk Sericin content (%)Mulberry 20 – 30 Tasar 5 – 15 Muga 7 – 8 Eri 4 – 5

Three different fractions are obtained from sericin based on

their relative solubility and histological staining, which

reveals existence of a three distinct layers of sericin in the

gland [5, 6]. Sericin of tasar silk is reported to contain at

least nine kinds of proteins laid down in a wavy form rather than

in layers. Recently, four stratified fractions have been

obtained from sericin identified using differing rates of

dissolution in hot water and UV absorption methods.

Sericin A – Soluble at pH 4 but insoluble in 75 percent alcoholSericin B – Sparingly soluble in hot water, resists proteolysisSericin C – Extremely insoluble in common solvents and isunaffected by enzymes.Sericin D – Small fraction, soluble in 75 percent alcohol

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Readily soluble fraction shows random coil conformation and

lesser soluble fraction shows tendency towards beta conformation

and increase in specific gravity and degree of crystallinity.

Association of the inner most fraction of sericin with a wax

constituent secreted by the gland accounts for its lower

solubility. The two outer layers are removed by alkali while the

third layer, which intimately attached to the fibroin is very

difficult to remove, requires longer duration [9]. Sericin

content of the natural silk can be estimated by boiling silk

using Marseille soap and calculating the weight loss [10]. Table

1 gives the composition of the various amino acids present in the

sericin component and its comparison with fibroin and wool

keratin.

Table 1 Amino Acid Composition (mole %) of sericin and fibroin

Amino Acid Sericin I

SericinII

SericinIII

SericinIV

Whole

Fibroin

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Sericin

Glycine 13.21 12.81 15.69 11.89 13.9 43.7Alanine 4.68 6.69 6.68 9.3 5.9 28.8Valine 2.97 2.21 3.21 4.16 2.7 2.2Leucine 0.86 0.96 1.27 6.26 1.1 0.5Isoleucine 0.59 0.57 0.85 3.50 0.7 0.7Serine 34.03 36.64 28.15 12.40 33.4 11.9Theronine 10.34 8.48 11.36 7.25 9.7 0.9Aspartic Acid 16.94 16.95 16.13 12.64 16.7 1.3Glutamic Acid 4.73 3.64 4.09 11.32 4.4 1.0Phenylanine 0.45 0.44 0.50 2.83 0.5 0.6Tyrosine 2.53 2.43 3.15 2.45 2.6 5.1Lysine 3.28 3.29 2.64 7.11 3.3 0.3Histidine 1.25 1.22 1.49 1.87 1.3 0.2Arginine 3.20 2.65 3.68 3.93 3.1 0.5Proline 0.58 0.63 0.66 2.75 0.6 0.5Tryptophan 0.19 0.20 0.25 0.23 0.2 0.3Cystine 0.17 0.15 0.12 0.13 0.1 0.2Methionine 0.04 0.04 0.04 0.12 0.04 0.1Hydroxy Amino Acids

46.90 47.54 42.66 22.10 45.7 17.9

Acidic Amino Acids

21.67 20.59 20.22 23.96 21.1 2.3

Basic Amino Acids 7.73 7.16 7.81 12.91 7.7 1.0Amino Acids with Sulphur Containing Side Groups

0.1 0.2

Amino Acids with Polar Side Groups

76.3 75.29 70.69 58.97 74.6 21.2

Amino Acids with Non-polar Side Groups

23.74 24.70 29.27 41.03 24.6 76.5

Degumming of Silk

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Degumming is defined as a process involving “ the removal of

sericin (silk gum) from silk yarns or fabrics, or from silk waste

prior to spinning at controlled conditions intended to have

little or no effect on the underlying fibroin” [8, 11, 12, 13,

14, 15]. Until 1879, silk degumming was carried out using the

solution of straw ash extracts which yielded good whiteness also.

Fibroin is water insoluble while sericin is soluble at elevated

temperatures but swells in presence of many chemicals [16].

Addition of surface active compounds like soap or nonionic

detergents aids the removal of sericin and also improves the

wetting properties of the degummed material. Hydrolysis prevails

when strong alkaline compounds are added to the degumming bath

and therefore control must be exercised over time, pH temperature

and concentration of the agents, which otherwise can adversely

affect the fibres under treatment. The removal of sericin in

chemical degumming is a combination of various effects such as

dispersion, solubilization and hydrolysis of different sericin

polypeptides. Fibroin is sensitive in alkali while sericin is

soluble in alkali [9]. This can lead to fibre degradation and

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subsequent loss in aesthetics and physical properties such as

dull appearance, surface fibrillation, poor handle, drop in

tensile strength and uneven dyestuff absorption during dyeing and

printing. Degumming can be carried out in a washing machine,

star frames and dyeing machine [17, 14, 18]. Fabrics can be

treated in open width form also, which is useful for printing

purpose [4, 9]. Tasar silk is more difficult to degum than

mulberry silk because of more mineral matters present in tasar

silk and harsh and chemically resistant nature [19, 20]. Table 2

gives the various methods followed for degumming.

Table 2 Methods of Degumming of SilkMethod CharacteristicsClassicalMethod

Treatment with “Marseille soap” followed by aboiling off and weakly ammoniacal rinsing

PhysicalMethod

Treatment with ultrasound at -180o C, Treatmentwith water under pressure at 121o C

EnzymeMethod

Treatment with enzyme (papain, trypsin, alcalase)followed by boiling off with hot water underpressure and weakly alkaline solution like sodiumbicarbonate, ammonia or acidic solution liketartaric, citric acid.

Acid Method Treatment with tartaric, succinic acid, dichloro,trichloro acetic acids for 30 minutes, nearboiling temperatures.

NewestMethod forContinuousProcess

Treatment with powder products containing sodiumcompounds (phosphate and carbonate), sometimeswith a soap additive. Treatment with liquidproducts containing additionally to sodium salts,anionic surfactants, chelating agents and fibre

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protective agents.Extraction with water

Sericin is water soluble at temperatures above 100o C while

autoclaved for prolonged periods, at least for 2 hours [8, 13,

21]. Since all the sericin is not removed by this method,

addition of nonionic synthetic detergents facilitates the removal

of sericin while maintaining the strength of the yarns [8]. Heat

treatment at high temperature results change in morphology, in

the case of mulberry silk whereas it remains unchanged in wild

silk. Degree of crystallinity and crystalline orientation

function appear to be reduced due to heat treatments in the case

of mulberry silk [22]. Wild silk fibres have noticeable

properties such as high chemical resistance, high Tg and high

thermolysis temperature due to the presence of tanning agents

which are not found in mulberry silk.

Soap Degumming

Conventional methods of degumming involves treatment of silk

fibres with soaps produced using various vegetable oils. A

systematic study of the degumming action of various soaps

obtained from various fatty acids has been carried out in the

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past [13, 23]. The degumming action of soap solution is due to

the alkali formed on the hydrolysis of soap. Neutralized soaps

solution has practically no scouring or degumming action on silk.

The traditional method of degumming involves the use of olive oil

(Marselle’s soap) soap. In soap degumming, the fabrics are

soaked for 6 hours in a solution of Marselle soap (3-5 g/l) at 40

– 50o C, then, the samples are boiled for 2 – 6 hours using soap

solution of 8 – 10 g/l at 90- 95o C. Though this is the gentlest

way of degumming silk it is expensive.

Acid Degumming

Though, both fibroin and sericin are susceptible for acid

attacks, the preferential attack on sericin over fibroin takes

place due to the specificity of the acid action on only few amino

acids. The dilute acids attack specifically the peptide bonds

adjacent to aspartic acid, and glutamic acid residues, which vary

significantly in the case of sericin and fibroin. Aspartic acid

residue is 1.0 – 1.9% (mol) in fibroin and is 14.6 -16.7 % (mol)

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in the case of sericin. Glutamic acid content is 0.8 – 1.4 %

(mol) in the case of fibroin and 4.42 – 7.9% (mol) in the case of

sericin. Various acids used for degumming have been categorized

[24] into three groups namely chloracetic acid, dibasic acid and

hydroxyl acid. In terms of weight loss, all the acids are

equally effective; considering weight loss and tenacity together

succinic acid gives the best results followed tartaric acid and

monochloro acetic acid. Acid degummed silks have better

aesthetics, comfort properties and dyeablity. But weight loss in

the case of monochloroacetic acid, acetic acid is less than

dichloroacetic acid and trichloro acetic acid. Degumming has

also been carried out using sulphuric acid, hydrochloric acids in

the pH range of 1 – 2.9. In the case of tartaric acid, at lower

temperature, the sericin removal is not complete irrespective of

the concentration whereas comparable results to that of soap

degumming can be achieved at 98o C and 4 -10 g/l strength.

Enzyme Degumming

In 1918, Kanegurachi Cotton Co., used an enzyme obtained from a

vegetable oil filter cake or from an animal body tissue known as

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proteolytic enzyme for degumming raw silk. Enzymes are used as

the alternatives for alkalis, acids or soaps to preserve the

physical properties, uniform removal of sericin and to reduce the

pollution levels [21]. The hydrophilic nature of sericin

accounts for relative ease with which it can be solubilized

during degumming and digested by proteolytic enzymes. Enzymatic

hydrolysis of sericin proteins can be achieved using various

types of enzymes namely trypsin, chymotrypsin, pepsin, bacterial

proteinases, papain, carboxypeptidase A & B, degummases and

luecine amino peptidase. Trypsin and (endopeptidase). Papain

has been recommended for degumming of silk and they

preferentially hydrolyse the peptide bonds formed by the carboxyl

groups of lysine and arginine while in neutral conditions,

fibroin is not attacked by the trypsin. The sericin is rich in

lysine content and in the case of mulberry silk, whereas the

arginine content of both sericin and fibroin is very low.

Complete degumming is not possible with these enzymes, so it

requires further soap treatment. Soap together with a

proteolytic enzyme at 50o C and a pH of 9 is found to be

effective for the degumming process [25]. Papain is sulphydryl

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enzyme, active at 70-90o C and pH of 5 to 7.5 whereas the

bacterial enzyme alcalase is active at 60o C and pH of 9 [26].

Degumming of silk with papain requires the pretreatment with hot

water to swell the sericin. Because of wide specificity, papain

removes most of the protein substrates extensively compared to

trypsin. It also hydrolyses derivatives of glutamine, histidine,

glutamic acid, leucine, glycine and tyrosine [27].

In the case of highly twisted yarns, proteases fail to remove the

sericin completely, which can be enhanced by mechanical

agitation, ultrasonication [14, 15, 28]. A combination of lipase

and protease has also been tried for the degumming [2, 29] in

which lipase is capable of hydrolyzing waxes into fatty acids and

alcohols and provide positive effects on wettability, comparable

to that of Marseilles soap treatment. By use of cellulase enzyme

along with a proteolytic enzyme complete removal of impurities

and improved wettability can be achieved [26]. Comparable dye

uptake and better luster can be obtained in the case of enzyme

treated silk than soap treated silk due to excess of alkali in

such treatments.

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Combined Degumming and Bleaching

Generally, silk fibre is bleached after degumming. Conventional

two stage process involves degumming of silk followed by

bleaching at 90o C for 1 hour using hydrogen peroxide, sodium

silicate and sodium carbonate [30]. A combined process for

degumming and bleaching has been developed using peroxide as a

means to reduce the time and energy [30, 31]. Combined degumming

and bleaching of silk using hydrogen peroxide in the presence of

sodium lauryl sulphate has been cost effective and also resulted

in lesser strength loss. Addition of sodium lauryl sulphate up to

22 g/l in combined bath improves whiteness level while retaining

similar strength.

Effect of Process Conditions and Parameters on Properties

Degumming imparts softness to silk and also better resistance to

wear in spite of loss in volume and weight. In physical

properties, no change is observed between mulberry and tasar in

terms of bending length, flexural rigidity and tenacity, while

crease recovery decreases [19, 32]. The shape of force-

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displacement curve changes significantly by degumming with

decrease in initial slope and proportional limit. Changes in

degree orientation and amorphous function in the fibroin is

difficult to measure after the degumming process [33]. Enzyme

degumming results in the separation of the twin filaments,

evident from the microscopic analysis [2]. The dull appearance

and stiff handle of raw fabric disappears and becomes shiny, soft

and scroopy. Mechanical agitation during treatment has proved to

change the aesthetics, tactile, thermal, comfort properties of

the fabrics.

In hard water, the degumming loss increases with increase in

concentration of detergent while no significant increase could be

observed in distilled water. During degumming not only sericin

but also waxes are also removed from the material leaving it

highly absorptive to dyes and chemicals. As the degumming time

increases, whiteness, K/S and fastness grade also increased [34].

In soap degumming process, with increasing time, the weight loss

also increases. Tenacity initially increases and then decreases

with decrease in elongation at break to a marginal extent [35].

15

Degumming of silk with papain results in more dye uptake, in the

case of reactive dyes, 1:1 metal complex dyes. Papain results in

a weight loss of 12-15% even with higher times than with alcalase

under similar conditions, which could be the possible reason for

the higher dye uptake of [34]. Alkaline, acidic and neutral

proteases have different specificity, which are evident from the

molecular weight and its distribution of sericin obtained from

these enzymes [2]. A comparative study of oxidative stable

endopeptidase, bacterial high alkaline strain, papain,

aspergillus pepsin I [2, 23, 36, 37, 38], which revealed very low

degumming efficiency (7.6 % weight loss) in the case of acid

protease compared to the alkali protease.

In the case of acid degummed silk, presence of residual acid

inhibit the dissociation of nucleophilic groups in the fibroin

and thus reduces the reactivity of reactive dyes especially in

the case of vinyl sulphone dyes [24, 39]. Degumming, with

organic acids, results in steep weight loss at high temperatures

and at shorter duration. In acid degummed silk, high tenacity is

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observed in 30 minute than at higher duration and very high acid

concentration leads to degradation of the sample. Table 6

compares the properties of acid degummed silk with soap degummed

samples.

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Table 6 Comparison between acid and soap degummed Silk

Parameter Soap Tartaricacid

Degumming Loss(%)

21.5 23.0

Tenacity (g/d) 3.3 3.7Modulus (g/d) 71.1 90.8Strain (%) 12.5 13.0

Assessment methods of degummed silk involve staining techniques,

dissolution methods and physical examinations [1, 4, 5, 6, 40,

41, 42, 43]. The physical methods of assessment include

assessment through handle evaluation, difference in weight of the

material before and after the degumming process and microscopic

examination for change in the cross section. Neocarmine W (Merk)

stains degummed silk into golden yellow and sericin to red.

Picrocarmine colours sericin red and fibroin yellow [20]. Sirium

Red F 3B 200 (Bayer) stains sericin into red colour and degummed

silk remains colourless in this treatment. Sericin in the

partially degummed silk and degummed silk can be identified by

staining with a mixture of CI Acid Blue 40 and CI Acid Orange 56

at 20o C for 10 min. Silk containing more than 10% sericin is

coloured yellow-red, and with less than 10% is coloured blue [5].

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Degummed silk with more of fibroin can be dissolved in Cuoxam

while sericin remains undissolved. Ninhydrin test is carried out

the identification of primary amine groups. Alpha amino acids,

except proline, react with tri-ketohydrindene hydrate known as

ninhydrin [44]. Purple colour imino derivatives obtained in this

reaction gives an indication of amino acids, which are generally

colourless (Fig. 1).

In the alkali and soap degumming process, degumming co-efficient

(k) of the process depends on factors such as temperature,

denier, agitation, liquor ratio, and the nature of soap and

alkali [1]. For alkalis, the coefficient is nearly independent

of concentration but for soaps, the coefficient decreases with

increasing concentration. The order of effectiveness of some

common alkali is 984 for sodium hydroxide, 827 for sodium

carbonate and 173 for sodium bicarbonate. With soaps, the

19

Fig. 1 Ninhydrin Reaction

degumming coefficient varies from 208 to 516 as the normality of

the solution decreases from 0.032 to 0.004.

Fixation of Sericin and Partial Degumming

Partial fixation or complete fixation of sericin using suitable

cross linking agents prior to degumming, improves the breaking

strength of the material. Fixation is due to the reaction between

amino groups in the proteins and cross linking agent, which

result in poor luster and low degree of whiteness. Cross linking

agents to fix up the sericin include cyanuric chloride,

triglycidyl isocyanurate or hexamethylene diisocynate, N, N

dihydroxy ethylene bisacrylamide, glutaraldehyde but they also

lead to significant discolouration [1, 14]. Chromium salts,

potassium oxalto chromate have, also, been used for this purpose.

Frequently entire silk sericin is not removed and only the

quantities sufficient to make the silk soft and lustrous and

workable in dyeing and bleaching are removed [13, 15, 45]. Cuite

silk represents completely degummed silk, souple silk is defined

as the silk in which the sericin is removed to 10 – 15% and 2 –

5% of the sericin from the raw silk is removed in the case of

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ecru silk. Half- boiled silk becomes bulky with the residual gum

present in the silk acting as a weak acid and helps in absorbing

more quantities of dyes without any difference in the fastness.

The exhaustion and colour strength of partially degummed silk are

significantly higher than fully degummed silk. The advantages of

partial degumming process [45] include sizing-free weaving

operation, improved colour strength and savings in amount of dye,

no deterioration in fastness properties, greater economy and

reduced pollution.

Applications of Silk Sericin

Silk sericin has potential applications in various fields, which

includes medicine, pharmaceuticals and cosmetics [46]. Silk

sericin blended with polyvinyl alcohol has the ability to form

hydrogel with good mechanical strength and water resistance.

Silk sericin membranes with their good biocompatibility and

adequate flexibility, can be used for wound healing and skin

scaffolds and artificial skin. Sulphonated sericin show

antithrombotic effect. Sericin grafted with methyl methacrylate

or styrene can be used for contact lenses. Sericin containing

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foods can relieve constipation, accelerates mineral absorption

and suppress bowel cancer and also has anti-tumor activity. In

the cosmetic front, sericin is used in skin, hair and nail

cosmetics. Skin lotions, creams and ointments have the ability

to reduce the wrinkles, aging effects and retain elasticity,

prevent epidermal water loss.

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