wool and silk pretreatment

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A LECTURE NOTE

ON

PREPARATORY PROCESSES FOR WOOL AND SILK

Adane HaileInstitute of Technology for Textile, Garment and Fashion Design

Bahirdar University

2004

PREPARATION OF WOOL

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IMPURITIES IN RAW WOOL

Keratin 33%

Dirt 26%

Suint 12% Impurities

Wax/lipids 28%

Mineral Matter 1%

Raw wool is dirty & consists of

impurities

Others: - Added lubricants and pigments

Dirt: Consists of dried grass, seed, dried oil and so on held by

means of adhesion forces of suint and wool fat.

Suint: This impurity comes from the secretions by sweat glands.

Typically contains potassium salts of fatty acids and amino acids.

Wax or wool grease: Mainly lanolin [Complex mixture of hydrocarbons]

Mineral matter: Oxides of certain metals (potassium & iron)

LARGE AMOUNT OF IMPURITIES LARGE AMOUNT OF IMPURITIES

ALKALINE & HEAT SENSITIVITY ALKALINE & HEAT SENSITIVITY

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AQUEOUS ALKALINE SCOURING

SOLVENT SCOURING

THERMAL SCOURING

FREEZE SCOURING

METHODS OF WOOL SCOURING

Scouring must be carried out so as to cause as

little harm to the wool fiber as possible, both

from chemical and mechanical (felting) point of

view.

Scouring must be carried out so as to cause as

little harm to the wool fiber as possible, both

from chemical and mechanical (felting) point of

view.

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AQUEOUS ALKALINE SCOURING

Alkaline scouring of wool is carried out in alkaline soap

solution.

Sodium carbonate is used as alkali in wool scouring.

Wetting agents & other scouring bath additives

Emulsification and solubilization

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Trough Soap Na2CO3 Time Temp No. (%) (%) (min) (0C)

1 0.8 0.2 2 .5-3 40-50

2 0.4 - 2-3 40-50

3 0.35 - 2 40-50

4 Water - 1-2 40-50

AQUEOUS SCOURING PROCESS IN STAGES

Wool scouring can be carried out on: Fiber, yarn or Fabric

SOLVENT SCOURING SOLVENT SCOURING High pressure jets of solvent remove the wax, dirt and suint High pressure jets of solvent remove the wax, dirt and suint

Solvents such as benzene and CCl4 (carbon tetrachloride)Solvents such as benzene and CCl4 (carbon tetrachloride)

Suitable solvent recovery plant is necessarySuitable solvent recovery plant is necessary

THERMAL SCOURINGTHERMAL SCOURING

Involves thermal liquefaction of wax by irradiation.Involves thermal liquefaction of wax by irradiation.

Mixing with grease absorbing material and RinsingMixing with grease absorbing material and Rinsing

FREEZE SCOURINGFREEZE SCOURING

Exposing wool to low temperature environment [-30Exposing wool to low temperature environment [-30oC] C]

Freezing of wax and dirt matter Freezing of wax and dirt matter

Mechanical breakage of frozen matter Mechanical breakage of frozen matter

RinsingRinsing

The scouring process for wool is primarily engaged in removal

of wax and water soluble impurities.

Vegetable matters in wool are removed by CARBONIZATION

CARBONIZATION OF WOOL

Carbonizing is done to remove the cellulosic

impurities from wool by treatment with acid or

acid producing salts. Primarily sulfuric acid is

used.

The ultimate effect is hydrolysis of cellulose

present in vegetable matters of wool fiber.

CONVENTIONAL CARBONIZING

PROCESS

SCOURING

ACIDIZING

DRYING AND BAKING

BURR CRUSHING AND DEDUSTING

NEUTRALIZINGCarbonizing liquor: H2SO4 (up to 7%) + Acid resistant

wetting agent

Drying @ 110 0C and baking @ 140 0C for a short

period of time

Removal of coloring matters from wool can be achieved

with similar mechanism as in cotton bleaching.

Possible bleaching processes for wool:

ALKALINE PEROXIDE BLEACHING

ACIDIC PEROXIDE BLEACHING

REDUCTIVE BLEACHING USING SODIUM HYDROSULPHITE

SEQUENTIAL OXIDATIVE AND REDUCTIVE BLEACHING

WOOL BLEACHING

Scoured wool varies in shade from light cream to

near black of pigmented wool.

Alkaline peroxide bleaching requires stabilizer

addition like in cotton bleaching. Bleaching is

carried out at lower alkalinity [8-10] at lower

temperature [~ 500C].

In the acid peroxide bleaching wool is treated with a

solution of hydrogen peroxide containing formic acid at pH

around 4 to 4.5 at room temperature. Drying takes place

for the bleaching reaction to take place.

The acid peroxide bleaching process is

advantageous: Lower chemical degradation of wool &

thorough rinsing after bleaching can be avoided.

Sequential bleaching: Treatment with peroxide bleach

bath and addition of reducing agent in the bath for

reductive bleaching.

Example: Thiourea

R S S R’ R SO S R’

RCH 2 S S CH 2 R’ + 2[H] RCH 2 SH + HS CH 2 R’

Effect on the disulphide linkage

Example: addition of stabilizers such as sodium pyrophosphate [Na4P2O7] in

reductive bleaching with sodium hydrosulphite

Fluorescent brightening for full white in acidic solutions

PREPARATION OF SILK

INTRODUCTION Silk fiber is composed of two proteins:

fibroin and sericin

The basic fiber substance is Fibroin made

from long-chain protein molecules.

Sericin [silk gum] surrounds the fibroin

filaments and holds them together.

Fibroin and sericin are composed of amino acids

with different composition and structural

arrangements. In particular the serine content of

sericin is higher.

R

-NHCHCO- R = -HOCH2

TYPICAL SILK FIBER COMPOSITION

Component %

Fibroin 70-80

Sericin 20-30

Waxy matter 0.4-0.8

Carbohydrates 1.2-1.6

Inorganic matter 0.7

Pigment 0.2

Impurities

Natural color of silk differs upon type of

silkworm and its feeding habit

DEGUMMINGSilk gum together with other impurities inhibit penetration

of chemicals used in subsequent silk processing.

Scouring of silk practically means removal of the silk gum

and other impurities such as waxy and mineral matter.

DEGUMMING

Cleavage of peptide bonds in

sericin by hydrolysis OR impurity

solubilization.

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Extraction with water

Boil off in soap

Degumming with acids

Degumming with alkalis

Extraction with water

Boil off in soap

Degumming with acids

Degumming with alkalis

CONVENTIONAL DEGUMMING METHODS

Acid degumming is due to hydrolysis of

sericin.

Alkali degumming involves solubilization of

sericin by converting –COOH groups to –

COONa.

Chemical Conc. Temp. [oC] Time

Soap 20-30% owf 90-100 1-2h

pH-10

Acid 0.05 mol/l 10060 minNI surfactant 3g/l

pH-1.5-2

Water 115 45 min

Alkali

Sod. Carb. 1:1 molar ratio 95 20min

Sod. Bicarb. pH 9.5-10.5

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SILK BLEACHING

The sericin remnants which are tenaciously

associated with the silk fibroin cause

discoloration in silk in addition to the naturally

occurring silk pigment.

Silk can be bleached by oxidative as well as

reductive methods. If a very high degree of

whiteness is required, a combination of

both methods in the form of double bleach

is applied.

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Selection of bleaching agent must consider

minimal damage to the silk fibroin.

Peroxide bleaching as oxidative method and

reductive bleaching using sodium hydrosulphite

are conventionally used.

The level of whiteness can be controlled by concentration of

bleach, length of treatment time, pH and temperature of

treatment bath.

The level of whiteness can be controlled by concentration of

bleach, length of treatment time, pH and temperature of

treatment bath.

PRETREATMENT OF SYNTHETICS AND BLENDS

Synthetic fibers generally do not contain naturally

occurring impurities like natural fibers.

The objective of scouring synthetic fiber fabrics is to

remove the dirt, spin finishes and processing lubricants.

Bleaching for removal of sighting colors.

SPIN FINISHES

PROCESSING LUBRICANTS

DIRT & SIGHTING COLORS

WATER SOLUBLE SIZE

Scouring for PC blends is mainly designed to remove the

natural oils and waxes from the cotton portion and the finish

oils which manufacturer add to polyester fibers when extruded.

Bleaching of PC blends are normally required to remove the

natural colors of cotton, sighting colors and if the polyester

portion is turned yellow at the time of heat-setting operation.

Alkaline hydrogen peroxide bleaching is the most preferred

system for polyester/cotton blends and bleaching.

COMBINED SCOURING/BLEACHING FOR ECONOMICAL PURPOSE