Download - 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