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3.1 FINISHES FOR FABRICS

Textile Science
Semester IV, B. Voc Fashion Design
Module 3.1.2
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Chemical Finishes

Chemical Finishes and Finishing

Chemical finishes are chemical re- agents or polymeric materials applied to textile
structures by a number of methods. The major types of chemical finishes used on
textile structures are listed in Table 1.

Chemical finishes can be applied by a number of methods including padding

(immersion in the treatment solution followed by squeezing to remove excess),
spraying, printing, foam application, or vapor techniques.

In addition, the finish can be added to the spinning bath prior to formation of man-
made fibers. Of these methods, padding is most important. Many finished fabrics
must be dried (to remove solvent) and cured (heated to cause a chemical reaction)
before chemical finishing is complete. Thorough wetting of the fiber by the finish
solution and spreading of the finish evenly over the fiber surface is critical in most
cases to get the desired effect. The location of the finish on the surface or within
the fiber is important, depending on the finish and its function.

Finishes Affecting Aesthetics, Comfort, and Service

Optical Finishes:
Optical finishes do little to affect the color of a textile substrate, but rather act to
destroy or mask color centres. They may either brighten the textile, making it
more reflective, or DE lustre the textile, making it less reflective, depending on
the treatment.

Bleaches are usually chemical oxidizing or reducing agents that brighten the textile
by attacking unsaturated molecules that make the textile appear off color. Chlorine
bleaches such as sodium hypochlorite (NaOCI) are strong oxidizing agents capable
of destruction of color centres on a textile substrate.

Unfortunately, sodium hypochlorite is fairly nonselective and attacks many dyes

and finishes and certain fibers, causing loss or change in color and a deterioration
in fiber properties. Sodium chlorite (NaCI02) and peracetic acid (CH3C03H) also
are used as strong oxidizing bleaches on some synthetic fibers at the mill to

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achieve desired whiteness. Oxygen bleaches such as hydrogen peroxide (H202)

and sodium perforate (Na3B03) are milder in oxidizing action and can be used on
sensitive fibers such as wool. With hydrogen peroxide, pH adjustment is critical in
getting the desired bleaching.

Sodium per borate bleaching must be carried out at elevated temperatures,

although chemical activators may be added to lower the effective bleaching
temperature. Oxygen bleaches are less likely than chlorine bleaches to damage
the fiber and dyes present on the fiber. Reducing agents chemically reduce and
saturate double bonds within color centres and find limited use as bleaches in mill
applications.

Reagents such as sodium dithionate (Na 2S206) and sodium formaldehyde

sulfoxylate (NaHS03·HCHO) are commonly used as reducing bleaches and in
compositions for stripping dyes from fibers. Fluorescent brightening agents are
colourless fluorescing dyes which mask yellow coloration on fabrics.

Bleaches and Fluorescent Brighteners:

Oxygen bleaches such as sodium per borate are often added to enhance the
whitening power of the formulation through destruction of color centres remaining
on the fabric. Fluorescent brighteners are added to nearly all synthetic fibers in
manufacture to cover yellow coloration through blue fluorescence of these
colourless dyes in the light. Fluorescent brighteners added to laundry formulations
are mixtures of brighteners which have affinity for all fiber types commonly found
in a wash load. Bleaches and brighteners also can be purchased and used
separately to enhance whitening of the textile substrate.

Delustering of fibers may be carried out through alteration of the fiber surface or
through addition of alight scattering and/or absorbing agent to the fiber substrate.
Chemical attack or etching of the fiber surface leads to a more irregular surface
morphology, thereby increasing light scattering and making the fiber dull. With
man-made fibers, titanium dioxide (Ti02) is added to the spinning solution before
fiber formation.

Titanium dioxide is an excellent light scattering agent, and this de lustring is

effective when added at 0.5%-2.0% levels to the fiber. Titanium dioxide can also

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be applied to the surface of natural fibers, if a binder such as urea-formaldehyde

resin is used to fix the de lustring to the fiber surface.

Hydrophilic and Soil Release Finishes:

Hydrophilic (water-seeking) finishes that promote absorption or transport of water
and aid in fiber wetting and soil removal in a textile construction are useful in
many fiber applications. In man-made fibers surface active agents such as non-
ionic polyethylene glycol derivatives can be added to the spinning solution prior to
spinning, which will make the fiber more wet table and hydrophilic.

Hydrophilic polymers such as copolymers containing acrylic acid can be fixed to

fiber surfaces to provide improved water absorbency and to limit penetration of
soils. Hydrolytic attack of the surface of hydrophobic manmade fibers such as
polyester improves fiber wetting, moisture transport, and soil removal.

None of the above absorbent techniques, however, are sufficiently effective to alter
the absorbency properties of hydrophobic fibers like polyester enough to totally
resemble naturally absorbent fibers like cotton and wool.

Softeners and Abrasion Resistant Finishes:

Softeners and abrasion resistant finishes are added to textile structure

(1) to improve aesthetics

(2) to correct for harshness and stiffness caused by other finishes on the textile
substrate.

(3) to improve the ability of the fibers to resist abrasion and tearing forces.

The softeners and abrasion resistant finishes are generally emulsions of oils or
waxes, surface-active agents, or polymers that lubricate the surface of individual
fibers in the textile substrate to reduce friction between fibers and permit them to
pass over one another more readily.

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Emulsions of oils and waxes and related derivatives have been used in the past as
softeners. Non-ionic and cationic detergents act as softeners, but lack
permanence.

Emulsions of polyacrylates, polyethylene, or silicones impart softening properties

and possess reasonable fastness. In addition, they may impart a fuller hand to the
textile.

Stiffening and Weighting Agents:

Textile auxiliaries that stiffen and weight fabrics have included temporary and
permanent sizes and metal salts applied alone or with a binding agent.

The sizes stiffen the fabric through formation of bonds between fibers, particularly
at fiber crossover points.

Temporary sizes include starch, naturally derived gums, carboxymethyl cellulose,

and polyvinyl alcohol. Acrylic binders, polyvinyl chloride, and polyvinyl acetate
emulsions act as permanent sizing agents. Metal salts may be used to weight
certain fibers such as silk. If the metal salt has affinity for the fiber, as is the case
for stannic chloride (SnCI4) and silk, no binder is needed.

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Urea-formaldehyde or acrylic resins can act as suitable binders for fixation of no

permanent metal salts onto the fiber surface. High intakes of metal salts used to
weight textile structures may cause fibers to become brittle and to be sensitive to
photo chemically induced damage.

Laminating Agents:
In recent years lamination of two textile structures together to form a composite
structure has become very important.

This technique requires use of adhesive materials that will not delaminate under
normal use, including flexing and bending, shearing forces, and cleaning. The
acrylics, polyurethanes, and a number of hot melt thermoplastic polymers are
used as adhesives.

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Some adhesives have reactive groups which on curing lead to strong textile
adhesive covalent bonds in addition to the physical forces normally responsible for
a good adhesive fiber bond.

Crease Resistant and Stabilizing Finishes:

When fibers are bent or deformed under various environmental conditions and
then allowed to recover, the degree of recovery will depend on the morphology
and inherent structure of the fiber. Most synthetic fibers show reasonable recovery
from such deformation, whereas the cellulosic and, to a lesser extent, the protein
fibers have poor recoveries, particularly under moist conditions.

Crease resistant finishes for cotton and rayon have been developed that give much
improved wrinkle recovery properties, whereas crease resistant finishes for wool
are still under development, since the process is complicated by felting shrinkage.

Crease resistant finishes for cellulosic are chemical cross-linking agents and
include wash-and-wear and durable press finishes. The wash-and-wear finishes
are generally urea-formaldehyde or melamine formaldehyde resins and are cured
on cellulosic as flat goods in the mill, whereas durable press treatments consist of
cyclic urea-aldehyde derivatives which are partially cured at the mill, after which
curing is completed after the fabric is made into a garment in apparel
manufacture.

Other di functional finishes, including epoxides, isocyanate, vinyl sulfones, and

aziridines, have been introduced as crease resistant finishes for cellulose. These
have met with only limited success owing to their higher cost and other
deficiencies. Since wool is already heavily cross linked, a different approach to
crease resistant finishing must be taken.

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The disulfide crosslinks in the textile structures must be chemically reduced

followed by a setting treatment with functional reagents such as capped
isocyanate derivatives.

Chemical treatments for setting of thermoplastic man-made fibers are generally

not used, since heat setting of these thermoplastic fabrics is an effective technique
for imparting crease resistance.

Urea formaldehyde resins also are often used on cellulosic to impart dimensional
stability to the textile structure and to prevent yarn slippage within the structure,
finishes capable of causing inter fiber bonding can act as effective stabilizing
finishes, although they may stiffen the overall structure.

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