Textile Chemical Finishing And Its Mechanisms
In final finishing, with its
great range of desired and undesired effects, the task of a textile finisher
can become demanding has to consider the compatibility of the different type of
finishing products and treatment, in particular their mutual influence on the
desired effects. With about different type of finishes and several finishing
agents, most of which are combined to give one-bath multipurpose finishes. Chemical
finishing need a solid basis of textile chemical knowledge and technical
understanding as well as some practical experience.
The term finishing, in a broad
sense covers all the processes which the fabric undergoes after leaving the
loom or the knitting machine to the stage at which it enters the market. This
the term also includes bleaching, dyeing, mercerizing etc. but normally the
term in restricted to the final stage in the sequence of treatment of woven
fabrics after bleaching and dyeing. However fabrics which are neither bleached
nor dyed are also finished. Some finishing processes such as creping of silk
and rayon, mercerization of cotton or crabbing of wool are carried out a part
of the fires phase of fabric treatment or over earlier, in the form of yarn. Hence
finishing is the term usually employed for processes. The appearance may by
qualitatively describe as clear or fibrous, fine or course, lustrous or matt, plain
or patterned and smooth or uneven.
These descriptions may be
considered as the two extremes in each pair and the actual fabric appearance
may range between them. The fabric may not have the best in all these pairs for
example; a clear finished fabric can be either lustrous or matt. Similarly the
handle of fabric may be soft or crisp, flexible or stiff and fall or compact.
The fabric texture may be close or open light or heavy, loose or firm flat or
raised and uniform or varied. Clarity of fabrics is necessary to display
colour, structure, and pattern or to present a smooth plain appearance and uniform
texture. A clear fabric should not have any fiber ends protruding form its
surface.
Mechanisms Of The Softening Effect.
Softeners provide their main
effects on the surface of the fabrics. Small softener molecules, in addition, penetrate
the fiber and provide an internal plasticization of the fiber forming polymer
by reducing of the glass transition temperature. The physical arrangement of
the usual softener molecules on the fiber surface is important and shown in
Fig.-1. It depends on the ionic nature of the softener molecule and the
relative hydrophobicity of the fiber surface. cationic softeners orient themselves
with their positively charged ends toward the partially negatively charged fabrics
(zeta potential),creating a new surface of hydrophobic carbon chain that
provide the characteristic excellent softening and lubricity seen with cationic
softeners. Anionic softener, on the other hand, orients themselves with their negatively
charged ends repelled away from the negatively charged fiber surface. This
leads to higher hydrophilicity, but less softening than with cationic
softeners. The orientation of non-ionic softeners depends on the nature of the
fiber surface, with the hydrophilic portion of the softener being attracted to
hydrophilic surfaces and the hydrophobic portion being attracted to hydrophobic
surface.
d- d-
(a)
(b)
d- d-
(c) (d)
d-
|
Hydrophobic part of softener molecule
cationic hydrophilic group
Anionic hydrophilic group
Non-ionic hydrophilic group
Fiber surface with partial negative charge.
|
Fig. 1 Schematic orientation of softeners on fiber surface
(a) Cationic softener and (b) Anionic Softener at fiber surface Non-ionic
softener at (c) hydrophobic and (d) hydrophilic fiber surface.
a) Cationic
Softeners.
The typical cationic softener
structure for example, N,N- distearyl-N, N-dimethyl ammonium
chloride(DSDMAC).Cationic softeners have the best softeners and are reasonably
durable to laundering. They can be applied by exhaustion to all fibers from a
high liquor to goods ratio bath they provide a hydrophobic surface and poor
rewetting properties, because their hydrophobic group are oriented away from
the fiber surface. They are usually not compatible with anionic product.
Cationic softeners attract soil, may
cause yellowing upon exposure to high temperatures and way adversely effect the
light fastness of direct and reactive dyes. Inherent ecological disadvantages
of many convential (unmodified) quaternary ammonium compounds (quaternaries)are
fish toxicity and poor biodegradability. But they are easily removed from waste
water by adsorption and by precipitation with anionic compound. Quaternaries
with ester groups, for example triethanol amine esters, are biodegradable, through
the hydrolysis of the ester group. The example of an ester quaternary in Fig.-2
is synthesized from triethanolamine, esterified with a double moler amount of
stearic acid and then quaternarised with dimethylsulfate.
Quaternary ammonium salt.
Amine
Salts.
Imidazolines.
Fig.-2. Chemical structure of typical cationic softeners.
b) Anionic Softeners.
Anionic softeners are heat stable
at normal textile processing temperature and compatible with other components
of dye and bleach baths. They can easily be washed off and provide strong
antistatic effects and good rewetting properties because their anionic groups
are oriented outward and are surrounded by a thick hydration layer. Sulfonates
are, in contrast to sulfates, resistant to hydrolysis Fig.-3.They are often
used for special applications, such as medical textiles, or in combination with
anionic fluorescent brightening agents
Alkylsulfate salt
Alkylsulfonate
salt
Fig.-3. Chemical
structures of typical anionic softeners.
c) Non-Ionic
Softeners Based On Paraffin And Polyethylene.
Polyethylene can be modified by
air oxidation in the melt at high pressure to add hydrophilic character (mainly
carboxylic acid group).Emulsification in the presence of alkali will provide
higher quality more stable products. They show high lubricity that is not
durable to dry cleaning they are stable to extreme pH conditions and heat at
normal textile processing condition, and compatible with most textile
chemicals.
Polyethylene
Ethoxylated fatty
alcohol
Ethoxylated fatty
acid
Fig.-4. Chemical
structures of typical Non-ionic softeners.
d) Amphoteric Softener.
Typical properties are good
softening effects, low permanence to washing and high antistatic effects. They
have fewer ecological problems than similar cationic products. Examples of the
betaine and the amine oxide type are shown in Fig.-5.
Alkyldimethylanime
oxide softener.
Betaine Softeners
Fig.-5.. Chemical
structure of typical amphoteric softeners.
e) Silicone Softeners.
None-ionic and cationic examples
of silicone softeners are shown in Fig.-6.They provide very high softeners, special
unique hand, high lubricity, good sewbability, elastic resilience, crease
recovery, abrasion resistance and tear strength. They show good temperature
stability and durability, with high degree of permanence for those products
that form cross linked films and a range of properties from hydrophobic to
hydrophilic.
Polydimethyl silicone
Cationic silicone
softener.
Fig – 6. Chemical
structures of typical silicone softeners.
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