Textile Chemical Finishing and Its Mechanisms

7/28/2019 Textile Chemical Finishing and Its Mechanisms

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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 offinishing 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 giveone-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 termalso 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 usuallyemployed 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 maybe 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, inaddition, 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 ofthe 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 thecharacteristic 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 fibersurface, with the hydrophilic portion of the softener being attracted to hydrophilic surfaces and

the hydrophobic portion being attracted to hydrophobic surface.


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+ + +

-

- - -

-

(a) (b)

- -

(c) (d)


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+

-

-

Hydrophobic part of softener molecule

cationic hydrophilic group

Anionic hydrophilic group

Non-ionic hydrophilic groupFiber surface with partial negative charge.

Fig. 1 Schematic orientation of softeners on fiber surface (a) Cationic softener and (b) AnionicSoftener 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 baththey 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 wayadversely 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 adsorptionand 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.

CH

R N R X

CH3

32

- X =HSO or- -

4

R =(CH ) CH2 n 3R = CH32

+

Quaternary ammonium salt.

R NH X3-

+

R = Long alkyl chain

Amine Salts.

CH3 (CH )2 16 CN

N

R3

CH

CH

2

2R = H or CH CH NH3 2 2 2

Imidazolines.

Fig.-2. Chemical structure of typical cationic softeners.


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b) Anionic Softeners.Anionic softeners are heat stable at normal textile processing temperature and compatible withother 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 tohydrolysis Fig.-3.They are often used for special applications, such as medical textiles, or in

combination with anionic fluorescent brightening agents

R SO3 R = Long alkyl chainO Na

Alkylsulfate salt

R SO3 R = Long alkyl chainNa

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 providehigher 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, andcompatible with most textile chemicals.

CH3 (CH )2 nCH3

Polyethylene

R 2 R = Long alkyl chainO(CH CH O) H2 m

Ethoxylated fatty alcohol

R2C O(CH CH O) H24 R = (CH ) CH4 2 3nm

Ethoxylated fatty acid

Fig.-4. Chemical structures of typical Non-ionic softeners.


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d) Amphoteric Softener.Typical properties are good softening effects, low permanence to washing and high antistaticeffects. They have fewer ecological problems than similar cationic products. Examples of the

betaine and the amine oxide type are shown in Fig.-5.

CH

R N O

CH

3

3

R = Long alkyl chain

Alkyldimethylanime oxide softener.

H C O

H C3

N CH C

CH3

3

R

O

3

N CH C

CH3

CH

R 2

C NR 2(CH )33

N

CH3

CH

C

H

Betaine Softeners

Fig.-5.. Chemical structure of typical amphoteric softeners.


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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, creaserecovery, 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.

Sio Sio Si

CH3

CH CH33CH3

CH3

CH3

CH3

CH3

Polydimethyl silicone

Sio Sio Sio Si

CH3

CH CH33CH3

CH3

CH3

CH3

Rn

X Y

R =(CH ) OCH CHCH N (CH )n

23 2 2

33

OH

Cationic silicone softener.

Fig6. Chemical structures of typical silicone softeners.

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Textile Chemical Finishing and Its Mechanisms