Article 2 Chemical Finishing and Its Mechanisms

8/2/2019 Article 2 Chemical Finishing and Its Mechanisms

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Textile Chemical Finishing And Its Mechanisms

By :- AVM Chemical Industries.

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 give one-bath multipurpose finishes. Chemical finishing need a solid basis oftextile 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 undergoesafter 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 inrestricted to the final stage in the sequence of treatment of woven fabrics after bleachingand 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 appearancemay 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 fabricappearance 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 beclose 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 fibersurface. cationic softeners orient themselves with their positively charged ends toward the

partially negatively charged fabrics (zeta potential),creating a new surface of hydrophobiccarbon 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. Theorientation of non-ionic softeners depends on the nature of the fiber surface, with the


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hydrophilic portion of the softener being attracted to hydrophilic surfaces and the

hydrophobic portion being attracted to hydrophobic surface.

+ + +

-

- - -

-

(a) (b)

- -

(c) (d)


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+

-

-

Hydrophobic part of softener moleculecationic 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)

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 arereasonably 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. Theyare 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 ecologicaldisadvantages 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 hydrolysisof 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 thenquaternarised with dimethylsulfate.

CH

R N R X

CH3

3

2- X =HSO or

-

-

4

R =(CH ) CH2 n 3R = CH32

+

Quaternary ammonium salt.

R NH X3-

+

R = Long alkyl chain

Amine Salts.


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CH3 (CH )2 16 CN

N

R3

CH

CH

2

2

R = H or CH CH NH3 2 2 2

Imidazolines.

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

b) Anionic Softeners.Anionic softeners are heat stable at normal textile processing temperature and compatiblewith 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, suchas 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 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.

CH3 (CH )2 nCH3

Polyethylene


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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.

d) Amphoteric Softener.Typical properties are good softening effects, low permanence to washing and highantistatic effects. 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


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C NR 2(CH )33

N

CH3

CH

C

H

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, elasticresilience, crease recovery, abrasion resistance and tear strength. They show good

temperature stability and durability, with high degree of permanence for those products thatform cross linked films and a range of properties from hydrophobic to hydrophilic.

Sio Sio Si

CH3

CH CH33CH3

CH3

CH3

CH3

CH3Polydimethyl silicone

Sio Sio Sio Si

CH3

CH CH33CH3

CH3

CH3

CH3

Rn

X Y

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

23 2 2 3 3

OH

Cationic silicone softener.

Fig 6. Chemical structures of typical silicone softeners.

Article 2 Chemical Finishing and Its Mechanisms

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