Saturday, December 12, 2020

PRE - TREATMENT PROCESS OF NATURAL CELLULOSIC FABRICS OR FABRIC PREPARATION FOR DYEING

FABRIC PRE- TREATMENT PROCESS OR READY FOR DYEING PROCESS (RFD):

We know that all the natural cellulosic fibres have many kinds of impurities in them. Few of them are contaminated during yarn manufacturing or fabric manufacturing process like sizing material, wax oils. There are so many impurities which get present in the fabric from fibre stage like wax, broken seeds and stems, immature fibre, pigment, and dust etc. The impurities present in the grey fabric creates difficulties during dyeing of these fabrics. Oil and wax influence the absorbency of the fabric. The broken seeds, broken leaf and stem influence the appearance of the fabric to be dyed. The any type of pigment present in the fabric results in the form of fullness of shade. The immature fibre shows poor dyeing affinity. These immature fibres absorb colour very hardly. Sometimes these immature fibres appear on the fabric surface like almost undyed fibres. The protruding fibres also presents on fabric surface. These impurities don't allow it to dye in it's grey stage form. In this way we need to remove all these impurities from the fabric before dyeing. "Now we can say that the fabric pre-treatment is a process in which all the impurities present in the fabric are removed from it by using chemicals in many stages".

OBJECTIVES OF PRE-TREATMENT PROCESS:

The main objectives of fabric pre-treatment process are given below:

1 - To remove all the natural impurities present in the fabric and to find almost impurities free fabric.

2 - To make the fabric able to uniform dye uptake.

3 - To improve the moisture absorbency of the fabric..

4 - To improve all the immature fibres present in the fabric.

5 - To achieve required degree of whiteness. 

The fabric pre-treatment process gets completed in below stages.

SINGEING:

Fabrics woven with cotton or viscose staple yarns show protruding fibre ends at the fabric surface; These these protrung fibres result on the fabric surface during use of garments and other items. The protruding fibres also disturb the surface appearance of the woven fabric and in dyeing produce an effect known as frosting. "It is therefore necessary to remove these protrung fibres from fabric surface by passing the fabric through a gas flame, a process known as singeing". 

• In singeing process, the fabric is passed through a fabric singeing machine.

• First of all, the fabric gets passed over a brush roller. This brush roller helps to raise the sticked protruding fibres on the fabric surface. 

Now the fabric enters in the protruding fibres burning zone.

Two rows of flames are used in this burning zone.

One flame row is used for bottom bottom surface of the fabric and other flame row is used for top surface of the fabric.

The L.P.G. is used for flame.

When the fabric gets passed between these rows of flames, the protruding fibres present on the fabric surface are burnt. 

In the standard process, the fabric is passed rapidly between two rows of gas flames at high speed (50-300 m/min).

Finally, the fabric goes into a quench bath to extinguish the sparks and cool the fabric. 

DESIZING:

The desizing process is applicable in case where the fabric contains size materials like starch, binder and softener etc. If the fabric doesn't have size materials, the sizing process is not applied during pre-treatment of the fabric.The process of removal of starch, gum, and lubricants (softener) from the fabric. The desizing is carried out by treating the fabric with some chemicals. 

There are so many methods used in desizing process. Few of the desizing methods are given below.

Acid desizing

Acid treatments degrade starch-based sizes and offer the advantage of removing calcium and magnesium salts from cellulosic fabrics. 2.0 % hydrochloric acid is used for short period of steeping. If overnight steeping is done, the concentration of the hydrochloric acid is kept 0.20 %. There should be no localised drying out of the fabric otherwise the hydrolytic damage of cellulosic fibres may be occured.

Enzymatic desizing

Enzyme treatments are used widely on cotton fabrics sized with starch.

Todays bacterial amylases are  used in majority.

The stability and activity of enzymes depend on many factors, including pH, temperature, and the presence of activators (e.g. metal ions) and wetting agents. 

in general the temperature of the enzyme liquor on the textile material is well below that required for optimum desizing. 

The enzyme desizing greatly depends upon processing time and temperature.


As the enzymatic degradation of starch size is a time- and temperature-dependent reaction, 

The fabric is washed in watch after size materials degradation in hot and cold water. The residual starch gets removed during washing.

Technician conducts iodine test to confirm complete removal of size materials in the desized fabric. A mixture of potassium iodide and iodine is used in this test. When we apply this mixture on the fabric surface, the colour of the surface looks yellowish where mixture is applied. Yellowish colour is the confirmation of complete removal of size materials. If the applied surface gets blackish, this indicates incomplete removal of size materials.

Oxidative desizing

Oxidative degradation of starch sizes, as an alternative to enzymes or acids, has been known for many years. Particularly prior to kier bleaching with hydrogen peroxide, dilute hypochlorite liquors were used for desizing by pad-steep processing. More recently hydrogen peroxide or persulphates have been generally associated with the term oxidative desizing.

The concept of oxidative desizing is particularly useful as a response to the extensive use of poly(vinyl alcohol) sizes. As indicated previously, the water solubility of these can be impaired prior to desizing. Oxidative desizing ensures degradation of these sizes and their subsequent removal. While this is not possible when size recovery is required.

Mercerizing

John Mercer discovered that the tensionless treatment of cotton fabrics with strong caustic soda improved the dye uptake and the tensile strength.

The lustre of the fabric gets improved in mercerizing.

The outer wall of the cotton fibre gets dissolved and fibre swelling takes place.

The internal reorientation of cellulosic structure also takes place.

The immature fibres present in the cotton fabric get dissolved in mercerizing.

Durability of the fabric gets improved.

The dimensional stability of fabric gets improved in mercerizing process.

All these properties depend on the alkali concentration, dwell time in alkali and treatment temperature. 

The concentration of sodium hydroxide necessary to achieve the fully mercerised effect is a 25% by wt solution. 

Mercerising may be carried out at various stages of the preparation sequence on one of two main types of mercerising machine, described as chain or chainless mercerisers. The chain merceriser applies tension directly to warp and weft whilst the chainless design only applies indirect tension across the weft. Consequently the fabric construction must allow for this width loss, or the fabric must be stentered to a greater width prior to mercerising, which in itself is difficult and sometimes impracticable.

SCOURING:

Scouring is a treatment with alkali that removes or destroys cotton waxes, coloured impurities and other non-cellulosic substances, such as pectic cell wall material and fragments of leaf and seed coat, that are present as impurities in cotton fibres. This leads to a more absorbent fibre with greatly enhanced wettability characteristics.

The sodium hydroxide and scouring agent are used. Temperature is kept between 50 - 90 degree.

Hydrogen peroxide bleaching

• In cellulosic fabric processing, the hydrogen peroxide is used for bleaching of natural cellulosic fibres widely.

• The broken seeds, leaf, and pigment material get removed in this process.

• Half white fabric results after this processing.

• The fabric received after peroxide bleaching is widely used for dyeing.

• This fabric is called R.F.D. ( ready for dyeing) fabric.

• The strength of hydrogen peroxide to being used in textile processing industries ranges from 27.5% to 50%.

• It is generally agreed that in bleaching with peroxide the first step is an ionisation of the hydrogen peroxide molecules in the presence of hydroxide ions to perhydroxide ions (HOO-) and protons (H+) that immediately combine with hydroxide ions to form water. 

The  pH 11.5 is maintained during peroxide bleaching. The process follows below reaction:

H2O2 + OH-  - HOO- + H+

H+ + OH-  H2O

Colour destruction and breakdown of other non-cellulosic impurities result from oxidation reactions and these may be represented, although as an oversimplification, by the release of an ‘active’ oxygen atom from the decomposition of the unstable perhydroxide ion.

HOO- - OH- + 0* o* + x - x-o

where 0* is ‘active’ (nascent) oxygen and X is an oxidisable substance


The rate of decomposition of hydrogen peroxide is dependent on the following variables:

• pH

• Temperature

• Types and concentrations of impurities

• Type and concentration of stabiliser.

• As the pH increases so does the rate of decomposition of the peroxide, the effect of pH being most marked between values 9 and 10. The effect of increasing pH promotes not only the decomposition of hydrogen peroxide according to Schemes 2.6 and 2.7 but the undesirable side reactions summarised as Scheme 2.8 are also accelerated.

• Temperature also has a marked effect; increasing the temperature speeds up the decomposition of peroxide, resulting in enhanced bleaching but also accelerating the undesirable side reactions. Normally the time and temperature of processing arei interrelated in this respect.


BLEACHING:

Bleaching of cellulosic fabrics fulfils a two following purposes. 

• Firstly it provides a high degree of whiteness for fabrics that need to remain white. 

Bleaching is applied for fabrics that are to be dyed to bright shades, prior bleaching enhances the whiteness of the cellulose and thereby imparts improved brilliance to the dyed effect. 

• Secondly, oxidative treatment improves the uniformity of fabric appearance by helping to destroy residual impurities such as seed husks.


1 - Sodium hypochlorite

2 - Sodium chlorite

Sodium hypochlorite bleaching

• Hypochlorite solutions provide a convenient form for handling chlorine, particularly for use in the concentrations required for textile preparation processes. 

• Commercially hypochlorites have been supplied either as a solution of sodium hypochlorite (NaOCI) or as bleaching powder (mainly calcium hypochlorite). 

• The strength of a solution of hypochlorite is normally expressed as the available chlorine content. 

• This term, also referred to as active chlorine content, relates to the chlorine which is formed on reaction with acids .

NaOCl + 2HCI - NaCl + H20 + Cl2


The determination of available chlorine in commercial hypochlorite solutions is usually by standard iodometric analytical methods.

Commercial products used in textile processing generally have an available or active chlorine content of either 150 g/l (for sodium hypochlorite liquor) or 35% by wt (for bleaching powder).

The liberation of chlorine, hypochlorites are used for bleaching cotton under alkaline conditions that lead to the release of ‘active oxygen’. 

The bleaching efficiency of the very slightly dissociated hypochlorous acid (HOCI) is based on its decomposition to hydrochloric acid and active (nascent) oxygen.

In practice only a small amount of active oxygen is formed in the initial stages of the reaction. When the active oxygen is used up for bleaching, more hydrochloric acid and active oxygen are formed, allowing the reaction shown to continue. 

The release of increasing amounts of hydrochloric acid causes the pH to drop to a level where more hypochlorous acid is generated by hydrolysis and this in turn increases the formation of active oxygen and more hydrochloric acid. 

Thus bleaching with hypochlorite can soon get ‘out of hand’, unless careful control of pH is exercised.

NaOCl + H2O - NaOH + HOCI

HOCI - HCI + 0

In hypochlorite bleaching close attention must be paid to the following points in order to avoid fibre damage.

a - Bleaching must always take place under alkaline conditions, and the hypochlorite bleach liquors should be buffered with soda ash to avoid excessive formation of active (nascent) oxygen. The pH at the beginning of the reaction should be 11.5-12.0.

b - To avoid increasing the rate of reaction, i.e. formation and breakdown of thehypochlorous acid, the bleaching temperature should not exceed 20-25°C.

c - To prevent excessive formation of hypochlorous acid, the pH must never be allowed to drop below 10-11 until the very end of the bleaching process.

A disadvantage of a hypochlorite bleach is that the bleached goods have atendency to yellow, whereas a peroxide bleach produces a stable white.

Jig bleaching with hypochlorite also requires treatment for about an hour at ambient temperature, depending on batch size. However, a more concentrated bath (2-4 g/l active chlorine at pH 11.5) is needed because of the short liquor ratio.

Sodium chlorite bleaching

Sodium chlorite (NaCIO2) is available commercially in the form of a white free-flowing crystalline powder (50% by wt sodium chlorite) or a yellowish clear liquid (24-26% by wt sodium chlorite). The powder form contains sodium nitrate and has better storage stability. Sodium chlorite is a powerful oxidising agent and advice provided by the manufacturer regarding handling, safety and storage should be followed.

The powder form is hygroscopic and its decomposition is accelerated by heat, catalytic contamination, acids and reducing agents. Mixtures of sodium chlorite with combustible substances or reducing agents can create hazards, particularly if subjected to heat, friction or impact. Even the liquid product may cause combustion if allowed to dry out on flammable materials. An explosive and toxic gas, chlorine

In addition to the chemicals already mentioned, chlorite bleaching baths contain two other components. Sodium nitrate is added as a corrosion inhibitor to protect stainless steel surfaces. An anionic surfactant is necessary to assist penetration and soil removal, but must be carefully checked for compatibility with sodium chlorite.

Jig bleaching with chlorite takes 1-3 h at 85-90°C and requires 5-7 g/l sodium chlorite and 2-3 g/l sodium nitrate at pH 3-4 and 5: 1 liquor ratio. Jig windows should be kept closed. After bleaching the fabric is given two ends in hot water, two ends antichlor treatment (0.2-0.4% sodium pyrophosphate or 0.5-1.0% sodium metabisulphite) and finally two ends in cold water.

Jig bleaching of certain cotton fabrics is still widely practised. It is possible to use an organic stabiliser to the exclusion of sodium silicate, thereby giving the bleached goods a better handle. A further advantage is the easier removal of organic stabiliser during washing at about 5:1 liquor ratio on the jig. The chemicals (2-5% hydrogen peroxide, 0.5-1.5% organic stabiliser and 1.0 to.0- 1.5% sodium hydroxide) are added over two ends and the temperature is raised gradually to 80-95°C. After bleaching for 1-3 h at top temperature the fabric is washed thoroughly over two ends in hot water and several ends in warm water.



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