Some Notes about Bleaching

Effect of Metals on Hypochlorite Bleaching

Copper and Iron catalyze the oxidaton of cellulose by Sodium hypochlorite degrading the fiber. Fabric must be free from rust spots and traces of metals otherwise bleaching will damage the fabric. 

Stainless stell equipments should be used and care must be taken that the water supply is free from metal and rust from pipes. Prescouring from chelating agents become an important step when bleaching with sodium hypochlorite. 

Weight Loss of Fabric in Bleaching

After bleaching operation the weight loss in the material takes place and it depends on different types of bleaching agents that are used. Due to the removal of coloring matters and fiber damage in the bleaching, textile material may lose considerable weight. In a study it was found that in plain weave fabrics, the weight loss was upto 11% for Sodium hypochlorite bleach and upto 8% in Hydrogen Peroxide bleach.

(Source:http://dspace.daffodilvarsity.edu.bd:8080/xmlui/bitstream/handle/123456789/519/Determination%20of%20weight.pdf?sequence=1)

 

 

 

Some Selected Notes on Textiles- Part 4

- Colorfastness to Light: In this the samples is exposed to a light source along with blue wool sample. The fading is observed and the rating is given.  The main difference between American and European Standards is that American standard is time bound. In European standards it is not a time bound test but fading bound test- means you have to keep on exposing the sample till the sample fades to that of blue scale. Factors affecting lightfastness are the type of dyes used, depth of shade, surface structure and finishing chemicals. 

- A cotton pigment printed fabric cannot be dry cleaned . In dry cleaning solvent called Perc- Perchloro Ethylene or Tetrachloroethylene. Pigments are very susceptible to dry cleaning- they will come out. Perc is the only solvent that do not blast. In exports they use MTO . The problem with perc is that it damages Ozone layer. Perc will take out the pigments from the print. However, if you dry clean with MTO, nothing will happen.  Perc does not smell as much as MTO does.  To test for dry cleaing, a bag is prepared where instead of steel balls, stainless steel discs are added. 

- To remove smell from pigment printed fabric, it has to be cured well. To do it, maintain a temperature of 150 deg and allow the fabric for 10 minutes. It will cause phosphoric acid to get liberated and pigment get fixed- for that Diammonium phosphate needs to be added – so fixation takes place under acidic condition.  

- When testing for shrinkage, the fabric is conditioned for 24 hours. Then it is subjected to washing process for 45 minutes to 1 hours. Then it is dried and reconditiond before remeasuring.

- When testing for shrinkage, soft water is used. Alum can be used for softening. Just move the alum on the upper surface for a few times. Now decant the upper portion and use the rest of the water for testing. One can also use wetting agent. 

- Drying can be line drying, wet drying, tumble drying or drip drying. 

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Some Selected Notes on Textiles- Part 3

- Color Fastness to Rubbing – There are two types of it, dry and wet. Dry rubbing is important for the materials like sofa covers where a person sits on it. Normally it is done using Croakometer with 10cm track length and 9.8 N force.

- For all colorfastness evaluation it is done either visually or using spectrophotometer. Normally worst rating is given.

- Colorfastness to water – It is done to measure colorfastness to water under intimate contact when getting wet, for example contact of inner wear to outer garments in rain. The sample is wet in distilled water , put between two acrylic sheets and then it is clamped under a force of 4.5 kg and put in incubator at 37 degrees Celsius for  4 hours.

- Colorfastness to Water is not the same as colorfastness to washing.  In measuring colorfastness to water we use distill water, whereas in colorfastness to washing, the detergent is used which makes the pH alkaline. A case is cited where a fabric had excellent colorfastness to washing but poor colorfastness to water. It can be due to the fact that the fabric was dyed with reactive dyes. There was some hydrolyzed dye present, but it had no reactivity and hence it was unable to react with water so there was no bleeding. On the other hand that hydrolyzed dye migrated in case of pressure and water hence had poor colorfastness to water.  Thus rigorous soaping could have solved the problem. Taking the other aspect, a fabric had poor colorfastness to washing but excellent colorfastness to water. It can be due to the fact the dye is probably sensitive to the pH.

- Colorfastness to Perspiration- Test is done similar to Colorfastness to water; the only difference is that a solution is prepared which simulate perspiration and the fabric is dipped in it, rest of the procedure is the same. Two types of solutions are prepared , one simulating alkaline perspiration with a pH of 8.0 and the other simulating acidic perspiration with a pH of 5.5. To prepare this solution Sodium Chloride and Lactic Acid are the main ingredients. The test conditions of 37+- 2 deg Celcius for four hours under a pressure of 5 kg. – Picture Source and Procedure. 

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Some Selected Notes on Textiles- Part 2

- Acid dyes are used for silks. There are three categories of acid dyes- Leveling or strong acid dyes, Milling or weak acid dyes and super milling or fast acid or neutral dyes. The colorfastness of fabric is decided by which of these dyes are used.

- Colorfastness to wet scrubbing is only for pigment printing. Pigment has no affinity to fiber yet it can be applied to all the fibers.  Here a brush is used to see the shade change. 

- Colorfastness to ozone is used only for Denim, which tends to turn yellow either due to atmospheric fumes or ozone. 

- REACH- Registration Evaluation Authorisation of Chemicals is a European document which enlists 800 chemicals which are carcinogenic. Out of which 56 chemicals are SVHC- substances of very high concern. Out of these 56, only 25 are related to textiles. 

- Presence of Alkyl Phenol Ethoxylate is a compound present in cheap detergents. 

- Colorfastness is the property of a color to retain its color under different conditions of use. 

- Colorfastness to washing is checked for many different fibers using a multifiber fabric. It is important to know the colorfastness against different fibers as we don’t know which all fibers will go together in a washing load. Please see Picture Source and Procedure here.

Washing Fastness Tester

- ISO grey scales are used. There are two types of shade cards- color staining and color change ( shade change).  Grey scales are used as opposed to colored scale because only intensity of color change is seen vis-a –vis intensity of grey scale. 

- Internationally a ½ point deviation is acceptable. Normally the evaluation should be done by minimum two people. This is done with D-65 light source for evaluation. 

- This scale is used  for all tests except light fastness ( For light fastness we use 8 scale- Blue wool scales are used)

- Cross staining is transfer of color from a darker to lighter component within the fabric. 

- A case is sited where there was a red dress with white piping. The red dress had a colorfastness of 4-5. But still it was enough to make the white piping pink which was not acceptable. For this case the minimum acceptable fastness was 5. 

- When measuring colorfastness to washing, the test replicates 5 typical home launderings. 

- The Load contains multi fiber test fabric, standard reference detergent, steel balls, washing devices and grade 3 water ( grade 1 is the purest). Addition of steel balls accelates the process. In testing silks steel balls are not used. 

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Some Selected Notes on Textiles - Part 1

- Strength of Cotton is in between silk and wool. Silk is the strongest fiber.

-  Cotton is less elastic than Silk.

- Cotton wrinkles as the cellulose from which it is made has hydrogen bonds that break when washed due to agitation or mechanical action. They are formed again when ironed but in different places. Read more...

- Perspiration is of two types- Alkaline and acidic. Majority of the perspiration is alkaline. Cotton is not affected by alkaline perspiration whereas silk does.

- Coloration principle involves physical absorption of water soluble dyes, transferring of dye on to fiber, dye becoming water soluble and is retained inside the fiber.

- Reactive dyes physically react with fibers and form covalent bonds. The process involves two steps: 1. Dyeing with the dye in presence of salt to effect as exhaustion as possible ( transferring of dye from solution to fabric) and 2. Chemically reacting the dye with water in presence of alkali like soda ash.

- Reactive dyes also react with the water in which they are dissolved.  This is called hydrolysis of dyes. These hydrolysed dye has to be removed from the fiber by proper soaping else color fastness will be a major issue.

- Disadvantage of reactive dyes are that the hydrolysis accompanies fixation, as told earlier. Also there there is approximately 30% wastage of dye due to hydrolysis. Only 70% of the dye react with fiber.  To remove hydrolysed dyes there is an excessive consumption of water. Also colored effluent discharge is a major problem. It also involves large amount of electrolyte ( Common salt) as a exhaustive agent. It is also costlier. 

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Process Control in Warping-1

Process Control in Warping

Warping is the process of winding together on a beam a specified number of warp ends from Creel. The creel is a convenient rack for holding spools, cones or cheeses while the yarn is withdrawn to the warp beam.

The following are the process control parameters in a warping department:

1. Minimising End breaks.
2. Quality of warping beam
3. Control of productivity

Importance of Minimising end breaks:

The stoppage of the machine due to an end break is likely to deteriorate the quality of the beam due to three reasons:

1. The rubbing of the beam by the drum which stops abruptly.
2. Owing to the difficulty in finding the broken end, there is a possibility of incorrect mending. It may lead to lappers during sizing.
3. There is some loss in the extensibility of broken end when the machine is stopped, it increases the probability of breakage in weaving and sizing.

The following points should be noted to improve the quality of warping beams:

1. condition of beam flanges:

If the beam flanges are damaged, the unwinding of yarn near the flanges will not be satisfactory. This will cause difficulties in sizing and weaving.

2. Stop Motions and Breaks:

Proper stopping of the warping machine after an end break ensures that the broken end on the beam can be traced easily.

3. condition of the driving drum:

On most warping machines the beam is driven by fricitonal contact with the driving drum. In order to get a package of the correct density, the pressure between the drum and the warper's beam has to be kept at fairly high level.

4. Barrel Diameter of the Beam

Beams of small barrel diameter give rise to high unwinding tension at sizing, particularly when the beam is about to become empty.

5. Cuts in Accessories in the path of yarn

Drop pins of stop motion, guide rollers, reed denting etc. should bot have any grooves.

6. Creel Fans

Fluff accumulated on the machine, particularly at thread guides, causes tension variations in the yarn. This fluff can pass on to the beam.

7. Length Measuring Motion

The length measuring motion should be accurate, otherwise estimation of beam count would be wrong and subsequently will give incorrect values of size percent which is commonly determined from the weights of yarns on the warper's and the size beams.

8. Density of the Beam

The beam should be firm, inadequate pressure between the beam and the drum causes soft beam. Adequate pressure should be maintained by making suitable mechanical adjustments.

Control of Productivity

The productivity at warping depends upon the machine efficiency and machine speeds. The speed is governed by the mechanical condition of the machine and its design. Machine efficiency depends on several factors, such as the breakage rate, the time taken to mend the machine stop, set length, length of yarn on supply package etc.

Practical Notes on Rope Dyeing for Indigo Dyed Denim

Practical Considerations in Rope Dyeing for Indigo dyed Denim.

The passage of yarn in rope dyeing is as follows:

Pre-scouring -->hot wash-->cold wash --> Dye baths--> hot wash-->cold wash--> application of softener

lets discuss these processes one by one:

Pre-scouring

1. The objectives of pre-scouring are the removal of wax content from cotton, removal of trapped air from cotton yarn and Making yarn wet

2. This is done at 90 o C

3. We use the following ingredients at pre-scouring stage:

Caustic Soda: Its quantity depends upon the quality of cotton fibres used in the mixing. Generally we take 2-4% of caustic soda. It removes the wax by the action of soapanification.
Wetting agent: It is anionic in nature
Sequestering Agent: Even with the use of water softening, it is very difficult to find the desired softness in water ( about 2-3 ppm) . So we use the agent to make the water soft.

4. Why Trapped Air should be removed. The reason for this can be understood as follows:
In 1 kg of yarn, there is approximately 2 litres of air. 1 litre of air decomposes 1.8 litres of Sodium Hydrosulphide. It will cause uneven dyeing and more consumption of Sodium Hydrosulphide ( hydro).

5. Absorbency of yarn may be checked after scouring.


Hot wash

As some caustic is carried by the yarn after pre-scouring, so hot water is given at 70-800C. If this is not done, this yarn will go into the dye-bath which will change the pH of the dye-bath.

Cold Wash

After hot wash, yarn temperature is more. To bring it back to its room temperature, cold wash is given to it.

INDIGO DYEING

1. Indigo is not a perfect vat color. It may be called a trash vat color. The constant of substantivity for other colors is 30, for indigo it is only 2.7. So there is a need of 5 to 6 dye baths and make the use of multi-dip and multi-nip facility to increase the penetration.
2. The dyeing is done at room temperature as indigo belongs to Ik class of vat dyes, where dyeing is done at room temperature and oxidation is done by air only and not by chemicals. If oxidizing agents are used, they will cause stripping of colors.

3. Indigo is not soluble in water. So it is reduced with Sodium Hydrosulphide. Then caustic soda is added to make sodium salt of vat colors to make it soluble. To reduce 1 kg of Indigo, 700 gms of sodium hydrosulphide is required. However some extra SHS needs to be taken to avoid some decomposition of SHS.

Practically it is prepared in the following sequence

-Take indigo
-Add caustic
-Then reducing agent

4. When caustic is added to indigo, it is an exothermic reaction. It is allowed to cool down, then before sending it to feeder, sodium hydro-sulphide is added. Reducing agent is not added first as it will be decomposed first, so consumption of it will increase. It is also not advisable to take solubalised vat, as offered by some companies due to the following reasons:

a. If it is used after 6 months, there will be a decomposition of sod. Hydrosulphide. It will become partially soluble. Then to make it soluble again, more SHS has to be added.
b. Transportation is difficult
c. Cost is more

5. Feeding System

Rat of flow of yarn is given by

((No of ropes x no of ends x speed of machine)/ count x 1.693 x 1000)

in kg of yarn / minute

So we can determine the rate of feed of indigo. It is very important that replenishment of indigo is there as any variation will result in the change of shade and also if level is more, there is a problem of over-flow.

6. If total capacity of dye bath for example is 15000 litres, then circulation must be 3 times the volume. If it is less then there are 100% chances of getting a lighter shade.

7. Core and ring dyeing effect
This effect is obtained by multidip-multinip facility

8. pH of the Dye bath should be kept in between 10.5-11.5. At this pH , sodium salt of Indigo is mono phenolic form. At this form, the strike rate of dye is very high. So after washing, there will be a better dye effect. At pH 11.5 to 11.7, at this affinity is less, so dye effect will be less prominent.

pH is controlled by the addition of caustic soda.

9. Testing of Hydro

TOTAL HYDRO
We take 10 ml of indio with SHS in 30-35 ml of water. It is set for one minute and shaken. As air will decompose SHS. So vacuum created will fetch the water from above. If 3 ml of water is required, then concentration of hydro is 3 gpl. As a thumb rule, concentration of total hydro should be min. 1.5 gpl.

REDUCED HYDRO

It is the hydro that is used for the reduction of Indigo. It should be around 0.7 ( 1000 kg of Indigo needs 700 kg of hydro to reduce it). For testing we take 10 ml of dye solution and 30 ml of water and 5-6 drops of 40% formaldehyde and shake it for one minute. The water that goes gives the readings of the reduced hydro.

Total Hydro- Reduced Hydro = free hydro

If Total hydro is min. 1.5 gm/lit. then free hydro must be min. 0.5 gms/ litre which acts as buffer

10. Also hydro reduction capacity is measured by mV meter which measures the Redox Potential.

It should be around 760-800

Through the day, the redox potential should be +- 20 mV of the norm. If it is more then the process control is a failure.

Caustic--> It is around 0.4 to 0.5 times the hydro used.

Washing
Rubbing fastness of indigo is very important. On a scale of (1-4), it is 2. Washing is done to improve the rubbing fastness.

Wash at 60 deg.--> Wash at 60 deg.--> Wash at room temperature--> wash with softener

Why Softener:

1. The rope is going to be opened at Long Chain Beamer. It the softener is not used, opening will be hampered.

2. It is generally 1.2% of the weight of the yarn. It is a cationic softener. It is always having pH in the range of 4 to 55. Softening is done at room temperature. If high temperature is used there is always some chance of tendering of yarn.

3. Concept of Buffer pH is given by Virkler USA, they say by addition of this, there is 40% less consumption of Indigo for same shade depth.

4. Metering Consumption

If solution is of 900 litres
10% Indigo-->90 litres
Hydro--> 90*.7 = 63 kg
Caustic--> 63*0.445= 28 kg.