Making a lot size for preparation or dyeing:

Grey stage fabric usually 60-70 m in length so that  several pieces are stitched together to make a processable length ( Lot size ).  Normally Polyester (Sewing thread ) yarn is used for stitching. Marked each piece properly for sort number, lot number etc. so that pieces can be identify at any stage of the processing. Two pieces are arranged selvedge to selvedge and the ends are stiched on a special end to end stitching machine so that pieces are joined without overlapping: Some of the important poits are given below:

•         Cotton threads: 3/40s, 4/30s, 4/36s, 6/28s and 6/36s are popular

•         Polyester threads are also used when carbonising is required

•    Stitches per inches:  6 to 7 for coarse, 7 to 8 for poplin and 8 to 10 for cambric

Stitching defects: Stitching defects are described below:

i)  Protruding Selvedge: It mostly occurs due to difference in width of the pieces. Some time careless handling also lead to a protruding selvedge.

It Causes: Fold or selvedge crease and the fold portion either less dyed or       remain undyed

ii)  Open End: Pieces are not locked in the stitch at and near the selvedge and due to careless handling.

It Causes: Such stitch will open out more and more every pull around guide roll during processing. It may induce Fold or crease formation at the selvedge, undyed/unprinted folds will result.

iii) Loose Thread: It occurs due to careless handling of stitching process.

It Causes: It may cause thread mark in dyeing and printing, lead entanglement in machine parts, etc. The loose thread may pick up soil and damage long lengths

iv) Fold: If the large folds are inserted in the stitch to accommodate width difference of two pieces, it will surely lead to damage

It Causes: It may cause warp stripes in dyeing and creases in dyeing and printing

v)Broken Stitch: It may be due to poor machine condition, poor quality of thread or poor method of operation.

It Causes: It may cause damage in dyeing and printing

Human Visual System

 Retina uses special cells called “rods” and “cones”. Rods “sees” in black, white & shades of grey and tell us the form or shape(Super-sensitive allowing us to see when it's very dark).Cones “senses” colour but need more light. Three types and each is sensitive to one of three different colours - red, green, or blue. Together these can sense combinations of light waves (To see millions of colours). Rods and cones together process the light to give you the total picture.

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PERCEPTION OF COLOUR

•         It involves a series of events which are interdisciplinary in nature. Perception of colours includes source of light, object that is illuminated and eye and brain that perceive the colour.

METAMERISM: It is a phenomenon observed when two specimens appears to have the similar colour under one set of viewing conditions, but different under another. The change in viewing conditions refers to change in source, observer or geometry of the observation.

TYPES OF METAMERISM

•         illuminant metamerism

•         observer metamerism

•         geometry metamerism

ILLUMINANT METAMERISM

•         When the colour of two specimens match under one illuminant but not with another illuminant, it is said to be illuminant metamerism.

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OBSERVER METAMERISM

•         When colour of two objects appear to match to one observer but not to the other, it is said to be observer metamerism. In this case observer may not be colour blind but the spectral sensitivity of colour receptors of one observer may be slightly different than the other.

GEOMETRY METAMERISM

•         Two objects which match in one arrangement of illumination, sample and observer may mismatch by altering the positions.

Kubelka-Munk  equations:

•         The mathematical basis for all color matching software is the Kubelka-Munk series of equations. These equations state that for opaque samples such as textile materials, the ratio of total light absorbed and scattered by a mixture of dyes is equal to the sum of the ratios of light absorbed and scattered by the dyes measured separately. Where absorption is defined as "K" and scattering is defined as "S", Kubelka-Munk states that _:

             (K/S) mixture = (K/S) dye 1 + (K/S) dye 2 + (K/S) dye 3 + ...

K/S is not a readily measurable quantity, but it can be calculated from the reflectance of a sample -- "R" -- by the Kubelka-Munk equation that states

                                 K/S= (1-R)²/2R

K/S  value is proportional to dye  concentration in the substrate. Means dark shade will have higher K/S and Light shade will have lower K/S value.

Now-a-days computer colour matching is used to determine the K/S value. 

CIE Color Systems

CIE Color Systems The CIE, or Commission Internationale de l’Eclairage (translated as the International Commission on Illumination), is the body responsible for international recommendations for photometry and colorimetry.

In 1931 the CIE standardized color order systems by specifying the light source (or illuminants), the observer and the methodology used to derive values for describing color. The CIE Color Systems utilize three coordinates to locate a color in a color space. These color spaces include:

• CIE XYZ

• CIE L*a*b*

• CIE L*C*h°

Measurement of colour:

Following methods are used:

i) Munsell Scale

ii) CIE colour system

Munsell Scale:

In 1905, artist Albert H. Munsell originated a color ordering system — or color scale — which is still used today. The Munsell System of Color Notation is significant from a historical perspective because it’s based on human perception. Moreover, it was devised before instrumentation was available for measuring and specifying color. This system assigned numerical vlaue to the three properties of the colour-Hue, Chroma and value.

Munsell saw that full chroma for individual hues might be achieved at very different places in the color sphere. For example, the fullest chroma for hue 5RP (red-purple) is achieved at 5/26

Another color such as 10YR (yellowish yellow-red) has a much shorter chroma axis and reaches fullest chroma at 7/10 and 6/10:

Methylene Blue absorption Method:

Carboxyl groups present in the cellulose may be determined by methylene blue absorption method. This method is based on the principle that when cellulose containing carboxyl group is titrated with a standard solution of methylene blue, under specified conditions, the dye (mehtylene blue) cation is quantitatively absorbed and retained by the carboxylate anion in the cellulose.

The drop in concentration of the dye in the treating solution can then be found out colorimetrically and the amount of carboxyl groups present in the cellulose can be calculated in terms of milliequivalent/100 g of dry cellulose. For this purpose pure methylene blue is required.

In this study first of all, a calibration curve is obtained with the known concentration of methylene blue solutions ( 2-16 mg/litre with increment of 2 mg/litre). The curve will be between absorbance and known concentrations of methylene blue solutions. 

After plotting the curve, in the second step, the concentration of  methylene blue solution left after treatment of cellulose is determined using colorimeter. The concentration of the dye in a solution to be estimated by finding out the absorbance of a solution at 620 nm  wavelength. Following reagents are used in this study:

Reagent-1: Mehtylene blue solution: 0.2 moles/litre

Reagent-2: Potassium dihydrogen phosphate solution: 0.625 milli mole/litre

Reagent-3: Sodium hydroxide solution: 0.4 mole/litre

To start study a stoke solution of methylene blue dye should be prepared by taking methylene blue, potassium di hydrogen phosphate and sodium hydroxide in the above proportion. 

Determination of copper number:

When cellulose is bleached i.e oxidised and hydrolysed by treatment with mineral acid solution , reducing groups are formed. These reducing groups are capable of reducing an alkaline solution of copper sulphate, when insoluble cuprous oxide is formed is proportional to the amount of the reducing groups present in the cellulose sample. This is expressed in terms of Copper Number, which is defined as a gram of cupric copper reducing to cuprous oxide by 100g of the cellulose sample under standard conditions of boiling in an alkaline medium.

To determine copper number following procedure is followed:

        Solution A: 100 g/litre hydrated copper sulphate

        Solution B: 50 g sodium bicarbonate + 150 g anhydrous sodium 

                          carbonate in 1 litre of distilled water

        Solution C: 100 g/litre ferric alum and

        Solution D: N/100 Ceric sulphate

Ø  Take cellulose sample whose copper number to be determined and conditioned it at 30^oC at 65% RH for 48 hours

Ø  Determine moisture content of the conditioned sample and note down the value

Ø  Take 0.25g of the dry sample in dry test tube in triplicate. For highly degraded sample, 0.125g of sample may be taken

Ø  Mix solution A and B in the ratio of 5:95 and boiled. Take 15 ml of this solution and add to all the tree test tube. Now each test tube will have cellulose sample and 15ml of mixture of solutions.

Ø  Mouth of these tubes closed using glass stopper and placed in the boiling water bath.  Along with these three test tubes, also placed three blank test tubes (only contain solution, No sample) in the boiling water bath. Boiling continue for three hours.

Ø  Cooled above test tubes and content of each test tube filter using Gooch crucible (Sinterd glass, AG3) and washed with distilled water (now the colour of solution become red).

Ø  The residue in the crucible also treated with first 10 ml and then 5 ml ferric alum solution (Solution C).  The residue also washed with successively with 10 ml and 5 ml 2N sulphuric acid. All these process are carried out on each test tube.

Ø  The solution so collected is titrated against N/100 ceric sulphate (Solution-D) using a few drop of ortho-ferrus phenathroline indicator (the initial red colour of the solution turns Green at the end point).

Ø  The blank alkaline copper solution gives a reading of 0.02 ml of N/100 ceric sulphate solution. This blank reading is subtracted from the actual reading in each case, where the cellulosic sample is treated.

Ø  The copper number is calculated from the amount of ceric sulphate solution consumed (after deducting blank reading) by ferrous sulphate formed by the reduction of ferric sulphate sulphate by cuprous oxide deposited on the cellulosic sample during the three hours of boiling.

Copper Number= (63.5 x V x N x 100)/ W x 1000

=(6.35 x V x N)/W

Where V is the titration reading (ceric sulphate solution) after deducting the blank reading, N is the normality of ceric sulphate solution and W is the weight of the bone dry cellulosic sample

Some facts of hot and cold mercerisation:

Hot mercerization: Hot caustic lye at about 60^oC penetrate better and more evenly to the yarn core of the fabrics. Swelling is better as compared to cold mercerization which helps to improve the dimensional stability of fabric. Hot mercerization ensure even dye penetration and imparts a softer handle and better appearance. Shorter impregnation time requirement reduce investment cost. For hot mercerizing, the impregnating section is equipped with an indirect steam heating system with temperature controlled to maintain uniform temperature.
   

  Cold mercerization:In the cold mercerizing the cold impregnating process impart more lusture to the fibre than the hot impregnation system mainly due to the greater shrinkage and high tension needed the re-stretch the fabric to the desired width. 

How to measure cotton degardation during bleaching and other processing?

It has been seen that during bleaching process the cotton (or cellulose) undergo oxidative degradation and form oxycellulose. Such degradation not only reduce the cellulose molecular chains or other word lower the degree of polymerization (DP) but also introduces new functional groups such as aldehydes, ketones and carbonyls.

The oxycellulose that form during degradation is of two types:

i) Reducing oxycellulose

ii) Acidic Oxycellulose

To measure the above two types of oxycellulose followoing two methods are adopted:

i) Copper number for reducing oxycellulose

ii) Methylene blue absorption test for acidic oxycellulose

The reducing oxycellulose, which act as reducing agent and copper number is the grams of copper which are reduced from cupric to the cuprous state by 100 grams of cellulose when boiled with fehling's solution.

Pure cellulose has no affinity for methylene blue, but the formation of acidic oxycellulose is generally accompanied by the appearance of carboxyl group which confer an affinity.

These tests are also helpful in distiguishing between oxycellulose from acid oxidation (low values of methylene blue absoption) and those from alkaline oxidation (high methylene blue absorption).

Heat setting of polyester:

Heat-setting is a heat treatment by which shape retention, crease resistance, resilience and elasticity are imparted to the fibres. It also brings changes in strength, stretchability,  softness,  dyeability and sometimes on the colour of the material. All these changes are connected with the structural and chemical modifications occurring in the fibre.

Heat setting is the process applicable to fabrics made from synthetic fibres like nylon, polyester in which the fabric is subjected to the action of high temperature for a short time to make it dimensionally stable so that the garments made from such fabrics retain their shape on washing and ironing.

The main aim of heat setting process is ensure that fabric do not alter their dimensions during use.

This is process for the stabilisation of synthetic fibres so they do not shrink on heating.

Polyester shrinks -7% in boiling water and 20% at 220 degree C.

Nylon-12% in boiling water 

Higher the pressure higher the shrinkage. So it is necessary to stablise dimensional stability of such fabrics so that they do not shrink during processing and during washing. The stablisation of dimensions of synthetic fibre/ fabrics by exposure to heat is possible, because these fibres resists shrinkage at high temperatures, if they are held tight at this temperatures for a short period. All synthetic fibres whether they are produced by melt, dry and wet spinning process are deficient in strength in their freshly formed condition.

To give them strength they are stretched several times to their original length. This high degree of stretching causes orientation of chain molecules. But this high degree of stretching is not sufficient to make these fibre stable. The single chain held together by vander waals  force and hydrogen bond, are not completely stretched. These chains start vibrating if energy is supplied to them in the form of heat. As further heat is supplied the intensity of vibrations increase and when it reached at a certain level  the vibration become so strong that some of hydrogen bonds are broken, thus there is breakdown of intermolecular hydrogen bonds so that the molecules are freed from the restraints which maintain orientation. This relaxation causes contraction of chains and therefore longitudinal shrinkage occurs . As the temperature increases, shrinkage also increase till a point is reached when the fibres melts. If the supply of energy is stopped as soon as the maximum potential energy is reached and the fibres are cooled rapidly, the hydrogen bonds gets stabilized. Heat set temperature should be above the glass transition temperature.

TG- transition temperature –the temperature at which the molecular movement starts in amorphous region of the polymer.

Glass Solid – Rubbery Solid

Melting point: at this point the forces holding the molecules in the crystalline regions of the fibre are overcome by the thermal energy and the polymer melts. In both polyester and nylon these temperatures are separated by about 150°C.

Three different stages of heat setting:

Heat-setting can be carried out at three different stages in a processing sequence i.e.

1. in grey condition (scarcely applied)
2. after scouring (Frequently applied); and
3. after dyeing (Scarcely applied) .

The stage of heat-setting depends on extent of contaminations and types of fibres or yams present in the fabric. Heat setting after dyeing could lead to the sublimation of disperse dyes (if not accurately selected).

The process grants excellent dimensional stability and good crease-proof properties. As far as operating conditions are concerned, the fabric must be treated in accurately controlled moisture and temperature conditions.

Heat setting of Some Fibers

Stenter Machine

Stenters are widely used for stretching, drying, heat-setting and finishing of Fabrics. The stenter frame is usually 80-100 feet long and 70-100 inches wide. The speed ranges from 10-45 m/min with a maximum setting time in the setting zone 30 sec at temperature ranging from 175 to 250~ depending upon the thickness and type of the material. 

 Iodine absorption test:

Heat set polyester absorb less iodine than the corresponding unheated material and this property is used for assessing the degree of heat setting of polyester. To do this test following is the procedure:

Take 1 grm of sample accurately weighed into a 250 ml stoppered flask and 30 ml of 0.1 N iodine solution (Prepared by dissolving 12.7 g iodine and 20 g potassium iodide in water. To this solution is added 100 ml of glacial acetic acid and 350 ml phenol (measured at 60^oC and finally diluted to 1 litre with water) is added and allowed to stand for at least 2 hours. After this period, the specimen is transferred to a sintered glass crucible (G1) and washed with water till free from iodine. The sample is then transferred to a 250 ml flask containing 50 ml chloroform. As the chloroform is powerful swelling agent for polyester, the iodine absorbed by the polyester quickly passed from the fibre to the chloroform. Exactly 10 ml of 0.1 N sodium thiosulphate is added and the mixture is titrated against 0.01 N iodine solution, using starch as indicator. Simultaneously a blank determination (without sample) is carried out under the same conditions. The absorbency of iodine is expressed in mg of iodine per gram of fibre i.e

Iodine absorption (mg/g)= {(X-Y) X 0.01 X 127}/ W

Where, X=ml of 0.01 N iodine required for blank

Y= ml of 0.01 N iodine required for sample

W=Weight in g of sample

If the polyester is blended with cellulose then later portion should be removed by carbonization (dissolving in sulphuric acid) and only polyester portion is taken for the above test.

Also note that the there is a significant effect of temperature on iodine absorption by polyester. With increase of temperature the iodine absorption value also increases. It is therefore necessary to carry out the test at the same temperature, in order to get reproducibility of the results.