chemical test of fiber

Chemical Tests
Chemical tests for fiber identification can only be conducted in well equipped laboratories. There are two primary methods to conduct chemical testing- stain and solvent.

Stain Method: Stain technique uses acid and alkali on different fabrics to identify their fiber contents. Most of the fibers have two color reactions when treated with stain. A fiber stained with dilute acetic acid turns to a specific color. The same fiber when stained with mild alkali like soda carbonate turns to a different color again specific to that fiber only. Acetate changes to light green color when acetic acid is used and turns orange when dilute carbonate of soda is used. Likewise, nylon turns beige in one and bright red in other. As double testing is done in this method, it is sometimes referred to as double-barreled stain identification.

Solvent Method: Various solvents are used in this method to distinguish one kind of fiber from another. However, there is no single solvent or chemical that can be used on all fibers. Additionally, different solvent procedures are adopted to separate and identify the fibers that are combined together. It becomes very difficult to use solvent methods in view of fibers that have similar chemical characteristics. Also, when more fibers are mixed to produce blended fabric, then also it becomes tough to identify the fibers with the help of solvent method. However, it is a very effective method for cross checking but in order to have accurate reports, the fabric has to be cleaned thoroughly and the finishing chemicals should also be removed completely. The fabric has to be unraveled, yarns have to be untwisted and the fibers have to be put in the solutions in as loose a condition as is possible.
As an example of solvent method, consider differentiating animal fibers from plant fibers with alkali. If wool or silk fiber has to be eliminated from a blended fabric then strong alkalies can be used because animal particles are destroyed in it. Five percent of caustic soda or sodium hydroxide is used in water. The action of the chemical is hastened by boiling the solution before immersing the sample fabric in it. The wool or silk fiber gets completely dissolved in it. The plant fibers remain unaffected. For differentiating them, acid has to be used as the dilute acids destroy plant fibers. A drop of sulfuric acid has to be put on the sample fabric which, in turn, is placed between two blotters and pressed with hot iron. If it contains cotton, linen or rayon then the fabric gets charred at the spot.

Physical Test of fibers

Identifying the fiber:

Identifying the fibers from textile materials mainly used physical, chemical test.

The tests for fiber identification done with the help of laboratory equipment are far more reliable than the nontechnical tests. However, technical knowledge and skill, particularly while handling chemicals, are the basic requirements for conducting these tests. The means of fiber identification can be broken down into five smaller groups or tests. They are burning, chemical, optical, staining, and density tests. Each test has its own advantages and disadvantages. Most are cheap and simple identification techniques, which is good. However, most fibers would need further analysis to be certain about the results. A burning test can help determine the class to which a fiber belongs by observing how the fiber burns. Observing how things smell and char when they are burnt are qualities than can help. However, have to be careful with burning tests in order not to over-generalize as to the type of fiber. The most important burning test pertains to flame resistance. Chemical tests are cheap and simple methods to find a number of elements, but the tests are not quantitative. Also, the number of elements that can be detected is limited. So for more accurate analysis, better and more expensive equipment is needed.

Physical test of fiber:

ü Microscope Test

ü Burn Test

Microscope Test

Microscopes having magnification of at least 100 power, can be successfully employed for testing and identifying the fiber contents of a fabric. Microscope test is very effective for testing the natural fabrics. Difficulties can be faced while testing synthetic fabrics as many of them have similar appearance. However, one must know, what the fibers look like under a microscope as many finishing processes like mercerizing and delustering, change the appearance of fibers under microscope. Apart from it, dark colored fabrics also cannot be tested with microscope as light cannot pass through dark substances. For such fabrics, either the textile dyes haveto be removed by stripping, bleaching etc. or they have to be chemically tested.

Natural fiber

Natural fibers have their own peculiar structures, spots, lines and other marks that help in identifying them. Following are some examples of natural fibers and how they look like under a microscope:

Cotton: The cotton fiber is a single elongated cell. Under a microscope, it looks like flat, spirally twisted ribbonlike tube with rough granular surface. However, mercerized cotton doesn't have natural twist. The finishing process makes them swollen, straight, smooth and round with a shining surface.

Linen: Linen fiber, under a microscope, looks like having multiple sided cylindrical filaments with fine pointed edges. The filaments show nodes at intervals. It, in fact, looks like a bamboo stick having joints that results into a little unevenness.

Wool: Wool fiber has irregular, roughly cylindrical, multi cellular structure with tapered ends. Under a microscope, three basic layers are shown- epidermis (outer layer), cortex (middle layer) and medulla (inner layer). Medulla is seen only in coarse and medium wool fibers and that too under a highly powerful microscope.

Silk: Raw silk fiber, composed of two filaments, has elliptical shape under the microscope. The two fine and lustrous filaments are shown clearly looking like transparent rods with triangular shape. Wild silk or tussah fiber has different appearance than the cultivated silk. It is flattened, coarse, thick and broader fiber having fine, wavy lines all across its surface whereas cultivated silk is narrower fiber with no marks on it.

Manmade fibers
Manmade fibers are difficult to identify through microscope because of similar appearance of many fibers. However, their certain distinguishable characteristics under a microscope have been mentioned below.

Rayons: Rayon fiber has uniform diameter with glass like shine. If delustered then rayon fiber shows marks similar to pepper, when viewed cross sectionally. Viscose fiber of rayon looks irregular when viewed cross sectionally.


Acetate: Acetate fiber looks lesser irregular than viscose rayon when viewed cross sectionally. It has indentations that look like occasional marks when viewed longitudinally.

Nylon: There are many variants of nylon fiber. However, generally it appears fine, round, smooth and translucent. Sometimes it has shiny appearance. If it looks dull, it will also be dotted under the microscope.

Aramid: If viewed longitudinally, aramid fiber looks smooth and straight. If viewed cross sectionally, it may be round or like peanut's shape.

Polyester: Generally, polyester fiber is smooth, straight. It looks round cross sectionally. However, with various finishing processes, its appearance changes in context of texture and luster.

Spandex: Spandex fiber have the outstanding characteristic of appearing like groups of fibers fused together. However, different variants of spandex show different characteristics too. The Lycra fiber looks like fused multifilaments cross sectionally. Individual fibers are dotted and in shape like that of dog-bone. If viewed longitudinally, they appear straight.

Polypropylene: When viewed cross sectionally, polypropylene fiber looks somewhat round but it looks straight and smooth when viewed longitudinally.

Glass: The glass fiber looks smooth, round, translucent, shiny and flexible.

Burn Test

To identify fabric that is unknown, a simple burn test can be done to determine if the fabric is a natural fiber, man made fiber, or a blend of natural and man made fibers. The burn test is used by many fabric stores and designers and takes practice to determine the exact fiber content. However, an inexperienced person can still determine the difference between many fibers to "narrow" the choices down to natural or man made fibers. This elimination process will give information necessary to decide the care of the fabric.
Natural fibers
Cotton is a plant fiber. When ignited it burns with a steady flame and smells like burning leaves. The ash left is easily crumbled. Small samples of burning cotton can be blown out as you would a candle.
Linen is also a plant fiber but different from cotton in that the individual plant fibers which make up the yarn are long where cotton fibers are short. Linen takes longer to ignite. The fabric closest to the ash is very brittle. Linen is easily extinguished by blowing on it as you would a candle.
Silk is a protein fiber and usually burns readily, not necessarily with a steady flame, and smells like burning hair. The ash is easily crumbled. Silk samples are not as easily extinguished as cotton or linen.
Wool is also a protein fiber but is harder to ignite than silk as the individual "hair" fibers are shorter than silk and the weave of the fabrics is generally looser than with silk. The flame is steady but more difficult to keep burning. The smell of burning wool is like burning hair.
Man Made Fibers
Acetate is made from cellulose (wood fibers), technically cellulose acetate. Acetate burns readily with a flickering flame that cannot be easily extinguished. The burning cellulose drips and leaves a hard ash. The smell is similar to burning wood chips.
Acrylic technically acrylonitrile is made from natural gas and petroleum. Acrylics burn readily due to the fiber content and the lofty, air filled pockets. A match or cigarette dropped on an acrylic blanket can ignite the fabric which will burn rapidly unless extinguished. The ash is hard. The smell is acrid or harsh.
Nylon is a polyamide made from petroleum. Nylon melts and then burns rapidly if the flame remains on the melted fiber. If you can keep the flame on the melting nylon, it smells like burning plastic.
Polyester is a polymer produced from coal, air, water, and petroleum products. Polyester melts and burns at the same time, the melting, burning ash can bond quickly to any surface it drips on including skin. The smoke from polyester is black with a sweetish smell. The extinguished ash is hard.
Rayon is a regenerated cellulose fiber which is almost pure cellulose. Rayon burns rapidly and leaves only a slight ash. The burning smell is close to burning leaves.

Thread twisting

Carding Machine

Yarn Processing(Bale Management)

Bale Management

About 15 different types of fibers are used to make yarn. These fibers fall into two categories, natural and synthetic. Natural fibers are those that are obtained from a plant or an animal and are mainly used in weaving textiles. The most abundant and commonly used plant fiber is cotton, gathered from the cotton bowl or seed pod when it is mature. In fact, cotton is the best-selling fiber in America, outselling all synthetic fibers combined.

Fibers taken from the plant leaf or stern are generally used for rope. Other plant fibers include acetate (made from wood pulp or cotton linters) and linen, made from flax, a vegetable fiber. Animal fibers include wool, made from sheep hair, and mohair, made from angora goats and rabbits. Silk is a protein extruded in long, continuous strands by the silkworm as it weaves its cocoon. Synthetic fibers are made by forcing a thick solution of polymerized chemicals through spinneret nozzles and hardening the resulting filament in a chemical bath. These include acrylic, nylon, polyester, polyolefin, rayon, spandex, and triacetate. Some of these fibers have similar characteristics to the natural fibers without the shrinkage problems. Other fibers have special properties for specific applications. For instance, spandex can be stretched over 500% without breaking. Fibers are shipped in bales, which are opened by hand or machine. The picker loosens and separates the lumps of fiber and also cleans the fiber if necessary. The carding machine separates the fibers and pulls them into somewhat parallel form. The thin web of fibers formed then passes through a funnel-shaped device that produces a ropelike strand of parallel fibers. Rollers elongate the strand, called a sliver, into a single more uniform strand that is given a small amount of twist and fed into large cans.

 Fibers are shipped in bales, which are opened by hand or machine. Natural fibers may require cleaning, whereas synthetic fibers only require separating. The picker loosens and separates the lumps of fiber and also cleans the fiber if necessary. Blending of different staple fibers may be required for certain applications. Blending may be done during formation of the lap, during carding, or during drawing out. Quantities of each fiber are measured carefully and their proportions are consistently maintained. 

Yarn Processing (Carding)

Carding

Carding is a section where individualization is done of fiber by various actions in a carding machine. It is called the heart of spinning. The carding machine is set with hundreds of fine wires that separate the fibers and pull them into somewhat parallel form. A thin web of fiber is formed, and as it moves along, it passes through a funnel-shaped device that produces a ropelike strand of parallel fibers. Blending can take place by joining laps of different fibers. From carding action sliver is formed.

Objectives of Carding

► Opening & Individual fibers.

► Elimination of impurities and dust.

► Elimination of short fibers.

► Disentangling of neps.

► Fiber blending & Orientation.

► Sliver formation.

Carding Machine Actions

A machine for combing and paralleling fibers of cotton, flax, wool, etc., prior to spinning to remove short, undesirable fibers and produce a sliver is the carding machine.

1. Carding action between cylinder and flat.  

Purpose   of the cylinder and   flats are

i)   To card the fibers which   means   to open the cotton, even to the   separation of   one fibre from all the others (fibre to fibre separation)

ii) To separate short fibers, dust and dirt and collect outside the cylinder under casing through spacing of the under grid. The wire points of flat oppose the direction of the cylinder wire point, i.e. point vs. point wire system is placed between cylinder and Flat. Flat slowly moves at the rate of 2.5 inches/min and rotates in the same direction of cylinder, because of these reasons, carding action takes place between cylinder end flats which means fiber to fiber separation is achieved. The flat gets full charge of cotton as the flats come in to position over the cylinder.

2. Heel and Toe arrangement

The wire surface of the flat is not parallel to that of   the cylinder. Wire point setting between cylinder and flat is wider   where the cotton enter and closed where the cotton leave. This arrangement is called as heel and toe arrangement. Object   of   heel and toe arrangement is   to   achieve   gradual carding action at each flat. Necessity: When the cotton is carried forward from one flat to another, the air current, centrifugal force, elasticity of fibre causes the fibre to rise from the surface of cylinder. These raised fibres would be rather severely dealt with if they were dragged into the narrow setting between cylinder and flats. This suddenness action is avoided by heel & toe arrangement.

3. Cylinder arch and concentric bends.

On each side of the main cylinder, a strong cast iron cylinder arch is fixed. Cylinder arches support the flats. On cylinder arch, front concentric bend, back concentric bend and   flexible   bend   are assembled   with   cylinder   grinding   and Front   plate   encloses the space between flat   and   doffer.  Back   plate is filed on back concentric bend.    This encloses   the space between flat and licker in.

4. Flexible bend

On each side of the cylinder, a strong cast iron cylinder arch is fixed. On cylinder arch, front concentric bend, back concentric bend and flexible bend are mounted. Flexible bend supports the flats.  Flexible   bend   is used to adjust and   correct    the   setting between flat and cylinder .Flexible bend is provided one on each side of the card.This bend is not flexible but their curvature can be altered, to correct the cylinder to flat setting. The repeated grinding of the cylinder wire points need accurate   adjustment    to   set    the flat as    close   as    0.010.To get wider setting between cylinder and flats, the lacking screw should be released first and then by operating the circular nut the regulating screw is raised.    The flexible bend will    also be raised, carrying the flat away from the cylinder. This will result in a wider setting between the flats and cylinder. When the correct setting is obtained, locking screw is fully tightened so that the position of flexible bend will be fixed till next adjustment.

5. Front plate or percentage plate

BEQ; what will happen if front plate setting is adjusted How will you control the % of flat waste On each side of the cylinder, a strong cast iron cylinder arch is fixed. On cylinder arch, front concentric bend, back concentric bend, flexible bend are mounted. Front concentric bend supports 3 pieces of front plate i) Top percentage plate setting:

Upper edge-10 thou to 60 Thou (10 Thou means 0.010 inch Lower edge-32 Thou. The object of this setting is to control flat strips waste the flats are stripped when they have left the cylinder area by an oscillating comb, and the strips hang at the front of the card like a curtain, being connected together along their lengths by some fibres which bridge from one strip to another. Top % plate setting is very important because it control the thickness and weight of the flat strip. If   this setting is too close- the flow of air over the    top edge   and   between   flat   is so fast   and   flats   are    effectively stripped   and   flat   waste   in the flat    is    transferred    to    the cylinder again which causes poor web buy results in lighter    flat strips. If   the   setting wider- flow of air round the   top    edge    is reduced   but, cotton is now moved from cylinder Wires    to    flats, bridging   them with them with good cotton, i.e. good material is lost as flat strip waste. The   best setting is that strip should barely hold    together by bridging fibres as they are stripped at the front of the* card. Middle   plate   provides    access to   the    cylinder   wire    for stripping and grinding.

6. Back plate

Back   plate    is fixed on back concentric bend.      This    plate encloses the space between flats and licker in.    The object of the back plate is to hold the fibres on cylinder wire and to   prevent undesirable air current.

Setting: Upper edge- 0.010" Lower edge-0.012" Wider   setting causes cloady web due to uneven    distribution of   fibre   across the cylinder because of    the    uncontrolled    air current.

7. Cylinder under casing.

 The objects of cylinder under casing are: i) To hold the good fibres on the cylinder ii) To permit the short fibres, dust, dirt particles to fall down and   collect outside the cylinder grid through spacing    of    under casing.

Settings: Back   0.012   “Middle - 0.032"   Front: 0.064"     This    setting influences   air   current   and production of   fly.     Wider   setting causes   loss of fibres.   The setting is wider at front    because, here the cotton first makes contact with under casing