Fiber properties

Fiber properties are essential thing to know for

various end uses and the products made from it

qualify on the basis of certain parameters.

The various properties are

High fiber length to width ratio

Tenacity (adequate strength)

Flexibility or pliability

Cohesiveness or spinning pliability

Uniformity

Fiber morphology

Specific gravity

Elongation and elastic recovery

Resiliency

Moisture regain

Flammability and other thermal reactions

Electrical conductivity

Abrasion resistance

Chemical reactivity and resistance

Sensitivity to environmental conditions.

High length to width ratio- A pre-requisite for processing of fibers into yarns

and fabrics is that their lengths must be more thanwidths. The minimum length to breadth ratio is100:1.

Length of fibers is also a basis for classifying theminto two groups staple and filament. Staple fibersare of relatively short length fibers; and filamentfibers are continuous length fibers.

The fineness of a natural fiber is a major factor inascertaining quality and is measured in microns(1microns= 1/1000millimeter). In general, finer fibersare softer, more pliable and have better drapability.They are, thus, considered superior and formbetter yarns and fabrics. Fineness of man-madefibers is controlled by the size of spinneret holes.

Tenacity-

Strength of textile fibers is referred to as theirtenacity. It is determined by measuring theforce required to rupture or break the fiber.Sufficient tenacity is required to withstand themechanical and chemical processing as well asmake textile products which are durable.

Tenacity is, directly related to the length of thepolymers, degree of polymerization, strength indry and wet conditions, and types of inter-polymer forces of attraction formed between thepolymers.

Its unit are -gram/denier(g/d) or gram/tex (g/t)

Flexibility-

Fibers should be flexible or pliable in order to

be made into yarns and thereafter into fabrics

that permit freedom of movement. Certain end

uses require greater flexibility, e.g., automobile

seat belts.

Uniformity-

Uniformity of fibers towards its length, ensure

production of even yarns which can then form

fabrics of uniform appearance and consistent

performance.

Cohesiveness or spinning quality-

It is the ability of the fiber to stick together

properly during yarn manufacturing processes.

Natural fibers have inherent irregularities in

their longitudinal or cross sections which permit

them to adhere to each other during fiber

arranging.

In case of synthetics, filament lengths aid in

yarn formation. Texturing introduces coils,

crimps, curls or loops in the structure of an

otherwise smooth filament. It is used to impart

cohesiveness.

Morphology-

It is the study of physical shape and form of a

fiber. It includes microscopic structure like

longitudinal and cross sections. These also

include fiber length, fineness, crimp, color and

luster.

Physical shape-

shape of a fiber include, its longitudinal

sections, cross section, surface contour,

irregularities and average length.

Luster-

It refers to the sheen or gloss that a fiber

possesses. It is directly proportional to the

amount of light reflected by a fiber.

This in turn is affected by their cross section

shape.

Among the natural fibers, silk or the queen of

fibers has a high luster, and cotton has low.

Cross sectional shape Degree of luster

Round High

Irregular Low

Kidney shaped Low

Oval High

Trilobal High

octagonal Low

Specific gravity-

The specific gravity of a fiber is the density related to that of water(at 4°c).

The density of water at that temperature is 1. fiber density will affect their performance and laundering.

If the specific gravity of a fiber is less than 1, it will float in water, making its washing and dyeing very difficult. E.g. Olefins fiber.

A related property is density which is defined as the mass per unit volume and measured in g/cubic cm.

Elongation and elastic recovery-

The amount of extension or stretch that a fiber

accepts is referred to as elongation. Elongation

at break is the amount of stretch a fiber can

take before it breaks.

Elastic recovery indicates the ability of fibers to

return to their original length after being

stretched. A fiber with 100% elastic recovery

will come back to its original length after being

stretched to a specific degree for specified

period of time. After removing and re-

measured.

Resiliency

Resiliency refers to the ability of a fiber to come

back to its original position after being creased

or folded. Resilient fibers recover quickly from

wrinkling or creasing. Good elastic recovery

usually indicates good resiliency. This property

is described qualitatively and ranges from

excellent to poor. Excellent resiliency is

exhibited by polyester, wool and nylon fibers.

Flax, rayon and cotton, on the other hand, have

a low resiliency.

Moisture regain-

The ability of a bone dry fiber to absorb

moisture is called moisture regain.

Measurements are done under standard testing

conditions (70°± 2F and 65% ±2% relative

humidity). Saturation regain is the moisture

regain of a material at 95-100% relative

humidity. Both regain and content are

expressed as a percentage.

Moisture regain= wt. of water in a material × 100

oven dry wt.

Moisture content= wt. of water in a material × 100

total wt.

Flammability and other thermal reactions-

Burning characteristics of fiber groups vary

from each other and can, thus be used as an

authentic identification method. Reaction to

flame can be further broken down into; behavior

when approaching flame, when in flame, after

being removed from flame.

Thermal characteristics of fibers are important

in their use and care like washing, drying and

ironing are selected on the basis of a fiber’s

ability to withstand heat.

Electrical conductivity-

It is the ability of a fiber to transfer or carry

electrical charges. Poor or low conductivity

results in building up of static charges. This

leads to the clinging of clothing and in extreme

cases can produce electrical shocks, which

produce crackling sound or even a tiny spark.

Acrylic is a poor conductor of electricity.

water is an excellent conductor of electricity

and fibers with high moisture regains will never

face the problem of static build-up.

Abrasion -

The wearing away of a material by rubbing against another surface is called abrasion. Different kinds of abrasion are identified, these are-

Flex abrasion- when a fabric bends/folds and rubs against another surface e.g. on elbow or knee areas.

Flat or plane abrasion: when a flat surface rubs against another surface e.g. on thigh area of a pair of jeans.

Edge abrasion- which occurs on the curved

Chemical reactivity and resistance-

Chemical reactivity plays a key role in

manufacture, application of finishes and care of

fabrics.

Resistance to acids, alkalis and organic

solvents in similar for fibers of one chemical

composition. Thus, cellulosic's are fairly

resistant to alkalis but get harmed by acids and

the reverse is true for protein fiber.

Morphology of cotton fiber Cellulosic fiber

70 % crystalline and 30 % amorphous region

Under a microscope, a cotton fiber appears as a very fine,

regular fiber.

length: 10 mm to 65 mm

Diameter: 11 micron meters to 22 micron meters.

Length to breadth ratio: 6000:1 (longest) to 350:1 (shortest)

Look like a twisted ribbon or a

collapsed and twisted tube.

Twists are called convolutions.

Chemical Composition of

Cotton Fiber Cellulose 94

%

Protein 1.3

%

Ash 1.2

%

Pectin 1.2

%

Oil, Fat and

Wax 0.6%

Trash 0.3

%

Cuticle is the outer waxy layer, exists very outside of the cotton fiber. Cuticle consists of pectin and proteinaceous materials. It serves as a smooth, water-resistant coating, which protects the fiber from chemical and other degrading agents. The waxy nature of the cuticle enables it to adhere tenaciously to the primary wall of the fiber. This layer is removed from the fiber by scouring.

primary wall is the original thin cell wall, which is immediately present underneath the cuticle . It is 200nm thick. It mainly consists of cellulose or network of small strands of cellulose, called fibrils. The fibrils spiral at about 70° to the fiber axis. This spiraling imparts strength to the primary cell, and hence, to the fiber. This makes for a well-organized system of continuous, very fine capillaries. The fine surface capillaries of each cotton fiber contribute greatly to cotton’s wipe-dry performance

Macro-structure of cotton

Macro structure of cotton fiber

The “secondary wall” layers of cellulose consist of concentric layers present beneath the primary cell wall , which constitutes the main portion of the cotton fiber. After the fiber has attained its maximum diameter, new layers of cellulose fibrils are added to form the secondary wall. Its fibrils are about 10 nm thick, but of undefined length. Near the primary cell wall, the fibrils of the secondary wall spiral at about 20° to 30° to the fiber axis. The fibrils are packed close together, again, forming small capillaries. As the fibrils change the direction of their spirals, a weak area exists in the secondary wall structure which results in weak areas are responsible for alternation of the direction of the twists of the convolutions.

The “lumen” is the hollow canal that runs the length of the fiber. It is filled with cell sap during the growth period. The lumen was once the central vacuole of the growing cotton fiber . After the fiber matures and the boll opens, the protoplast dries up, and the lumen naturally collapses, leaving a central void in each fiber. When the sap evaporates , its constituents remain behind to contribute to the color of the cotton fiber . As the sap evaporates, the pressure inside the fiber become less than the atmospheric pressure on the outside. This caused the fiber to collapse inward resulting in the bean or kidney-shaped cross-section of the cotton fiber.

The microscopic appearance of

cotton

Cross-sectional view of cotton fiber is kidney-shaped

swelling almost round when moisture absorption takes place.

This shape occurs from the inward collapse of the cotton fiber when it dries out. The cross-section tends to provide an indication of the relative dimensions of the lumen and fiber walls.

Under the microscope, each cotton fiber is a single elongated cell that is flat twisted and ribbon like with a wide inner hollow canal as lumen. These twists are also referred as convolutions Formation of convolutions occurs after the cotton boll bursts open. Then the limp, sap-filled cotton seed hairs begin to dry out and their cell walls collapse inward which decreases the size of the lumen. Shrinkage, twisting of the cotton seed hair results in the removal of sap from the lumen and at last in the formation of convoluted fiber.

The seed end of the fiber is quite irregular. The main part of the fiber, about three-quarters to fifteen-sixteenths of its length, is quite regular, with a thick fiber wall, canal along the centre of the fiber called the lumen, and about sixty convolutions per centimeter. The fiber tip is less than one-quarter of the fiber length, and tapers to a cylindrical, pointed tip with no convolutions. The convolutions give cotton an uneven fiber surface, which increases inter-fiber friction and enables fine cotton yarns of adequate strength to be spun.

The convolutions and kidney-shaped cross-section of the cotton fiber enable it to make only random contact with the skin, which is more comfortable and compatible to human skin. Moisture absorbance of cotton fiber is due to the countless minute air spaces which exist because of the convolutions and kidney-shaped cross-section of cotton fibers, thus making them more comfortable to wear.

Structure: The cotton fibre is short (1/2 inch -2 long inch) and

cylindrical or tubular as it grows. The cotton fibre is essentially

cellulose consisting of carbon, hydrogen and oxygen. Bleached

cotton is almost pure cellulose raw cotton contains about 5% of

impurities.

Strength: Cotton fibre is relatively strong which is due to the

intricate structure and 70% crystalline.

Elasticity: Cotton is relatively inelastic because of its crystalline

polymer system and for this reason cotton textile wrinkle and

crease readily.

Hygroscopic moisture: Cotton does not hold moisture so well as

wool or silk but absorbs it and so feels damp much more

quickly. It also rapidly spreads throughout the material.

Electrical property: The hygroscopic nature ordinarily prevents

cotton textile materials from developing static electricity.

Absorbency: As cotton has cellulose it is a good absorbent of

fibre.

Physical properties

Thermal properties Cotton fibres have the ability to conduct

heat energy, minimizing any destructive heat accumulation

thus they can withstand hot ironing temperature.

Drape ability: Cotton does not have good body to drape well

in shape. The type of construction of the fabric may improve

this property.

Resilience: Cotton wrinkles easily some wrinkle resistant

finishes may reduce this property.

Cleanliness and wash ability: Though cotton absorbs dust

due to its rough nature. It can be washed easily in the hot

water and strong soaps without damaging the fibre.

Luster: The natural cotton has no pronounced lustre. This can

be improved by the mercerization finish of the cotton(that is

sodium hydroxide treatment).

Shrinkage: The fibre itself does not shrink but cotton fibre

which has been stretched in the finishing process tends to

relax back creating shrinkage.

Heat conductivity: Cotton is the better conductor of heat than

wool or silk but not as good as rayon

Action of acids and alkalis Strong acids will destroy the fibres

immediately. Dilute inorganic acids will weaken the fibre and if left dry

will rot it. Therefore after treatment with acidic solutions cotton articles

should be thoroughly rinsed in water. They are affected very little by

organic acids. They are also quite resistant to alkalis even to strong

caustic alkalies at high temperature and pressure. In 8% NaOH

cotton fibres swells, spirals, twisted uncoil and shrinks and become

thicker. The resultant fibre is smoother, lustrous, and stronger and

has increased water and dye absorption.

Effect of bleaching: These have no effects until used in uncontrolled

conditions and with heat.

Effect of sunlight and weather: Ultraviolet rays of sunlight affect the

strength of fibre and change the colour to yellow when exposed to

prolonged period. Pollution also effect fibre.Concentrated and diluted

mineral acids like sulphuric acids will discolor fibre .

Affinity to dyes: Cotton takes in dyes better than linen but not as

readily as silk and wool. If a mordant is used cotton is easy enough to

dye mordant colours, direct or substantive dyes should be applied to

the cotton.

Chemical properties

Properties of cotton fibers

Property Evaluation

ShapeFairly uniform in width, 12-20 microns; length varies from ½ to 2½ inches;

typical length is ⅞ to 1¼ inches.

Luster low

Tenacity (strength)

Dry

Wet

3.0-5.0 g/d

3.3-6.0 g/d

Resiliency low

Density 1.54/1.56 g/ccm

Moisture absorption

raw:conditioned

saturation

mercerized: conditioned

saturation

8.5%

15-25%

8.5-10.3%

15-27%+

Dimensional stability good

Resistance to

acids

alkali

organic solvents

sunlight

microorganisms

insects

damage, weaken fibers

resistant; no harmful effects

high resistance to most

Prolonged exposure weakens fibers.

Mildew and rot-producing bacteria damage fibers.

Silverfish damage fibers.

Thermal reactions

to heat

to flame

Decomposes after prolonged exposure to temperatures of 150˚C or over.

Burns readily.

Test

Fiber

Soda

ash

40%

sol.

Causti

c soda

25%

sol.

Sodi

um

hypo

chlo

ride

Hyd

ro

chlo

ric

acid

40%

Nitr

ic

acid

15%

Nitric

acid

70%

Sulp

huric

acid

15%

Sulp

huric

acid

70%

Burning

in Flame

Microsc

opic

View

Remarks

Cotto

n

swells Swells

&

Shines

Whit

ened

Turn

s

yello

wish

Ope

ns

up

looses

strengt

h

Dissol

ves

slowly

Disso

lves

on

heat

ing

Disso

lves

quick

ly

Burns

continuo

usly

leaving

grey ash

of

burning

paper

smell

Longitud

inal

twists.

Resistanc

e to

alkalis.

IDENTIFICATION OF COTTON FIBRE

Wool fiber Wool is the natural protein fiber from the fleece of

sheep

Length of the fiber ranges from 5cm for finest to 35cm for the coarsest wools.

Diameter for finer 14μm, coarse 45μm,

Length width ratio ranges from 2500:1 for the fine and shorter, 7500:1 for coarse and longer

Colors vary from off white to light cream.

The wool fiber is a crimped, fine to thick, regular fiber.

Fine wools have 10 crimps per centimeter, while coarse wool has less than 4 crimps per 10 centimeters. As the diameter of wool fibreincreases, the number of crimps per unit length decreases.

The number of crimps per unit length may be taken as an indication of wool fiber diameter or wool fiber fineness. As the diameter of the wool fiber increases the crimp per unit length decreases

The crimped configuration prevents wool fibers from aligning themselves too closely when being spun into yarn. As a result it is possible to have wool textile materials with air spaces occupying about two-thirds of the volume. The warmth of wool fabrics is due more to the air spaces in material than to the fiber

Microscopic appearance of

wool Longitudinal microscopic appearance of wool is

the overlapping surface cell structure. These surface cells, known as epithelial cells and commonly known as scales, which point towards the tip of the fiber

The cross section of wool fibre is usually oval in shape

Macro-structure of woolThe micro structure of wool fiber consists of three main

components

Cuticle : The cuticle is the layer of overlapping epithelial cell's surrounding the wool fiber. There are three cuticle.

Epi Cuticle: The epicuticle is the outermost layer covers of the wool fiber.

Exocuticle : The overlapping epithelial cell forms the exocuticle.

Endocuticle: The endocuticle is the intermediate connecting layer bonding the epithelial cell of the cortex of the wool fiber.

Cortex: The cortex or core, of the fiber forms about 90% of the fiber volume. It consists of countless, long, spindle shaped cells or cortical cells. It is composed of two regions known as orthoand para cortex. The ortho cortex absorbing more dye than paracortex. The ortho and para cortex spiral around one another. Fine wool fibers have about 20 such cells, whereas coarse wool fibers have about 50 cortical cells across diameter of their cross-section.

Medulla: Coarser fibers have a hollow space running lengthwise through the center

Wool polymer Wool polymer is a linear, alpha-keratin polymer which has a

helical configuration. Steps in the formation of wool polymer are not known. So the repeating unit of wool polymer is amino acid which is linked to each other by the peptide bond (-CO-NH-). As a result, it is not possible to determine the extent or degree of polymerization for wool. It consists of a long polypeptide chain constructed from 18 amino acids.

wool polymer is about 140 nm and about 1nm thick in its normal relaxed state , the wool polymer has alpha keratin structure stretching of the wool fiber tend to stretch, straighten with unfolded configuration called called beta-keratin. A beta-keratin wool polymer always tends to return to its relaxed alpha keratin structure.

Amorphous : Wool polymer system is extremely amorphous, as it is about 25 to 30% crystalline. The spiraling of the proto-fibrils, micro-fibrils and macro-fibrils does not imply a well aligned polymer system.

The complexity of the wool polymer is due to important chemical groupings it contains and the inter-polymer forces of attraction.

Polar peptide groups: The oxygen of the carbonyl groups (-CO-) is slightly negatively charged and as a result will form hydrogen bonds with the slightly positively charged hydrogen of the amino groups (-NH-) of another peptide group.

Salt linkages or ionic bonds: carboxyl radicals (-COOH) and (-NH2) as side groups of amino acids which are basically the acidic and basic groups, salt linkages or ionic bond will forms.

Covalent bonds: cystine, the sulphur containing amino acid which is present in wool, makes the wool polymer system the only one with cystinelinkages, also known as di-sulphide bonds. Cystinebonds are covalent bonds, they occur within and between wool polymers.

Van der Waals forces

Physical propertiesTenacity:

When wool absorbs moisture, the water molecules gradually force sufficient polymers apart to cause a significant number of hydrogen bonds to break. Water molecules hydrolyze salt linkages in the amorphous regions of the wool fiber. Breakage of these inters –polymer forces of attraction are apparent as swelling of the fiber and results in a loss in tenacity of the wet wool textile material.

Wool is comparatively weak fiber

Wool is composed principally of proteins which are polycondensation products in which the different amino acids are linked together to form a polypeptide chain:

They possess a large number of highly polar peptide linkages which can give rise to inter- and intra-molecular hydrogen bonding. While these bonds contribute much toward increasing the strength of the fiber, such close spacing of these groups along the molecular chain would be detrimental to other desirable fiber properties. They contain relatively large side chains (R groups in the scheme of the polypeptide chain) which prevent close packing of the protein molecules and thus decrease the extent to which hydrogen bonding can occur. The low tensile strength of wool is due to the relatively few hydrogen bonds that are formed.

Elasticity: wool has very good elastic recovery and excellent resiliency. The ability of wool fibres to recover from being compressed is due to

a) crimped configuration of wool fiber

b) alpha–keratin configuration of the wool polymer

Covalent bonds can stretch, but they are strong. The disulphide bonds in the amorphous parts of the strand or fibre are able to stretch when the strand is extended. When the strand is released the disulphide bonds pull the protein molecules back into their original positions

3) Hygroscopic nature: absorbent nature of wool is due to the polarity of the peptide groups, salt linkages and amorphous nature of its polymer system. The peptide groups and salt linkages attract water molecules which readily enter the amorphous polymer system of the wool fiber.

Conductivity of heat:

It has a low conductivity of heat and therefore makes it ideal for cold weather. The resiliency of the fibre is significant in the warmth properties of the fabric. Wool fibres do not pack well in yarns because of the crimp and scales, and this makes wool fabric process and capable of inserting much air. Air is one of the best insulators since it keeps body heat close to the body. The medulla of the wool fibre comprises air spaces that increase the insulating power of the fibre.

This strand can take up moisture in vapor form. Absorbency is a factor also in the warmth of clothing. In winter, when people go from a dry indoor atmosphere into the damp outdoor air, the heat developed by the fibre in absorbing moisture keeps to protect their bodies from the impact of the cold atmosphere

Chemical properties

Effect of acids

Wool is more resistant to acids than to alkalis. Acid hydrolyze the peptide group but leaves the disulphide group. The polymer weakens but does not dissolve though it become very vulnerable to further degradation. it is essential to neutralize wool after acid treatment.

Effect of bleaches

No method is known for bleaching wool permanently. The effective method of bleaching wool is to use a reducing bleach followed by an oxidizing bleach. Reducing bleach such as sodium bisulphite, sodium sulphite converts discoloration on the fibre surface to colourless compounds. Due to the application of oxidizing bleach the colourless compounds are converted into water soluble compounds and then can be rinsed off

Effect of alkalis

Wool dissolve readily in alkaline solution. Alkali dissolve the hydrogen bonds, disulphide bonds and salt linkages. Prolong exposure to alkies cause fragmentation and complete destruction of wool fibres

Effect of sunlight and weather

Sunlight cause yellowing or dullness of wool fabric. The ultraviolet rays of sunlight degrade the peptide and disulphide linkages; degradation products cause wool fibre to absorb more light and to scatter the incident light even more to give yellowing or dulling effect on fabric.

Dye ability of wool fiberOrtho-cortex absorb more dye than the para-cortex

Different staining is due to the different composition of the para-cortex and the ortho-cortex. The chemical composition of the para-cortical cells shows a higher cystine content than ortho-cortical cells. Cystine is a sulphur containing amino acid, capable of forming di-sulphide cross-links. This increased cross-linking tends towards greater chemical stability resulting in less dye absorption of para-cortical cells.

The cortical cells of the wool fibre consist of a number of macro-fibrils. These macro-fibrils held together by a protein matrix. Each macro-fibril consists of micro-fibrils of indeterminate length. And each micro-fibril composed of eleven proto-fibrils these protofibrils spiral about each other. Finally, each proto-fibril consist of three wool polymers (alpha keratin polymers), which also spiral around each other. It is the fibrillar and spiralling structure, within the cortical cells, which contributes towards the flexibility, elasticity and durability of the wool fibre.

Silk fiber Silk is a natural protein fiber

Cultivation of cocoons for the filament

Silk filaments are 600-1700 m long and

Diameter ranges from 12-30 µm depending

upon the health, diet and state under which

the silk larvae extruded the silk filaments.

Fiber length to breadth ratio is 2000:1

It is off-white to yellow in color

Micro structure Silk polymer is linear, fibroin polymer. It is

composed of 16 different amino acids.

Silk polymer occurs only in the beta-configuration. Length of the silk polymer is about 140nm which is slightly longer than wool polymer, and about 0.9 nm thick. The important chemical groupings of the silk polymer are the peptide groups which give rise to hydrogen bonds, and the carboxyl and amine groups which give rise to the salt linkages.

Polymer system of silk is composed of layers of folded linear polymers. This results in 65-70% crystalline polymer system. The major forces of attraction between silk polymers are hydrogen bonds, which are effective across a distance of less than 0.5nm. This ensures that fibroin polymers must be closer than this given distance.

Alanine and Glycine contribute 65-70%

of silk protein

Cross section of silk fiber

Physical properties

Physical characteristics of fiber are determined by the structure of the macromolecule composing the fibroin. Partly the macromolecule comprises of amino acids with a low molecular weight, result in a series of crystalline regions which gives a high degree of tenacity. The rest of the macromolecule is characterized by the presence of amorphous areas enclosing amino acids of a higher molecular weight. Crystalline and amorphous zones give combination of strength, flexibility and elasticity

Tenacity - The silk filament is strong. This strength is due to its linear, beta configuration polymers and very crystalline polymer system. These two factors permit many more hydrogen bonds to be formed in a much more regular manner. Silk loses strength on wetting. This is due to water molecules hydrolyzing a significant number of hydrogen bonds and in the process weakening the silk polymer.

Specific gravity - silk is less dense than cotton. It has a specific gravity of 1.3. Silk fibre are often weighted by allowing filaments to absorb heavy metallic salts; this increases the density of the material and increases its draping property.

Elasticity Very crystalline polymer system does not permit the amount of polymer movement which results in plastic nature of silk than elastic. Hence, if silk textile material is stretched, the silk polymers which have beta-configuration will slide past each other. This stretching results in rupturing of hydrogen bonds. After stretching, the polymers do not return to their original position, which leads to distortion and wrinkling or creasing of the silk textile material.

Effect of heat Heating of silk fiber remains unaffected for a long period at 1400C. It is more sensitive to heat compared to wool, which is due to the lack of any covalent cross links in the polymer system of silk, compared with the di-sulphide bonds which are present in wool’s polymer system. Peptide bonds, salt-linkages and hydrogen bonds of silk fiber decompose quickly at 1750C.

Hygroscopic nature: Moisture regain of silk is about 11% compared to cotton which has 8.5% this is due to the very crystalline polymer system. The amount of moisture absorbed by silk depends on whether it is raw or degummed silk or on the species of silk and in the humidity

Electrical properties - Silk is a poor conductor of electricity and tends to form static charge when it is handled. This causes difficulties during processing, particularly in dry atmosphere.

Hand feel - The handle of the silk is described as a medium and its very crystalline polymer system imparts a certain amount of stiffness to the filaments. This is often misinterpreted, in that the handle is regarded as a soft, because of the smooth, evenand regular surface of silk filaments.

Drapes Property - Silk fibre is flexible enough and if silk fibre is used to make garments, then the fabric drapes well and this is why it can be tailored well too.

Abrasion resistance - Silk fabric possess good abrasion resistance as well as resistance to pilling.

Effect of sunlight - Silk is more sensitive light than any other natural fibre. Prolonged exposure to sunlight can cause partially spotted color change. Yellowing of silk fibre is generally occurred due to photo degradation by the action of UV radiation of sunlight. The mechanism of degradation is due to the breaking of hydrogen bonds followed by the oxidation and the eventual hydrolytic fission of the polypeptide chains.

Investigation of fiber structure

Fiber structure helps to acquire knowledge about fiber an its composition for improving use of fiber in textile.

Measuring of fiber structure manly introduce the fiber composition, length, weight, thickness, fineness, flexibility, stability etc

So fiber investigation methods and study of fiber structure is very important to knowing about the fiber properties

Methods for investigation of fiber

structure◦ The absorption of infrared radiation and

Raman scattering of light;

◦ Optical and X-ray diffraction

◦ Optical microscopy

◦ Electron microscopy and electron diffraction

◦ Optical properties

◦ Thermal analysis

◦ Density method

Absorption of infrared radiation

and Raman scattering When electromagnetic waves interact with

matter, they are scattered and absorbed.

In infrared spectroscopy, radiation with wavelengths between 1 and 15 μm is absorbed at certain characteristic frequencies, which yield structural information.

Elastic scattering does not give molecular information, though light scattering does give larger-scale information

the variation in absorption can be found and plotted against wavelength

Absorption spectrum of nylon

The wavenumber at which absorption takes place depends primarily on the nature of the two atoms and of the bond between them. Thus there will be absorption frequencies characteristic of such groupings as C--H, C--O, ,O--H, N--H, C--C and so on. To a smaller extent, the absorption frequency is influenced by the other groups in the neighborhood

For example, the absorption frequency for a carbon–hydrogen bond in a terminal group, —CH3, is different from that for the same bond in a chain, —CH2—.

Application Identification of the presence of certain

groups in the molecule, leading to the

determination of its chemical formula.

The method can also be used in routine

analysis to identify and estimate

quantitatively the presence of given

substances, even in small quantities in a

mixture, by observation of their

characteristic spectrum.

It can be used to determine the amount of

water in fibre

Other structural information can also be obtained. If the infrared radiation is polarised, then the oscillation of the atoms will vary from a maximum for one orientation to a minimum for an orientation at right angles. The variation in the absorption spectrum with the direction of polarisation can therefore be used to investigate the degree of orientation of the molecules in a fibre.

For example, in nylon, the >N—H, >CH2 and >C=O absorption bands all show weak absorption. It is due to the vibration direction of the electric vector is along the molecular chain and strong absorption when it is vibrating perpendicular to the chain axis.

Raman spectroscopy It is the incidence of the photons shifts electrons from one

state to another. The energy of the change comes from the photon. Consequently the scattered photon has a different energy and hence a different frequency. The effects are manifested in the visible region.

Raman spectra are influenced by material structure in a way similar to that described for infrared absorption spectra, but the greater complication of the interaction yields more directional information.

Raman spectroscopy has become a powerful tool for investigating fibre structure as a result of the development of Raman microscopes. With a spot size less than a fibrediameter, spectra can be obtained from single fibres.

If the fibre is mounted on an extension stage in the microscope, it is possible to observe the shift in the spectral lines with fibre extension. In this way it is possible to show which parts of the structure are changing.

An account of the use of Raman spectroscopy in various ways in the study of aramid, polyester and carbon fibres is given by Young

Optical diffraction When a beam of light is

passed through a photographic slide, the light is scattered in many directions.

By using a lens in the right place, we can recombine this scattered information about the picture into an image on a screen.

Example: There is a characteristic diffraction pattern from a single slit. The difference between the image that must be focused at a particular place and the angular diffraction pattern that can be intercepted anywhere is shown in Fig.

The use of polarized light in either of the above two techniques changes the pattern and thus, in principle, increases the available information about structure if it can be interpreted.

A diffraction grating of regularly spaced lines, illuminated normally by parallel light, will give a set of fringes, with the maxima of the bright bands at angles φ defined by the relation:

◦ nλ = a sinφ

◦ Where n is an integer, λ the wavelength of light and a the spacing of the lines in the grating.

X-ray diffraction

X-radiation (composed of X-rays) is a form of electromagnetic radiation.

X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3×1016 Hz to 3×1019 Hz) and energies in the range 100 eV to 100 keV.

The wavelengths are shorter than those of UV rays and longer than those of gamma rays.

X-radiation (composed of X-rays) is a form of electromagnetic radiation.

X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3×1016 Hz to 3×1019 Hz) and energies in the range 100 eV to 100 keV.

The wavelengths are shorter than those of UV rays and longer than those of gamma rays.

X-ray diffraction

The condition that a particular reflection should occur is that the layer of atoms should make the required angle with the X-ray beam. This will happen for a series of orientations of the crystals distributed around a cone. The X-rays will be reflected around a cone of twice this angle, as shown in Fig.

Electron microscopy and Electron diffraction

Electron diffraction refers to the wave nature of electrons. However, from a technical or practical point of view, it may be regarded as a technique used to study matter by firing electrons at a sample and observing the resulting interference pattern. This phenomenon is commonly known as the wave-particle duality, which states that the behavior of a particle of matter can be described by a wave.

Normal optical microscope we can find out up to 0.5 Å only.

By using of electron microscope we can able to find out up to 5 Å.

The rays from electron source are condensed on the specimen.

Here only dry sample can be examined.

Contrast in the image depends on the variation in scattering of the electrons by parts of the specimen of differing density.

Electron microscopy and Electron diffraction

Electron microscope method is better to examining the surface of the fiber

The main use of EM in fiber science has been in the range of medium to high magnification, which is near or beyond the limit of the microscope .