Textile Fibre Properties and Structure

Textile Fibre Properties and Structure

Introduction
The study of the structure & physical properties of fibers, yarns & fabrics forms the unified subject that is legitimately. It is an essential part of education of any textile technologist.

Physical structure of fibres

Crystallinity and orientation, Basic concepts of methods for investigating fibre structure, e.g. X-ray diffraction, optical and electron microscopy infra-red absorption, relations between fibre properties and structure of fibre.

Detailed study of fibre properties

Mechanical properties – tensile, flexural and torsional properties, stress/strain relations under various conditions, Modules of elasticity, plasticity, creep and relaxation.

Effects of moisture – Effect of water on fibre e.g. swelling.

Frictional properties – Importance in drafting experimental methods of measurement. Effect of lubricant and dyes.Relationship of frictional properties of knitting, stitching and sewing.

Optional properties – Reflection, refraction, scattering, polarization, birefringence.

Thermal properties – Absorption and emission of radiation, Energy changes asociated with changes of state including transition temperature of fibres. Moisture content and heat of wetting

Physical structure of fibres

Crystallinity and orientation, Basic concepts of methods for investigating fibre structure, e.g. X-ray

diffraction, optical and electron microscopy infra-red absorption, relations between fibre properties and structure of fibre.

Detailed study of fibre properties

Mechanical properties – tensile, flexural and torsional properties, stress/strain relations under various

conditions, Modules of elasticity, plasticity, creep and relaxation.

Effects of moisture – Effect of water on fibre e.g. swelling.

Frictional properties – Importance in drafting experimental methods of measurement. Effect of

lubricant and dyes.Relationship of frictional properties of knitting, stitching and sewing.

Optional properties – Reflection, refraction, scattering, polarization, birefringence.

Thermal properties – Absorption and emission of radiation, Energy changes asociated with changes of

state including transition temperature of fibres. Moisture content

Parameters/properties of textile materials

1. Strength (tensile,

tearing, bursting)

2. Durability

(serviceability)

3. Longevity (more than

it can perform)

4. Degradation

5. Weight

6. Thickness

7. Pliability

8. Absorbency

(modalà

regenerated, more

absorbency, more

strength)

9. Air permeability

10. Stiffness

11. Softness

12. Compressibility

13. Elasticity

14. Extensibility

15. Friction/surface

characteristics

16. Electrical

17. Thermal

18. Crease resistance

19. Crease recovery

(Cotton bad crease

recovery, polyester

good crease

recovery)

20. Drape

21. Luster

22. Regain

23. Handle

24. Breaking length

25. Water resistance

Factors on which properties of textile materials depend

i. Fibres properties

a. Chemical content

b. Molecular wt./chain length

c. Molecular arrangement

ii. Yarn properties

a. Fibres properties

b. Fibres arrangement

c. TPI

iii. Fabric construction

a. EPI

b. PPI

c. Warp count

d. Weft count

iv. Fabric structure

a. Woven fabrics (plain, twill, satin etc.)

b. Knit fabrics (plain, rib, interlock, purl, warp)

c. Felting/bonding

d. Braiding

v. Fabric engineering

a. Yarn specification

b. Fabric specification

c. Yarn crimp

d. Weave structures

vi. Fabric finishing

a. Mechanical and chemical

Properties of fibres depends on the following

a. Chemical constitution and properties

b. Molecular structure and dimension

c. Arrangement of fibres forming molecules

d. Auxiliaries used for fibres manufacture

e. Conditions and technique of man-made fibres manufacturing

Fibre

By Textile Institute fibre is defined as units of matter characterized by flexibility, fineness and a high ratio of length to thickness (1000:1). For textile fibres a sufficient high temperature stability and a certain minimum strength and extensibility required. Elastic upto breaking extension 5-50%. Since glasses and crystalline solid are less extensive whereas rubber are much more extensible. Cellulose, proteins and variety of synthetic fibres are partly oriented, partly crystalline linear polymers.

Fibre structure depends on the following information

1.       Chemistry of fibre material (preparation, composition, molecular formula and reaction)

2.       Absorption of infrared radiation

3.       Optical and x-ray diffraction studies

4.       Optical properties

5.       Optical microscopy

6.       Electron microscopy/diffraction

7.       Thermal analysis

8.       Density

9.       Nuclear magnetic resonance

10.   General physical properties.

Crystallinity

If a substance exposed to x-rays, gives sharp and well defined x-ray diffraction patterns, then the substance is called crystalline substance and this property is called crystallinity.Maximumcrystallinity is possible if the polymer is annealed just at its melting temperature for sufficient long time. Crystallinity is expressed as degree of crystallinity as below,

Degree of crystallinity =                here, C- refraction Index, Ccr –the value of property when the polymer

is crystalline, Cam- the value of property when the polymer is amorphous.

Amorphousness

If a substance exposed to x-rays, gives diffused and broad x-ray diffraction patterns, then the substance is called amorphous substance and the property is called amorphousness.

Orientation

The molecular orientation of fibre is the alignment of long chain molecules relative to the fibre axis. When the molecules of a material are highly oriented they are parallel to each other and parallel to the axis of fibre. There is no 100% crystalline or 100% amorphous material.

Requirements of fibre formation

1.       Polymer must have straight and long chain molecules

2.       The chain must be more parallel to one another

3.       Molecular weight of the polymer should be high (DP)

4.       Polymer must have attraction among the molecules

5.       Polymer should have resistance to different chemicals

6.       Polymer should be soluble in some solvents from which it can be spun(not for melt)

7.       Hygroscopic nature of the polymer should be hydrophilic

8.       There should be freedom in molecular movement in order to give required extensibility.

Methods for investigation of fibre structure

1.       X-ray diffraction method

2.       The absorption of infra-red radiation

3.       Optical microscopy

4.       Electron microscopy

5.       Nuclear magnetic resonance

X-ray diffraction method

When a beam of x-ray strikes on a crystal, it strongly reflects and it strikes on the layers of atoms

at a certain angle θ. In this case, initial angle is equal to reflected angle.Let the distance between atomic layers=d, wave length of x-ray =λ

Here, <FBD =90⁰ and <FBC =θ So, <CBD=90-θ, As, <BCD=90⁰ So, <CBD + <BDC = 90 => 90- θ+ <BDC = 90 =><BDC = θ

Now, sinθ=     => BC =BDsinθ =dsinθ.

Again, <FBD= 90⁰ and <EBA =θ, so, <ABD = 90-θ and similarly, <BDA =θ

Now, sinθ= => AB=BD sinθ =dsinθ Now, AB + BC = dsinθ + dsinθ =2dsinθ

^ð  Nλ=2dsinθ. Where, n be the integer no.

So, if d is greater atomic layer distance increased but crystallinity decrease.

Absorption of infrared radiation

When infrared radiation composed of electromagnetic waves with wavelengths between 1 and 5µm is passed through a material, it is mostly highly absorbed at certain characteristic frequency. By using an infrared spectrometer, the radiation in absorption can be found and plotted against wave number. This is illustrated in the following which is absorption spectrum of nylon.

Here, full-line represents electric vector perpendicular to fibre axis and broken line represents electric vector parallel to fibre axis.

The peaks occur where the frequency of the electromagnetic waves corresponds with the natural frequency of vibration between two atoms in the material. If these are associated with electric dipole then the vibration of electric field set up the vibration and energy is absorbed from the radiation.

The wave no. at which absorption takes place depends primarily on the nature of the two atoms and of the bond between the, =NH, =CO,ΞC-CΞ,=C=C=, -OH, ΞC-O- etc.

To similar extent, the absorption frequency is influenced by the other groups in the neighborhood.

Instrument consist of-

1.       Source of infrared radiation

2.       Diffraction grating

3.       Sensitive heat detector

4.       Output display.

Advantages of infrared radiation absorption method

1.       The first use of infrared absorption is an aid to the identification of the presence of certain groups in the molecule, leading to the determination of its chemical formula

2.       The method can be used in routine analysis to identify and estimate quantitatively the presence of given substance, even in small quantities in a mixture

3.       It can be used to determine the amount of water in fabrics.

4.       It can analyze the degree of orientation of molecules in a fibre

5.       It can determine the molecular configuration of a molecule.

6.       It can give information about molecules of crystalline and non-crystalline region.

Electron microscopy

Electrons are particles that can act as if they were waves with a wavelength of order of 0.005nm; and furthermore they can be focused since their path [rays] can be bent by electric and magnetic fields in the same way that light rays are bent by lenses. It is therefore possible to form an image by an electron microscope with a limit of resolution that is far smaller than is possible with an optical microscope.

Why?

-          Allows a sample to be examined at far higher magnification and lateral resolution than is possible with light microscopy

-          Large depth of field/whole surface remaining in focus at a time

-          Huge range of contrast

The electron microscope illustrated schematically in the following figure is

exactly analogous to the optical microscope. The rays from an electron source are condensed on the specimen and then focused by electric or magnetic fields acting as lenses to give a magnetic image on a fluorescent screen or photographic plate.

Difficulties

-          Obtaining sufficient contrast

-          Ordinary transmission/confusion of depth of focus

-          Observed may be caused by the section-cutting [for high contrast]

Uses

-          Cut face/fibre structure

-          Internal details may also be shown by peeling a layer a layer off the fibre to expose a new internal surface for replication.

-          Fragments left behind after mechanical/chemical degradation

-          Information same as X-ray diffraction [orientation/crystalline]

Types

1.       Scanning electron microscope [detector mounted on the same side of the sample]

2.       Transmission electron microscope [ detector mounted on behind the sample]

Condenser lens

-          Create a de-magnified image f the gun crossover

-          Control minimum beam spot size

-          Affects the beam convergence at the sample

-          Controls diameter of the illuminated area

Aperture

-          Control the intensity of illumination

Objective lens

-          Form an inverted initial image which is subsequently magnified

-          Form a diffraction pattern in the back focal plate

Objective Aperture

-          Selects electrons that will contribute to the image

-          Improves contrast of the final image

-          Placed in back plane of image

Intermediate lens

-          Magnify initial image formed by objective lens

-          Focuses on initial image or diffraction pattern formed in back focal plane

Projector lens

-          Magnifies size of image

-          Can have various strength

Relation between fibre properties and structure

1.       Tensile strength is –

a.       Proportional to covalent bond energy

b.      Proportional to molecular weight

c.       Proportional to crystallinity

d.      Proportional to fibre orientation

2.       Extensibility is-

a.       Reciprocal to degree of molecular orientation

b.      Reciprocal to degree of crystallinity

c.       Proportional to helical angle of fibril

d.      Proportional to chain length

3.       Moisture regain is-

a.       Proportional to no. of polar group

b.      Reciprocal to degree of crystallinity

4.       Electric property is-

a.       Proportional to degree of crystallinity

b.      Proportional to molecular orientation

5.       Electric conductivity is-

a.       Reciprocal to degree of crystallinity

b.      Proportional to no. of polar group

c.       Proportional to electrolytic content

Reference Books

1.       Mechanics of Yarn and Fabric Structure and Properties- J.W.S. Hearle

2.       Physical Testing of Textiles-B.P. Saville.

3.       Proper properties of textile fibre- Morton.

4.       Principle of Textile Testing- J.E. Booth

5.       Man-made Fibres- Moncrif