Characteristics of synthetic fibre

Characteristics of synthetic fibre

Both natural and man-made fibres are mainly composed of the compounds belonging to high polymers or macromolecules. Macromolecular structure is necessary for production of materials of high mechanical strength and high melting point. The natural fibres are found to consist chain molecules of linear macromolecular type. Further, the chain molecules are oriented into parallel bundles in the process of growth. Based on these investigations it is assumed that a polymer must satisfy minimum requirements, if it is to serve as a fibre. These requirements are mentioned below:

 

Flexibility

The polymer must be a linear flexible macromolecule with a high degree of symmetry. The effective cross sectional diameter should be less than 15 A. The polymer should not contain any bulky side groups or chains.

 

Molecular Mass

The polymer must have a comparatively high molecular mass. The average length of its molecular chain should be in order of 1000 A or more

 Configuration

The molecule must have the capacity to adopt an extended configuration and state of mutual alignment.

 

 Crystallinity

 A polymer should have at least a high degree of intermolecular cohesive power. This indicates that the molecular chains should have sufficient number of sites of attraction.

 

 Orientation

A high degree of orientation of the molecules in the polymer is a pre- requisite for producing good tensile strength.

Wet spinning

Wet spinning

Wet spinning is applied to polymers which do not melt and dissolve only in non-volatile or thermally unstable solvents. In this process, the polymer is dissolved in a suitable solvent, and the solution is extruded through a spinnerene to a liquid bath containing low molecular weight chemicals. This chemical is a non solvent to the polymer but is freely miscible with the solvent used to prepare polymer dope 

In this process, the method of solution preparation, filtration and extrusion i.e., polymer dope preparation is very similar to those used for dry spinning. But more dilute polymer solution is used for complicated solidification process .Polymer contentvaries between (5 % to25 %) it 20 to 5000 poise viscosity.

For extrusion, the same general principles are used for the design of the spinnerettes. But the spacing of the holes in the wet spinning is closer than the dry spinning because the polymer stream he moves in a liquid medium after extrusion. Presence of liquid restricts t coalescing of the filaments during solidification

The spin-block is not a compacted one like that of melt spinning and dry spinning because of the presence of special spin bath with chemicals. Instead, the design of the spin block is such the spinneretter are submerged in spin bath and connected to the spin block by transfer e of pipes. Also, the spinnerette pack present in spin chemically inert material.

The solidification process of the polymer solution consists of extraction of the solvent by the chemicals present in the spin bath. This process is an f spin inward and outward mass transfer process in the polymer stream of bath. Inward and outward mass transfer is more complex phenomenon in wet spinning.


The extracted fibre can be stretching in the spin-bath because of its gel- like structure. The degree of stretching can be as high as 30 times only a slow rate of transition allows higher stretch. Sometimes to have more degree of stretching, the fibre usually passes through several baths containing The extruded fibre ca the co-adulating chemical.

Because of high liquid content in the wet spun fibres, it is difficult to wind them on take-up bobbins. Also, drying of the fibres before drawing
renders difficulties for further processing. So instead of winding in a bobbin in take-up, sometimes centrifugal spinning method is generally adopted to collect the filaments after solidification. 

In this method, the fibre is passed to the inside of a rotating container into which the yarn is collected with the aid of centrifugal force. The material gathered in the rotating container forms a stable package or cake. The cake can be removed v.hen rotation stops. 

An alternative process can be used, where the fibres after solidification can be post-treated continuously. The cakes or the fibres can be washed repeatedly to remove any residual solvent or any other chemical present in the fibre.

The advantages and disadvantages of wet spinning process can be summarized below

 Advantage

(a) It can be used for any polymer. Even polyethylene, polypropylene and nylon can be converted from polymer to their fibre by wet spinning technique.

(b)  Fibre can attain maximum strength, which can be comparable with maximum theoretical strength, particularly for flexible polymers. Comparatively melt spinning process can not result in maximum theoretical strength

(c) The process can be continuous.

Disadvantage

(a) The production rate is low

(b) One or more than one bath is requiredfor completely removal of the solvent from the polymer

(c) Post-spinning operations are morel length

(d) Because of lengthier process, it is more costly.

(e) The formation of exact fibre cross section is difficult to control because of inward and outward mass transfer process.

Dry spinning

Dry spinning

Dry spinning is similar to the melt spinning. The polymer concentration in the dope ranges from 20 % to 30 % High polymer concentration results in higher solution viscosity and difficulties in material handling.

 However, physical properties of the resulting fibre are better with higher polymer concentration. The polymer solution is generally heated ‘to a higher temperature to reduce dope viscosity, The polymer  solution is extruded into a dry spinning cell. It is a jacketed vertical box, where heated

air is circulated to evaporate the solvent. After extrusion, in the solidification stage, the solvent is removed by evaporation in presence of heat and suitable inert gas current. 

Because of this, the spacing for holes in spinnerette plate s wider. For such operation, the solvent should have certain properties like low boiling point, low heat of evaporation, good heat stability, inertness, ease of recovery and free from explosion hazards and toxicity.

Industrial fibre production process consists of the following operations

(a) Dissolution of the polymer in the solvent.

(b) Mixing and ripening of the polymer solution.

(c)  Metering of the polymer solution through the extruder.

(d) Filtration of the solution to remove mechanical impurities and undissolved polymer particles

(e) Extrusion of the polymer solution through the spinnerette into a vertical drying chamber, at 100-150 Kg/cm2.The diameter of the hole is in between 0.2-0.4 mm.

(f) Solidification of the polymer solution by means of heated inert gas, mostly air.

(g) Application of spin-finish for lubrication and anti-static effect.

(h) Collection of the filaments in take-up bobbin.

Advantages of dry spinning

Dry spinning process has the following advantages:

(a) it is a recommended process for heat-sensitive polymers

(b) High spinning speed can be achieved.

(c) Lower solvent is required because of high polymer concentration in the dope.

(d) The dry spinning method is a relatively flexible one and spinning conditions can be modified.

(e) The process is more suitable for filament yarns.

(f) The post spinning operations are simple like that of melt spinning.

Disadvantages of dry spinning

The process has the following disadvantages

(a) The process requires solvent and so solvent recovery process.

(b) Because of the presence of solvents, there is a limit to maximum spinning speed.

(c) As the evaporation rate of the solvent is higher than the diffusion rate, a circular cross-section of the fibre is difficult to achieve.

(d) Additional post spinning operation is required for complete solvent removal

 (e) spinnerette hole configuration in the spinnerette is very much critical for uniform outward mass transfer.  

The fibres which are formed by this method and solvent with their boiling points are given in Table

 Polymers and Solvents in Dry Spinning

Polymer                     solvent     Boiling point of the solvent

Cell-acetate      →       Acetone             →   56°C

Cell-triacetate  →        Methylchloride   →   41°C

PVC                 →        Acetone             →   56°C

 PAN/PU         →         DMF                   →    153°C

PVA                →       Water                    →   100°C 

Melt spinning

Melt spinning

The melt spinning process can be applied to the production of filaments when the polymer on melting will give a viscous liquid without decomposition. So an important requirement of the polymer to be melt spun is that it must have a melting point and should not degrade when softened by heat. Hence a polymer that is degradable at the spinning or extrusion temperature is certainly not suitable for melt spinning.

The process of melt spinning is simple  involves feeding and melting of the polymer forming the melt. The melt is extruded through a spinnerette at a constant rate, and pressure of 70-150 Kg/cm2 with application, of lubricant and antistatic agents, to a spinning line of 4-6 mts long. The filaments are solidified by a cooling medium, usually air. After solidification, the filaments are collected on bobbins. In view of the high temperature used, the standard of engineering precision of the extrusion apparatus must be of high order, and must be capable of producing filaments and fibres with high tolerance

Melt spinning process is the preferred one because of the following advantage

(a) The process is simple and economical

(b) Any production rate i.e. lowest to highest can be obtained with minor charge

(c) There is no requirement of solvent or chemical and its recovery.

(d) The properties of the finished product can be controlled by suitable manipulation.

(e) The process is more versatile.

However, melt spinning process, at present, suffers with following disadvantages.

(a) Only limited polymers, not all the polymers can be converted into fibres by melt spinning process. For example cellulose and PVC do not give a stable melt and so these polymers have to form into filaments by solution spinning method.

(b) Structural development is poor because of higher solidification rate.

(c) Because of inadequate structural development, maximum strength cannot be achieved

(d) Higher production at also leads to higher waste Optimum properties.

(e) Accurate temperature control is very critical for uniform and optimum properties.  

Polymerisation & type of Polymerisation

Polymerisation

The polymerisation techniques used in industry are different for condensation and addition polymerization this is basically due to the differences in mechanisms in the two types of polymerisation.

Condensation polymerization

 This type of polymerisation can be conducted by either high temperature or by low temperature method. Most of the industries are using high temperature method. In this method, the polymerisation is most frequently conducted in molten monomers in an autoclave or reactor at temperatures above 200°C.Alongwith the' monomers, sometimes, inert gases to avoid side reactions, blocking agent for viscosity control, and delustering agents are used.

 Several hours are required to complete the polymerisation reaction. As the reaction progresses, viscosity increases. After the reaction, the system is evacuated to remove the low molecular weight product. 

Further, the whole mass is pushed out very quickly and quenched in water, followed by cutting, washing and drying. In general, monomers with low reactivity require high temperature polymerisation. If the monomer reactivity is high or if it is modified for high reactivity, then the polymerisation reaction can be conducted at lower

temperature. For example, Nylon 6,6 can be produced by reaction with adipic acid and hexa methylène diamine at temperatures above 250°C.But if instead of adipic acid, its acid chloride is used for, reaction, then the polymerisation can be done at low temperature i.e., below 40°C.0

Addition polymerization

 There are several methods of conducting åddition polymerisation of vinyl compounds or any other double bonded compounds. The methods vary only in the physical state of dispersions. Depending upon the state of dispersions, the different methods are: (a) Gas phase,

(b) Bulk

(c) Solid phase

(d) Solution

(e) Emulsion and

(f) Suspension polymerisation

Gas phase Polymerisation

This method is carried out with the monomer in the gaseous state. Polymer formation begins on the walls of the reactor or on the surface of already formed polymer. This method is used to produce polyethylene.

Bulk Polymerisation

This method is carried out in the liquid monomer at definite conditions like temperature, pressure etc. If the resulting polymer is soluble in monomer,
the viscosity of the medium gradually increases in the course of polymerisation. Then the polymer is a monolithic block of polymer. If the polymer is insoluble in monomer, the polymer is obtained as a powder or as a porous material. Polystyrene and poly(methyl methacrylate) are produced by bulk polymerisation method.

Solid-phase Polymerisation

This polymerisation method proceeds at temperatures below the melting point of the monomer. Only selected monomers can be polymerised at temperatures below their melting point. The polymerisation can be initiated by the action of light, irradiation with x-rays or gamma rays or by any other high energy particles. This method has not become popular.

Solution Polymerisation .

In this method, the reaction medium is a suitable solvent, where monomers, initiators and modifiers are added. Polymer may be formed in two ways.

(i) If the polymer like the monomer is soluble in the solvent, then the polymerisation results in a solution of polymer, called a lacquer. The lacquer may be used directly for further processing or application. Alternately, the polymer may be precipitated.

(ii) If the polymer is insoluble in the solvent, then the polymer precipitates and is separated from the solvent. This method is used to produce high density polyethylene, polypropylene and their copolymers.

Emulsion Polymerisation

 In this method, the liquid monomer is not soluble in the reaction medium or dispersion medium. Here, the medium is usually water and the monomer is dispersed in the medium (water) to form an emulsion. Initiators, emulsifiers and other additives are added to the medium. The reaction mass remains in the form of an emulsion till the polymerisation is over.

 During polymerisation, the monomer emulsion (monomer droplets) is slowly transferred into polymer emulsions (polymer droplets).The polymer droplets are smaller than that of the monomer droplets. Polyacrylonitrile, poly(vinyl acetate), poly(vinyl chloride) and poly(methyl methacrylate) are produced by emulsion polymerisation method.

Suspension Polymerisation

This method is similar in principle to emulsion polymerisation. But here the droplets are larger. Also, the polymerisation proceeds with the formation of large granules in the suspension of polymer in water

Source of synthetic fibre

Source of synthetic fibre

the main sources for formation of synthetic fibres are:

(1) Petroleum,

(2) Natural gas deposits and

(3) Coal.

Petroleum contains gaseous, liquid and solid hydrocarbons. These hydrocarbons, depending upon their molecular structure fall into three main classes, the paraffin’s, the naphthenes and the aromatic hydrocarbons Petroleum contains many different liquid and solid hydrocarbons of all the ee abovementioned classes in solutions as well as hydrocarbons of mixed structure. Also, petroleum field gases have greater, amount of gaseous hydrocarbons

 Natural hydrocarbon gases consist of simplest paraffin called methane hydrocarbons, Natural gases occurring in sedimentary rocks also contain hydrocarbons.


A large amount of aromatic compounds are found in coal-tar, a by product in the manufacture of coal gas or coke. Benzene and its homologous are present in the light oil fraction of the coal tar.

 Crude naphtha and phenol are present in the middle oil fraction of coal tar Petroleum paraffin hydrocarbons, natural gas and coal tar yield different types of saturated and unsaturated hydrocarbons other intermediate hydrocarbons. 

These materials serve as raw materials for the production of synthetic fibres. At present synthetic fibres are more prominent than the natural fibres. The main advantages of synthetic fibres are as follows:

(a) High resistance to chemicals,

(b) High resistance to micro-organisams,

(c) Low flammability

(d) High elasticity and so high resistance to distortion and creasing,

(e) High resistance to abrasion.