Drawing is is necessary to remove the region of natural draw of the fibres. When the fibre is drawn beyond the NDR, polymer chains form an elastic network with extended conformation in the direction of draw(i.e. along the fibre axis). This gives fibres higher yield point,strength,initial modulus and recovery compared to its as spun form.
Figure shows, a stress strain plot of an as spun fibre.The elastic region is small and is bounded by the yield point,the fibre shows deformation without much resistance till all the chains are unfolded and a new improved(more elastic) network of polymer is formed.In this region ,polymer chains assume new extended conformation and do not recover back to their original state when stresses are released.This known as the region of Natural Draw.
2. True Stress vs True Strain
It is important to remember that the stress-strain curves that are usually drawn while explaining the properties of fibres and process of drawing are based on load per unit original denier and strain is calculated based on the original length of the specimen.However at any point during tensile drawing ,the true stress is different because the diameter or the denier of the material is continuously reducing .Similarly,the true strain must be calculated based on the actual length of the fibre at that moment. Therefore, as shown in equation (1) ,
true stress is given byϬ=Ϭ⁰×DR………………………………………(1)
Whereas true strain is given by the integral of dl/l in the limits of original length to final length, as shown in equation (2)
If a stress strain curve is converted in to true stress-true strain curve then their behavior changes.
3. Effect of spinning speed on maximum draw ratio
The natural draw ratio in the as spun fibre is a function of its spinning speed.The extensibility is about 400-500% when the fibres with the spinning speed of 1000 to 1500m/min to form LOY filament(Low oriented yarn). The POY(Partially oriented yarn),which are spun at around 3000-4500 m/min, have the extensibility of around 70-90%. The extensibility significantly reduces at higher speed due to stress induced crystallization.A natural draw of about 10% may still be observed at very high spinning speed of 6000m/min. or higher.Therefore, the fibres spun at very high speed may also have to be drawn though to a much smaller extent.
4. Drawing Machines
Drawing is carried out by stretching the filament yarn between two sets of rollers.The drawing ocurs between feed rollers and take up rollers (Draw rollers) at room temperature.or at an elevated temperature by providing a heater plate.The drawn material is given a small twist. and is wound on a bobbin.The path of yarn undergoing drawing is regulated by proving guide pins or alternately placing the feed and take up(drawing) rollers perpendicular to each other.An idle roller at an angle is provided with each other ser (i.e. feed or draw) to allow the yarn to be wound in multiple loops separately on each on these rollers.This gives a better grip to the yarn does not slip easily under drawing tension.The ratio of surface speed of the draw(V2) and feed roller (V1) is called the machine draw ratio.
The fibre after drawing may relax and recover some parts of the machine draw ratio applied to it.Eventually,the fibre may attain the machine draw ratio depending upon the amount of relaxation the fibre undergoes.The speed at which the fibre travel between the two rollers determines the rate of drawing.An estimate of this draw rate can be worked out as follows:
The average speed between the roller is =(V1+V2)/2, if the distance between the draw rollers is L, then the time spent by the fibre during drawing is=(L/(V1+V2))/2.
5. Net Drawing
Cold drawing or neck drawing of polymer is a frequently used technique for obtaining polymers in an oriented state.The term cold drawing was initially applied to amorphous polymer stretched in a temperature range between brittle point and glass transition.The cold drawing occure when a fibre sample is forced to extend at a rate faster than the rate of molecular mobility or relaxation (i.e.a little below Tg),At the Tg,the rates become comparable and the neck disappears.
6. Effect of parameters on drawing behavior
- The morphology of as spun yarn
- The draw ratio
- The draw rate
- The draw temperature
- The moisture content of the fibre
7. Structure development during drawing
Drawing is associated with significant changes in the structure of the fibre. Due to high stresses developed during drawing ,The orientation of fibres increases, crystallinity usually changes,and the type of crystals may undergo transformation. The morphology of structure may also change in the connectivity between amorphous and crystalline phase. Overall change is to lower Gibbs free energy(G) by lowering enthalpy (H) of the system.
a)Orientation development: After drawing there is no doubt The network draw ratio reflects the state of deformation of the molecular network present in the undrawn and drawn fibres.Its is interest to look at the development of molecular orientation with network draw ratio.
b)Change in crystallinity during drawing: Whereas the degree of orientation always increases in the course of drawing, the crystallinity can change in both directions.Three types of behavior can be distinguished
- Deformation is accompanied by additional crystallization and increase of crystallization.
- Deformation does not effect phase structure;an undrawn amorphous sample remains amorphous after drawing ;crystalline sample does not change its degree of crystallinity.
- Deformation is accompanied by partial destruction of the original structure and reduction of crystallinity.
c) Change in crystal type during drawing: In many fibres,initial crystal type may get transformed as the fibre is subjected to drawing stress.In this crystal type of high energy may get transforms to alpha crystalline phase on drawing,similarly gamma crystals in the nylon 6 posses hydrogen bonding between antiparallel chains while gamma type structure posses hydrogen bonding between parallel chains.The gamma type structures generally have a shortened crystallographic repeat distance in the direction of chain axis.In material where there is one type of crystal,Imperfect crystallites formed during spinning stress get transformed in to more stables and perfect crystals during drawing.
d) Changes in long -range morphology of the fibre: The morphology of the structure may also change from micellar to fibrillar,with concurrent changes in connectivity between amorphous and crystalline phases.Overall changes is to lower Gibbs free energy(G)by lowering enthalpy (H) of the system.Long range order is important in fibre morphology. This decide the fibrillar structure, which is more load bearing. Also decide the manner in which crystalline and amorphous regions are connected to each other for load transfer.
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