Drawing or stretching of filament yarn is usually accomplished by conveying the spun fibre through two sets or two series of rollers separated by a heater called godet. The output roller or rollers rotates at faster speeds than the respective input rollers. The filament yarn passes through the heater which is placed between the output and input rollers. The ratio of output roller to input roller speed is called draw ratio or stretch ratio. Three different levels of temperature and stabilising overfeed used to produce drawn yarn and properties related to filament yarn were tested.
Polyester partially oriented yarn of 250/48 was used to produce drawn yarn. The fully drawn yarn produced using Himson draw winder DTW-1000AS machine. To produce samples, three draw ratio of 1.55, 1.65 & 1.75, drawing temperature 160 degrees, 170 degrees and 180 degrees C and stabilising overfeed of 2.5, 3.5 and 4.5 per cent were used and the parameters like speed of 400 MPM, traverse speed, oiler roller speed etc. were kept same. Total samples were reduced to nine samples and were produced using L9 orthogonal statistical tool as follows.
The material used in this study was PET partially oriented yarn of 250/48D. Tensile strength of material was found 528gf with breaking elongation of 78.18 per cent.
Linear density of the drawn yarns was tested by using ASTM standards 1907-07, on the metric wrap reel.
Mechanical properties of the oriented materials were measured at room temperature. The samples were tested on an Instron tensile tester. The results obtained were expressed in terms of tensile strength and breaking elongation.
The birefringence measurements of the as spun yarns were made on simple microscope using refractive index method. In this method the birefringence was determined by directly measuring two principal refractive indices parallel (nǁ) to the fibre axis and perpendicular (n┴) to the fibre axis and calculating the difference ∆n= nǁ - n┴.
Shrinkage refers to the decrease in length of specimen when it is placed in a hot environment. Here the produced polyester samples were placed in hot air for a specific period of time and the shrinkage percentage is calculated over the change in length due to hot air.
Shrinkage % = [(L0-Lf)/L0]*100
Where, L0 & Lf are the initial length and final length.
Densities of produced samples were determined by using flotation technique and then crystallinity was calculated. The volume fraction crystallinities percentage of yarn was estimated from densities of sample using following expression.
Density crystallinity % = [pc (pc -pa)/ p (pc - pa)]*100
Where p, pa, pc, are densities of sample standard amorphous and standard crystalline phase respectively. The values for pa and pc are given below.
Pa= 1.335g/cc and pc=1.455g/cc
Results and discussion
Physical properties and structural characteristics of yarns were summarised in Table 2.
Physical and some structural properties characteristics of yarns
Denier of yarn
It is seen that denier decreases with increase in draw ratio due to stretching of filaments. The effect of the draw ratio on the linear density of drawn yarn was shown in fig. 1. For as-drawn filaments the calculated linear densities were less than the measured linear density. This difference is due to the filament contractions after drawing. It was seen that increase in stabilising overfeed denier of yarn increases steadily because increase in stabilising overfeed reduces tension in filament giving more relaxation to fibre which leads to increase in linear density of filament. It is seen that the effect of the draw ratio on the linear density of as-drawn filaments. For as-drawn filaments the calculated denier was less than the measured denier. This difference is due to the filament contractions after drawing. ANOVA shows draw ratio significantly affects denier of yarn.
Strength and elongation
Micro fibrils arranged into fibrils of filament length are proportional to draw ratio. The deformation in the structure is caused by sliding of fibril while drawing. As draw ratio increases, volume fraction of micro fibrils decreases. The increase in fibre strength on drawing increases as a result of transformation of micro fibrils into extended chain interfibrillar molecules. The increase of the strength with draw ratio is possibly due to the increase of molecular orientation. Same trend of increase of strength of filament with increase in temperature from 160 to 180c due to drawing at high temperature builds up a crystal structure giving the fibre high tenacity. The elongation values increase as stabilising overfeed increases. Due to relaxation, tension on filament decreases on extended chain molecules and we get more elongation.
The draw force is found to be increasing with respect to increasing draw ratio. The molecular orientation developed is dependent on the stress developed during drawing or stretching. It was seen that birefringence values increase with increase in temperature due with increasing temperature build up a perfect structure gives increase in crystallinity and orientation.
Density of samples is measured and crystalline fraction is calculated. Drawing as expected, increases bulk density of filaments. Also it is seen that increase in stabilising overfeed percentage decreases density crystallinity percentage. In case of polyester, the chains contain bulky benzene rings in main chain which prevent chains from attaining lowest state of energy level. As drawing proceeds, the chains pack up in an orderly way and in such a manner so that the energy associated with them decreases. Thus, more crystallinity seen at 2.5 percentage and decreases with increase in stabilising overfeed percentage.
It was seen that shrinkage decreases with increase in draw ratio. In case of polyester, the chains contain bulky benzene rings in main chain which prevent chains from attaining lowest state of energy level. As the drawing proceeds, the chains pack up in an orderly way so that the energy associated with them decreases. This helps in release of internal stresses, lowering shrinkage values. At higher draw ratio, higher orientation is produced. Shrinkage was increased at lower drawing speed and at low draw ratio. It was observed that a direct relationship exists between shrinkage and filament denier i.e., with the increase in the denier, shrinkage increases and vice versa.
It seems that increasing drawing temperature, shrinkage values decreases. This may be due to high temperature values that make the perfect crystal formation. It was also seen that increase in stabilising overfeed increases shrinkage. This is because overfeed increases molecular relaxation. Because of overfeed, the chains pack in disturbed positions and at lowest energy by reducing tension. This causes internal stress in chains once again because of disturbed position and secondly because of immobility of bulky groups. Thus, increase in internal stress causes increase in hot air shrinkage.
Different drawn yarns were produced using combinations of draw ratio (1.55, 1.65, 1.75), drawing temperature (160, 170, 1800c) and stabilizing overfeed (2.5, 3.5, 4.5%). The results were statistically analysed and the conclusions were:
- Draw ratio significantly affects linear density of yarn. As draw ratio increases, denier of filament decreases.
- Increase in draw ratio results in more crystallinity region in filament. This results in more molecular orientation and hence strength of yarn increases causing reduction in elongation of yarn. There is reduction in hot air shrinkage of filament with draw ratio due to higher orientation of molecule.
- Increase in drawing temperature results in increase in strength of yarn and with decrease in elongation of yarn. As drawing temperature increases, it results in better orientation.
- With increase in stabilising overfeed initially, birefringence value increases. After that it decreases due to disturbance in molecular chain.
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The authors are thankful to the management of DKTES Textile & Engineering Institute, Ichalkaranji and DKTES Center of Excellence in Nonwoven for giving permission for testing and carrying out this work. The authors are also thankful to Welspun Syntex Ltd, Mumbai and Sandeep Ostwal, Vice President, Welspun Syntex Ltd Mumbai for supplying polyester and for his guidance.