Elastic Performance Coeffi cient and Recovery of Modifi ed Polyester / Polyvinyl Alcohol Ring Spun Yarn

The structural modifi cation of yarn is opening up new fi elds of application. In the present study, the structure of polyester/polyvinyl alcohol (PVAL) blended ring spun yarns was modifi ed with a dissolution of PVAL, while the yarns were prepared using various percentage of the PVAL fi bre. The elastic recovery and elastic performance coeffi cient (EPC) were measured before and after the dissolution of PVAL for a comparative assessment. Fibre fi neness and twist multiplier were also selectively altered to study the eff ect. The experiments were carried out to study the elastic recovery at 2% and 4% extension, while EPC was calculated using 30% and 50% of breaking load of respective yarns. The t-test result confi rmed some signifi cant diff erence in EPC and recovery behaviour of the yarns on modifi cation through the removal of PVAL. Fibre fi neness and applied twist were found to infl uence the behaviour. Modifi ed yarns were found to exhibit improved elastic recovery properties.


Introduction
Th e structure of a spun yarn governs its mechanical properties. Any structural change leads to a change in its properties [1] and expected to widen its scope of application. Structure of a yarn can be modifi ed during and/or post manufacturing process. Such modifi cation can be brought about either by mechanical or by chemical process or by combination of the processes. One of the ways of post manufacture modifi cations can be a suitable treatment. Such modifi cation in yarn structure not only changes the aesthetic properties but strongly infl uences other physical and mechanical properties of the yarn and in the products made out of it. Textile products are generally subjected to stress and strain of repetitive nature and such modifi cation may aff ect its time dependent behaviour or even infl uence the useful life. Th e mechanical failure of a material as a result of repeated loading and unloading occurs if the structure is incapable of absorbing and dissipating the imparted energy without the occurrence of failure as either a permanent deformation or an actual rupture of the components, depending upon the enduse requirement [2]. Th e elastic recovery plays a special role as one of the mechanical properties of yarn [3]. It is a time-dependent phenomenon, which is not only dependent on the structure but also on the duration, level of stress and level of strain on which the material is subjected to. Th e longer it is held at a given extension, the lower is the level of recovery [4,5]. Both shape retention and durability of a textile material are likely to be aff ected due to the repetitive nature and level of applied stress causing delayed elastic and plastic aft er eff ects. Th e ability of a material to retain its original properties aft er repeated use, hence the reproducibility of stress-strain properties exhibited by a material following cyclic loading and unloading is very important and is defi ned as its elastic performance. Precisely, the degree to which any material duplicates perfectly elastic material has been termed as elastic performance coeffi cient. Th e elastic per forman ce coeffi cient refl ects the eff ect of imme diateelastic recovery, primary-creep, and se con dary-creep defl ections [6,7]. High immediate elastic recovery at low average tensile strain is an important property in determining the crease recovery of the fabric. Sett [8] has shown the yarn compactness, fi bre orientation and fi bre mobility/rearrangement to be the key factors in governing viscoelastic behaviour and elastic recovery of jute blended yarn. Chattopdhyay [9] reported that a yarn with poor structural integrity consumes less energy during deformation and accordingly the recoverable energy is also less. A low recovery and high permanent set for air jet spun yarn while higher recovery and less permanent set for ring yarn compared to rotor spun yarn was reported by Tyagi [10,11]. A signifi cant eff ect of add on fi nish on recovery properties for air jet spun yarn was also reported by Tyagi [12]. Guthrie [13] while studying elastic recovery of viscose rayon fi bres have found its dependence on the time of applied extension and on the time allowed for their recovery. A higher delayed elastic recovery for rotor spun yarn while higher permanent set for MJS yarn was reported [14]. Manich [15] reported fi bre orientation in yarn structure to result better elastic characteristics and higher permanent set. With the removal of one component in a blended yarn, elastic behaviour is expected to be decisively changed with possible internal structural modifi cations. Such removal is expected to infl uence moisture management behaviour and likely to alter the mechanical characteristics of yarn. In the present study, an attempt has been made to study the eff ect of structural modifi cation through removal of one component on the recovery behaviour of blended yarns.

Material
Th e detail of material and process is given in Table 1. Yarn was spun on cotton spinning system while the blending of fi bres was done at blow room to ensure homogeneity in mixing. Th e yarns were divided into two groups. One of the groups was taken for treatment to remove PVAL and will be referred as modifi ed yarn. Th e parent yarn and the modifi ed yarn aft er dissolution of PVAL are designated as X and Y yarn respectively. Th ough dissolution of PVAL renders the yarn to virtually a homogenous yarn but in the discussion both the yarns will be referred as blended yarn. Factorial design used to prepare the sample is given in Table 2. Twenty seven types of yarns were prepared for the study.

Conditioning and mass irregularity
Th e yarns were conditioned at a tropical atmosphere of 27 ± 2 o C and at 65 ± 5% RH. Th e mass irregularity was measured using UT-3 evenness tester and was found to lie between 11.5 to 12.4% (cut length of 1 mm).

Elastic recovery
Th e recovery parameters of yarns were determined following ASTM D1774-79 standard [16]. Th e immediate elastic recovery (IER), delayed elastic recovery (DER), and permanent set (PS) together representing elastic recovery behaviour were obtained at two extension levels of 2% and 4%. Th irty observations were taken for each yarn sample to get result at 95% confi dence limit. A typical extension recovery curve is shown in Figure 1. Th e yarn was extended up to a predetermined level 'G' and immediately retracted up to 'O' , the origin though point 'C' on tex/2 g load line. Aft er allowing the yarn to relax for 3 min, it was again extended till it crossed the tex/2g load line at 'B' . Recovery parameters were calculated from the following equations: Extension Recovery B C Figure 1: Evaluation of elastic recovery components

Elastic performance coeffi cient
In order to calculate EPC, the yarns were subjected to 10 cycles of loading on Zwick UTM. A traverse rate of 120 mm/min was used while two levels of conditioning loads, viz; 30% and 50% of average breaking load of respective yarns were used. A typical repeated load-defl ection diagram (initial and conditioned cycles) is shown in Figure 2. On the basis of the diagram [6] EPC was calculated by using the following equation: where A L0 is area under loading curve of 1 st cycle, A Lc is area under loading curve of last cycle, A Rc is area under unloading curve of last cycle, a 0 is defl ection length of loading curve of 1 st cycle and a c is defl ection length of unloading curve of last cycle.

Load
De ection a 0

Observed structural changes
In order to study the changes in the structure scanning electron microscope (SEM) images of both X and Y yarns were taken. Typical SEM images are given in Figure 3. Following observations are made from the images: a) the removal of PVAL has led to a reduction in the diameter of the yarn, b) the angle of helix has reduced, c) voids have been generated within the structure thereby causing slackness in fi bres. Th e reduction in diameter, helix angle and generation of voids can signifi cantly alter the arrangement and confi guration of fi bres in the structure. Such a change not only expected to change the inter fi bre cohesion but also the stress distribution pattern in the constituent fi bres whenever the structure is subjected to loading.

Statistical treatment
A pair wise t-test for the results was carried out. Th e result at 95% confi dence limit is given in Table 3.

Mechanisms of recovery in staple yarn
A material which off ers good immediate elastic recovery can contribute to crease resistance, wrinkle resistance, fatigue resistance and fi nally can provide better comfort characteristics [17]. Elastic recovery represents recovery of the material on withdrawal of load and when a staple fi bre yarn is subjected to loading its elastic recovery on withdrawal of load is expected to be infl uenced by: a) the property of constituent fi bres, b) composition and arrangement of fi bres in the yarn, c) frictional characteristics of the constituent fi bres, d) fi bre packing and hence compactness and interfi bre cohesion, e) ability of structure to maintain its integrity, f) imperfections including voids and looseness in the structure. When one component of a blended yarn is removed, it is expected to infl uence most of the factors cited above. Accordingly, the eff ect is expected to be refl ected in the tensile recovery properties as well. When load is applied in a yarn, the stress will develop in it due to the stretching of constituent fi bres. Th e level of stress and its distribution in individual fi bres, however, may be infl uenced by the radial position of fi bre and its arrangement and confi guration in the structure. Removal of component is likely to change the radial position of a fi bre due to its freedom of radial movement in the structure. When a load is applied, the extension in a yarn may occur due to one or more of the following reasons: a) straightening of the fi bre, b) stretching of individual fi bres and their eventual breakage, c) slippage of individual fi bre. Th e generation of stress and level of induced strain energy will depend on the mechanism involved in the extension of the yarn. Slippage and breakage of fi bres lead to irrecoverable extension, though straining of constituent fi bres will help the structure to recover due to induced strain energy. In case of fi bre breakage, a loss in strain energy is imminent. Accordingly the structure cannot recover to its original length. Similarly extension due to slippage of fibre is also irreversible and causes loss in energy. Th e ability of a material to recover is dependent on its capability of absorbing energy imparted through application of stress and of releasing this energy on removal of the stress without causing any major structural changes in the yarn viz; geometric and inherent. Geometric changes refer to the changes which do not cause translation of the component fi bre, while the inherent change refers to the locational change of the constituent fi bres [6]. In the former case, the system can use the elastic energy to regain its original position while in the later, translation of fi bre causes loss in energy and hence it cannot come back to its original position. Th e deformation which takes place due to the alteration of internal structure of material is diffi cult to recover. Th e geometric form of a material also has a defi nite eff ect on both the magnitude and distribution of applied external loads. Elastic property depends upon the inherent elastic characteristics of fi bres, yarn count, and yarn twist. Any deviation in the elastic characteristic under tensile load arises purely as a result of changes in the stress distribution amongst the fi bres only because of the geometry [18,19].
Yarn twist can also infl uence the distribution of stress in a yarn, as it changes the radial position and helix angle of fi bres. Higher the yarn twist the greater is the fi bre tension for a given load. It implies that the load on individual fi bre for particular load is a function of helix angle. Th e distribution of stress (f) has been given by the following equation [20]: where N is the yarn twist (turns/cm), R is the yarn radius (cm), A and B are constants, and determined from the stress strain diagram of individual fi bre.

Immediate Elastic Recovery
Th e immediate elastic recovery (IER) refers to the ability of the textile material to recover from deformation immediately on withdrawal of load and is measured by the recovered length with respect to the total extension imposed. Depending on the structure, a yarn recovers to diff erent extent and at different speed. IER is ideally associated with displacement of the constituent fi bres from their position of equilibrium and with their spontaneous and immediate return on withdrawal of load. Slackness in the structure, ensuring less resistance can help such recovery, if the loading does not cause the extension to exceed the yield point. IER will be better if the breakage and/or translation of the fi bres can be prevented. It is observed from the Figure 4 that IER of Y is always higher irrespective of level of extension, fi bre fi neness, blend ratio and twist multiplier. Th e IER of Y decreases with coarse fi bre, increased blend ratio and twist multiplier. IER increases as openness of yarn structure increases on dissolution of one component while the parent yarn X is relatively compacted. Th e level of stress in individual fi bre on application of load will be dependent on its radial position. A fi bre travelling through a longer path is already at a higher stress level. It is also possible that loading may cause extension to such fi bres beyond yield point causing a reduction in IER. In a compacted structure possibility of damage to such fi bre is more while in a loosened structure the possibility of damage to the fi bre at same radial position is less due to the accumulation of slackness in fi bre. Dissolution of PVAL led to a more open structure and additional openness provides enough space to the fi bres for rearrangement in the structure. Th e openness may also help fibre straightening protecting them from any appreciable breakage and/or displacement. Readjustment of fi bres in a loosened structure may prevent the fi bre extension exceeding a limit causing loss in elastic energy. Openness in the structure can also off er less hindrance in recovery leading to higher IER.

Delayed Elastic Recovery
Th e delayed elastic recovery (DER) refers to the ability of the textile material to recover from deformation with time. DER can be seen as hindered elastic recovery as some of the displaced fi bres continue to return spontaneously for some time. Aft er withdrawal of load the inter fi bre cohesion/entanglement and stored elastic energy together can help in restoration. In an entangled and compacted mass of fi bres, the mutual support can help in regaining original confi guration. Such mutual interaction may even help a fi bre at lower energy level to restore its original confi guration. In a relatively open structure, where the inter-fi bre cohesion reduces, the displaced/extended fi bres get less assistance from the surrounding fi bres in the recovery process. Th e recovery is expected to be mainly by virtue of stored elastic energy. Hence on removal of a component the increased opening of the structure does not facilitate time dependent recovery much. It is observed from the Figure 5 that Y yarns show relatively lower DER irrespective of the level of extension, fi bre fi neness, blend ratio and twist multiplier. Th e value of DER at both extensions for Y yarns, were increased with coarser fi bre and higher blend ratio, while it remains unchanged with twist multiplier at lower extension. However, at a higher extension the DER reduces with increase in twist multiplier.
On removal of a component the compactness of the structure and hence inter fi bre cohesion reduces. Reduction in inter fi bre cohesion does not support in time depended recovery and hence DER reduces for the modifi ed structure (Y). deformation in constituent fi bres in the structure is also less. When the level of strain is less the extent of irrecoverable deformation is expected to be less. However, at low level of strain, the imposed strain energy in the structure and in the constituent fi bres is also expected to be less which may aff ect DER. Accordingly, at higher strain, the amount of stress will increase and lead to higher deformation which may not be recovered even with time despite higher level of strain energy. At higher strain, non-recoverable fibre strain may also add to the deformation in the structure. Hence delayed elastic recovery at lower strain level is more than that at higher strain level. Th e DER is hence, infl uenced by the level of imposed strain energy, deformation in the yarn structure and deformation in the constituent fi bres.

Permanent Set
Th e permanent set (PS) refers to the change or deformation in structure of the textile material which cannot be recovered at all. Th is may cause remarkable change in the shape of textile product aft er use and hence undesirable. Th ough PS can also exist at lower load but is generally detected aft er the structure exceeds its elastic limit. It is caused due to irreversible shift of constituent fibres. Such a shift is expected to be infl uenced by a) displacement of individual fi bres, b) straightening of slack fi bres without imposing strain and its subsequent failure to restore to the preloading confi guration, c) stabilization of the rearrangement of fi bres due to load application and hindrance in recovery offered by the modifi ed structure. It is observed from the Figure 6 that yarns Y result lower permanent set than X in all cases under study irrespective of level of applied extension. Th e inter fi bre cohesion in an opened structure being less the restoration of confi guration become more diffi cult. Th e PS in the opened structure shows a tendency to increase. On removal of a component, stress is distributed among less number of fi bres which may incidentally cause more deformation in fi bres. When coarser fi bres are used, the number of fi bres further reduces thereby increasing the stress per fi bre. So use of coarser fi bre leads an increase in load per fi bre and hence the PS is more. Slackness in the structure changes the stress distribution pattern in the constituent fi bres. Th e geometric position of the fi bres might show more change than the inherent properties of the fi bres. Hence, the PS is low.

Elastic Performance Coeffi cient
Th ough the elastic performance coeffi cient (EPC) refers to the ability of the textile material to recover from deformation on repeated loading, the degree to which material can duplicate perfect elastic material has typically been termed as elastic performance coeffi cient. Th e results are represented in Figure 7. It is observed from the fi gure that the EPC of the yarn after removal of one component improved compared to that of the parent yarn. With increase in the applied load the diff erence in EPC reduced. Th e fi bre fi neness, blend ratio and applied twist had also infl uenced EPC. EPC improved with the use of fi ner fi bre and 15% PVAL blended yarn resulted highest value both for parent and modifi ed yarn. However, at higher conditioning load, EPC is higher in yarn with lowest percentage of PVAL. Th e twist level, however led an initial increase of EPC and then a fall. On loading, the level of stress in each fi bre will be dependent on their respective radial position. Fibres away from the central position will experience higher stress. When the structure is compact, the possibility of readjustment is less. If some additional spaces are created in the structure by removing one of the components the remaining fi bres will have a scope to change the radial position and hence level of stress will be reduced on application of load. Th e additional spaces created in the structure allow slackness to accumulate in fi bres. It is possible for such fi bre to withstand to higher level of load without undergoing any inherent deformation. Th is leads to the area under loading and recovery curve to change and hence the removal of PVAL leads to an improvement in EPC. Th e EPC was also found to be infl uenced by fi bre fi neness, blend ratio and applied twist. In the parent yarn no signifi cant change in EPC was observed while it declined in modifi ed yarn as the fi bre became coarser. For a particular count, the number of fi bres in the cross-section will be more when fi ner fi bres are used. Accordingly, higher number of load sharing components (leading to reduction in load/fi bre) and slackness in the structure might have caused less stretching of the fi bres. Th is leads to the minimization of deformation of the structure and of constituent fi bres resulting better recovery. Th e EPC of modifi ed yarn (Y) is higher when EPC is calculated at lower load. When the applied load increases the diff erence in EPC of X and Y reduces. In the modifi ed yarn it is higher at 15% PVAL blend at lower conditioning load. It is higher at 10% blend in at higher conditioning load. Locational change of the fi bres and possibility of inherent change in properties of fi bre at higher conditioning load are responsible for minimum diff erence in EPC at higher load. A marginal reduction in EPC at higher twist may be attributed to the higher internal stresses in the individual constituent fi bres than those in the fi bres in yarn at lower twist. Th e conditioning load essentially is the sum of the components of fi bre loads parallel to the yarn axis. So at higher twist an increase in stress on individual fi bre leads to the possibility of secondary creep to dominate and hence the EPC reduces. Change in helical position due to the removal of PVAL and subsequent creation of space in the structure have caused some slackness. In such a case, even a low level of elastic energy can also help in recovery due to less resistance from inter fi bre contact and accordingly EPC is higher.

Conclusions
Elastic recovery and elastic performance coeffi cient can be infl uenced by the properties of fi bres, yarn composition and twist. Fibre fi neness, twist and PVAL% in parent yarn and its dissolution in modifi ed yarn resulted considerable infl uence on elastic recovery and elastic performance coeffi cient. From the study, following conclusions can be made: the parameters, fi bre fi neness, twist multiplier and blend ratio were found to infl uence both elastic recovery behaviour and EPC. it is evident from the Figures 4-6 that modifi ed yarn has higher immediate elastic recovery and lower delayed elastic recovery and permanent set indicating modifi ed yarn has more ability to recover with application of stress. similarly, a look on the fi gure 7 reveals that the modifi ed yarn has higher elastic performance coeffi cient indicating that higher ability to withstand and recover with repeated stress application. dissolution of PVAL seems to have caused by a reduction in yarn diameter.
structure of the modifi ed yarn becomes more open with reduction of helix angle. Th e generated voids in the structure can reduce inter fi bre cohesion which can alter the stress distribution pattern in the yarn. Th e stress distribution pattern is also expected to change due to the change in radial position of fi bres.