Indian Journal of Fibre & Textile Research Vol. 32, March 2007, pp. 80-87 Static and kinetic frictional characteristics of staple fibres and woven fabrics A Das a & S M Ishtiaque Department of Textile Technology, Indian Institute of Technology, New Delhi 110 016, India Received 7 February 2006; accepted 21 March 2006 An instrumental has been developed for measuring static and kinetic frictional characteristics of staple fibres and fabrics. The influence of fibre length and fineness on static and kinetic frictional characteristics of polyester and viscose staple fibres has also been studied. A comparative study has been carried out to ascertain the difference in frictional forces measured by an attachment fitted with Instron tensile tester. The developed instrument is also used to study the frictional characteristics of acrylic staple fibres at different levels of finish application. A wide variation in frictional characteristics is observed among different varieties of cotton, even within same variety from different place of origin. It is also observed that warp density and fabric thickness have good correlation with static and kinetic frictional properties of both warp-on-warp and weft-on-weft motions. Keywords: Cotton, Frictional index, Kinetic friction, Polyester, Staple fibre, Static friction, Viscose, Woven fabric IPC Code: Int. Cl. 8 D03D, G01N33/36 1 Introduction Textile scientists and technologists have recognized the contribution of inter-fibre or fibre to other material frictions in controlling fibre flow during spinning process, affecting physical properties of yarn and fabric with deformation behaviour of fibre assembly. The knowledge of the frictional values of individual fibres is very important to predict whether the processing will be satisfactory or not, when other physical properties are satisfactory. The processability of a staple fibre is affected, in general, by two factors, namely (i) geometrical structure of each fibre and (ii) surface properties. The detailed studies have already been reported 1-5 on the frictional properties of fibres. The frictional properties of fabrics have been accepted for a long time in the subjective evaluation of smoothness or roughness and perhaps taken for granted. One of the factors which influences the subjective judgment of fabric is undoubtedly the static and/or kinetic coefficient of friction between the cloth surface and the fingers. It has been widely reported that the classical friction laws by Amontons fail to describe the frictional behaviour of materials that deform viscoelastically, such as textiles. Firstly, the coefficient of friction of both fabric-to-fabric 6-8 and fabric to other a To whom all the correspondence should be addressed. E-mail: apurba_das@hotmail.com / apurba@textile.iitd.ernet.in material 9-11 is not independent of the normal force but decreases with the increase in normal force up to some constant value. Secondly, the friction is dependent on the geometric area of contact between the two bodies. In spite of its great importance in textiles, the measurement of friction has been given least importance for practical purpose. A large number of studies has been reported on frictional behaviour of textiles and most of the researchers have used either some attachments fitted with common tensile testers like Instron 7, 12 where chances of errors are there due to friction between connecting thread and pulley, or they have used some complex systems to carry out specific friction-related studiey. 8 The present work was therefore undertaken with the objectives (i) to design and develop an instrument for measuring static and kinetic frictional behaviour of staple fibres as well as fabrics. The instrument should be an independent one and not to be attached with any other instrument; and (ii) to characterize the friction-related parameters of different types of staple fibres and woven fabrics with the help of the above instrument. The effects of place of origin of cotton variety; length and fineness of polyester and viscose staple fibres; and different levels of surface finish for acrylic staple fibres, on frictional characteristics have also been studied.
DAS & ISHTIAQUE: STATIC AND KINETIC FRICTIONAL CHARACTERISTICS OF FIBRES & FABRICS 81 2 Materials and Methods 2.1 Materials Different varieties of cotton (J-34, V797, S-6 and DCH-32), polyester, viscose and acrylic staple fibres and wide range of woven fabrics were used for the study. Two varieties of cotton (J-34 and V797) were used to study the effect of places of origin on frictional characteristics. In case of synthetic and blended fabrics, apart from other physical parameters, different combinations of warp and weft yarns were chosen. The details of staple fibres and fabrics are given in Tables 1 and 2 respectively. 2.2 Design and Development of Instrument The schematic diagram of the instrument for measuring the frictional behaviour is shown in Fig.1. The movable jaw (J1), which is connected with the motor (M), holds the bottom layer (B) of fibre or fabric. The other jaw (J2), which is fixed, holds the top layer (A) of fibres or fabrics which is attached with the load cell (L). A known load (N) with standard foot area is placed over the layers of fibres or fabrics, which applies the normal load. The bottom layer is supported by highly polished stainless steel adjustable platform (T); for staple fibres short platform and for fabrics long platform are used. When the jaw (J1) starts moving, it pulls the bottom layer of fibre/fabric (B). As the bottom layer starts moving, the resistance force (frictional force) due to inter-surface friction between layers A and B generates and detected by load cell of bonded strain gauge type with rated capacity of 0-2000 gf. The output is recorded on a personal computer through an amplifier and an A/D converter, fitted within control panel (C). The communication settings are done at 300-baud rate from control panel to computer through RS-232 port. The developed instrument is very simple in operation and not to be attached with any testing instrument like Instron. It is fully computer controlled, i.e. the data analysis is done through dedicated software. Various friction-related parameters are available as per the requirement. Both staple fibres and fabrics can be tested in the same instrument; only the base platform needs to be changed. There are no chances of frictional loss due to pulley arrangements, as in case when some attachments are fitted with tensile testers like Instron. Friction between textile materials, i.e. fibre-to-fibre and fabric-to-fabric, as well as between textile and non-textile materials can be tested in this type of instrument. Table 1 Details of fibre samples Fibre Sample Variety Length a Linear type code (place) mm density b Cotton CJ 1 J-34 (Bhiwani) 25.4 4.16 CJ 2 J-34 (Mehem) 25.7 4.19 CJ 3 J-34 (Hissar) 25.5 4.12 CJ 4 J-34 (Narwana) 25.2 4.18 CJ 5 J-34 (Sri Ganganagar) 25.5 4.10 CV 1 V797 (Kadi) 24.8 4.77 CV 2 V797 (Harij) 24.6 4.75 CV 3 V797 (Radhanpur) 24.8 4.69 CV 4 V797 (Virangam) 24.5 4.66 CD DCH-32 30.1 3.10 CS (Shankar-6) 28.8 3.98 Staple polyester PL 1-34 1.4 PL 2-44 1.4 PL 3-51 1.4 PD 1-44 1.0 PD 2-44 1.2 PD 3-44 1.4 PD 4-44 2.0 Staple viscose VL 1-38 1.5 VL 2-44 1.5 VL 3-51 1.5 VD 1-44 1.2 VD 2-44 1.5 VD 3-44 1.75 Staple acrylic AC - 44 1.4 a For cotton it is 2.5% span length and for synthetic fibre it is mean length. b For cotton the unit is μg/in and for synthetic fibres the unit is denier. 2.3 Methods 2.3.1 Sample Preparation The fibre samples, in the form of fringes of 30 mm width, were prepared with the help of a common fibre sampler. The base of the fringes was then fixed carefully with the adhesive tape so that no fibre distortion takes place. Care was taken while preparing the fibre samples so that the distribution of the fibres along the width of the fringe remains almost same. Also, the fibres should be parallel to each other and no distortion of fibre alignment should be there during clamping of the fringes. The amount of fibres in the fringes should not vary too much. In the case of fabrics, the sample size was kept at 130mm 50mm. With a particular type of fibre, a large number of samples was tested and consistent results were
82 INDIAN J. FIBRE TEXT. RES., MARCH 2007 Table 2 Details of fabric samples Fabric Fibre content Fabric Fabric sett, thd/cm Yarn count, tex Weight Thickness code P T structure P T P T g/m 2 mm C 1 C C Plain 19.3 13.8 16.4 16.4 57.9 0.13 C 2 C C Plain 52.0 21.3 14.8 14.8 106.0 0.20 C 3 C C Plain 18.1 18.1 30.7 30.7 116.4 0.44 C 4 C C Plain 34.6 21.6 19.7 19.7 121.0 0.29 C 5 C C Plain 39.4 29.5 15.1 15.1 122.6 0.15 C 6 C C Plain 37.0 21.6 21.0 21.0 128.7 0.27 C 7 C C Plain 19.7 18.1 31.2 46.9 143.0 0.49 C 8 C C Plain 18.1 14.6 32.4 64.2 161.7 0.63 S 1 PMF PMF Plain 47.2 25.2 9.1 9.1 74.1 0.20 S 2 PMF PSF Plain 44.1 29.9 9.1 19.7 100.5 0.22 S 3 PMF PSF Plain 36.2 22.0 19.7 19.7 123.5 0.27 S 4 PMF, P/V P/V Plain 36.2 28.7 9.1, 19.7 19.7 125.2 0.28 S 5 P/V P/V Plain 25.2 18.9 32.8 32.8 153.4 0.36 S 6 PMF, PV PMF, P/V Plain 18.2 18.2 19.7, 19.7 19.7, 19.7 154.3 0.37 S 7 PMF P/V Plain 23.6 20.5 36.9 31.9 158.7 0.40 S 8 PMF P/V Plain 25.9 18.1 37.0 50.9 199.3 0.46 S 9 P/V P/V Twill 31.1 21.3 42.2 42.2 224.0 0.44 S 10 P/V P/V Plain 14.1 13.8 107.4 107.4 331.0 0.54 C Cotton; PMF Polyester multifilament; PSF Polyester staple fibre; P/V polyester/viscose; P Warp; and T Weft. obtained. From the same fabric, two types of samples were cut, i.e. in warp direction and in weft direction. In the present study, the fabric-to-fabric frictional parameters are reported both for warp-on-warp and weft-on-weft directions. For each testing two identical samples were taken, i.e. either warp for warp-on-warp friction or weft for weft-on-weft friction. Before frictional study, all the fibre and fabric samples were conditioned in a testing atmosphere of 27±2 o C and 65±2% RH for at least 24 h. For fabric samples, the conditioning was done with the technical face resting upwards. 2.3.2 Test Procedure The frictional indices were measured between two samples of the same fibre or fabric. The sample, forming the bottom layer, was mounted on the movable jaw (J1) and rested on the polished supporting platform (T) (Fig. 1). Another sample, which forms the top layer, was then attached with the fixed jaw (J2). A known weight (40.82 gf for fibres and 52.46 gf for fabrics) with certain foot dimension (30mm 15 mm for fibres and 30mm 50mm for fabrics) was then placed over the sample layers to impart the normal force (N) in between the fibre or fabric layers. The effect of weight of fibre or fabric Fig. 1 Schematic diagram of instrument for measuring frictional properties samples was neglected. The fabric samples were clamped in the jaws in such a way that the technical face sides of both top and bottom layers come into contact. As the bottom layer starts moving, the frictional force (F) generated and measured with the help of load cell and computer. The speed of the movable jaw was kept constant at 10 mm/min for all the fibre and fabric samples. Average of five readings was taken for fabrics and of twenty readings for fibres. The static and kinetic frictional values were obtained from the computerized data and from the
DAS & ISHTIAQUE: STATIC AND KINETIC FRICTIONAL CHARACTERISTICS OF FIBRES & FABRICS 83 friction trace. A typical friction trace of fabric is displayed in Fig. 2. The static frictional force (F s ) was taken as the highest peak at the beginning of the motion, whereas the mean of the peaks and troughs was regarded as the kinetic frictional force (F k ) at that point where the friction trace becomes horizontal. The static and kinetic frictional forces of fibres and fabrics are then converted into frictional indices by dividing them with normal load (N) (Tables 3-5). For fabric samples, friction amplitude (F a ), i.e. the height of the stick-slip pulse, was also measured from the frictional trace in the same way as reported 12 earlier. The differential frictional force (F s -F k ) was also measured for all the fabric samples. 3 Results and Discussion 3.1 Frictional Properties of Fibres Table 3 shows that the static and kinetic frictional indices of staple fibres vary widely, depending on the type of fibre. In all the fibres, the static frictional indices are higher than kinetic frictional indices. The difference in frictional indices determines the spinning performance of fibres and handle of fibres, yarns and fabrics. If the difference is large, the material will have a coarse, crunchy feel and will give a fabric that rustles like silk, owing to the marked 'stick-slip' motion. 13 This tendency can be reduced by any lubricant finish, which reduces the static frictional index and thus the difference between static and kinetic frictional indices. In the two primary operations, i.e. lap making and carding, emphasis must be laid on the properties of the fibre that promote mutual coherence among the fibres; in subsequent operations the slipping of the fibres becomes the more important feature. In general, it is found that the greater the difference between the static and kinetic coefficient of friction, the better the qualities of the lap and more satisfactorily the carding of the fibre will proceed. The amount and type of oil present on the fibre, and geometrical and surface characteristics govern the static and kinetic friction indices. It is also evident from Table 3 that both the static and kinetic frictional indices of different varieties of cotton (J-34, V797, DCH-32 and Sankar-6) vary widely. Even for same variety of cotton (J-34 and V797), the static and kinetic frictional indices vary widely (samples CJ 1 - CJ 5 and CV 1 - CV 4 ) even when their physical parameters are same and these differences are statistically significant in most of the Fig. 2 A typical fabric-fabric friction trace for woven structure Table 3 Static and kinetic frictional indices of fibres Fibre type Sample Static Kinetic code frictional frictional index index Cotton CJ 1 0.415 (0.44) 0.305 (0.36) CJ 2 0.408 (0.36) 0.303 (0.33) CJ 3 0.470 (0.28) 0.362 (0.28) CJ 4 0.450 (0.51) 0.345 (0.35) CJ 5 0.492 (0.55) 0.352 (0.33) CV 1 0.362 (0.42) 0.212 (0.19) CV 2 0.387 (0.26) 0.237 (0.30) CV 3 0.419 (0.47) 0.250 (0.29) CV 4 0.355 (0.28) 0.215 (0.22) CD 0.327 (0.33) 0.255 (0.24) CS 0.275 (0.46) 0.230 (0.42) Staple polyester PL 1 0.445 (0.22) 0.327 (0.19) PL 2 0.482 (0.24) 0.332 (0.16) PL 3 0.622 (0.21) 0.325 (0.12) PD 1 0.590 (0.38) 0.355 (0.22) PD 2 0.595 (0.14) 0.355 (0.12) PD 3 0.482 (0.19) 0.332 (0.16) PD 4 0.477 (0.16) 0.315 (0.18) Staple viscose VL 1 0.355 (0.25) 0.250 (0.18) VL 2 0.392 (0.22) 0.255 (0.18) VL 3 0.400 (0.19) 0.252 (0.14) VD 1 0.385 (0.13) 0.250 (0.14) VD 2 0.392 (0.26) 0.255 (0.20) VD 3 0.370 (0.20) 0.240 (0.16) Staple acrylic AC 0.445 (0.18) 0.387 (0.15) Values in parentheses indicate the CV% of frictional indices. cases. From these results, it is expected that different varieties of cotton and also of same variety from different places of origin will behave differently during spinning and subsequent processes.
84 INDIAN J. FIBRE TEXT. RES., MARCH 2007 Table 3 also shows that with the increase in fibre length, the static frictional index increases for both polyester and viscose staple fibres (samples PL 1 - PL 3 and VL 1 -VL 3 ) but the kinetic frictional index remains almost unchanged. This is due to the fact that in case of higher fibre length the number of contact point is more, resulting in higher initial frictional resistant. As the fibre becomes finer the static and kinetic frictional indices increase initially but when the fibres become too fine the frictional index values drop marginally. The same phenomenon is observed both for polyester and viscose staple fibres (samples PD 4 - PD 1 and VD 3 - VD 1 ). The higher friction for finer denier fibre is due to more inter-fibre contact area. Table 4 Effect of surface finishes and methods of testing on frictional indices of acrylic staple fibre Treatment Static frictional index Kinetic frictional index A B A B Nil 0.445 0.500 0.387 0.425 0.2 % PEG600 0.462 0.520 0.405 0.452 0.4 % PEG600 0.472 0.535 0.425 0.482 0.1 % AVF5/32 0.465 0.527 0.415 0.470 0.4 % AVF5/32 0.497 0.567 0.440 0.505 A Values obtained in the developed instrument; B Values obtained with an attachment fitted with Instron tensile tester by thread and pulley. PEG600 is a type of lubricant. AVF5/32 is a lubricant cum antistat. Table 4 shows that the addition of lubricant affects adversely the static and kinetic friction of the staple acrylic fibre, i.e. the frictional indices increase with the application of lubricant and these indices further increase as the lubricant add-on increases. The increase in add-on of high viscosity finish on fibres sets up hydrodynamic friction where the film-to-film friction comes into play. A relative greater force is required to shear this viscous film, which results in increased fibre-to-fibre friction. The similar observations have also been reported by Gowda. 14 It is also evident from Table 4 that the developed instrument shows lower static and kinetic friction indices than the indices measured by the attachment fitted with Instron tensile tester by thread and pulley arrangement. This is due to extra force required to rotate the pulley in the later system, added with the actual frictional force. But in the developed instrument there is no such friction loss, thus giving the reproducible and reliable results. A similar trend was also observed for all the fibres and fabrics. 3.2 Frictional Properties of Woven Fabrics Table 5 shows the frictional properties of cotton and synthetic/blended fabrics. In most of the fabrics, it is clear that the static frictional index in weft-onweft direction is higher than that in warp-on-warp direction. This may be due to the fact that as warp yarns form definite parallel lines along the length of Table 5 Frictional properties of woven fabrics Fabric code F s /N F k /N F a, gf F s -F k, gf P T P T P T P T C 1 1.28 1.31 0.87 0.88 8.60 8.75 21.51 22.18 C 2 0.54 0.76 0.31 0.45 5.61 5.84 12.30 16.52 C 3 1.59 1.59 1.26 1.21 10.44 11.09 17.75 19.75 C 4 1.07 1.37 0.79 0.99 7.54 7.81 14.19 19.72 C 5 1.32 1.39 0.73 0.83 12.44 12.96 31.00 29.10 C 6 1.18 1.20 0.77 0.82 9.64 10.62 21.8 20.11 C 7 1.71 1.74 1.31 1.31 5.55 11.05 21.01 22.40 C 8 1.67 1.86 1.34 1.32 6.64 13.30 17.30 27.98 S 1 0.47 0.85 0.33 0.62 10.73 11.66 7.34 12.17 S 2 0.72 1.01 0.45 0.67 6.71 7.56 14.4 17.86 S 3 0.88 0.95 0.62 0.63 13.92 14.56 13.62 16.52 S 4 0.93 1.16 0.57 0.83 11.02 11.25 18.2 17.48 S 5 1.48 1.67 1.07 1.15 13.38 13.80 20.92 27.00 S 6 1.17 1.17 0.81 0.80 13.77 13.80 18.86 18.90 S 7 0.94 1.03 0.65 0.62 5.50 12.2 15.25 20.66 S 8 0.96 1.30 0.73 1.05 8.42 11.73 11.65 13.50 S 9 2.04 1.88 1.50 1.46 20.00 10.92 27.93 21.70 S 10 1.53 1.69 1.09 1.31 7.20 10.46 22.69 19.72 F s /N Static frictional index; F k /N Kinetic frictional index; F a Friction amplitude; F s -F k Differential friction force; P Warp-on-warp motion; and T Weft-on-weft motion.
DAS & ISHTIAQUE: STATIC AND KINETIC FRICTIONAL CHARACTERISTICS OF FIBRES & FABRICS 85 fabric due to higher tension during weaving and as these pass through dents of reed, it helps in smooth movement of fabrics in warp-on-warp direction. The earlier workers 7 have also reported the similar trend. But in case of fabric sample S 9 (twill fabric), the trend is totally different, i.e. weft-on-weft static frictional index is lower than that of warp-on-warp. This is mainly due to the fact that apart from the resistance force offered by the yarn surface and ridges of warp or weft yarn, the additional resistance by twill lines plays a major role. In the fabric samples C 3 and S 6, both warp-on-warp and weft-on-weft static frictional indices are found to be equal. This may be due to the reason that both the thread density and the yarn count of warp and weft are equal. It is also clear from Table 5 that the kinetic frictional index of all the fabrics is much lower than that of static frictional index and the kinetic friction of weft-on-weft motion is, in general, higher than that of warp-on-warp motion. The friction amplitude is greater for weft-on-weft motion than for warp-onwarp motion for all the fabrics, except the fabric sample S 9, which is of twill structure. A similar trend was also observed by El Mogahzy and Broughton. 2 In case of fabric sample S 1, which is made out of 100% filament yarns in both warp and weft, the friction amplitude of warp-on-warp is high at 10.73 when the kinetic frictional index is as low as 0.33. The ratio between friction amplitude and kinetic frictional index becomes very high, which indicates crunchy feel and gives a fabric that rustles owing to the marked stick-slip motion with lower frictional resistance. Differential frictional force does not show any specific trend but for most of the fabrics it is higher for weft-on-weft direction (Table 5). Tables 6 and 7 show the regression equations of static and kinetic frictional properties respectively for cotton fabrics. It is clear from these tables that the warp density and fabric thickness have got very good correlation with static and kinetic frictional indices of both warp-on-warp and weft-on-weft motions. The linear regression fits (least square), as shown in Figs 3 and 4, indicate the trend between the frictional indices with warp density and thickness respectively for cotton fabrics. The positive correlation between fabric thickness and frictional index may be due to the fact that the higher fabric thickness means higher compressibility, which results in higher contact points between fabric surfaces, causing higher frictional resistance. But for fabric samples C 1 and C 5, in spite of very low thickness the static frictional indices are high, which may be due to typical physical structure of fabric. Table 8 shows that there are very good positive correlation between static and kinetic frictional indices of both warp-on-warp and weft-on-weft for cotton as well as for synthetic and blended fabrics. When the frictional index data of cotton, synthetic and blended fabrics are combined together, it also shows very good correlation between the frictional indices. Table 6 Regression analysis of static frictional properties and other physical parameters of cotton woven fabrics Y X Regression equation Correlation coefficient Standard error (r) (F S /N) P Warp density, thd/cm Y = -0.0264X +2.0824-0.88 0.197192 (F S /N) P Fabric thickness, mm Y = +1.459X +0.8208 +0.67 0.306057 (F S /N) T Warp density, thd/cm Y = -0.0227X +2.0776-0.85 0.197006 (F S /N) T Fabric thickness, mm Y = +1.4847X +0.9200 +0.77 0.235442 (F S /N) P Static frictional index in warp-on-warp motion; and (F S /N) T Static frictional index in weft-on-weft motion. Table 7 Regression analysis of kinetic frictional properties and other physical parameters of cotton woven fabrics Y X Regression equation Correlation coefficient Standard error (r) (F K /N) P Warp density, thd/cm Y = -0.0259X +1.6986-0.92 0.157524 (F K /N) P Fabric thickness, mm Y = +1.6791X +0.3806 +0.82 0.221405 (F K /K) T Warp density, thd/cm Y = -0.0203X +1.5807-0.88 0.154995 (F K /K) T Fabric thickness, mm Y = +1.3741X +0.5297 +0.82 0.181300 (F K /N) P Kinetic frictional index in warp-on-warp motion; (F K /N) T Kinetic frictional index in weft-on-weft motion.
[ 86 INDIAN J. FIBRE TEXT. RES., MARCH 2007 Table 8 Regression analysis of frictional properties of woven fabrics Y X Fabric group Regression equation Correlation coefficient Standard error (r) (F S /N) P (F S /N) T Cotton Y = 0.8541X +0.2964 +0.96 0.105783 (F S /N) P (F S /N) T Synthetic/Blended Y = 0.8206X +0.3755 +0.93 0.120535 (F S /N) P (F S /N) T Combined Y = 0.8169X +0.3629 +0.95 0.108119 (F S /N) P (F K N) P Cotton Y = 0.9027X -0.2428 +0.96 0.113208 (F S /N) P (F K N) P Synthetic/Blended Y = 0.7484X -0.0518 +0.99 0.041744 (F S /N) P (F K N) P Combined Y = 0.8223X -0.1321 +0.97 0.082246 (F S /N) T (F K /N) T Cotton Y = 0.8441X -0.2075 +0.97 0.071647 (F S /N) T (F K /N) T Synthetic/Blended Y = 0.8258X -0.1404 +0.96 0.073342 (F S /N) T (F K /N) T Combined Y = 0.8141X -0.1448 +0.97 0.071090 (F K /N) P (F K /N) T Cotton Y = 0.8039X +0.2316 +0.98 0.060312 (F K /N) P (F K /N) T Synthetic/Blended Y = 0.8540X +0.2527 +0.85 0.143881 (F K /N) P (F K /N) T Combined Y = 0.7879X +0.2744 +0.92 0.109866 Fig. 4 Scatter plot of static and kinetic frictional indices as function of fabric thickness Fig. 3 Scatter plot of static and kinetic frictional indices as function of warp density of fabric This indicates that by knowing only on frictional index, e.g. static frictional index of only one direction, one can predict the rest other indices with the help of regression equations. 4 Conclusions 4.1 The developed instrument is very easy to handle and the frictional properties of both staple fibres and fabrics can be measured in the same instrument. No friction loss is observed. 4.2 Kinetic frictional index is always lower than the static frictional index for all the staple fibres and fabrics studied. 4.3 For different varieties of cotton and even within same variety with different places of origin the static and kinetic frictional indices vary widely. 4.4 With the increase in fibre length the static and kinetic frictional indices increase for both polyester and viscose staple fibres. 4.5 As the fibre becomes finer the static and kinetic frictional indices increase initially and then drop marginally in case of very fine fibre. The same trend is observed for both polyester and viscose staple fibres. 4.6 In general, the static and kinetic frictional indices of weft-on-weft is higher than that of warpon-warp and the kinetic frictional index is always lower than the static frictional index for all the fabrics. Friction amplitude is greater for weft-on-weft motion than for warp-on-warp. 4.7 Warp density and thickness have very good linear correlation with the static and kinetic frictions of cotton fabrics. 4.8 Very good positive correlations are observed in between static and kinetic frictional indices in both warp-on-warp and weft-on-weft motions. References 1 du Bois W F, Text Res J, 29 (1959) 451. 2 EL Mogahzy Y E & Broughton R M, Text Res J, 63(1993) 465.
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