A Novel Approach for Measurement of Fiber-on-fiber Friction

F98S-09 Page 1 A Novel Approach for Measurement of Fiber-on-fiber Friction Number: F98S-09 Competency: Fabrication Team Leader and members: Y. Qiu, NCSU; Y. Wang, Georgia Tech; J.Z. Mi, Cotton Inc. Graduate Students: Michael A. Laton, Xin Shao, Georgia Tech; Chuyang Zhang, NCSU. URL: http://www.ntcresearch.org/current/year8/.htm Goal To develop a new method for measurement of fiber-on-fiber friction in textile manufacturing processes based on the frictional energy dissipated in dynamic loading of a fiber assembly, including (1) to develop a new theoretical model for characterization of inter-fiber friction from frictional energy loss of a fiber bundle subjected to dynamic loading; (2) to design and build a set of instrument for measurement of fiber-on-fiber friction energy; (3) to determine the effect of various environmental factors such as moisture content, temperature, loading frequency and fiber mechanical properties on friction energy loss of fibrous assembly. Abstract Study of energy loss due to friction provides a new approach and opportunity for fiber-on-fiber friction research. Based on the dynamic mechanical theory of polymeric materials, a theoretical model of kinetic friction energy loss has been developed. In order to verify the theory, a systematic experimental study was carried out using a cotton and a polyester roving with different twist levels (0, 1.5, and 3.0 tpi), different cyclic loading frequencies (1 and 10 Hz), and different gage lengths (9, 50, and 60 mm). Twist affects friction energy loss of fibers in cotton and polyester rovings. The lower the twist in the cotton, the lower the loss tangent value at both 9 mm and 50 mm gage lengths. The same is true for polyester at 9 mm gage length. For polyester at 60 mm gage length, the lower the twist, the higher the loss tangent value. Friction energy loss due to inter-fiber friction depends on frequency. For cotton at 50 mm, the higher the frequency, the higher the loss tangent. For cotton at 9 mm with no twist, the lower the frequency, the higher the loss tangent. When twist was added, the opposite occurred. For polyester at both 50 mm and 9 mm gage lengths, the higher the frequency, the higher the loss tangent. Gage length plays an important role in fiber-on-fiber friction. For shorter gauge lengths, loss tangent versus curves stabilized more quickly. For cotton, the longer gauge length produced higher loss tangents. For polyester, the longer gauge length generally produced lower loss tangents. The load level applied during test is also critical to the nature of fiber-on-fiber friction due to the existence of different geometry of yarns. For cotton at 50 mm and polyester at 60 mm, the loss tangent increased with the force due to the twisted structure. For cotton and polyester at 9 mm,, the loss tangent decreased when the force was decreased.. 1

F98S-09 Page 2 1. Introduction Fiber-on-fiber friction plays an important role in textile manufacturing processes and end-use behavior of textiles. The fundamental knowledge of fiber-on-fiber friction behavior obtained in this project will help the US textile and related industries develop new, better products, and improve the manufacturing processes for better quality and efficiency. As one of the most important fiber characteristics, fiber-on-fiber friction is responsible for the behavior of fibers in textile manufacturing processes and the performances the final products (1, 4, 6, 10, 12). It is frictional force between the neighboring fibers that holds the fibers together in a yarn or yarns in a fabric. In spinning, knitting and weaving processes, the friction of fibers creates tension on yarns. The friction of fibers translated to the yarn friction properties also determines properties of fabrics. Tensile, bending, and shear properties of a fibrous assembly is largely dependent on the friction between fibers. Due to its paramount importance, much attention has been paid to fiber-on-fiber friction. Almost all the approaches to date measure the frictional force in either a tensile or a compressive test of a fiber bundle or the like. The friction properties of fibers are then characterized according to the classic definition of friction between two flat surfaces (e.g., the frictional force is assumed to be equal to the normal force times the friction coefficient). Because of the complex nature of fiber friction, the friction coefficient value has been found to be strongly dependent on testing conditions. (5) In textile processes such as yarn spinning, knitting, braiding and weaving, the fibers are more likely to be subjected to dynamic loading and thus the friction phenomenon is no longer the same as that in a "classic" static friction test. For a fibrous assembly under dynamic loading, the compression force applied normal to the fibers is very hard, if not impossible, to determine. Furthermore, the heat generated as the result of the frictional energy release directly affects the properties of the fibers and thus the process controlling parameters. The energy dissipated due to fiber-on-fiber friction is a direct reflection of the friction phenomenon. However, little attention has been paid to the energy loss due to friction between fibers in a dynamic loading process. Therefore, we propose to use the energy dissipation to characterize fiber-on-fiber friction. Friction is a common phenomenon studied by scientists for long time. In 1699, Amontons stated the two classic laws of friction, namely (1) the frictional force is independent of the contact area between the two bodies sliding on one another, and (2) the frictional force is proportional to the load applied perpendicular to the surfaces. In other words, the ratio between the frictional force and load is a constant and is defined as the coefficient of friction. The sources of frictional force is considered to come from (1) adhesion or the attraction between molecules on the two surfaces, (2) the mechanical ploughing action due to surface roughness. For metals, the contacting areas between the two bodies are believed to be pressed exceeding their yield point and thus the two pieces are welded together at those points. The shear force required to break up these bonding points is proposed as the source of frictional force. Obviously the higher the load applied normal to the surface, the larger the real area of contact, and the higher the frictional 2

F98S-09 Page 3 force. Therefore, Bowden and his coworkers (Bowden) proposed that the frictional force is proportional to the real area of contact. This also explains the difference between the static and kinetic friction coefficients. The ploughing mechanism may be assumed when surface roughness is significant. A good case of this mechanism is wool fiber that has scales on surface. When movement of the two neighboring fibers is at with-scale direction the friction is substantially smaller than when it is at anti-scale direction (3, 5, 10). Amontons proposed simplest and the earliest law for friction as F = µ N (1) where F is the frictional force, N is the normal force, and µ is the frictional coefficient. This holds rather well with metals and some other hard materials. For fibrous structures and soft materials such as polymers, they are less valid. Gralen and coworkers (5) found that static and kinetic friction coefficients between two viscose rayon fibers decreased as the normal force increased. Their assumption is that the deformation of fiber surfaces is partially plastic and partially elastic. It is the elastic deformation of the fiber surface upon application of the normal force that gives the second term in the relation between frictional force and load as expressed in the following equation: F = αn + β lr (2) where R is the radius of the fiber, l is the contact length, and α and β are constants. Gralen et al were able to confirm their theory with experiments using a twist friction meter in testing nylon and polyester fibers, though the β value seems to vary rather significantly. In contrast, α value seems to be relatively stable and almost a constant around 0.25 for both fibers. Bowden and Young (bowden) proposed a power law of friction for fibrous materials as F = an n (3) where a and n are constants. The assumption of this model is that the asperities have plastic deformation on the contacting surfaces. This model seems to be the most popular one among all the models proposed for fibrous materials. Howell (howell) verified this theory by means of a yarn on yarn friction test in which a yarn was slided perpendicularly across another yarn. More recently, Gupta and El Mogahzy (7) did a more detailed study of the basis of the power law theory and proposed the composition of the constants a and n as functions of material and morphological factors. Hertel and Lawson (8) reported a shear friction measurement of textile fibers using a simple oscillating pendulum. The energy loss due to inter-fiber friction was calculated from the difference between the amplitudes of two consecutive swings, and the normal force applied to the top plate of the devise. A shear friction calculation was given as: 2 K( A0 An ) T S = 2 (4) n ( A0 + An ) where K is a constant determined by the load and the specimen area, A 0 and A n are the amplitudes of the first and the nth swing, and T is the applied normal force. This method explored the dynamic and static frictional energy loss in fiber-on-fiber shear friction. However, in that study, the shear friction, S, was not clearly defined and the energy loss 3

F98S-09 Page 4 due to intermolecular friction of the fiber was neglected. In addition, this method can only estimate the frictional energy at very low frequencies due to the slow swing nature of the pendulum mechanism. El Mogahzy and Broughton (2) created Auburn Beard Test of fiber-on-fiber friction in which the friction between two beards of fibers were measured at various conditions. The effect of different finishes on friction between fibers was examined. This method falls into the category of classic friction test. Studying fiber on fiber friction of cotton, Oxenham and Hassanin (11) used a new test method in which an analysis of compressional behavior of a fiber mass was performed. A slip-stick mechanism was assumed and the noise of the compressional load versus displacement curve was interpreted as the result of inter-fiber friction. 2. Theoretical basis for frictional energy loss measurement Dynamic mechanical properties of polymeric materials have been studied extensively (9). When a polymeric material is under cyclic loading, the strain and the resultant stress of a polymeric material is not in the same phase due to the viscoelastic nature of polymer as discussed in every polymer physics textbook. The stress and the strain of a polymeric material when subjected to a sinusoidal loading can be expressed as σ = σ0 sin( ωt + δ) (5) ε = ε sinωt 0 where σ is the stress, ε is the strain, δ is the phase angle and ω is the angular speed. Therefore the stress σ = σ0sinωtcosδ + σ0cosωtsinδ (6)= = ε0e sinωt + ε0e cosωt where σ 0 σ 0 E = cosδ and E = sinδ ε0 ε0 and E is the real part of the modulus and E is the imaginary part of the modulus. The phase angle is thus given by E tanδ = (7) E The real part of the modulus, E is called the storage modulus related to the elastic energy stored in the polymer system and released periodically. The imaginary part of the modulus is also called loss modulus associated with energy loss due to intermolecular friction. When a single polymeric fiber is cyclic loaded, these two parts can be calculated using conventional dynamic mechanical analysis instrument. However, when a fibrous structure is similarly loaded the two parts of the modulus will not be so simply divided. In addition to the intermolecular friction that generates the loss modulus of the fiber, inter-fiber friction will result in more energy loss, resulting in a higher apparent loss modulus. In order to calculate the energy loss due to inter-fiber friction, the following equation has been proposed (9) : 4

F98S-09 Page 5 η* ω tanδ = (8) k 4 f where η = η+ ηeq = η+ Aω η* is the apparent viscosity of the fibrous structure, η is the viscosity of the fiber, η eq is the equivalent friction viscosity, A is the magnitude of the periodic loading, ω is the angular frequency of the loading, f is the coefficient of friction and k is the real modulus of the fiber. By testing single fibers and the fiber assembly at different frequencies, it is possible to determine the frictional energy loss due to inter-fiber friction. 3. Experiment procedure A 520-Tex cotton roving and a 520-Tex polyester roving were tested and 3.0 tpi. The cotton roving was subjected to cyclic tensile loading at two different frequencies: 1 Hz and 10 Hz with two gage lengths: 9 mm and 50 mm using a Dynamic Mechanical Analysis System. The polyester roving was subjected to the same two frequencies with two gage lengths of 9 mm and 60 mm. At 9mm gage length, the roving has either no twist or 3.5 twist per inch (tpi). At 50 or 60 mm gage lengths, the roving were twisted at 1.5 or 3.0 tpi. The oscillation displacement was 80 µm for all tests. A newly designed testing system is still under adjusting and calibration as shown in Figure 1. 3.1. Results and discussion 3.1.1. Effect of twist The results of the tests are presented in Figures 2-5. For the cotton roving, the loss tangent value decreases as the twist decreases from 3.0 tpi to 1.5 tpi at 50 mm gage length. At 9 mm gage length, the loss tangent was higher for the twisted roving than the untwisted roving. The same is true for polyester at 9 mm gage length. On the other hand, at 60 mm gage length, for polyester, a higher loss tangent value was obtained with lower twist. Increase of twist may increase frictional energy loss due to increased pressure on the fibers when twist is applied or increased. However, twist may have an opposite effect, namely restricting the relative movement of fibers in the roving. The balance of the two opposite factors determines the net outcome. The results for 9 mm gage length evidently indicate that the application of twist to the rovings increased the contact pressure between the neighboring fibers and thus increased energy consumption. The movement of the fibers was restricted with and without twist because the gage length was substantially shorter than average length of the individual fibers. The increased part of energy loss is mainly due to inter-fiber friction because very limited amount of friction exists when no twist is applied. On the other hand, at higher gage length, i.e. 50 mm or 60 mm, almost no fiber was held at both ends. Therefore, the load applied to the roving was transferred purely through friction. In other words, friction is critical for long gage length. The inter-fiber pressure increase due to increased twist seems to be more dominant in the case of cotton roving, while the decreased relative movement seems to be 5

F98S-09 Page 6 more important for polyester roving. This should depend on fiber surface, mechanical and morphological properties. 3.1.2. Effect of frequency According to Equation 8, loss tangent should increase with frequency, which is exactly the case for cotton at 50 mm at both twist levels, i.e. the higher the frequency, the higher the loss tangent. The same is true for cotton at 9 mm with twist and for polyester at both gage lengths and twist levels. The only exception is for cotton at 9mm gage length without twist, the opposite occurred, namely the lower the frequency, the higher the loss tangent. 3.1.3. Effect of gage length Gage lengths used in this study were purposely selected such that one gage is significantly below the average fiber length and the other exceeds the length of the longest fibers. Therefore for 9 mm gage length, most of fibers were held at both ends and limited relative axial direction movement of the fibers was allowed. Indeed, as shown in the figures, for shorter gauge lengths the loss tangent versus time curves stabilized more quickly due to absence of slippage. For the long gage lengths, almost no fiber in the roving was held at both ends and thus, fiber movement was a sure thing. Therefore the stabilization of loss tangent curves is hard to achieve as shown in the figures. The value of loss tangent was affected greatly by inter-fiber friction due to the relative movement of the fibers. For cotton, the longer gauge length produced higher loss tangents mainly due to better cohesion of cotton fibers and large movement of the fibers. For polyester, the longer gauge length generally produced lower loss tangents, implicating a reduced inter fiber pressure perhaps as a result of poor cohesion. 3.1.4. Effect of load level The applied load level determines where on the stress-strain curve the fibers are cyclically tested. Higher load will lead to higher amount of plastic deformation and thus should increase the energy loss due to molecular movement. Another factor is the interfiber friction, which is associated with the geometry of the roving. If no twist or little twist is present, the higher the load level, the straighter the fibers and thus the less interaction, especially the lateral movement during disentanglement is expected, leading to a lower frictional energy loss. This is indeed the case for cotton and polyester at 9 mm, where the effect of twist became less significant and the lower the force, the higher the loss tangent. When twist is present, however, the higher load level tends to induce higher interfiber pressure, resulting in higher energy loss. For cotton at 50 mm and polyester at 60 mm, at both twist levels, the higher the force, the higher the loss tangent. 4. Conclusions 1. Twist affects friction energy loss of fibers in cotton and polyester rovings. The lower the twist in the cotton, the lower the loss tangent value at both 9 mm and 50 mm gage 6

F98S-09 Page 7 lengths. The same is true for polyester at 9 mm gage length. For polyester at 60 mm gage length, the lower the twist, the higher the loss tangent value. 2. Friction energy loss due to inter-fiber friction depends on frequency. For cotton at 50 mm, the higher the frequency, the higher the loss tangent. For cotton at 9 mm with no twist, the lower the frequency, the higher the loss tangent. When twist was added, the opposite occurred. For polyester at both 50 mm and 9 mm gage lengths, the higher the frequency, the higher the loss tangent. 3. Gage length of test specimens plays an important role in fiber-on-fiber friction. For shorter gauge lengths, loss tangent versus curves stabilized more quickly. For cotton, the longer gauge length produced higher loss tangents. For polyester, the longer gauge length generally produced lower loss tangents. 4. The load level applied during test is also critical to the nature of fiber-on-fiber friction due to the existence of different geometry of yarns. For cotton at 50 mm and polyester at 60 mm, the loss tangent increased with the force due to the twisted structure. For cotton and polyester at 9 mm, the loss tangent decreased when the force was decreased. 5. Future work The theoretical model has to be further developed to explain the physics of the phenomenon. The testing instrument needs to be further perfected to perform the tests with wider range of frequencies and various testing conditions including temperature and moisture regulations. Reference 1. Ajayi, J. O. "Effects of Fabric Structure on Frictional Properties." Text. Res. J. 62, no. 2 (1992): 87-93. 2. El Mogahzy, Y. E., and R. M. Broughton. "A New Approach for Evaluating the Frictional Behavior of Cotton Fibers." Text. Res. J. 63, no. 8 (1993): 465-75. 3. Frishman, D., A. L. Smith, and M. Harris. "Measurement of the Frictional Properties of Wool Fibers." Text. Res. J. 18, no. August (1948): 475-80. 4. Galuszynski, S., and P. Ellis. "Frictional Forces in the Heald Eye." Text. Res. J. 53, no. 8 (1983): 462-68. 5. Gralen, N., B. Olofsson, and J. Lindberg. "Measurement of Friction Between Single Fibers. Part VII Physicochemical Views of Interfiber Friction." Text. Res. J. 23, no. September (1953): 623-29. 6. Grey, S. J., and G. A. V. Leaf. "The Nature of Inter-Fiber Frictional Effects in Woven Fabric Bending." J. Text. Inst. 76, no. 5 (1985): 314-22. 7. Gupta, B. S., and Y. E. El Mogahzy. "Friction in Fibrous Materials. Part I: Structural 7

F98S-09 Page 8 Model." Text. Res. J. 9, no. 61 (1991): 547-55. 8. Hertel, K. L., and R. Lawson. "Shear Friction in Textile Processing." Textile Bulletin, no. May (1968): 23. 9. Murayama, T. Dynamic Mechanical Analysis of Polymeric Material. New York, USA: Elsevier Scientific Publishing Company, 1978. 10. Olofsson, B., and N. Gralen. "Measurement of Friction Between Single Fibers. V. Frictional Properties of Viscose Rayon Staple Fibers." Text. Res. J. 20, no. 7 (1950): 467-80. 11. Oxenham, W., and H Hassanin. "A Novel Technique for Assessing the Frictional Characteristics of Cotton."Cotton Quality Measurements Conference. 12. Zurek, W., E. Tyc, M. Piasecki, and I. Frydrych. "Frictional Restraints During the Knitting Process." Text. Res. J. 59, no. 6 (1989): 329-36. Figure 1. The fiber-on-fiber friction testing machine. 8

F98S-09 Page 9 0.45 0.4 Tan δ 0.35 twist/load range/hz 1.5 / 5-10 / 1 1.5 / 5-10 / 10 1.5 / 10-20 / 1 1.5 / 10-20 / 10 3.0 / 5-10 / 1 3.0 / 5-10 / 10 3.0 / 10-20 / 1 3.0 / 10-20 / 10 0.3 0.25 0.2 1 5 9 13 17 21 25 29 33 37 41 45 49 Time Figure 2 Tan δ for Cotton at 50 mm guage length. 0.44 0.39 0.34 tw ist/load range/h z no / 5-10 / 1 no / 5-10 / 10 no / 10-20 / 1 no / 10-20 / 10 tw / 5-10 / 1 tw / 5-10 / 10 tw / 10-20 / 1 tw / 10-20 / 10 Tan δ 0.29 0.24 0.19 0.14 0.09 1 5 9 13 17 21 25 29 33 37 41 Time Figure 3. Tan δ of Cotton at 9 mm gauge length. 9

F98S-09 Page 10 Tan δ 0.23 0.21 0.19 0.17 0.15 twist/load range/hz 1.5 / 5-10 / 1 1.5 / 5-10 / 10 1.5 / 10-20 / 1 1.5 / 10-20 / 10 3.0 / 5-10 / 1 3.0 / 5-10 / 10 3.0 / 10-20 / 1 3.0 / 10-20 / 10 0.13 0.11 0.09 0.07 1 5 9 13 17 21 25 29 33 37 41 45 49 Time Figure 4. Tan δ for Polyester at 60 mm gauge length Tan δ 0.28 0.26 0.24 0.22 0.2 0.18 0.16 twist/load none/ 5-10 /1 none/ 5-10 /10 none/ 10-20 /1 none/ 10-20 /10 tw / 5-10 /1 tw / 5-10 /10 tw / 10-20 /1 tw / 10-20 /10 0.14 0.12 1 6 11 16 21 26 31 36 41 46 Time Figure 5. Tan δ of Polyester with 9 mm guage length. 10