ABSTRACT. Bauer-Kurz, Ina. Fiber Crimp and Crimp Stability in Nonwoven Fabric

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1 ABSTRACT Bauer-Kurz, Ina. Fiber Crimp and Crimp Stability in Nonwoven Fabric Processes. (Under the direction of Dr. William Oxenham and Dr. Donald A. Shiffler.) In nonwovens, crimp characteristics of synthetic fibers are, along with finish, major contributors to processing efficiency, web cohesion, fabric bulk and bulk stability. However, the meaning of measurable crimp parameters and their influence on processing and fabric characteristics has not been quantified. The purpose of this study is to quantify the mechanical fiber behavior during crimp removal, and relate it to fundamental fiber properties, nonwoven fabric properties, and processibility in nonwoven equipment. Single fiber tensile tests in the crimp removal region have been performed on various fibers with the Textechno FAVIMAT and have also been monitored optically. Based on empirical evidence, a basic understanding of the physical crimp removal mechanism is obtained. A methodology is developed, to identify the true crimp removal region of the whole single fiber load-extension curve during a tensile test. A mechanical model accounting for the nonlinear load-deflection behavior during crimp removal is developed. According to this model, a logarithmic function can be used to describe the material behavior in the crimp node during crimp removal. This function is fit to experimental data and delivers two fitting parameters that characterize the shape of the experimental load-extension curve in the crimp region.

2 The extracted characteristic crimp parameters are being evaluated in terms of fiber material characteristics, such as fiber type, crimp processing settings and carding performance during nonwoven production. A dependence of the shape of the crimp removal curve on crimping settings during crimp production is established. The characteristic crimp parameters are also correlated to the sequence of processing stages during nonwoven production and cylinder speed during carding.

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4 ii DEDICATION My dissertation is dedicated to my husband, Dr. Harald Kurz. I will never forget how selflessly he accepted and even encouraged my decision to earn the doctorate degree in the USA, even though it meant the geographical separation of the two of us for more than 3 years. Still, he has been my biggest support during my studies at North Carolina State University. I could not have accomplished my degree without his loving tolerance and advice.

5 iii BIOGRAPHY The author, Ina Bauer-Kurz, was born on September 12, 1969, in Germany. She received her degree Diplom-Ingenieur in Mechanical Engineering, equivalent to an M.S. degree, in December 1997 from the Rheinisch-Westfälische Technische Hochschule Aachen (RWTH Aachen), Germany. Minoring in Textile Machinery and Technology, Ms. Bauer-Kurz first came to the College of Textiles at North Carolina State University in November 1996, to complete her thesis in Mechanical Engineering under the combined direction of Professor Burkhard Wulfhorst, RWTH Aachen, Germany, and Professor William Oxenham, North Carolina State University (NCSU). This thesis was supported by a honorarium of the Walter-Reiners-Stiftung des Deutschen Textilmaschinenbaues. In January 1998, Ms. Bauer-Kurz was accepted by NCSU to pursue her Ph.D. in Fiber and Polymer Science. Her Ph.D. research has been sponsored by an assistantship of the Nonwoven Cooperative Research Center.

6 iv ACKNOWLEDGEMENTS My sincere thanks go to the many people who supported and inspired me intellectually and mentally during my almost four years of life in the USA and study at the College of Textiles of North Carolina State University. I am very grateful having experienced the support and warmth of my American family - my friends Marc and Susan Krouse, Dr. Kate Piatek, Russ Parker and Christine Stephenson. I would like to thank my advisors Dr. William Oxenham and Dr. Donald A. Shiffler for their great patience and support throughout my research project. Dr. Oxenham has always been an inspiration for me with his smart curiosity, impressive intellect and humorous and caring kindness. I have learned for my life from working with him. I will also never forget the interesting and challenging discussions with Dr. Shiffler, often beyond the actual research subject, who has always been generous in sharing his broad life and industrial working experience with me. My gratitude is extended to Dr. Jeffrey W. Eischen, whose reliable support in mechanical engineering questions has been very important to me, and to Dr. Hawthorne Davis, who honored me as a committee member and with his interest and help especially in fiber imaging. I would also like to thank Dr. Richard Peterson for serving as my graduate representative. My research would not have been possible without the financial support from the Nonwovens Cooperative Research Center at NCSU. I appreciate very much the research assistantship provided by the NCRC during my Ph.D. studies, as well as the generous instrumental equipment available for my use and the intellectual and material support from industrial member companies, in particular Wellman, KoSa, Hollingsworth, FiberVisions, DuPont and Albany International.

7 TABLE OF CONTENTS LIST OF FIGURES... vii LIST OF TABLES...x NOMENCLATURE... xi 1 INTRODUCTION Research Objective Research Outline Practical Relevance of this Research Deliverables LITERATURE REVIEW What is Crimp? Crimp in Wool Fibers Morphological and Chemical Structure of Wool Helical Spring Model Load-Extension Curve of Wool Crimp in Synthetic Cellulosic Fibers Crimp in Cotton Fibers Crimp in Man-Made Fibers Staple Fiber Crimp Technology of Stuffer Box Crimping Buckling Theory Applied to Stuffer Box Crimping Mechanics of Triangular Crimp Morphology of Crimped Structures Bicomponent Crimp Why is Crimp Important? Impact of Crimp on Processing Impact of Crimp on Other Fiber Properties Impact of Crimp on Products Quantitative Crimp Parameters Geometrical Parameters Wave Length, Crimp Frequency and Crimp Length Crimp Angle Crimp Amplitude and Crimp Index Crimp Width and Crimp Depth Crimp Degree Effective Crimp Diameter ECD and Effective Wave Number EWN Single Fiber Bulking Capacity Crimp Curvature and Torsion Mechanical Crimp Parameters Crimp Stability Crimp Parameters from the Load-Extension Curve of a Fiber Bulk Parameters... 44

8 2.4 Crimp Measurement Bulk Methods Rotor Ring Instrument Measurement of Cohesion Length of Slivers Standard Test Method For Bulk Properties Of Textured Yarns [D4031] Single Fiber Methods Measurement of Crimp Geometry Cotton Inc. Fiber Tester ( Hook Method ) Lenzing VIBROTEX Textechno FAVIMAT Summary and Conclusions INSTRUMENTATION Tensile Testing with the FAVIMAT Count Measurement with the FAVIMAT Optical Observation of Crimp Removal EXPERIMENTS Observations of Physical Mechanism of Crimp Removal Crimp Tests with Different Fiber Materials Fibers with Different Crimp Production Settings Carding Experiments Crimp Removal Tests with Carded Fibers DATA ANALYSIS Averaging of Single Data Curves Analysis of Load-Extension Curve Physical Model of Crimp Removal Data Extraction and Analysis with C Programs Analysis of Fitting Parameters Influence of Fiber Type on Crimp Parameters Influence of Crimp Settings on Fiber Crimp Removal Influence of Processing Stage on Fiber Crimp Removal Influence of Carding Settings on Fiber Crimp Removal SUMMARY AND CONCLUSIONS FUTURE WORK REFERENCES APPENDIX...112

9 vii LIST OF FIGURES Figure 1.1: The Fundamental Problem with Measuring Fiber Crimp from the Single Fiber Load-Extension Curve... 3 Figure 1.2: The Use of Single Fiber Crimp Characteristic for Process and Product Control... 3 Figure 1.3: Relevance of Quantitative Crimp Parameter k for Processing and Product Properties... 4 Figure 2.1: Crimp Definitions [1]... 5 Figure 2.2: Cross-Section of a Wool Fiber [9]... 6 Figure 2.3: Bilateral Structure of a Coarse Wool Fiber [9]... 6 Figure 2.4: Planar Projection of Helical Crimp Model [22, 35]... 8 Figure 2.5: Typical Force-Extension Curve for a Wool Fiber [38, 39] Figure 2.6: Stress in a Wool fiber under Small Tension [38] Figure 2.7: Stained Cross Section of a Crimped Rayon Spun from a Conjugate Jet [44] Figure 2.8: Cross Section of a Crimped Rayon Filament with Ruptured Skin [44] Figure 2.9: Cross Section of a Crimped Rayon Filament with Unbalance of Serrations and No Ruptured Skin [44] Figure 2.10: Production of Crimped PET Staple Fibers [42, 43] Figure 2.11: Crimp Formation [56] Figure 2.12: Two Types of Figure 2.13: Crimping Mechanism [55] Figure 2.14: Plane Model and Photomicrograph of Zigzag Shaped Staple [8] Figure 2.15: Extension of Circular Arc-Nomenclature [61] Figure 2.16: Load-Elongation Curve of a Crimped Fiber [78] Figure 2.17: Initial Portion of Load-Elongation Diagram of Uncrimped Fiber, a and Highly Crimped Fiber, b [2] Figure 2.18: Empirical Fit of Power Law to Crimp Removal Curve Analogous to Pressure Curve of Press Felts [62, 63] Figure 2.19: Crimped Fiber Structure, a Function of Bend Geometry [53] Figure 2.20: Correlation between Finish, Crimp and Key Processing Properties [55] Figure 2.21: Idealized Geometry of the Stuffer Box Crimped Fiber [43] Figure 2.22: Irregular Crimp Wave with Crimp Descriptors [91] Figure 2.23: Curve in Cartesian Coordinates [97] Figure 2.24: Force-Extension Curve in the Crimp Stability Test [39]... 42

10 viii Figure 2.25: Schematic of Rotor Ring Instrument [55, 82] Figure 2.26: Diagram of Hook Method for Single Fiber Strength [47] Figure 2.27: Example of Force Transducer Output Wave Form [47] Figure 2.28: Crimp Removal of an Irregularly Crimped Fiber with Low Crimp Frequency and Extrapolation of Crimped and Straightened Fiber Length [94] Figure 2.29: Characteristic Crimp Lines for Acrylic Fiber - A Regularly Crimped, B Irregularly Crimped [94] Figure 3.1: Textechno FAVIMAT Single Fiber Crimp Tester Figure 3.2: Measuring Unit of Textechno FAVIMAT Figure 3.3: Technical Features of Textechno FAVIMAT Figure 3.4: Clamp Movement with the Crimp Stability and Count Test Figure 3.5: Reproducibility of Count Data under Different Testing Conditions Figure 3.6: Frequency Distribution of Count for a 3.0 den PET Sample of 25 Single Fibers Figure 3.7: Load-Displacement Curves Produced by a Crimp Test of 3.0 den PET Fibers Figure 3.8: Force and Extension at 1 cn/tex in Dependence of Fiber Count (3.0 den PET Fibers) Figure 4.1: Load-Extension Curve and Fiber Shape during Crimp Removal for a 6 den PET Highloft Fiber Figure 4.2: Load-Extension Curve of a 0.9 den (1 dtex) Acrylic Fiber Figure 4.3: Load-Extension Curve of a 16 den (17.78 dtex) Nylon Fiber Figure 4.4: Crimp Removal Mechanism of a 16 den Nylon Fiber from DuPont Figure 4.5: Crimp Removal Mechanism of a 0.9 den Acrylic Fiber from Sterling Figure 4.6: Crimp Removal Mechanism of a 9 den PP Fiber from FiberVisions Figure 4.7: Crimp Removal Mechanism of a 3 den PET Fiber from Wellman Figure 4.8: Flow Chart with Sample Schedule for 3 den PET Fibers Figure 4.9: MASTERCARD [2] Figure 5.1: Stress-Strain Curves for 25 Fibers of B1, Sample A Figure 5.2: Stress-Strain Curves for 25 Fibers of B2, Sample A Figure 5.3: Stress-Strain Curves for 25 Fibers of B3, Sample A Figure 5.4: Averaging Single Fiber Load-Extension Curves Figure 5.5: Averaging of Load-Extension Curves for 25 Fibers of Bale 1, Sample A.. 78 Figure 5.6: Principle of Averaging of Single Fiber Load-Extension Curves Figure 5.7: Averaging of Different 3 den PET Fibers Figure 5.8: Textechno Criteria for Crimp Removal Point [6]... 80

11 ix Figure 5.9: Modeling of the Load-Extension Curve Figure 5.10: Determination of the End of the Slack Region Figure 5.11: Determination of the Start of the Tensile Region Figure 5.12: Identification of Crimp Region of a Load-Extension Curve Figure 5.13: Two Mechanical Models for the Extension of Folded Structures Figure 5.14: Dynamics for Joint and Frame Model Figure 5.15: Refined Joint Model Figure 5.16: Load-Extension Relationship for One Crimp Bow and Assumption of Linear Torsional Spring Figure 5.17: Theoretical Stress-Strain Relationship for Crimp Angel of 100 o, Spring Constant k = Figure 5.18: Calculation of the Crimp Angle from Fiber Extension Data Figure 5.19: Calculation of Load versus Crimp Angle from Load versus Fiber Extension for a 3 den PET Fiber Figure 5.20: Identification of Crimp Region from Experimental Load-Angle Data Figure 5.21: Power-Law Function to Fit Load-Angle Data in Crimp Region Figure 5.22: Transformation of Power-Law into Linear Equation Figure 5.23: Linear Regression of Experimental Data in Crimp Region Figure 5.24: Algorithm of the C-Program for the Data Analysis Figure 5.25: Relating the Power-Law Fit to the Crimp Removal Model Figure 5.26: Interpretation of Moment in Crimp Node in Terms of Fiber Structure Figure 5.27: Shape of Power-Law Function in Dependence of Fitting Parameters Figure 5.28: α Values for 3 Different 3 den PET Fibers Figure 5.29: β Values for 3 Different 3 den PET Fibers Figure 5.30: ln(α) in Dependence of Processing Stage Figure 5.31: β in Dependence of Processing Stage Figure 5.32: Influence of Feedroll-LickerIn Clearance on ln(α) Figure 5.33: Influence of Feedroll-LickerIn Clearance on -β Figure 5.34: Influence of Flat Clearance on ln(α) Figure 5.35: Influence of Flat Clearance on -β Figure 5.36: Influence of Cylinder Speed on ln(α) Figure 5.37: Influence of Cylinder Speed on -β

12 x LIST OF TABLES Table 2.1: Texturing Procedures for Filament Yarns [6, 54] Table 3.1: Test Options Offered by the Textechno FAVIMAT Standard Software Table 4.1: 3den PET Test Material for Carding Trials Table 4.2: Experimental Plan for Carding of 3 den PET Fibers Table 4.3: Settings for Carding Experiments with 3den PET Table 4.4: Parameters of FAVIMAT Tests for Carded Fibers Table 5.1: Crimp Removal Point Experimental Data and Theories Table 5.2: Fitting Parameters for Fibers of Different Material Table 5.3: Regression Characteristics for Crimp Removal of 3 den PET Fibers... 98

13 xi NOMENCLATURE A Cartesian Coordinate Point; Designation of Processing Stage a Fiber Radius ACME Automatic Crimp Measurement Instrument B Single Fiber Bulking Capacity; Cartesian Coordinate Point; Designation of Processing Stage Bi Label for Crimped Fibers BS Bulk Shrinkage b Vector Orthogonal To n, t C Moment Couple; Designation of Processing Stage C am Crimp Amplitude C c Crimp Content C d Crimp Depth C f Crimp Frequency C h Crimp Height C I Crimp Intensity Factor C i Crimp Index C p Crimp Potential Function CPLI Crimps Per Linear Inch C S Crimp Sharpness C St Spatial Crimp Frequency C W Crimp Width c i Constant D Cartesian Coordinate Point; Designation of Processing Stage d Fiber Denier E Young s Modulus; Cartesian Coordinate Point; Uncrimping Energy; Designation of Processing Stage ECD Effective Crimp Diameter EI Bending Rigidity EWN Effective Wave Number F Force; Cartesian Coordinate Point; Designation of Processing Stage F v Pretensioning Force f Resonance Frequency FFT Fast Fourier Transform F 0 Uncrimping Force F 20 Force at 20% Extension G Torsional Modulus; Flexual Rigidity GI p Torsional Rigidity h 0 Extrapolated Rise of a Crimp Bow I z Moment of Inertia of Bending K Curvature; Spring Constant K g Crimp Degree k Constant; Hookean Slope L Crimped Fiber Length; Crimp Width L b Crimped Fiber Length After Load L 0 Straightened Fiber Length l Leg Length of Crimp Bow; l i Web Thickness of Fiber Material i l k Buckling Length l 0 Side Length of Crimp Bow; Initial Length M Moment

14 xii m N n Mass Finite Number of Points Number of Crimps per Fiber; Finite Number n Spatial Vector in Direction of Radius 0 Origin of Coordinate System P Load; Force P critical Critical Buckling Load P K Closing Force of Stuffer Box P R Frictional Force P T Load of Feed Rollers PX 2 0 /G Dimensionless Stress R Resilience; Fiber Radius R E Recovery Coefficient for Uncrimping Energy R F Recovery Coefficient for Uncrimping Force r Radius r 2 R-Square S Minimum Sum of Squares; Crimp Stability SS Skein Shrinkage T Torsion; Moment of Torsional Spring T t Count in dtex t Time t b Time at break t Tangent Vector to a Curve T Increment in Torsion U Total Strain Energy U b Strain Energy Due to Bending U t Strain Energy Due to Torsion V Voltage W Work X Distance; Overall Fiber Extension X/X 0 Dimensionless Strain x Cartesian Coordinate (x,z) Plane Coordinates y Cartesian Coordinate (y,z) Plane Coordinates z Cartesian Coordinate Φ Crimp Angle Φ α β δ δ 0 δ c ϕ λ θ σ σ g σ z σ Change in Crimp Angle Fitting Parameter of Curve Shape; Angle Fitting Parameter of Curve Shape Distance; Displacement Initial Fiber Length with Slack and Crimp Length of Crimped Fiber Crimp Angle Wave Length of Crimped Fiber Pitch Angle of Helical Coil Strain; Fiber Tension Tension Where Fiber is Straightened Tension Where Fiber is Recrimped Strain Differential

15 1 INTRODUCTION Crimp in synthetic staple fibers is an essential parameter, influencing processing performance and product quality. To make initially straight man-made fibers processible in conventional textile equipment designed for natural fibers, they need to have certain frictional properties and cohesion. These parameters are controlled by an artificially introduced waviness and the application of finish, imitating the waviness and coating of wool fibers or the natural unevenness and wax of cotton fibers. The complicated interrelation of crimp and finish makes the relationship between settings during crimp production, crimp properties of synthetic fibers and processing performance extremely difficult to quantify. A second reason for the lack in control over crimp and resulting processing performance is the absence of objective parameters for crimp measurement itself. Up to the present time, attempts have been made to describe crimp in terms of geometry of the fiber path in the plane or in space. Beyond the practical problems of high variability and tediousness of the measurements, the question arises, Is the geometrical aspect of crimp even relevant in processing? In addition to the frictional properties of the fiber surface, other important factors for the fiber-to-fiber cohesion during processing, are the changes in fiber length and crimp caused by a load applied to the fiber, and the stability of the crimped structure to loads. These aspects of crimp can be characterized by the load-extension behavior of a fiber; however, the loads occurring during crimp removal of single fibers are extremely small, so that until recently, measurement systems were not sensitive enough to detect them. The literature review in the second chapter of this thesis explains the importance of fiber crimp and gives an overview of current crimp measurement techniques. Despite the great importance of crimp and crimp stability for fiber processing performance and product quality, there is no quantitative knowledge of how crimp production parameters, fiber crimp properties, subsequent fiber processing performance and fabric properties are correlated.

16 2 This lack of process and product control compels the nonwovens industry to find ways to measure fiber crimp quantitatively. Their urgent need is to dissociate the effect of fiber crimp from the effect of finish on processing performance and product properties, in order to gain knowledge for the optimization of both parameters. The shortcomings of crimp assessment techniques in use at present focuses the research reported in the subsequent chapters on developing an industrially applicable methodology to quantify crimp in terms of single fiber load-extension behavior. Meaningful, objective parameters, which quantitatively describe the load-extension curve of a fiber in the crimp removal region, could be correlated to crimp production settings in the stuffer box environment, or used as measures of the loads applied to the fibers during processing. As part of the presented research, a mechanical model for the crimp removal mechanism has been developed. Based on this model, a material-characteristic function is determined for the stiffness of the crimp node during crimp removal, delivering a theoretical relationship between fiber load and extension. The fit of this function to experimental load-extension curves delivers two parameters that characterize the mechanical crimp behavior of a fiber. These fitting parameters are being interpreted in terms of fiber material, crimp settings during production and carding performance. The data analysis also includes the identification of the start and the end of the crimp removal region within the fiber s whole load-extension curve. 1.1 Research Objective When a crimped fiber is clamped in a tensile tester and continuously subjected to an increasing load, the stress-strain curve of the fiber during crimp removal can be calculated from the load-extension response, as shown in Figure 1.1. However, it has not been determined yet which part of the load-extension curve of a fiber can be attributed to crimp removal. Also, it seems difficult to extract quantitative parameters from the loadextension curve that are useful to characterize fiber crimp.

17 3 Load Artifact Stress- Strain Curve Tensile Region Extension How do you know you are measuring crimp? How can you determine start and end of crimp removal? How can you use the stress-strain curve to characterize crimp? Figure 1.1: The Fundamental Problem with Measuring Fiber Crimp from the Single Fiber Load-Extension Curve The global objective of this work is to understand what fiber crimp is and what impact it has on fiber behavior. More explicitly, the purpose of this research is to quantify the mechanical fiber behavior during crimp removal and relate it to fundamental fiber properties, nonwoven fabric properties and processibility in nonwoven equipment, as depicted in Figure 1.2. Single Fiber Processing Parameters Crimp Properties & Stability Cardability Web Properties Fabric Properties Figure 1.2: The Use of Single Fiber Crimp Characteristics for Process and Product Control 1.2 Research Outline Essential steps of this research are: 1. Project preparation Literature review and planning Collection of fibers used in nonwoven production for experiments Choice of testing instrument 2. Experiments Experimental plan Fiber crimp testing Carding experiments 3. Data analysis Analysis of experimental load-extension data Development of physical model for fiber load-extension behavior

18 4 Extraction of key parameters to describe fiber crimp 4. Practical relevance of fiber crimp parameters Correlation of crimp parameters and fiber parameters Correlation of crimp parameters and carding settings and performance 1.3 Practical Relevance of this Research The practical relevance of a meaningful characterization of mechanical fiber crimp behavior is illustrated in Figure 1.3. If a parameter k was identified, that quantifies fiber crimp at a certain processing step, this parameter could be used to give information about the forces acting on the fiber during the processing step, e.g. carding, or the number of carding cycles, to which the fiber has been subjected. Also, for a comparison of web properties, resulting from different fiber materials 1 and 2, crimp parameters k could be correlated to measurable web parameters such as thickness l. Crimp Behavior Carding Parameters k k Crimp Web Behavior Properties Web Properties l 2 force # of cycles 1 1 k 1.4 Deliverables Figure 1.3: Relevance of Quantitative Crimp Parameter k for Processing and Product Properties In order to find objective measures to describe fiber crimp, the experimental approach of this project includes the collection of stress-strain data for various fibers, accompanied by the following deliverables necessary for better process and product control in nonwoven fabric production: 1. A quantitative definition of fiber crimp parameters 2. A physical model relating single filament crimp parameters to fiber properties such as modulus, stability and geometry 3. A mechanism to determine the interaction of textile processing, such as carding, crimp and crimp stability and fabric properties.

19 5 2 LITERATURE REVIEW 2.1 What is Crimp? Crimp in a textile strand is defined as the undulations or succession of waves or curls in the strand, induced either naturally during fiber growth, mechanically, or chemically [1, 2]. Crimp in a fiber is thus considered as the degree of deviation from linearity of a nonstraight fiber [3, 4, 5]. Yarn crimp in textile fabrics is the waviness or distortion of a yarn caused by interlacing in the fabric, and is defined as the difference in distance of a length of yarn lying in a fabric and the same length of the straightened yarn [1, 6]. Crimp in filament yarns is the bulk in textured yarns [1] and is almost directly a product of fiber crimp. Fiber crimp is the waviness of a fiber expressed as waves or crimps per unit length [1] or as the difference between the lengths of the straightened and crimped fiber (expressed as a percentage of the straightened length) [6]. Figure 2.1 summarizes the different definitions of crimp. Crimp CRIMP of Yarns in Textile Fabrics CRIMP in Wool Fibers (Helical Crimp) Staple Fiber CRIMP CRIMP in Man-made Fibers (Planar Crimp) CRIMP in Filament Yarns CRIMP in Cotton Fibers (Convolutions) Figure 2.1: Crimp Definitions [1] Crimp in Wool Fibers Wool is outstanding among textile fibers, because of its unique three-dimensional fiber crimp. This crimp causes a high degree of bulk or loftiness, good thermal isolation and

20 6 pleasant tactile properties. Wool crimp is fiber inherent and very stable. Wool fibers reveal a reversible spring-like return when dried after wetting, or release after stretching. Thus, the geometrical shape of wool fiber crimp essentially depends on temperature and humidity. Wool crimp not only determines the quality of end-use products, but also the fiber performance during processing [5, 7, 8, 9, 10, 11, 12, 13, 14, 15]. Fiber crimp, especially its amplitude, seems to be the most important factor for the classification of wool quality by visual assessment and handle [9, 16, 17, 18, 19, 20, 21]. While in general, finer wool fibers have a higher fiber crimp [9, 22], the universality of this correlation has been disproved by several experimental measurements [16, 17, 23, 24] and crimp is shown to also depend on breed, environmental conditions and body part of the sheep [9, 22, 23] Morphological and Chemical Structure of Wool Cell Membrane Ortho-Cortex Microfibril Microfibril Macrofibril Para-Cortex Epicuticle Exocuticle Endocuticle Protein and Nuclear Remnants Figure 2.2: Cross-Section of a Figure 2.3: Bilateral Structure of a Coarse Wool Fiber [9] Wool Fiber [9] As shown in Figure 2.2, a wool fiber consists of a cuticle on the outside, the cortex as the body of the fiber and, in some coarse wools, a medulla within the cortex [9]. A characteristic property of wool is felting, caused by movement of fibers relative to each

21 7 other. Due to their scaliness, wool fibers have a higher friction against deformation toward the tip than against the root and thus cannot move reversibly in both directions in a fiber assembly. With relative motion, entanglements of the fibers are hooked and drawn closer together [9]. Felting is hindered by crimp [25]. Intense fiber crimp keeps adjacent fibers at a distance to each other and prevents sticking together along the fiber length. Thus, highly crimped fibers will have less contacting surface to hook to each other, resulting in less felting tendency. The cross-section of wool fibers is generally circular or oval. Despite this geometrical symmetry, wool fibers are asymmetrical bilaterally, Figure 2.3. Two cortex sections of different constitution, called para- and ortho-cortex, twist spirally around each other along the entire fiber. This structure creates an inherent elasticity and ultimately a coiling of the fiber [8, 10, 14]. The para-cortex faces the inside of a crimp curl [8], and the ortho-cortex faces the outside. The ratio of para- and ortho-cortex determines the crimp shape [9]. A clearly segmented cortex produces three-dimensional helical crimp or well-defined uniplanar crimp waves in a wool fiber [26]. Several complimentary theories exist about the chemical and morphological differences of para- and ortho-cortex [4, 5, 14, 26]. Chemical evidence for the bilateral structure of wool fibers was found from experiments revealing different reaction to chemicals. The ortho-cortex stains darker with most acid dyes and swells more in alkali [8, 27, 28, 29, 30, 31]. Chemically, wool is composed of long chain molecules aligned along the fiber. The main protein, Keratin, consists of high-molecular weight polypeptide chains of carbon and nitrogen, with attached hydrogen and oxygen atoms and organic radicals such as sulfur [9].

22 Helical Spring Model The fiber interaction, caused by the closeness of packing in an unopened wool tuft, forces all the fibers in the same direction. Since the ortho-cortex will still be at the outside of the crimp waves, the single fibers are forced into twisting [5, 7, 8, 9, 14, 18, 32]. The stable form of a single wool fiber however is helical and can be explained in terms of internal stresses caused by differential contraction of the para- and the ortho-cortex [7]. With moisture regain, Cystine bonds break down in the ortho-cortex, causing disorientation and shrinkage. The para-cortex, however, shrinks less since it has a higher orientation and is more stable [33]. Thus, the helix opens when the fiber is wetted [7, 26, 29, 34]. Heat treatment increases this crimp pullout [26, 29]. In general, crimp in wool fibers is increased by conditions hindering swelling or promoting contraction, and decreased by conditions promoting swelling or hindering contraction [34]. The helical equilibrium configuration of an ideally uniform wool fiber with perfect orthopara asymmetry can be modeled by a spring, as projected in a plane as shown in Figure 2.4 [28, 29, 35]. 2πr θ l 0 a, E, G 2πr θ 2l 0 2l a r L θ E G l 0 l n C f = fiber radius = radius of curvature = πrn = length along the fiber = pitch angle of the coil = Young s modulus = torsional modulus = crimp width (3D) = πr = extended length of 1 crimp = number of crimps in fiber = l -1 = crimp frequency Figure 2.4: Planar Projection of Helical Crimp Model [22, 35] The total extension x is the sum of the extension due to pulling out the helix x 1 caused by bending and twisting of the spring, and the perfectly elastic longitudinal deformation of the spring material itself x 2. Extensions due to shear stress and effects of creep and stress relaxation are neglected [22].

23 9 The following geometrical relationships for curvature K and torsion T can be derived from Figure 2.4 [35]: l 2πr = 2πr = 1 2πrC f C f C f 2 2 With ( ) ( ) 2 ( ) 2 and l = Equation 2.1 sin θ = 2πrC f Equation ( 2π ) 2 sin θ K = = r C f r Equation 2.3 sinθ cosθ T = = 2πC 1 ( 2 ) 2 f πrc f r Equation 2.4 The strain energies per unit length of fiber due to bending, U b, and to torsion, U t, are given by U b = 1 2 EI ( K ) 2 1 Ut p 2 The total strain energy U (ignoring strain energy due to fiber tension) is Equation 2.5 = GI ( T ) 2 Equation 2.6 U = U b + U t Equation 2.7 For an energy minimum of U, the crimp radius r for an equilibrium helical configuration in dependence of the ratio of bending rigidity EI to torsional rigidity GI p can thus be derived mathematically. In order to model the extensional behavior of a wool fiber in response to a load, various researchers derived mechanical relationships for the elastic axial extension of a helical spring in response to an axial load [8, 22, 35, 36, 37]. The common conclusion from these various equations is, that the work W necessary or the force F applied to stretch a helically crimped fiber, is proportional to the square of the crimp frequency, C f, as crimps per extended fiber length, and linear proportional to the bending rigidity EI of the fiber [35]. Since the bending rigidity EI increases for a fiber of constant material, if the diameter and thus the mass increases, the derived relationship supports the common conception that crimp and fiber denier are inversely related in wool fibers [8, 35]. These relationships W ~ F ~ C 2 f and W ~ F ~ EI were supported by

24 10 comparisons of experimentally measured and theoretically calculated spring constants for the over-all load-extension behavior of the helically coiled spring [35] Load-Extension Curve of Wool Figure 2.5: Typical Force-Extension Curve for a Wool Fiber [38, 39] The force-extension curve of a wool fiber extended at a constant rate of extension is shown in Figure 2.5. The extrapolation of the Hookean slope k to the abscissa is defined as the point of 0 extension at which F 0 is measured. Thus, at F 0, the fiber is essentially straight, while the fiber crimp has been removed in the region below the point designated as 0 extension. From almost any point below the knee of the curve, a wool fiber will recover elastically after load removal. Following the Hookean region, is a region of flow with a slow increase in force with extension, characterized by the force at 20% extension F 20 [38]. As demonstrated with the experimental load-extension curve for a wool fiber in Figure 2.5, during the initial stages of crimp removal, the extensional behavior is controlled by fiber bending. Fiber extension due to torsional stresses and deformation of the fiber is negligible. Later on during crimp removal, the overall fiber extension is controlled by

25 11 both fiber bending and extension. Finally, as the crimp removal approaches completion, this behavior is controlled by the extensional modulus of the fiber [22, 35]. Figure 2.6: Stress in a Wool Fiber under Small Tension [38] Experiments however indicate, that the Hookean slope, and to some extent even F 20, depend on the fiber crimp [38]. This dependence may be interpreted in terms of nonuniform local stresses across the diameter of a crimped fiber along its length, as depicted in Figure 2.6. The outside of a crimp is lagging behind the inside along a force-extension curve in dependence of the crimp radius and the fiber diameter. Thus, the force increases slower for the crimped fiber than for the uncrimped fiber [22, 35, 38, 40] Crimp in Synthetic Cellulosic Fibers The increasing use of viscose in mixtures with wool provoked the production of crimped rayon staple fibers, with a bilateral structure resulting from an asymmetrical distribution of skin around the core [4, 8, 14, 41, 42]. Upon stretching and releasing, or swelling and shrinking, the side with thicker skin will take the inside of a crimp bend due to its higher elasticity and contraction force in the differential shrinkage of the two halves [8, 43, 44, 45]. The three general principles of producing an unbalanced cross section in rayon fibers are conjugate filaments, broken skin, and irregular serrated cross sections [41]. If two different viscoses are forced out of a conjugate jet hole side by side, one of them will

26 12 develop a thicker or normal skin, whereas the other component will only develop a thin or no skin, under determined chemical conditions of the coagulation bath [41, 44, 45], as shown in Figure 2.7. Figure 2.7: Stained Cross Section of a Crimped Rayon Spun from a Conjugate Jet [44] Figure 2.8: Cross Section of a Crimped Rayon Filament with Ruptured Skin [44] Figure 2.9: Cross Section of a Crimped Rayon Filament with Unbalance of Serrations and No Ruptured Skin [44] Unbalanced skin-core structures as shown in Figure 2.8 and Figure 2.9 are obtained without the use of special jets [41, 44]. The filaments either have little zippers opening the skin and allowing the interior to pop out, or a combination of thicker skin on one side and more crenellations on the other side, caused by a special coagulation bath where the flow of the bath acts more on one side of the filament than on the other [28, 41, 44, 45] Crimp in Cotton Fibers Cotton fibers do not reveal distinct bends like wool, giving geometrical evidence for the presence of crimp. However, during length or mass measurement of cotton fibers, especially with high volume cotton testing instruments (HVI), the experimental results are distorted by the degree of linearity of the fibers. While this problem has initially been attributed to inappropriate sampling techniques and statistical methods [46], recent studies relate it to the residual fiber crimp still present in the fiber during the measurements. With HVI, using tapered specimens of randomly clamped fibers, the

27 13 deviations in length measurements can be correlated with the brushing action during sample preparation [47]. Thus, cotton fibers have crimp in the sense of overall length reduction due to fiber-nonlinearity. For accurate values in weight measurement, cotton fibers have to be straightened and crinkles have to be removed without overstretching the fibers [48]. While the general shape of the load elongation curve of cotton fibers is not altered by previous stretching of the fiber, the designated uncrimping region is highly non-elastic and the uncrimping force and energy depend essentially on the history of the fiber [49] Crimp in Man-Made Fibers Man-made fibers are processed into woven, knitted, and nonwoven fabrics [50]. In order to make synthetic fibers processible with traditional textile equipment, originally designed for natural fibers, the elastic behavior and shape of wool is imitated by crimping the synthetic fibers [5, 51]. Crimping synthetic fibers also increases the bulkiness of the card web or sliver and changes the hand of the produced fabric [21]. The major advantages of man-made fibers are the extensive possibilities of influencing their properties, and their price stability. However, it is often difficult to control fiber quality, since the influence of the numerous processing parameters may not be isolated and quantified [50]. Accurate knowledge about the quantitative effects of crimping on processing and products is still lacking [21]. For synthetic fibers, crimp varies in type and intensity. Crimp parameters can be controlled over a wide range by different crimping techniques and settings, summarized in Table 2.1 [2]. Crimp may be incorporated into the fiber during fiber spinning at least latently, or the uncrimped fiber can be crimped after spinning [4]. The most popular method for staple fiber crimping is stuffer box crimping, since the crimp results achieved are the most predictable [52]. Common crimping and texturing procedures are summarized in Table 2.1. Most texturing methods depend on the thermoplastic properties of the synthetic fibers. The setting of the

28 14 fiber crimp is caused by structural changes in the material at a molecular level through a process of crystallization and crystalline reorganization [53]. Table 2.1: Texturing Procedures for Filament Yarns [6, 54] Method Description Shape of Crimp Application False Twist Texturing Multi-filament yarn is highly twisted, thermally set at temperature higher than glass transition, cooled untwisted to stabilize the crimp (only possible with partial tow) Threedimensional Curls Filament yarns only, Thermoplastic materials Stuffer Box Texturing Yarn is fed through a nip into a stuffer box & folded against the box pressure Two-dimensional triangular crimp Dominant method for staple fibers Impact Texturing Yarn is plasticized & subsequently impacted onto cooling surface Threedimensional Carpet yarn Edge Crimping Heated yarn is passed over dulled knife-edge causing crystallite rupture at bend inside Three-dimensional helical crimp Gear Crimping Heated yarn is passed between gear wheels & crimped shape is thermo-set Two-dimensional triangular crimp Staple fibers Knit-Deknit Yarn is knitted into fabric that is thermally-set & unraveled Planar loops Filament yarns only Air-Jet Texturing Yarn is over-fed through turbulent air stream inside a jet assembly so that entangled loops are formed in filaments Threedimensional curls & twists Filament yarns only Bicomponent Crimping Yarn is composed of bicomponent fibers in asymmetric cross-section & subjected to heat relaxation / wet process with differential shrinkage of components Threedimensional helical crimp Filaments and staple fibers

29 Staple Fiber Crimp Crimped man-made staple fibers are produced by cutting of previously crimped continuous filaments. Since for staple fiber production, the filaments are collected in big cables of about 1-3 million dtex, the applicability of crimping methods is restricted and false-twist and torsional twisting methods cannot be applied. The most common techniques are stuffer box crimping, producing a characteristic two-dimensional triangular zigzag crimp, and bicomponent crimping, caused by an asymmetrical structure in the filaments, producing a three-dimensional helical crimp similar to wool [45, 55]. The production process of synthetic staple fibers, with the example of PET fibers, from the polymer to the final fiber is depicted in Figure Melt Spinning Dry Spinning Wet Spinning Adjustable Roller Crimper Preheat Stufferbox Crimping Heated Drying Cable Collection Adjustable Roller Filament Cutting Bale Press Multifilament Tow Cans of Undrawn Fibers Pretension Guides Feed Rolls Finish Bath 1. Stage Draw Rolls Heating Channel 2. Stage Draw Rolls Heating Channel Heat Setting Rollers Cooling Cable Formation Spray Finish Figure 2.10: Production of Crimped PET Staple Fibers [42, 43] Technology of Stuffer Box Crimping With stuffer box crimping, two draw-in rollers pinch the heated and softened fiber tow and feed it into a chamber at rates of m/min [56, 57, 58]. The tow is compressed into the stuffer box through the inlet, crimped against the pressure in the chamber and withdrawn after thermosetting the crimp [4, 8, 54, 55]. The chamber usually measures

30 16 about 20 mm in height, mm in width and mm in length [55]. So-called doctor blades guide the filaments into the stuffer box [56, 57] and provide the pressure in the chamber. They are either tapered towards the exit side of the box, or a hinged piece is added to the end of one doctor blade [55, 56], as depicted in Figure PRIMARY CRIMP FEED ROLLERS UPPER DOCTOR BLADE FIBER TOW SECONDARY CRIMP LOWER DOCTOR BLADE Figure 2.11: Crimp Formation [56] CRIMPER ROLLS UNCRIMPED FIBER P T DOCTOR BLADE CLEARANCE PRIMARY CRIMP SECONDARY CRIMP P R DOCTOR BLADE ANGLE P K Figure 2.12: Two Types of Figure 2.13: Crimping Mechanism [55] Stuffer Boxes [56] in the Stuffer Box The primary crimp in the tow is formed at the tangent point of the crimper rolls, called nip. The fiber cake collecting in the crimper box causes pressure at the nip, which provokes the filaments to buckle. This pressure also causes an additional folding action in the tow, called secondary crimp, of much greater amplitude and lower frequency than the primary crimp [8, 52, 56, 57].

31 17 As the tow exits from the box, the primary crimp remains in the filaments, while the secondary crimp unfolds, promoting cohesion of the tow bundle [55, 56]. Crimp frequency and amplitude of primary and secondary crimp are determined by processing conditions such as tow feed speed, and design of the stuffer box such as roll width and box depth, which control the back pressure in the chamber [8, 54, 56, 57]. The crimp stability is determined by the temperature and humidity conditions of the fiber tow and in the stuffer box, and the consecutive setting conditions, like temperature, humidity and time period in the heater. Furthermore, the crimping results depend on the capacity of the fibers to accept and retain crimping, which varies widely among synthetic fibers, in combination with the finish used (e.g. PET is set in hot air at 120 to 160 C, whereas Nylon is set at 100 to 140 C with saturated steam. Usual setting times are 5 to 8 minutes) [4, 43, 52, 55, 56, 59]. Technological changes in fibers, such as lower fiber denier and skin/core structures, make crimping more difficult. Inventions for the improvement of crimping performance include systems with flexible temperature control. This may prevent the filaments from sticking together and reduce burnishing damage to the fibers, caused by higher temperatures. On the other side, it may improve the thermosetting effect resulting in better crimp stability, which does require high temperatures [8, 55, 57]. In spite of the intensive blending during further processing, crimp irregularity, between single fibers and along single fibers, causes problems in processing performance. Stuffer box crimping is sensitive to irregularities in tow feed such as [52, 55, 56] Non-uniform tow tension upon feeding Non-parallelism of filaments Non-uniform tow density Uneven lubrication of the tow Temperature dependence of finish characteristics Irregular tow feed & pressure built-up caused by rapid nip roller wear Thus, an important parameter for uniform crimp quality is the careful preparation of the fiber tow fed to the crimper [52]. Furthermore, it is highly desirable to gain more control over the crimping process itself. Gear crimping does not produce satisfactory crimp

32 18 results, like too-flat-arced crimp bows [43], and is practically limited to smaller tow sizes. However, it is thought of as alternative to stuffer box crimping, since it might reduce crimping irregularities by replacing the uncontrolled pressure conditions in the stuffer box with the imprinting of a specific and constant crimp geometry independently of tow irregularities [55]. Better control of crimp production could also be obtained with a self-adjusting stuffer box system that detects, and compensates for, irregularities in the fiber input. This solution, however, requires quantitative knowledge of how processing conditions during crimping influence crimp parameters, which is still lacking today Buckling Theory Applied to Stuffer Box Crimping In order to relate crimp geometry to the processing conditions during crimping and to fiber properties, crimping of a fiber in the stuffer box has been modeled as buckling of a filament. An elastic column (representing the fiber) vertically fixed at its lower end, and loaded with a vertical compressive load P at its upper end, will buckle at a critical load P critical. Here, the Euler s buckling equation applies: π EI l k = Equation P where l k = buckling length E = elastic modulus P = compressive load acting on single fiber I = axial areal inertia moment EI = buckling resistance [43]. with P = P K + P R - P T Equation 2.9 where P K = closing force of stuffer box P R = frictional force between tow and chamber wall P T = load of feed rollers [43, 55], shown in Figure The buckling equation for a column can only be applied qualitatively to the situation in the stuffer box due to the following deficiencies [55]:

33 19 The filaments have friction between each other and thus cannot be treated independently as single fibers [21] The filaments are spatially restricted to buckling in the box The inertia of the filament tow in the chamber is not considered, and the filaments do not have a constant elastic modulus for big deflections as occurring during buckling [21, 43, 55] There is a large number of factors involved beyond the actual single fiber characteristics, determining the pressure conditions in the chamber in a complex way, such as total tow denier, chamber width, nip roll diameter [55]. However, some qualitative conclusion during crimping can be drawn [21, 54]: A bigger compressive load reduces buckling length & causes tighter buckling angles higher closing pressure of the chamber reduces side length of crimp bows The elastic modulus of the fiber influences the buckling length [21] smaller drawing ratio, higher tow temperature, higher humidity cause lower elastic modulus & smaller side length of crimp bows [60] The diameter of the delivery rollers influences the load acting on the fiber tow bigger roller diameter increases perpendicular load on fibers & thus reduces axial compressive force, resulting in higher side length of crimp bows Finish influences the buckling length, since it influences the fiber-to-metal friction (tow-to-chamber friction) & also the surface friction between single fibers The tow thickness influences the crimp since it determines the interaction of the single fibers in the tow [43]

34 Mechanics of Triangular Crimp M 1 M 2 M 3 M 4 M 5 M 6 2θ M 7 M 8 M 9 F Figure 2.14: Plane Model and Photomicrograph of Zigzag Shaped Staple [8] In the case of a zigzag spring, the external stress is concentrated in the junction points of the elements, and only the bending force responds to the external load [8]. The external load is thus derived as function of the angle of adjacent zigzag elements: F = 1 0 θ θ lm cosθ k 2n Equation 2.10 where F = load θ 0 = half the initial angle between the elements θ = half the angle between the elements after loading n = number of elements l m = length of fully extended zigzag k = constant [8]. The constant k is determined by dθ/dm, where M is the moment caused by F acting on the elements. k is assumed to be proportional to the reciprocal of EI z, where E is the modulus of elasticity, and I z the moment of inertia of bending. Equation 2.10 expresses

35 21 that the load causing a defined deformation of the spring is proportional to the number of elements per extended length, and also to EI. The overall spring constant K is given by lm 1 θ θ0 K = k = Equation n F cosθ This model was supported by experiments with steel wire [8]. K values calculated from experiments with three kinds of zigzag shaped acetate staple fibers were also concluded as supporting the theory [8]. However, the calculated 5 to 6 values for K showed a standard deviation of about 20%. Therefore, the presented experimental results of acetate staple cannot be used as proof for the proposed model. The model is helpful to illustrate the stress-concentration in the tips of the crimp nodes during crimp removal. However, for the large deflections during decrimping caused by deformation of a small part of the fiber, the tip of the node, linear-elastic behavior of the material and thus E = constant cannot be assumed. This also means that a linear spring-like behavior obeying dθ/dm = constant is not applicable for polymeric fiber behavior during crimp removal. A crimped fiber will not have a linear load-extension response of the form F = K x, where x is the overall fiber extension. Another model was developed for the correlation of applied external load and internal stress concentrations in the crimp nodes based on the assumption of a two-dimensional sine-wave crimp and simple beam theory [21]. This model however is not useful due to the assumed crimp shape, which does not occur in actual crimped staple fibers. Furthermore, no practically relevant relationship between load and extension was derived. The qualitative conclusion from this model is, that a reduction in fiber strength for crimped fibers is due to the crimping geometry and the resulting stress concentrations, and not to fiber damage potentially introduced during crimping [21]. In a third model, the fiber is assumed as a beam of homogeneous material and constant circular cross-section, randomly bent as an arc in a plane. Material behavior is linear elastic and the effects of shear are neglected [61].

36 22 Figure 2.15: Extension of Circular Arc-Nomenclature [61] Constant terminal couples C and forces P are acting on the filament DEF of flexural rigidity G, of original length l 0 and radius r (Figure 2.15). Extension of the filament as response to C and P is caused by an opening of the arc and the extension of the filament [61]. Equating bending moments leads to 4 differential equations for the extension of the system in dependence of C and P. The numerical solution leads to a relationship between the parameter X/X 0 as the dimensionless strain and (PX 2 0 )/G as the dimensionless stress in the filament [61]. Even though this model can be customized from the general arc-shaped geometry to the triangular saw-tooth crimp shape, it is not useful to explain the crimp removal mechanism for stuffer-box crimped fibers. The basis idea of this model is that the fiber deformation is caused by unbending of a homogenous material, and not by an opening of the crimp angle. With stuffer box crimp however, the essential property of the crimp node is the inhomogenity of material and its physical properties in crimp legs and node, causing the load-extension response to be a function of the deformation in the top of the crimp node rather than a function of homogenous bending of the whole fiber. Due to the difficulty in deriving a quantitative load-extension response based on the mechanics of crimp removal, attempts have been made to describe crimp removal behavior empirically [54, 63]. The shape of a load-extension curve of a stuffer-box crimped fiber in the crimp region, as shown in Figure 2.16, differs from the loadextension curve of a straight fiber. Since the point A, where the curve diverges from the zero line, is difficult to locate precisely, it has been common practice to set the origin of

37 23 the curve at O, the extrapolated point corresponding to a hypothetical straight fiber. The difference in straight and crimped length is then given by the distance AO [12, 78, 96]. Figure 2.16: Load-Elongation Curve of a Crimped Fiber [78] a b C A B Figure 2.17: Initial Portion of Load-Elongation Diagram of Uncrimped Fiber, a and Highly Crimped Fiber, b [2] Since the crimp removal part of the load-extension curve is concave upward and the tensile part is concave downward, the merging region approximates a straight line [2]. This straight-line portion can be extrapolated for the separation of the crimp and stretching portion, shown by the dotted line in Figure 2.17b. The true separation of these

38 24 portions however is the dashed line. Use of the dotted line extrapolation, results in a too low crimp content, stiffness or modulus, of the fiber, and in too high crimp permanence, and in an apparent change of modulus with stretch [2]. The load required to unbend crimps in a fiber is smaller than that to stretch the fiber. Thus, with a tensile test at constant rate of extension, the load on the fiber rises very slowly at first during crimp removal, then more rapidly as the crimps approach complete straightening out, and then much more rapidly as the fiber is actually stretched [2, 39], as can be seen from Figure The crimp portion of Figure 2.16 looks in general like an exponential or logarithmic function. In analogy to a model for the load-compression behavior of paper press felts [62], e.g. a power law is suggested to fit the load-extension behavior of single staple fibers in the crimp region [63]. Load P δ c P = α 1 δ δ 0 β δ 0 δ C δ* Extension δ Figure 2.18: Empirical Fit of Power Law to Crimp Removal Curve Analogous to Pressure Curve of Press Felts [62, 63] With the boundary conditions [62] P = 0 for δ = δ c dp/dt = 0 for δ = δ c P for δ δ*, the equation δ P = α 1 δ δc β may fit experimental data as demonstrated in Figure 2.18 [63]. Here, δ 0 is the initial distance of the fiber, where the crimp and also slack are

39 25 still present and δ c is the distance of the crimped fiber, where slack is completely removed and crimp removal starts. α characterizes the crimp removal force and β is a parameter for the shape of the crimp removal curve [62, 63] Morphology of Crimped Structures Small Figure 2.19: Crimped Fiber Structure, a Function of Bend Geometry [53] Drawing orients the amorphous regions enabling the subsequent stuffing environment to permanently set the three-dimensional crimp geometry [53]. Figure 2.19 shows schematically how three distinct regions in a crimped fiber develop structurally after the fiber is deformed in its stuffing environment then set with heat (and sometimes, for Nylon, with moisture). Crystalline rearrangement is achieved by heating the fiber to a temperature high enough to cause melting of crystallites, which can then reform in the relaxed configuration [64]. The outside region of the bend being most extended, is also the most oriented, while the inside of the bend, being the most compressed, is the least oriented. With heat, these regions crystallize differently under the different stress fields, and thus stabilize their respective structures, resulting in a

40 26 permanent memory of this geometrical configuration. Thus, any decrimping forces are resisted below the fiber s glass-transition temperature. If deformed, the crimped structure can be redeveloped with heat, since the fiber remembers its most stable configuration [53]. The final level of crimp developed, the uncrimping resistance to tensile stress, and the crimp permanence, highly depend on the fiber s micro-structural evolution from crimping through heat setting. The initial level of crimp may be identical for two fibers, but produce a yarn with different final levels of crimp due to crimp permanence differences [53]. The superior settability of PET, e.g., in comparison to Nylon or Polypropylene fibers, may be caused by the low crystallinity of the unset filament. Development of crystallinity during heat setting of the crimp is important for achieving good crimp permanence with respect to time and temperature [64]. In experiments, crimped fibers drawn past the Young s Modulus peak, a point of permanent deformation, still recovered most of their original crimp. The morphology changes make the crimp points even more resistant to tensile stress than the straight regions between crimp points [53] Bicomponent Crimp An alternative to stuffer box crimping, especially for non-thermoplastic material, is bicomponent crimping. Man-made bicomponent fibers, such as the Acrylic fiber Orlon Sayelle by DuPont or a Nylon 66 / 6 combination, are produced by spinning the filaments from two different substances through conjugate nozzles. The crimp results from the structurally asymmetric cross-section and thus different shrinking behavior of the component polymers, analogously to the para- and ortho-cortex concept in wool fibers. Upon heating or wetting and drying, an intensive three-dimensional crimp, causing high bulk, is produced. The component with the higher heat shrinkage is always situated in the inner part of the crimp helix. Bicomponent fibers barely compact under heat and pressure, and they recover easily from hot compacting with steam [11, 59]. Bicomponent

41 27 crimp is very stable, similar to wool. Its disadvantage is the requirement of special, sensitive spin nozzles, and the added expense of two polymer systems [4, 8, 11, 42, 43, 59]. Bicomponent crimp is often produced as latent crimp and first developed in the yarn [6, 8, 43, 53]. Besides the side-by-side conjugation of the two components in bilateral fibers, one component may also be eccentrically enclosed in the other. This type is useful for components that cannot be bound to each other stably enough and thus likely to be separated by mechanical treatment [8]. In addition to the load-extension relationship theoretically derived for a helically crimped fiber, the dependence of the equilibrium crimp configuration on the characteristic parameters of the two components in the bilateral fiber was investigated. For the case of uniform compression on one side of a filament and uniform tension on the opposite side, the crimp frequency was predicted as function of the differential shrinkage of the two components based on the theory of a bimetal thermostat. The differential shrinkage is derived with mechanical strain energy considerations and depends on the ratio of thickness a 1 /a 2 and elastic moduli E 1 /E 2 of the two components, the differential change in length σ due to temperature, and the total thickness h of the fiber [35, 36, 43, 54, 59, 65]. C f kc p σ h = Equation 2.12 where C f = Crimp frequency k = Constant, slightly dependent on modulus ratio of the two components σ = Strain differential or differential shrinkage of the two components h = Fiber thickness (proportional to square-root of fiber titer) C p = Crimp potential function, reflecting relative amounts of components & their distribution, or measure of the strain asymmetry. The derived equations for curvature and crimp frequency indicate an inverse correlation between fiber diameter and crimp frequency, well known in wool technology [26, 35].

42 Why is Crimp Important? Fiber crimp characteristics have a big influence on the processing performance of the fibers. Crimp also contributes essentially to the properties of intermediate fiber assemblies, yarn and finished fabrics [1, 3, 7, 8, 18, 21, 22, 35, 42, 49, 55, 66, 67, 84]. Fiber crimp imparted to synthetic fibers, which are initially straight, makes it possible to process these fibers with existing machinery designed for natural fibers. Straight, slick synthetic fibers would not have sufficient cohesion for carding, combing, drawing, roving, and spinning [52]. In nonwoven processes, crimp and crimp retention during processing are major contributors to processing efficiency, cohesion, fabric bulk and bulk stability [68] Impact of Crimp on Processing Fiber crimp is necessary in processing to provide appropriate fiber-to-fiber cohesion for carding, drawing and spinning [4, 45, 56]. The card web itself needs sufficient strength through fiber cohesion not to fall apart [52, 57, 79]. The cohesion of the fibers also determines the amount of fly liberated during processing [78]. However, the crimp is not only needed to hold fibers together, but also to keep them apart in order make the card web bulky and lofty [2] and to make drafting easier [52, 56]. Too much fiber crimp however, may cause neps during processing and makes drafting difficult [45]. Today s high performance machinery and the constantly increasing production speeds cause the strains on fibers to increase. This makes crimp removal less controllable. Thus, it becomes even more difficult to determine optimal processing conditions in dependence on fiber crimp characteristics [55]. Fiber crimp is as important as finish in its influence on processing. Finish affects crimp formation, since it determines the fiber-to-fiber friction in the tow and the fiber-to-metal friction during crimping. The static fiber-to-fiber friction depends only on the fiber surface and thus on the finish. The other fiber frictional properties however, such as Dynamic fiber-to-fiber friction Static fiber-to-metal friction Dynamic fiber-to-metal friction

43 29 depend on fiber crimp as well, since the crimp determines the mean distance between adjacent fibers in the structure. Only if this distance is sufficiently small, these kinds of friction will be fully effective [55]. With synthetic fibers, the overall friction level changes in the course of fiber processing, because the fibers are progressively decrimped during carding, combing, drawing and spinning [2, 12, 18, 42, 50, 55, 80]. The residual crimp during processing is determined by the initial crimp level, fiber crimp permanence and the strain to which the fibers have been exposed during previous processing [2, 55]. Finish Static Fiber-to-Fiber Friction Dynamic Fiber-to-Metal Friction Static Fiber-to-Metal Friction Dynamic Fiber-to-Fiber Friction Crimp Crimp Geometry Crimp Stability or Respective Residual Crimp Basic Processing Properties Practical Processing Properties Ease of Opening Bulking Capacity Figure 2.20: Correlation between Finish, Crimp and Key Processing Properties [55] For fiber manufacturing, high tow package & bale density is favorable [55]. During subsequent processing however, the staple tufts must be easy to open. These contradictory requirements can be satisfied with a relatively low static fiber-to-fiber friction, responsible for the ease of fiber separation, and a springy crimp with distinct nodes [55]. During carding, crimp improves the fiber-to fiber cohesion due to hooking of the crimp bows, which facilitates the construction of a card web. Too high crimp however may cause fiber breaks and fiber sticking in the carding elements [43]. Carding is the processing step imposing the most stress to the fibers before drawing. Thus, card settings interact with inherent fiber crimp stability, determining the crimp pullout and the

44 30 respective residual crimp for further processing such as drawing and spinning or web production in nonwovens. Because of crimp pull-out during processing, and the resulting increase in fiber-to-fiber contact, the processing properties at the last drafting passage or the flyer are largely influenced by dynamic fiber-to-fiber friction due to finish, whereas in carding and the first drafting passage, fiber crimp has a major influence [55]. A crimped fiber passes the drawing field less controlled than a straightened fiber, since it has a smaller overalllength and thus a greater distance to the transporting rollers [43]. This might lead to drafting irregularities [82]. High fiber crimp results in excessive tuft and sliver volume and may cause sliver accumulations on the rollers of the drawframe and insufficient packing density on the roving package. High crimp also causes drafting problems, because the required drafting force rises [50]. On the other side, insufficient sliver bulk may cause lapping tendency of fibers, since the fiber-to-cylinder contact is very close [55]. Too little fiber crimp may cause unwanted fiber slipping past the machine surface. In general, bulky fibers, e.g. Acrylic fibers, have a high sliver volume and therefore need large sliver trumpets and guide passages. Moreover, because of the high total volume at the infeed, doubling must be reduced [50]. In ring spinning, intense fiber crimp causes a wide spinning triangle, which impairs fiber anchoring and may result in end breaks. In rotor spinning, high crimp produces a relatively wide fiber ring, which is difficult for the open-end to tie up to. This results in a higher minimum number of fibers required for spinning and more end breaks. However, too little crimp causes a too compact fiber ring, since the open yarn end ties off the fibers more difficulty [55] Impact of Crimp on Other Fiber Properties Crimped fibers show a considerable variation in breaking stress and strain of individual fibers, caused by randomly occurring defects along the length of a single fiber such as

45 31 polymer impurities and defective crimp nodes. The distribution of breaks in crimped fibers is skewed to the left, since it is easy to make a fiber weak with a crimp defect, but hard to make it stronger than the ultimate strength of the polymer itself [69]. This effect is increased by the large production scale of the staple fiber crimping process. With ropes of significant thickness, filaments near the crimper heating medium relax more, thus lose more orientation and are more likely to be physically damaged [69]. Crimp in fibers reduces the elastic modulus. This may be explained by stress differentials in the crimp nodes. Since the length along the outside of a crimp is greater than it is along the inside, stresses will not be distributed uniformly in the fiber [5, 38, 74]. Crimp does not influence properties beyond the Hookean region, e.g. the stress at 20% extension [18]. Residual crimp in Cotton fibers prepared for bundle strength testing with HVI equipment affects the measurement of linear density, or bundle mass, as well as the ultimate force required to break the fibers. This measurement error cannot be eliminated completely in practice, but it can be reduced substantially with additional brushing in sample preparation [49, 70, 71, 72, 73]. HVI instruments measure the fiber weight per length (i.e. in mg/meter) either by resistance of fiber material to airflow, or with light scattering methods. The specific weight indicated by the instrument however is influenced in both cases by different fiber crimp levels, since crimp increases the air resistance, or causes changes in light scattering from the fiber surface [73]. Fiber crimp makes the assessment of fiber length in a staple more difficult. Staple length is defined under ANSI/ASTM D1234 as the length of a staple without stretching or disturbing the crimp of the fibers [77]. With crimped fibers, it has to be considered that the fibers in the staple may be taken to be appreciably shorter than their true straightened length [78].

46 Impact of Crimp on Products Crimp prevents the fibers from lying flat and tight in a yarn as the case in a non-textured filament yarn. The fibers are kept at distance to each other, so that air chambers are built in the yarn or fabric. The effective crimp level in the final product depends more on the crimp permanence than on the initial level of crimp, since crimp may be pulled out of the fiber differently during processing. Consequently, yarns and fabrics with quite different properties [53], such as bulk, elasticity and air content may be produced from fibers with the same initial crimp level. Fiber crimp improves the following desirable properties of yarns and fabrics, such as knits, wovens and nonwovens [1, 2, 4, 8, 11, 18, 21, 22, 40, 43, 50, 51, 53, 54, 64, 66, 75, 78, 85]: Wool-like esthetics & visual appearance Warm, dry, soft handle without slickness Bulk, loft, hairiness, voluminosity, lightness, tuft Covering power of yarns & filling capacity of fibers in assemblies Greater extensibility, compressibility, recovery, elasticity & resilience Better wrinkle resistance & recovery Less flexural rigidity, better drape Good thermal isolation, air permeability, moisture absorption, higher wear comfort due to porosity Low crimp results in a lean, silky fabric with high luster, since the low crimp causes less diffusion of light. Low crimp is desirable for Cotton type fabrics, in hard-twist yarns for high-strength application, sewing threads, filter fabrics, and other industrial textiles [52, 84]. In carpet manufacture, fiber crimp determines the carpet properties in interrelation with a second shape-setting operation: Helical twist is heat-set in plied yarns. Without set to relieve the torque caused by the twisting, the two plied ends of the yarn would unravel in the tuft [53]. More crimp gives a carpet more body and better cover, thus allowing the manufacturer to use less fiber material per carpet area. However, the resultant surface wears worse and has poorer endpoint definition, since less structural reorganization is available to set the twisted configuration of the yarn, if too much energy is spent for

47 33 crimp setting. Tuftlines varying within a carpet in their respective levels of crimp, cause optical streaks as differences in light scattering behavior [53]. With assemblies or fabrics of straight fibers, the application of a lateral load causes energy absorption by direct compression and eventual collapse of the fibers. With highly crimped fibers however, the initial energy absorption occurs by bending reactions, since the work required to bend a fiber is much smaller than the work required to stretch or compress a fiber. The energy absorbed in a single bending reaction is small, but with a large number of bending reactions, as in the case of assemblies of highly crimped fibers, the energy absorption of a load impact differs greatly from that one of assemblies of straight fibers. With compressive loading, an uncrimped fiber will eventually collapse in only one or two places. A highly crimped fiber however will behave spring-like under compression [21]. In fiber assemblies, an increase in crimp thus causes a decrease in the initial compressional Young s modulus and thus reduces the resistance to compression [9] under small loads. This conclusion was theoretically derived with a model of crimped fibers in a random assembly and mathematical probability of the fiber orientation distribution, and corresponds well to the common knowledge that assemblies composed of highly crimped fibers are soft [85, 86]. Furthermore, it was shown that the Poisson s ratio increases with an increase in fiber crimp [85]. 2.3 Quantitative Crimp Parameters Although crimp is one of the most important characteristics of wool and man-made fibers, there is no consensus how to quantify crimp in terms of measurable parameters [87]. Due to the irregularity, variability and complexity of shape and behavior of crimped fibers, there is no agreement on what properties should be used to describe crimp characteristics [5, 22, 88].

48 34 The characterization of fiber crimp can broadly be divided into static parameters describing the geometrical shape of the crimp bows, and dynamic parameters describing the tensile fiber behavior attributed to the crimped shape of the fiber [49]. In terms of the tensile response to crimp removal in fibers, empirical parameters have been derived from the load-extension curve in the crimp region [22]. Geometrical crimp descriptions include simple physical parameters such as the number of crimp waves or their amplitude, but also sophisticated functions of fiber helices and their statistical evaluation in terms of spatial volume [5, 7, 32, 42, 61, 89, 90] Geometrical Parameters For a mathematical description of fiber crimp, either a series of triangles or circular arcs can be considered. Both idealized geometrical shapes resemble an oscillation, which is described by two parameters, commonly the wavelength λ and amplitude A [4, 43]. Thus, for the complete geometrical description of crimp, two parameters are necessary. A problem with the commonly used parameters, such as crimp percentage or frequency measured on a flattened fiber, is that the fiber cannot be assumed to be crimped in one distinct plane. The high non-uniformity of crimp shapes makes the objective determination of these parameters difficult due to the likelihood of visual errors, the questionable decision where a crimp bow begins and ends and the high variability coefficient resulting in low statistical confidence. Furthermore, these parameters are very sensitive to any handling treatment of the fiber. Even with objective measurement techniques, the results may thus be subjective because of subjective treatment of the fiber for sample preparation. The static crimp parameters also depend on the load that is acting on the fiber during measurement, e.g. counting of the crimp bows in the crimped fiber. Even if the fiber is positioned loosely on a smooth surface, frictional forces will be acting on the fiber. For the crimp ratio, also the straightened length of the fiber has to be known, which

49 35 introduces the additional difficulty of measuring accurately the straightened, but unstretched length on the fiber. At present, nobody knows exactly how to determine the load associated with this fiber state [5, 94, 97]. Simple parameters such as the leg length and the crimp angle, even though dependent on the loads applied during measurement, have the advantage of simple correlation to the crimp manufacturing process [43] Wave Length, Crimp Frequency and Crimp Length The wave length λ of fiber crimp is twice the distance between two crossings of the fiber with the zero axis. It cannot be measured directly, since fiber crimp is by far too irregular and the small measuring quantities wavelengths in the order of 1 to 3 mm - cause problems. Therefore, the crimp length l c, as the average length of fiber in one crimp, is sometimes used to describe crimp [15], where lc 1 2 λ [ mm] or [ inch] = Equation 2.13 More commonly used is the crimp frequency C f of a fiber, defined as twice the average of the inverse of the wavelength [4]. It is also called crimp number or crimp count, and characterizes the number of crimp bows or waves C n per unit length of straightened fiber L 0 [1, 2, 3, 6, 15, 17, 35, 78, 90, 92]. The unit length L 0 is taken as 1 inch in the US, whereas in Europe, 100 mm or 1 cm is used [4] Crimp Angle C f = λ or Equation 2.14 inch 100mm The angle α between the leg of a crimp wave and the zero line may be used to characterize crimp geometry [4]. The crimp angle ϕ is the angle between the two legs of a crimp bow, as shown in Figure ϕ indicates the sharpness of a crimp [91].

50 36 l 2l 0 ϕ α A with n = number of crimp waves per fiber l 0 = side length of one crimp bow l = width of one crimp bow λ = 2l = wave length of crimped fiber α = angle between crimp leg, fiber axis ϕ L 0 L A = crimp angle = 2n l 0 = extended length of fiber = n l = crimped length of fiber = crimp amplitude Figure 2.21: Idealized Geometry of the Stuffer Box Crimped Fiber [43] Crimp Amplitude and Crimp Index The crimp amplitude A is the maximum distance of a crimp bow from the zero axis. Since the measurement of the amplitude of single crimp bows is practically impossible, an average crimp amplitude of the fiber is derived geometrically with Pythagoras from length measurements of the crimped and the uncrimped fiber [4]. The crimp index C i is an indirect measure of the crimp amplitude [1]. It is also called crimp ratio, crimp percentage, crimp contraction or crimp retraction and is the ratio of the difference of extended length L 0 and crimped length L c of a fiber, in percent of the extended length of the fiber L 0 [2, 5, 15, 35, 66, 67, 78, 80, 85, 93, 94]. C i describes the crimp potential of a textile fiber as its ability to contract under tension [1, 6]. C i L0 Lc = 100 [%] Equation 2.15 L 0 A crimp index of zero (C i = 0) indicates that the fiber is straight with no crimp. A crimp index of one (C i = 1) indicates that in the relaxed state, the fiber is in collapsed loop form in the case of helical crimp, or ideally plied together at zero length in the case of planar crimp. For a given crimp frequency C f, the crimp index C i is a measure for the crimp amplitude [35].

51 37 Sometimes, the crimp index is denoted as percentage of the crimped length [1, 67] as C k L0 Lc = 100 [%], where L c C C i k = Equation Ci Crimp Width and Crimp Depth These parameters were defined for the use in numerical image analysis of crimped fibers. The crimp width C w is the distance between the midpoints of successive valleys of a crimp, as shown in Figure The crimp depth C d is the perpendicular distance between a peak of a crimp and a line joining the valleys of the adjacent crimp waves. C w, C d or C w /C d characterize the size of individual crimps. The midpoints are determined by numerical regression of fitting curves [88]. For an idealized triangular crimp bow, C w corresponds to λ / 2 and C d corresponds to 2A. Figure 2.22: Irregular Crimp Wave with Crimp Descriptors [91] Crimp Degree The crimp degree K g (Kräuselungsgrad in German) is defined as L 0 / L c, where L 0 is the length of the straightened fiber and L c is the length of the crimped fiber [4, 80, 92]. It is related to the crimp index by C K 1 [67] Equation 2.17 i = g Effective Crimp Diameter ECD and Effective Wave Number EWN For three-dimensional description of fiber crimp geometry, the parameters effective crimp diameter ECD and effective wave number EWN were introduced. The ECD measures the average spatial amplitude of a crimped fiber [95, 96] and is calculated from width values of the planar projections of a rotating fiber under defined loads. The ECD

52 38 describes the average volume occupied by the fiber in space as a cylinder defined by the fiber [5, 96]. It corresponds to the crimp amplitude A of a three-dimensionally crimped fiber. 2 ECD = 2 r dl L, Equation 2.18 where r is the distance between each point of the fiber and the z-axis connecting the endpoints of the fiber. L is the clamping distance and thus the crimped length of the fiber [5, 85]. In practice, measurements are carried out at a finite number of points N. With linear regression analysis, the ECD is approximated by ( S N ) ( S N ) ECD = 2 Equation 2.19 x + where S x and S y are the minimum sums of squares of the distances of N fiber points in two perpendicular planes (x, z) and (y, z) as defined in Equation 2.20 [49, 95, 96]. S x = N i= 1 ( x x) i 2 N xi z i= 1 N z i= 1 i 2 i 2, Sy = ( y y) N i= 1 y i 2 N yi z i= 1 N z i= 1 i 2 i 2 Equation 2.20 The EWN completes the fundamental concept of fully describing crimp by means of the ECD. It is calculated from width values of the plane projections of a rotating fiber and the crimp ratio [5]. The EWN can be interpreted as an average wave number of the fiber [5]. C C 1 =, Equation 2.21 L C l l n n EWN = = 0 n 0 where C n is the total number of waves in the fiber of extended length L 0, and l 0 the curve length of one complete wave. The EWN characterizes the crimp frequency C f of threedimensionally crimped fibers. Since the calculation of C n and thus the EWN is based on the wavelength of the fiber, it takes different values for a helix model and a sine curve. They are distinguished as EWN1 and EWN2 [5, 96]. From the helix model, the EWN1 is calculated as 0

53 39 1 EWN1 = Cr ( 2 Cr ) [5, 49]. Equation 2.22 π D eff Accounting for the three-dimensional nature of crimp, originally based on the helical shape of a wool fiber [95], the ECD does not provide any practically meaningful information on fiber crimp with a periodic two-dimensional shape, overlapped by a random three-dimensional fiber path, like stuffer-box crimp. Furthermore, the problem encountered with all static geometric crimp parameters persists: The ECD depends on the load applied, and what load is the right load to be applied during measurement is indeterminate. Thus, the ECD is not useful for a correlation of fiber crimp production in the stuffer box and resulting fiber crimp characteristics. However, the ECD does give a good measure of the effective volume of a fiber and thus may be correlated with the volume occupied by a loose fiber assembly e.g. in webs and slivers, thus allowing prediction of product properties from fiber crimp characteristics Single Fiber Bulking Capacity For the determination of a parameter defined as single fiber bulking capacity B, a grid of parallel lines is placed over a crimped fiber, parallel to its overall fiber axis. The number of intersections of the fiber with the grid lines is counted. The bulking capacity B of a single fiber is then defined by where I B = Equation 2.23 nl I = number of grid lines intersected n = total number of intersections per fiber L = crimped length of fiber [3] B is chosen as a measure for the volume occupied by a fiber in space, related to the resulting volume of webs and slivers and their bulking capacity (potential to elastically recover their volume after pressure). However, the definition of B is arbitrarily chosen depending on the grid system and thus not a meaningful and objective parameter.

54 Crimp Curvature and Torsion The two parameters, crimp curvature K and geometrical torsion T, derived from the helical spring model for wool fibers in chapter , may be used to accurately describe the spatial path of any curved fiber [7, 15, 67, 97]. Geometric torsion should not be confused with mechanical or fiber torsion, commonly called twist [97]. The mathematical definitions of curvature K and torsion T are l dt K = n ds and l db T = Equation 2.24 n ds Figure 2.23 shows a curve in three-dimensional space with the Cartesian coordinates x, y, z. T and K at any point on this curve can be defined in terms of the three orthogonal vectors n in direction of the radius of curvature, t tangent to the curve and orthogonal to n, and b orthogonal to n and t. K and T are constant along the length of a perfectly cylindrical helix. The curvature is the averaged reciprocal value of the radius of a crimp bent. The dimensions of K and T are reciprocal length units [67, 97]. Z n t ds X b Y Figure 2.23: Curve in Cartesian Coordinates [97] Mechanical Crimp Parameters There are more aspects to fiber crimp than its geometry. The load-extension behavior of a fiber at loads much smaller than the breaking load will have a major influence on the fibers processing behavior and also on the resulting product properties. Thus, dynamic

55 41 crimp parameters have been established characterizing the relationship of loads and extensions in this initial region of the fiber stress-strain curve, which can be attributed to fiber crimp removal [39, 49] Crimp Stability The crimp stability S, also called crimp recovery or crimp permanence, is the amount of crimp recovered after the fiber has been stressed mechanically and/or thermally. Its value depends on the applied load, on the duration of the load and on the relaxation time. It is expressed as the ratio of values of crimp index measured before and after a specified mechanical and/or thermal treatment of the fiber [1, 6, 12, 35, 43, 68]. S L0 Lb = L0 L0 Lb = L0 L L0 L Equation 2.25 L 0 where L 0 is the straightened fiber length, L is the crimped fiber length, and L b the length of the crimped fiber after the load [43]. If the crimp is fully recovered, L b = L and S = 1. If the fiber stays straightened, L b = L 0 and S = 0 [80]. A load-extension diagram of a tensile test for the determination of S is shown in Figure The crimped fiber is straightened at a constant rate of extension up to a preset load level F CS. It is then released again at a constant rate of movement, kept at a minimum load of 0.01 cn/tex, and straightened. S is calculated from the values of crimped and straightened length of the two crimp removal cycles of the test. The crimp stability is more important for fiber processing performance than the initial crimp characteristics of the fiber from the bale and a good measure for determining the impact of processing on fiber crimp characteristics, by comparing fibers from the bale and after various processing steps. The interpretation of crimp stability in terms of physical and chemical fiber structure, measured in cyclic tensile tests, however is extremely difficult, since the results will be determined by various testing conditions such as rate of extension, applied load during cycles and its endurance and relaxation time. A

56 42 practical problem for the interpretation of crimp stability is that a fiber with perfectly spring-like crimp and no hysteresis, and a fiber with no crimp at all, will both have 100% crimp stability [68] Crimp Parameters from the Load-Extension Curve of a Fiber T 1 : LOADING TIME T 2 : RELAXATION C B Figure 2.24: Force-Extension Curve in the Crimp Stability Test [39] In Figure 2.17 and Figure 2.24, the uncrimping region of the fiber is the beginning portion of the curve until the start of the Hookean region. Visual examination of fiber during extension shows that the crimp is removed at the end of this region [54]. The length AB in Figure 2.17 is proportional to the crimp index [2]. The uncrimping energy E corresponds to the amount of energy necessary to uncrimp a fiber. It is proportional to the area enclosed between the initial part of the load-elongation curve, the elongation axis, and the Hookean slope-line represented by the area ABC in Figure 2.24 and Figure 2.17b. The crimp force F 0 is either approximated as the force corresponding to the elongation at the intersection of the prolonged Hookean slope line with the horizontal axis, at B in Figure 2.17b, or as the force where the curve peels off from the straight Hookean slope, see Figure It is considered as the force necessary to uncrimp the fiber [2, 39, 49]. The crimp content C c is defined as F X 0 C c =, Equation 2.26 d L0

57 43 where d is the denier of the fiber and L 0 is the extended fiber length. The crimp content of single fibers may be calculated with the uncrimping energy from the load-extension curve. The distance X is the difference between the abscissa corresponding to the ordinate 0.20 F 0 on the force-extension curve and the point B in Figure 2.17b or Figure X is an arbitrary index chosen to provide information about the fiber contraction due to crimp [2, 39]. Repeated loading, unloading and recovering influences the load-extension behavior of crimped fibers. The fibers have a hysteresis between two crimp removal cycles as shown in Figure 2.24 [5]. Recovery coefficients for uncrimping energy R E and uncrimping force R F were defined to compare fiber crimp before and after a certain treatment [49]. Ei E1 RE = L L, F0, i F0,1 RF = Equation 2.27 Li L1 i 1 where E i, F 0,i and L i denote the values of the uncrimping energy, the uncrimping force, and the uncrimped length from the i th extension cycle [49]. The load-extension curve during crimp removal is the most important aspect of fiber crimp for processing performance, since it characterizes the fiber s reaction to externally applied loads [2, 38, 39]. Furthermore, the measurement of load-extension behavior caused by fiber crimp is more accurate than geometrical fiber crimp description, since it does not depend on subjective visual evaluation or arbitrary definitions of shape characteristics. The crimp removal force F 0 and the uncrimping energy E have a direct physical meaning. However, their determination requires a thorough analysis of the loadextension curve.

58 Bulk Parameters The parameters skein shrinkage SS and bulk shrinkage BS measured on tows of crimped fibers [1] are defined as ( L b L a ) SS = 100 [%] L b Equation 2.28 ( C C ) BS = 100 b a [%] Equation 2.29 C b with L b = length of skein under heavy load before heating [mm] L a = length of skein under heavy load after heating [mm] C b = length of skein under light load before heating [mm] = length of skein under light load after heating [mm]. C a The ease of opening and the bulking capacity of fiber assemblies are parameters characterizing the willingness of fiber assemblies to separate in single fibers and to expand upon pressure release. They have not been defined quantitatively and depend on a combination of crimp and static fiber-to-fiber friction. A high crimp frequency contributes to high bulking capacity, especially in combination with a high crimp index. A low static fiber-to-fiber friction supports a great ease of opening [55]. The filling capacity of a fiber mass is defined as the ratio of the over-all volume (including the air volume) under a defined pressure to the total fiber weight. The compressibility of a fiber mass is defined as the percentage reduction in volume resulting from a specified increase in the applied pressure. The ability of the fiber mass to recover from compression is characterized by the amount of energy freed by the material upon load removal. This resilience of the fibers in bulk is expressed as a percentage of the energy expended by the testing machine in compressing the material between the same limits of pressure. For a perfectly elastic material, the resilience would be 100%, while for a perfectly plastic material, it would be zero [75]. The over-all specific volume, compressibility, recovery and resilience of fiber assemblies can be measured with a cylinder with removable walls and vertically movable lid. The

59 45 overall specific volume decreases with increasing pressure asymptotically approaching a minimum volume. The resilience R [75] is defined as Energy returned by the on removal of load R = specimen 100 [%] Equation 2.30 Energy expended by machine in deforming the specimen Cohesion properties of fiber assemblies like the ease of opening and the bulking capacity are extremely important for the processing performance of the fibers. They are a measure for crimp and static fiber-to-fiber friction resulting from fiber finish, and their interrelationship. Bulk parameters in general have the advantage of a much lower variability and higher reproducibility than single fiber characteristics. However, it is difficult, if not impossible, to extract the effect of fiber crimp in distinction to the influence of fiber finish, which is however necessary to relate processing to crimp production and to get better control over the fiber crimping process. Furthermore, a universally valid, quantitative parameter to access the bulking capacity of fiber assemblies has yet to be determined. 2.4 Crimp Measurement For man-made fibers, regular diagnostic tests for quality assurance of crimp in terms of crimp frequency and percentage are becoming more and more common [78, 79, 84]. However, in industrial practice, fiber crimp is primarily characterized by observing the length change with load of multi-fiber ropes and tows, or the resulting fabric behavior e.g. as volume change under compressive loading [68], since single fiber crimp testing is time-consuming and produces high variability Bulk Methods The characterization of fiber crimp by measurement of fiber assemblies is much easier, less time consuming and drastically reduces measurement scatter in comparison with single fiber methods. Fiber crimp in the measurement of sliver cohesion can be attributed to the very slow increase in force in the initial portion of the load-extension diagram. Differences in ease of opening and bulk capacity of a tow or sliver correlate to

60 46 differences in crimp stability of the fibers. While these bulk parameters are of practical relevance and can be qualitatively correlated with processing performance of fiber assemblies e.g. in drawing or carding, they do not give quantitative information on single fiber crimp [43, 55, 82]. Bulk properties relate to fiber-to-fiber friction resulting from the combination of fiber crimp and finish. Thus, measurement values are not of practical use for controlling fiber crimp production and optimizing fiber crimp independently of finish Rotor Ring Instrument FIBER FEED FEED ROLLER OPENING ROLLER ROTOR RING (SLIVER OF PARALLEL FIBERS) FEED PIPE ROTOR Figure 2.25: Schematic of Rotor Ring Instrument [55, 82] The Rotor Ring instrument was developed for the measurement of ease of opening and the bulk capacity of fiber assemblies. The instrument corresponds to a rotor-spinning unit without yarn-taking and -winding elements, see Figure After extensive opening of the fiber material, a circular fiber sliver is formed in the rotor. The width of this ring sliver, measured in mm on the relaxed sliver after removal from the instrument, is directly proportional to the bulking capacity. The drive power demand of the opening roll corresponds to the opening work, which is directly proportional to the dynamic fiber-to-fiber friction [55, 82].

61 Measurement of Cohesion Length of Slivers The tow cohesion of card and draw-frame slivers can be measured with simple tensile tests at a slow rate of extension. The maximum force is interpreted as the characteristic cohesion length of a sliver and is interpreted as a measure for processing performance depending on fiber-to-fiber friction and fiber crimp [82] Standard Test Method For Bulk Properties Of Textured Yarns [D 4031] This test method covers the measurement of the change in length of a tensioned skein of textured yarn due to change in crimp characteristics caused by exposure to wet or dry heat. The method is limited to crimped, continuous multi-filament strands ranging from 1.7 to tex (15 to 8000 denier) [1]. A skein of yarn of defined linear density is subjected to a crimp development medium using a specified loading routine. As the crimp is developed or shrinkage occurs in the yarn, the skein changes in length. The lengths of the skein under specified tension forces are used to calculate the value of bulk shrinkage, crimp contraction, skein shrinkage, or crimp recovery, depending on the loading and exposure procedures [1]. In industrial fiber production, this method is applied on a regular basis for quality control of tows of crimped staple fibers before cutting Single Fiber Methods Measurement of Crimp Geometry Measurement methods for crimp geometry range from tedious subjective techniques to sophisticated computerized three-dimensional assessments [5, 15, 87]. The Standard Test Method for Crimp Frequency of Man-Made Staple Fibers [D3937] describes how to determine the crimp frequency of man-made staple fibers for quality assurance purposes in industrial practice. It is applicable to all crimped fibers provided the crimp can be viewed two-dimensionally as a sine-wave configuration. Counting the number of

62 48 crimps along the fiber length and the measurements of crimped and straightened length are either performed on single fibers with low magnification, on projected images of single fibers or on fiber tufts [1, 82, 92, 93, 94]. The fiber straightening is either done manually, by pulling the fiber straight on a board along a ruler with two fingers, or with help of a crimp balance. A crimp balance consists of a clamping unit with a movable lower clamp, providing a scale for length measurement, and a mechanical device for the determination of the load applied to the clamped fiber. With torsional or crimp balances, the fiber may be subjected to more defined and more accurate loads, and the lengths and length changes may be measured more accurately than with manual methods [82, 94]. For handling purposes, the fiber is provided with a pre-tensioning weight sufficiently small not to produce any perceptible change in the crimp, and clamped into the gages of the crimp balance. For the determination of the crimped fiber length, a small tension is applied to the fiber. For a 4 dtex fiber, e.g., the initial tension is found suitable as 48 µn/tex, resulting in applied loads of 19.6µN. For measuring the straightened length, a tension of 0.98 cn/tex or a load 3.92 mn is recommended. These values are chosen arbitrarily, and the testing of fine fibers and the necessary application of very small, suitable loads is limited by the sensitivity of the crimp balance. For crimp stability measurements, the fiber is stressed under a defined load for a defined time period, relaxed and recrimped again [43, 79, 80]. The tensile load on the fiber may also be increased stepwise. The movable clamp is adjusted with each load increase. The stepwise fiber length changes due to crimp removal become smaller and smaller, and finally invisible to the human eye. It is assumed that at this point, the decrimping process is completed and the straightened fiber length can be measured [4, 12, 43, 80, 94]. The tension, with which the last length increase could be detected, is called crimping resistance. This method of invisible length changes is more accurate than observing the crimp nodes disappearing. With the attempt of removing small remaining crimp nodes, the test continues to high tensions already in the region of fiber stretching [80].

63 49 A drawback of crimp balances is, that the applied pre-tension and the straightening tension are either set according to a specific nominal count that may differ essentially from the count of each single fiber [94], or are still estimated visually. Furthermore, the load sensitivity and resolution with commercial crimp balances are small compared to the small loads acting on single fibers in the crimp region. Various prototype devices for two- or three-dimensional image analysis of the crimp geometry of single fibers have been developed. The only commercialized measuring unit up-to-date however is the optical sensor optionally implemented in the Textechno FAVIMAT, as described in section The automatic crimp measurement instrument ACMI built by DuPont and used for research purposes, consists of a fiber-tensioning device with a load sensor and an optical scanner measuring the fiber geometry in spatial coordinates for any discrete length of the vertically mounted fiber. Crimp frequency and extensibility parameters, as well as curvature and torsion, are calculated by fitting a polynomial to successive segments of the fiber with the least-square method [15, 97]. Another measuring system, designed for the determination of the three-dimensional fiber geometry and the effective crimp diameter ECD under defined tensions, consists of a rotating clamping device and a projection system [5, 23, 32, 87]. Due to limitations of the rotary crimp apparatus, the spatial crimp geometry and the ECD were further calculated from the fiber projections on two perpendicular planes [96]. Based on the assumption that crimp in synthetic fibers is mainly planar, systems to measure crimp geometry two-dimensionally have also been developed [87, 88, 91, 93]. For the image capture, the fibers need to be attached to glass microscope slides, free of or under a defined tension [87, 93]. Recent two-dimensional crimp image analysis is done with a CCD camera and/or a scanner and sophisticated statistical software [87, 91]. All basic geometrical crimp parameters can be measured or calculated (Figure 2.22, page 37), including crimped length, straightened length, crimp percentage, crimp frequency, crimp

64 50 height C h, crimp width C w, crimp angle C a, crimp amplitude C am, and extrapolated rise of a crimp bow h 0. Furthermore, a spatial frequency C sf as the dominant crimp frequency obtained by the Fast Fourier Transform (FFT), a crimp intensity factor C I and the crimp sharpness C s are defined and calculated [88]. Visual counting of crimp bows in single fibers or crimp index measurements with crimp balances are useful for detecting major differences in fiber crimp for quality control purposes. Due to the lack in accuracy and reproducibility of these methods, they are not useful in research and development, where minor differences are important. Major difficulties are [1, 2, 3, 4, 5, 12, 13, 15, 18, 43, 67, 78, 80, 82, 91, 93, 94, 97]: Fiber crimp is very irregular in terms of frequency, shape or amplitude. Often, it is subjective whether to interprete a certain bend or loop in a fiber as crimp bow. Large bends are straightened out in processing. However, smaller and sharper bends are likely to persist throughout processing. The question arises, which crimps should be ignored as large curls and flat waves of low frequency, and which crimps of higher frequency and sharper waves should be counted. Single fiber testing is very difficult and time consuming due to random crimp shapes and thus high variance of single values (variation coefficient 25 to 60%) and much time consumption of each single test (1 2 min/fiber). The high variance requires large sample size, leads to even more time consumption, and makes it even more important to assure random sampling. To measure the crimped fiber length, the fiber must be overall aligned. This requires force, and the length of the fiber varies with the magnitude of this aligning force. Some bends seem to disappear with alignment due to the unstable state of the crimped fiber. Fibers show hysteresis of crimp under load. Thus, the history of a fiber influences crimp, which has to be considered carefully in crimp measurements. Crimp is generally analyzed two-dimensionally. However, this produces inaccurate results, since the crimp bows perpendicular to the plane are ignored. A cylindrical helix model is suitable for conjugate fiber crimp, but not for mechanically crimped fibers. It is difficult to determine the straightened, but unstretched length of a fiber. Initial true fiber strain is not visible, but may essentially alter the observed fiber length. Due to the crimp irregularity, true fiber strain may even occur in some parts of the fiber, while in other parts, crimp nodes are still visible.

65 51 During the separation of single fibers from the staple and handling for sample preparation, the fibers are already subjected to tensile loads. Practically, this cannot be avoided and may result in partial crimp removal and alter the crimp characteristics, especially with fibers of low crimp stability. Carded material is separated into single fibers, but an essential part of the fiber crimp has already been removed through the carding action. The crimp frequency counted in a chip of fibers may differ from the actual crimp frequency of the single fiber, since the geometry of the single fiber in the staple is very much influenced by frictional interactions with other fibers in the staple. Geometrical crimp shapes and parameters have not been quantitatively correlated to physical characteristics or the structure of fibers. Thus, no practical relevance of geometrical crimp parameters for processing performance of crimped fibers or resulting product properties has been established yet. With the availability of sophisticated scanning and computerized data evaluation devices, image analysis provides an efficient and objective means for quantifying crimp morphology two- or even three-dimensionally [4, 87, 88, 94, 97]. The difficulty persists however, to determine and apply the load for the perfectly crimped state of the fiber. Parameters identifying a bend or a non-alignment in a fiber as a crimp can only be defined arbitrarily. The straightened fiber length can be determined accurately, even for irregularly crimped fibers, without actually straightening the fiber, by scanning along the fiber and summing up the lengths of incremental fiber pieces of the crimped fiber. The meaning and practical relevance of the geometrical crimp parameters thus obtained is still questionable. Even a perfect geometrical description of fiber crimp does neither give information about how the fiber will behave during processing, nor relate to important settings during crimp production such as temperature or pressure conditions in the stuffer box. The information content of a comparison of fiber images before and after a certain processing step is critical, because of the extremely high variation of geometrical shapes between fibers. In laboratory tests, the very same fiber can be visualized with image analysis before and after treatment, but in production, this is not possible. It is thus difficult to distinguish between the effect of the processing step on the fiber geometry and the effect of the inter-fiber variance on the measured fiber geometry. Thus, image

66 52 analysis is a useful tool for fiber crimp visualization, but only useful for the production process when combined with an analysis of the load-extension behavior of crimped fibers Cotton Inc. Fiber Tester ( Hook Method ) A prototype instrument was developed to determine the breaking strength of single Cotton fibers. The signal from the force transducer can also be interpreted in terms of fiber crimp in Cotton fibers present at clamping. To facilitate loading, airflow is used to pull the fiber ends between the open jaws, resulting in a pre-tension of less than 0.1 g in the fiber at clamping [47, 72]. Figure 2.26: Diagram of Hook Method for Single Fiber Strength [47] Figure 2.26 shows a diagram of the apparatus. The hook is mounted directly to a force transducer. By a sufficiently great hook radius, it is assured that fibers do not break in the hooking zone. The distance between the hook and the clamps is 3.2 mm. The voltage output from the transducer is proportional to the fiber tension. The transducer has 70 ms delay time. The hook elevator moves at a constant speed of 1 mm/sec (31.5% displacement rate) [47].

67 53 Figure 2.27: Example of Force Transducer Output Wave Form [47] In the typical voltage output in Figure 2.27, 70 ms delay time is recorded before the computer starts the hook elevator. The part of the curve between t = 0 and t = t 0 characterizes the crimp in the fiber. The curve between t = t 0 and t = t b is the voltage ramp representing the stress-strain response of the fiber. V b is the voltage at fiber break at time t b. The breaking force is proportional to V b /2 due to the looped geometry. The elongation is computed using the time period t b t 0. t 0 is determined at the intersection of y = V 0 and the regression line (y - V 0 ) = a (t - b), where the regression parameters a and b approximate the values m and t 0 in Figure 2.27 ( (y - V 0 ) = m (t - t 0 ) for fiber extension). The percent crimp in cotton fibers is calculated [47] as % Crimp ( 1mm ) 100t hook speed 100t 0 0 = s gage length 3.2mm = Equation 2.31 The suction mechanism for clamping is very advantageous for fiber crimp measurements, since fiber handling is made easier and variability in fiber mounting due to handling is reduced. Orienting and clamping the fiber with help of airflow is a gentle, controllable and little variable mean to prepare the fibers for the actual crimp removal test. Unfortunately, the Cotton Inc. tester is only rigidly set up for Cotton fibers, with a defined clamping length and a fixed speed, and is not commercialized. Furthermore, the implemented force transducer is not sensitive enough to actually quantify the voltages and thus loads occurring during crimp removal, as can be seen from the low resolution in Figure 2.27.

68 Lenzing VIBROTEX The single fiber tensile tester VIBROTEX has been designed especially for fiber crimp measurement, having a load sensitivity of 10-3 cn and a measuring range of 2 cn. The upper clamp of the testing unit is connected to a load cell, whereas the lower one moves vertically at a constant rate of extension. Outputs are gage distance and measured load. In order to determine tensions instead of forces, the nominal titer is provisionally set before testing [94, 98]. For better visualization of the extensive load range in dependence of elongation, a logarithmic amplifier for the load is incorporated in the instrument. In the test display, the elongation is plotted on the x-axis linearly to the fiber extension, while the load is plotted on the y-axis in a logarithmic scale. The testing speed of the instrument is variable from 0.1 mm/min upwards. Testing may be performed in direction of crimp removal or crimp recovery, to obtain a hysteresis for the characterization of crimp stability [94]. Approximate straightness of the half-logarithmic diagram was observed with various regularly crimped fibers. Thus, the crimp removal curve is approximated by an exponential function of the shape M L σ = σ 0 e. Equation 2.32 For irregularly crimped fibers, the curve becomes concave upwards, but is still approximated by two similar exponential functions with exception of a transition area. The deviation of the decrimping curve from the ideal straight line is a measure for crimp regularity [94]. The standard set-up for crimp testing includes an initial extension of the fiber up to almost complete crimp removal. The actual measurement starts after a rest period in this state. The fiber is allowed to recrimp almost completely by reducing the clamping distance. It is assumed that the two ends of the load-extension curve during recrimping will proceed linearly in the half-logarithmic scale and can be extrapolated from the four points (σ 1 L 1 ) to (σ 4 L 4 ), each two located in one of the linear portions of the measured part of the load-extension curve.

69 55 The two tension limits σ g and σ z, at which the fiber is assumed to be either straightened or completely recrimped, are defined by agreement, by existing standards as 0.01 cn/tex and 1 cn/tex [99], or on the basis of preliminary testing. The corresponding fiber lengths L g and L z are not reached in the measurement cycle, but can be extrapolated from (σ 1 L 1 ) to (σ 4 L 4 ), as shown in Figure 2.28 [94]. Figure 2.19 shows the mean crimp diagrams of two acrylic samples. Both samples reveal the same crimp removal extension and the same load-extension response in a normal tensile test beyond the crimp removal region. However, the load-extension responses during crimp removal differ radically [80]. The crimp frequency of the fine, regularly crimped sample A is twice as high as the crimp frequency of sample B [94]. If the acrylic samples A and B can be approached with straight lines, as shown in Figure 2.29, the question arises how the curves continue into the tensile region. Supposedly, they have the same tensile response. This however would mean, that one of the fibers must have a sudden change in slope between crimp and tensile region, which is never the case in a continuous extension process. A further problem with the suggested logarithmic analysis of portions of the curve, to obtain partially linear crimp and decrimp slopes, is that there is no physical significance attached to these slopes [68]. Figure 2.28: Crimp Removal of an Irregularly Crimped Fiber with Low Crimp Frequency and Extrapolation of Crimped and Straightened Fiber Length [94]

70 56 Figure 2.29: Characteristic Crimp Lines for Acrylic Fibers A Regularly Crimped, B Irregularly Crimped [94] Textechno FAVIMAT The Textechno FAVIMAT is a single fiber tensile tester capable of measuring the very small forces occurring during crimp removal. The testing unit is countersunk in the instrument and can be closed from environmental influences with a sliding door. The clamps are operated by compressed air. The gage length is variable between 5 and 100 mm [100]. In addition to the extremely high resolution of cn at a measuring range of 160 cn, the compensating force measuring system guarantees high stability with respect to external vibrations. The instrument includes a measuring head for the determination of the vibrational fiber denier [100]. In addition to standard tensile tests, alternating load tests and relaxation tests may be performed [100]. For crimp tests, initially crimped fibers or filaments are tested for their tensile behavior at very low tensions. The curve of crimp force versus elongation, the crimp extension ( percent crimp ) and the decrimping point, where the crimp extension turns into material extension are determined [84]. An additional optional measuring system with an opto-electronic sensor scans the crimped fiber two-dimensionally and permits the evaluation of crimp geometry in terms of crimp frequency and amplitude [84].

71 Summary and Conclusions Even though the production of staple crimp in synthetic fibers offers great opportunities for the customization of fiber processing properties and quality of the final product, little research has been done to correlate crimp production parameters, resulting fiber crimp properties and subsequent fiber processing performance and fabric properties. This might be due to the fact that the effect of fiber crimp is often underestimated in favor of fiber finish, since both parameters interfere and interdependently influence the processing performance of fibers. Furthermore, measuring fiber crimp quantitatively and accurately remains a problem. Geometrical descriptors are generally unsuitable for an objective characterization of crimp, since the variability of geometrical crimp shapes between and within fibers is immense, while the measuring methods are tedious and time consuming. Even with computerized image analysis, subjective criteria have to be determined whether to consider a crimp as a crimp or whether to interprete a certain wave in a fiber just as an overall curve in the fiber axis. It is common knowledge that intensive fiber crimp results in bulky fabrics, but this vague qualitative statement is neither useful for improvement of the crimping process nor for control of the processing performance of a crimped staple fiber into a fabric. The measurement of bulk properties of fiber assemblies is useful for the prediction of fiber behavior during processing and for comparative quality control. However, the effects of single fiber crimp and of fiber finish cannot be separated and bulk methods are thus not useful to gain control over the crimp production process. More important than the geometrical aspect of staple fiber crimp is the extensional response and the crimp stability resulting from loads applied to the fiber. This behavior is characterized by the load-extension curve of a fiber during crimp removal or cyclic crimp removal and recovery. Crimp removal force and uncrimping energy are meaningful parameters, but it is difficult to locate their position on the load-extension curve. Only

72 58 recently, load cells of sufficient sensitivity to measure stress-strain data of single fibers in the crimp removal region have been embodied in commercial instruments [68]. With the resulting availability of almost continuous data of the whole crimp removal region, it could be quantified how the shape of the load-extension curve of a fiber influences the crimp removal during processing and thus the processing performance. Not only the initial crimp level imparted to the fiber during crimping, but also the crimp stability is important. Fiber crimp stability results from inherent material properties and settings during crimp productions, and determines with processing settings together how much crimp is left after each processing stage of the fibers and finally in the product. For an optimization of the crimp production process and the fiber processing performance, quantitative, meaningful parameters need to be established characterizing the crimp pull-out and thus the fiber extensional behavior as response to applied loads. With a quantitative correlation between load and crimp pullout, processing conditions and crimp parameters could be adjusted to each other by altering machine parameters and crimp production settings.

73 59 3 INSTRUMENTATION There are currently two commercial instruments specifically designed for the measurement of fiber crimp available in the U.S. and Europe: The VIBROTEX by Lenzing, Austria, and the FAVIMAT by Textechno, Germany. The essential feature of these two instruments in comparison to traditional tensile testing devices such as the INSTRON is the high sensitivity and resolution of the force measuring head, which can measure the extremely small forces occurring during crimp removal of single fibers. For the research project at North Carolina State University, the FAVIMAT was chosen due to the following advantages: Several companies in the US nonwoven industry already use this instrument for tensile fiber testing Greater flexibility in testing procedures (gage length, testing speeds, testing sequences) Higher load resolution ( cn FAVIMAT vs cn Vibrotex) Linear display of load values vs. logarithmic display of load Easier access to raw data Fineness measurement available in same testing unit after crimp test Testing unit protected from environmental influences Figure 3.1 shows the measuring instrument Textechno FAVIMAT, Figure 3.2 shows the countersunk measuring device with the two measuring heads for force and fiber count, and Figure 3.3 lists the major technical data of the FAVIMAT. Figure 3.1: Textechno FAVIMAT Single Fiber Crimp Tester