CHARACTERIZATION OF NECK BONDED LAMINATE

NONWOVEN WEBS AND PHOTO PRINTING PAPER

By

VINAY BHUMANNAVAR

Bachelor of Engineering in Mechanical Engineering

Visvesvaraya Technological University

Belgaum, India

2006

Submitted to the Faculty of the Graduate College of the

Oklahoma State University in partial fulfillment of the requirements for

the Degree of MASTER OF SCIENCE

December, 2009

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CHARACTERIZATION OF NECK BONDED LAMINATE

NONWOVEN WEBS AND PHOTO PRINTING PAPER

Thesis Approved:

Dr. Hongbing Lu

Thesis Adviser

Dr. J. K. Good

Dr. R. D. Delahoussaye

Dr. A. Gordon Emslie

Dean of the Graduate College

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ACKNOWLEDGMENTS I take this opportunity to express my gratitude towards my advisor Dr. H. Lu for his support, guidance and encouragement throughout my research. His inspirational words have instilled my belief in perseverance and self driven progress. I am also thankful to Dr. J. K. Good and Dr. R. D. Delahoussaye for their suggestions, and acceptance to be in my committee. Thanks to Web Handling Research Center (WHRC) and Hewlett-Packard (HP) for the financial support provided through the completion of this project. I am grateful to Mr. Ron Markum for his valuable suggestions and advice with regards to the experiments performed at the WHRC. I would like to acknowledge and thank Mr. Vijay Subramanian, Mr. Yao Ren and Mr. Siddharth Vaijapurkar who have always supported and helped me with fruitful discussions in times of confusion. Mr. Jerry Dale and Mr. John Gage deserve a special mention for their help with machining of experimental setups. I thank Ms. Sarah Staggs for her help with SEM imaging of nonwovens. I am also thankful to Mr. Arjun Reddy, Mr. Karthik Dasari, Mr. Amol Patil, Mr. Rahul Mirani, Miss Aparna Jain, and Mr. Kyunghan Chung for their help with experimental setups for creep and camber tests, and microscope observation. I would like to thank and express my heartfelt appreciation towards all my colleagues at the WHRC, who made this a pleasant experience and helped me at some point or the other. I am thankful to my fiancée for her patience and support during the past two years. I am indebted to my parents and sister for believing in me, and giving me a chance to pursue higher education. Their love and encouragement are my motivation to work harder every day.

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TABLE OF CONTENTS

Chapter Page I. INTRODUCTION ......................................................................................................1

Webs, Rolls and Nomenclature................................................................................1 Thickness Variations and Associated Defects .........................................................1 Influence of Viscoelastic Behavior ..........................................................................2 Nonwoven Webs and Neck Bonded Laminates.......................................................2 Laminated Photo Printing Paper ..............................................................................3 Thesis Objective.......................................................................................................3 II. REVIEW OF LITERATURE....................................................................................5 Non-homogeneity in Nonwoven Webs ....................................................................5 Elastic Property Characterization and FEM Modeling ............................................6 Research Potential ....................................................................................................8 III. CHARACTERIZATION OF NECK BONDED LAMINATE NONWOVEN .......9 Measurement of Thickness ...................................................................................10 Characterization of In-Plane Properties .................................................................11 In-Plane Elastic Properties ..............................................................................12 Effect of Specimen Size ..................................................................................19 Camber Quantification .....................................................................................20 Determination of Poisson’s Ratio ....................................................................21 In-Plane Viscoelastic Properties ......................................................................22 Stress Relaxation ..............................................................................................24 Creep ................................................................................................................24 Time-temperature Superposition .....................................................................25 Improved Relaxation Approach .......................................................................26 Viscoelastic Characterization in MD, CMD ....................................................29 Validation by Creep .........................................................................................31 Characterization of Out-of-Plane Properties ..........................................................35 Temperature Effects on Nonwoven web ................................................................37 Coefficient of Thermal Expansion ...................................................................40

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IV. CHARACTERIZATION OF LAMINATED PHOTO PRINTING PAPER……43 Observation of Laminate Structure ........................................................................43 Standards Used for Uniaxial Testing .....................................................................49 Characterization of Elastic Properties of HP Raw Base Photo Paper ....................54 Characterization of Elastic Properties of HP Advanced Photo Printing Paper ......59 Characterization of Elastic Properties of HP Vivid Photo Printing Paper .............65 Characterization of Out-of-Plane Properties of HP Photo Papers .........................69 Prediction of Polyethylene Film Properties by Simulation of Lamination ............70 V. CONCLUSIONS ....................................................................................................70 Neck Bonded Laminate Nonwoven Webs .............................................................70 Photo Printing Paper ..............................................................................................71 VI. FUTURE WORK...................................................................................................72 VII. REFERENCES .....................................................................................................73

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LIST OF TABLES

Table Page 1 Coefficients of Prony series ...................................................................................30 2 Coefficients of Prony series for CMD ...................................................................35 3 Coefficients of Prony series fitted to Master curve at 167 0F ................................39 4 Coefficients of Polynomial expressions fitted to stack test data. ...........................68

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LIST OF FIGURES

Figure Page 2.1(a) FE model for MD, (b) FE model for CMD. .....................................................6 2.2(a) Modeling thermal bond points, (b) Construction of Mesh ...............................6 2.3 Representation of constrains and boundary conditions used. ...............................7 3.1(a) Fiber orientation and bond pattern, (b) Damage caused to fibers ....................9 3.2 Setup used for measurement of thickness. ..........................................................10 3.3 Pressure vs. Thickness plot generated and polynomial fitted. ............................11 3.4(a) Stress-strain relationship in MD and (b) CMD ..............................................12 3.5(a) & (b): Comparison of MD and CMD strains .................................................12 3.6 Testing Setup ......................................................................................................13 3.7 Machine compliance ...........................................................................................13 3.8 (a) Specimen dimensions-ASTM 5035, (b) Specimen locations ........................14 3.9 Stress-strain relationship of web in MD from along the width. ..........................15 3.10 Stress-strain relationship of web in MD at various radial locations .................15 3.11 a) Necking at 10% strain (b) Fiber Pull-Out (c)Multi-point failure ..................16 3.12 Failure observed in CMD..................................................................................18 3.13 Stress-strain relationship of web in CMD at various radial locations ..............18 3.14 Stress-strain relationship of web in 450 orientation at various radial locations 18 3.15(a) 450 specimen (b) Onset of wrinkling (c) Failure at top notch ......................18 3.16 Comparison of Stress-strain relationship in MD, 450 orientation, and CMD ...19 3.17 Stress-strain relationship of varying specimen sizes. (Dimensions in inches ...19 3.18 Nonwoven web laid out for Camber quantification ..........................................20 3.19 Graph illustrating camber modeling by modified Shelton’s method ................20 3.20 Screenshot of DIC for determination of Poisson’s ratio ...................................21 3.21 Comparison of stress-strain relationship in 450 by experiment and equation....22 3.22 Stress-strain relationship of web in MD at various strain rates ........................23 3.23 Stress-strain relationship of web in CMD at various strain rates ......................24 3.24 Demonstrative illustration of typical relaxation curve of a polymer ................27 3.25 Step strain in theory ..........................................................................................27 3.26 Error due to ramp strain history ........................................................................27 3.27 Demonstration of Curve shifting of multiple curves ........................................28 3.28 Demonstrative extraction of relaxation data from ramp of T3 .........................28 3.29 Setup used for relaxation experiments ..............................................................29 3.30 Ramp responses MD at 70 0F and 135 0F .........................................................30 3.31 Raw relaxation data collected from experiments ..............................................31 3.32 Master curve obtained by TTS & Prony series fit ............................................31

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Figure Page 3.33 Setup for creep test............................................................................................32 3.34 Creep compliance of Nonwoven web ...............................................................33 3.35 Comparison of relaxation and creep data..........................................................34 3.36 Master Curve and Prony series fitted for CMD ................................................35 3.37 Radial Stress-Strain response of a stack of nonwoven web ..............................36 3.38 Radial creep compliance of nonwoven web .....................................................37 3.39 Approximate temperature profile during transportation ...................................38 3.40 Master Curve at 167 0F and Prony series fitted to it .........................................39 3.41 Setup used for determination of CTE ...............................................................40 3.42(a) Ux def field & (b) Uy def field before compensation ..................................41 3.43(a) Ux def field & (b) Uy def field after compensation .....................................42 4.1 Layer structure of HP Raw .................................................................................43 4.2 Laminate structure of HP Advanced ..................................................................44 4.3 Laminate structure of HP Vivid .........................................................................45 4.4 Setup Figure ........................................................................................................46 4.5 Failure by ASTM D828 ......................................................................................46 4.6 Failure by TAPPI 494 OM6 ................................................................................46 4.7 JIS 2201 Specimen ..............................................................................................47 4.8 Double Dog-bone specimen ................................................................................47 4.9 3D model created in ProENGINEER .................................................................47 4.10 Aluminum template machined ..........................................................................47 4.11 Failure regions of specimens made in accordance with ISO 527 .....................48 4.12 Stress-Strain curves obtained during a repeatability test ..................................49 4.13 Stress-strain relationship of HP Raw in MD.....................................................50 4.14 Stress-strain relationship of HP Raw in CMD ..................................................50 4.15 Stress-strain relationship of HP Raw in 450 orientation ...................................50 4.16 Comparison of stress-strain curves of HP Raw ................................................51 4.17 Comparison of stress-strain curves of HP Raw 450 by equation and theory.....52 4.18 Stress-strain relationship of HP Raw in MD at various strain rates .................53 4.19 Stress-strain relationship of HP Raw in CMD at various strain rates ...............54 4.20 Stress-strain relationship of HP Raw in 450 at various strain rates...................54 4.21 Stress-strain relationship of HP Advanced in MD ............................................54 4.22 Stress-strain relationship of HP Advanced in CMD .........................................55 4.23 Stress-strain relationship of HP Advanced in 450 orientation ..........................56 4.24 Comparison of stress-strain curves of HP Advanced ......................................56 4.25 Comparison of stress-strain curves of HP Advanced 450 equation and theory .57 4.26 Stress-strain relationship of HP Advanced in MD at various strain rates.........58

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Figure Page 4.27 Stress-strain relationship of HP Advanced in CMD at various strain rates ......58 4.28 Stress-strain relationship of HP Advanced in 450 at various strain rates ..........59 4.29 Stress-strain relationship of HP Vivid in MD ...................................................59 4.30 Stress-strain relationship of HP Vivid in CMD ................................................60 4.31 Stress-strain relationship of HP Vivid in 450 orientation ..................................61 4.32 Comparison of stress-strain curves of HP Vivid ..............................................61 4.33 Comparison of stress-strain curves of HP Vivid 450 equation and theory ........62 4.34 Stress-strain relationship of HP Vivid in MD at various strain rates ................63 4.35 Stress-strain relationship of HP Vivid in CMD at various strain rates .............64 4.36 Stress-strain relationship of HP Vivid in 450 at various strain rates .................64 4.37 Stack test results of HP Raw ...........................................................................65 4.38 Stack test results of HP Advanced ...................................................................66 4.39 Stack test results of HP Vivid ..........................................................................67

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CHAPTER I

INTRODUCTION

Webs, Rolls and Nomenclature Many of the commodities used today are available in the form of thin sheets and rolls. Products such as diapers, polythene covers, etc. are produced in the form of sheets, and stored as rolls at some point during the production process. Such thin continuous strips and sheets of material for which the length dimension is very large relative to the width, and the width dimension is very large relative to the thickness, are termed as ‘Webs’. These may be webs of paper, polythene, nonwovens, metal sheets, carpets, countertops, etc., and the process of winding, unwinding and storage of such webs is called web handling. Properties in different directions have unique effects on webs and rolls, and hence on their winding and storage respectively. It is therefore essential to establish the nomenclature required to distinguish properties in all directions. The lengthwise direction of the web which is same as the direction of web-line during a winding process is known as the Machine Direction (MD), while the width of the web is referred to as the Cross Machine Direction (CMD). The Z direction which constitutes the thickness is known as the Radial Direction in a wound roll. Thickness Variations and Associated Defects Webs are produced either by extrusion or assimilation of fibrous structures held together by chemical or thermal bonds. Both techniques have evolved over time, but not to the extent that inherent thickness variations along the width of the webs can be completely ruled out. The resulting change in basis weight causes uneven winding tensions to be applied at each of these points. Resulting defects include baggy lanes, wrinkling, buckling, local strains, etc., and may transform into something more drastic when the web is rolled, or act as hindrances during processes such as printing, converting and slitting. The change in thickness has been historically quantified as the change in basis weight recorded along CMD, or as a profile of MD strains at various CMD locations. The properties have been averaged over the width to calculate winding parameters. On certain occasions, a somewhat uniform thickness profile is obtained by adding an extra layer of opposing basis weight profile to compensate for the

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inherent basis weight variations. In either case marginal improvements have been observed, with the issue of local variations and consequent problems still unresolved. Influence of Viscoelastic behavior Another important factor influencing the choice of winding parameters and resulting roll quality is the time dependent behavior of webs. Many webs manufactured today contain polymers as the significant constituent. Most polymers display time dependent behavior, i.e. the material properties change over a period of time under constant stress or strain. Such materials are termed as ‘Viscoelastic’ materials, and though strictly speaking most of the materials in nature are viscoelastic, we only focus on those which change their properties to a great extent over the product’s lifespan. Such material behavior is not suitable for webs, as they are rolled at a particular stress level onto a core, and stored or shipped to various locations. The resulting loss of radial pressure can cause roll defects such as telescoping and falling apart of rolls. Although one may envision higher winding tensions as a solution to this problem, such actions may result in higher overall CMD strains or aggravate the severity of localized strains. Nonwoven Webs and Neck Bonded Laminates (NBL) Classified as a fabric, nonwovens essentially comprise of materials that are neither woven nor knitted, but instead bonded together by chemical, mechanical, solvent or thermal treatment. Cotton and polymer fibers of length 0.25-5 inches constitute a significant portion of raw materials used, owing to which nonwovens have become the alternatives to Polyurethane foams for a number of applications. Unless reinforced with additional polymer films by chemical or thermal bonding, nonwovens generally lack strength. The orientation of fibers often depends on the intended use of the nonwoven, which is mostly for products related to hygiene, medical needs, and filtration. Diapers, feminine hygiene products, adult incontinence products, wipes, bandages, surgical gowns, petroleum filters and geotextiles products such as soil stabilizers are some examples. The most commonly used nonwovens are the Staple and Spun Bonded nonwovens. Staple nonwovens are made in multiple steps of spinning and are wet/dry laid, while Spun Bonded nonwovens are manufactured in one single step. The subject material of this study- Premium NBL nonwoven made by Kimberly-Clark, uses Spun Bonded nonwoven with area density of 0.85 ounce per square yard (Osy) as one of the constituent materials along with Kraton rubber, and is designed for use in diapers. Spun Bond webs from 60 inches diameter and 120 inches wide rolls are passed through an oven and stretched to a reduction of 57% in width. This web is then laminated with Kraton rubber under a nip load, following which the laminate is cooled and slit into 11 slits of 5 inch width, 30 inch diameter rolls wound at a tension of 0.7 pli. While the stretching of constituent Spun Bond is meant to reduce the severity of viscoelasticity, the addition of Kraton with an opposite basis weight profile is

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meant to reinforce the web and compensate for previous thickness variations. The overall area density of the resulting NBL is approximately 4.17 Osy. Laminated Photo Printing Paper Laminated paper is used in the manufacturing of many products such as packaging materials, adhesive tapes, insulations, poster and photograph printing papers, books, etc. It is usually manufactured as a combination of layers of paper (made of polymer/natural fibers forming the base), and polymer films. An additional layer of coating is occasionally desired to maintain specific end user needs such as optimum reflectivity and surface finish. The lamination of such polymer film and miscellaneous coatings along with the base paper results in a product, whose properties are influenced by parameters such as stacking sequence and stacking orientation, lamination pressure, lamination temperature, etc. The effective thickness of polymer film added changes after lamination and hence needs to be determined to calculate its effective properties within the laminate. Thesis Objective The major problems associated with the winding, storage and shipping of NBL nonwovens are the Slit Width Growth (SWG), floppy edges and falling apart of rolls during seasonal shipping. SWG can be caused due to multiple reasons, the first one of them being improper winding tensions applied during the final winding process. This problem was partially solved by monitoring the winding tension real time using load cells. The other reason speculated to be the cause for SWG is the viscoelastic recovery of the material in CMD either from stretching of the Spun Bond or from the stresses developed within the roll due to applied winding tensions and storage. Floppy edges and camber are mainly a result of inconsistent basis weight profiles along the CMD. In this study, the in-plane stress-strain relationship of the nonwoven web was measured, and hence the effects of possible density variations were determined. In-plane and out-of-plane viscoelastic behavior has been characterized to approximate the material properties over a period of 3-4 weeks. Further, high temperature properties of the material have been studied as the webs are to withstand temperature of about 165 0F while they are shipped in containers traveling through equatorial regions. The elastic in-plane and out-of-plane properties of three different laminated photo printing papers manufactured by Hewlett-Packard (HP) were characterized as a latter part of this study. The first product was the base paper known as HP Raw photo base paper, which is used as a base layer in the manufacturing of the other two products, namely HP Advanced photo printing paper and HP Vivid photo printing paper. On initial visual inspection, HP Advanced and HP Vivid photo printing papers were speculated to consist of a Polyethylene film laminated on to HP Raw base paper, along with Silica coating for a glossy finish. In order to better understand the lamination structure and obtain approximate thickness values of individual constituents in HP Advanced and HP Vivid photo printing papers, samples of

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all three products were studied under a high resolution microscope. The measured thickness values, along with elastic stress-strain data of base and laminated papers obtained from uniaxial tensile tests enabled the simulation of lamination in Abaqus. This inverse method enables the determination of effective modulus values of constituent Polyethylene film.

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CHAPTER II

REVIEW OF LITERATURE

Non-homogeneity in Nonwoven Webs Non-homogeneities in nonwoven webs lead to thickness variations along the CMD during winding, and result in local strains while being stored as a roll. Any effort to characterize the mechanical properties would hence depend on the specimen location in the roll, and along the CMD. Kim [1] established a relationship between the Fiber Orientation Distribution (ODF) and mechanical anisotropy of thermally point-bonded nonwovens. The nonwoven fabrics were produced with different bond areas from the precursor webs and the ODF was determined using the Fourier method. The method was fairly successful in that there is a good agreement between the simulated and measured ODF at various orientation angles. The planar stress-strain relations [2] expressed in equation (1) were used to predict tensile properties at various azimuthal angles and compared with experimental data.

�������� � �� �� 0�� �� 00 0 ��

�������� (1)

where Sij are the components of elastic compliance, ε� are the strains, and σ�are the stresses. The work also studies anisotropy ratio as a function of tensile modulus of nonwovens produced at different temperatures and pressures. However, there is no way to determine and model the OFD along different points in CMD for various radial locations. At best, the properties need to be determined experimentally and plotted as a function of CMD location and roll radius. Chhabra [3] implemented statistical measures to characterize mass distributions in fiber substrates. An image analysis technique which represents the mass distribution by the intensity of reflected/transmitted light was used to quantify the uniformity in nonwovens.

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Elastic Property Characterization and FEM Modeling Spun Bonded nonwovens are known to display mechanical behavior dependent on the shape factor of specimen chosen. Often the constitutive behaviors for such webs are defined within ranges of specimen sizes and applied strains. Hou, Acar and Silberschmidt [4] [5] reported in their study, the tensile properties of low density thermally bonded nonwoven at various shape factors. The deformation behavior and performance of nonwoven at different loading stages, as well as cyclic loading conditions have been reported. At low strains the nonwoven shows completely random mechanical behavior when specimens of different sizes were chosen. This partly contributes to the motivation behind present research to select specimens from multiple locations to study the tensile behavior of nonwoven. In the work quoted above, the experiments conducted were also compared with results of image analysis and FE models. The models developed consider stress concentrations at points of thermal bonding, but do not consider different types of bonds, nor give a relationship depicting changes in properties as the distance between bond points is changed. Figure 2.1 [5] shows the FE model developed for MD and CMD, while figure 2.2 [5] shows the consideration of thermal bond points and relevant mesh construction.

(a) (b)

Figure 2.1: (a) FE model for MD (b) FE model for CMD. Picture source: Xiaonan Hou, Memis Acar, Vadim V. Silberschmidt. Finite Element Analysis

of thermally bonded nonwoven material [5].

(a) (b)

Figure 2.2: (a) Modeling thermal bond points (b) Construction of Mesh. Picture source: Xiaonan Hou, Memis Acar, Vadim V. Silberschmidt. Finite Element Analysis

of thermally bonded nonwoven material [5]. Singh, Biggers and Goswami [6] developed a more efficient FE model depicting the non-uniform deformation of Spun Bonded nonwovens taking into consideration effect of jaws during uniaxial tensile tests, nonlinear nature of deformation and low shear stiffness of fabric. Variation of Poisson’s ratio is shown as a function of longitudinal strains. The study

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discusses the effect of fiber buckling and material nonlinearity. The model enforces a more complex and accurate boundary condition to correctly predict the deformation of nonwoven into post linear regions showing necking during a uniaxial tensile test. Figure 2.3 [6] shows the boundary conditions used to define the problem.

Figure 2.3: Representation of constrains and boundary conditions used.

Picture source: Smita Bais-Singh, Sherill B. Biggers Jr., Bhuvenesh C. Goswami, Finite Element Modeling of the non-uniform deformation of Spun Bonded nonwovens [6].

Mueller and Kochmann [7] considered a single fiber to be a spar in their model and modeled its behavior in a uniaxial tensile test at various temperatures. These results were then used to define the model for thermo-bonded nonwovens. Kim, Pourdeyhimi, Abhiraman and Desai [8] made an effort to account for the structural changes in nonwoven fabrics during load-deformation experiments, but the results are restricted to cases where the nonwoven itself is relatively homogeneous. Good and Qualls [9] presented a viscoelastic winding model to predict roll properties for orthotropic webs. The model incorporates the nonlinear radial modulus as a function of interlayer pressure. The viscoelastic behavior is modeled by generalized Maxwell model. Researchers have successfully modeled nonwovens using Finite Elements, and have attempted to define the nonwoven behavior as a function of fiber distribution. However, the FE models developed to date do not consider any variations and inconsistencies in fiber distribution or bonding pattern. Further, little effort has been made towards considering time dependent material properties and resulting challenges in winding and storage.

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Research Potential In terms of elastic material characterization a model can be developed to account for the localizations in rolls of inhomogeneous nonwoven webs. The localizations can be defined as functions of fiber orientation and bonding pattern. This will facilitate modification of already developed orthotropic viscoelastic models to incorporate nonwoven non-homogeneity. The present study is focused toward characterizing some of the compliance matrix components for an orthotropic nonwoven web, and studying the material behavior at high temperature. With regards to laminated papers, there is potential to characterize the time dependent behavior of constituent papers, films and coatings, and study the influence of lamination parameters on above mentioned properties and quality of laminated paper. An inverse method adopted in this study to determine effective elastic properties of Polyethylene film in the laminate could be extended to the viscoelastic domain. .

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CHAPTER III

CHARACTERIZATION OF NECK BONDED LAMINATE NONWOVEN

The NBL nonwoven used for this study has an area density of 4.17 Osy and contains Kraton rubber as reinforcement. A glass transition temperature (Tg) of 212 0F and relatively higher strength makes Kraton a logical choice as a thermoplastic elastomeric reinforcement. However, substantial amount of polymer fibers present in the Spun Bonded web and the presence of elastomer makes the resulting laminated web a potentially viscoelastic material. The characterization of this nonwoven hence focuses on elastic and viscoelastic properties influencing the winding and storage of these webs, and endeavors to determine properties at high temperatures. The stretching of Spun Bonded web, orientation of fibers and bond pattern employed to hold the fiber-matrix together govern the strength and stretch-ability offered by the resulting web. Figure 3.1 (a) and (b) show the fiber orientation and bond pattern as observed under a Field-Emission Scanning Electron Microscope (SEM).

Figure 3.1(a): Bond pattern in NBL web. Figure 3.1(b): Fiber damage.

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Considerable damage to the fibers as a result of width reduction at elevated temperatures was observed. The bond patterns were observed to be periodic alternating circular and elliptical bonds. Measurement of Thickness Although the industry standard for thickness used in determination of stress-strain relationship is the ‘Average In-roll Caliper’, this study uses a thickness value that corresponds to a web sample taken from the roll and allowed sufficient recovery time. Traditional practice involves measuring the thickness of a stack of web, and dividing the result by the no. of layers in the stack. However, such an approach cannot be applied to webs that are highly pressure sensitive as the caliper used exerts a small amount of pressure. In order to determine the thickness as a function of pressure, a stack of web with known no. of layers was subjected to varying pressure levels, and a pressure vs. thickness plot was generated. Experimental setup involves use of aluminum platens 2.5 inches in diameter and self aligning linear bearings for application of load. Figure 3.2 shows the setup used to measure the thickness.

Figure 3.2: Setup used for measurement of NBL web thickness. Picture to the right shows

speckles created to track displacements.

Digital Image Correlation (DIC) is an optical technique used for measurement of displacements, deformations and strains from successive/continuous images representative of the displacement. The method involves image tracking and registration, and presents results including contributions from higher order terms. Implementation of this method in this study is done by creating speckles on to the surface of interest and processing the images in software called WinDIC. WinDIC is capable of tracking relative displacements of these speckles and obtaining displacements on the order of 0.1 pixel. Figure 3.3 shows the pressure vs. thickness plot generated by applying a range of pressures to the stack of web and measuring resulting displacements.

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Figure 3.3: Pressure vs. Thickness plot generated for single layer of NBL web.

The thickness illustrated in the graph is for a single layer of web. A polynomial fit was employed to extrapolate the results and obtain approximated thickness value of 0.026 inches. Characterization of In-Plane Properties The emphasis in characterization of elastic in-plane properties with regards to web material is primarily on the stress-strain relationship in MD, CMD and other intermediate orientations. Focus towards such an approach is driven by the need to determine whether the nonwoven material is anisotropic, orthotropic or isotropic. Further, as the web is designed for use in diapers, it is meant to strain easily in CMD while exhibiting relatively much higher strengths in MD. Prior to experiments conducted on NBL nonwoven, preliminary experiments were conducted on 12 inch wide Spun Bonded nonwovens made by Fiberweb®. These Spun-bonds are similar to the raw material used in manufacturing of NBL, and are also used in the making of diapers. In order to obtain a measure of variations in stress-strain relationship due to inconsistent basis weight profiles, uniaxial tensile tests were conducted by selecting specimens along various points on the CMD. Figure 3.4 (a) and 3.4 (b) show a comparison of stress-strain relationships observed at three locations along the width, in MD and CMD respectively.

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(a) (b)

Figure 3.4(a): Stress-strain relationships in MD, and (b) CMD for Spun Bond. A more efficient way to visualize this is to track the strains of specimens in MD and CMD along the width at a particular stress level. Figure 3.5 shows this trend observed in specimens selected from three different radial locations in the roll.

(a) (b)

Figure 3.5(a) Comparison of MD, and (b) CMD strains of Spun Bond respectively along the width from various radial locations.

The stretching of Spun Bonded web (used in making of NBL) and addition of Kraton rubber as reinforcement is meant to reduce variations such as above resulting from the original Spun Bonded web. To validate the same, similar approach was adopted to investigate the in-plane elastic properties of the NBL nonwoven web. In-Plane Elastic Properties In order to accommodate full width specimens of NBL nonwovens, and also use the same for viscoelastic characterization, a setup was machined for uniaxial tensile testing. Because testing at 450 orientations were to be carried out to check for anisotropic behavior, the setup was made rigid. However, an adapter was attached to allow rotations about X axis to maintain clamping jaws in the same plane during the test. Figure 3.6 shows the setup machined for testing purposes.

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Figure 3.6: Uniaxial tensile testing setup.

The tests were conducted on an Instron 4202 screw driven tensile/compression testing machine. A data acquisition card-NIDAQ693 was interfaced with a PC to acquire data, and voltage data were converted into respective load and displacement readings by means of a LabVIEW code. Machine or Setup compliance refers to the displacements occurring due to the mechanical inconsistencies such as loose grips, rotations, etc, and need to be corrected in order to accurately obtain the true displacements. A test specimen relatively rigid compared to the material being tested is loaded and the displacements are recorded for load ranges that are expected to be encountered during testing of the material. Because the nonwoven being tested is soft, an aluminum plate was used to prepare the specimen for compliance testing. Figure 3.7 shows the compliance observed. For loads encountered during the testing of the nonwoven within linear elastic range, the compliance was observed to be less that 0.5% of NBL web displacement, and hence was negligible.

Figure 3.7: Machine compliance of the uniaxial tensile testing setup.

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To obtain true representative behavior of the nonwoven, ASTM D1117 was chosen as the testing standard. It is essentially an assimilation of various testing standards, and redirects to ASTM 5035 for testing the breaking strength of nonwoven fabric using a constant rate of elongation apparatus. The standard requires specimens to be chosen from locations which are at a distance of at least 1/10th of width from the edges. The specimen sizes were scaled according to the dimensions mentioned by the standard in order to accommodate specimens within 5 inches in length. Equation (2) [10] gives a relationship for total specimen length as per ASTM 5035.

L = K +2C (2) where, L = Specimen Length, inches. K = Gage length, inches. C = Clamp face width / extension, inches. Figure 3.8 (a) represents the specimen dimensions as per the standard, while figure 3.8 (b) illustrates the locations and orientations of web samples selected for testing. All tests were conducted at elongation rates of 12 inch/min as specified by the standard.

Figure 3.8(b): Orientations of specimens in NBL web. In order to observe any basis weight variations, the first set of tests conducted focused on stress-strain relationship of the nonwoven web at three different locations along the width. However, because the web is only 5 inches wide, this could not be accomplished for CMD and 450 orientations. Figure 3.9 shows the stress-strain relationship specimens of nonwoven web in MD, selected from 3 different locations along the width of the web. The observed results show fairly consistent results, and affirm the expected results after stretching of Spun Bond and addition of Kraton.

Figure 3.8(a): Specimen dimensions as per ASTM 5035

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Figure 3.9: Stress-strain relationship of NBL web in MD along the width.

In order to observe any possible inconsistencies in stress-strain relationship due to variation in interlayer radial pressures, tests were then conducted by selecting web specimens from 4 different radial locations. The radii chosen were simply equidistant (4.5 inches) from each other, starting at the core and reaching the outer layer of the roll. For MD, two samples were chosen for each radial location, one from the edge and one from the center. Figure 3.10 illustrates the stress-strain relationship of web in MD at various radial locations.

Figure 3.10: Stress-strain relationship of NBL web in MD at various radial locations.

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Fairly consistent results were observed with no inconsistencies seen within the linear range of 6%. The only difference observed is in the failure strains of the web (with no particular trend observed) when moving from one radial location to another. The failure mechanism is mostly characterized by necking at 10% strain followed by fiber pull-out at low strain rates. Occasionally, initiations of multi-point failure regions were observed as indicated in figure 3.11 (c). This indicates the development of local strains, thereby suggesting that the variations have only been reduced and not eliminated. Unlike failures at 30-40% strain, failures in CMD occur at about 180% strain as the web is designed to strain easily in this direction. Figure 3.12 shows the failure mechanism observed in CMD. The high strain experienced by the web, results in multi-point necking with failure occurring at the center neck. Figure 3.13 illustrates the stress-strain relationship in CMD at various radial locations.

(a) (b) (c) Figure 3.11(a): Necking at 10% strain in NBL web. (b) Fiber pull-out (c) Multi-point failure Figure 3.14 shows the stress-strain relationship of web in 450 orientation. Based on the previous experience of testing Spun Bond, the clamp was made rigid so as to not allow load output due to shear affect the load read from load cell. Holding the clamp rigid resulted in the development of twin opposite notches at the top and bottom of specimen, with failure occurring at one of the notches as shown in figure 3.15.

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(a) (b)

Figure 3.12: Failure observed in CMD of NBL web.

Figure 3.13: Stress-strain relationship of NBL web in CMD at various radial locations.

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Figure 3.14: Stress-strain relationship of NBL web in 450 orientation at various roll radii.

(a) (b) (c)

Figure 3.15: (a) 450 NBL specimen. (b) Wrinkling at 15% strain. (c) Failure at top notch. The nonwoven is a good example to display the potential of obtaining desired properties by varying the fiber orientations. Multiple tests conducted showed that even a slight change of 50 from the MD resulted in a large drop in modulus. Figure 3.16 illustrates a comparison of stress-strain relationship of web in MD, CMD and 450 orientations. The modulus in MD, CMD and 450 orientations were measured to be 13700 psi, 200 psi and 335 psi respectively. The material is thus orthotropic, with its properties symmetric about the MD and CMD.

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Figure 3.16: Comparison of stress-strain relationship of NBL web in MD, 450 and CMD.

Effect of Specimen Size For the in-plane elastic properties observed during the tests to be representative of webs true behavior and useful for constitutive modeling, the stress-strain relationship should not vary with specimen size chosen. Figure 3.17 (a) and (b) confirm the same by displaying the results of three tests conducted on specimens measuring 4 X 2 inches, 6 X 1 inches, and full width specimen of 20 X 5 inches. Owing to slit width of just 5 inches, the test was conducted for MD only.

Figure 3.17: Stress-strain relationship of varying specimen sizes of NBL web.

(Specimen dimensions in inches)

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Camber Quantification Camber has been historically defined as the inability of a web to lie flat on a plane when un-tensioned. On occasions, webs do lay flat, but adopt a circular arc rather than being constrained to a straight line [11]. For Spun Bonded nonwoven webs, thickness variations and resulting induced unequal winding tensions cause a high amount of camber. The addition of Kraton however increases the density, hence compensating for thickness variations to certain extent. To quantify the amount of camber present in the roll supplied, 120 feet of web material was laid on floor (figure 3-18), and their deviations from straight line grids were measured. A modified Shelton’s approach based on radius of curvature was used to model the camber present, but the fit was not satisfactory. A better way to quantify camber in such situations would be to define them as a function of applied tension. Figure 3.19 shows the measured camber values. This could also be caused due to high density and resulting friction between the web and the floor.

Figure 3.18: NBL web laid out for camber quantification.

Figure 3.19: Graph illustrating camber in NBL web modeled by modified Shelton’s method.

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Determination of Poisson’s Ratio Poisson’s ratio is defined as the ratio of transverse contraction to longitudinal extension when subjected to a stretching force such as uniaxial tension. For materials which exhibit time dependent behavior, Poisson’s ratio is also a function of time. For structures made by assimilation of constituents such as fibers, polymer fragments, binding matrix, etc., the Poisson’s ratio really is a representation of shape changes occurring as a result of fiber reorientation, rather than realignment of inter-atomic bonds with the deformation to resist any volume change. It is the same reason which makes the Poisson’s ratio of nonwovens appear close to 3.0, and not 0.2-0.4 which would be the value obtained by consideration of a single fiber. Owing to the above mentioned complexities, the study focuses on implementing DIC to accurately measure the Poisson’s ratio in the elastic range such that it is useful for constitutive modeling and implementation of roll and winding models. Unlike speckles created by using flat paint, speckles on nonwoven were made by using a sharpie permanent marker and allowing sufficient drying time. This approach was adopted to avoid any property changes occurring due to reinforcement by flat paint. Figure 3.20 shows the speckles created and grid pattern employed processing of images. The Major and Minor Poisson’s ratio were measured to be 2.33 and 0.1 respectively.

Figure 3.20: Screenshot of DIC processing images taken for determination of Poisson’s ratio

of NBL. For verification purposes, the obtained Poisson’s ratio was used in the equation of Young’s modulus at intermediate orientations for an orthotropic fiber composite. GLT in equation (3) is evaluated by a constraint rather than an equality and is expressed by equation (4). The equations neglect the effect of minor poisons ratio in the evaluation of modulus and hence are approximations. �

�� � ������� � �����

�� � �� � �

��� �!��" Sin�2θ (3)

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G)* + ���,�-!��. (4)

where Eθ is Young’s modulus in any intermediate direction (psi), EL is Young’s modulus in longitudinal direction (psi), ET is Young’s modulus in transverse direction (psi), GLT is Shear Modulus (psi) and θ is the intermediate direction angle which is equal to 450 in this case. Figure 3.21 shows a comparison of the stress-strain relationship in 450 by experiment and use of equation (3). An average error of 3.3 % was observed.

Figure 3.21: Comparison of stress-strain relationship of NBL in 450 by experiment and

equation (3) In-Plane Viscoelastic Properties Elastic materials recover fully with no energy lost within the elastic range when the applied stress is released. Further, the stress at a particular point of time does not depend on previous stress history, therefore not displaying any time dependent behavior. It therefore seems fair enough to represent such a behavior by using springs, the stiffness of which would relate to strength of elastic material being represented. However, there exist materials in nature that display dependence on applied stress history, i.e. the stress-strain relationship at a particular point would depend on the stress history the material has been subjected to previously. If the applied stress history is represented as a

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function of time, the behavior of the material at any point of time can be expressed similarly. Such dependence of material property on time is due to the ‘viscous’ component present in it, and the contributions of both elastic and viscous components in exhibited material behavior are responsible for terming them as Viscoelastic. The NBL Nonwovens contain a significant quantity of polymers and Kraton, making them a candidate potential of displaying highly viscoelastic behavior. Visualizing the Spun Bonded web’s stress-strain relationship as a bilinear curve, the web is stretched into the second linear region at elevated temperatures and then mixed with Kraton, thereby reducing the extent of inherent viscoelastic properties. To investigate the possible viscoelastic behavior of NBL nonwoven web, the stress-strain relationship of the web was measured at various strain rates. Figures 3.22 and 3.23 illustrate the stress-strain relationship of web in MD and CMD respectively. It can be seen from both the graphs that the web shows little difference in stress-strain relationship between strain rates of 5x10-2 s-1 and 5x10-4 s-1, which are the two extremes of strain rate applied during testing. This would mean that the mechanical behavior of the web is not strongly dependent on the strain rate. However, it is essential to characterize the properties over a period of at least 3 weeks, which constitutes the storage and transportation time before the roll is unwound and used. There is opportunity to record the stress-strain relationship of material held at a constant strain, known as Relaxation, or observe the increasing strains when the material is subjected to a constant stress, known as creep. Both phenomena have been briefly discussed.

Figure 3.22: Stress-strain relationship of NBL web in MD at various strain rates.

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Figure 3.23: Stress-strain relationship of NBL web in CMD at various strain rates.

Stress Relaxation Relaxation refers to the relieving of stress over a period of time by a material when it is held under a constant strain. The prescribed strain levels are typically within 5%, exceeding which would require the use of large deformation models. The stresses encountered in nonwoven rolls wound at 0.7-2 pli range from 1.5 to 3% strains. Defined as a function of time, the stress-strain relationship is given by: �,/. � �01,/. (5) where �0 is the step strain applied and 1,/. is the relaxation modulus.[12] However, the application of step strain in reality is not a possibility. The consequent issues and modified approaches have been discusses further. Creep Creep refers to the response of a material while subjected to a constant stress. The application of stress is a step function similar to that of strain in relaxation. Creep strain per unit of applied stress, known as ‘Creep Compliance’ is usually used as a measure of creep: 2,/. � 3,4.

56 (6)

where �0 is the step stress applied. J(t) is known as the creep compliance [12].

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Time-temperature Superposition Although the equations to determine the time dependent behavior are simple, the actual execution of experiment over a period of month (or in some cases decades) is not always a feasible option. Fortunately, there is a way to deal with this by conducting accelerated tests. The concept is to change testing environment/parameters and find its equivalence with time. By doing so, properties can be determined for short terms and then defined for longer times. For elaboration purposes the case of Time-temperature superposition is considered here. The reader is free to choose between other methods of acceleration such as Time-stress superposition, accelerating tests at various moisture levels etc. Time-temperature superposition states that temperature affects material behavior in a way as to stretch (or shrink) real time behavior data above (or below) reference temperature. In other words, short term property data obtained at certain temperatures can be used to describe behavior at longer times at any other desired temperature (called reference temperature) [12]. The principle can be represented by the following expression: 1,7, /. � 1,70, 9. (7) where t represents real time, T represents temperature and 9 represents ‘reduced time’. In this case the properties are measured short time at higher temperatures and then used to describe properties at longer times, but at lower temperatures. This process is implemented by shifting the short term curves at higher temperatures in time towards lower temperature curves. 9 � 4

:;,<. (8)

Here =< is known as the shift factor and represents the amount in time by which the curves need to be shifted, and the relation between =< and temperature aids determination of reduced time 9. A popular way of doing this is by using an equation proposed by Williams, Landel, and Ferry [12]:

log�0=4,7. � log 4A � BC,<D<6.

BE-,<D<6. (9)

where T0 is the reference temperature at which the properties are desired, and K1 and K2 are material constants. The values of these constants for a number of materials are defined, and also can be obtained by employing curved fitting to data that is being shifted. The method is applicable to both relaxation and creep experiments provided the curves shifted purely represent relaxation/creep data and do not contain effects of ramp stress/strain history. The method is an excellent way to obtain long term properties, but the success of the same depends on temperature sensitivity and linearity of subject material. An example discussing the implementation of this method on relaxation behavior will be discussed along

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with an improved approach which proposes a way to extract relaxation data from ramp strain portions. Improved Relaxation Approach Knauss and Zhao [13] proposed a method, which involves extraction of relaxation data from ramp strain portions of curves. This enables extension of relaxation curves beyond conventional norms, hence reducing the number of experiments required to acquire long term data. The method is explained and implemented to characterize long time relaxation data of NBL nonwovens. Figure 3.24 shows a demonstrative illustration of typical relaxation curve for a polymer plotted in log-log scale to make use of its capability of displaying long term data. For such a curve the ‘ten-times’ rule is generally applied in order to extract the relaxation data. The rule essentially states that, because in reality a step strain history (figure 3.25) is not feasible, a ramp strain history is applied (figure 3.26), the result of which is that the following stress response will not approach the relaxation function until approximately ten times the time required to reach �0. Because of this issue, a significant amount of relaxation data is lost. This loss translates into requirement of more relaxation curves (and hence experiments) to facilitate shifting in time scale (figure 3.27, assumed rise time for ramp = 1 second). The resulting curve at the end of shifting is known as master curve and represents the material properties at the reference temperature. Knauss and Zhao proposed the following equations to enable extraction of relaxation data from ramp and post ramp portions of the curve. Equation 10 is used to fit data to ramp portion and obtain coefficients of relaxation modulus. Equation 11 is used for the post-ramp region. The coefficients thus obtained are used to represent relaxation modulus as a function of time by means of Prony series, as shown in equation 12. A detailed description of the method may be obtained from [13]. �,/. � F G 1H/ � ∑ 1J9J K1 M � 4

AN"O P / Q /0 (10)

�,/. � F R 1H/0 � ∑ 1J9J STM UVND�WX STMYUVN WX Z / + /0 (11)

1,/. � 1H � ∑ 1JM�D UVN" (12)

where ε0 � R t, t Q t0 , R t0, t + t0

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The advantage of using this approach is the extension of relaxation curves at extreme temperatures as highlighted in figure 3.28, by which the need for intermediate curves (and hence experiments) is eliminated.

Figure 3.24: Demonstrative illustration of typical relaxation curve of a polymer.

Figure 3.25: Step strain in theory. Figure 3.26: Error due to ramp strain history.

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Figure 3.27: Demonstration of Curve shifting of multiple curves. Tr is reference temperature.

Figure 3.28: Demonstrative extraction of relaxation data from ramp of T3 eliminates the need

for experiments at T1 & T2. The success of this approach is dependent on temperature sensitivity of the material, noise present in data, number of data points and curvature of relaxation curves. The method employs rigorous simplified trust region nonlinear optimization routine, and recommends the use of about two relaxation times for every decade.

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Viscoelastic Characterization in MD, CMD The method was successfully employed to characterize the long term behavior of NBL nonwoven at higher ends of room temperature, and was validated by inter-conversion of creep data obtained from a conventional creep test. At a strain level of 5%, the loads encountered for sample sizes used for elastic property characterization were too small, and hence with the resolution of 0.1 lb the noise recorded was very high especially for CMD. Therefore, in order to maintain consistency all tests were carried out with sample sizes of 4.5 X 2 inches. Given that the sample sizes did not have an effect on stress-strain relationship, this is an acceptable approximation. The tests were carried out in a temperature chamber heated by a solenoid coil, and the temperature was controlled using a Omega® CN900A temperature controller coupled with relay and feedback loop. The thermocouple used for sensing temperature was a K-type Alumel-Chromel thermocouple surrounded by Ceramic material to maintain constant temperature readout. Figure 3.29 shows the setup with a specimen loaded for testing.

Figure 3.29: Setup used for relaxation experiments of NBL.

Because the Nonwoven web displays orthotropic characteristics, it is imperative that it needs to be characterized in MD and CMD for its time dependent behavior. For MD, relaxation tests at 5% strain were conducted with a rise time of 10 seconds at 70 0F and 135 0F respectively. Figure 3.30 shows the ramp response observed for both the tests. It can clearly be seen that the data at higher temperatures is non-smooth relative to data recorded at room temperature. Figure 3.31 shows the raw data collected from the experiments.

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Extraction of relaxation data was performed using equation 10 and 11. Figure 3.32 shows the master curve at 700F obtained by curve shifting and Prony series fitted to it. Table 1 shows the coefficients of Prony series fitted to the master curve. (values in log)

E Coefficients (Psi) Relaxation times (seconds) E∞ = 1.903 E1 = 0.1162 ζ1 = 9.087 E2 = 0.9948 ζ2 = 12.59 E3 = 0.295 ζ3 = 13.57 E4 = 0.9996 ζ4 = 19.01

Table 1: Coefficients of Prony Series for master curve at 70 0F for MD Young’s relaxation modulus of NBL.

Figure 3.30: Ramp responses in MD of NBL at 70 0F and 135 0F.

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Figure 3.31: Raw relaxation data in MD of NBL.

Figure 3.32: Master curve at 70 0F for MD Young’s relaxation modulus of NBL obtained by

Time-temperature superposition & Prony series fit.

Validation by Creep Long term creep experiments were performed and compared with the relaxation data obtained by implementation of Time-temperature superposition. Nonwoven web of length

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171.5 inches was hung from a wall at stress levels of 90 psi at room temperature. The resulting displacements were recorded using Nikon DSLR camera and images were processed using DIC to obtain accurate displacements. Figure 3.33 shows the setup, while figure 3.33 shows the creep compliance plotted as a function of time in log scale.

Figure 3.33: Setup for creep test of NBL. The initial portion of data is usually disregarded as the applied stress is not in reality a step stress. Further, the release of loads results in vibrations which can result in noisy data. The true creep compliance therefore is illustrated by displacements at times far away from initial portions of loading. Inter-conversion of creep data into relaxation data is not a straight forward computation, and involves approximations of the convolution integral relating the two: ] 1,/ ^.2,^._^ � /40 (13)

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Figure 3.34: Creep compliance of NBL web in MD at room temperature. Implementation of Laplace transforms to convert one from the other is a common practice. In Laplacian domain the relation is given by: 2,.`̀ `̀ ` 1,.`̀ `̀ `̀ � �

aE (14)

The accuracy of inter-conversion is a function of equations used to represent creep and relaxation, curvature of both the curves being converted, and conversion method used. Several approximations have been proposed [14], and in this study the method proposed in [12] was used. Figure 3.35 shows the comparison of converted creep data with the relaxation data obtained previously.

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Figure 3.35: Comparison of NBL MD relaxation modulus master curve at 70 0F from Time-temperature superposition and relaxation data obtained by conversion of creep data of NBL

at room temperature.

Converting the values into non-log scale, an average error of 26% was observed. However, the error significantly reduces with a small improvement in defining the slope of the source curve. In an effort to keep the overall logarithmic deviation of one curve to the other at a minimum, the curvature is reduced to straight lines of smaller slopes. Several methods for inter-conversion proposed in literature involve computation of gamma functions, the values of which for non-integers are merely integral approximations. The increase in number of terms used to describe log-log slopes of source curve (and hence function) results in computation of higher number of corresponding gamma functions while inter-conversion, hence compromising the accuracy. Therefore a good balance is sought between describing the source function accurately and keeping the computations of gamma functions at a minimum. A significant amount of accuracy is compromised while choosing the creep compliance data for conversion. The accuracy gets better with the amount of compliance data discarded from the initial loading region. The last reason believed to be responsible for inaccuracies is the control of temperature during the creep test. The test is performed over a period of 15 days, and to maintain isothermal conditions for webs of length greater than 100 inches is tedious and unfeasible. The characterization of nonwoven in CMD was conducted at higher ends of room temperature (90 0F). The loads encountered at strains of 5% are on the order of 0.7 lb, and with the resolution of load cell being 0.05-0.08 lb, the noise recorded was very high. Therefore the data obtained was smoothened multiple number of times before Time-temperature superposition could be applied. Figure 3.36 shows the relaxation behavior in

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CMD after shifting. Table 2 provides the coefficients of Prony series fitted to the master curves.

Figure 3.36: Master curve for Young’s relaxation modulus in CMD at 90 0F for NBL and

Prony series fitted to master curve.

E Coefficients (psi) Relaxation times (seconds) E∞ = 2.221 x 10-14 E1 = 1.285 ζ1 = 10 E2 = 0.9318 ζ2 = 100 E3 = 2.221 x 10-14 ζ3 = 1000 E4 = 2.221 x 10-14 ζ4 = 10000 E5 = 2.221 x 10-14 ζ5 = 100000 E6 = 2.221 x 10-14 ζ6 = 1000000

Table 2: Coefficients of Prony series fitted to master curve for Young’s relaxation modulus in CMD at 90 0F for NBL.

Characterization of Out-of-Plane Properties The Z direction in a wound roll constitutes the Radial direction. Historically the Radial modulus has been known to exhibit nonlinear behavior when described as a function of applied radial stress. Pfeiffer [15] first described a way to characterize this radial property by means of a ‘Stack Test’. The test essentially involves making one inch stacks of web material and applying compressive load by means of platens whose size are smaller than that of web.

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The relation between normal pressure and normal strain as expressed by Pfeiffer is given by: b � �c � d�,MBE3e 1. (14)

where K1 and K2 are constants unique to the web material. Utilizing full width of the roll slits, a stack of nonwoven web measuring about one inch in thickness was subjected to radial pressures on platens measuring 5 inches in diameter. The test was conducted on MTS servo hydraulic tensile/compression tester. Figure 3.37 shows the stress-strain response.

Figure 3.37: Radial stress-strain relationship of NBL web obtained by stack test.

Pfeiffer’s equation was fit to the experimental data obtained by stack test, and the comparison of the same is shown in figure 3.37. The values of K1 and K2 are 0.953 psi and 7.54 psi respectively. While the previous tests merely provides an idea of the web strength in radial direction, a more descriptive data is required to understand the web response in a wound roll over a period of time. The decay of stress as a result of relaxation can result in loss of radial pressure, resulting in falling apart of rolls, or difficulties in unwinding. In order to characterize the radial viscoelastic properties, the radial creep properties of the web were characterized for a period of 14 days. A setup similar to that used for measurement of web thickness was used to characterize creep. DIC was employed to record displacement and obtain corresponding strains. Figure 3.38 shows the creep compliance of the web in radial direction at temperature of 70 0F.

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Figure 3.38: Radial creep compliance of NBL web at room temperature.

Temperature Effects on Webs The nonwoven laminate is produced in the US and shipped to Korea, Australia, Europe and many other places. Transported in containers for about 3 weeks over equatorial regions, the webs encounter high humidity and temperatures of about 165 0F. The problem is often referred to as ‘seasonal shipping’ in the industry. The challenge here for the nonwoven is to not just face high temperatures, but also withstand thermal fatigue along with humidity variations for the time period it spends in shipping. Figure 3.39 shows the approximate temperature profile the web is subjected to over a period of one day during transportation.

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Figure 3.39: Approximate temperature profile the NBL web is subjected to during

transportation through equatorial regions.

Tests carried out at elevated temperatures till now have been shifted for the purpose of obtaining long term data at lower temperatures. In order to obtain realistic description of what the webs would go through in those three weeks, the primary objective for the latter part of this study was to characterize the behavior of nonwoven at high temperatures. Historically, Time-temperature has been implemented for transient temperature conditions by employing some form of modified reduced time. A relatively simpler implementation proposed by Morland and Lee [12] suggests the use of following reduced time:

ζ,t. � ] gh′i�j*kh′lm′.

h0 (15)

where t’ is just any time prior to t. However, to simplify the experimental process the web was characterized for constant temperature of 167 0F. Due to the inability of the load cell to detect loads on the order of 0.2 lb at such high temperatures, the web has not yet been characterized in CMD. However, accelerated creep tests are a recommended option to be pursued. The experiments were thus performed in MD at elevated temperatures, and Time-temperature superposition was employed to obtain relaxation behavior of web at 167 0F. Figure 3.40 shows the master curve and Prony series fitted to it. Owing to sudden change in curvature at longer times, the data pertaining to properties after 21 days had to be discarded.

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Figure 3.40: Master curve of MD Young’s relaxation modulus of NBL at 167 0F and Prony

series fitted to it.

Table 3 shows the coefficients of the Prony series fitted to the above master curve.

E Coefficients (psi) Relaxation times (seconds)

E∞ = 1

E1 = 2.655 ζ1 = 10

E2 = 0.5314 ζ2 = 100

E3 = 1.066 x 1011 ζ3 = 1000

E4 = 7.156 x 1010 ζ4 = 10000 Table 3: Coefficients of Prony series fitted to master curve of MD Young’s relaxation

modulus of NBL at 167 0F.

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Coefficient of Thermal Expansion (CTE) Coefficient of thermal expansion may be expressed as a function of volumetric, area or linear strain. Because of the high temperature environments the web is exposed to, the determination of CTE is important. Further, it is one of the 9 parameters of the compliance matrix for an orthotropic material, and hence needs to be determined to aid in constitutive modeling. Conventional methods of using strain gauges are suitable only for bulk materials and cannot be applied to webs as it could reinforce the portion it is attached to. Although it is possible to obtain an estimate of CTE for MD and CMD by subjecting the web under a preload to higher temperatures and deriving the strain by observing the decay in stress, the method does not consider relaxation occurring as time progresses. Since the decay in stress is continuous, a particular instant of time cannot be chosen to extract the exact strains. Therefore DIC is used to track the relative displacements (and hence thermal strains), which are expressed as linear functions of position [16]. Speckles on nonwoven web were created using Sharpie® permanent marker and after allowing sufficient drying time, the web was placed in a temperature chamber. Images were recorded before and after the increase in temperature. Figure 3.41 shows the setup made to house the specimen during the experiment.

The obtained displacements were fit into the following equations [16] using surface fitting tool in Matlab®. o,p, q. � =0 � =�p � =�q r,p, q. � s0 � s�p � s�q (16)

where a0, ....., b2 are the coefficients obtained by surface fitting.

Figure 3.41: Setup used for determination of CTE of NBL.

The strains are calculated as partial derivatives of the above equations. Figures 3.42(a) and (b) show the contours of displacements occurring as a result of the increasing temperature. Although much of the noise was reduced, certain amount of rigid body rotations are observed. The rigid body rotations can be computed by determining the rotation angle [16].

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Figure 3.43(a) and (b) show the contour plots of displacements after the application of rotation compensation. However, it is to be noted that a relatively larger grid of smaller density was used during image processing, which enables such smooth contour plots. This leads to approximations in computed values of CTE, and for accurate determination of the same a fine grid pattern should be used. Although a fine grid might not produce smooth contour plots, the results obtained are more accurate.

Figure 3.42(a) Ux deformation field of NBL before rotation compensation.

Figure 3.42(b) Uy deformation field of NBL before rotation compensation.

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Figure 3.43(a) Ux deformation field of NBL after rotation compensation.

Figure 3.43(b) Uy deformation field of NBL after rotation compensation.

The CTE was measured to be 42.3x10-6/ 0F in MD and 61.09x10-6/ 0F in CMD.

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CHAPTER IV

CHARACTERIZATION OF LAMINATED PHOTO PRINTING PAPER

Observation of Laminate Structure In order to obtain the thickness of constituent films and overall laminated structure, edges of samples measuring 2 mm X 10 mm were observed using a Nikon Eclipse Inverted Metallurgical microscope. Figure 4.1 shows the structure of HP Raw photo base paper as observed under the microscope.

Figure 4.1: Layer structure of HP Raw photo base paper.

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Raw Photo Base paper was observed to contain a silica coating measuring about 15.285±3.1 microns in thickness, while the Base paper measured 177.6±7.1 microns. Figure 4.2 shows the structure of HP Advanced photo printing paper as observed under the microscope.

Figure 4.2: Laminate structure of HP Advanced photo printing paper.

The stacking sequence observed confirmed initial speculations of Silica coating on a Polyethylene film, which in turn was stacked on Base paper. The absence of any clear line of demarcation between the multiple layers led to the calculation of effective thickness along with certain standard deviation using the accompanying software. The deviation measured is a function of measurement locations chosen in the magnified image. To obtain a representative thickness value, 100 locations were chosen including areas with maximum and minimum thicknesses for each layer’s thickness measurement. The overall thickness was measured to be about 244.3±8.1 microns. The thickness of Base layer was measured to be 176±5.3 microns, while that of Polyethylene and Silica layers were measured to be 42.80±3.4 microns and 22.1±2.9 microns respectively. Figure 4.3 shows the laminated structure of HP Vivid photo printing paper.

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Figure 4.3: Laminate structure of HP Vivid photo printing paper.

A similar approach was adopted to measure the layer thicknesses. The measured thickness value of base paper layer was 156.38±5.6 microns, while that of the PE layer and Silica coating were 46.09±3.4 microns and 16.86±2.1 microns respectively. The overall thickness was measured to be 215.298±8.9 microns. Standards Used for Uniaxial Testing ASTM D828 is a widely used standard for testing the tensile properties of paper and paperboard using a constant rate of elongation apparatus. The standard issues specific guidelines for preparation of samples for testing, fixture dimensions and testing procedures. A setup capable of accommodating specimen widths up to 6 inches was fabricated using 60-61 grade aluminum. To ensure that no slippage occurred, samples were held together with setup’s jaw clamps by making use of additional C-clamps. The compliance of the setup was measured and observed to be not negligible for the loads encountered during testing of laminated samples. Curve fitting was employed to obtain a function relating compliance displacements to load data and compliance correction was made to all tests carried out using the setup. Specimens of length 7.1 inches and width 1 inch were prepared by using a X-Acto paper cutter. An Instron 4202 screw-driven tensile tester was used to record displacements and tests were carried out at elongation rates of 1 inch/min. A national instruments NIDAQ693 data acquisition card interfaced with a PC was used to acquire data, and a

46

LabVIEW code was used to convert voltage readings into corresponding load and displacement data. Almost all the tests carried out using this standard resulted in failures at regions of non-uniform strain, hence rendering the results not usable. Consequently another widely used standard, TAPPI 494 OM6, which is also used to determine tensile properties of paper and paperboard, was used in an attempt to obtain acceptable failure patterns. The TAAPI standard is similar to ASTM D828 in essence, and only prescribes a slightly different shape factor. Tests carried out using this standard also resulted in failures at the end regions of gage length. Figure 4.4 shows the testing setup, while Figure 4.5 and Figure 4.6 show failure patterns observed by ASTM D828 and TAPPI 494 OM6.

Figure 4.4: Testing Setup. Figure 4.5: Failure of HP Vivid by ASTM D828.

Figure 4.6: Failure of HP Vivid by TAPPI 494 OM6.

With straight strip type specimens resulting in non-acceptable failure patterns, a dog bone type specimen was sought to obtain acceptable data. JIS Z 2201 and its variants were then used as testing standards, and DIC was to be used for recording strain data once acceptable failure patterns could be obtained. Both notched dog bone and notched double dog bone type specimens yielded failures at the regions of cross section changes, i.e. at the end of gage length. Figures 4.7 and 4.8 show the samples and failure regions observed as a result of JIS Z 2201 and double dog bone type specimens respectively.

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Figure 4.7: JIS 2201 Specimen for HP Raw. Figure 4.8: Double Dog-bone specimen. (All dimensions in Inches) At this point it was speculated that the sharp changes in cross sections of notched specimens were causing failures at regions of non uniform strain. Therefore, a standard prescribing un-notched dog bone type specimen - ISO 527, was used to prepare samples for characterization of tensile properties of all three products. A 3D model of ISO 527 was created using ProENGINEER Wildfire and Edgecam was used to generate machine G/M codes for use in CNC machine. An aluminum template was thus machined and used to cut samples for testing purposes. 10 different tests were carried out to ensure that repeatable results and failures at regions of uniform strain would be obtained. Figure 4.9 shows the 3D model created in ProENGINEER, while Figure 4.10 shows the aluminum template machined for cutting samples.

Figure 4.9: 3D model created in ProENGINEER. Figure 4.10: Aluminum template machined.

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Figure 4.11 shows three extremes of failure regions obtained by employing ISO 527, with all three regions lying in central region of uniform strain bands.

Figure 4.11: Failure regions of specimens of HP Vivid made in accordance with ISO 527.

The strain data was obtained by Digital Image Correlation, and synchronized in time with load data obtained from the PC. For tests conducted at higher strain rates, a relation between strain data obtained from DIC and machine velocity was established in order to compensate for lack of sufficient data points, and hence obtain smoother curves. For making use of the DIC, speckles were created using fast drying flat paint on the samples. To ensure that the speckles did not interfere with the material properties, the paint was sprayed at lower density on the polythene side of the samples. Samples prepared as prescribed by ISO 527 were tested at an elongation rate of 1 Inch/min. Images at regular intervals of 3 fps were captured by a Nikon D70 DSLR camera. Subsequent images within the linear range were processed in WinDIC to calculate Poisson’s ratio of the samples tested. A repeatability test was carried out to ensure that the results obtained by this method were reliable. Figure 4.12 shows the stress-strain curves obtained for three tests using samples prepared from HP Vivid photo printing paper.

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Figure 4.12: Stress-strain curves obtained during a repeatability test on HP Vivid photo

printing paper. The results obtained show that the method yields fairly repeatable results with the difference only observed in failure strains. Characterization of In-Plane Elastic Properties of HP Raw Base Photo Paper In-plane properties of HP Raw photo base paper were characterized using ISO 527 as the testing standard. The stress-strain responses were fit into appropriate polynomial and exponential equations. The coefficients of these equations for each of the curves have been mentioned. Figure 4.13 shows the stress-strain relationship of the material in the Machine Direction (MD). Average modulus within the linear region of 0.8% strain was observed to be 1.04 Gpa. The Major Poisson’s ratio was measured to be 0.26. The stress-strain relationship is fitted into: σ = axε6+bxε5+cxε4+dxε3+exε2+fxε+g, where σ is in psi. The coefficients in psi are: a = -1.254x1014, b = 1.254x1013, c = -4.247x1011, d = 6.078x109, e = -2. 392x107, f = -1.391x105 and g = -19.5.

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Figure 4.13: Stress-strain relationship of HP Raw photo base paper in MD.

Figure 4.14 shows the stress-strain relationship of the material in the Cross Machine Direction (CMD). Average modulus within the linear region of 0.8% strain was observed to be 0.82 Gpa. The Minor Poisson’s ratio was measured to be 0.33. The stress-strain relationship is fitted into: σ = axe(bxε)+cxe(dxε) , where σ are in psi. The coefficients in psi are: a = 1.84x107, b = -2.762x106, c = 1.558x105, d = -88.68.

Figure 4.14: Stress-strain relationship of HP Raw photo base paper in CMD.

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Figure 4.15: Stress-strain relationship of HP Raw photo base paper in 450 orientation.

Figure 4.15 shows the stress-strain relationship of the material in the 450 orientation. Average modulus within the linear region of 0.8% strain was observed to be 0.88 Gpa. The stress-strain relationship is fitted into: σ = axε6+bxε5+cxε4+dxε3+exε2+fxε+g, where σ are in psi. The coefficients in psi are: a = -3.856x1012, b = 5.697x1011, c = -2.818x1010, d = 4.539x108, e = 7.155x105, f = 9.336x104, g = 58.85. The comparison of stress-strain curves of HP Raw photo base paper in MD, CMD and 450 orientation in figure 4.16 indicates that the material is anisotropic.

Figure 4.16: Comparison of stress-strain curves of HP Raw photo base paper.

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In order to determine the accuracy of data measured, a comparison of stress-strain relationship in 450 orientation from experiment was made with that obtained by calculation of Young’s modulus by equation (4). Figure 4.17 shows the comparison of the two curves. To investigate the dependence of mechanical behavior of the material on strain rate applied, samples were tested at three different strain rates. The results of the tests conducted as shown in figure 4.18, figure 4.19, and figure 4.20 display a considerable dependence of mechanical behavior on strain rate indicating possible viscoelastic properties.

Figure 4.17: Comparison of stress-strain relationship for HP Raw photo base paper in 450

orientation by experiment and equation (4)

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Figure 4.18: Stress-strain relationship of HP Raw photo base paper in MD at various strain

rates.

Figure 4.19: Stress-strain relationship of HP Raw photo base paper in CMD at various strain

rates.

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Figure 4.20: Stress-strain relationship of HP Raw photo base paper in 450 at various strain

rates.

Characterization of In-Plane Elastic Properties of HP Advanced Photo Printing Paper Figure 4.21 shows the stress-strain relationship of the material in the MD. Average modulus within the linear region of 1% strain was observed to be 2.25 Gpa. The stress-strain relationship is fitted into: σ = axε2+bxε+c, where σ are in psi. The coefficients in psi are: a= 5032000, b = 412300, c = -247.5. The Major Poisson’s ratio was measured to be 0.29.

Figure 4.21: Stress-strain relationship of HP Advanced photo printing paper in MD.

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Figure 4.22 shows the stress-strain relationship of the material in the CMD. Average modulus within the linear region of 1% strain was observed to be 1.56 Gpa. The stress-strain relationship is fitted into: σ = axe(bxε)+cxe(dxε), where σ are in psi. The coefficients in psi are: a = 3004, b = -5.6196, c = -3010, d = -84.25. The Minor Poisson’s ratio was measured to be 0.38. Figure 4.23 shows the stress-strain relationship of the material in the 450 orientation. Average modulus within the linear region of 1% strain was observed to be 1.79 Gpa. The stress-strain relationship is fitted into: σ = axe(bxε)+cxe(dxε), where σ are in psi. The coefficients in psi are: a = 4542, b = 7.714, c = -4767, d = -71.75.

Figure 4.22: Stress-strain relationship of HP Advanced photo printing paper in CMD.

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Figure 4.23: Stress-strain relationship of HP Advanced photo printing paper in 450

orientation.

The comparison of stress-strain curves of HP Advanced photo printing paper in MD, CMD and 450 orientation in figure 4.24 indicates that the material is anisotropic.

Figure 4.24: Comparison of stress-strain curves of HP Advanced photo printing paper.

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In order to determine the accuracy of data measured, a comparison of stress-strain relationship in 450 orientation from experiment was made with that obtained by calculation of Young’s modulus by equation (4). Figure 4.25shows the comparison of the two curves. To investigate the dependence of mechanical behavior of the material on strain rate applied, samples were tested at three different strain rates. The results of the tests conducted as shown in figure 4.26, figure 4.27, and figure 4.28 display a considerable dependence of mechanical behavior on strain rate indicating potential viscoelastic properties.

Figure 4.25: Comparison of stress-strain relationship for HP Advanced photo printing paper

in 450 orientation by experiment and equation (4)

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Figure 4.26: Stress-strain relationship of HP Advanced photo printing paper in MD at various

strain rates.

Figure 4.27: Stress-strain relationship of HP Advanced photo printing paper in CMD at

various strain rates.

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Figure 4.28: Stress-strain relationship of HP Advanced photo printing paper in 450 at various

strain rates.

Characterization of Elastic Properties of HP Vivid Photo Printing Paper Figure 4.29 shows the stress-strain relationship of the material in the MD. Average modulus within the linear region of 1% strain was observed to be 1.70 Gpa. The stress-strain relationship is fitted into: σ = axε2+bxε+c, where σ are in psi. The coefficients in psi are a= -3.236x106, b = 3.078x105, c = -123.2. The Major Poisson’s ratio was measured to be 0.20.

Figure 4.29: Stress-strain relationship of HP Vivid photo printing paper in MD.

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Figure 4.30 shows the stress-strain relationship of the material in the CMD. Average modulus within the linear region of 1% strain was observed to be 1.04 Gpa. The stress-strain relationship is fitted into: σ = axe(bxε)+cxe(dxε), where σ are in psi. The coefficients in psi are: a = 2819, b = 6.227, c = -3015, d = -81.83. The Minor Poisson’s ratio was measured to be 0.23. Figure 4.31 shows the stress-strain relationship of the material in the 450 orientation. Average modulus within the linear region of 1% strain was observed to be 1.23 Gpa. The stress-strain relationship is fitted into: σ = axe(bxε)+cxe(dxε), where σ are in psi. The coefficients in psi are: a = 3.848x104, b = -14.22, c = -3.861x104, d = -20.32.

Figure 4.30: Stress-strain relationship of HP Vivid photo printing paper in CMD.

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Figure 4.31: Stress-strain relationship of HP Vivid photo printing paper in 450 orientation.

Figure 4.32 shows a comparison of stress-strain curves of HP Advanced photo printing paper in MD, CMD and 450 orientation.

Figure 4.32: Comparison of stress-strain curves of HP Vivid photo printing paper.

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In order to determine the accuracy of data measured, a comparison of stress-strain relationship in 450 orientation from experiment was made with that obtained by calculation of Young’s modulus by equation (4). Figure 4.33 shows the comparison of the two curves. To investigate the dependence of mechanical behavior of the material on strain rate applied, samples were tested at three different strain rates. The results of the tests conducted as shown in figure 4.34, figure 4.35, and figure 4.36 display a considerable dependence of mechanical behavior on strain rate indicating potential viscoelastic properties.

Figure 4.33 Comparison of stress-strain relationship for HP Vivid photo printing paper in 450

orientation by experiment and equation (4)

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Figure 4.34: Stress-strain relationship of HP Vivid photo printing paper in MD at various

strain rates.

Figure 4.35: Stress-strain relationship of HP Vivid photo printing paper in CMD at various

strain rates.

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Figure 4.36: Stress-strain relationship of HP Vivid photo printing paper in 450 at various

strain rates.

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Characterization of out-of-plane properties of HP Base, HP Advanced, and HP Vivid Photo Printing Papers The Z direction of the web which corresponds to its thickness forms the radial direction in a wound roll. The radial modulus is as important a parameter, and much like the tensile tests conducted for determining in-plane strength of the web, a compression test on a stack of web is carried out to determine the radial modulus of the web material. In order to characterize the radial modulus of HP Raw photo base paper, a stack of web measuring just over an inch was subjected to compression between two relatively rigid platens of 5 inch diameter on a MTS servo-hydraulic tension/compression testing machine. Because the total strain expected to be reached is low, a fairly low strain rate is used in order to obtain sufficient data points. Figure 4.37 show the stress-strain curve obtained as a result of the test conducted.

Figure 4.37: Stack test results of HP Raw photo base paper.

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Due to highly non linear nature of the curve at low strains, the data could not be fit into Pfeiffer and Hakiel’s equations. Hence the data was fit into a 9th order polynomial equation: σ = a1xε9+a2xε8+a3xε7+a4xε6+a5xε5+a6xε4+a7xε3+a8xε2+a9xε+a10, where σ are in psi. Figures 4.38 and 4.38 display the strain-strain curves obtained as a result of stack tests conducted on HP Advanced and HP Vivid respectively.

Figure 4.38: Stack test results of HP Advanced photo printing paper.

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Figure 4.39: Stack test results of HP Vivid photo printing paper.

A similar 9th order polynomial equation was fit into the data obtained by tests on HP Advanced and HP Vivid photo printing papers.

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The coefficients of the equation are as follows:

Coefficients (psi)

HP ADVANCED HP RAW HP VIVID

a1 -1.36x1014 -2.815x1012 -4.921x1012

a2 5.465x1013 8.592x1011 -9.953x1012

a3 -8.911x1012 7.32x109 4.015x1012

a4 7.422x1011 -3.555x1010 -6.32x1011

a5 -3.157x1010 6.18x109 5.281x1010

a6 4.827x108 -4.908x108 -2.429x109

a7 8.866x106 1.934x107 6.1x107

a8 -1.787x105 -2.272x105 -501800

a9 1121 1573 2318

a10 -0.8855 -2.08 -1.955

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Prediction of Polyethylene Film Properties by Simulation of Lamination The elastic properties obtained by experiments were used as inputs to a simulation of lamination in Abaqus. The properties of Raw base paper and Silica are compared to the properties of Advanced and Vivid photo printing papers to back out the properties of constituent Polyethylene films. As per the experimental realizations Raw, Advanced and Vivid photo papers were described as orthotropic materials. The Polyethylene film to be laminated was assumed to be orthotropic, and the thickness for the same was extracted from microscope observations. For simplicity, a Poisson’s ratio of 0.3 was assumed for the Polyethylene film and the silica layer was assumed to be isotropic. With the experimentally determined inputs and assumed values for silica layers, the following values were obtained for properties of Polyethylene film in the elastic range: EL = 325728 psi, ET = 226328 psi, and G12 = 101756 psi.

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CHAPTER V

CONCLUSIONS

Neck Bonded Laminate Nonwoven Webs. The stretching of Spun Bonded web at elevated temperatures is partially successful in reducing the severity of inherent viscoelasticity. All the experiments conducted in MD, CMD and 450 orientations for NBL nonwoven web at various locations along the CMD and radii of the roll displayed consistent responses with localizations observed only during failures. The web is designed to carry substantial weight, and still be able to expand considerably in CMD. The modulus value ratio of > 65 (MD : CMD) and failure strain rates of 180% in CMD confirm the expected results. The same is confirmed by experimental measurement of a Major Poisson’s ratio of 2.33 and a Minor Poisson’s ratio of 0.1. It is to be noted that, though Poisson’s ratio here is merely a function of fiber orientations rather than shape changes to resist any change in volume. Unlike what is seen with many Spun Bonds, specimens of various sizes displayed consistent results, thus rendering the obtained stress-strain relationships useful for constitutive modeling in future. The camber in the material is observed to be low with a maximum bow of 2 inches observed for a web length of 120 feet. However, a better method of modeling camber, perhaps one based on the applied tension, needs to be used. Owing to the presence of significant amount of polymers the web was suspected to be viscoelastic in nature. Even though a low dependence of web on strain rate was observed, the web was characterized in MD and CMD for a period of about one month. Improved relaxation approach by Knauss and Zhao [13] was used to characterize the relaxation behavior of the web by using just two experiments for each direction. The variation in relaxation data and that converted by creep is thought to be primarily due to multiple approximations of slopes of log-log curves during computations of gamma functions of the same. A significant loss in modulus (~80%) was observed over a period of 115 days with the web subjected to constant strain of 5%. This is contradictory to what was expected on observation of web’s dependence on strain rate.

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Stack test on nonwoven revealed a not so severe nonlinear behavior and Pfieffer’s equation sufficed to fit the data. Radial viscoelastic properties of the web were characterized by a radial creep test. The web was observed to be highly sensitive to high temperatures, and a loss of over 63% of its strength was observed at temperatures encountered during transportation to international locations. The web was also observed to have high coefficients of thermal expansion when compared to other nonwoven webs and elastomers containing Kraton rubber. The method used to characterize the CTE needs to be improved upon by consideration of finer grids and isolation of vibrations and noise produced during the experiment. Photo Printing Paper The observation of HP Raw photo base paper under a high resolution microscope showed the presence of a silica coating on the paper layer. Both HP Advanced and HP Vivid photo printing papers were observed to consist of base paper layers stacked under Polyethylene and Silica layers, in that order. Uniaxial tensile tests on HP Base, HP Advanced and HP Vivid photo papers revealed that all three were anisotropic. DIC was used to track the relative displacements of speckles sprayed on to the material and Poisson’s ratio was thus determined. HP Advanced seemed to be the strongest of the three materials. Further, a significant dependence of mechanical behavior on strain rate applied indicates that the material exhibits time dependent behavior. In order to characterize the elastic properties in the Z direction, a compression stack test was conducted. The resulting nonlinear behavior was fit into appropriate polynomial equations as Pfeiffer and Hakiel’s models failed to simulate the obtained results. The properties determined from experiments were used in the simulation of lamination in Abaqus and the effective properties of constituent Polyethylene film were determined as EL = 325728 psi, ET = 226328 psi, and G12 = 101756 psi.

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CHAPTER VI

FUTURE WORK

In view of the problems associated with the unwinding and storage of NBL Nonwoven webs, the first thing to accomplish would be to measure and quantify the amount of camber present as a result of present quality control methods being employed. The camber thus quantified needs to be free of parameters such as friction between web and adjacent surface, density effects etc. Secondly, the elastic properties need to be characterized starting from a single fiber and moving on to larger sample sizes. The pattern emerging in Poisson’s ratio, moving from edge to the center, can help in determining the right nip loads to be applied while laminating Kraton with the Spun Bond. High temperature properties in CMD and radial directions need to be characterized in order to fully understand the effect of temperature on the roll as a whole. Further, the effect of temperature history profiles needs to be determined. A way to do this would be to subject the web to short term high temperature profiles and obtain long term low temperature profile data. Many polymers have been characterized for temperature histories, and a variation of similar approach could be used in this case. Transportation at sea level is usually accompanied by high humidity. It is therefore imperative that the effect of moisture be considered along with variations in temperatures. In view of modeling nonwoven completely, other constants of compliance matrix used to represent an orthotropic material need to be determined. Off-axis biaxial tests are one way to obtain shear modulus of the web. In the viscoelastic domain, further characterization of web in 450 orientation would allow constitutive modeling and hence provide all the inputs required for implementation of winding and roll models. However, it is important to consider the effect of localization. One way to do this would be to study the localizations throughout the roll and describe the pattern observed in terms of fiber orientation distribution defined as a function of roll and web location. For laminated photo printing paper, the time dependent properties could be characterized and an approach implemented in this study could be extended to the viscoelastic domain. It would provide an effective way to determine if lamination aggravates or alleviates the time sensitive properties of Polyethylene film added on to the base layer.

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CHAPTER VII

REFERENCES

1. Han Seong Kim. Relationship between fiber orientation distribution function and mechanical anisotropy of thermally point-bonded nonwovens. Fibers and Polymers, Vol 5, No.3, 177-181. Dept. of Textile Engineering, Pusan National University, Pusan, Korea, 609-735. 2004. 2. W. F. Chen, A.F. Saleeb. Constitutive Equations for Engineering Materials, p175, John Wiley and Sons. New York, 1982. 3. Rajeev Chhabra. Nonwoven uniformity - Measurements using image analysis. INJ Spring 2003, The Procter & Gamble Company. Cincinnati, Ohio. 2003. 4. Xiaonan Hou, Memis Acar, Vadim V. Silberschmidt. Tensile behavior of low density thermally bonded nonwoven material, Journal of Engineering fibers and fabrics, Vol 4, Issue 1, 2009. Wolfson School of Mechanical and Manufacturing Engineering, Loughborough university, LE11 3TU, UK. 2009. 5. Xiaonan Hou, Memis Acar, Vadim V. Silberschmidt. Finite Element Analysis of thermally bonded nonwoven material, 8th World congress on computational mechanics, (WCCM8), 5th European Congress on computational methods in applied science and engineering (ECCOMAS 2008), July 30- July 5, 2008, Venice, Italy. Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, LE11 3TU, UK. 2008. 6. Smita Bais-Singh, Sherill B. Biggers Jr., Bhuvenesh C. Goswami. Finite element modeling of the non-uniform deformation of spun-bonded nonwovens, Textile Res. J. 68(5), 327-342 (1998), School of textiles, fiber and polymer science, Clemson University, Clemson, South Carolina 29634, USA. 7. Dieter H. Mueller, Markus Kochmann. Numerical Modeling of thermo-bonded nonwovens, INJ Spring 2004, University of Bremen, Germany. 8. Han Seong Kim, Behnam Pourdeyhimi, Agaram Abhiraman, Prashant Desai. Characterization of structural changes in Nonwoven fabrics during load-deformation experiments. JTATM, Volume I, Issue I, Sept. 2000, NCSU, Georgia Tech, and Fiber Visions, Inc., USA.

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9. W. R. Qualls, J. K. Good. An orthotropic Viscoelastic Winding Model Including a Nonlinear Radial Stiffness. Journal of Applied Mechanics, March 1997, Vol 64 / 201. 10. American Society for Testing and Materials; Standard Test Method for measuring breaking strength of fabrics by constant rate of elongation apparatus: ASTM 5035. 11. J. J. Shelton. Effects of Web Camber on Handling, 4th International Conference on Web Handling, 1997, Oklahoma State University, Oklahoma. 12. William N. Findley, James S. Lai and Kasif Onaran. Creep and Relaxation of Nonlinear Viscoelastic Materials, North-Holland Publishing Company, 1976. 13. Knauss, W.G and Zhao, J. Improved Relaxation Time Coverage in Ramp-Strain Histories, Mechanics of Time-Dependent Materials, Vol 11, 199-216. 2007. 14. S. W. Park, Y. R. Kim. Inter-conversion between Relaxation Modulus and Creep Compliance for Viscoelastic Solids, Journals of materials in Civil Engineering, P 76-82, Feb 1999. 15. J. K. Good, David. R. Roisum. Winding Machines: Mechanics and Measurements. DEStech Publications, Inc. 2007. 16. Pan Bing, Xie Hui-min, Hua Tao, Anand Asundi. Measurement of Coefficient of thermal expansion of films using digital image correlation method. Polymer Testing 28(2009) 75-83. 17. American Society for Testing and Materials; Standard Test Method for Tensile Properties of Paper and Paperboard Using Constant-Rate-of-Elongation Apparatus: ASTM D828, 2002. 18. Tensile properties of paper and paperboard (using constant rate of elongation apparatus): TAPPI 494 OM6. 19. Determination of tensile properties unidirectional fiber-reinforced plastics: ISO 527-5, 2009.

VITA

Vinay Bhumannavar

Candidate for the Degree of

Master of Science Thesis: CHARACTERIZATION OF NECK BONDED LAMINATE NONWOVEN WEBS

AND PHOTO PRINTING PAPER Major Field: Mechanical Engineering Biographical: Education:

1. Completed the requirements for the Master of Science in Mechanical Engineering at Oklahoma State University, Stillwater, Oklahoma in December, 2009.

2. Completed the requirements for the Bachelor of Engineering in Mechanical Engineering at Visvesvaraya Technological University, Karnataka, India. July, 2006.

ADVISER’S APPROVAL: Dr. Hongbing Lu

Name: Vinay Bhumannavar Date of Degree: December, 2009 Institution: Oklahoma State University Location: Stillwater, Oklahoma Title of Study: CHARACTERIZATION OF NECK BONDED LAMINATE NONWOVEN

WEBS AND PHOTO PRINTING PAPER. Pages in Study: 75 Candidate for the Degree of Master of Science

Major Field: Mechanical Engineering Scope of study and Findings: The objective of this study is to characterize the mechanical properties of neck

bonded laminate nonwoven webs and photo printing paper. The stress-strain relationships in various orientations of the orthotropic web were studied, and Poisson’s ratio was determined using Digital Image Correlation. The experimental results have been compared with theoretical equations of Young’s modulus in 450 orientation, and an error of 3.3% was observed. The viscoelastic properties of the web were characterized using time-temperature superposition and compared with actual relatively long term data of over three weeks. The out-of-plane elastic and viscoelastic properties of the web were determined. Effect of higher temperatures on the web was studied and the thermal coefficient of expansion was determined. The elastic properties of three different photo printing papers were characterized and the stress-strain relationships were fit into appropriate equations. Poisson’s ratios for all the materials were determined using Digital Image Correlation. Dependence of all three photo paper’s mechanical behavior on strain rate was determined. The elastic out-of-plane properties of the three papers was characterized and fit to appropriate equations.