Abstract Method for Low Areal Density

ABSTRACT

METHOD FOR LOW AREAL DENSITY MATERIAL STRUCTURE CHARACTERI- ZATION: SOFT X-RAY FORMATION AND COMPRESSIBILITY MEASUREMENT

by Guizhou Wang

In this paper, a new method of characterizing nonwoven structure and a modification of paper towel compression study were discussed. Though nonwoven technology is not a new one, there is a lack of research on its structural properties. Due to its bulky nature, we used soft X-ray to take the radiographic images of nonwoven and paper towel samples. After preprocessing, the soft X-ray films of nonwoven samples were used to calculate the areal density using specific Matlab code on the workstation. Then the compressibility measurements were performed on the modified Continuous Micro Compression Tester for paper towel samples. Direct visualization of the compression process was obtained.

METHOD FOR LOW AREAL DENSITY MATERIAL STRUCTURE CHARACTERIZATION: SOFT X-RAY FORMATION AND COMPRESSI- BILITY MEASUREMENT

A Thesis

Submitted to the Faculty of Miami University In partial fulfillment of the requirements for the degree of Master of Science Department of Chemical, Paper and Biomedical Engineering by Guizhou Wang Miami University Oxford, Ohio 2015

Advisor: Dr. Steven Keller Reader: Dr. Shashi Lalvani

Reader: Dr. Douglas Coffin Contents

Chapter 1: Introduction ...... 1

1. Background ...... 1

1.1 Materials ...... 2

1.1.1 Tissue Paper ...... 2

1.1.2 Nonwovens ...... 6

What is nonwoven? ...... 7

The manufacturing of nonwovens ...... 7

1.2 Structure Characterization ...... 9

1.2.1 Formation Measurement ...... 9

1.2.2 Local Thickness Mapping ...... 10

1.2.3 Z-directional Compressibility Property ...... 11

2. Statement of problem and objectives ...... 14

2.1 Problem Statement ...... 14

2.2 Objectives ...... 15

Chapter 2: Formation Characterization for Low Areal Density Materials ...... 16

1. Principles for Soft X-ray Formation Measurement ...... 17

2. Materials ...... 18

3. Methods ...... 19

3.1 Soft X-ray Imaging ...... 19

ii 3.2 X-ray Film Development...... 20

3.3 Image Preprocessing and Processing ...... 20

4. Results and Discussion ...... 21

4.1 Textile-based Nonwovens ...... 21

4.2 Paper-based Nonwovens ...... 25

4.3 Extrusion-based Nonwovens ...... 29

4.4 X-radiographic Grammage vs Gravimetric Grammage of Nonwovens ...... 33

4.5 Formation Analysis ...... 34

Chapter 3: Compressibility Measurement for Paper Towel ...... 35

1. Review of Feng’s Compressibility Measurement ...... 36

2. Materials ...... 40

3. Experiment Methods ...... 41

3.1 Strain gauge calibration ...... 42

3.2 Determination of Z-strain ...... 45

3.3 Young’s Modulus ...... 46

4. Results and Discussion ...... 47

4.1 Compressive Response of CWP Embossed Features ...... 47

4.2 Compressive Response of TAD Patterns ...... 51

4.3 Young’s Modulus ...... 54

4.4 Comparison of CWP & TAD ...... 57

Chapter 4: Conclusions and Future Work ...... 60

i ii 1. Conclusions ...... 60

2. Future work ...... 61

References ...... 62

i ii List of Tables

Table 1-1. Basic Nonwoven Fabric Manufacturing Systems. 8

Table 2-1. Nonwoven Samples. 18

Table 2-2. Comparison of Gravimetric and X-radiographic Grammage. 33

Table 2-3. Comparison of COV. 34

Table 3-1. Compression Test Samples. 40

Table 3-2. Collapse Value of CWPs and TADs. 58

Table 3-3. Young’s Modulus of CWPs and TADs. 59

iv List of Figures

Figure 1-1. A Yankee Dryer with a tissue web dried on it. 3

Figure 1-2. Schematic figures of a tissue web before and after creping and rewetting after- wards. 4

Figure 1-3. A Through Air Dryer with a tissue web dried on it. 4

Figure 1-4. Schematic figures of a TAD tissue web before and after wetting. 5

Figure 1-5. Depictions of three embossing types. 6

Fig. 2-1 AXR Minishot Instrument for soft X-ray formation measurement. 19

Fig. 2-2. Structural maps of T1. 22

Fig. 2-3. Structural maps of T2. 23

Fig. 2-4. Structural maps of T3. 24

Fig. 2-5. Structural maps of P1. 26

Fig. 2-6. Structural maps of P2. 27

Fig. 2-7. Structural maps of P3. 28

Fig. 2-8. Structural maps of E1. 30

Fig. 2-9. Structural maps of E2. 31

Fig. 2-10. Structural maps of E3. 32

Fig. 3-1. Two Sided Compression (A) Versus Single Sided Compression (B) 36

Fig. 3-2. Compression of Tissue and Towels. 38

Fig. 3-3. Compression of Ideal Solid Foam. 39

Fig. 3-4. Continuous Micro Compression Instrument. 41

Fig. 3-5. Out of limit test for both strain gauges. 43

Fig. 3-6. Strain gauge force calibration. 44

Fig. 3-7. Stain gauge deflection calibration. 45

Fig. 3-8. Compression Plots of CWP1 Embossed Features. 48

Fig. 3-9. Feng’s work as a comparison. 48

Fig. 3-10. CWP1 embossed point 1. 50

Fig. 3-11. Compression Plots of CWP2 Embossed Features. 51

Fig. 3-12. Compression Plots of TAD1. 52

Fig. 3-13. Compression Plot of TAD1 Embossed Point 1. 53

Fig. 3-14. Compression Plots of TAD2. 53

Fig. 3-15. Compression Plot of TAD1 Embossed Point 1. 55

Fig. 3-16. Young’s modulus of each sample. 56

vi ACKNOWLEDGEMENTS

First, I want to give my deepest and sincere thanks to my advisor, Dr. Keller! Thanks for his patience, guide, and thoughtful ideas to help me both on my research and thesis during my graduate study here.

I also want to thank Dr. Lalvani and Dr. Coffin, for being my committee members and giving me precious suggestions and advices for my thesis.

I would also like to give my gratitude to Michael Weeks, for the LabVIEW programming of the micro compression instrument utilized in this research.

Finally, I want to thank all the professors, technicians, graduate students and my friends for their help and encouragement throughout my study here.

vii

Chapter 1: Introduction

1. Background

Low density (LD) fibrous materials hold an important place in society. From consumer goods such as hygienic tissues, kitchen towels, and nonwoven textiles, to more specialized applications such as filters, fuel cell components or thermal insulation, LD fibrous products are ubiquitously a part of everyday life.

This research focuses on low density (low grammage and low volumetric apparent density) web materials that fall within the categories of, 1) paper products comprised solely of wood based fibers, such as (toilet tissue, facial tissue and kitchen towel, and 2) nonwoven products that contain a considerable fraction of, if not entirely composed of, synthetic polymer fibers, such as wipes, liners, filters and disposable textiles. The significance of these materials for low cost personal hygiene, bacterial control and the filtration or isolation of haz- ardous chemicals or contagions makes further understanding of their structure and their response to mechanical forces important for both process and product development.

The global market for tissue products is approximately $57.4 billion USD (2014). The global market for nonwovens is about $26.5 billion USD (2013) and expected to reach $42.1 billion USD by the year 2020. With the ever increasing demand for these products in developing countries for the disease control and improving the standard of living, there is considerable interest in reducing production cost (energy and raw material) while meeting the regional demands for useful products. To facilitate improvements in the manufacturing process or the products themselves, it is essential to have a comprehensive characterization of the fibrous material structure to allow a cause and effect relationship between process steps and end use properties or behavior. The goal of this thesis is to examine specific as- pects of fibrous web characterization that have not yet been explored deeply.

1.1 Materials

1.1.1 Tissue Paper

Paper, which is defined as any fibrous structure felted from a fluid system onto a grid [1], is one of the most important inventions in history and also one of the most widely used materials in daily life nowadays. Among all the paper products, the market for tissue paper grades, which include facial tissues, paper towels, and napkins, is huge and is reported to be approximately $57.4 billion USD (2014). Similar to all other paper products, the manufacturing of tissue papers involves proper fiber selection to achieve the end use quality requirements with minimum costs, as well as the optimal formation processes to distribute the selected fibers in water and form a uniform wet web. However, unlike other paper products, such as printing papers and packaging boards for which manufacturers and customers place high value on fine scale material uniformity, controlled non-uniformity is intentionally introduced into tissue papers through many methods. Such methods include creping which creates corrugations and increases bulkiness, through-air Drying (TAD) which im- parts greater bulking of the web with regular TAD patterns, and embossing which is used for bonding multiple plies or for decorative purposes.

With the ever increasing consumer demand for more products and better quality, such as absorption and softness, while providing sufficient strength, it is recognized that a deeper understanding of the paper structures and how the structural elements and the manufacturing processes influence the final products is needed. The fact that tissue products are much bulkier and more deformable under much lower range of applied stress, and have small induced patterns in the range of 0.5 mm to 5 mm, increases the difficulty of separating the contributions of each process to the structure and the interdependency of structural features. As a consequence, the number of published papers dealing with is limited. In this paper, the focus is on paper towel and on a broad array of low grammage nonwoven materials. Sev- eral retail paper towel samples were selected to represent the products produced by the two main drying methods during the manufacturing: conventional wet pressing (CWP) and

through air drying (TAD). More details are given in the following sections. Since embossing is also typically introduced to tissue papers, especially for products dried by the CWP, a brief introduction to the processes is also included here.

 Conventional Wet Pressing (CWP)

In the conventional wet pressing process, a wet tissue web from the forming section is pressed onto a large heated cylinder which is referred to as the Yankee dryer.

Figure 1-1. A Yankee Dryer with a tissue web dried on it [2]

As depicted in Figure 1-1, a chemical solution containing adhesive compounds and release agents is continuously sprayed onto the surface of the dryer. The water is pressed out from the web at the pressure roller and evaporated rapidly by the heat transferred via conduction from the drum and through convection due to hot air outside of the web. As the water evap- orates, a bond-defined web structure is formed by hydrogen bonding between the fibers. The dried web is scraped off the dryer by a special doctor blade which causes the web to

buckle, forming cross direction ridges. This process is called creping. After creping, the tissue web becomes thicker, bulkier, more deformable and softer. However, when the web is rewetted, the cellulose fibers usually swell and straighten out, which results in the partially returning of the web to the bond-defined shape it had before creping, as shown in Figure 1- 2.

Figure 1-2. Schematic figures of a tissue web before and after creping and rewetting after- wards. [2]

 Through Air Drying (TAD)

As compared to conventional wet pressing, the through air drying process does not uniformly densify the tissue web and thus the bulk is maintained at both microscopic and mac- roscopic levels. In this process, the wet web from the previous sections is first transferred from one fabric to a second fabric which is slower under careful conditions, then placed on a three-dimensional through air drying fabric or belt. The moist web is then dried by the passing hot air through the belt/web assembly which creates a morphology in the dried web corresponding to that of the TAD fabric. Part of the process is depicted in Figure 1-3.

Figure 1-3. A Through Air Dryer with a tissue web dried on it.[2]

The hot air provides high heat transfer rates when passing through the web and dries the moist web without significant compression. One more thing to mention is that the web bond-defined morphology is inherently three-dimensional, since the web is in a bulky state as hydrogen bonds formed. So when the dry web is rewetted, the shape of the web will re- main relatively unchanged, as seen in Figure 1-4.

Figure 1-4. Schematic figures of a TAD tissue web before and after wetting.[2]

 Embossing

Embossing is also very common process utilized in tissue paper production, with the purpose to bond different layers, produce greater bulkiness and/or decorate the product. There are mainly three different embossing types: traditional, nested and foot-to-foot. For the traditional type, it embosses all plies simultaneously and is mainly used to produce bathroom tissue. As for nested and foot-to-foot types, each ply is embossed separately after which, the plies are aligned and bonded together. The difference between these two types is: for nested embossing, the peak of one ply is matched to the valley of the other; while for foot- to-foot type, the peaks of two adjacent plies are matched together. The schematic depiction of each embossing type is shown in Fig. 1-5. As compared to the traditional embossing method, the two later ones can improve the bulkiness and absorption ability to a great level for multi-ply products.

Figure 1-5. Depictions of three embossing types: A, traditional; B, nested; C, foot-to-foot.

1.1.2 Nonwovens

Unlike woven textiles, nonwoven fabrics are typically manufactured by the stochastic deposition of fibers, and produce structures that are similar in some respects to those made using wood based fibers in papermaking. But unlike paper products, whose main bonding mechanism is from hydrogen bonding, the fibers in nonwovens mainly involve synthetic fibers bonded together by chemical adhesive, thermal melting or mechanical interlocking forces. Nonwovens can provide specific properties such as absorbance, softness, filtration, liquid repellence, strength, thermal insulation and so on. They can mimic the texture of woven fabrics, or may be much bulkier for use in sound absorption, insulation or protective padding. In combination with other materials, they can be used for home furnishings, engineering, health care and consumer goods. Due to these advantages, the market for these products is ever increasing. According the World Nonwovens Market report[3], the world- wide demand for nonwovens is forecast to rise 5.3% annually to 9.0 million metric tons in 2017. A similar forecast from different perspective was given in another report saying the global market that was about $26.5 billion USD (2013) is expected to reach $42.1 billion USD by the year 2020. Though many reports pointed out that rate of increase varies from region to region, the global trend for nonwoven market is very promising.[4-6]

Therefore research interests in the development and investigation of nonwovens is ever growing, of which structural characterization is of much importance. The similarity of structure and behavioral properties between nonwovens and paper tissue and towel products, e.g. the web structural hierarchy and bulkiness suggests that the methods used to study the properties of both paper towel and tissue might find similar application for nonwovens.

 What is nonwoven?

Several definitions have been made by different associations, including the American Soci- ety for Testing and Materials (ASTM), the International Standards Organization (ISO), the Association of the Nonwoven Fabrics Industry (INDA), to distinguish nonwovens from other fabric materials, like paper and woven fabrics. Although similarities exist in these definitions, INDA’s is used in this paper due to the idea of adopting fiber composition and fiber length to diameter ratio into the definition. The following is the detailed definition [7]:

A sheet, web, or batt of natural and/or man-made fibers or filaments, excluding paper, that have not been converted into yarns, and that are bonded to each other by any of several means.

 The manufacturing of nonwovens

The basic concept in manufacturing nonwovens is to transform fiber-based materials into flat, flexible, porous, sheet structures with fabric characteristics [8]. Based on the technology utilized in the processes, the manufacturing methods can be divided into four groups: textile, paper, extrusion, or hybrid (combination).

Among them, the use of textile technology involves garneting, carding and aerodynamic forming methods to produce nonwovens. Due to the handling of fibers in dry state, fabrics manufactured by these systems are referred to dry laid nonwovens, carrying descriptive terms like garneted, carded or air laid. Other structures formed from tow or bonded by stitching filaments or yarns are also included in this category.

The paper technology base, which is specifically designed for relatively short fibers sus- pended in a fluid, involves dry laid pulp and wet pulp (modified). The basic principle is to disperse the fibers into a fluid and then stochastically deposit them onto a screen through which the fluid flows and upon which the sheet forms.

The extrusion-based process uses polymer extrusion methods to produce nonwovens. It includes spunbond, meltblown and film systems. Fabrics made by these systems are referred to as polymer laid nonwovens; or more individually as spunbond, meltblown and textured or apertured film nonwovens.

More detailed information can be found in the Table 1-1[8]. It also shows the typical four elements during the manufacturing processes in the table which include fiber selection and preparation, web formation, web consolidation and finishing.

Table 1-1. Basic Nonwoven Fabric Manufacturing Systems

Textile Paper Extrusion

Garnetting Carding Air Laid Fiber Air Laid Pulp Wet Laid Spunbond Meltblown Film

Fiber Natural and Manufactured Textile Fibers Natural and Manufactured Fiber/Pulp Fiberforming Ploymer chips Selection Mechanical, and Electrostatic, Aerodynamic, Perforate, Cast; Mechanical Opening Preparation Mechanical Opening and Volumetric Blending Aerodynamic, Fiber Cast and Gravimetric Feeding Wet Slurry Filament Orientation and Aperture Orientation Shattering

Mechanical Fluid Pattern Web Collection on Heat, Heat Parallel Fiber layers Random Fiber Matts Layering on Formaiton Isotropic Fiber Conveyor Stretch, Randomized Batts Controlled Fiber Conveyor Layers Screen or Shape Perforate, Crosslapped Layers Layers Screen

Mechanical Mechanical Mechanical Cooling Stitchbonding, Needlepunching, Hydroentangling Hydroentangling Needlepunching Web Chemical Consolidatio Sprayed Latex or Powder; Saturated, Printed, or Frothed Latex; Solvent n (Bonding) Thermal Thermal Calendar, Radiant or Convection Oven, Vaccum Drum or Mold, Laminating, Sonic Welding Slitting, Winding Finishing Other Application-Dependent Physical or Chemical Surface Treatments

1.2 Structure Characterization

Due to their production methods, there exists a hierarchy of structure in nonwovens and paper products, which is of critical importance for their end use properties and performance. The hierarchical structure includes fiber dimensions, formation, areal density and TAD or embossed features. The structure impacts web compaction (compressibility), softness, ten- sile strength to support handling and reuse and the ability to absorb liquids. To address these often conflicting objectives, optimization of the structure and proper selection of raw material is used to design and engineer a rather complex composited material. To support this effort to improve the inner structure, an important first step is to thoroughly characterize the contribution that each process step has in developing the structure. As mentioned above, the properties of nonwovens have similarity to that of paper towel and tissue. This suggests that the analytical methods used for towels and tissues might also be useful to characterize nonwovens. A quick review of the structural characterization of these parame- ters is now provided.

1.2.1 Formation Measurement

Formation is an indicator of how the fibers and other components are distributed in the sheet. It was defined by Corte as the local basis weight variation or local mass distribution[9]. Initially, it was approximated by the “look through” performed by an experienced human observer. However, this method is considered very subjective. With the development of related technologies, visible light transmission, β-transmission and soft X-ray transmission formation measurement of papers were introduced. The formation meter[10] is the first objective method trying to characterize the formation which measures the optical density by light transmission. Corte used β-transmission to measure the distribution of mass density instead of the distribution of optical density[9]. Norman and Wahren [11] applied β-transmission radiography on X-ray film to study the mass formation. Cresson [12] introduced a video β-transmission system to rapidly scan film into a computer system. Kel- ler et al. [13] utilized a storage phosphor screen instead of film to study β-transmission and found that β-transmission was not suitable to measure low density or bulky material due to

the material thickness that causes a standoff distance effect which further causes the defo- cussing of features as the distance increases. Farrington[14] demonstrated that soft X-ray could be used to characterize paper formation, although radiographs suffered from a geo- metrically based spatial non-uniformity of radiation intensity. Tomimasu [15] compared β- radiography with the other three methods, which included electron beam, light transmission and soft X-radiography. He found that β-radiography and electron beam imaging are supe- rior to light transmission but inferior to soft X-radiography in terms of spatial resolution. Most recently, Feng [16] has improved the soft X-ray methods and successfully corrected the spatial nonuniformity artifact to measure several retail paper towel samples. In this investigation, we use the approach used by Feng to study characterize the structure of nonwoven materials.

1.2.2 Local Thickness Mapping

The widely used thickness measurement standard is the TAPPI method T-410, in which the paper sample is pressed to 50kPa between two 16 mm diameter steel platens and the separation is measured. This is a simple test, but it has three major drawbacks. First, it preferentially measures the thickest regions and may not even contact thinner regions with less mass, thus making it dependent on distribution of mass, i.e. formation, and on densification processes such as pressing and calendering. Secondly, it represents an approximated average thickness of the paper within the 16 mm diameter region of the platens, that requires multiple tests to account for the variability of the paper at larger scales within the sample. Lastly, and most importantly, for highly deformable papers and nonwovens, the sample will be subject to 50 kPa pressure of compressive force, which de- forms the paper from its original thickness. Due to the fact that tissue papers are creped or embossed during the manufacturing processes, they tend to be much more compressible (easy to deform even under very low pressure), have more induced features that cause less uniformity at various length scales. Therefore, the TAPPI standard method or any other contacting method is considered unsuitable to obtain an accurate thickness of tissue and towel papers, as well as many nonwoven materials that have similar structures. For these reasons, the use of non-contacting thickness measurement, such as those based on laser

range sensing, provide more useful thickness values when examining the thickness of bulky materials. More recently, Huang utilized the twin laser profilometry (TLP) to map the local thickness variations and out-of-plane deformation of several paper towel over 12.5mm12.5mm regions with the resolution ofm.

1.2.3 Z-directional Compressibility Property

Compressibility is one of the most important properties that manufacturers and customers would concern about for the paper towel tissue and nonwovens. It is defined as the absolute change in the thickness of the material at the direction which the stress is applied [17]. Compared to printing grades, paper towel is much bulkier, more deformable even under much lower range of applied stress and has small induced patterns which falls into the range of 0.5mm to 10mm. Whether it is dried through the CWP or TAD method, the radial forces that occur in the winding process can have detrimental effect on bulky webs that do not have the compressive strength or resiliency. When webs are rolled properly, the wound in tension is greater in the center and decreases towards outer layers. Thus the radial force compresses the web and can cause irreversible crushing of the web structures to a different extent through the entire roll. From the manufacturers’ perspectives, the tendency is to over engineer the compressive strength of web features to ensure that crushing does not occur, making the product more rigid. On the other hand, consumers will detect this hardness and consider it less soft as a result. Clearly for paper towels, a balance must be found between compressibility for softness and resiliency for winding and storage. The study of local compressibility, especially of engineered features in paper towels has heretofore rarely been the subject of published works in the literature.

Several investigations have previously reported on the mapping of compressibility of paper, including Van Eperen [18], Liu [19], Leporte [20], Pawlak [21], and Feng [16]. Among them, Van Eperen [18] modeled compressibility as one of the four factors which were closely related to softness. An Instron universal testing machine was used in his research to perform compression measurement on tissue specimen using multi-plies and a

66.4 cm2 area. Liu[19] used a Thwing-Albert Model 89-100 instrument to study the compressibility of tissue on a 2×2 in area at several pressure levels up to 3.571 kPa. Since these two studies performed the compression measurement on a relatively large area, they found a poor correlation for compressibility and softness. More recently, Leporte [20] introduced the idea of “micro-compaction” by utilizing the Fiber-Load-Elongation-Record (FLER). A probe with a diameter of 0.79mm was used for the micro-compaction measurement on retail tissue samples which were comprised of 1-ply, 2-ply and 4-ply. Inasmuch as the limits of the basis weight and thickness measurement methods used in his research, only a poor correlation of the distribution of responses to loading was found. More recently Pawlak [22, 23] mapped the local variation of compressibility in a single paper. He placed the sample vertically in the sample frame. One side of the sample was backed with a smooth surface, the other side was subject to a continuous indentation. Later, Feng [24, 25] adopted and modified Pawlak’s method to map compressibility of paper towel samples.

Due to the bulky nature of paper towel tissue, Feng’s approach was also used in this study, although the sensitivity was substantially improved upon to improve the resolution of the compressive response of the delicate embossed and TAD features. The method is referred to as continuous micro-compression measurement. In Feng’s work [24] several retail paper towels were studied by using two probes diameters, 2.37mm or 1.83mm. A two-sided compression method was developed by pressing the out surfaces of the sample with two identi- cal probes positioned in opposition, the deformation of load cells was used to determine the local compressive responses. By comparing the load – deformation plots at different loca- tions within the samples, he was able to demonstrate the differences in behavior between the CWP and TAD samples and the variability with a single sample. However, due to the size of the probes, the tests measured the compressibility of the web through the thickness, and not the specific collapse of individual features, such as the TAD domes or cylindrical (embossed) fine scale features. The force sensor utilized in Feng’s work had a maximum load of 2N or 205g, which provided insufficient sensitivity to measure the small forces (< 10 kPa) typically encountered during the collapse of weaker towel structures, such as the TAD protrusions. The results he reported were most sensitive to the compressive responses

of the planar web structure and gave only limited sensitivity to the collapse of important structural features such as embossments and TAD domes. Also, he did not correlate the compressive response to visual observations of the change of shape that the web undergoes during the test. Such observations contribute significantly to the understanding of the mechanisms that control crush resistance and the response of the web to external loading.

2. Statement of problem and objectives

2.1 Problem Statement

Several investigations have been conducted to study the structure of bulky fibrous materials, especially for paper towel and tissue. These studies use methods such as mapping areal mass distribution that may also be valuable applied to nonwoven materials. As for the formation of nonwovens, Branca [26] attempted to use β-transmission radiography, which is widely used as an off-line formation imaging method, but found that resolution of imaging was limited. Images showed a blurring artifact that was caused by web thickness, the dif- fuse source and the large stand-off distance (distance between the sample and detector). β- transmission was essentially shown to be ineffective for precise imaging with these bulky nonwovens materials. We proposed to use the soft X-ray method demonstrated by Feng [24] that was used in imaging the distribution of mass for bulky paper towel samples. How- ever, since nonwovens might have greater bulk or grammage than paper towel or tissue, the question remains whether this method would be suitable or not for this application.

For the compressibility of paper towel samples, the need existed to develop a new method, based on the earlier work of Feng to increase the strain sensitivity, and implement an opposing probe configuration, that enables the measurement of the compressive response of individual features in paper towels. Furthermore, since it has not been attempted, the direct correlation with visual observation of feature collapse, as with video imaging, is essential to associate compressive behavior with the manner in which the structure fails.

2.2 Objectives

There are two major objectives for this investigation. The first was to determine the suitability of soft X-ray for use in the mapping of the areal mass distribution of various nonwoven samples. Once attained, such maps would be used to provide information about structural formation, such as first order statistics, i.e. formation numbers.

The second one is to modify the continuous micro-compression instrument and bring the direct visualization of compression into reality. Once designed and built, the instrument will be used to test the compressive response at each point for several representative paper towel samples. The relationship between the compressive responses and the local formation will be discussed afterward.

Chapter 2: Formation Characterization for Low Areal Density Materials

As an indicator of how the fibers and other components are distributed in the sheet, the characterization of areal mass formation is of great importance. It is the very first step to look into the inner structure of sheet. In order to get the best position to study the sheet structure, there is always a balance between the size of the sampled area and the attainable resolution of the technique. Along the way of development, three different methods based on light transmission, β-transmission and soft X-ray transmission respectively, have oc- curred simultaneously. With regard to bulky fibrous materials such as many nonwovens, Keller and Pawlak [13] utilized a storage phosphor screen to study β-transmission. They found that β-transmission was not suitable to measure low density or bulky material due to the standoff distance effect, where the spatial resolution of the mass is a function of the distance between the mass and the detector (film). More recently, it has been shown by Feng [24]that soft X-ray could be used to measure the formation of the thick and bulky structure of paper towel samples. Since nonwoven samples are usually composed of organic polymers and contain little or no inorganic matter, the method used by Feng to map formation is expected to perform well for the measurement of nonwoven formation. However, this hy- pothesis requires verification.

Details of soft X-ray formation measurement will be discussed in detail in this chapter. Samples within three groups of nonwovens were chosen to represent the textile-, paper- and polymer extrusion-based manufacturing methods. Formation maps were obtained and statistically analyzed. Obvious differences were found for each group. Comparisons between the areal density based on the soft X-ray method and gravimetric method were also discussed in the last section.

1. Principles for Soft X-ray Formation Measurement

The primary principle of the method is the X-ray beams are attenuated when they encoun- ter matter on the travelling path by the Beer-Lambert Law:

−푘,푤 퐼표푢푡 = 퐼푖푛 ∙ 푒

Where 퐼표푢푡 is the X-ray intensity after penetrating the material, 퐼푖푛 is the incident intensity, k’ (m2g-1) is the mass absorption coefficient, w (gm-2) is the material basis weight.

As for the spatial non-uniformity of X-ray intensity, the correction method used by Feng is adopted in this study. This involves application of the cosine fourth law [27], to create a spatially uniform baseline of intensity from which variation in mass is represented by differences in detected intensity, Iout at small scale sample areas, referred to as pixels. This study also uses a Mylar step wedge with different thicknesses of the plastic film as an internal standard for each X-ray film so that the local areal density can be accurately determined from which the formation could be analyzed.

First, the mean areal density is obtained by using the following equation:

푁푥 푁푦 훽푖.푗 훽̅ = ∑ ∑ 푁푥푁푦 푖=1 푗=1 where (i, j) is the element (pixel) in the ith row of the jth column in the areal density map; β is the local areal density value; N indicated the total number of row, x, and column, y, in the selected area.

Then, standard deviation of grammage, 휎훽, is calculated based on 훽̅. After which, the coefficient of variation, COV, is obtained by:

휎훽 퐶푂푉 = 훽

2. Materials

The low areal density materials of interest in this research are specifically nonwovens and paper towels. Representative samples were selected from each of these two groups. The nonwoven samples were chosen from Nonwoven Fabrics Sampler and Technology Refer- ence and divided into three minor groups to represent each of the three producing methods: textile, hybrid paper and polymer extrusion. Samples were trimmed to the dimension of 70 mm×70 mm. Gravimetric grammage were measured using the TAPPI standard T410[28] and record in Table 2-1.

Table 2-1 Nonwoven Samples

Sample Web Formation Grammage Sample Group Fiber Bonding ID Method /(g/m2) T1 Polyester Card Thermal Powder 41.2 Textile-based T2 Rayon Card, Perforate Latex Print 51.7 Nonwovens T2 Rayon, Polyester Card Hydroentangled/Pattern 64.0 P1 Wood Pulp, Synthetic Air Laid Hot Air, Embossed 60.0 Paper-based P2 Glass Wet Laid Chemical 84.9 Nonwovens P3 Wood Pulp, Synthetic Wet Laid Chemical 70.2 Extrusion- E1 Polyester Spunbond Thermal Area 92.4 based E2 Biocomponent Spunbond Thermal Area 104.3 Nonwovens E3 Polypropylene Meltblown Entangled, Cooled 19.3

3. Methods

3.1 Soft X-ray Imaging

An AXR Minishot X-ray cabinet (East Heaven Corporation, 06512), shown as Fig. 2-1, was used to perform soft X-ray formation measurement.

Fig. 2-1 AXR Minishot Instrument for soft X-ray formation measurement. Left: X-ray In- strumentation. Right: Illustration of exposure chamber internal structure and spatial placement of paper board template, sample, X-ray film, wood grid and plastic board.

The method reported by Feng [24] was used here to get the soft X-ray films. Due to the sensitivity of spatial artifacts, guides were used with the dimension of 8 in ×10 in to be certain that for each exposure, the film would be fixed to the center of the support shelf. A pa- perboard template to support and handle the sample was used. It had a square opening of

7.5 cm×7.5 cm in the center to expose the sample and another opening of 1.5 cm×7.5 cm beneath the center opening, for placement of the Mylar films step wedge with known grammages. In order to maximize the final image quality, the sample was attached to the central opening and placed between the template and X-ray film. The X-ray tube acceleration voltage and exposure time were set to be 6kV and 16 minutes respectively.

3.2 X-ray Film Development

After exposure, the X-ray films was developed in a dark room using reagents from A.R Si- mon (Baltimore, MD) according to the film manufacturer’s suggested processing. The pro- cedure used to develop the soft X-ray films is the traditional wet condition treatment. First, the fixed X-ray films were immersed in the developer tank for five minutes. To prevent film defects such as streaks, every 30 seconds film hangers were moved up and down during development to ensure that the developer solution as even as possible in the tank. After developing, films were rinsed with clear water in the rinse tank for three minutes, to prevent the forming of stripes and fog on films and the rapid neutralization of fixer solution by developer. The films were then transferred to immerse in the fixer tank for another five minutes to remove the unreacted silver halide from the emulsion layer, with the purpose to render the image formed permanently in the development step. The films were then put into the final wash tank for 30 minutes before drying, to remove all the residual fixer and unreacted soluble silver halide to extend the longevity of the films. After the washing step, films were hung to drain and carefully wiped by paper towel before air dry.

3.3 Image Preprocessing and Processing

An Epson V700 high resolution scanner was used to scan the X-ray films in transparency mode at 4800 dpi and 16 bit gray level depth. Adobe Photoshop was then used to crop and divide the image by an overlay mask to isolate the test area and realign to a reference so that the cosine fourth correction can be applied. This was accomplished using a custom routine executed in Matlab. Areal density calculations (based on the Mylar film calibration curve) were then performed using a custom routine in Matlab 12b on a Hewlett-Packard HP Z600 Workstation computer.

4. Results and Discussion

In this section, the formation maps for nonwoven samples generated from soft X-ray images are discussed and compared.

4.1 Textile-based Nonwovens

Structural maps for nonwoven samples T1, T2 and T3 were shown in Figure 2-2, Figure 2- 3 and Figure 2-4 respectively. To assist in the interpretation of these graphs, basic information of carding and hydroentanglement is presented here.

Carding is the mechanical process which disentangles, cleans and intermixes fibers to produce a continuous web for subsequent process. It is achieved by passing fibers through dif- ferentially moving surfaces which are covered with card clothing.

Hydroentanglement is the bonding process for wet or dry fibrous webs made by carding, air-laid or wet-laid processes. It entangles the web with a patterned weave which, as a result, gives the web a lacy appearance.

Since all these three samples utilized the carding web formation method, it was reasonable to assume that the fiber orientations of the web should be generally in one direction, or more specifically, the machine direction. This was demonstrated in the X-ray images. In these figures, most of the fibers aligned in the machine direction, which is shown as verti- cal orientation in the figures. Besides this, X-ray images also provide additional information.

In Figure 2-2, there were many bright spots in the X-ray images which are not observed in the other two figures. The thermal powder method was used for bonding the T1 sample while the others use fusion of individual fibers. The bright spots are where the thermal powder adhesive is fused. In Figure 2-3 and Figure 2-4, nicely shaped and organized openings were observed, but there were differences between them. Besides carding, T2 also used a perforating process during the web formation. Therefore, the shapes of openings are

quite uniform and there are much less fibers stretched out to the openings. For T3, the visible openings are created from the hydroentanglement bonding method used to form the structure. As a consequence, the opening shapes were less uniform and also more fibers were observed in the openings.

A T1 B

10 mm 5mm

C D

10 mm 1 mm

Fig. 2-2. Structural maps of T1: A is X-ray graph for T1 to determine the local grammage with a resolution of 5.3 μm; B and D are the expanded view of the central area; C is the light transmission photograph of T1.

A T2 B

10 mm 5mm

C D

10 mm 1 mm

Fig. 2-3. Structural maps of T2: A is X-ray graph for T2 to determine the local grammage with a resolution of 5.3 μm; B and D are the expanded view of the central area; C is the light transmission photograph of T2.

A T3 B

10 mm 5mm

C D

10 mm 1 mm

Fig. 2-4. Structural maps of T3: A is X-ray graph for T3 to determine the local grammage with a resolution of 5.3 μm; B and D are the expanded view of the central area; C is the light transmission photograph of T3.

4.2 Paper-based Nonwovens

Structural maps for nonwoven sample P1, P2 and P3 are shown in Figure 2-5, Figure 2-6 and Figure 2-7, respectively. Due to the web formation method differences, these samples were quite different as compared to the three textile-based samples. Sample P1, P2 and P3 all had a stochastic distribution of fibers similar to that of paper. Therefore, they had finer porous structures. The X-ray images of P1 and P3 looked quite similar because their basis weights were very close and they were both made of wood pulp, though they had different appearances in light transmission photographs due to densification of the structure by the different embossing patterns. This confirms a previous finding by Branca [26, 29] which stated that the embossing process does not relocate the mass in the plane.

Despite the stochastic fiber deposition, P2 was much different from P1 and P3. The reason is that P2 was composed of glass fibers instead of wood pulp. As the nature of glass fibers, they are very straight and have uniform shapes. That was why straight and uniform fibers were observed in Figure 2-6. However, an accurate grammage is not attainable, since the mass absorption coefficient for glass differs greatly from that of lignocellulosic or petro- leum based polymers, and an appropriate was not developed for this sample.

A P1 B

10 mm 5mm

C D

10 mm 1 mm

Fig. 2-5. Structural maps of P1: A is X-ray graph for P1 to determine the local grammage with a resolution of 5.3 μm; B and D are the expanded view of the central area; C is the light transmission photograph of P1.

A P2 B

10 mm 5mm

C D

10 mm 1 mm

Fig. 2-6. Structural maps of P2: A is X-ray graph for P2 to determine the local grammage with a resolution of 5.3 μm; B and D are the expanded view of the central area; C is the light transmission photograph of P2.

A P3 B

10 mm 5mm

C D

10 mm 1 mm

Fig. 2-7. Structural maps of P3: A is X-ray graph for P3 to determine the local grammage with a resolution of 5.3 μm; B and D are the expanded view of the central area; C is the light transmission photograph of P3.

4.3 Extrusion-based Nonwovens

Structural maps for nonwoven sample E1, E2 and E3 are shown in Figure 2-8 to 2-10 respectively. Due to the fiber deposition on the moving conveyor by this forming process, the fiber alignment of each of the three samples is principally oriented in the machine direction. E1 and E2 were quite similar both in the X-ray images and the light transmission photograph. This is because both samples used the same web forming and bonding methods, and their grammages were also very close. Fiber edges are clearly visible in both figures.

However, the E3 sample was much different from E1 and E2. This likely resulted from two major differences: the mean grammage and the method used for forming the web. The grammage of E3 was less than 20 g/m2, while that of E1 or E2 was about 100 g/m2. There- fore, the X-ray images of E3 appear darker than the images for E1 and E2, an indication that there was much less mass in exposed area. As for the meltblown forming method used for E3, fibers are supposed to be finer, although this cannot be verified by either the light transmission photograph or the X-ray images. The possible explanation is grammage is so low, and the imaging methods have insufficient resolution.

A E1 B

10 mm 5mm

C D

10 mm 1 mm

Fig. 2-8. Structural maps of E1: A is X-ray graph for E1 to determine the local grammage with a resolution of 5.3 μm; B and D are the expanded view of the central area; C is the light transmission photograph of E1.

A E2 B

10 mm 5mm

C D

10 mm 1 mm

Fig. 2-9. Structural maps of E2: A is X-ray graph for E2 to determine the local grammage with a resolution of 5.3 μm; B and D are the expanded view of the central area; C is the light transmission photograph of E2.

A E3 B

10 mm 5mm

C D

10 mm 1 mm

Fig. 2-10. Structural maps of E3: A is X-ray graph for E3 to determine the local grammage with a resolution of 5.3 μm; B and D are the expanded view of the central area; C is the light transmission photograph of E3.

4.4 X-radiographic Grammage vs Gravimetric Grammage of Nonwovens

Table 2-2 shows the comparison of the mean grammages of nonwoven samples measured gravimetrically and using X-radiographic imaging. From this table, it may be seen that for most of the nonwoven samples (T1, T3; P1, P3; E1, E2), X-radiographic grammage closely approximated that measured gravimetrically, with a relative ∆W of ± 10%. This confirms that soft X-ray can be used accurately map the formation of most nonwovens.

The three outliers, T2, P2 and E3 had sufficiently different mass absorption coefficients since T2 utilized latex print as the bonding method; P2 used glass fiber as the raw material; E3 was made of polypropylene. Such differences in chemical composition can be overcome if the material is homogeneous and a more appropriate internal calibration step wedge is used instead of Mylar. However, if the web is comprised a blend of compounds that have widely varying absorption coefficients, then soft X-radiography cannot be used to accurately map formation. This suggested that before using soft X-ray to study the formation of nonwovens, close examination of the material composition is recommended to ensure the suitability.

Table 2-2. Comparison of Gravimetric and X-radiographic Grammage

Sample Gravimetric/ Relative Sample Group Fiber X-ray/(g/m2) ID (g/m2) ∆W T1 Polyester 41.2 37.2 9.6% Textile-based T2 Rayon 51.7 61.9 -19.8% Nonwovens T3 Rayon, Polyester 64.0 69.9 -9.3% P1 Wood Pulp, Synthetic 60.0 64.6 -7.7% Paper-based P2 Glass 84.9 245.4 -188.9% Nonwovens P3 Wood Pulp, Synthetic 70.2 71.0 -1.1% Extrusion- E1 Polyester 92.4 85.3 7.7% based E2 Biocomponent 104.3 92.6 11.2% Nonwovens E3 Polypropylene 19.3 13.0 32.4%

Notes: Relative ∆W = (Gravimetric – X-ray) / Gravimetric.

4.5 Formation Analysis

Table 2-3 shows the comparison between samples regarding the coefficient of variation (COV). In this table, it can be seen that the COV of P3 is the lowest; COVs of the paper- based group (P1, P2 and P3) are lower than those of other groups. This is in line with what has been observed from the above graphs. In these graphs, the paper-based group shows the most uniform fiber distributions, among which the fibers in P3 distributed most uniformly.

The COVs of the extrusion-based group also show good correlations with the graphs. In this group, E3 has the highest COV due to the lowest basis weight it has.

As for the textile-based group, the fusion spots in T1 makes the mass distribution less uniform, therefore it has a relatively higher COV. When it comes to T2 or T3, the COV is much higher since there are regular patterns or openings in the samples which makes formation measurement not suitable here. Therefore, they are not included here.

Table 2-3. Comparison of COV

Sample ID COV T1 60 P1 36.4 P2 30.7 P3 24.1 E1 38.9 E2 54.5 E3 129.4

Chapter 3: Compressibility Measurement for Pa- per Towel

The end use properties, including absorptivity, flexibility, softness, strength and others, of soft hygienic products such as tissues, napkins and towels, stem from the bulky fibrous structures. Features like creping, through air drying (TAD) patterning and embossing are intentionally used to bulk the structure while maintaining strength between different layers. Earlier studies have been conducted to characterize the compressive responses of various regions of several different retail towels that include these bulking structural features. A micro-compression instrument was previously developed to address this problem [24]. It consisted of two axially aligned cylindrical probes used to compress the sample from both sides, recording stresses up to 400 kPa. The goal of this research study was to measure and characterize a much lower range of compressive responses of towel which influence its end use properties mentioned above. The focus was on recording the compressive response curves that show the yield stress, limiting web thickness at 80 kPa and the Young’s modulus, characteristics of the collapse of TAD patterning and embossments found in paper towel samples. A modified micro compression instrument was developed to optimize the load sensitivity. The compressive responses with sensitivity to 1.0 kPa were recorded for each 0.5 μm step as specimen were compressed between a 2.37 mm diameter probe and a planar backing plate. The mass of the fibrous web within the test region was also determined so that apparent structural density could be accounted for. Video imaging of the web compression and collapse event was recorded which enabled a correlation between the mode of structural deformation and the load measured on the sensor. This further enhanced the ability to interpret the compressive behaviors of the different towels. The results showed that conventional wet pressing embossment had a greater yield stress and Young’s modulus, while through air drying pattern had a larger web limiting thickness.

1. Review of Feng’s Compressibility Measurement

To better understand the work in this research, this section provides a review of the method developed by Feng [24] for compressibility measurement. In that work, two sided compression (using two opposing probes to press simultaneously from each side) were used to measure the compressibility, an example of which is shown in Fig. 3-1(A). It is easy to see that this method is effective for the non-embossed and embossed regions when only the web collapse is of interest. It is not suitable for studying the structural collapses of embossed features and TAD domes with dimensions on the order of several millimeters. When the scale of embossed region is large enough, the back probe enters the embossed region partially or completely so that feature collapse is not observed. Only contact with the web and subsequent compression is recorded. In this case, the embossed region would be treated as non-embossed region in that only web compression is measured. That is one of the main reasons that Feng’s approach was not entirely useful for investigating the collapse of individual embossed features.

A B

Fig. 3-1. (A) Two Sided Compression; (B) Single Sided Compression

To overcome this issue, Single sided compression was introduced in this investigation, as illustrated in Fig. 3-1(B). A back plate was utilized instead of back probe to support the 36

sample during the compression. Only the front probe was moved during loading to press the sample.

Meanwhile according to Rowe[30] when the compression pressure approaches 70 kPa, the relative thicknesses of towel and tissue are equal to flattened paper. This means under this pressure, towel and tissue will no longer have their bulky and soft nature, void spaces will be eliminated as fiber-fiber contacts increase to that of much denser papers. Therefore, the low stress range, namely around 10 kPa is of most interest to those studying and designing these bulky materials. In Feng’s work [24], he was able to differentiate the embossed and non-embossed patterns, but with a much lower sensitivity of the load sensor and a quite large spanning of stress values, up to 400 kPa. This provided little information about the compressive response of more delicate features such as the TAD belt and fabric patterns, which significantly influence the softness of the materials.

2 Relative Thickness Relative

Rowe and Volkerman 1965 0.05 0.1 0.5 1 5 10 Pressure, psi 0.340.34 0.690.7 3.43.44 6.89 6.9 34.434 68.969 Pressure,Pressure, kPa kPa

Fig. 3-2. Compression of Tissue and Towels

In order to study compression properties of towel samples under fine scale of stress, a second important modification of the micro-compression instrument was made. A more sensitive load cell was affixed to the existing load cell so that much lower range of stress values could be recorded. The use of this tandem sensor configuration permitted the accurate continuous recording of stress values under 10 kPa. What’s more, a digital camera was in- stalled on the platform to record images simultaneously during the compression testing of each sample, which in turn enabled the directly viewing of the different stages of collapse of features that corresponded to a given applied load. With the help of this technology, the measurement of compressive responses for paper towel was brought to a brand new stage.

To better understand the compression behavior of paper towel, the compression of ideal solid foam is introduced here, which illustrated in Fig. 3-3. Three phases describe the whole process of compression: Elastic Deformation of the Structure, Collapse of the Struc- ture (Plastic Deformation) and Compacted Structure (Deformation of Wall Material). Simi- lar phases were found in the compression test of paper towel, which were discussed in a later section.

Phase III Phase II Phase I Phase I: Elastic Deformation

of the Structure Stress

Phase II: Collapse of Structure, Plastic Deformation

Phase III: Compacted Structure, Compressibility Deformation of Wall Material Platen Direction of Compression Separation

Fig. 3-3. Compression of Ideal Solid Foam [22]

2. Materials

Two Conventional Wet Pressing (CWP) samples and two Through Air Dried (TAD) samples with similar grammage were tested here using the modified compression micro compression instrument. All measurements were based on single ply. If the sample was two-ply or multi-ply, then only the top-ply was separated and tested.

Table 3-1. Compression Test Samples

Drying Gravimetric Soft X-ray Grammage Sample Name Method Grammage/(g/m2) Grammage/(g/m2) Deviation/(%) CWP1 CWP 21.91 21.51 0.02

CWP2 CWP 25.33 27.19 -0.07

TAD1 TAD 21.58 20.38 0.06

TAD2 TAD 22.07 24.96 -0.13

Notes: Grammage Deviation = (Gravimetric Grammage - Soft X-ray Grammage)/ Gravimetric Grammage.

3. Experiment Methods

Micro compression testing was performed on the platform shown in Figure 3-4. For this study, a small back plate was moved in place to contact the back of the sample, while the front probe, with diameter 2.37mm, was used to apply a flat surface to compress the sample. There were two other modifications from the original configuration [24] included the addition of a more sensitive strain gauge and a digital camera (CMOS 1.3 Megapixel Sen- sor Color Board Camera 24 C1.3XDIG) mounted adjacent to and directed toward the front probe. This allowed images to be captured simultaneously during the compression test. During the measurement, A3200 and LabView 2012 software were used to control the movement of the platform and recording the data sets. Samples were held vertically by the frame with an opening of 70 mm×70 mm in the center. The back plate was first moved towards the sample until it contacted the back surface of the sample. Then, the front probe was advanced toward the sample until it reached a predesignated force limit. Compressive data sets and pictures were recorded, time stamped and saved during the process.

Figure 3-4. Continuous Micro Compression Instrument

3.1 Strain gauge calibration

Two strain gauges were used in the test. The idea of introducing the large load cell (S251 Miniature Platform Load Cell) was to expand the range of sensitivity when the small load cell (SMD3277-030) reached its limit. In order to be certain that the strain gauge was maintained in the linear range of performance, a test of the senor limits was first performed. A calibration curve was developed to convert strain amplifier voltage (mV) to force (mN), and further convert to stress (kPa) based on the area of the probe, 2.37 mm. Meanwhile, the load cell itself was strained during compression, and so a deflection calibration curve for load cell also needed to be generated. Both load cells were calibrated before the test and re- calibrated periodically in order to maintain the accuracy of system.

First, for sensor limit test, the front probe was brought into contact with the back plate, and then advanced with a step rate of 0.001 mm/s until the linear range was passed. The front probe defection value was plotted against the relative force data with the unit of mV, the plots for both the small and large load cells are shown in Figure 3-5.

From the figure, it is easy to see that both load cells maintain linearity until the deflection was around 0.4mm. Therefore, to be sure of a linear translation of compression and load cell response, -25.00 mV was set as the limit for the small load cell in subsequent testing.

Force calibration was then performed for the load cells. The load cells were first held vertically using three prong ring stand clamps. An analytical balance was used to calibrate the load cells. The actual force measured with the analytical balance was plotted against the relative force, as shown in Fig. 3-6. As evident in the plot, linear correlations were excellent for both load cells.

-5

-10

-15 Relative -20 Force (mV) -25

-30

-35

-40 0 200 400 600 800 1000 1200 1400

Deflection (μm)

-10

Relative-20 Force -30 (mV) -40

-50

-60 0 200 400 600 800 1000 1200 1400

Deflection (μm)

Figure 3-5. Out of limit test for both strain gauges: upper one is for small strain gauge; bottom one is for the large one.

0.0

-5.0 y = -0.0101x - 0.9299 R² = 1 -10.0 Amplifier -15.0 Output (mV) -20.0

-25.0 y = -0.051x - 7.2785 R² = 1 -30.0 0 100 200 300 400 500

Force (mN)

Figure 3-6. Strain gauge force calibration: red dots are for the large strain gauge; blue is for small one.

The last step of load cell calibration was the deflection calibration. The front probe and back plate were brought into contact first. The front probe was then advanced towards the back plate with step rate of 0.001mm/s. Excellent linear correlation was shown in Fig. 3-7.

400 y = 1.048x - 3.0311 350 R² = 1 300

250

Force 200 (mN) 150

100

0 0 50 100 150 200 250 300 350 400

Deflection (μm)

Fig. 3-7. Strain gauge deflection calibration

3.2 Determination of Z-strain

As mentioned in the calibration section above, the load cell would also deflect during the compression test. Compensation for this is needed in order to determine the actual compressive strain that occurs under the probe during a test. The true strain can also be used to determine the thickness of the sample as the compression test progresses. The sample thickness was calculated using the following equation:

푍 = 퐷푖푛푖푡푖푎푙 − 퐷푓 − 퐷푏 + 퐷푑

Where Z is the sample thickness, µm; 퐷푖푛푖푡푖푎푙 is the initial distance between the front probe and the back plate, µm; 퐷푓 is the front probe z-directional position, µm; 퐷푏 is the back plate z-directional position, µm; 퐷푑 is the deflection of the probe, µm.

The initial distance,퐷푖푛푖푡푖푎푙, between the front probe and the back plate was found to be 21.467 mm.

3.3 Young’s Modulus

Since each sample was undergoing deformation during the compression test, Young’s modulus was introduced here to study the sample compressive elasticity.

푆푡푟푒푠푠 휎 퐸 = = 푆푡푟푎푖푛 ∆푙⁄푙0 where E is the Young’s modulus, kPa; 휎 is the stress exerted on the sample, kPa; ∆푙 is the absolute thickness change under compression, μm; 푙0 is the original thickness of sample, μm.

Among them, the determination of original thickness 푙0 should be considered more carefully. After viewing the video images and compressibility curves taken during the compression test, it was observed that the sample was first pushed by the probe toward the back plate where it made initial contact. At that point, a linear increase in compressive stress with strain was observed immediately after, this is an indication that elastic deformation that was occurring. The starting point thickness value of the linear elastic region, 푙0, was taken at this point.

4. Results and Discussion

4.1 Compressive Response of CWP Embossed Features

Examples of the compression curves for the embossed points of the CWP1 are given in Figure 3-8. To appreciate the increased sensitivity of the modified micro-compression instrument, compare this plot with the results from the earlier work by Feng [24] shown in Figure 3-9. Samples were taken from the same roll, and so the response should be the same within statistical variation. From these two graphs, it is apparent that the modified system provides additional information about the low stress response that was measured to a much lower level of precision from the earlier study. Though both plots indicate an initial plateau (yield) stress and a sharp increase when thickness is less than 50 µm, the decrease in stress that occurs when the feature abruptly fails, just prior to rapid loading of the flattened web, from about 100 to 150 µm, see Figure 3-8, was not detected in the earlier study. There are likely two reasons for this. First, the single sided compression configuration focuses loading directly onto the embossed feature while the two sided compression reduces the loading zone to only a small region near the apex of the embossment. The modified instrument isolates the feature while maintaining parallel platen compression of the feature as collapse progresses. A more accurate representation of the compressive response is the result. The other reason was the installation of the more sensitive tandem sensor which could enhance stress measurement than the original design used by Feng [24].

80 70 CWP – 1 60

Stress 40 (kPa) 10 mm 30

0 0 100 200 300 400 500 600 700 800 900 1000

Thickness (μm)

Fig. 3-8. Compression Plots of CWP1 Embossed Features.

400

350 Embossed Regions 300

250 Non-embossed 200 Regions

150 Stress (kPa) Stress

100

0 0 50 100 150 200 Thickness (m)

Fig. 3-9. Feng’s work as a comparison (format was kept as original)

Another big improvement which was not shown in the graph is the implementation of digital image recording of the collapse event. With the help of the images taken by the camera, close observations of the whole process for each test point was possible. As a consequence, for each compression curve in Figure 3-8, the collapse event was divided into four different phases, similar in some respects to the compression of ideal solid foam introduced above, see Figure 3-3 : web deflection, which was the initial plateau; phase I (elastic region), which was the linear part of the curve after plateau; phase II (feature collapse), which was the pressure drop region; and phase III (web compaction), which was the region where the stress increased sharply. In Figure 3-10, embossed points 1 of sample CWP1 was given as an example to illustrate the division of these four different phases. The limiting web thickness and yield stress were also marked in that graph, which were about 30 μm and 20 kPa respectively. A differential curve of stress was also added to the bottom graph in Figure 3- 10 to help find the exact collapse value. After applying the same process to each test point of CWP 1 sample, the average yield stress and limiting thickness for CWP1 embossed features were achieved: 18(±3) kPa and 26(±7) μm.

Limiting Web Thickness 80

70 Phase Phase Phase Web III II I Deflection 60

Stress 40 (kPa) 30

20 Yield Stress

Web 10 Web Feature Elastic Deflection Compaction Collapse Region 0 0 50 100 150 200 250 300 350 400 450 500

Thickness (μm)

25 0.9

Yield 0.7 Stress CWP-1 Embossed Point 1 20 0.5 0.3 15 0.1 Stress -0.1 (kPa) 10 -0.3 -0.5 5 -0.7 -0.9 0 -1.1 0 50 100 150 200 250 300 350 400 450 500

Thickness (μm)

Fig. 3-10. CWP1 embossed point 1. Top: Compression curve with different phases. Bot- tom: Compression curve with differential curve of stress.

The compression curves of CWP2 embossed features are shown in Figure 3-11. Similar patterns are evident, although the yield stress is more varied as compared to CWP1. The average yield stress for CWP2 embossed features was 45 (±11) kPa. The main reason of the scattering is that the embossed features in CWP2 were taller in profile than those of CWP1.This caused them to have more chances to contain sidewall defects in the embossed features, or collapse in an off axis direction. And there appear to be three groups of limiting thickness shown in the figure, 30 (±5) μm, 82 (±5) μm and 114 μm respectively. This was most likely due to the overlapping of the different layer of the structure as it folds in on itself during collapse.

80 70 CWP – 2 60

Stress 40 (kPa) 10 mm 30

0 0 100 200 300 400 500 600 700 800 900 1000

Thickness (μm)

Fig. 3-11. Compression Plots of CWP2 Embossed Features

4.2 Compressive Response of TAD Patterns

Compression curves for the TAD1 sample are shown in Figure 3-12. The general trend for each test point was about the same in this graph. However, since the dimensions (protrud- ing height and dome width) of the TAD patterns were much smaller than that of the CWP embossments, the phase II (feature collapse) was not as pronounced in the compression plot. Still, yield stress can be determined by closer examination of the stress values and adding the differential curve of stress, Figure 3-13 as an illustration. This illustrates that the average yield stress of TAD1 features, which was 7 (±3) kPa, is on average much lower than CWP embossment structures. The average limiting thickness of TAD1, which was 57 (±13) μm, was greater than that of CWP samples.

80 70 TAD – 1 60

Stress 40 (kPa) 10 mm 30

0 0 100 200 300 400 500 600 700 800 900 1000

Thickness (μm)

Figure 3-12. Compression Plots of TAD1

25 0.9

TAD-1 Embossed Point 1 0.7 20 0.5

15 0.3 Stress (kPa) 10 0.1 Yield Stress -0.1 5 -0.3

0 -0.5 0 50 100 150 200 250 300 350 400 450 500

Thickness (μm)

Figure 3-13. Compression Plot of TAD1 Embossed Point 1

Compression curves for TAD2 are shown in Figure 3-14. The general trends are quite similar to those of TAD1. However, just like the differences between CWP1 and CWP2, TAD2 compression curves were also more scattered than TAD1’s. As may be seen in the graph, TAD2 sample could also be separated into two groups of limiting thickness, one at 38 (±1) μm and the other at 66 (±5) μm. The reason for this was the same as mentioned previously, where the collapse of the feature causes more than one layer to form under the test probe during the compression event. The average yield stress of TAD2 was 11 (±3) kPa.

80 70 TAD – 2 60

Stress 40 (kPa) 10 mm 30

0 0 100 200 300 400 500 600 700 800 900 1000

Thickness (μm)

Figure 3-14. Compression Plots of TAD2

4.3 Young’s Modulus

For any deformable web material, elastic deformation may happen to a greater or lesser extent depending on the nature of the fibers, the strength of the interfiber bonding, and the arrangement of fibers in space. This holds for paper towels, which have relatively low density, are soft and easy to deform, seen Figure 3-15 as an example. In this figure, the compression curve of embossed point 1 of CWP1 sample was plotted. The elastic region, which is the linear part of the compression curve, was marked there as the region with a constant slope, and a distinct plateau in the differential plot. Therefore, Young’s modulus was introduced here as a useful material characteristic that can help quantify the resiliency of the web to compressive forces.

25 1 CWP-1 Embossed Point 1 0.8 20 0.6 0.4 0.2 15 0 Stress -0.2 (kPa) 10 -0.4 -0.6 5 Elastic Region -0.8 -1 0 -1.2 0 50 100 150 200 250 300 350 400 450 500

Thickness (μm)

Figure 3-15. Compression Plot of TAD1 Embossed Point 1

In Figure 3-16, the compression curve for the first tested embossed feature or TAD pattern of each sample is shown. In these plots, the linear relation was found for each sample. Also, the Young’s modulus was 28.7 kPa, 30.5 kPa, 8.6 kPa, 13. 8 kPa respectively. The average Young’s modulus for each sample was obtained by performing the same process for each tested points of each sample. The results are 23.7 (±5.5) kPa, 37.8 (±9.6) kPa, 9.7 (±2.6) kPa and 10.6 (±3.8) kPa for CWP1, CWP2, TAD1 and TAD2 respectively. Since the original thicknesses were different from point to point and sample to sample, normalized values based on CWP1 average original thickness were also calculated to give a better understanding of the data sets which were shown in the following part.

10 9 CWP-1 Embossed Point 1 8 y = 28.478x + 2.8436 7 R² = 0.9914 6 Stress 5 (kPa) 4 3 2 1 0 0 0.025 0.05 0.075 0.1 0.125 0.15

Strain

10 9 CWP-2 Embossed Point 1 8 y = 30.524x + 2.1382 7 R² = 0.9867 6 Stress 5 (kPa) 4 3 2 1 0 0 0.05 0.1 0.15 0.2

Strain

5 4.5 TAD-1 Point 1 4 3.5 y = 8.6276x + 0.6282 3 R² = 0.9969 Stress 2.5 (kPa) 2 1.5 1 0.5 0 0 0.1 0.2 0.3 0.4 0.5 0.6

Strain

5 4.5 TAD-2 Point1 4 3.5 y = 13.779x + 1.017 3 R² = 0.9937 Stress 2.5 (kPa) 2 1.5 1 0.5 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Strain

Figure 3-16. Young’s modulus of each sample

4.4 Comparison of CWP & TAD 57

Table 3-2 shows the values of yield stress and limiting thickness for CWPs and TADs samples. It was found that the grammage of each sample’s test points would have an influence on the yield stress for each group. For instance, CWP2’s yield stress (45 kPa) was greater than CWP1’s (18 kPa); TAD2’s (11 kPa) was also greater than TAD1’s (7 kPa). The reasonable explanation would be given the similar manufacturing processes, the higher the grammage, the more materials there would be to withstand the stress. When the yield stresses between the CWPs and the TADs are compared, it appears that TADs are more deformable, or in other words, they are more easily deformed under pressure. On the other hand, the limiting thicknesses of CWPs are smaller than TADs’, which suggests that TAD samples tended to dissipate stress more easily than CWPs.

Table 3-2 Collapse Value of CWPs and TADs

Limiting Soft X-ray Sample Name Yield Stress/(kPa) Thickness/(μm) Grammage/(g/m2) CWP1 18 (±3) 26 (±7) 19.8 (±3.1)

CWP2 45 (±11) 30 (±5) 32.3 (±4.0)

TAD1 7 (±3) 57 (±13) 19.0 (±3.5)

TAD2 11 (±3) 38 (±1) 24.0 (±4.1)

In Table 3-3, it also gives the Young’s modulus for each sample. Once again, it appears that the grammage has an influence on the Young’s modulus, and the trend is the same as for the limiting thickness, or more specifically, Young’s modulus will increase when grammage increases given the same or similar manufacturing processes. Meanwhile, it is also revealed that TADs tended to have smaller Young’s modulus as compared to CWPs. TADs are apparently less rigid than CWPs, which is intuitive from the manufacturing processes where CWP hare much more rigidly bonded regions throughout the web, while TADs have intermittent regions where bonding limited. When considering the normalized values, there

is a minor conflict as the normalized Young’s modulus of CWP2 was greater than that of CWP1. The possible explanation is that grammage is not the only factor that can affect the Young’s modulus, other factors, for instance the size and structure of the embossed feature, the extent of creping and the fiber furnish are also expected to have a significant effect.

Table 3-3 Young’s Modulus of CWPs and TADs

Soft X-ray Sample Name Original/(kPa) Normalized//(kPa) Grammage/(g/m2) CWP1 23.75 (±5.52) 23.75 19.8 (±3.1)

CWP2 37.82 (±9.60) 21.52 32.3 (±4.0)

TAD1 9.70 (±2.55) 8.78 19.0 (±3.5)

TAD2 10.63 (±3.79) 9.38 24.0 (±4.1)

Chapter 4: Conclusions and Future Work

1. Conclusions

1. The soft X-ray method used by Feng [24] was selected to study the formation of nonwoven samples. High resolution X-ray radiographs with pixel size of 5.3 μm were obtained for 9 samples. Details of key features resulting from different forming or bonding process were evident in these radiographic images. The standard formation numbers were determined for those samples that had a stochastic distribution of fibers, i.e. were not formed into regular patterns. X-radiographic grammage was also calculated and compared to the gravimetric grammage for each sample. The results suggested that soft X-ray is a viable method for use in studying the formations of most nonwoven materials whose major components are organic polymers with little or no inorganic matter. Other than these materials, soft X-ray may also be used, if the material is of relatively uniform composition, and an appropriate internal grammage standard is selected to determine the mass absorption coefficient.

2. The continuous micro compression platform was modified for use in studying several retail paper towel samples. Direct visualization of the compression behavior of the probe was achieved by the installation of an online video imaging system. Data sets with much finer range of force sensing, <10 kPa were obtained by the installation of a more sensitive load cell on the tandem sensor. Because of these two major revisions, the collapse response curves, Young’s modulus values, yield stress and limiting thicknesses were determined for embossed features and TAD patterns from 4 samples. The results showed that in general, TADs tended to be more deformable under pressure and also more resilient, since they had smaller yield stress and Young’s modulus values as compared to the CWPs.

2. Future work

1. 2D fiber orientation maps should be determined for nonwoven samples since the fiber orientation readily apparent in the soft X-ray images. This would be very useful to study the relationship between the manufacturing processes and physical properties.

2. Perform more tests on additional paper towel samples to verify the findings of this research project. If possible, try to determine the relationship between softness and the yield stress and Young’s modulus.

3. Mass absorption coefficient of different materials should be found or be able to calculate if soft X-ray is utilized to study materials made of components other than organic polymers such as lignocellulosics.

4. Customized Matlab code should be developed to process the raw experiment data automati- cally instead of manually. This could help to save the time costs and minimize the chances of error.

References

[1] Kouris M, American Paper Institute. Dictionary of paper. 5th ed. Atlanta, Ga.: TAPPI Press; 1996. [2] Lindsay JD, Chen FJ, Bednarz J. High-bulk tissue laminates for building materials and other purposes. Appita Journal. 2005;58:349-52. [3] ReportBuyer. World Nonwovens Market. The Wall Street Journal. 2014. [4] Industry N. Global nonwovens market poised for growth. 2013. [5] Prigneaux J. Challenges & Opportunities for Nonwovens Global & Regional Market Trends. EDANA. 2012. [6] Corporation N. Nordson Nonwovens Trends. 2014. [7] Butler I. The nonwoven fabrics handbook. 1999. [8] Vaughn EA. Nonwoven Fabric Sampler and Technology Reference. INDA. 1998:1. [9] Corte H. On Distribution of Mass Density in Paper. Papier. 1969;23:381-&. [10] Davis MN. Paper Inspection Process and Apparatus. 1935. [11] Norman B, Wahren D. measurement of mass distribution in paper sheets using a beta radiographic method. Sven Papperstidn. 1974:10. [12] P. CTMTHL. Characterization of paper formation. I, Sensing paper formation. Tappi J. 1990:7. [13] Keller DS, Pawlak JJ. beta-radiographic imaging of paper formation using storage phosphor screens. J Pulp Pap Sci. 2001;27:117-23. [14] Farrington TE. Soft-X-Ray Imaging Can Be Used to Assess Sheet Formation and Quality. Tappi Journal. 1988;71:140-4. [15] Tomimasu H. Comparision of four paper imaging techniques: β-radiography, electrography, light transmission, and soft X-radiography. Tappi J. 1991:12. [16] CHI F. Soft X-Ray Formation Measurement of Low Density Material and Compressive Response Characterization. M S Thesis. 2012. [17] Gavelin G. The Compressibility of Newsprint. Sven Papperstidn. 1949. [18] Van Eperen RHH, Keith W. ; Wink, W. A. (Wilmer A.) ; Van den Akker, Johannes A. Softness of sanitary tissues. Project 2220 progress report five : a progress report to members of group project 2220. 1962.

[19] Liu J, Hsieh J. Characterization of facial tissue softness. Tappi Journal. 2004;3:3-8. [20] Leporte LE. Experimental studies of selected physical properties of paper tissues in relation to their subjective softness. Project 2817, report three : a progress report to members of Group Project 2817. The Institute of Paper Chemistry 1971. [21] Pawlak JJ, Keller DS. Measurement of the local compressive characteristics of polymeric film and web structures using micro-indentation. Polym Test. 2003;22:515-28. [22] Pawlak JJ. The Local Compressive Properties of Paper in the Z-direction as Related to Paper Friction: State University of New York, ESF,Syarcuse, NY; 2001. [23] Pawlak J, Keller D. Relationships between the local sheet structure and Z-direction compressive characteristics of paper. Journal of Pulp and Paper Science. 2004;30:256-62. [24] Feng C. Soft X-Ray Measurement of Low DensityMaterials and Compressive Response Characterization [M.S.]. Oxford, Ohio: Miami University; 2012. [25] Keller DS, Feng C. Mapping the Compressive Properties of Low Density Fibrous Webs. 2011 Progress in Paper Physics Seminar. Graz, Austria2011. [26] Keller DS, Branca DL, Kwon O. Characterization of nonwoven structures by spatial partitioning of local thickness and mass density. Journal of Materials Science. 2012;47:208-26. [27] Max R. The Cos4 Law of Illumination. Journal of the Optical Society of America. 1945;35:6. [28] Press T. Tappi Standard Test Methods T410 om-02. 2002. [29] Branca D. Uniformity of Low Density Fibrous Structures and The Effects of Manufacturing Processes on Apparent Density: SUNY College of ESF; 2007. [30] Rowe S, Volkerman RJ. Thickness measurement of sanitary tissues in relation to softness. TAPPI. 1965;48:54A-6A.

Appendix: Bounty Extra BountyExtra Embossed Point 1

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Bounty Trap and Lock Bounty Trap and Lock Embossed Point 1 Bounty Trap and Lock Embossed Point 1

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Bounty White Bounty White Embossed Point 1

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Bounty Basic Bounty Basic Embossed Point 1

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Cascade Cascade Point 1 Cascade Point 2

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Marcal Marcal Embossed Point 1 Marcal Embossed Point 2

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Scott Scott Embossed Point 1 Scott Embossed Point 2

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Sparkle Sparkel Embossed Point 1 Sparkel Embossed Point 2

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White Cloud White Cloud Embossed Point 1 White Cloud Embossed Point 2

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