Modern Textile Characterization Methods

Mastura Raheel, Mastura Raheel

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351430487
• Edition: 1
• Topic: Scienze fisiche
• Subtopic: Chimica

This work details current advances in assessing the characteristics of polymers, single fibres and fibrous systems, and associated processes based on evolving theories in the physical, chemical and mechanical sciences. It focuses on recent develpments in selected characterization methods - such as Fourier transform infrared spectroscopy, Fourier transform nuclear magnetic resonance, electron diffraction, x-ray diffraction and electron microscopy - applicatble to polymers, fibres and textiles.

1

Introduction: Developments in Textile Characterization Methods

MASTURA RAHEEL University of Illinois at Urbana-Champaign, Urbana, Illinois

Textile characterization must take into consideration an in-depth understanding of the nature of fiber-forming materials (polymers), fiber structure, its physical, mechanical, and chemical properties, and how these properties relate to further engineering operations that result in fabrics/textiles and finished products. The end-use performance of finished products will depend upon all these factors, and can be predicted on the basis of fundamental theories of fiber science and sound characterization methods.

Fundamental theories of fiber science have evolved from the classical theories of physics, chemistry, polymer science, and engineering. The greatest advances in textile materials have been where linear laws of classical physics or physical chemistry can be applied. The difficulties increase when it becomes necessary to take account of quantum and relativistic effects and chemical interactions. Textile systems generally are extraordinarily complex, and the effects of treatments almost invariably go beyond the bound of linearity. Thus predictive mathematical models may very well be nonlinear or only yield empirical statistical correlations. Major strides have been made in the last decade or so in the use of sophisticated methods and mathematical models to characterize textile materials and predict end-use performance. Textile characterization is important at all stages of textile production and processing in order to achieve a product that meets perceived performance needs. The aim of textile characterization is to understand the material structure and behavior as well as the processes sufficiently to be able to predict their consequences, and so to be able to set up control techniques that will lead to products with specified properties.

There are numerous well-known organizations, such as the International Standards Organization (ISO), the American Society for Testing and Materials (ASTM), the American Association of Textile Chemists and Colorists (AATCC), the European Standardization Committee (CEN), and various others, that develop standard test methods for evaluating and predicting performance of fibrous systems. However, generally, there is a significant time lag between the developments in textile characterization methods and their acceptance as standard methods. The literature is replete with innovative uses of standard methods as well as newer methods and instrumentation for characterizing polymers, fibers, textiles, and their auxiliaries. It is not the intent of this book to include all physical, mechanical, and chemical methods for characterization of fibrous materials, but rather to focus on recent developments in selected characterization methods and their applications to fibrous systems, based on evolving theories of physical, chemical, and engineering sciences.

The book begins with polymer characterization methods. Polymers, the fiber-forming materials, have (or can be manipulated to have) characteristic structures and physicochemical properties. These features have profound impact on fiber and textile properties. In Chapter 2 P. H. Geil, a renowned polymer scientist, discusses in great detail polymer characterization methods. The specific areas of polymer characterization covered in Chapter 2 include (1) chemical structure, including composition and configuration, (2) physical structure, including crystallinity and morphology-related aspects, and (3) physiochemical properties.

Geil mentions the use of traditional methods of characterizating various aspects of polymers but focuses mainly on recent advances in polymer characterization methods. For example, polymer chemical composition and configuration analysis begins with the traditional analytical chemistry techniques of elemental analysis by atomic absorption spectroscopy, x-ray dispersive analysis, and reaction of specific groups in a polymer with specific reagents, but the thrust of his discussion is on Fourier-transform infrared spectroscopy (FTIR) and FT nuclear magnetic resonance (FT-NMR) methods. He explains the theoretical basis of these analytical techniques and provides practical guidance about sample preparation, the analytical technique, and interpretation of results. Also, he describes the usefulness of these techniques in studying textile fibers. Molecular weight determination is described using chromatography processes and also by simpler techniques such as solution and melt viscosity methods. The significance of molecular weight characterization on solution spinning and melt spinning of fibers is described.

The physical structure of polymers and fibers requires a range of techniques for characterization because of the range of size scales, particularly in fibers. The structures of interest fall into the size scale of the individual molecular segment; the relative number of regular and random conformations and their arrangement in space, that is, the degree of crystallinity and orientation; the size and shape of the crystalline and amorphous regions; and the organization and interaction of these crystals in larger structures. Characterization of all these aspects is discussed in great detail with illustrations and examples of polymers and fibers by using a range of techniques. The techniques described include FTIR, electron diffraction (ED), x-ray diffraction [both wide-angle (WAXD) and small-angle scattering (SAXS)], and electron microscopy (EM) [both scanning (SEM) and transmission (TEM)]; also many probe microscopes are described. Geil cautions about the problems in utilizing several techniques (especially electron diffraction) that primarily depend upon appropriate sample preparation. He suggests sample preparation methods and describes their representative results and potential difficulties.

In Chapter 3, W. R. Goynes discusses the importance of structural characterization of fibers and textiles using scanning electron microscopic (SEM) techniques. He focuses on the specifics of sample preparation and microscope operating conditions, bringing to attention the difficulties of obtaining meaningful signals and interpreting those signals. The significance of back-scattered electrons in interpreting changes in elemental composition of fibers/materials is introduced, and the importance of x-rays for elemental analysis is emphasized. Goynes concludes with examples of textile characterization using SEM as a powerful tool. It is well known that surface morphology and characteristic structural features of fibers are dramatically revealed by scanning electron microscope; however, Goynes also presents the effects of physical and chemical treatments on changes in the fibers’ characteristic features. This characterization method also provides valuable information regarding process evaluation and product quality control.

Chapter 4 focuses on analytical pyrolysis as a technique to identify and detect small changes in polymers, fibers, and other textile auxiliaries. Analytical pyrolysis (or thermolysis) is a nonoxidative process in which polymers or large molecules break down into characteristic smaller molecules. Instrumental analysis of these pyrolysates, which are structure-specific volatile compounds, provides information about the structure and identity of the parent compound. I. R. Hardin discusses the mechanism of pyrolysis, the types of reactions that occur to give rise to complex mixtures of products, and how these volatile fragments are separated and analyzed using gas chromatography (GC) alone or in conjunction with mass spectrometry or Fourier-transform spectroscopy. He elaborates on these techniques with examples of identifying or detecting small changes in polymers, finishes, and dyes.

Chromatographic and spectroscopic methods are employed for characterization of a wide variety of polymers, fibers, textiles, and textile auxiliaries. In Chapter 5, Y. Yang presents the scientific basis and application of conventional liquid chromatography (LC) for dye identification, separation, and purification. Also, as a powerful tool, LC is employed for analysis of textile finishing processes such as flame retardant, stain resistant, durable press, and others. Packing textile material into the column as a stationary phase is an innovative method for the investigation of pore structure and dyeing and finishing behavior of the specific textile employed as a stationary phase. This technique is useful as well for studying dyeing and finishing mechanisms in textile systems. Yang provides the basic concepts of liquid chromatography as a tool for textile and related materials characterization, and focuses on pore structure and surface area analysis as it relates to textile wet processes. The subjects of color identification, separation and purification, dyeing thermodynamics, sorption isotherms, dye compatibility and dye–fiber interactions are discussed in depth. In a related topic, K. R. Beck, in Chapter 6, focuses on characterization of durable-press finishes for cellulosic textiles using chromatographic and spectrophotometric methods. Beck, a pioneer in the use of chromatographic techniques for analyzing textile finishes, describes analysis of durable press chemicals utilizing thin-layer, gas, and high-performance liquid chromatographic methods as well as spectroscopic methods. The spectroscopic methods included are ultraviolet-visible, near infrared, infrared, nuclear magnetic resonance, and mass spectrometry. Beck illustrates the use of these methods in determining molecular structure, mixture composition, and properties of durable press agents, as well as the mechanism of cross-linking reactions.

In Chapter 7, N. R. Bertoniere describes a technique based on the principles of gel-permeation chromatography. Her focus is on the development of reverse gel permeation column chromatography to assess pore size distribution in cotton cellulose. This method was developed at the Southern Regional Research Center, New Orleans, La. over a period of years by Bertoniere and associates. Bertoniere describes the experimental problems with columns made from cotton cellulose by various methods and proposes meaningful solutions. Reverse gelpermeation chromatography as a tool to elucidate pore structure in different varieties of cotton and jute fibers is described. The effects of caustic mercerization and liquid ammonia treatment on pore size distribution of cotton are explained; the progressive losses in the accessible internal volume of cotton with increasing the degree of cross-linking is used to illustrate increases in resilience accompanied by losses in strength. Of significance is the use of this method in following the differences among conventional cross-linking agents and formaldehyde-free cross-linking agents with respect to the degree to which they alter the pore size distribution in the cross-linked cotton. Bertoniere explains why formaldehyde-free reagents differ in the weight add-on required to impart easy care performance to cotton fabric. Research in this area is ongoing.

Chapter 8, authored by L. Rebenfeld et al., focuses on characterization of pore structure in fibrous networks as it relates to absorbency. They discuss the discontinuous nature of textile materials, their heteroporous nature, and the deceptively high level of porosity in textile materials—which is directly related to absorbency. Nevertheless, the porosity of a textile material is strongly affected by lateral compressive forces to which the material is subjected, hence the pressure dependence of liquid absorption characteristics of textiles. While porosity is an important physical quantity, the dimensions of the pores give a more descriptive way of characterizing the porous nature of a network. Rebenfeld and associates delineate pore volume, which determines liquid absorption capacity, from geometric considerations such as pore throat dimensions that influence liquid flowthrough processes, which in turn affect filtration or barrier properties of porous materials. On the basis of the heteroporous nature of fibrous materials, they introduce the concept of pore sizes and their distribution as unimodal, bimodal, and trimodal. To characterize the pore structure in terms of pore volumes and pore throat dimensions they describe the instrumentation of mercury porosimetry used until recently, and the new instrumentation developed at the Textile Research Institute by the authors. These analytical methods are particularly well suited for textiles and other compressible planar materials.

Another topic that has presented much difficulty in the past is that of characterizing single fibers as to their mechanical properties. S. Kawabata, a renowned researcher in the area of polymers, fibers and textiles, presents in Chapter 9 the theoretical basis of direct measurement of the mechanical properties of single fibers. Kawabata describes the advantages of direct “micromeasurement” of single fiber mechanical properties and discusses anisotropy in mechanical properties and the difficulties in measuring very small force and deformation in a single fiber. The mechanical anisotropy of the fiber strictly reflects the microstructure of the fiber and has great implications on the micromechanics of fiber/resin composites. Kawabata also presents the instrumentation developed by the author for this purpose.

The next four chapters focus on new developments in analyzing textile attributes (handle, color, protective qualities) that are not easily measurable as compared to specific textile properties. Chapter 10 deals with objective measurement of fabric hand. S. Kawabata and M. Niwa, the leaders in this area of research, present the significance of fabric hand or handle evaluation on the perception of garment appearance, comfort and tailorability. They analyze and correlate fabric hand judgments by experts with specific fabric properties that express fabric hand characteristic and that can be measured objectively. This is described as objective system for hand evaluation. The nonlinear mechanical properties of a fabric that describe fabric hand, including the weighting system for these properties and the equations that describe these weighting systems, are presented. The mechanical parameters are measured by a set of four instruments known as the Kawabata system or KESF system. Recently, an automated KESF system has been developed.

Color and colorimetry is another elusive but rapidly evolving area of study. P. T. F. Chong, in Chapter 11, provides an extensive background in basic colorimetry and describes the color measuring systems, as well as the developments in color measuring instruments. On the basis of his extensive experience as a color scientist, Chong provides valuable insights into instrument setup, calibration, and verification, as well as sample preparation and color measurement. This is followed by an in-depth presentation of the application of color measuring systems in the textile and textile-related industries. The major applications discussed are color matching, color quality monitoring or screening the color of the products against preset tolerance in color requirement, colorant strength evaluation, and whiteness/yellowness evaluation. Chong also discusses aspects of colorant solution evaluation including colorant strength, dye solubility, solution stability, dye exhaustion characteristics and so on.

Chapter 12 deals with characterization of chemical barrier performance of textile systems. J. O. Stull describes the types of barrier materials, standards pertaining to chemical barrier performance of these materials, and an overview of barrier testing approaches. Three testing approaches are discussed in detail; those pertaining to resistance of material to degradation, chemical penetration resistance, and permeation resistance. The complexities of textile substrate (homogeneous single layer, coated, laminated, microporous, or containing adsorptive components), testing techniques, test conditions, and the impact of multicomponent chemical challenges are brought to focus. For example, using different test methods, or even the same method but different test conditions, can provide different results for the same material and chemical combination. Thus, selection of test method and conditions must be appropriate to the product’s application and expected performance.

Degradation resistance testing may show how material/products deteriorate or are otherwise affected, but will not always demonstrate retention of barrier characteristics with respect to specific chemicals. Degradation testing is most useful when retention of specific physical properties is desired or as a screening technique for other chemical barrier testing techniques. Penetration testing should only be used if the wetting or repellency characteristics of materials are to be evaluated. This type of testing is appropriate for the evaluation of material performance against liquid chemicals and can be used for microporous and continuous film-based materials. Vapor transmission test methods are used to measure gross vapor penetration of chemical vapor or gas challenges over relatively short periods of time. This characterization technique is applicable to any film-based material or adsorbent-based material. Chemical permeation testing, however, provides a barrier material’s total chemical resistance and can detect very small amounts of permeating chemical. Thus, permeation testing provides the most rigorous of all chemical resistance test methods. Several techniques are presented to provide flexibility in test conditions and applications. Since there are a number of techniques to characterize the barrier performance of materials, careful selection of a test method and its parameters depends on the understanding of the material (textile/product) and its application.

In Chapter 13 P. L. Brown introduces a topic of much interest and concern among health care providers and others, the barrier properties of textiles against microorganisms. Recently, the focus on preventing transmission of infectious microorganisms through barrier materials has grown to include both infection control and personal protection. One major reason for this growth is the risk associated with exposure to blood-borne pathogens as perceived by the health care community. Other potentially hazardous microorganisms (not blood-borne) include Prions, Muerto Canyon virus, and multiple-drug-resistant forms of Mycobacterium tuberculosis, staphylococci, and enterococci, to name a few. In addition, biotechnology workers dealing with recombinant DNA, laboratory technicians handling cultures of human pathogens, and veterinary and agricultural workers dealing with zoonotic agents also risk exposure. However, each work environment with potential microbiological hazard may require a different strategy and risk reduction decision.

The basic performance objectives of personal protective clothing products against biohazards are allowing fluid flow, such as air or liquid, while limiting the transfer of potentially pathogenic microbes being transported with them, or else preventing the transfer of fluids and indirectly preventing the transfer of microbes. These two objectives are fundamentally different and require different experimental approaches to the analysis and characterization of the barrier properties of the respective materials to microorganisms. Brown, with his extensive experience as a research scientist and protective product specialist, provides an extensive theoretical background about the types of biohazards, textile substrates, and characterization methods for assessing barrier properties of textiles. He discusses the limitations of laboratory test methods and emphasizes the need for understanding the different microbial, physical, chemical, and thermal stresses imposed on textiles (and finished products) used in personal protection and infection control.

Recognizing the complexities of the different end-use environments for microbial barrier textiles and various stresses that can be imposed on their barrier integrity, Brown discusses developing a realistic strategy related to product evaluation in the laboratory. He suggests developing a feasible testing hierarchy based on combinations of various tests. The degree of hazard associated with exposure to the microbes will dictate how carefully the end-use application for the textile will need to be investigated, how conservative the modeling and experimental...