Commercially, polymeric fibers are perhaps the most important of all; in terms of production volume as well as the vast range of applications. There are two broad categories of polymeric fibers: natural and synthetic. Natural fibers can be from the vegetable kingdom such as cotton, sisal, jute or from the animal kingdom such as wool, silk, etc. Natural fibers are mostly polymeric in nature. There are, however, some natural fibers that occur in rock formations. These fibers are minerals and can, therefore, be treated as a ceramic, e.g., asbestos and basalt. We describe these natural mineral fibers in Chapter 7. In this and Chapter 4, we describe polymeric fibers. In this chapter, we first briefly review some of the fundamental aspects of polymers and then describe the natural polymeric fibers. We devote Chapter 4 to the synthetic polymeric fibers, which have seen a tremendous advancement in the last half of the twentieth century. A vast range of natural polymeric fibers is available and they find large scale commercial applications. Much research effort, however, is focused on a very special natural fiber originating in the animal kingdom, spidersilk. The idea here is to learn about the processing, structure, and properties of silk fibers, especially spidersilk fibers, which are very strong and stiff. More about silk fiber later in this chapter. The volume of other natural fibers such cotton, jute, sisal, ramie, etc. in industrial and nonindustrial applications has always been quite large because of their many attributes such as the wear-comfort of cotton and the fact that all natural fibers represent a renewable resource. The main disadvantage of natural fibers is the immense variability in their physical, chemical, and mechanical attributes.
First, in order to understand the processing, structure, and properties of polymeric fibers, the main focus of Chapters 3 and 4, it will be useful to review some general and basic concepts regarding the structure of polymeric materials, and especially some of the important features associated with the macromolecular chains that do not have their counterparts in metals and ceramics. Readers more versed in these aspects of polymers may skip straight to Section 3.2. In what follows, we first examine the salient structural aspects of polymers and then describe the processing, structure, and properties of some important natural polymeric fibers.
The term fiber conjures up an image of flexible threads, beautiful garments and dresses, and perhaps even some lowly items such as ropes and cords for tying things, and burlap sacks used for transporting commodities, etc. Nature provides us with an immense catalog of examples where materials in a fibrous form are used to make highly complex and multifunctional parts. Protein, which is chemically a variety of complexes of amino acids, is frequently found in nature in a fibrous form. Collagen, for example, is a fibrous protein that forms part of both hard and soft connective tissues. A more well-known natural fiber, which is essentially pure protein, is silk fiber. Silk is a very important natural, biological fiber produced by spider and silkworm. It is spun from a solution; the solution, in this case, being produced by the silkworm or the spider. Silkworm silk has been commercialized for many years. However, scientists and engineers are beginning to realize the potential of silk, in general, and spidersilk, in particular.
Indeed, materials in a fibrous form have been used by mankind for a long time. Yarns made of fibers have been used for making fabrics, ropes, and cords, and for many other uses since prehistoric times, long before scientists had any idea of the internal structure of these materials. Weaving of cloth has been an important occupation in most ancient societies. The term fabric is frequently employed as a metaphor for societal characteristics. One talks of the social fabric or moral fiber of a society, etc. It is interesting to note that an archeological excavation of a 9000-year-old site in Turkey led to the discovery of a piece of fabric, a piece of linen, woven from the fibers of a flax plant (New York Times, 1993). Normally, archeologists date an era by the pottery of that era. It would appear from this discovery that textile fabrics came even before pottery. There is also recorded use of sutures as stitches in wound repairs in prehistoric times (Lyman, 1991). An ancient medical treatise, about 800 BC, called The Sushruta Samhita, written by the Indian surgeon Sushruta, describes the use of braided fibers such as horse hair, cotton fibers, animal sinews, and fibrous bark as sutures. Incidentally, the word suture comes from the Sanskrit word, sutra meaning filament or thread.
Fibrous materials are ubiquitous. Nature makes extensive use of the fibrous form of matter because fibers are extremely flexible, allowing for complex shapes to be formed. Fibers can be natural (cotton, jute, etc.) or synthetic (polyamide, polyethylene, etc.). Synthetic fibers, however, form a large part of our economy. Among synthetic fibers, the so-called high performance fibers (HPF) represent a very small segment by volume. Small though this segment is, it is a very vital one. It is driven by special functions that require specific properties unique to these fibers. Usually HPFs have very high levels of at least one of the following properties: stiffness, strength, ability to resist high temperatures, optical signal transmission, and high resistance to chemicals. Since the publication of the first edition of this book, there has been tremendous progress in the field of fibrous materials. As an example, carbon fibers (one of the HPFs) have entered the era of large scale commercial use in civilian aircraft. Impressive developments in our understanding of spidersilk fiber, processing of synthetic polymeric fibers, metallic fibers and ceramic fibers are recorded in this second edition. Impressive gains have been made in making high strength, lightweight organic fibers such as aramid, polyethylene, etc. as well as very high temperature ceramic fibers such as alumina, silicon carbide, etc. Metallic fibers are used for reinforcement (think of tires), cords and ropes for elevators, bridge cables, etc. Less well known are the superconducting metallic filaments. Other applications of materials in a fibrous form include:
• clothing, garments, carpets, cables, ropes and cords;
• reinforcement of polymers, metals, ceramics, and cement for structural purposes;
• geotextiles for soil-stabilization;
• optical fibers for communication purposes, etc. All the data (audio, video, and data) transmission that we take for granted today, indeed the whole field of fiber optics, would not be possible without the availability of specialty glass fiber. As an aside, we should mention that Charles Kao was awarded the Nobel Prize in physics in 2009 for his work on fiber optics.
The basic theme of the second edition of this book continues to be the triad of processing–structure–properties of various materials in fibrous form. It is the only book that covers the whole range of fibrous materials available.
Metals in bulk form are quite common materials and are extensively used in engineering and other applications. Metals can provide an excellent combination of mechanical and physical properties at a very reasonable cost. One of the important attributes of metals is their ability to undergo plastic deformation. This allows us to use plastic deformation as a means to process them into a variety of simple and complex shapes and forms, all the way from airplane fuselages to huge oil and gas pipelines to commonplace aluminum soda pop cans and foil for household use. What is less appreciated, however, is the fact that metals in the form of fibers or wire have also been in use for a long time. Examples of the use of metallic filaments include: tungsten filaments for lamps; copper and aluminum wire for electrical applications; steel wire for use as a reinforcement fiber for tires and in cables for use in suspension bridges; niobium-based filamentary superconductors; and, of course, strings for various musical instruments such as violin, cello, guitar, piano, sitar, etc. Highly ductile metals such as gold and silver can be drawn into extremely thin filaments. Extremely fine filaments of such noble metals have been used for ages as threads in making Indian women's traditional dress called saree.
Let us first review some of the important characteristics of metals, in particular the characteristics that allow metals and their alloys to be drawn into fine filaments, and then describe the processing, structure, properties, and applications of some important metallic filaments. Readers already familiar with the basic attributes of metals may skip directly to Section 6.2.
General characteristics of metals
Metals are generally crystalline materials, although it is possible to produce amorphous metals. One technique involves very rapid cooling rates, say, greater than 106 K s−1. Another technique involves supercooled liquids that fail to crystallize and result in bulk metallic glasses, an important class of amorphous metals. Nevertheless, most common metals are crystalline. Crystalline metals have the following three common crystal structures:
(a) face centered cubic (FCC);
(b) body centered cubic (BCC);
(c) hexagonal close-packed (HCP).
Figure 6.1 shows these structures. Examples of FCC metals are aluminum, copper, gold, silver, and γ-iron (austenite), etc. They are all very ductile and can be drawn to less than 100 µm in diameter.
The term glass or a glassy material represents a rather large family of materials with the common characteristic that their structure is noncrystalline. Thus, rigorously speaking, one can produce a glassy material from a polymer, metal, or ceramic. An amorphous structure is fairly common in polymeric materials. It is less so in metals, although metallic glass, generally in the form a ribbon, can be produced by rapid solidification, i.e., by not giving enough time for crystallization to occur. In this chapter we describe silica-based inorganic glasses because of their great commercial importance, as a reinforcement fiber for polymer matrix composites and as optical fiber for communications. Communication via optical glass fibers is a well-established field. All the audio, video, and data transmission that we take for granted today, indeed the whole field of fiber optics, would not be possible without the availability of specialty glass fiber. Charles Kao was awarded the Nobel Prize in physics in 2009 for his work on fiber optics. The Internet as we know it became possible mainly because of undersea cables. The undersea cables for communication became possible because of the optical fiber. As Blum (2012) describes it: light enters the optical fiber at one shore and comes out on the other, which may be as far as two continents away.
Crude optical glass fiber bundles were used to examine the insides of the human body as far back as 1960. Since then tremendous progress has been made in making ultra pure, controlled composition fibers with very low optical signal attenuation. The total worldwide shipment of optical fibers runs into many billions of US$. Before we describe the processing, structure, and properties of glass fiber, it would be appropriate to digress a bit and describe for the uninitiated, albeit very briefly, the basic physics behind the process of communication via optical glass fibers.
In this chapter, we describe the basic physics behind optical communication followed by processing techniques, composition, structure, and properties of glass fibers of different kinds. A new type of glass fiber called photonic bandgap fiber is described. Finally, applications of different types of glass fibers are described.
Basic physics of optical communication
By far the most important and simple phenomenon that is made use of in optical wave guides is refraction of light. Figure 8.1 shows this phenomenon.
This book is about materials in fibrous form, precisely what the title says. Perhaps the only thing that needs to be emphasized is that the materials aspects of fibers are highlighted. The main focus is on the triad of processing, microstructure, and properties of materials in a fibrous form. I have kept the mathematics to the bare minimum necessary. More emphasis is placed on physical and chemical insights. Although all kinds of fibers are touched upon, there is a distinct tilt toward synthetic, nonapparel-type fibers. This is understandable inasmuch as the second half of the twentieth century has seen tremendous research and development activity in this area of high performance fibers, mainly for use as reinforcement in a variety of matrix materials.
The field of fibrous materials is indeed very vast. To compress all the information available in a reasonable amount of space is a daunting task. My aim in writing this text has been to provide a broad coverage of the field that would make the text suitable for anyone generally interested in fibrous materials. I have provided ample references to the original literature and review articles to direct the reader with a special interest in any particular area.
The plan of the book is as follows. After an introductory chapter, some general terms and attributes regarding fibers and products thereof are described in Chapter 2. This chapter also serves to provide a mutually comprehensible language to textile and nontextile users of fibers. There is no gainsaying the fact that many definitions, units, and terms about fibers owe their origin to the textile industry. Thus, it behooves a materials scientist or engineer to take cognizance of those and be at home with them. At the same time, it is not unreasonable to expect that a textile engineer should know the stress–strain curves of fibers in engineering units. This general chapter is followed by Chapters 3 and 4 on natural and synthetic polymeric fibers, respectively. Chapter 5 covers metallic fibers, which are quite widely used in a variety of engineering applications, although generally not so recognized. Chapter 6 describes ceramic fibers where much innovative processing work has been done during the last quarter of the twentieth century.
In this chapter, we define some important terms and parameters that are commonly used with fibers and fibrous products, such as yarns, fabrics, etc., and then go on to describe some general features of fibers and some important products thereof. These definitions, parameters, and features serve to characterize a variety of fibers and products made therefrom. Thus, we exclude items such as fiber reinforced composites wherein fibers are added to a matrix. It is worth emphasizing that these definitions and features are generally independent of fiber type, i.e. polymeric, metallic, glass or ceramic fibers. They depend on the geometry rather than any material characteristics.
Fiber is the fundamental unit in making textile yarns and fabrics. Fibers can be naturally occurring or synthetic, i.e. manmade. There are many natural fibers, mostly organic but also some inorganic. Examples of natural organic fibers include cotton, jute, sisal, silk, wool, etc. while inorganic fibers occurring in nature include asbestos, wollastonite, and basalt. There is a much larger variety of synthetic fibers available commercially. Polymer fibers such as polypropylene (PP), polyethylene (PE), polyamide (PA), polyethylene terephthalate (PET), polyacrylonitrile (PAN), polytetrafluoroethylene (PTFE), aramid, etc. are well established, commercially available, synthetic fibers. Metallic wires or filaments have been available for a long time. Examples include steel, aluminum, copper, tungsten, molybdenum, gold, silver, etc. Among ceramic and glass fibers, glass fiber for polymer reinforcement became available early in the twentieth century; optical glass fiber for telecommunication purposes made its debut in the 1950s, while ceramic fibers such as carbon, silicon carbide, alumina, etc. became available from the 1960s onward. We shall describe these fibers separately, in detail, later in this book.
One can transform practically any material, polymer, metal, or ceramic, into a fibrous form. As we pointed out in Chapter 1, historically and traditionally, fibers formed part of the textile industry domain for uses such as clothing, upholstery and draperies, sacks, ropes, cords, sails, and containers, etc. Gradually, their use entered the realm of more engineered items such as conveyor belts, drive belts, geotextiles, etc. With the advent of high modulus fibers, the use of fibers has extended to highly engineered materials such as composites.
Understandably, therefore, some of the terms commonly used with fibers have their origin in terminology derived from the textile industry. We examine some of this terminology next.
Carbon fibers have become established engineering materials. In view of their commercial importance, we devote a separate chapter to them. Carbon is a very versatile element. It is very light, with a theoretical density of 2.27 g cm−3. It can exist in a variety of forms, glassy or amorphous carbon, graphite, and diamond. Carbon in all these forms can be found in nature. Carbon in the graphitic form has a hexagonal structure and is highly anisotropic. The diamond form of carbon has a covalent structure and is an extremely hard material. The latest additions to different forms of carbon are Buckminster Fullerene, or Buckyball, with a molecular composition such as C60 or C70, and carbon nanotubes. Carbon nanotubes (CNTs) are long, thin cylinders of carbon atoms arranged in a graphitic lattice structure. They include single-walled and multi-walled carbon nanotubes. In this chapter, we follow the same sequence as in previous chapters: processing, structure, properties, and applications of carbon fibers. However, in order to understand these aspects of carbon fiber, it is helpful to review the basic structure and properties of graphite.
Structure and properties of graphite
Carbon fiber is a generic name representing a family of fibers. Over the years, it has become one of the most important reinforcement fibers in many different types of composite, especially in polymer matrix composites. It is an unfortunate fact that the terms carbon and graphite are used interchangeably in commercial practice as well as in some scientific literature. Rigorously speaking, graphite fiber is a form of carbon fiber that is obtained when we heat the carbon fiber to a temperature greater than 2400 °C. This process, called graphitization, results in a highly oriented, layered crystallographic structure, which, in turn, leads to significantly different chemical and physical properties from non-graphitic forms of carbon (see Section 9.2). The atomic arrangement of carbon atoms in the hexagonal structure of graphite is shown in Fig 9.1. The c-axis is perpendicular to the basal plane. A graphite single crystal will thus have hexagonal symmetry and its elastic properties will be transversely isotropic in the layer plane. Such crystal symmetry requires five independent elastic constants.
Fracture of brittle materials, in general, involves statistical considerations. Materials have randomly distributed defects on their surfaces or in their interior. Fibrous materials, as we saw in Chapter 2, have a large surface area per unit volume. This makes it more likely for them to have surface defects than bulk materials. The presence of defects at random locations can lead to a scatter in the experimentally determined strength values of fibers, which calls for a statistical treatment of fiber strength. Clearly, such a scatter will be much more pronounced in brittle fibers than in ductile fibers such as metallic filaments. This is because ductile metals will yield plastically rather than fracture at a flaw of a critical size. Thus, most high performance fibers, with the exception of ductile metallic filaments, show a rather broad distribution of strength because they are highly flaw sensitive. Since the distribution of flaws is of statistical nature, the strength of a fiber must be treated as a statistical variable. To bring home this important point of variation in strength of a fiber as a function of fiber length, we show, in Figs. 11.1 through 11.3, the variation of tensile strength of some fibers as a function of gage length: high modulus carbon fiber (Fig. 11.1), boron fiber (Fig. 11.2), and Kevlar 49 aramid fiber (Fig. 11.3). In all cases, the strength decreases as the gage length increases. Intuitively, one can see that the probability of finding a critical flaw (which corresponds to the failure strength) increases as the volume of brittle material increases. In the case of a fiber, this translates into an increase in the probability of finding a critical flaw as the fiber length increases. Sometimes this phenomenon is termed the size effect, i.e., the average fiber strength decreases with increasing fiber gage length. An increase in fiber diameter can have similar effect. In this chapter, we provide a brief review of the statistical variation of fiber strength.
Variability of fiber strength
Variability of strength in brittle materials is analyzed by Weibull statistics, which is based on the assumption that we can regard the brittle material as consisting of a chain of links (Weibull, 1939, 1951). The failure of a material occurs when the weakest link in the chain fails. This is called the weakest-link assumption. We can regard a fiber as consisting of a chain of links.
In this chapter, we provide a description of the processing, structure, and properties of high temperature ceramic fibers, excluding glass and carbon, which are dealt with in separate chapters because of their greater commercial importance. Before we do that, however, we review, ever so briefly, some fundamental characteristics of ceramics (crystalline and noncrystalline). Once again, readers already familiar with this basic information may choose to go directly to Section 7.3.
Some important ceramics
We provide a summary of the characteristics of some important ceramic materials that have been converted into a fibrous form.
Bonding and crystalline structure
Ceramics are primarily compounds. Ceramics (excluding glasses) generally have a crystalline structure, while silica-based glasses, a subclass of ceramic materials, are noncrystalline. In crystalline ceramic compounds, stoichiometry dictates the ratio of one element to another. Nonstoichiometric ceramic compounds, however, occur frequently. Some important ceramic materials are listed in Table 7.1. Physical and mechanical characteristics of some ceramic materials are given in Table 7.2. It should be noted that the values shown in Table 7.2 are more indicative than absolute.
In terms of bonding, ceramics have mostly ionic bonding and some covalent bonding. Ionic bonding involves a transfer of electrons between atoms that make the compound. Generally, positively charged ions balance the negatively charged ions to give an electrically neutral compound, for example, NaCl. In covalent bonding, the electrons are shared between atoms. The characteristic high strength as well as brittleness of ceramic materials can be traced to this type of bonding which make the Peierls–Nabarro potential very high, i.e., inherent lattice resistance to dislocation motion is very high. Thus, crystalline ceramics have crystal imperfections such as dislocations but, unlike in metals, they are not very mobile. Also, the number of slip systems available in ceramics is fewer than that in metals. Thus, unlike metals, the stress concentration at a crack tip in a crystalline ceramic cannot be relieved by plastic deformation, at least not at low and moderate temperatures. This has led to attempts at toughening ceramics by means other than large scale dislocation motion, for example, by incorporating fibers or second phases (Chawla, 2003).
We devote a whole chapter to fibers produced by a process called electrospinning. Although the process is thought to have originated in the early twentieth century, it was not until 1995 when Doshi and Reneker (1995) sort of rediscovered the process and used the term electrospinning for the process. Reneker and his group are credited with pointing out the diverse range of applications for electrospun nanofibers. It is a very versatile technique for making nanofibers, generally polymeric fibers although ceramic fibers have also been made by this technique. Most electrospun fibers are nanofibers because their diameters are less than 100 nm. There has been a tremendous increase in the use of the electrospinning technique and applications of nanofibers produced in fields as diverse as health care and filtration in aggressive environments (Laudenslager and Sigmund, 2015). The starting material can be in solution form or melt form. There are essentially three components in the process: a high voltage supply, a capillary tube with a needle, and a screen to collect the fibers. The high voltage creates an electrically charged jet of polymer solution or melt out of the needle. The solvent in the jet evaporates (or if a melt is used, it solidifies) and an interconnected web of small fibers is collected on the collector screen. Initially, this technique was used for making polymeric nanofibers. The technique has been used for the preparation of metal oxide/ceramic nanofibers; e.g., silica, zirconia, titania, nickel oxide, barium titanate, lead zirconate titanate, and other oxide materials (Ramakrishna et al., 2005).
It turns out that fibrous nanomaterials or nanofibers as processed by electrospinning are attractive for many applications because of their intrinsically high porosities and large surface areas. Porosity or voids in materials, as highlighted by Gladysz and Chawla (2014), are not always undesirable. Electrospinning is a simple, versatile technique for generating nanofibers from a variety of materials.
In this chapter, we describe the basic process of electrospinning, followed by some examples of nanofibrous structures produced by this process and applications of electrospun nanofibers.
Under the action of an electrostatic field, a droplet of a conducting polymer solution at the tip of a capillary is deformed into a conical shape; this shape is called the Taylor cone. The Taylor cone is formed because of equilibrium between the surface tension of the droplet and the applied electric field. Figure 5.1 shows the Taylor cone schematically.
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