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Thursday, February 8, 2018

Fundamental Properties of Textile Fibers

In a broad sense the word fiber is used for various types of matter – natural or manmade, forming basic elements of textile fabrics and other textile structures. Fibers are the fundamental building blocks of all products defined as a textile, regardless of application or construction. Fibers come in various forms and guises, which can have an impact on the performance, aesthetics, handle of a textile material. Natural fibers were used for millennia to create garments and tools that contributed to the expansion and development of humanity. More recently, science and industry has created a series of fibers and filaments derived from man-made polymeric material through chemical and extrusion processing, most notably nylon and polyester. This chapter will introduce the most common textile fibers from both natural and synthetic sources along with the properties of such fiber and filaments. Providing a comprehensive list of the subtle relationships between form and function for each fiber is beyond the scope of the text, but it should provide an overview of the fundamentals. This will enable the reader to assess critically how fiber type will impact on the properties of the final fabric. It is essential that the reader appreciates that appropriate selection of fiber is of fundamental importance when considering a desired design, application, or visual effect. To this end, the influence of fiber type, form, shape, diameter, and finish on properties and design is appraised by considering the major fiber types, the common production route, and how the manufacture can influence the types of yarns and fabrics available. The economic and societal implications are also of importance with the volumes, costs, and environmental impact of each class of fiber being considered.

Technologists have defined the term Textile fibres as those fibres which can be spun into a yarn or made into a fabric by interlacing, or interlooping in a variety of machines including weaving, knitting, braiding, felting, bonding, etc.

Various textile fibres
Figure-1: Various textile fibres
General Properties of Textile Fibers:
A fiber is simply considered to be a linear strand with flexibility and a length many times its width. This differentiates it from other assemblies such as tapes, films, and rods. For the designer, fibers and filaments could be considered the smallest element in a textile construction. The properties of a fiber will determine how it appears, how it drapes, how it conforms, and how it stretches. The designer should be aware that there is a full gamut of aesthetic finishes that can be generated by variation of this simple element.

Fibers are typically available in either staple or continuous filament. Staple fibers are elements with a length that is limited via a natural or man-made process. In contrast, continuous filaments are considered to be uncut and could be as long as many hundreds of kilometers. A continuous filament product can be easily converted in to a staple form with a length cutter. The natural materials (except silk) are available exclusively as spun staple products, whereas the synthetic filaments are often available in more formats, with monofilament yarns, multifilament yarns, and staple being the principal categories.

Fiber structure:
Whether staple or filament, natural or synthetic, the fundamental principles of physics apply to each fiber type. These properties are determined by the dimensions and molecular structure of the fiber. Fibers are constructed from long chains of atoms known as polymers. These structures can be formed naturally (cellulose, keratin, collagen, asbestos) or may be formed through synthetic chemical processing (polyethylene, nylon, polyester). Regardless of the method of synthesis, it is the length, molecular structure, and net orientation of these polymers that will govern not only the mechanical properties (strength, stiffness, elasticity) but also the processing in to fabrics (filament manufacture, fabric manufacture, dyeing) and the behavior of such materials (crease resistance, water adsorption, wicking). For example, cotton and flax are comprised largely of cellulose, a chain of carbon, oxygen, and hydrogen linked in the structure shown in Figure-2.

Chemical structure of cellulose
Figure-2: Chemical structure of cellulose (n is the degree of polymerization).
The length and arrangement of these chains determine the strength and stiffness of the fiber. Cellulose chains group together and arrange into linear fibrils, which, like a bunch of twigs, is strong and difficult to bend. It is the hydroxyl (–OH) chemical groups in the cellulose chain that allow for the adsorption and wetting of cellulose (hydrophilicity), which adds to the comfort and moisture response of cotton and linen products. The cellulose chains will also readily form hydrogen bonds between hydroxyl groups and with water. The hydroxyl–hydroxyl bond arranges the chains into a regular, patterned arrangement known as a crystal. There will also be regions of disorder where chains terminate or cannot form a regular structure. This combination of order and disorder (semicrystallinity) provides textile fibers with the strength and ductility necessary to form yarns and fabrics. The chemical groups in cellulose are also responsible for the phenomenon of creasing in cellulose where the hydroxyl groups form hydrogen bonds in new positions, breaking the initial conformation and creating a crease. The original structure can be recovered by resetting the hydrogen bonds with the addition of heat, water, and pressure (ironing). In contrast, poly(ethylene terephthalate) (PET) is a linear chain constructed of aromatic and ester linkages (Figure-3). The chemical groups in PET are less likely to form hydrogen bonds and thus polyester sportswear garments show excellent crease recovery and may not need ironing after washing. The incorporation of PET fibers into a cotton yarn increases its crease recovery, as the PET fibers are inclined to return to original state, thus creating easy-care fabrics.
Chemical repeat of poly ethylene terephthalate
Figure-3: Chemical repeat of poly(ethylene terephthalate) (n is the degree of polymerization).
Fiber dimensions:
The principle property that often determines the markets and application for a fiber is the diameter or fineness. The fineness of a fiber determines the flexibility of a fiber and the fiber assembly. Fiber fineness is typically expressed in microns when referring to diameter and in decitex or denier when referring to linear density. The finer the fiber, the more readily it will bend and is one of the key factors in determining softness and comfort levels.

Textile fibers have to be sufficiently fine that they can be formed into yarns and fabrics and do not itch or demonstrate a prickle effect on the skin. Short segments of fiber do not bend upon skin contact until they reach a threshold value beyond which they buckle. For coarse fibers, this threshold force exceeds the force required to activate the nerve sensation. Fiber fineness will determine the number of fibers within a yarn, with more fibers within a cross section generating more frictional contact and allowing for the creation of stronger yarns. There are established methods for quickly measuring the fineness of natural materials: the cotton industry uses the micronaire system and large segments of the woolen trade use the CSIRO laserscan.

Length is another key metric in determining the processability of a material. This is often one of the pricing considerations with natural materials, with length being ranked the most important parameter for ring and air jet spun cotton yarns (Gordon, 2007). Longer fibers can make yarn processing easy and could be used in a finer and stronger yarn. In contrast, short fibers can not only increase yarn hairiness and bulk but also can significantly reduce the processing yield as fibers are lost during spinning. For natural fibers, any supply is a distribution of lengths from long to short and there are objective means to characterize the average length and the variation and uniformity within a sample. With synthetic materials, staple length is controlled accurately through filament cutting and can be easily optimized to work well with existing processing equipment.

Shape and cross section:
Not all fibers could be considered to be circular and the shape of a given fiber will affect bending and rigidity, handle, and luster. Nonround fibers are common in natural fibers and can be engineered into man-made fibers. With manufactured fibers, these shapes can be altered using new spinneret designs and processing techniques. Smooth circular fibers are typical for man-made fibers, but they can be irregular and include features such as crenellation, which can have a marked impact on optical properties.

Mechanical Properties of Textile Fiber:
The response of fibers to mechanical stimulus is arguably the most important property, as without sufficient strength or flexibility there can be no yarn and no fabric. The mechanical properties, such as strength, stiffness, elasticity, and flexibility, will determine the behavior during processing and the resulting fabric properties. The strength and properties of a fabric and yarn is a complex combination of fiber and interfiber frictional properties; however, it should be realized that the yarn or fabric strength can never exceed the strength of the aggregate of textile fibers.

The elastic characteristics of a fiber determine how well it will recover from deformation and are of far more importance for many applications than the actual breaking point. Fibers with excellent elastic recovery will lend themselves to applications where tension and deformation is applied regularly and for prolonged periods. Spandex is an elastomer with fantastic elastic recovery, allowing nylon hosiery to recover very well from stretches and not sag or deform permanently. In comparison, a viscose sweater demonstrates less elastic behavior and is likely to deform at the areas of repeated strain such as elbows and necklines.

Optical and Aesthetic Properties of Fibers:
The visual aspect of a fiber is determined by its size and shape, the internal microstructure, and the surface texture. Fibers can generate wildly different aesthetic effects through the variation of one or more of these values. This variation can be easily seen in the difference in light reflectance, luster, and gloss effects seen between polyester and woolen products. A smooth surface will generate a more specular reflection, creating a gloss effect, whereas a rough surface will generate more diffuse light refection and a matt effect (Figure-4).

Specular and diffuse reflection on a smooth and rough surface
Figure-4: Specular and diffuse reflection on a smooth and rough surface.
Alongside surface reflection, there are a number of additional light interactions that will influence the luster and visual effect of a fiber. In broad terms, how light is reflected by a fiber is governed by the following physical properties:
  • Surface texture: A rough fiber surface will generate reflections in multiple directions creating more diffuse reflections.
  • Refractive index: This is determined by the relative velocity of light through a fiber in comparison to light in a vacuum. Changes in refractive index will skew how light is reflected and may also cause birefringence and dichroism within a fiber.
  • Adsorption: Light that penetrates within a fiber can be absorbed by molecular agitation. Visible spectrum light is readily adsorbed by dyes, pigments, and additives to generate colored reflection light.
  • Shape: The shape of the fiber will greatly influence the luster and shine of a fiber. Light is reflected depending on the angle of incidence and so complex shape variations generate different levels of shine and luster. Fibers with circular cross sections typically generate more specular reflection and so appear more lustrous.