2. Textile Structures
The term textile is derived from the Latin “textilis” and the French “texere”, meaning “to weave”. It originally referred only to woven fabrics; then, it was extended to fabrics produced by other methods, such as threads, cords, ropes, braids, lace, embroidery, nets, and fabrics made by weaving, such as woven, knitted, braided, or non-woven fabrics
[18][19].
The different operations that begin with fiber production and proceed to fabric realization are as follows: fiber production, yarn production, fabric production, pre-treatment, dyeing and printing, and finishing treatment
[20]. Technically, the basic principle of weaving involves two series of filaments: the first (i.e., the warp) is fixed, while the second (i.e., the weft) is inserted within the structure formed by the warp.
The weaving process consists of three steps: (i) shedding, during which the warp sheet is divided into two patterns, one spaced, separated, and lifted up from the other so to create an opening between the warp series; (ii) filling insertion, which provides for the insertion of a yarn inside the created opening; and (iii) beat-up, in which the filling yarn is pushed inside the weave of the fabric. Using this method, 2D planar weaving structures, i.e., with in-plane-oriented fibers, can be produced
[19][21].
Other typical processes exist, depending on the interlocking type, that can generate another type of weaving, such as knitting, braiding, and non-woven materials. In detail, four main categories of textile structures can be recognized
[20].
Woven fabrics are made by weaving two sets of interlaced yarn components (warp and weft) using orthogonal interlacement at 0° and 90° intervals. In woven fabrics, warp yams are positioned vertically or longitudinally, and weft yarns are orientated horizontally.
Braided fabrics are created by placing fibers in the bias direction according to a specific braided angle. The first set of tows is called the axial yarns, while the others are called the braided yarns.
Knitted fabrics are constructed by intermeshing loops and segments of fibers.
In non-woven fabrics, the fibers are randomly oriented in the structure in a discontinuous way.
In the 19th century, three-dimensional (3D) structures were a group of fibers in multiaxial orientation (i.e., fibers arranged in three dimensions).
The most common architectures in woven fabrics are plain weave, twill and sateen. The plain weave is the main basic weave realized through filling yarns passing alternatively over and under other yarns.
In the twill structure, the filling threads are woven over and under two or more warp yarns, producing a characteristic diagonal pattern. Numbers reflecting the twill structure in the form of x/y are frequently provided. This indicates that x warp strings and y weft yarns are used to build up the diagonal stripe. For example, 3/1 twill means the alternation of three warp strings and one weft yarn to build the structure.
In the sateen weave, the filling threads are interlaced with the warp at widely spaced intervals, creating the appearance of a continuous surface.
3. Common Materials Constituting Textile Structures
Textile fibers can be broadly classified into two main categories (natural and synthetic fibers), as shown below (
Figure 1).
Figure 1.
Materials commonly used in textile fibers.
Natural fibers originate from animals, vegetables/plants, or mineral sources.
Examples of animal-based fibers are wool, silk, and hair, whereas plant-based fibers are constituted by seed, bast, leaf, wood; finally, mineral fibers include asbestos, fibrous brucite, and wollastonite
[22]. In terms of their compositions, plant-based fibers are constituted by bio-based polymers such as cellulose, semi-cellulose, lignin and other components in small percentages (impurities, ash, and extractives)
[23]. Plant-based fibers, rather than animal-based fibers, can attain superior performances, higher strengths, and stiffnesses. Silk is an exception, as it can have very high strength but is very expensive, has lower stiffness, and is less widely available
[24]. Natural fibers are hydrophilic in nature due to the presence of hydroxyls and other polar groups, and they have a high moisture absorption and a low evaporation rate, as well as poor dimensional stability since they swell when exposed to water
[23].
Asbestos (as a part of mineral-based natural fibers) is often avoided due to the health hazards associated with its inhalation/ingestion as well as the potentially carcinogenic response in humans. In many countries, the use of asbestos fibers is even forbidden
[24].
Synthetic fibers, also known as man-made fibers, are typically made from synthetic materials, such as petrochemicals (polyamide, polyester, polypropylene). However, some synthetic fibers, such as rayon, modal, and lyocell, are made from natural cellulose (bleached wood pulp)
[22].
Compared to synthetic fibers, natural fibers possess very good characteristics in terms of sustainability, given the recyclability and renewability of the raw materials, the biodegradability, the relatively low cost and density, the low energy requirements during the manufacturing process, and the low carbon dioxide contribution during the growth and production
[25].
However, being mainly hydrophilic in nature, the tensile characteristics of cellulosic fibers are highly dependent on the relative humidity of the environment. Moreover, the higher the fiber length of the cellulosic fibers, the lower the tensile strength. This is attributed to the higher probability of them containing defects in longer fibers and to the number of weak links or imperfections. Furthermore, the tensile properties of cellulosic fibers are susceptible and sensitive to measurement conditions, such as speed, initial gauge length, moisture, temperature, different cross-section of the fiber at a different point, and surface treatment, chemical treatment, upgrading treatment, water treatment, and drying. During the process used to produce cellulosic fibers, the different phases (plant growth, harvesting, fiber extraction and supply) can alter significantly the quality
[26][27].
A comparison in terms of the density, Young’s modulus and tensile strength among plant-based fibers, animal-based fibers, asbestos and synthetic fibers is reported in
Table 1.
Table 1.
Components, density (g/cm
3
), Young’s modulus (GPa), and tensile strength (MPa) of the main fibers.
Some chemical structures of the plant-based and animal-based components are displayed in
Figure 2.
Figure 2.
Chemical structures of some fiber components: cellulose
; keratin
; lignin
; and fibroin
.