Textile Chemistry

Any of a number of textile fibers produced from chemical substances of natural origin or synthetic origin; the latter are also known as synthetic textile fibers. Among the natural sources of manufactured textile fibers are plant cellulose and protein, rubber, metals, and nonmetallic inorganics. The synthetic textile fibers are produced from organic intermediates derived from petroleum, coal, and natural gas. With the exceptions of glass and metal textile fibers, the manufactured textiles are made from very long chainlike molecules called linear polymers. These polymers may be naturally occurring (cellulose from cotton or wood pulp) or may be synthetic (polyester). 

Irrespective of their chemical nature, textile fiber-forming polymers must possess the following characteristics:  

(1) great length—at least 200 monomer units must be joined in a chain; (2) a high degree of intramolecular and intermolecular attraction, whether through primary chemical bonds or other attractive forces; (3) the ability to be oriented along the axis of the fiber; and (4) the ability to form well-ordered crystals or pseudocrystals. All of these parameters are sensitive to the chemical nature of the polymer and the processes of manufacture of the fiber. In turn, they establish the properties of the fiber, such as strength, flexibility, resilience, and abrasion resistance, which contribute to their usefulness in various end uses for apparel, home furnishings, and commercial and industrial applications. Only a fraction of those substances capable of forming textile fibers prove to have all of the characteristics necessary for commercial success. The fiber types of major importance in the United States are classified by composition as follows: cellulosic (composed of regenerated cellulose, cellulose diacetate, and cellulose triacetate); synthetic (composed of polyamide, polyester, polyacrylic, polyvinyl, and polyolefin resins); and inorganic (composed of glass and metal). 

All of the manufactured fibers are produced according to the same principles: 

(1) the textile fiber-forming material must first be made fluid; (2) the fluid is forced under pressure (extruded) through tiny holes into a medium which causes it to solidify; and (3) the solid textile fibers are further processed to obtain their optimum properties. Typically one of three procedures is used to produce fibers. In wet spinning, (for example, the production of rayon by the viscose process), the polymer is dissolved in an applicable reagent to form the fluid (dope). The fluid is then pumped through metal plates (spinnerets) containing many small holes into a liquid bath of appropriate composition. A chemical reaction between the spinning dope and the bath causes the fiber to solidify. In dry spinning, the polymer is again dissolved in an appropriate solvent and extruded through a spinneret. However, the liquid bath is replaced by a stream of warm gas (usually air) which evaporates the solvent and allows the polymer to solidify as a filament. Cellulose diacetate and triacetate are produced in this manner. In melt spinning, Nylon, polyester, and the other thermoplastic fibers are produced by melt spinning. No solvents or reagents are required since the polymer can be melted without appreciable decomposition. Thus, the fluid consists of hot molten polymer which, upon extrusion into a stream of cold air, solidifies into a filament. Depending upon the end use, filaments may be produced in various sizes ranging from finer than a human hair to thick bristles for toothbrushes. They may also be produced with different cross-sectional shapes, such as round, lobed, square, or dogbone. After extrusion, filaments are usually stretched (drawn). Drawing causes an increase in order (crystallinity) by extending the molecules of the fiber so that they pack more closely together, and orients the molecules along the longitudinal axis of the textile fiber. Higher orientation and increased crystallinity raise the strength of the fiber, decrease its stretch, and improve its elasticity. Often, the manufactured fibers are textured to improve their comfort properties. textile Fabrics made from smooth, straight filament yarns are not as comfortable as those made from yarns spun from the shorter natural fibers. Texturing introduces irregularities (crimp) along the length of the filament and leads to bulkier filament yarns which are closer to spun yarns in their performance. Advances in polymer and fiber technology have led to the development of fibers with exceptionally high temperature resistance and extremely high strength. These properties are desirable in applications such as upholstery and floor coverings in aircraft and other mass-transit vehicles, protective clothing for fire fighters and other emergency personnel, body armor for soldiers and police officers, tire cords, and industrial belting. Metallic textile fibers of silver and gold have been used for millennia to decorate fabrics. Today metallic fibers serve useful as well as decorative purposes. These fibers are formed by drawing metal wires through successively finer dies to achieve the desired diameter. Although gold and silver are the easiest to draw, modern methods have allowed the manufacture of steel, tantalum, and zirconium textile fibers. Because they are electrical conductors, metal textile fibers have been blended into textile fabrics to reduce the tendency to develop static electrical charges. Glass fibers are prepared by the melt spinning of previously formed glass marbles, and the molten filaments are drawn down to very fine dimensions. It is the fineness of the textile fibers that gives them their flexibility and allows them to be used in textiles. Unfortunately, the fibers are so stiff that when broken they can penetrate human skin. Thus, they are not well suited to use in apparel or upholstery. Glass is widely used in curtains and drapery because of its total resistance to the degrading effects of sunlight, its low cost, and its flame resistance. It provides a nonrotting, nonsettling insulating material for homes and industrial uses.textile Fiber properties include the physical, mechanical, chemical, biological, and geometrical characteristics of textile fibers. Some of the more important ones are tensile strength, elongation at break, modulus of elasticity or stiffness, fatigue under repeated stress, resilience or ability to recover from deformation, moisture absorption and wettability, electrostatic properties, friction, color, luster, density, and resistance to light, heat, weathering, abrasion, laundering, mildew, insects, chemicals, and solvents; and finally a number of geometric features, such as diameter, cross-sectional shape, and crimp. Such properties play an important part in determining whether or not the fiber can be made into a textile fabric that will be wrinkle-resistant, pleasing to the touch, comfortable, easy to clean, durable, and attractive in color, luster, drape, and general appearance. With a knowledge of textile of the physical properties of the available fibers, the textile engineer can choose the best textile fiber or best blend of several fibers to fit the intended use. The final result, however, is also dependent upon the proper choice and control of additional factors such as the yarn and fabric structure, the weave pattern, and the finishing of the cloth.

Textile Chemicals:

The enormous number of chemicals used in textile processing may be divided broadly into two categories: those intended to remain on the textile fiber, and those intended to wet or clean the textile fiber or otherwise function in some related operation. The former includes primarily dyes and finishes. The latter group consists mainly of surface-active agents, commonly known as surfactants.

Preparation:

Preparation is a term applied to a group of essentially wet chemical processes having as their object the removal of all foreign matter from the fabric. This results in a clean, absorbent substrate, ready for the subsequent coloring and finishing operations. The operations constituting preparation depend primarily on the fibers being handled. Synthetic fibers contain little or no natural impurities, so that the only materials that normally must be removed are the oils and lubricants or water-soluble sizes needed to facilitate earlier processing. This is generally accomplished by washing with water and a mild detergent capable of emulsifying the oils and waxes. On the other hand, natural fibers contain relatively high amounts of natural impurities, and in addition frequently are sized with materials presenting difficulties in removal. In the case of cotton, prolonged hot treatment with alkali, usually sodium hydroxide, and strong detergent is necessary to break down and remove the naturally occurring impurities. Special scours are necessary for cleaning such materials as wool and silk. The protein textile fibers are very sensitive to alkali and strong detergents; they are usually washed with mild soap or sulfated alcohols. After other impurities are removed from the fiber, it is usually desirable to remove any coloring material. This process is known as bleaching. By far the major bleaching agent in use is hydrogen peroxide, which is efficient in color removal, while still being considered relatively controllable and safe for use.

Mercerization:

Mercerization is a special process applied only to cotton. The textile fabric or yarn is treated with a strong sodium hydroxide solution while being held under tension. This process causes chemical and physical changes within the fiber itself, resulting in a substantial increase in luster and smoothness of the fabric, plus important improvements in dye affinity, stabilization, tensile strength, and chemical reactivity.

Coloring:

Although many textiles reach the consumer in their natural color or as a bleached white, most textiles are colored in one way or another. Coloring may be accomplished either by dyeing or printing, and the coloring materials may be either dyes or pigments.textile Dyeing essentially consists of immersing the entire fabric in the solution, so that the whole fabric becomes colored. On the other hand, printing may be considered as localized dyeing. In printing, a thickened solution of dyestuff or pigment is used. This thickened textile solution, or paste, is applied to specific areas of the fabric by means such as engraved rollers or partially porous screens. Application of steam or heat then causes the dyestuff to migrate from the dried paste into the interior of the fiber, but only in those specific areas where it has been originally applied.

Finishing:

Finishing includes a group of mechanical and chemical operations which give the textile fabric its ultimate feel and performance characteristics. Many desirable characteristics may be imparted to the textile fabric through the application of various chemical agents at this point. Softeners are used to give a desirable hand or feel to the fabric. These chemicals are generally long fatty chains, with solubilizing groups which may be cationic, nonionic, or occasionally anionic in character. They are essentially surfactants constructed so as to contain a relatively high proportion of fatty material in the molecule. Conversely, certain types of polymeric material such as polyvinyl acetate or polymerized urea formaldehyde resins are used to impart a stiff or crisp hand to a fabric. See also Polyvinyl resins; textile Urea-formaldehyde-type resins. It is in finishing that the so-called proof finishes are applied, including fire-retardant and water-repellent finishes. A fire-retardant finish is a chemical or mixture containing a high proportion of phosphorus, nitrogen, chlorine, antimony, or bromine. A truly waterproof fabric may be made by coating with rubber or vinyl, but water-repellent textile fabrics are produced by treating with hydrophobic materials such as waxes, silicones, or metallic soaps. Many other types of highly specialized treatments, such as antistatic, antibacterial, or soil-repellent finishes, may be applied to fit the fabric to a particular use.