Overview of Functional and Speciality Fibers

Abstract

The development of functional and speciality fibers, which have equivalent performances of easy-care properties of man-made fibers, superior touch and appearance of silk and wool, and outstanding moisture and water-absorbing abilities of cotton, was started shortly after the birth of man-made fibers. Afterward, the application of functional and speciality fibers has expanded toward high-touch, moisture and water absorbability, heat insulation and exothermicity, enhanced color developability, antistatic and conductivity, antibacterial and deodorant, flame retardant and proofing properties, and have contributed to our comfortable, healthy, safe, and environment friendly life. In the development of the functional and speciality fibers, Japan has cultivated the world’s top level of development capability. The driving force of this Japanese R&D originates with the development of technologies that consist of the trinity of polymer, fiber and yarn, and post-processing modifications, together with biomimetic approaches, to learn the structure and functions of living organisms in the natural world. In this chapter, the key production technologies of functional and speciality fibers are presented with comments on representative examples of the fibers.

1 Introduction

It was over 70 years ago that the history of man-made fibers started with the sale of women’s stockings made of nylon, the first man-made organic textile fiber. Nylon fiber was an invention of note not only because it is derived from “coal, water, and air” but also because it is “as strong as steel” and is “as fine as the spider web.” Since then, macromolecular design has been conducted by human hands to construct various polymer structures that are suitable for making man-made fibers. As a result, various kinds of man-made fibers were created and widely used to various purposes to bring revolutionary changes in our life [1].

Looking back on the history of man-made fibers, their progress can be divided into four stages. (1) The first stage was in “the dawn of man-made fibers.” In this stage, the original easy-care properties of man-made fibers such as wash and wear characteristics, wrinkle resistance, and durability were highlighted, accomplishing remarkable developments with the invention of various fibers. (2) The second stage was in “the time for imitating natural fibers.” Here, man-made fibers were improved with an idea, “how to make them closer to natural fibers.” By using the elaborate structures and ingenious functions of natural fibers as models, much effort has been made to impart functional properties such as superior touch and appearance exhibited by silk and wool and outstanding hydrophilic nature of cotton to man-made fibers. (3) The third stage was in “the time of new man-made fibers called “Shin-gosen”.” Here, the fibers having high sensitivity and textures that are intrinsic to man-made fibers and not available even with natural fibers were investigated. For example, bulky yarns exceeding silk fibers and superior yarns having downy hair touch as peach skin were created. (4) In recent years, we have entered the fourth stage, i.e., “the time of market-oriented functional and speciality man-made fibers.” People have been placing so much importance on advanced functionalities that are required for better quality of life and environment, for improved safety, and for saving and recycling of energy and natural resource. Such a surge of social needs to a better life urges man-made fibers to be more comfortable, healthy, safe, and environment conscious, calling for a big change in the social trend from the product-oriented development to the market-oriented development. Table 12.1 summarizes a genealogy of the man-made fibers mentioned above, whereas Fig. 12.1 shows a map of functions and specialities that have been accomplished by various functional man-made fibers [2–4].

Table 12.1 Genealogy of the man-made fibers

Fig. 12.1

Map of functions and specialties

In the development of man-made fibers, Japan has cultivated and accumulated a strong potential, which should be of top rank in the world, particularly in developing those of high-touch and elaborate functionalities. The driving force of the Japanese R&D ought to be based on the trinity of technological developments in polymer synthesis, fiber fabrication, and fiber processing/dyeing, as well as on the biomimetic or bioinspired approaches that can be attained by learning the structures and functions of living things in nature.

This chapter offers a basic knowledge on the functional and speciality man-made fibers by reviewing (1) the world production amounts of man-made fibers in 2013, (2) the modification technologies of man-made fibers (chemical modification of polymers, structure modification of fibers, and post-processing technologies), and (3) the functional and speciality man-made fibers developed with advanced technologies as well as those developed by the biomimetic approaches.

2 Production Amount of Man-Made Fibers [5]

The total amount of fibers produced in the world in 2013 was 84,494,000 tons in which the amount of chemical fibers reached 57,615,000 tons (68.2 %) with the rest 26,879,000 tons (31.8 %) occupied by natural fibers. The production of man-made fibers amounted to 52,706,000 tons, dominating 91.5 % of the production of chemical fibers. Among the man-made fibers, polyester fibers amounted to 45,284,000 tons in total including both filament and staple forms, whereas nylon and acrylic fibers amounted to 4,135,000 and 2,001,000 tons, respectively. Namely, the three major man-made fibers, i.e., polyester, nylon, and acrylic, occupied 97.6 % of man-made fibers (Fig. 12.2).

Fig. 12.2

Man-made fiber production in 2013

While most of the acrylic and nylon fibers are utilized as staple fibers and filament yarns, respectively, the polyester fibers are used both as filament yarns and staple fibers in large quantity. Polyester fibers have the widest application not only to textile use but also to home and industrial uses. This large share of polyester fibers can be attributed to the following reasons:

• 1. Polyester fibers are superior to other fibers in physical and chemical properties. In particular, they show advantages in strength, heat resistance, chemical resistance, etc.

• 2. Their raw materials are inexpensive, getting an economical advantage.

• 3. The productivity is very high because of their excellent melt spinnability. Differing with dry and wet spinning methods, melt-spinning method needs no organic solvent and is preferred in terms of environmental load.

• 4. Various kinds of technologies are available for modification of polyester fibers. The modification can be done at any stage of fiber production, i.e., polymer production, fiber making, and post-processing such as dyeing and finishing.

One of the most famous examples for the modification of polyester fibers and textiles is “alkali-reduction treatment” by which textiles of polyester fiber are allowed to become softer but stiffer with anti-drape and resilient properties. Accordingly, the textiles of alkali-treated polyester fibers can give a feeling similar to that of silk textiles from which the surface sericin is removed by the similar alkali treatment. The man-made silky fibers thus developed have created a new market. During the alkali-reduction treatment, polyester fibers are hydrolyzed with alkali and narrowed sequentially with dissolution of the fiber surface. The fibers become thinner without losing mechanical properties, which are close to those of native silk fibers. Since this technique is used only for polyester, big advantage has been taken by polyester fibers over the other man-made fibers [6]. In addition to this easy modification, the overwhelming share, reaching 88 % of the three major man-made fibers, has made use of polyesters for developing functional and speciality fibers.

In spite of these advantages of polyester fibers, nylon fibers, consisting of aliphatic polyamides, are used in considerable amount for moisture-absorbing application, because nylon shows highly hygroscopic nature in comparison with polyester (official moisture regain: nylon 4.5 % and polyester 0.4 %).

In addition, cross-linked acrylic fibers containing a large number of hydrophilic groups are made from acrylonitrile fibers, whose primary structure is shown by ^–CH₂CH₂CN^–. By converting the pendent cyano groups (CN group) to hydrophilic groups (metal salts of carboxylic acids), the acrylic fibers can attain such a highly hygroscopic nature as to exceed that of natural fibers. Some of the acrylic fibers currently manufactured are known to show a moisture absorption coefficient of about 40 % under the standard environment of 20 °C and 65 %RH.

3 Modification Technologies of Man-Made Fibers [7, 8]

Key technologies that have driven the evolution of man-made fibers are divided into three categories: “technology for chemical modification of polymers,” “fiber modification technology,” and “post-processing technology.” Below, these three technologies are explained by taking polyester fibers as examples.

3.1 Technology for Chemical Modification of Polymers

Although “polyester” generally represents a class of polymers connected with ester bonds (^–CO^–O^–), it is also used to specify poly(ethylene terephthalate) (PET) consisting of terephthalic acid and ethylene glycol as dicarboxylic acid and diol components, respectively (Fig. 12.3). Thus, when one simply says “polyester fiber,” it means PET fiber.

Fig. 12.3

PET structural formula

Generally, two methods are utilized in the chemical modification of polymers, copolymerization and polymer blend (with organic and inorganic modifiers). In the copolymerization method, it is a merit that the function of the modifiers lasts semipermanently because an appropriate amount of modifying units are directly connected to the PET main chains as comonomers. However, it involves demerits that the melting temperature and degree of crystallization likely decrease in as much as to make the heat resistance decrease. Accordingly, the variety of the comonomer design is not high.

In the polymer blend method, on the other hand, the choice of the modifiers is wider because of requiring no chemical reaction. However, the durability of the function of modifiers is inferior to that realized by the copolymerization method, because the modifiers are not connected to the polymer backbones. In particular, inorganic modifiers likely cause many problems about processability, for example, filter choking during spinning. Even organic modifiers cause thermal degradation somewhere in the thermal history of polymerization and melt-spinning processes.

Table 12.2 compares the principal technologies for chemical modification of polymers used for polyester fibers. The cationically dyeable polyester fiber is made by copolymerization of sodium 5-sulfoisophthalic acid as a modifier, whereas the polyester fiber dyeable with disperse dye at atmospheric pressure is made by copolymerizing polyalkylene glycol as a modifier. Since the latter copolyester is easily alkali soluble, it can be utilized for manufacturing special modified cross-sectional fibers with sharp edge and super extra-fine fibers. Here, sea-island or splittable conjugate yarns are made by the conjugate melt-spinning process with the copolyester as one component, then fabricated into woven or knitted fabrics, and finally subjected to dyeing and finishing to dissolve the easily alkali-soluble polyester to obtain fibers having such finest structures.

Table 12.2 Principal technologies for chemical modification of polymers used for polyester fibers

3.2 Fiber Modification Technology

Different from the polymer modification technology depending mainly on functionalization with chemical elements, the fiber modification technology utilizes the functionalization by changing the physical properties and morphology of fibers, for example, fiber diameter (from super-extra-fine to ultra-thick), mechanical properties (such as strength elongation, thermal shrinkage, thermal stress, etc.), fiber cross-sectional shape (triangular, multilobed, hollow, flat), and conjugate structure (such as sheath core, side by side, sea island, etc.). One of the key technologies in melt-spinning process consists in the design of spinning machine, in particular, cap design and pack channel design. Furthermore, a wide variety of modification technologies have been developed in the drawing and yarn texturing processes. For example, composite yarns combining filaments having different shrinkages and those interlacing filaments having different physical properties are manufactured. Core-sheath double-layered yarns made by the process of false-twist texturing the yarns consisting of filaments having different physical property are also manufactured. Table 12.3 compares the representative technologies for fiber modification of polyester using specific mechanical processes (spinning, drawing, and yarn texturing).

Table 12.3 Representative technologies for fiber modification of polyester using specific mechanical processes (spinning, drawing, and yarn texturing)

The filament yarns and textured yarns described above are fabricated into woven or knitted fabrics and subjected to post-processings such as dyeing and finishing to finally attain the functional properties engineered by the polymer chemical modification and/or fiber modification.

3.3 Post-processing Modification Technology

In the post-processing of woven or knitted fabrics, dyeing is the main process. The dyed fabrics are then subjected to various chemical and/or physical processings such as refinement, relaxation, preset, alkali-reduction, and other functionality imparting treatments and finally heat set under dry or moist heat conditions. Table 12.4 summarizes the main technologies for the post-processing of polyester woven or knitted fabrics.

Table 12.4 Main technologies for the post-processing of polyester woven or knitted fabrics

As mentioned above, the alkali-reduction treatment is a process for manufacturing silky man-made fiber textiles and is utilized only for silk and polyester fibers. The weight reduction of polyester fibers can easily be performed by contacting with concentrated aqueous solution of alkali, because the main-chain ester bonds are easily hydrolyzed. Since polyester is hydrophobic and does not swell in an alkaline solution, the hydrolysis proceeds from the fiber surface in a stoichiometric manner, and the hydrolysis products (alkali salt of terephthalic acid and ethylene glycol) are allowed to dissolve away and diffuse in the alkali bath. Such a mechanism working on polyester fiber is favorable for the homogeneous thinning of fibers from the surface. The process control of weight reduction is possible by measuring the quantity of alkali consumption even in the industrial production (Fig. 12.4). The fiber thinning can bring a fine space between the woven fibers of fabric and an increase in mobility of the fibers. In consequence, the handling of the fabric becomes soft and flexible, while the physical properties such as strength do not substantially decrease in values per cross-sectional area of a fiber [6].

Fig. 12.4

Alkaline hydrolysis of PET

Although the bath absorption method similar to the dyeing process with disperse dyes is utilized to impart functional properties to fibers, its applicability is rather limited because the modifiers having a solubility parameter close to that of polyester are limited in number.

The most generally applied to fiber modification is the pad method in which a modifier is fixed onto the fiber surface with a binder resin. In this pad method, the handling of fabrics likely becomes harder, and the durability of functional properties is not as high as that imparted by the aforementioned fiber modification. However, the pad method shows advantages over the polymer and fiber modifications in easiness of production in small lots and in variety of modifiers available. The requirements for the modifiers are milder, particularly concerning the heat resistance, particle size, and so on.

4 Biomimetic Man-Made Fibers Having Specific Structures and Functions [7, 8]

The potential functionalities and specialties are first contrived to polymers and/or fibers by using the aforementioned polymer and/or fiber modification technologies and intensified in the post-processing stages, including the dyeing step to produce highly functionalized and specialized fibers.

Table 12.5 shows the high-level functional and speciality man-made fibers that can be differentiated by the modification stages, i.e., polymer, fiber, and their elemental technologies. It is evident that many of the speciality fibers were developed by envisaging the trinity of polymer, fiber, and post-processing technologies dealing with high sensitivities and functionalities.

Table 12.5 High-level functional and speciality man-made fibers that can be differentiated by the modification stages

The blend and copolymerization with special modifiers are used in the polymer technology, while the fibers with modified cross section, conjugate fibers, composite yarns combining filaments with different shrinkage, and composite yarns of false-twist texturing filaments are elements of the fiber modification technologies. The alkali-reduction treatment, dyeing, and functionality imparting processings are important in the post-processing technology.

Simultaneously, another important key concept that has led the development of these speciality fibers is the “form of synthetic fibers.” It represents that the structure and function of living things in nature can serve as the sources of idea, which is sometimes represented by a word “bioinspired” or “bio-mimic.” For example, such inspiration is gotten from the function that the cornea of a moth having a submicron concave-convex surface structure does not reflect light for camouflage. Similarly, the capillary action of trees for absorbing water, squeaky feeling of a tussah silk fabric, sand-washed silk fabric, natural and suede leathers, and the color depth of black-dyed wool fabric and the function of the concave-convex surface structure (waxy substance) of a lotus leaf or a taro leaf that repels drops of water are examples of functions and structures from which new ideas are inspired in designing man-made fibers.

5 Conclusion

This chapter includes the following contents:

• 1. Most of the functional and speciality man-made fibers are composed of polyester, which occupies a predominantly high share in the world production of man-made fibers.

• 2. The modification technologies are based on the trinity of polymer, fiber and yarn, and post-processing modifications, for which Japan has taken a leadership.

• 3. The functional and speciality fibers developed in Japan depend on the modification technologies based on biomimetics.

In the following chapters, functional and speciality man-made fibers are explained in detail in the following order related with the functions:

• 1. High-Touch Fibers and “Shin-gosen” (see Chap. 13)

• 2. Moisture and Water Control Man-Made Fibers (see Chap. 14)

• 3. Heat-Controllable Man-Made Fibers (see Chap. 15)

• 4. Light-Control Man-Made Fibers (next issue)

• 5. Antistatic and Conductive Man-Made Fibers (next issue)

• 6. Antibacterial and Deodorant Man-Made Fibers (next issue)

• 7. Flame-Retardant Man-Made Fibers (next issue)