Manufactured Fibre Technology

A textile fibre is a long thin object with a high ratio of length to thickness. It is characterized by a high degree of fineness and outstanding flexibility. In addition, it should have dimensional and thermal stability and minimum levels of strength and extensibility consistent with the end use. Fibres should also be capable of being converted into yarns and fabrics. There are a number of other requirements that a fibre must satisfy, but those noted above are relatively more significant.

A fibre must ultimately satisfy the needs of the consumer. The consumer is not concerned with the method of production of the fibre, nor with its chemistry and physics; the consumer looks for durability, comfort, adequate dimensional stability and other useful properties in addition to aesthetic appeal. However, the fibre producer must have a good understanding of how fibre structure controls its properties so that the fibre can be ‘engineered’ by suitable choice of the polymer and its molecular weight, orientation, crystallinity, morphology, etc. By so doing, the fibre producer can expect to meet the needs of the consumer. The starting point to gain such an understanding is to consider the fibre-forming polymers against the backdrop of the available polymers so that the structural requirements for a useful fibre-forming polymer become clear. This is the primary purpose of this chapter. The emphasis is on a physical understanding of the structure-property relationships in very simple terms; the resulting loss of accuracy should not detract from the main theme that is developed.

The inspiration and the knowledge needed to develop the spinning techniques used for fibre manufacture were provided by the spider and the silkworm. These creatures showed that the following steps were required for extruding thin continuous filaments: (1) acquisition of spinnable liquid, (2) jet formation, and (3) jet hardening. Add to this bobbin winding, and we get manufactured fibres in the spun state. The manufactured fibres may need further drawing so that they have adequate properties.

As stated in the previous chapter, melt-spinning is the simplest method of fibre manufacture, mainly because it does not involve problems associated with the use of solvents. It is therefore the preferred method, provided the polymer gives a stable melt. When polymer granules or chips form the starting material for melt-spinning, they are first dried and then melted in the extruder. The homogenized and filtered melt is squirted through narrow channels into a quench chamber where solidification of the fluid filament bundles is achieved (Fig. 4.1). Finally, spin finish is applied before the filament bundles are wound on tube rolls. In larger modern plants, polyester and nylon are produced in continuous polymerization units in which the melt is directly transported from the final polymerizer to the melt-spinning unit. In the case of polypropylene, since polymerization leads to a solid product, it is separate from the spinning process.

The production of poly(ethylene terephthalate) (PET), nylon and polypropylene (PP) fibres or filaments involves melt-spinning of the molten polymer followed by solid state drawing and annealing. The final fibre properties such as tensile strength, elongation, shrinkage and dyeability are determined by structural parameters such as orientation and crystallinity. The fibre morphology is the result of the combined influence of the spinning, drawing and annealing steps. Although the desired orientation and crystallinity are achieved primarily through the control of process parameters in the drawing and annealing steps, the fibre line processability in terms of drawability and level of broken and fused filaments is determined by the orientation and uniformity of the as-spun fibres. Also the final product quality parameters such as the coefficient of variation of fibre denier and tenacity, level of over-length fibres, etc. are influenced mainly by the spinning process variables. This is because the basic spatial arrangement of the polymer molecules with respect to the fibre axis is determined in the melt-spinning step.

If a polymer can be melted under reasonable conditions, its conversion to a fibre by melt-spinning is preferred over the solution-spinning process, mainly as the former does not involve the use of solvents and the problems associated with their use, namely their removal, recovery, the associated environmental concerns and the low spinning speeds. When melt-spinning cannot be carried out, either because the polymer degrades before melting or the melt is thermally unstable, distinction as to the type of process is made depending upon whether the solvent is removed by evaporation by heating (dry-spinning) or by coagulation in another fluid which is compatible with the spinning solvent, but is itself not a solvent for the polymer (wet-spinning). Historically, the formation of fibres by dry-spinning was the first to evolve as a method for manufacturing polymer-based fibres, as pointed out in Chapter 3. In the present chapter, the solution-spinning processes will be briefly described.

Manufactured fibres require spin finishes for efficient processability and conversion to textile materials for various applications. Although a spin finish is a layer only a few molecules thick on the surface of the fibre, it is one of the most important variables dictating the performance, quality and uniformity of processing.

The drawing or orientation stretching process is a vital post-spinning operation for a melt-spun fibre. An undrawn fibre deforms inelastically under low loads and has a poor stress value. Such a material has very little utility for most textile applications. Through the drawing operation the fibre is orientationally strengthened due to alignment of the molecular chains along the fibre axis and shows enhanced recovery. Drawing also induces changes in the levels of crystallinity and sometimes in the crystalline form. Both semicrystalline fibres with a lamellar morphology in the undrawn state and amorphous fibres with an entangled network of molecular chains are transformed into a fibrillar structure through the process of drawing. An increase in draw ratio increases the orientational order as well as conformational conversions, resulting in a structure which has much higher strength, modulus and dimensional stability compared with its undrawn state.

Synthetic filaments such as nylon and polyester are extruded as continuous filaments and drawn afterwards to the required level to impart strength and stability. The drawn nylon and polyester filaments are semi-crystalline and oriented and exhibit thermal shrinkage when they are heated to temperatures above the glass transition but still well below the crystalline melting point. The decrease in length arises mainly from the relaxation of molecular chains. The magnitude of shrinkage is dependent upon structural parameters such as orientation and crystallinity of the fibres and upon external variables such as temperature, tension, time, etc. Thus, drawn but unset fibres, although possessing good tenacity and elasticity properties, cannot be used for most textile and technical purposes since they do not possess dimensional stability. Furthermore, twisted or doubled filaments have a strong tendency to curl; this has a deleterious effect on further processing. Apart from this, planar structures like fabrics produced from these unset, continuous filaments or fibres display unfavourable creasing behaviour when in use. Therefore it is necessary to ‘set’ the drawn or twisted filaments in order to attain resistance against shrinkage, and dimensional changes, curling and creasing. This can be achieved by the action of heat in the absence or presence of swelling agents, with or without tension. In practice, hot water, saturated vapour or dry heat treatments are used.

Characterization of manufactured fibres is an integral part of the fibre production process, as every manufacturing unit has to ensure that its products are as per the specifications. Though fibre characterization has significant areas of commonality with polymer characterization, the emphasis is often different, necessitating adaptation of the techniques to meet specific needs and sometimes the use of special techniques. However, fibre characterization has received relatively less attention than polymer characterization [1‐3].

Testing of manufactured fibres, filaments and their yarns is important from the points of view of quality control, process control, product development and process optimization. Fineness, crimp, shrinkage, tensile and evenness are some of the properties tested depending on the material and end use requirements. The exact norms for these properties are not normally available but, depending on the product and end use requirements of the customer, each plant establishes certain norms for each important property. Besides satisfying these norms, the fibres and yarns should have consistent properties. The principles and methods of evaluating these properties are discussed in this chapter.

Poly(ethylene terephthalate), or PET, is the most outstanding member of the family of polyester fibres. Polyester is defined by the International Standards Organisation (ISO) as ‘a polymer comprising synthetic linear macromolecules having in the chain at least 85% (by mass) of an ester of a diol and terephthalic acid’ [1]. A comprehensive compendium on PET dealing with historical, technical, research and commercial aspects has been recently published [1].

Nylon 66 and nylon 6 are two important members of a group of polymers known as polyamides. The structural units of a polyamide are joined together by an amide, -NH-CO-, group. A polyamide manufactured from aliphatic monomer(s) is commonly designated as nylon. However, the US Federal Trade Commission has defined nylon as a manufactured fibre in which the fibre-forming substance is a long-chain synthetic polyamide in which less than 85% of the amide linkages are attached directly to two aromatic rings, while a polyamide in which at least 85% of the amide links are joined to two aromatic groups is known as an aramid. Aramid fibres are mainly used for industrial applications and are described in Chapter 18.

The widely used conventional commodity yarns of polyamide (nylon 6 and nylon 66) and polyester (polyethylene terephthalate) are usually melt-spun from the standard polymer and the filaments have a round cross-section. The so-called ‘speciality yarns’ are also melt-spun either from a chemically modified polymer or through a process involving physical modification or special equipment. The wide spectrum of such yarns includes such diverse products as deep-dyeable nylons, cationic-dyeable polyesters, antistatic fibres, flame resistant fibres, hollow fibres, microporous fibres, fibres of different cross-sections and fine, ultra fine and micro fibres. The modified products offer a number of advantages and also lead to value addition to the product; this makes their production attractive. Their industrial production may involve some special considerations and these will be highlighted in this chapter.

Next to polyester and polyamides, acrylic fibres occupy an eminent position in the family of synthetic fibres. The importance of acrylic fibres has been shown by their phenomenal growth and their popularity throughout the world. Acrylic fibres have replaced wool in many major applications, particularly in hand knitting and hosiery garments. The majority of knitting yarns are usually bulky yarns which go into the manufacture of pullovers, sweaters, socks, etc. Blankets and carpet are other applications where acrylic fibre competes with wool because of its high elasticity, colour brilliancy, voluminosity, ease of washing, resistance to pilling and good light and colour fastness. Because of these properties, and also due to the ease with which modifications can be made during synthesis, spinning and finishing, acrylic fibres have experienced a tremendous growth since their introduction by Du Pont, USA, in 1950.

Polypropylene is the first stereoregular polymer to have achieved industrial importance. The fibres from polypropylene were introduced to the textile arena in the 1970s and have become an important member of the rapidly growing family of synthetic fibres. Today polypropylene enjoys fourth spot behind the ‘big three’ fibre classes, i.e. polyester, nylon and acrylic. However, as opposed to other commodity fibres, its use as apparel and household textiles has been rather limited; the bulk of the fibre produced is used for industrial applications. Nevertheless, the textile and household uses are growing.

Rayon, which is one of the oldest manufactured fibres, is a regenerated cellulose fibre with a wide spectrum of properties. Historically, rayon faced a strong challenge from synthetic fibres like nylon, polyester and acrylics, which came much later, but in spite of this competition it has retained its place as a major textile fibre. The important considerations in favour of rayon are that the essential raw material for its production, namely cellulose, is abundantly available and a renewable source. Moreover, its hygroscopicity and easy dyeability are additional assets. Furthermore, rayon fibres can be produced with a wide range of properties, particularly mechanical properties, so far unmatched by any other fibre, natural or manufactured.

The emphasis in this book has up till now been on manufactured commodity fibres, like poly(ethylene terephthalate), nylon 6, nylon 66, polyacrylonitrile, polypropylene and viscose rayon, which have been described individually in separate chapters. While describing these fibres, it was emphasized that they are used not only as apparel and household textiles but also as industrial fibres or technical textiles. Taking polyester, which is a well-known apparel and household textile fibre, as an example, it is interesting to note that, in the USA, industrial polyester yarn consumption in 1991 was over 50% of the total polyester filament yarn consumption [1], with tyre cord alone accounting for 38%, the rest being used in seat belts, V belts, coated fabrics, cordage, hoses and other applications. In the recent past, the pattern for fibre production has shown significant changes, with the industrial nations moving towards lower volume but much higher value-added products for industrial markets, while the developing countries are expanding their production capacities for commodity fibres [2].

The Textile Institute defines a fabric as ‘a manufactured assembly of fibres and/or yarns, which has substantial area in relation to its thickness and sufficient mechanical strength to give the assembly inherent cohesion’. Fabrics are most commonly woven or knitted, but the term includes assemblies produced by lace-making, tufting, felting, net-making and the so-called non-woven processes. The distinctive characteristics of the ‘sheet material’ (fabric) arise from the manner in which the fibres are arranged in the planar structure. Woven and knitted fabrics are made by interlacing and interlooping of linear assemblies of filaments and fibres; non-wovens are made by bonding of web-like arrays of fibres or filaments. The webs may be made from fibres of discrete lengths (ranging from a few millimetres to a few metres) by the carding or wet-laying process, or they may be produced by laying or blowing filaments as they are being melt-extruded. The fabrics made by these latter processes are commonly known as spunbonded or spunlaid and melt-blown non-woven fabrics.

During the production of fibres, a certain amount of waste is generated in pre-and post-spinning operations. Since almost all synthetic fibres are non-biodegradable, their disposal poses serious problems. Recycling of thermoplastics (polymers and fibres) has therefore become a subject of vital importance, keeping in view the long-term environmental effects of waste disposal. The current concern regarding the disposal of industrial and post-consumer waste in diminishing landfill sites and the general impact of wastage on the environment have focused attention on developing effective reclamation and recycling policies. More and more fibre producers are therefore establishing corporate recycling policies for various polymers and fibre wastes created during manufacturing or post-consumer waste [1‐18]. In this chapter, various aspects relating to the reuse of PET, nylon 6, nylon 66, polypropylene and acrylic fibre waste will be considered.