Fiber formation is the process whereby fibers are formed either through natural processes or from raw materials in the case of syntheticfibers. Natural fibers are formed from natural processes occurring in plants and animals and are harvested as agricultural products. Man-madefibers, however, are either formed from polymers of synthetic origin or from those of natural origin. The synthetic polymers are products of organic polymerization reactions. Natural polymers, on the other hand, are products of processing of natural materials, e.g. wood pulp. To form thefibers, the polymers must be taken through a process called‘extrusion’or ‘spinning’.
3.3.1 Formation of Man-Made Fibers
Man-madefibers are synthesized or are processed from natural and synthetic raw materials. Consequently, producers of man-madefibers have greater control of the characteristics of the fibers that are produced. They can also be modified both physically and chemically and tailor made to achieve desirable properties. This versatility of man-madefibers has broadened their area of applications and made them very popular with manufacturers and consumers alike.
The starting materials for all man-madefibers are polymers. The polymers are turned into fiber by the process of extrusion or spinning. The term extrusion is preferred to spinning as the latter term is a borrowed one from the process of
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turning staplefibers into yarn. A key equipment in the making of man-madefibers is thespinneret. The spinneret is a plate with many tiny holes through which the polymer in a liquid form is passed at high speeds. Often, the polymers when synthesized are in a solid form and to be able to extrude them intofibers they must first be converted into a liquid form either by dissolution in a solvent or by melting.
There are three common methods of making man-made fibers. These are wet extrusion, melt extrusion and solvent-dry extrusion. The method used for any particular fiber depends on the ease of conversion of the polymer from the solid state into liquid and also on the behavior of the polymer when subjected to heat treatment. If the polymer can be converted into liquid form easily by heating without damage, then the melt extrusion method may be used. If, on the other hand, it is liable to decomposition or damage when heated but can be dissolved in a low boiling point solvent which can later be easily evaporated then the solvent-dry method is used. If, however, the polymer will suffer damage upon heating and cannot be dissolved in a low boiling point solvent then the wet extrusion method is used.
3.3.1.1 Melt Extrusion Method
Melt extrusion is the simplest of the man-made fiber production processes. The method is mostly used for thermoplastic polymers (i.e. polymers that melt when heated). Among the polymers produced by this method are polyamides (e.g. Nylon 6 and Nylon 66), polyester, polypropylene and polyethylene. These polymers have the characteristics that they can melt to at least 30 °C above their melting points without decomposition. This ensures that the molten polymers have sufficient fluidity to be extruded.
The method involvesfirst heating the polymer to melt it, thenfiltering the melt to remove foreign particles, extruding the melt through a spinneret andfinally cooling the emerging streams of molten polymer in a stream of air current. The heating is done in a Melter in an atmosphere free of oxygen. Oxygen must be prevented from the Melter as it can attack the molten polymer. This is achieved by carrying out the heating in a nitrogen atmosphere. The polymer to be melted is fed from a hopper in the form of chips into the Melter, which is heated to the correct temperature depending on the melting point of the polymer. The molten polymer from the Melter collects into a pool from where metering pumps at high pressures are used to pump the melt through pipes into the spinning head assembly. Before the melt reaches the spinneret, it isfiltered to remove foreign particles that could potentially block the holes of the spinneret and hence interrupt theflow of the molten polymer, which could lead to the formation of broken filament fibers. The pool of molten polymer, pump,filter, spinneret assembly and spinneret are kept at 20–30 °C above the melting point of the polymer. The spinneret is a metal plate, roughly 50 mm in diameter containing fine holes. It may contain anywhere from 1 to 50 holes if extruding continuousfilament yarns, or over 100 holes if extruding tows for staple yarn production. The moltenfilaments from the spinneret emerge into a stream of cooling air current, which rapidly solidifies them. The solidfilament travels down a
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conditioning tube to cool further, and a lubricant is applied to lubricate and create non-electrostatic properties for thefiber.
3.3.1.2 The Solvent-Dry Extrusion Method
This is the least used of the majorfiber production techniques. The method is used for polymers that for one reason or the other are not amenable to the melt extrusion processes. In this method, the solid polymer is converted to a liquid state by dissolving in a volatile solvent to form a solution. The solution is subsequently extruded into a stream of heated gas where the volatile solvent is rapidly evaporated away leaving the filaments. Examples of the fibers produced by this method are cellulose diacetate, cellulose triacetate, polyacrylonitrile, modacrylics and polyvinyl chloride (PVC).
It is important in this method to get the concentration of the solution right in order to get afilament of the right strength and flexibility. After dissolution, the solution isfirstfiltered and pumped into storage tanks where bubbles are removed from the solution by application of vacuum. When necessary, pigments may be added to the solution at this stage to provide color for thefiber. Finally, the solution is pumped into heated extrusion heads in which one or more spinneret are located.
The polymer concentration in the solution is about 25% (wt). At this concentration, however, the viscosity of the solution is quite high, and it is therefore heated so that it will flow sufficiently to facilitate its extrusion through the tiny holes of the spinneret. Before the solution is extruded it passes through an additionalfilter in the spinneret assembly unit over the top of the spinneret itself. This serves to safeguard any unwanted particles passing to the spinneret and disrupting the extrusion pro- cess. The extrusion heads are often sited within the spinning cell so that the evaporating gas passes around them. Metering gear pumps, one for each spinneret, are used to direct the solution to the spinneret. The extrusion occurs at a very high speed, in the order of 500–1000 m per minute. This causes the pressure on the spinneret plate to be great, consequently thicker spinneret plates, often made of stainless steel, are used. The solution emerges from the spinneret into a heating cell where a current of hot gas (often air or nitrogen), either in the direction thefilaments are moving or countercurrent to it, heats the filament or evaporates the solvent away. The heating cell is long enough to allow sufficient solvent to evaporate away such that thefilaments do not fuse together nor stick to any surface they come into contact with. The filament emerging from the heating cell is contacted with an applicator that lubricates on thefilament to reduce both friction and static formation in subsequent operations. Finally, thefilaments must be washed to remove residual solvent from the extrusion stage and then dried.
An important component of this method is the solvent recovery and recycle processes that must be done for economic and environmental considerations.
Approximately, 3–6 kg of solvent is extruded per kilogram offiber. It is possible to achieve solvent loss of less than 10% with the use of very efficient solvent recovery methods. The solvent dry extrusion method is most economical for the production of continuousfilament yarn. This is because the dynamics of the method allows for only a small number offilaments to be produced from a spinning head. The holes in
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the spinneret are spaced and this makes for wide spacing of thefilament to allow for evaporation of the solvent without any fibers sticking together. The temperatures used in this method are lower than that for melt extrusion, so this process can be used where the polymer would be degraded by high temperatures. The limitation of this method is that it depends on the existence of a volatile solvent for the disso- lution of thefiber. Among the solvents commonly used are acetone for cellulose diacetate, and dichloromethane for cellulose triacetatefibers. Dimethyl formamide is another solvent used for some acrylic fibers, but its use is limited as it is not particularly volatile. It boils at 153 °C, so the air into which thefibers are extruded must be heated to between 130 and 200 °C to be able to adequately evaporate the solvent. Another disadvantage of the method is that the solvents used are expensive, toxic,flammable and potentially explosive when mixed with air.
3.3.1.3 The Wet Extrusion Method
Among the polymers for which this method is applicable are acrylic polymers and cellulose. Examples offibers made by this approach are courtelle, acrilan and viscose rayon. The method is used for the preparation offibers from those polymers, which do not melt without decomposition and which will neither dissolve in volatile sol- vents. Consequently, they are dissolved in high boiling solvents and extruded into a chemical bath of the same solvent where thefilaments solidify. Examples of solvent used in this process are dimethyl formamide, dimethyl acetamide or aqueous solu- tions of inorganic salts such as zinc chloride or sodium thiocyanate. There are two variations of the wet extrusion method. These are: (i) the purely physical process method and (ii) the chemical regeneration and physical method.
i. Physical Process Method
In this method, the polymer solution or dope isfirst deaerated and thenfiltered before being extruded through the spinnerets. The pressure necessary for the extrusion is provided by a gear pump, which also serves as a metering device.
The spinneret itself is immersed in a coagulating bath that contains a dilute solution of the solvent in water. The bath is of such composition that the tiny streams of polymer undergo both physical and chemical changes during which the polymer precipitates upon entering it, formingfirst a gel and subsequently a fiber. To avoid corrosion of the spinneret, it is made of corrosion-resistant precious metal alloys. The size of a hole in the spinneret is smaller than what pertains to spinnerets used for the melt extrusion and the wet-dry extrusion processes. Large numbers of holes per spinneret can be used due to the liquid coagulation in the extrusion bath. As many as 20,000 holes per spinneret of size of 7.5 cm diameter face or 167,000 holes on a rectangular faced spinneret. The method is therefore economical for the production of staplefiber and tow. Due to low extrusion rates of the method, continuous cleaning, drawing and after treatment of the fiber are possible. An important aspect of this method is the recovery and recycling of the solvents used, which is always desirable as the solvents tend to be expensive.
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ii. Chemical Regeneration and Physical Process Method
This process involves,first, a chemical step followed by a physical step hence the name. The method is used for the production of viscose rayon fiber. The polymer from which the fiber is made, cellulose, cannot be dissolved in its original form as such it isfirst chemically modified by a reaction with a suitable reagent before it is dissolved. The resultant solution is extruded in the same manner as the physical processes described earlier, except that the coagulation bath contains an acid, which neutralizes the sodium hydroxide in the modified cellulose. Full regeneration of thefilaments to cellulose does not occur in the bath. This allows them to be drawn and so orientate the cellulose molecules while they are still plastic. The need for chemical modification of the cellulose arises from the fact that it is obtained from wood pulp and therefore has very high molecular weight. At that high molecular weight, they cannot be taken through the extrusion process. Therefore, it isfirst chemically changed to soda cellulose by reaction with sodium hydroxide. In that form, it is oxidized using oxygen to reduce the molecular chain length to suitable levels. The degraded soda cellulose is further treated with carbon disulfide to form cellulose xanthate, which when dissolved in alkali solution forms a brown solution called viscose.
The viscose isfiltered, then allowed to stand for a few days during which time the xanthate degrades back to cellulose. It is this solution that is filtered and subsequently extruded in the manner described earlier.
3.3.2 Preparation of Natural Fibers
The naturalfibers cannot be used in the form in which they are harvested. They mustfirst be cleaned and rendered in a form such that they can be used to make a yarn. The nature of treatment that thesefibers must be taken through before making yarns out of them depends on the source of the particular fiber. The specific methods used to process the different naturalfibers are described below.
3.3.2.1 Cotton Fiber
The raw harvested cotton is taken through several physical processes before it is ready to be spun into yarn. These processes are ginning, picking, carding and sliver combing.
Ginning: The cotton from the farm contains seeds and these are removed in a cotton gin. The process of removing the seeds is calledginning. The cotton gin is made up of a number of different rollers with teeth on them. As the cotton passes through the gin, the seeds are removed. The seedless cotton is then put into bales and sent for further processing.
Picking: Following the removal of the seeds, the vegetable matter in the cotton is removed in a process calledpicking. The machine used for this process is called a picker’. In the picker, the cotton from the cotton gin isfirst beaten with a beater bar
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tofluff it up and piled onto a screen, from where it is fed through rollers to remove the vegetable matter.
Carding: The carding machine consists principally of one big roller, which is surrounded by smaller ones. All the rollers are covered with small teeth which get progressivelyfiner along the line of passage of the cotton. Large bats of cotton from the picker are fed into the carding machine where they are lined up nicely to make them easier to spin. The cotton leaves the carding machine in the form of large ropes calledsliver.
Combing: The slivers, several at a time, arefirst combined in a combing machine to get a consistent size. This, however, produces a very thick rope of cotton fibers.
Therefore, directly after being combined the slivers are separated into rovings. It is from these rovings that cotton yarns are spurn. When the roving must be processed by machine, they are separated such that they are about the width of a pencil.
3.3.2.2 Wool Fiber
The processes described here apply generally for all animal hairfiber with slight modifications; therefore, the preparation of individual animal hairfibers would not be described separately in this section. After shearing, the wool is taken through the physical processes of skirting, cleaning, carding and combing before it is ready to be turned into yarn.
Skirting: During this process, the fleece (sheared wool) that is unsuitable for spinning or requires extra effort to spin into yarn are disposed of. Shortfleece and fleece smeared with sheep dung fall into this category.
Cleaning:This is done to rid thefleece of vegetable matter (sticks, twigs, burs and straw) and lanolin. Actually, vegetable matter could be prevented from the fleece by having the sheep wear a coat all year round. When spinning‘in the grease’ is the goal then only the vegetable matter is cleaned (picked) from thefleece. In this case, the lanolin remains in thefleece and is only washed away after spinning or allowed to remain and not removed at all when a water repellent garment is the target. When cleaning is intended to remove both vegetable matter and lanolin, then thefleece is washed in soap either by swishing it around in a tub and rinsing until thefleece is clean of soap and dirt or soaking thefleece in hot water in a washing machine made for the purpose and melting the lanolin away. It is important to note that thefleece must not be rubbed against itself when washing. Otherwise, it may become felt and would be impossible to spin into yarn.
Carding: Carding is the process of getting the fleece into a form that will facilitate spinning. Carding when done by hand yields a loose woolen roll offibers called arolag. Carding can also be done using a drum. In the latter case, a mat of flat fibers of rectangular shape called a bat is the product. Carding with a mill returns afleece in a roving, which is a stretched bat. The roving is very long and often the thickness of a wrist. Carding by hand or by drum is a very lengthy process, therefore carding is often done using a carding mill.
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Combing: Combing is yet another method used, albeit less often, to align the fibers parallel to the yarn prior to spinning. This method is good for spinning a worsted yarn. Rolags from hand-carding are useful for producing woolen yarns.
3.3.2.3 Bast Fibers
Bastfibers are obtained from the stalks of dicotyledonous plants. Examples of such plants are jute,flax, hemp, sunn or sann, kenaf, nettle, urena and ramie. Thefiber is extracted from the stalk of the plants mostly by microbial means in a process called retting. Other processes, for instance dressing, are sometimes used to extract the fiber from the stalk.
i. Processing of Jute and Flax Fibers
The methods for the extraction of the fibers from jute and flax (commonly calledlinen) are similar. In both cases, retting is the method used to extract the fiber. During retting, the harvested stalk of the plant is exposed to microbial action in clean, slow-moving water, which causes the fiber bundle to separate from the woody stem. They are then washed and dried. Acid is produced from the retting process, which would attack any metal container. As such retting of flax stalks is made to take place in a plastic, concrete or wooden container.
Retting could also be carried out by submerging the stalks in a standing pool of water in a closed container at about 26.7 °C. After 4 h, a complete change of water is effected, and 8 h after that the scum is washed off the top by the addition of more water. Subsequently, the scum is washed off every 12 h until the retting process is over. At 26.7 °C, the retting process takes about 4 or 5 days. If the temperature is lower than 26.7 °C, the process takes longer. The end of the retting process is marked by a soft and slimy feel of the bundles and a popping out of a few of thefibers out from the stalks. It is better not to let the bundles sit in the water long enough than to let them sit there too long, as they always can be submerged again if found to be wanting later, but the reverse problem cannot be solved. In this case, the fibers are rotted away with the stalks. In the case of ramie fiber, after retting, the ribbons of fiber are first stripped from the stem. The resultant product calledchina grassis then boiled to separate the ribbons intofibers for spinning.
ii. Dressing theflax
In this process, the flax is broken, scotched and hackled. The fibers are thereafter removed from the straw and cleaned well enough to be spun. As a first step, the straws are broken into short segments using a breaking machine. It is then hanged vertically, and the edge of a knife is scrapped along thefibers to pull away the pieces of stalk (this latter process is called scotching). Finally, the fiber is pulled through various sized hackles (a bed of nails-sharp, long, tem- pered, polished steel pins driven into wooden blocks at rectangular spacing). At the end of this process, the fiber emerges split and polished and ready to be carded or spun.
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