(659h) Fiber- Forming Materials: Fiber Technology, Fiber Processing and Applications

Polymers have some versatile applications such as fibers, films, coatings, foams or molded and fabricated articles and exhibit a wide range of properties in terms of strength, rigidity, toughness, flexibility, elasticity, resilience, optical clarity, chemical and solvent resistivity, etc. These properties are inherently built into polymeric materials and can be further modified by adding plasticizers, fillers, coloring agents, stabilizers, etc. Fiber technology: Fiber- forming materials possess rigidity and stiffness and undergo irreversible deformation as nylon 6,6 monofilament does not stretch, nor does it deformed so easily. It offers considerable resistance before it can be deformed to a limited extent. For imparting these properties, the molecules in these materials should have a very high degree of polymerization. They should neither exhibit localized segmental mobility nor overall chain mobility. The material should have a very high crystallisability. Fibers: From consumer’s point of view, fibers can be classified into three types, viz., comfort fibers, safety fibers and industrial fibers. Comfort fibers are those used for making lightweight, soft garments and garments. Comfort fibers should have adequate strength and extensibility, softness, good moisture regain, and preferably some flame retardancy, and should be capable of being dyed. For instance, cotton, silk, wool, nylon, polyesters and acrylics. Safety fibers are those used making carpets, curtains, seat covers, draperies and so on. Comfort fibers can be rendered flame- retardant and made to behave as safety fibers by adding small quantities of substances containing atoms such as B, N, Si, P, Cl, Br or Sb. Since these substances serve as flame- retardant agents and delay the ignition and flame spread, they cannot reduce smoke and toxic gases if these treated fibers get involved in a fire. For instance, aromatic polyamides, polyimides, polybenzimidazoles and polyoxy diazoles. Industrial fibers are used as reinforcing materials in composite structures. These fibers are also called structural fibers as they possess very high modulus, strength, thermal stability, toughness and durability. Structural fibers are used to reinforce articles such as rigid and flexible tubes, pipes and tyres and also in composite structures called fiber- reinforced plastics used in the construction of boats, cars, planes and even buildings. Fiber spinning: Fibers are made from polymers by a process called ‘spinning’. There are three principal spinning methods, viz., melt spinning, dry spinning and wet spinning. In the melt- spinning process, the polymer is used in the molten state, while, in the other two cases, the polymer is used as a solution in an appropriate solvent. In all the three cases, however, the polymer (either in the molten or in the solution form) is streamed through a spinneret which is a special kind of plate with extremely fine holes for the fibers to emerge. Melt spinning: Polymer chips are electrically heated and melted in a heating grid. This converts the solid polymer into a viscous mobile liquid. Sometimes, during this heating process, cross- linking or thermal degradation of the polymer may occur leading to formation of lumps. These lumps can be easily removed from the hot molten polymer by using a filter pack. Furthermore, the hot molten polymer should be protected from the surrounding oxygen to avoid oxidative degradation. Dry spinning: Many polymers such as polyacrylonitrile or polyvinyl chloride are converted to fibers on a large scale by using the dry spinning process. The polymer is dissolved in an appropriate solvent to get a solution of high concentration. The viscosity is controlled by increasing the temperature of the solution. The hot viscous solution is pumped through the spinneret when fine continuous jets of the solution come out of the orifices. The filament is formed from these jets by the simple evaporation of the solvent. Solvent evaporation can be enhanced by passing dry nitrogen in a counter- current manner. Wet spinning: Wet spinning, like dry spinning, employs a fairly concentrated polymer solution. The increase in polymer solution viscosity is controlled by spinning the solution at an elevated temperature. Wet spinning also converts a viscous polymer solution into fine jets through a spinneret. These jets, however, are led into a coagulation bath containing a large volume of a non- solvent which can precipitate the polymer from its solution. Fiber forming materials: Polyimides: Polycondensation of pyrometallic anhydride and p,p’ – diamino diphenyl ether results in the formation of polyimides. The reaction, however, cannot be carried out in a single stage. To start with, the reaction is carried out in suitable solvents, such as DMF, at around 50⁰C, when a polyaddition reaction takes place with the formation of polyamic acid:During the second stage, the polyamic acid is cast as a film, the solvent is evaporated and the film is baked at 300⁰C in a nitrogen atmosphere, when the condensation reaction takes place.The product formed during the second stage from polyamic acid is insoluble and infusible and has to be formed in the final required shape and form. Polyimide finds extensive application in electrical industry as the insulation coating of electromagnetic wirings. In practice, the wire is coated with polyamic acid and baked at 300⁰C under an inert atmosphere when the polyimide is deposited on the surface of the wire. Polyimide can withstand temperatures up to 425⁰C for short time exposures without undergoing any degradation or deformation. For this reason, the polymer finds application as surface coating in supersonic aircraft. Polyamides: Polyamide are prepared by the melt polycondensation between dicarboxylic acids and diamines. The aliphatic polyamides are generally known as nylons. A polyamide made from hexamethylene diamine and adipic acid is written as nylon 6,6. Commercially, nylon 6,6 and nylon 6,10 are extremely important. Nylons made by the self- polycondensation of an amino acid or by the ring- opening polymerization of lactams, which is made from caprolactam. Nylon 6,6 is used as a plastic as well as a fiber. It has good tensile strength, abrasion resistance and toughness up to 150⁰C. Also, it offers resistance to many solvents. However, formic acid, cresols, and phenols dissolve this polymer. A large quantity of nylon 6,6 is used to produce tyre cord, monofilaments and ropes, it is also used to make textile fibers fro use in dresses. Nylon 6,6, being tough plastic, is used as a good substitute for metals in gears and bearings. Nylon 6,10 is used to make brushes and bristles. They have high melting points in the range of 435- 450⁰C. Fully aromatic polyamide (or aramides), prepared either by the self- condensation of aromatic amino acids or by polycondensation between aromatic diacid chlorides and aromatic diamines, are also gaining industrial importance in recent years. Polycondensation in these cases are carried out by melt, solution or interfacial techniques. Aramides have a very high melting point ( > 500⁰C). Fibers made of them have extremely high modulus and find application in reinforced plastics with unusually high strength- to- weight ratios. Fibers spun from poly (paraphenylene terephthalamide), commercially known as ‘Kevlar’ fibers. Polyacrylonitrile: Polyacrylonitrile (PAN), also known as polyvinyl cyanide. It is produced from acrylonitrile by the radical polymerization technique using peroxide initiators. Acrylonitrile monomer can be obtained from acetaldehyde and hydrogen cyanide. PAN is soluble in dimethyl formamide, dimethyl sulphoxide, adipo nitrile, and so on. It has a remarkable resistance to heat upto around 220⁰C and exhibits very good mechanical properties. Polyacrylonitrile is used to produce what are known as PAN fibers. Polyester: A polyester such as polyethylene terephthalae (PET) has a high melting point because of presence of the aromatic ring under the trade name of terylene or Terene. In the synthesis reaction,trans- esterification is followed by polycondensation, i.e., step polymerization. Terephthalic acid (TPA) process- Ethylene glycol (EG) is reacted with TPA. The reaction is performed at 240- 260⁰C and 300- 500 kPa yield bis(hydroxyethyl) terephthalate (BHET). Dimethyl terephthalate (DMT) process- EG is reacted with DMT. The reaction is performed at 140- 220⁰C and 100 kPa yield BHET. The 1^st polymerization step is trans- esterification between BHET molecules, displacing EG, at 250- 280⁰C and 2-3 kPa. The resulting oligomers are then polycondensed at 270-280⁰C and 50-100 kPa. Because of its high crystalline melting temperature (270⁰C) and stiff polymer chains. PET has good mechanical strength, toughness, and fatigue resistance up to 150- 175⁰C as well as good chemical, hydrolytic, and solvent resistance.Cellulose: Cellulose is a naturally occurring, linear, stereo- regular polysaccharide, made up of β- D (+) glucose residues. Cellulose is a versatile polymer which is found in plenty in nature in the form of cotton, hemp, jute, flax, etc. Also, a good percentage of wood consists of cellulose and has an extremely high degree of crystallinity. It has a very high melting point and, in fact, it decomposes before beginning to melt. It is generally resistant to dissolution in several solvents, though it can swell in some, such as water, owing to the presence of hydrogen bonds. Wood pulp, rayon and cellophane (all the three derived from wood cellulose). Cotton cellulose differs from wood cellulose primarily having a higher degree of polymerization and crystallinity.