By pre and post spinning production process of producing fibers a certain amount of waste is formed. Discarding of synthetic fibers causes a serious problem due to their non-biodegradable properties. Hence recycling of non-degradable textile material is a crucial subject considering the long-term environmental effect of waste disposal. Land filling and blazing of this waste is not the right answer for this problem.
Decreasing landfill sites and the general blow of wastage on the environment has centered interest on useful reclamation and recycling policies. More and more fiber manufacturers are planning recycling strategies for different types of wastes, generated during manufacturing or post-consumer waste. Recycling is the most feasible method to decrease the solid waste. The primary aim of textile recycling is to reproduce the by-production of textile and fiber industries to have useful end products hence to achieve into the original stream. It has the greatest economic value by carrying the waste back into the original system in the form of monomers.
The main aim for any recycling system utilized by the fiber manufacturer is change of low bulk density wastes into high bulk density, free flowing granules. During the switch over process, it is essential to make sure that simply a little stress is applied on the polymeric material to prevent any change in the material properties. In the world, polyester is found as a largest recycled polymer. Fibers, films, sheeting and food/non-food contact bottles, etc, are the products in which recycled materials are used.
Recycling is generally done according to the source and morphology of the polymer and fibrous waste, so that the waste is transformed into a polymer or as monomer, which can be use again in the similar plant for fiber manufacturing or in the manufacturing of value-added products like adhesives, resins, thickeners, etc.
Synthetic fibers are used to a huge amount in the textile industry; some of them fibers are polyester, nylon6, nylon66, polyacrylic, and polypropylene. In which, polyester is the broadly used as synthetic polymer and it is used in such great quantity of amount and it is clear that a large number of course of action have been increased to implements the recycling of polyester fibers.
Source of waste
Textile waste can be categorized either by pre-consumer or post-consumer. Pre-consumer textile waste includes by-product materials from the textile, fiber and cotton industries. In general, waste for this industry is any inferior quality material produced by fault of machinery, operators or raw materials during the production process. Also, most of the reactions of synthetic fiber manufacturing are equilibrium, hence, not provide hundred percent conversions. The untreated monomer is considered as a waste, which has to be transformed. Also, while processing of spinning, weaving various types of wastes are sourced. Due to this entire basis about 4-15 per cent waste is sourced in this industry.
Kinds of waste
The waste, which is considered to recycling, is a combination of various kinds of wastes and holds a variety of impurities. The waste might be trim fibers of short length or might have knotted mass of long filaments. It might as well include polymeric lumps. The waste includes impurities like dyes, chemicals, knitting oils finishing, etc. It is very essential to eliminate all these impurities before proceed further for the recycling procedure.
The factors behind right method of selection
The factors behind the selection of waste:
. Amount of chemical or mechanical harm
. Economical viability of the process
. End use of the recycled product
. Kind and quantity of impurities
. Use of polyester waste
. Quality of waste
A large number of wastes are generated during the production of polyester. The polyester waste includes chips, continuous filaments, polymer lumps and staple fibers also. The waste is generally impure with dyes, finishes, knitting oils and other fibers like nylon and acrylic and because of mixing of waste with molded items the waste may also be impure with papers, metals, pigments that are present in it. Therefore, the quality of waste, amount of chemical or mechanical harm, economy of the process etc. , has to be considered while selecting of suitable process to remove inferior waste.
Recovered as polymer
Spinnable polymer can be regained from low damaged, clean filaments or fiber waste, in addition to re-extrusion ensured by granulation or by dissolution in an appropriate non-depolymerising solvent and separating out the polymer in the powder form.
The physico-chemical properties of the fibers gained by the frequent extrusion and spinning are adequate up to three recycling. During the extrusion melting of fibrous waste and due to the high heat insulation properties of fibrous mass an uneven heat transfer occurred, while polymer experienced important thermo-oxidation and mechanical degradation. An inert atmosphere of the drop in molecular weight can be prevented by high heat insulation properties of fibrous mass.
The fiber waste is slash to a standard size and frequently carried to a forced feed, which is also perform as a drying unit. By optimal drying of the waste preceding to re-extrusion, hydrolytic degradation is decreased. The extruder is set with one or two melting sectors to eliminate low boiling adulteration and the part of the finish oils present in the waste.
The degassed, molten and homogenous filtered mass is cast as a plane piece, frozen in water bath and crushed. These crumbled articles mix together with virgin crumbled articles to make staple fibers. The crushing or granulation operation can be avoided by supplying the melt from the extruder straight away with a spinning system.
On the other hand, the dense fiber waste is set up into a twin-screw extruder and the dissolved material is achieved and it is subjected to decrease the pressure at 260-290 degrees C to make polyester with an enhanced level of polymerization. This procedure provides polyester, which is quite stiff, therefore is not applied for apparel wear. This polyester can be twisted into fibers, which are usually utilized for nonwoven and fiberfill purpose. Such materials are similar to those manufactured from virgin polymers.
Recovered as powder by dissolution
Melting the waste in an appropriate solvent and then cascading the polymer, and obtain spinnable polymers from polyester waste requires solvent selection and should have the following requirements:
The polymer be supposed not to passed in depolymerisation in the solvent
. The polymer has to be rapidly and ably achieved from the solution.
Polyester waste can be melted in the hot polar solvents and after that the solution frozen at a stable speed to precipitate elements of preferred size, which are recollected by filtration. And flocculate of the polyester done by shock quenching. Solvents containing carbocyclic rings are appropriate for melting polyester. They melt polyester at 160-240 degrees C in an aggregation of 10-40 per cent by weight of the polymer in the solution. When the polymer solution is gradually cooled to near 100 degrees C, the polymer separated as amorphous gel or paste.
Selection of quenching medium and its process
Shock quenching is done by the additive liquid quenching mode, which is blended with the solvent. Below its freezing point such a medium remains the solvent in the liquid form. On the other hand, polyester solution can be dry or wet spun into inert gaseous quenching medium (N2, CO2, etc) and the remaining solvent in the polymer can be detached by abstraction with an appropriate solvent. Acetone, dichloromethane, di-methyl formamide, etc. , are the pre-treated quenching medium for polyester solution in naphthalene.
Chemical recycling of polyester
For regaining back to in its monomers form i. e.in the form of Dimethyl Terephthalate or Terphthalic acid polyesters following procedures are adopted:
1. Glycolysis of polyester waste
Poly (ethylene terephthalate) dilutes in boiling glycol under atmospheric pressure and at a fast speed, while heating under pressure. Oligomers of ethylene terephthalate are obtained by cooling the solution.
2. Hydrolysis of waste
Process with water: With water at 150-200 degrees C under pressure polyester waste can be hydrolyzed. About 2-4 times amount of water is added compare to weight of the waste. Transesterification catalyst such as sodium acetate is used for hydrolysis.
Process with base chemicals: A base like caustic soda lye (4-20%) or ammonium hydroxide can be used.
Process with acid: With a strong acid such as sulphuric acid, nitric acid and phosphoric acid to produce terephthalic acid in good yield without the use of elevated pressure and temperature polyester waste can be hydrolyzed. The reaction is accomplished within quite a few minutes with sulphuric acid of concentration of 87 per cent at 60-95 degrees C.
3. Methanolysis of polyester waste
In this process, the waste is taken with methanol under pressure to gain dimethyl terephthalate and ethylene glycol. The reaction done at 180-210 degrees C temperature at 2-4 MPa. The production of the reaction is choosing by the ratio of waste to methanol (by weight) and is a vital factor. It may be changes from 1:2 to 1:10. Maximum ratio is to be considering of 1:4. The temperature for reaction is done at 185 degrees C. The reaction is catalyzed by transesterification catalyst such as zinc acetate, manganese acetate, etc.
Recovered from nylon 6 waste
A huge number of wastes are sourced in the manufacturing of nylon 6 materials, such as yarns, cord, rod, blocks, and molded materials. The waste comprises polymeric lumps, entangled masses of filament, drawn fibers with or without finish cables, rejected bobbins and abnormal batches of polymer. In addition, ten per cent of the low molecular weight oligomers formed while polymerizing process.
Classification of two categories found for Nylon 6 waste. . Liquid waste :Composed of water extractable from in different steps of polymerization
. Polymeric fiber waste or solid waste: Achieved while polymerizing, spinning and textile processing.
Twenty to twenty five per cent oligomers together with organic and inorganic compound are produced while recovery process of Caprolactam. Under reduced pressure the excess achieved after distillation of Caprolactam and is consistently impure with inorganic materials such as MnO3, Na2HPO4 KHSO4, K2SO4, NaHPO4 and Na3PO4. The characteristics and figures of adulteration rely on the method applied for purification and distillation of Caprolactam.
In water and dilute solution of Caprolactam the cyclic oligomers are soluble. While processing through chemical purification and concentration, these oligomers get divided from raw liquor and are laid on the surface of the equipment. NaOH is used in the oligomeric waste while 6-aminocaproic acids and sodium salt are existed.
Structure of distillation residue
An attributed structure of distillation residue is:
Open chain oligomers ~ 65 components
Cyclic oligomers ~ 20 components
Inorganic substances ~ 10 components
Caprolactam ~ 5 components
Caprolactam or 6-Aminocaproic acids’ recovery from solid waste:
The produced solid waste would get about 8-10 per cent and furthermore from it Caprolactam or 6-Aminocaproic acids can be regained.
It occupies depolymerisation of nylon 6 and purification of Caprolactam. Depolymerisation of nylon 6 waste using superheated steam in the existence of inorganic or organic acids like HNO3, HCOOH, benzoic acid, and HCl brings this out. On commercial scale of production this procedure has a get important value. To make a dilute aqueous solution of Caprolactam, in a traditional procedure, superheated steam is permitted by a molten mass at 250-300 degrees C.
As increases the molecular weight of polymer depolymerisation also increases. Before changing it to 70 per cent liquor the solution is act according to multi-stage purification, which is partly distilled in the presence of appropriate alkali to regain pure Caprolactam. With the existence of catalyst such as alkali, metallic sodium and phosphoric acid and its salts, this procedure may also be applied under reduced pressure.
At the beginning of the procedure the reaction rate gets fast, but you will get poor product purity wise. With existence of acid catalyst, it provides high production of Caprolactam under the vacuum method. To regain 92 per cent of the Caprolactam with TPA, isophthalic acid or adipic acid is used. The efficiency of the product can be achieved by adding phosphoric acid.
6-Aminocaproic acids’ recovery
In the occurrence of aqueous alkali metal salt or acid salt of 6-Aminocaproic acids, Hydrolysis of nylon 6 waste is achieved. With dilute HCl and at 90-100 degrees C temperature the hydrolysis process of nylon 6 waste is achieved within three to four hours. To achieve a dilute aqueous solution of aminocaproic acid salt, the reacted mass is discharged in water. By filtration before passing the acid solution through a strong cation exchanger resin, the non diluted products such as pigments, additives and fillers are separated. Then, SO3H type cation exchanger takes up aminocaproic acid and HCl is cleaned by demineralized water. Washing is done till the extraction is free from chlorine ion. Concentrated, crystallized and pure aminocaproic acid is occupied with NH4OH to achieved dilute aqueous solution.
Most of the ionic impurities are removed over ion exchange resin and during crystallization; hence an extremely pure aminocaproic acid is received through this procedure. By dilute sulphuric acid procedure, nylon 6 waste can also be hydrolyzed. In the existence of excess of water and the sodium salt of aminocaproic acid it can be made by sodium hydroxide and is transformed into pure aminocaproic acid.
Nylon 66 yarn waste
By continuous hydrolysis process of the polymer waste in sulphuric acid, the adipic acid and hexa methylene diamine can be achieved from nylon 66 polymer. Achieving the adipic acid by crystallization after every hydrolysis step of diamine is also achieved by distillation after neutralizing the acid.
The hydrolysis and crystallization of adipic acid to be taken step by step involving frequent filtrations and time for optimum recovery of adipic acid. Hence it is not resourceful for huge waste.
Day by day global environment is becoming a trouble. For recycling of non-degradable material it requires vital steps to develop innovative, economical and more fruitful procedure. Right now, many textile industries and institutions are doing more and more research to cope up with the global environment related issues and they are still trying to come up with better methods to recycling of polyester waste. Definitely, we are responsible to leave a breathable world and to live with our future generations, hence to take care of the environment by recycling of non-degradable of textile material has became an important issue.
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