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Recycling of non-degradable textile material

Vishal Warke, Pradip Chandratre

During the production of fibres, a certain amount of waste is generated in pre- and post-spinning operation. Since almost all synthetic fibres are non-biodegradable, their disposal causes serious problems. Hence their recycling is a subject of vital importance, keeping in view the long-term environmental effect of waste disposal. Land filling and incineration of this waste is not the solution.

Diminishing landfill sites and the general impact of wastage on the environment has focused attention on effective reclamation and recycling policies. More and more fibre producers are devising recycling policies for various types of wastes created during manufacturing or post-consumer waste. Recycling is the most viable approach to reduce the solid waste. The main objective of textile recycling is to reprocess the by-production of textile and fibre industries back into the original stream or into useful end products. Putting the waste back into the original system in the form of monomers has the greatest economic value.

The key objective for any recycling system used by the fibre producers is conversion of the normally low bulk density wastes into high bulk density, free flowing granules. During the conversion process, it is necessary to ensure that only very little stress is applied on the polymeric material to avoid any change in the material properties. The world’s most recycled polymer is polyester. Some of the products in which recycled products are used are fibres, films, sheeting and food/non-food contact bottles, etc.

Depending on the source and morphology of the polymer and fibrous waste, recycling is generally done so that the waste is converted into a polymer or as monomer, which can be reused in the same plant for fibre production or in the production of value-added products like adhesives, resins, thickeners, etc.

In the textile industry, synthetic fibres are used to a large extent. Polyester, nylon6, nylon66, polyacrylic, polypropylene are some of the fibres used. Polyester is the most widely used synthetic polymer. Since it is used in such abundance, it is obvious that a large number of processes have been developed to tackle the recycling of polyester fibres. Therefore this paper focuses on recycling on synthetic fibres and polyester in particular.

Origin of waste

In general, fault of machinery, operators or raw materials leads to production of inferior quality material, which is waste for the industry. Also, most of the reactions of synthetic fibre production are equilibrium reactions and do not give 100 per cent conversions. The unreacted monomer is waste, which has to be recovered. Also during operations like spinning, weaving different types of wastes are generated. Due to all these reasons about 4-15 per cent waste is produced in the industry.

Types of wastes

The waste, which is subjected to recycling, is a mixture of different types of wastes and contains various impurities. The waste might be staple fibres of short length or might contain entangled mass of long filaments. It might as well contain polymeric lumps. The waste contains impurities like dyes, finishing chemicals, knitting oils, etc. It is necessary to remove all these impurities before going ahead with the recycling process.

Selection of process

The selection of waste depends on following factors:

• Quality of waste
• Degree of chemical or mechanical damage
• Type and amount of impurities
• End use of the recycled product
• Economical viability of the process

Utilisation of polyester waste

In the manufacturing of polyester, a significant amount of waste is generated. The polyester waste consists of chips, continuous filaments, staple fibres and polymer lumps. The waste is usually contaminated with dyes, finishes, knitting oils and other fibres such as nylon and acrylic. The waste may also be contaminated with papers, metals, pigments that are present because of mixing of waste with moulded articles. Therefore the selection of suitable process depends on the quality of waste, degree of chemical or mechanical damage, economy of the process used and mode of disposal of secondary waste.

Recovery as polymer

Spinnable polymer can be recovered from low damaged, clean filaments or fibre waste, either by re-extrusion followed by granulation or by dissolution in a suitable non-depolymerising solvent and precipitating out the polymer in the powdered form. The physico-chemical properties of the fibres obtained by the repeated extrusion and spinning are satisfactory upto three recyclings. However, during the extrusion-melting of fibrous waste, polymer undergoes significant thermo-oxidation and mechanical degradation as a result of uneven heat transfer due to the high heat insulation properties of fibrous mass. By heating the fibrous waste below its melting point in an inert atmosphere the drop in molecular weight can be avoided.

The fibre waste is cut to a standard size and continuously delivered to a forced feed, which also acts as a drying unit. By optimum drying of the waste prior to re-extrusion, hydrolytic degradation is reduced. The extruder is equipped with one or two degassing zones to remove low boiling impurities and the part of the finish oils present in the waste.

The molten, degassed and homogenous filtered mass is cast as a sheet, cooled in water bath and granulated. These granules may be blended with virgin granules to produce staple fibres. The granulation process can be bypassed by feeding the melt from the extruder directly to a spinning system.

Alternatively, the compressed fibre waste is introduced into a twin-screw extruder and the molten material thus obtained is subjected to reduced pressure at 260-290ø C to produce polyester with an improved degree of polymerisation. This process gives polyester, which is rather coarse, hence is not used for apparel wear. This polyester can be spun into fibres, which are generally used for nonwoven and fibrefill applications. Such products are comparable to those produced from virgin polymers.

Recovery as powder by dissolution

Spinnable polymers from polyester waste are obtained by dissolving the waste in a suitable solvent and then precipitating the polymer. The solvent selected should satisfy the following requirements:

• The polymer should not undergo depolymerisation in the solvent
• The polymer must be rapidly and efficiently recovered from the solution.

Polyester waste can be dissolved in the hot polar solvents and the solution then cooled at a constant rate to precipitate particles of desired size, which are recovered by filtration. The polyester can be flocculated by shock quenching. Solvents having carbocyclic rings are suitable for dissolving polyester. They dissolve polyester at 160-240ø C in a concentration of 10-40 per cent by weight of the polymer in the solution. The polymer precipitates as amorphous gel or paste when the polymer solution is gradually cooled to about 100ø C. Shock quenching is accomplished by the addition of a liquid quenching medium, which is preferably immiscible with the solvent. Such a medium keeps the solvent in the liquid form even below its freezing point. Alternatively, polyester solution can be wet spun or dry spun into inert gaseous quenching medium (N2, CO2, etc) and the residual solvent in the polymer can be removed by extraction with a suitable solvent. The pre-treated quenching medium for polyester solution in naphthalene include dimethyl formamide, acetone, dichloromethane, etc.

Chemical recycling of polyester

Polyesters can be recycled back to itsmonomers ie, Dimethyl Terephthalate or Terphthalic acid by using one of the following processes:

A] Methanolysis

B] Glycolysis

C] Hydrolysis

Methanolysis of polyester waste

In this process, the waste is treated with methanol under pressure to recover dimethyl terephthalate and ethylene glycol. The reaction is carried out at 180-210ø C at 2-4 MPa. The ratio of waste to methanol (by weight) is an important factor, which decides the yield of the reaction. It may be varied from 1:2 to 1:10. Ratio of 1:4 is optimum. The temperature for reaction used in the industry is 185ø C. The reaction is catalysed by transesterification catalyst such as zinc acetate, manganese acetate, etc.

Glycolysis of polyester waste

Poly(ethylene terephthalate) dissolves in boiling glycol under atmospheric pressure and more rapidly when heated under pressure. On cooling the solution, oligomers of ethylene terephthalate are obtained.

Hydrolysis of waste

With water: Polyester waste can be hydrolysed with water at 150-200ø C under pressure. The amount of water used is about 2-4 times the weight of waste. The hydrolysis is catalysed by transesterification catalyst such as sodium acetate.

With base: A base like caustic soda lye (4-20%) or ammonium hydroxide can be carried out

With acid: Polyester waste can be hydrolysed 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. With sulphuric acid of concentration 87 per cent at 60-95ø C, the reaction is complete within several minutes.

Recovery from nylon 6 waste

In the production of nylon 6 material, such as yarns, cord, rod, blocks, and moulded articles, a large amount of waste is generated. The waste includes polymeric lumps, cables, entangled masses of filament, drawn fibres with or without finish, rejected bobbins and abnormal batches of polymer besides 10 per cent of the low molecular weight oligomers produced during polymerisation. Nylon 6 waste is broadly classified under two categories:

1. Liquid waste, which is composed of water extractable from various stages of polymerisation

2. Polymeric fibre waste or solid waste which is obtained during polymerisation, spinning and take up including that obtained during textile processing.

Liquid waste

During caprolactum recovery 20-25 per cent oligomers together with organic and inorganic compound are generated. The residue obtained after distillation of caprolactum, under reduced pressure, is invariably contaminated with inorganic substances such as MnO3, KHSO4, K2SO4, NaHPO4, Na2HPO4 and Na3PO4. The nature and amount of the impurities depends upon the process used for purification and distillation of caprolactum.

Recovery of Oligomers

The cyclic oligomers are sparingly soluble in water and dilute solution of caprolactum. During the process of concentration and chemical purification, these oligomers get separated out from the extracted liquor and are found deposited on the wall of the equipment used for the process. The presence of 6-aminocaproic acid and sodium salt may be present in the oligomeric waste, especially when NaOH is used for distillation of caprolactum. A typical composition of distillation residue is:

Open chain oligomers - 60-65 parts
Cyclic oligomers - 15-20 parts
Inorganic substances - 10 parts
Caprolactum - 5-10 parts

Solid waste

Amount of solid waste generated is around 8-10 per cent. Caprolactum or 6-Aminocaproic acid can be recovered from it.

Recovery of Caprolactum

It involves depolymerisation of nylon 6 and purification of caprolactum.Depolymerisation of nylon 6 waste

This is carried out by using superheated steam in the presence of inorganic or organic acids such as HNO3, HCOOH, benzoic acid, and HCl. This process has attained significant value on commercial scale. In a conventional process, superheated steam is passed through a molten mass at 250-300ø C to produce a dilute aqueous solution of caprolactum. The rate of depolymerisation increases with increase in molecular weight of polymer. The solution is subjected to multi-stage purification before converting it to 70 per cent liquor, which is fractionally distilled in the presence of suitable alkali to recover pure caprolactum. This process may also be carried out under reduced pressure, in the presence of catalyst such as alkali, metallic sodium and phosphoric acid and its salts. The reaction rate is fast initially, but the purity of product is poor. The process under vacuum in the presence of acid catalyst gives high yield of caprolactum. In this process, an adduct of caprolactum with TPA, isophthalic acid or adipic acid is used to recover 92 per cent of the caprolactum. The yield can be improved by using phosphoric acid.

Recovery of 6-Aminocaproic acid

Hydrolysis of nylon 6 waste is carried out in the presence of aqueous alkali metal salt or acid salt of 6-Aminocaproic acid. The hydrolysis of nylon 6 waste is rapid with dilute HCl 90-100ø C and completes within three to four hours. The reacted mass is poured in water to get a dilute aqueous solution of aminocaproic acid salt. The undissolved material such as pigments, additives and fillers are removed by filtration before passing the acid solution through a strong cation exchanger resin. ‘SO3H’ type cation exchanger absorbs aminocaproic acid and HCl is washed away with demineralised water. Washing is carried out till the eluant is free of Cl-. Pure aminocaproic acid is eluted for with NH4OH to obtain dilute aqueous solution, which is concentrated and crystallised. A highly pure aminocaproic acid is obtained because most of the ionic impurities are removed over ion exchange resin and during crystallisation. Similarly nylon 6 waste can also be hydrolysed using dilute sulphuric acid. Alternatively it can be done using sodium hydroxide in the presence of excess of water and the sodium salt of aminocaproic acid is converted into pure aminocaproic acid.

Nylon 66 yarn waste

Adipic acid and hexa methylene diamine can be recovered from nylon 66 polymer by repeated hydrolysis of the polymer waste in sulphuric acid, recovering the adipic acid by crystallisation after each hydrolysis step of diamine is recovered by distillation after neutralising the acid.

The hydrolysis and crystallisation of adipic acid have to be carried out in a stepwise manner, requiring frequent filtrations and time for optimum recovery of adipic acid. Therefore it is not efficient for high volume of waste.

Conclusion

The global environment is worsening day by day. It is very necessary to develop newer and more efficient processes for recycling of non-degradable material. Right now, in the textile chemistry division of the University Department of Chemical Technology, Mumbai, research is being carried out on recycling of polyester waste. It is our duty to leave a breathable world for our future generations by taking proper care of the environment now.Vishal Warke and Pradip Chandratre, UICT, authored and presented this paper at a national level technical symposium, at Bannari Amman Institute of Technology, Sathyamanglam, Tamil Nadu, and bagged second prize.)