By guest author Subrata Das, Department of Fashion Technology and 2Department of Textile Technology Bannari Amman Institute of Technology Sathyamangalam -638401, India
Polyester is a manmade fiber, synthesised from petrochemical products by the polymerization process. A majority of polyester used in textile is polyethylene terephthalate (PET). About 60 % of the PET produced globally is used to make fibers for textiles and about 30% is used to manufacture bottles. The annual global production of textile grade virgin polyester is estimated to require about 70 million barrels of oil. This necessitates the adoption of recycling solutions to reduce environmental damage. Recycled Polyester Textile (rPET) is recovered from end-of-life apparels, bottles, and other polyester products. The energy required for the production of rPET is lesser than that required for virgin polyester. It can be either mechanically or chemically recycled, with feedstock consisting of either pre- or post-consumer waste that can no longer be used for its intended purpose. The process of extracting polyester causes environmental damages. Polyester is produced by polymerization of ethylene glycol and dimethyl terephthalate. The usage of polyester had been increased several times since its discovery. It is also non-biodegradable. It takes about 35 to 42 year to decompose and so polyester waste disposal has to be reduced to reduce pollution. Hence recycling of the polyester is the most suitable method to reduce the environmental impacts.
Among the total global fibre production, 49% is polyester fibre. More than 63,000 million tonnes of polyester fiber are produced annually. Polyester is the mostly used fibre in apparel production sector. The chemicals used during the extraction of polyester and the by-products after the extraction are toxic and harmful. The cause water pollution and air pollution. They also cause health issues to those concerned in the process. Recycled polyester can be used to solve these problems.
The clear plastic water bottles or PET are used as the raw material for recycled polyester. The pre-consumer waste and post-consumer waste which cannot be used further can be recycled. If not recycled these polyester go as waste into the landfills. The recycled polyester can be used as fibre, foam, sheets and bottles. At the period of 1994, nearly 500 million plastic bins were recycled and 45 million ton fibres were produced. These polyester fibres were used to manufacture carpets and household products. The polyester waste can be recycled either mechanically or chemically.
Upcycling of Polyester
The clear plastic water bottles or PET are used as the raw material for recycled polyester (rPET). PET is the name of the resin used to make light, transparent bottles for drinks, juices, soap and etc. PET is 100% recyclable. The first PET bottle was recycled in the year 1977. Recycling of plastic bottles can prevent them from going into landfills that cause pollution.
Nearly 10 plastic bottles are recycled to produce 450 grams of recycled polyester fibre. The energy consumption to produce recycled polyester fibre is 8 times lesser than the energy consumption to produce virgin polyester fibre About 7.4 cubic sq. yard of land space can be saved if 1 ton of PET .bottles are recycled. One foot square sized carpet can be made by recycling 10 one litre PET bottle. Fibre from recycling thirty five, one litre bottle can be used for fibre filling the sleeping bag.
The triangular symbol with number “1” inside it in the plastic beverage bottles that we use in our day to day life represent that the bottle can be recycled. The symbol may also have an abbreviation “PET” or “PETE” below it. PET stands for polyethylene terephthalate. It is also called as polyethylene terephthalate ester (PETE).
PET bottles can be recycled by two methods:
In mechanical recycling the plastic bottle is washed, shredded and made into polyester chips. The fibres are then made using those chips. In chemical recycling the waste plastic is made into its original monomer and then the fibres are produced. Chemical recycling is not successful due to its cost and large quantity should recycled is chemical recycling method is used. Economically chemical recycling is not possible, so practically mechanical recycling method is used. Some of the worldwide major companies recycling PET bottle are Tex Amerixa, Napcar, USA and Fleissner GmbH and Co.
Process involved in production of recycled Poliyester INVOLVED IN PRODUCTION OF RECYCLED POLYESTER
The process of recycling of PET bottles in mechanical method is shown in Figure 1.
Collection and sorting of PET bottles: The used PET bottles are collected from the consumers or wastes. The bottles are brought to the place where the bottles are baled and compacted. The bottles are cleaned before baling. The bottles after compaction are transported to the flake production unit. The bottles are sorted according to the requirement (colour/quality). The clear bottles are sorted for the production of white polyester yarn and the green bottles are used for the production of green polyester yarns. The transparent bottles have higher economic value. The bottles may be sorted manually or automatically. Automatic sorting is done by colour identification method. The labels in the bottles are removed and cleaned. The labels and bottle caps are either sold or disposed into landfill. The bottles are then sterilized and dried.
Conversion of PET bottles to flakes: The bottles are chopped into flakes by the shredder with rotating blades. The size of the flakes should be 0.4 – 8 mm. These flakes are then allowed to float in water. Higher density materials are separated from the flakes. The flakes are then heated and dried to remove the moisture content. The flakes may also be re-grinded to convert them to finer flakes. The flakes may be washed using detergent and 2% NaOH with hot water and then rinsed with cold water or using tetrachloroethylene. Then the flakes are dried at a temperature of 17oC for nearly 6 hours. The flakes are then transported to the next process.
Production of fibre: The flakes may be directly melted and extruded to produce fibres (flake – fibre) or converted into chips/pellets and processed(flake- pellets – fibre). Melt extrusion method is used to produce fibres. Melting temperature of the flakes should be greater than 240oC. The water content in the flakes should be less than 0.02%.
Flake – Fibre: The moisture content in the flakes are removed by heating and drying. Then they are melted into liquid polymer at high temperature and extruded through spinneret. The polymer, before extrusion, is filtered through a dye plate that consists of tiny hole. The filaments are passed through denier setter before further processing. The filament is then cooled and hardened. The filament sent over the air rollers and then it is stretched. The filaments can be cut into staple fibres. In this process, nearly 1% of the flakes are disposed as solid wastes.
Flake – Pellets – Fibre: The moisture content in the flakes are removed by heating and drying. Then the flakes are melted into liquid polymer at high temperature and filtered. The polymer is cooled and made into pellets and then dried. The pellets are then transferred to spinning unit where the pellets are melted and spun into filament/fibres. For every 1.03 kg of fibres produced, 0.05 kg of solid waste is produced. The fibre can be given fluffy wool texture by passing it over a crimping system. The outline of this mechanical recycling process is shown in the Fig.1.
The fibre/filament can be dyed if required. The fibres are spun into yarn and made into bales that are ready to be converted into fabric.
The recycled polyester fibre can be used separately to produce fabric or can be blended with other fibre to acquire a required property from the fabric. Some of the strong, durables items like coat, jacket, bags, shoes, and accessories can be produced from these fibres. Sleeping bags, carpets, fiberfill, carboot linings are some of the recycled polyester fibre products.
In this process, the PET polymer is converted into monomer by depolymerisation. Depolymerisation is achieved by various means like methanolysis, glycolysis, hydrolysis, ammonolysis, aminolysis. Among these methanolysis, glycolysis and hydrolysis are mostly preferred method. The chemical equations of depolymerisation are shown in figure 2. In chemical recycling the properties of virgin polyester can be retained. This method of recycling is costlier than mechanical recycling. Chemical recycling is economically possible only in large scale.
In chemical recycling, the glycolysis process is achieved by converting PET to oligomer bis-hydroxyl ethylene terephthalate (BHET). The temperature range for glycolysis process is 180 – 250oC with time period of 0.5 to 8 hours. This process is carried out in the presence of catalyst. Zinc acetate is mostly used catalyst with 0.5% weight. PET degradation is carried out most frequently using ethylene glycol, diethylene glycol, propylene glycol, and dipropylene glycol. The oligomer is then filtered before repolymerisation of the PET. The polymer is then spun into fibres. During recycling, for every 1.05 kg of PET flakes, 0.05kg of PET goes as solid waste. Figure 3. shows the chemical recycling by the process of glycolysis.
In methonolysis process, PET along with methonal is converted into dimethyl terephthalate (DMT) and ethylene glycol (EG) with high temperature and high pressure. Pressure of 2-4 MP and temperatures of 180-280 °C are used. Zinc acetate, magnesium acetate, cobalt acetate, and lead dioxide can be used as catalyst. Zinc acetate is the mostly used catalyst. At the end of the reaction the catalyst should be deactivated. The DMT obtained is precipitated and crystallised. Continuous and batch method is sed for the production.
This is the last process of chemical recycling. The PET waste is recycled into TPA and EG. This method is related to the development of PET synthesis from TPA and EG. The elimination of ethanol is done by this technique. Acid hydrolysis, alkaline hydrolysis, and neutral hydrolysis can be performed from this process. In acid hydrolysis concentrated sulphuric acid is most frequently performed. The minimum weight of sulphuric acid used is 87%. This allows the process to take place in a pressure less apparatus. This reaction takes place for 5 minute under atmospheric pressure. In the first stage, the ground PET waste is mixed with sulfuric acid for concentration not less than 87 wt % at temperatures of 85-90 °C. In alkaline hydrolysis, aqueous solution of NaOH of concentration 4-20wt% is used. The process takes place for 3-5 h at temperatures of 210- 250 °C, under pressure of 1.4-2 MPa. Then in neutral hydrolysis is carried out with the use of water or steam. The process usually runs at a pressure of 1-4 MPa at temperatures of 200-300 °C.
Virgin Polyester vs Recycled Polyester
The biggest difference between rPET and PET is its impact on the environment. The requirement of energy and water for the production recycled polyester is lesser then that required for the production of virgin polyester. The energy consumption for the production of recycled polyester is only 2/3 of the energy required for the production of virgin polyester. Recycled polyester requires only 10% of water used for production of virgin polyester. Increasing the usage of recycled polyester reduces the need for petroleum as virgin polyester is extracted from petroleum. The durability, strength and performance of recycled polyester is similar to that of the virgin polyester.
Application of recycled polyester in fashion industry
Recycled polyester, also known as rPET, is obtained by melting down existing plastic and re-spinning it into new polyester fibre. While much attention is given to rPET made from plastic bottles and containers thrown away by consumers, in reality polyethylene terephthalate can be recycled from both post-industrial and post-consumer input materials. But, just to give an example, five soda bottles yield enough fiber for one extra-large T-shirt. More than half of the polyester in its products will be recycled in this current year by Adidas and the company has committed to using only recycled polyester from 2024. The company also plans to produce a record 15 to 20 million pairs of shoes with plastic waste collected from beaches and coastal regions in 2020, compared to 11 million pairs last year, five million in 2018, and one million in 2017. The use of recycled plastic in its products is part of the efforts to avoid plastic waste and stop the pollution of the world’s oceans. Many fashion designers are adopting sustainable clothing to support the environment. The recycled polyester textiles market in Asia Pacific is anticipated to expand at a rapid pace in the near future. Rise in population and rapid growth of the textile industry in emerging economies such as India and China are some of the factors that are expected to fuel the recycled polyester textiles market. Product awareness through brand promotion and capacity expansion by various textile producers are likely to boost the use of recycled polyester in fashion industry.
Features of Recycled Polyester
Constituent – Virgin polyester is extracted from petroleum by polymerization of ethylene glycol and dimethyl terephthalate. Increasing the usage of recycled polyester reduces the need for petroleum. Recycled polyester reduces the need for extraction of crude oil and natural gas from earth. The pollution caused during the extraction of virgin polyester can also be reduced.
Lesser energy and water consumption – The energy consumption for the production of recycled polyester is only 2/3 of the energy required for the production of virgin polyester. Recycled polyester requires only 10% of water used for production of virgin polyester.
Reduces pollution – The plastic bottles if not recycled go into waste as landfills. Landfills cause land and soil pollution which harms some of the living creatures. The United States Environmental Protection Agency reported that about 26 million tons of plastic were received to the country’s landfill. In United Kingdom, Waste and Resources Action Programme reported an estimate that nearly 140 pounds worth fabric is dumbed in the landfills. When these bottles are burnt they release hazardous gases and cause land pollution. If not dumped as landfill these plastic are let into ocean. It harms the aquatic environment. Many million tons of plastic are let into the ocean every year. The aquatic creatures mistake the plastic as plants and consume them. Hence in 100% of turtles and 60% of seabirds the plastic is found. If the pollution continues after few years there will more plastic than aquatic life in the ocean. Recycling of the plastic gives a second life to it. Hence recycling plastic reduces land, air and water pollution. Recycling of polyester emits lesser hazardous gas in the environment than the gas released during the extraction of virgin polyester.
Closed loop system – The garments can be produced from recycled polyester without any degradation in quality. The garment produced from recycled polyester can be recycled again. Thus the cycle continues and forms a closed loop system.
Environment and health impact – The chips obtain from PET bottles during the mechanical recycling are found to be in different shade. Few many be in creamy yellow shade, others in yellowish white shade. To obtain uniform white shaded chips they are bleached using chlorine. Usage of chemicals can cause pollution. While dyeing the recycled polyester fibre it is difficult to obtain uniform shade among the fibre lot. Hence repeated dyeing of fibres is required. This consumes a lot of energy, chemicals and water. Also few reports say that some of the recycled fibres take 30% more dye to attain the equivalent shade to that of virgin polyester fibre.
Antimony oxide is used as catalyst in the manufacturing process of PET and polyester. Though antimony is used in very small amount, it is said to be hazardous. During the recycling process, antimony is released by the PET bottles. Antimony is considered as cancer causing substance and hence may lead to health problems.
Microplastic – Studies say that recycled polyester release microplastics. Although it is believed that recycling of plastic and PET bottle prevent them from going into oceans, a paper published in 2011 on the journal Environmental Science Technology says that 85% of human-made debris on shorelines around the world are microfibers. In UK a recent study by a team from Plymouth University found that more than 700,000 plastic fibers are released into the environment by each cycle of washing machine.
Constraint in garment recycling – If a virgin polyester garment is to be processed to produce recycled polyester there are certain limitations, most of garments are made from blended fibre and hence it is difficult to recycle those garments. If certain finishing is given to the garment, it makes the recycling of garment difficult or impossible. Repeated recycling of polyester can cause degradation in its properties.
As the apparel, fashion, and retail industries move to become more sustainable, an area of interest is the use of recycled fiber, yarn, fabric, and product content in the development and production of new products. The decision to use recycled materials in products must occur during design and product development and continue throughout the manufacturing processes. There are several recognized stages in recycling collection, processing, and then use in a new product. Recycled materials used in textile and apparel products can be obtained throughout the textile and apparel supply chain and post-consumer collection methods. The use of recycled raw materials aligns with the larger movements of global industries toward a circular economy through closed-loop production cycle.