The aim of this report is to investigate ways of recycling polyethylene terephthalate to produce a greater, purer yield of terephthalic acid. Terephthalic acid is important within the textile industry with it being a great contributor in the production of polyester fibres, as well as it having many uses within the pharmacology sector. With 150,000 tonnes of non-biodegradable polyester taking up large spaces in UK landfills, the increasing production of non-biodegradable waste is an important issue within the field of environmental chemistry.
A. Eun Seon Kim, B. Chang Hwan Lee, and C. Seong Hun Kim conducted a study looking into possible pre-treatments of polyester. The aim of these pre-treatments was to increase the rate of reaction and percentage hydrolysis of the polyester. Samples of polyester were soaked in either benzyl alcohol or phenethyl alcohol and left to dry for 4 hours. Alkaline hydrolysis was undertaken using these soaked polyesters as well as a non soaked control sample. Afterwards, these soaked samples of polyester were neutralised with an acid. The percentage shrinkage was calculated as an indicator of yield purity and the extent of hydrolysis. Results of this experiment showed that pre-treatments of alcohol can be used to remove the crystallinity of the fibre and therefore increase the percentage shrinkage after hydrolysis occurs.
Annually, the UK produces approximately 23 million tonnes of waste each year. Of this, 5% is textile waste, which makes the UK the 4th largest producer of textile waste in Europe.[1,2] With the UK alone filling landfills with 350,000 tonnes of clothing each year, textile waste is becoming a major issue. This textile waste can be as much as 92 million tonnes worldwide, all produced within the fashion industry. Unfortunately, professionals have suggested that this will increase by 60% by 2030 if a recycling method is not readily available. Most clothing, around 95%, can be reused, re-worn, and upcycled in some way; however, this is not the reality all over the world, with only <20% being recycled. Just one person in the UK produces, on average, 70kg of textile waste per year.[2,6] According to Figure 1, 57% of clothing waste ends up in landfills per year, with roughly 25% being incinerated.
As well as filling landfills, this large quantity of clothing waste has a serious negative effect on the environment. Polyester is the most common textile used in the fashion industry and the most damaging textile to the environment, using double the amount of energy as cotton to process and releasing many harmful chemicals such as
Figure 1: Textile waste statistics as found in 2017
carcinogens, antimony, cobalt, manganese salts, sodium bromide, and titanium dioxide into the environment during production. If released into the air or water untreated, these chemicals can have a great impact on the environment. For example, antimony pollutes the soil, has possible toxic effects on small animals, and can cause lung damage. Bromide ions may also be introduced to the environment after the dissociation of various salts or the degradation of bromide containing compounds. These ions are harmful to the human body and may cause nausea or paralysis; in some cases they are corrosive to tissue.
Polyester is an oil based fibre and therefore stays in our landfills for decades and potentially even hundreds of years. When water is passed over these fibres, microfibers are washed off and enter the waterways. Whilst microplastics only seem to directly affect aquatic animals through ingestion, these toxins accumulate and concentrate and enter the human food chain, which in turn is passed into the wider environment. A common polyester, used in food packaging and water bottles, is polyethylene terephthalate, also referred to as PET. Due to its biodegrading resistance, polyethylene terephthalate stays in landfills for a very long time, causing the size of landfills to increase. Unfortunately, the rotting rubbish in these landfills creates methane and C02, both of which are greenhouse gases; these contribute to global warming. Therefore, the recycling of polyethylene terephthalate is becoming increasingly important.
PET is formed in a condensation reaction, the combination of two molecules with an additional product of water, between ethylene glycol and terephthalic acid, with each polymer connected by ester bonds. To break these bonds and therefore break the polyester back down into its monomers, a hydrolysis reaction needs to occur, which is the breaking down of a molecule using water. This could either be with an acid or base catalyst. Acid hydrolysis reacts with more of the fibre by penetrating the molecule, but it is a very slow reaction. On the other hand, base hydrolysis has a quicker rate of reaction but only reacts on the surface of the fibre. The products of base hydrolysis are not pure, so they need to be filtered and reacted further. Enzymatic hydrolysis, in which enzymes react in the presence of water in the cleavage of the bonds, has been found as a better way to perform a similar reaction. The alcohol swells the polyester, allowing the OH- ions to diffuse into the fibre and access more of the molecule.
Textile and Clothing Waste
There are three types of textile waste: production, pre-consumer, and post consumer. Production waste involves the material cut offs, ends of rolls, clothing scraps and prototype products, making up roughly 15% of the total waste. Damaged and unsold products are a part of pre-consumer waste. As damaged products aren’t usually fixed and sold, these usually end up in the landfills. The largest contribution to textile waste comes from post-consumer waste, those that are thrown away due to being outgrown, worn out or even out of fashion. Whilst 40% of people say they would potentially buy second hand clothes, only 20% buy these on a regular basis. Second hand clothes can include those bought from charity shops, car boot sales, or online second hand shops.
Most textiles can be broken down into 4 main fibre types: polyester, polyamide, cotton, and animal fibres. Other fibres, such as silk, spandex, and rubber can be classified within these 4 fibre categories (Figure 2). Animal fibres are a type of natural fibre consisting largely of proteins. There are 20 animal fibres, including wool, silk, and cashmere. Another type of natural fibre is cotton. Cotton is derived from the genus Gossypium plant and is found as a ball encasing seeds – this cotton is almost pure cellulose. Cotton can be combined with polyester to produce a poly cotton blend. Cotton is soft and lightweight, so it is used for fishing nets, denim, and even coffee filters. Whilst cotton production causes a significant amount of deforestation, the whole of the plant is used during textile production, so it leaves no plant waste. Polyamide, also known as nylon, is a type of synthetic fibre, much like polyester, with monomers linked by amide bonds. Some uses of polyamide are tights, gym clothes, riot gear, and firefighting gear.
Polyester is another synthetic polymer, but it is much more common than polyamide. Two types of spinning processes can form these fibres: the filament process and the spun process. The filament process involves long pieces being twisted together, whilst the spun process forms a staple from the combination of shorter pieces. Due to its high water resistance and strength, polyester is used for film, cushioning, attic insulation, and even canoes.
Figure 2: Classification of textile fibres
Even after decomposition, polyester persists in the ecosystem and is one of the greatest contributors to the world’s microplastics. Every time one item of polyester clothing is washed, 2000 fibres (microplastics) are washed off. Making up 49% of all textiles, polyester is an important focus within the textile and environmental field. There are several forms of polyester, but the form focussed on within this report is polyethylene terephthalate. The annual worldwide production of PET is around 40 million tonnes and is increasing at approximately 7% per year. Around 65% of this PET is used to make fibres, with 5% used for film and 30% for packaging.
Polyester is produced in a condensation reaction between terephthalic acid, a crystalline solid, and ethylene glycol, a colourless liquid. Terephthalic acid is produced when xylene, a volatile liquid hydrocarbon, undergoes oxidation with dilute nitric acid. Terephthalic acid has a molecular formula of C6H4(CO2H)2. The intermediate ethylene oxide is produced when ethylene forms ethylene glycol. The molecular formula of ethylene glycol is C2H6O2. H2O is also produced in this condensation polymerisation reaction. Conditions for this to occur are at a pressure of 400 kPa and 290 degrees Celsius. A higher yield of polyethylene terephthalate is produced at higher temperatures due to the reaction being endothermic – the equilibrium shifts to the right according to Le Chatelier’s Principle. Higher pressures also shift the equilibrium to the right due to a lower number of moles on the right hand side. Therefore, higher pressures and temperatures are ideal when producing a higher yield of PET. Esters link each monomer in this long chain polymer, which can be broken down in this reversible reaction. These ester links are formed when hydroxyl and carbonyl groups react. When forming the polyester, a lone pair of electrons is donated from the oxygen in ethylene glycol to the carbon in the terephthalic acid, forming a coordinate bond.
Polyesters are classified as aliphatic, semi-aromatic, and aromatic. This is due to the framework of the polyester’s main chain. Aromatic reactants improve the hardness, rigidity, and heat resistance of the polyester molecule. Aliphatic acids and diols, however, improve the processing ability and lower the melting or softening point. They also increase the flexibility of the polyester molecule. Polyethylene terephthalate (as pictured in Figure 3) exhibits a hard and rigid structure and is ideal for rigid plastics.
Figure 3: Condensation polymerisation of ethylene glycol and terephthalic acid to form PET
Condensation polymerisation of TPA and EG
The mass average of a polymer is defined as the average weight of the molecules that make up that polymer. Characteristics of the polymer change with molecular weight. After finding the average molecular weight and the basics of the polyester structure, it is important to research which characteristics of PET change and why. As a general rule for the properties of the polyester regarding molecular weight, as molar mass increases, the strength, viscosity and stretch of the polyester increases. Viscosity is the consistency of a material due to internal friction. Higher viscosity means it is harder to process the polyester using the typical method of depolymerisation. Resistance also increases; more damage is taken to the main chains of the molecules before the strength of the molecule is changed. Entanglement is defined as the cross linking of polymers – if the chain crosses the plane 3 times or more, it is said to be entangled. As the degree of entanglement increases, so does the stretch of the material. However, this increase only goes up to a point depending on the molecular weight.
Despite the range in polyester molecular weights and chemical properties, all polyesters undergo the same types of formation reactions, where only the final material’s properties change but the chemistry itself doesn’t change. Looking towards building a new recycling process, the chemistry of polyester should first be reviewed to determine how to effectively recycle polyester.
To recycle the polyester, first it must be broken down into its monomers. Polyesters can undergo hydrolysis reactions to break the ester bonds to form terephthalic acid and ethylene glycol. A hydrolysis reaction is a solvolysis reaction involving the breakdown of a substance using water. However, this alone is too slow, so an acid or base catalyst is used to increase the rate of reaction. Acids penetrate the polyester in order to access more of the molecule and its bonds. However, alkalis can only react on the surface of the molecule. The products of both acid and alkali hydrolysis are shown in Figure 4.
Alkali based hydrolysis is more frequently used in the depolymerisation of PET. It is completed under reflux with OH- ion containing compounds such as NaOH. Each ester linkage is hydrolysed to a carboxylate salt (-COO-Na+) and an –OH group. Much like the base hydrolysis reaction, acid catalysed hydrolysis is also performed under reflux conditions. The linkage in this polyester molecule is hydrolysed to a carboxyl group (COOH) and again an –OH group. The most common acid catalyst used is H2SO4, as this produces the greatest percentage yield on average. However, this reaction is rarely used due to the rate of reaction being so slow compared to that of base hydrolysis.
Figure 4: Acid/Base hydrolysis of PET
Both hydrolysis reactions are carried out under high temperatures and pressures. This means that these reactions are very costly, and a balance of cost, yield and rate needs to be established.
Unfortunately, by-products are also formed, decreasing the purity of the yield of ethylene glycol and terephthalic acid. Some of these by-products are ethanal, carbon monoxide, ethyne, 4-methylstyrene, 4-methylbenzoic acid, and 4-methylbenzaldehyde. As purity decreases, the reaction becomes less cost effective. Usually, unwanted products can be sold to other companies for a profit; however, as the by-products in this hydrolysis reaction are only formed in small amounts, this unfortunately isn’t possible.
The properties of these by-products have been investigated to determine their toxicity. Ethyne (Figure 5), also known as acetylene, is commonly used in portable lighting as well as in welding and cutting gases. Acetylene produces a reducing zone when burned in the presence of oxygen; this reducing zone can clean the surface of metal. This is also the hottest fuel gas when combined with oxygen. As this gas is stored under high pressures, there is also a high risk of explosion if heated when mixed with air. Inhalation can also be fatal, as ethyne displaces the oxygen in the body and causes rapid suffocation, with the lethal dose at 50pph/5M.
Figure 5: Structure of ethyne
Carbon monoxide or CO (Figure 6) is another by-product of the depolymerisation of polyethylene terephthalate. CO is used in the reduction of ores and in pharmaceutical manufacturing, as well as in many other reactions. Much like ethyne, this gas is stored under high pressures and has a high risk of explosion if exposed to high heat. When inhaled, carbon monoxide is exceedingly more toxic than ethyne. When CO is introduced into the blood, it displaces oxygen and deprives the heart, lungs and other organs of oxygen. A higher lethal dose is at 5000 ppm (0.50%) in a 5 minute interval.
Figure 6: The structure of carbon monoxide
New technology to recycle textile waste
Separation of Terephthalic acid (TPA):
After the acid/base hydrolysis reaction has occurred, a way to separate the products to produce a purer sample of terephthalic acid needs to be found. A general method to separate the products of hydrolysis is through gravity filtration.
Firstly, the solution is filtered using gravity filtration (Figure 7). This is done to remove any soluble by-products – terephthalic acid is insoluble, so it doesn’t pass through the filter paper. Afterwards, deionised water is used to wash the residue to remove any remnants or impurities still remaining on the surface of the solid.
Figure 7: Gravity filtration
After the remnants have been both washed and dried to a constant weight, NaOH is used to dissolve the sample. NaOH is used to increase terephthalic acid’s solubility (Figure 8). This is done to remove any insoluble impurities, and the filtration step is repeated. Re-precipitation is performed with concentrated acid to recrystallize the terephthalic acid.
Figure 8: Addition of NaOH to TPA
Finally, this recrystallized terephthalic acid is filtered, washed and dried once more to create a pure sample of terephthalic acid.
Hydrolysis produces crude terephthalic acid. However, for this to be used in new plastics and fibres, purified terephthalic acid needs to be highly pure at over 99% purity. If the purity of the terephthalic acid is lower, the products will be a slightly yellow colour, which decreases optical transparency.
4-Carboxybenzaldehyde (CBA) is a common by-product of TPA production at a 0.5% yield. Since approximately 40,000,000 tons of terephthalic acid are produced per year, CBA is a relatively large-scale industrial chemical. CBA is an organic compound with the formula C8H6O3. 4-carboxybenzaldehyde is made up of a benzene ring with both an aldehyde and a carboxylic acid functional group. The AMOCO process can be used to remove this aldehyde functional group; it works by oxidising the CBA/p-xylene to produce terephthalic acid. The aqueous solution of terephthalic acid is treated with hydrogen at a temperature between 300 F and around 450 F and a pressure at which the solution remains liquid in the presence of a noble metal catalyst (Mn/Co).
Catalytic hydrogenation (Figure 9) is used to purify TPA in the presence of platinum. Catalytic hydrogenation is hydrogenation (the adding of hydrogen to a compound) of a compound with a double or triple bond. It reduces the compound and leaves fewer bonds between the carbons. The process of catalytic hydrogenation utilizes a metal catalyst such as nickel, palladium, or platinum. However, this is extremely expensive due to a number of factors. Firstly, the reaction takes a long time to occur and therefore needs a high amount of energy to increase the rate of reaction. In addition, the platinum catalyst is very costly. Platinum is a relatively scarce element, with its mining production less than that of gold. As such, it tends to trade at higher per-unit prices.
Figure 9: Catalytic hydrogenation
Whilst acid/ base hydrolysis is a valid way of breaking down polyethylene terephthalate, the yield of TPA isn’t high enough. A way needs to be found to increase the yield of TPA. Enzymatic hydrolysis is a process involving the breakdown of a compound with the addition of water. 
The extent of the enzymatic hydrolysis of polyethylene terephthalate depends on the degree of the crystallinity, or the degree of order within the structure of the molecule. Highly ordered structures of the polyethylene terephthalate cannot be attacked by the enzyme. The glass transition temperature of PET is around 70 degrees Celsius, which reduces the level of crystallinity of the polyester. The flexibility of the amorphous regions increases and becomes more susceptible to enzymatic hydrolysis. Therefore, enzymatic hydrolysis of PET is favoured at temperatures of over 70 degrees Celsius. However, the intermediates of this reaction are inhibitors of polyester hydrolase. Polyester hydrolase enhances hydrolysis of the ester bond, and carboxyl esterase enhances further hydrolysis of intermediates. Non-enzymatic natural degradation of PET can take hundreds of years. PETase significantly reduces the time required to degrade PET, taking on average only a couple of days.
A pre-treatment of the polyester may help to improve the rate of reaction and percentage of hydrolysis by reducing the crystallinity of the polyester. Research conducted by A. Eun Seon Kim, B. Chang Hwan Lee, and C. Seong Hun Kim regarding possible pre-treatments is detailed below.
Several samples of polyester were obtained and soaked in either a solution of benzyl alcohol or 2-phenyl ethanol for four hours.
Benzyl alcohol (Figure 10) has the formula C6H5CH2OH. Due to its polarity, low toxicity, and low vapor pressure, it is a very useful solvent. Benzyl alcohol has many uses including as a wax, shellac, paint, lacquer, and epoxy resin coating.
Figure 10: The skeletal formula of Benzyl Alcohol
Phenethyl alcohol, otherwise known as 2-phenylethanol, is an organic compound that consists of a phenethyl (C6H5CH2CH2) group attached to an –OH. The structure of 2-phenylethanol is shown in Figure 11. Phenethyl alcohol is a colourless liquid which is miscible with most organic solvents; however, it is only slightly soluble in water (2 ml/100 ml). 2-phenylethanol can be prepared via two routes. The most common route is the Friedel-Crafts reaction. This reaction occurs between benzene and ethylene oxide whilst in the presence of aluminium trichloride (equation 1).
(1) C6H6 + CH2CH2O + AlCl3 → C6H5CH2CH2OAlCl2 + HCl
Figure 11: 2-Phenylethanol
After being soaked in either solution, the polyester was then left to dry for another four hours. Next, alkaline hydrolysis was undertaken with a 4% weight of NaOH. Finally, the fabric was then neutralised with acetic acid. In addition, a control situation was performed, where a polyester sample was hydrolysed in the same conditions without the pre-treatment first.
The results show that pre-treatments actually improved hydrolysis and that over time, percentage shrinkage was greater when a pre-treatment of the alcohol was used compared to when there was no pre-treatment of the control polyester, as seen in Figure 12. Whilst both pre-treatments increased hydrolysis, a treatment of benzyl alcohol showed the greatest result.
The alcohol pre-treatment swelled the polyester, which allowed the OH- ions to diffuse into the polyester and access more of the fibre. This investigation has shown that benzyl alcohol and 2-phenyl ethanol treatments modify the structural and morphological characteristics of PET fibres. When PET fibres are treated with a benzyl alcohol or 2-phenyl ethanol reagent, a disorientation of the amorphous phase and dissolution of the amorphous region occur in the treated fibres. The weight loss and crystallization of PET was affected greater by 2-phenyl ethanol, compared to benzyl alcohol.
The degree of crystallinity of the fibres was found to have increased after hydrolysis; however, this wasn’t said to be due to the pretreatment.
Figure 12: The results of the pretreatment of hydrolysis
PET – No pre-treatment
PET-b – Pre-treatment of benzyl alcohol
PET-p – Pre-treatment of phenethyl alcohol
Practical applications of this research involve using enzymatic hydrolysis to break down PET into its monomers, which are then recycled into new products. The amount of PET in landfills and in turn the production of greenhouse gases will be reduced. Greenhouse gases, such as carbon dioxide, are released when plastics in landfills are heated under the sun. The decrease in these greenhouse gases is incredibly important for the decrease in the rate of climate change.
Current research by Wolfgang Zimmerman (2020) puts forward the idea; “by producing biocatalysts which implement a protein engineering technique, the recycling of polyethylene terephthalate can be developed further for industrial applications.” Future research into this topic may include how dyes within the polyester have an effect on its depolymerisation and whether a pre-treatment of an acid will have a greater impact on the purity of the yield of terephthalic acid.
In conclusion, acid/base hydrolysis is a valid way to depolymerise polyethylene terephthalate to its monomers ethylene glycol and terephthalic acid. Alkali based hydrolysis is the method most frequently used, as it can penetrate the fibre and access more of the bonds within the molecule. However, it has been found that the yield of terephthalic acid/ethylene glycol is low, and many by-products are formed.
A different method was discovered to produce a higher yield of terephthalic acid quicker – enzymatic hydrolysis. To increase the rate of reaction and percentage shrinkage, a pre-treatment was used. This could be, for example, phenethyl alcohol or 2-phenylethanol. Afterwards, the pre-treatment had to be neutralised by acetic acid. To summarise, a combination of enzymatic hydrolysis and a pretreatment of an alcohol can be used to increase the yield and purity of terephthalic acid. The environmental importance of this study is critical in reducing the quantity of polyesters found in landfills.
Conflicts of interest
There are no conflicts to declare.
Thanks goes to Edward Randvirr for all the time and commitment he invested into helping with this project and all the advice he offered along the way. Sarah Burke, the project coordinator, who has given so much help and guidance throughout the project; thank you.
Notes and references
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