Chemical
Recycling of
Polycondensation
Polymers
Literature Thesis by: Roxy Strijdhorst (12308242)
5-5-2020
Primary Supervisor: Prof. dr. Gert-Jan Gruter Secondary Supervisor: Chris Slootweg
List of Abbreviations
3HB 3-hydroxybutytric acid 3MB Methoxybutanoic acid
BD 1,4-butanediol
BHET Bis(2-hydroxyethyl) terephthalate Bmim 1-butyl-3-methylimidazolium
BPA Bisphenol A
CA Crotonic acid
CHA Cyclohexylamine
DEA Diethanolamine
DEC Diethyl carbonate DEG Diethylene glycol
DEGMEE Diethylene glycol monoethyl ether DET Diethyl terephthalate
DETA Diethylenetriamine
DMAP N,N-di-methylaminopyridine DMC Dimethyl carbonate
DMI 1,3-dimethyl-2-imidazolidinone DMS Dimethyl succinate
DMSO Dimethyl sulfoxide DMT Dimethyl terephthalate DPG Dipropylene glycol DPT Dipropyl terephthalate EA Ethanolamine EC Ethylene carbonate EG Ethylene glycol
FDCA 2,5-furandicarboxylic acid HDMA Hexamethylene diamine HMF 5-hydroxymethylfurfural
IL Ionic liquid
MDA Methylene diphenyl amine
MHPT Methyl-(2-hydroxypropyl) terephthalate MK5 Montmorillonite K5 MK10 Montmorillonite K10 PBS Poly-butylene succinate PBT Poly-butylene terephthalate PC Polycarbonate PDO 1,3-propanediol
PEF Poly-ethylene furandicarboxylate PEIT Poly-ethylene Isosorbide Terephthalate PET Poly-ethylene terephthalate
PHA Poly(R-3-alkanoate) PHB poly(3-hydroxybutyrate) PLA Poly(L-lactic acid)
PTT Poly-trimethylene terephthalate
PU Polyurethane
SoC Substances of concern
TDA Diaminotoluene
THF Tetrahydrofuran
0. Abstract
Plastic, due to its wide range of applications, is currently used in many industries such as the packaging, automotive, clothing, medical material and electronic industry. The demand for packaging plastics, as a result, has increased to 311 million tonnes per annum in 2014. Only 14% of these plastics are collected for recycling and from what is recycled, it is mostly processed through open and closed-loop mechanical recycling. This compromises the properties of the plastics after a number of cycles. Chemical recycling is an alternative approach. This review will discuss available technologies for chemically recycling various polyesters into their corresponding monomers through solvolysis.
1. Introduction
Polymers have been instrumental in advancing the standard of living in the modern society. Synthetic polymers are useful and versatile materials that can be easily mass produced into consumer goods with automated machinery at a low cost1. They are used globally as packaging materials, films, fibres,
clothing due to their strength, resistance, and durability1. The annual consumption of these plastics
has grown to 311 million tons per annum in 2014 from 15 million in 1964. Over the next 20 years, this number is expected to double2.
The properties that make synthetic polymers desirable also make them a threat to environmental pollution as they are not easily degraded in natural conditions3. Furthermore, many plastics have a
short duration of use, 40% of synthetic polymers have a product lifespan of less than one month4.
Thus, they accumulate in landfills and other garbage dump sites1. This is problematic due to land
pollution and space constraints5. Additionally, synthetic polymers are synthesized from
petrochemicals, representing about 6% of global oil consumption2. The depletion of oil reserves also
poses a threat to the availability of these materials6.
Two major waste streams are generated from the polymer industry. The first is manufacturing waste that is generated in the production process of these synthetic polymers. The second is used and discarded plastic waste called “post-consumer waste”. Because of the mentioned problems, the polymer industry is a hazard to a clean environment from both waste streams7. This is a complex
problem that requires the cooperation of the industry, the government, and the public1.
Using monomers derived from renewable resources to synthesize polymers reduces the industry’s dependence on oil reserves. These polymers may have properties that are not achievable with typical petrochemical feedstocks due to their varying chemical structures. In addition, their industrial production initially (first 10-20 years) not as large scale and thus not as low-cost as current synthetic polymers8. Though this addresses the depleting resources and climate change issues, biodegradable
plastics can also have the potential to persist in the natural environment for decades, depending on the type of polymer and the local conditions such as ultraviolet light exposure, temperature, and presence of the appropriate microorganisms9.
Figure 1 shows the global packaging plastic flows in 2013. It shows that only 14% of plastic packaging is collected for recycling. The rest is sent to be incinerated (14%), or is not recovered at all (72%). The polymers that are not recovered are either landfilled or leak out of the system of collection in place through illegal dumping or mismanagement. For this reason, better sorting and recycling techniques of polymers must be developed. Not only does recycling tackle the waste problem, it
conserves valuable and scarce energy and feedstock. The large amount of energy stored in the chemical bonds of polymers are recovered instead of wasted1.
Recycling 1 ton of plastic saves, on average, 130 million kJ. An estimate performed by Rahimi et al states that using recycling has the potential to save US$176 billion annually and add US$38 billion to the US economy alone. This estimate was based on crude oil prices from Bloomberg Markets in January, 2017. In addition, using carbon-containing plastics that are already above ground as a raw material for the synthesis of new plastics has no further contribution to climate change through the use of more oil reserves. Hence, plastic waste should be treated as a feedstock for the synthesis of new materials6.
Some issues arise in the process of recycling. Plastics generally contain additives that enhance the material properties or reduce the cost of production. Additives include pigments, plasticizers, fillers, stabilizers and flame retardants. Some of these substances can have harmful and irreversible effects on human health and the environment. They are called substances of concern (SoC), however, they are present in low concentrations and may not necessarily be harmful. SoCs become an issue when closing material loops as SoCs accumulate from all stages of the product’s life cycle. Potentially harmful substances may remain in the loop and be transferred to newly synthesized materials which can deteriorate the material properties or be harmful for human health. Hence, using recycled materials for food-contact applications requires major health and safety concern2.
Figure 1 Plastic packaging global flow in 20132
Chemical recycling is a technique that recovers the monomeric/oligomeric components of a polymer. It is a technique that aligns most with sustainable development principles which is to develop to meet the present generation’s needs without compromising the future generation’s ability to meet their needs10. There are two main types of chemical recycling, solvolysis and pyrolysis, this will be
discussed in more detail in the next section of the review. This review will focus on the chemical recycling of common polycondensation polymers through solvolysis. Polycondensation polymers
contain functional groups, namely ester and amine groups, that serve as sites for the scission of the polymer chain to recover the various monomers. Pyrolysis will not be regarded in this paper due to the energy intensive nature of the process7. This review will focus on scientific paper and patents
that focus on monomer recycling through three key steps: 1) proper depolymerization that convert polymers into monomers or oligomers, 2) purification techniques that afford pure monomeric/oligomeric products and 3) repolymerization into a desirable polymer from the recovered monomers/oligomers. Step 1 is particularly important as the depolymerization should selectively produce a monomer that is suitable for repolymerization. A complex mixture of monomers is not desired. Furthermore, removing the additives through a purification step makes repolymerization more effective and also makes chemical recycling more tolerant to produce sensitive plastics such as those to be used for food packaging applications11.
2. Types of Recycling
According to the Ellen McArthur Foundation, there are a few different types of recycling: a) Mechanical recycling in closed loops (Primary recycling)
Primary recycling, also called closed-loop recycling, is a method wherein high-quality post-consumer waste material is mechanically reprocessed into the same purpose with similar characteristics as the original material5,6. It is possible to use pristine, uncontaminated
industrial manufacturing waste in this process12. This is a popular recycling technique as it is
simple and low-cost and may be conducted “in-plant” using materials with controlled history1,4. The main example of this type of recycling is recycling a PET bottle into a PET
bottle. Currently only 2% of packaging is recycled in this way2.
b) Mechanical recycling in open loops (Secondary recycling)
In this process, polymers (typically post-consumer polymers) are reprocessed into pellets through melt extrusion. Before this, contaminants, such as food residues, in the polymer are removed12. The mechanical recycling process includes sorting polymeric waste, size
reduction and melt filtration. The chemical structure remains unchanged in the process. Products of this type of recycling are of different characteristics than the original, typically they are of lower value5,6. A disadvantage of mechanical recycling is that the polymer
properties deteriorate with every extrusion cycle. This is a result of the polymer chains breaking due to heat treatment and the presence of impurities4. This limits the number of
times that polymers may be subjected to this type of recycling6.
c) Chemical Recycling (Tertiary recycling)
Tertiary recycling uses chemical processes to recover the monomeric or oligomeric components of the plastic4. Chemical recycling can be done in two ways: solvolysis and
pyrolysis. Solvolysis involves the use of solvents such as water and alcohols to degrade the polymer. Solvolysis can only be applied to condensation polymers due to the presence of functional groups that can be broken. The monomers are purified to produce new polymers12.
Pyrolysis performs degradation through heat without the presence of oxygen and air. Typically, this process yields a pyrolysis oil that can be used as a feedstock for hydrocracking6.
d) Incineration
This is a process in which the energy stored in the chemical bonds of polymers is recovered as thermal energy. The heat generated in the process may be used to heat steam that can be used to produce electricity with turbine generators5. It is the most efficient way to decrease
it does not require collection, separation and sorting which are often difficult1. However, risks
of incineration as an energy source is the release of toxic substances into the atmosphere. Additionally, the ash residue may contain catalyst residues which may be toxic4. Incineration
is common to generate energy and electricity, although, globally it emitted 16 million metric tons of CO2. If the current trajectory is followed, by 2050 incineration could add up to 4.2 Gt
of CO213.
3. Polyesters
Polyesters are a class of polymers that have an ester functional group present in the main chain. This ester group serves as the site at which the polymer can be broken down into its component monomers for chemical recycling14. This section will elaborate on the chemical recycling techniques
for different types of polyesters.
3.1. Poly-Ethylene Terephthalate
Poly-ethylene terephthalate (PET) can be synthesized in two ways. First is the esterification of terephthalic acid (TPA) and ethylene glycol (EG) (Figure 2a). The second, less popular method, is the transesterification of dimethyl terephthalate (DMT) with EG (Figure 2b). Both reactions produce bis(2-hydroxyethyl) terephthalate (BHET) which can undergo polycondensation to produce PET4.
PET is a light, tough, transparent and easily processable polymer15. For these reasons, PET is globally
used primarily in the manufacturing of fibers, bottles and other forms of food packaging16,17. The
demand for PET is growing and already exceeds 50 million tons of global consumption18. Due to its
large presence, it is one of the most present waste materials in landfills19. Hence, the introduction of
a large-scale recycling infrastructure is necessary20. Some chemical recycling techniques have
already been established for PET and are discussed further.
3.1.1. Hydrolysis
Hydrolysis of PET produces TPA and EG as products. This can be performed in conditions that are neutral, basic, or acidic. Hydrolysis is an actively studied option, however, due to the difficulty of purifying TPA as it is of low solubility and low vapor pressure, it is not yet commercially viable21.
Acidic hydrolysis is generally performed with the use of concentrated sulfuric acid (>14.5 M). However, other acids such as nitric or phosphoric acid may be used. Following acid treatment, the solution is neutralized by sodium hydroxide to form a TPA sodium salt. Finally, the salt is acidified and TPA is obtained through precipitation. This is performed from 60-100 °C at atmospheric pressure. Drawbacks of this process is the difficult separation of EG from sulfuric acid, the substantial amount of waste generated due to the corrosive nature of the acid1,12,21.
Alkaline hydrolysis typically employs an aqueous solution of KOH or NaOH at 4-20 wt% to produce EG and a disodium or dipotassium terephthalate salt. The reaction takes place in 3-5 hours at 210-250 °C at a pressure of 1.4-2 MPa. Yields of 98% TPA can be achieved3. By introducing oxygen into
the NaOH and PET solution, TPA and oxalic acid are produced as products in high yield. Oxalic acid is a more valuable product than EG15. Using sodium hydroxide in nonaqueous solutions such as
methanol, EG and different ethers as solvents or co-solvents have proven to increase the rate of hydrolysis1,3,12.
Neutral hydrolysis utilizes water or steam to obtain TPA and EG. Campanelli et al found that performing PET hydrolysis with molten PET increases the depolymerization rate. At temperatures of 265 °C for 2 hours with a water:PET ratio of 5.1:1, complete depolymerization was observed. Furthermore, the use of metal acetates such as zinc and sodium increases the rate as these catalysts electronically destabilize the polymer-water interface22. Neutral hydrolysis produces fewer
inorganic salts and other forms of waste than acid/alkaline hydrolysis. However, the TPA obtained is difficult to purify. It is typically performed by filtration of TPA that is dissolved in a solution of caprolactam or an aqueous sodium hydroxide solution. Or by recrystallization of TPA from caprolactam. EG can be obtained through distillation1,21.
3.1.2. Enzymatic Degradation
Few examples of enzymatic degradation of PET exist. This is due to the long degradation times required which hinders industrialization. This only increases with the increasing crystallinity of the PET to be depolymerized23. Müeller et al were the first to report enzymatic hydrolysis using the
hydrolase Thermobifida fusca. In three weeks 50% of weight loss of PET films were reported24. A
more recent example by Ronkvist et al employed cutinases. The cutinase Humilica insolens showed a weight loss of 97% for PET films after 96 hours of incubation at 70 °C. Analysis of the degradation products show that they consist of TPA and EG25. Pellis et al. explored the use of ultrasound to
increase reaction rates, they found that ultrasound-activated Thermobifida cellulosilytica degraded PET powder of 8% crystallinity 6.6 times faster than without ultrasound. Activation was less effective for more highly crystalline PET or for PET films due to the decrease in surface area23.
3.1.3. Alcoholysis
3.1.3.1. Glycolysis
Glycolysis is a method of chemical recycling wherein glycols such as EG, together with a transesterification catalyst, are used to break the ester bonds and replace them with hydroxyl terminals. The main product of glycolysis is BHET and other PET oligomers (Figure 3)1. Glycolysis is
Figure 3 Glycolysis of PET17
Many types of catalysts are available for the glycolysis of PET. Metal acetates may be used. Ghaemy et al determined that the activity of metal acetate catalysts is decreasing in the order of Zn+2 > Mn+2 > Co+2 > Pb+2 for PET fibers and obtained 75% BHET and dimer at 198 °C with zinc
acetate16. Goje et al also found that zinc salts had more catalytic activity and that the addition of
cyclohexylamine and NaOH further increased the reaction rate. PET conversion was 98.66% with a particle size of 127.5 µm, for 90 minutes at 197 °C. DMT and EG were obtained in yields similar to conversion20. A titanium (IV)-phosphate catalyst achieved a yield of 97.5% BHET at 300 °C for 2.5
hours. The same reaction with zinc acetate yielded 62.8% BHET19. Zinc, however, has the tendency
to accumulate in the environment with detrimental effects. Therefore, Shukla considered simple chemicals such as acetic acid, potassium sulfate, sodium sulfate, and lithium hydroxide. These catalysts afforded BHET yields of >60%26.
Silica nanoparticles impregnated with the metal oxide catalysts was a modification to increase the surface area of the active site. Imran et al deposited zinc, manganese and cerium oxides on silicon nanoparticles. A maximum BHET yield of 90% was obtained after 80 minutes at 300 °C and 1.1 MPa. The EG:PET ratio was 11:1 and the PET:Catalyst ratio was kept at 1:0.0127.
Wang et al were the first to use ionic liquids (IL) in the glycolysis of PET (Figure 4). ILs are salts with a melting point of less than 100 °C. Its main advantage as a catalyst is that it is easily separable and reusable. Furthermore, they are thermally stable, non-volatile and exhibit low flammability and strong solvability. 1-butyl-3-methylimidazolium bromide [Bmim]Br was used as a catalyst at 180 °C and achieved 100% PET depolymerization after 8 hours. The solubility of PET increased with recycled IL potentially due to the presence of water in recycled IL28. Yue et al reported that basic IL
such as [Bmim]OH showed better activity than [Bmim]Br and [Bmim]Cl, 71.2% BHET was obtained with 100% PET conversion after 2 hours at 190 °C17.
Using other solvents with glycols have also been successful in catalyzing glycolysis. Wang et al used urea in a mass ratio of EG:Urea:PET as 0.1:4:1. 100% conversion was achieved at 160 °C for 2.5 hours. Using density functional theory, it was deduced that H-bond formation between the oxygen in urea and EG lengthens the O-H bond in EG making hydrogen loss easier which subsequently makes the nucleophilicity of the oxygen increase. This accelerates the rate of glycolysis as nucleophilic attack on PET is faster18.
Heating EG to above the supercritical level (446.7 °C and 8 MPa) is another method to depolymerize PET. This technique eliminates the need for catalysts which may present difficulties in separation. A
reaction was performed at 450 °C at 15.3 MPa. The reaction was in equilibrium quickly and after 30 minutes 93.5% BHET was achieved29.
Figure 4 Potential mechanism of the depolymerization of PET with the IL [bmim]Cl17
Microwave-irradiation has also proven to assist in glycolysis. Chaudhary et al have proven that glycolysis could break down PET into BHET in 30 minutes instead of many hours with conventional heating. 44% BHET was obtained after 30 minutes, however, a second glycolysis step with the remaining oligomers was performed instead of prolonged heating and BHET yield reached close to 100% after 10 minutes with a PET to EG ratio of 1:630.
3.1.3.2. Methanolysis
Methanolysis is generally more expensive than glycolysis, however, it is able to chemically recycle PET of lower quality as it is more tolerant towards contamination21. Furthermore, the methanol and
EG can be easily recycled1. It is a process wherein methanol is used to produce DMT and EG.
Methanolysis can be performed in a few ways. One way is the use of liquid methanol. Typical transesterification catalysts are used such as zinc/magnesium/cobalt acetates. However, zinc acetate is the most common1. This is performed at temperatures ranging from 180–240 °C at pressures from
2–4 MPa to get a yield of 80-85% DMT. The high pressure is to keep the methanol liquid12.
Methanol in the vapor phase is also a viable depolymerization medium. Naujokas and Ryan of Eastman Chemical Company proposed a process wherein PET is dissolved first in oligomers of DMT and EG. Superheated methanol in the vapor phase (at lower pressures than in the liquid phase) is passed through the solution31. DMT is then removed as a vapor, making purification easier as DMT
and EG form an azeotrope, making separation challenging21,31. Therefore, a higher yield is obtained
than in liquid phase, though the reaction is longer31.
Supercritical fluids are denser than in the liquid state and have high kinetic energy. Hence, they are of suitable reactivity for PET depolymerization. Sako et al used supercritical methanol at 300 °C at pressures above 8 MPa for 30 minutes. 100% depolymerization and high monomer/oligomer recovery was achieved32. However, the use of supercritical methanol is costly due to the intense
3.1.4. Aminolysis
This chemical recycling process produces
the TPA diamide:
bis(2-hydroxyethylene)terephthalamide
(BHETA). The reaction is frequently carried out in aqueous primary amine solutions. Collins used methylamine, ethylamine, ethanolamine (EA), and anhydrous n-butylamine at temperatures ranging from 20-100 °C33. Shukla used EA with catalysts
such as glacial acetic acid, sodium acetate and potassium sulfate (Figure 5). High yields of >80% were obtained with 1wt% sodium acetate and potassium sulfate after 8 hours34. Spychaj et al used polyamines,
for instance diethylenetriamine, triethylenetetramine, p-phenylenediamine, triethanolamine and their mixtures. It was determined that the basicity and steric hindrance of the amine controls the rate of degradation. The products could be used as a feedstock for resins with the potential for acting as the polyol component in the synthesis of polyurethanes35.
3.1.5. Ammonolysis
Ammonolysis uses ammonia to degrade PET into terephthalamide in an EG environment (Figure 6). Blackmon described a process wherein PET postconsumer bottles were subjected to ammonolysis at 120-180 °C for 1-7 hours at 2 MPa. The amide is then filtered and rinsed with water to obtain a yield of >90% and high purity. Low-pressures may also be applied using zinc acetate as a catalyst in a 0.05wt% at 70 °C providing a yield of 87%36.
Figure 6 Ammonolysis of PET36
3.2. Poly-butylene Terephthalate
Poly-butylene terephthalate (PBT) was introduced 17 years after PET37. It is an important
engineering plastic due to its great mechanical properties. PBT is estimated to have a global production of more than 1.21 million tons38. PBT is more resistant to photooxidative yellowing,
accepts dyes better, and has better elastic recovery properties in comparison to PET39. Furthermore,
its mild melting temperature of 225 °C and fast crystallization makes it ideal for injection molding for durable commodities40,41. For these reason PBT has found applications in fibers, high-performance
yarns, bottles, sheets, and films39. PBT is generally synthesized by the polycondensation of
1,4-butanediol (BD) and DMT using a weak basic catalyst, however, TPA can replace DMT37,40.
3.2.1. Hydrolysis
PBT hydrolysis with the presence of NaOH in a batch reactor was also examined by Goje et al. It was found that PBT conversion increases linearly with reaction temperature. Meanwhile the conversion is inversely proportional to the PBT particle size at the start of the reaction. The ideal conditions were determined to be 450 rpm agitator speed at 140 °C for 90 minutes with an initial PBT particle size of 127.5 µm to attain up to 97% yield of BD and TPA through a salting out technique39. Goje et al studied
the same parameters without the use of NaOH in an autoclave reactor for the auto-catalyzed hydrolysis of PBT. The ideal particle size was the same, however, at 245 °C for 10 minutes at 450 rpm. Almost 100% yield of the monomers were achieved42.
3.2.2. Alcoholysis
3.2.2.1. Methanolysis
Previous studies have reported the smooth depolymerization of PET with high temperature and high pressure conditions. Hence, Shibata et al studied the potential to do the same with PBT in methanol. Changing the pressure from 6-14 MPa while keeping temperature constant had little effect as after 20 minutes DMT and BD could be recovered in high yields. Varying the temperature had a larger effect, the depolymerization took place in much higher rates above the melting point of PBT (227 °C) suggesting that the miscibility of PBT in the molten state and methanol is an important parameter43.
Yang et al investigated the use of different alcohols. Methanol, ethanol and propanol were used in supercritical conditions to obtain DMT, diethyl terephthalate (DET), and dipropyl terephthalate (DPT), respectively along with BD. The reactivity of supercritical methanol was the greatest, obtaining the highest yields of 98.5% DMT and 72.3% BD after 75 minutes at 310 °C44.
Figure 7 Alcoholysis of PBT44
Pan et al proposed a mechanism for supercritical ethanolysis of PBT. Higher temperatures allow for easier diffusion of ethanol into the polymer chain due to the weakening of intermolecular forces. A yield of 97.7% DET and 89.4% BD was obtained under the optimal mass ratio of 10:1 ethanol/PBT at a temperature of 240 °C for 60 minutes38. Huang et al also studied the characteristics of
depolymerization of PBT in supercritical conditions. The depolymerization process takes place in three steps: a subcritical region, a transitional region and a supercritical region. The subcritical region was a swelling process with minor decrease in molecular weight. In the transitional period PBT dissolution was more rapid. Finally, thorough depolymerization through random chain scission takes place in the supercritical region45.
Shukla examined the depolymerization of PBT in alkali conditions using solutions of 40% and 10% NaOH in water, methanol, and ethanol. It was found that the alcohols provided higher weight loss than the aqueous solutions most likely due to the hydrophobicity of PBT. Methanol gave a higher weight loss than ethanol possibly because its smaller size allowed for easier access to the PBT surface. However, PET showed higher weight loss in the same conditions in comparison to PBT46. Goje et al
did a similar study with the same solvents only using KOH instead of NaOH as it has greater reactivity. Furthermore, cyclohexylamine (CHA), tetrahydrofuran (THF), and dimethyl sulfoxide (DMSO) was added to increase depolymerization. The addition of the solvents did increase the depolymerization, the highest yield obtained was only 38% of both TPA and BD in methanolic alkaline solution which was almost equal to the PBT conversion47.
3.2.3. PBT Co-polyesters
To increase polymer sensitivity to hydrolytic degradation, research into adding hydrophilic moieties derived from carbohydrates into the polymer chain has become of interest. Alla et al explored this option for PBT by copolymerizing BD and up to 50% 2,3,4-tri-O-methyl-pentitols to produce PBT copolyesters. A decrease in crystallinity was observed and sensitivity to hydrolysis significantly enhanced40. Lavilla et al replaced either BD or DMT with 2,3:4,5-di-O-methylene-galactarate. It
formed polymers containing a bicyclic structure which improved the polymer properties. Both polymers displayed significantly faster hydrolysis rates in comparison to PBT41,48.
3.3. Poly-trimethylene Terephthalate
Poly-trimethylene terephthalate (PTT) is a polymer synthesized from the transesterification and polycondensation of 1,3-propanediol (PDO) and TPA or DMT (Figure 8). Usually tetraisopropyl titanate is used as a catalyst49. Whinfield and Dickson first synthesized PTT in
1941, however, due to the lack of an economically viable source of PDO, it was not commercialized. Shell Chemical Co. in the 90s developed a technology that made the production of PDO cheap through the hydroformylation of ethylene
oxide, making PTT a price competitive polymer50. The odd number of methylene moieties between
the terephthalate units gives PTT an extended zigzag structure different to its homologous PBT and PET. This makes PTT resilient, chemically resistant, and stable, with excellent dyeability and elastic recovery 50–52. These properties make PTT a useful material for fibers for carpets and textiles,
packaging, film and engineering thermoplastics52.
3.3.1. Hydrolysis
Gao et al used hot compressed water (supercritical water above 200 °C at high pressure) as it displays physical and chemical properties that alter between the extremes of gas and liquid-like behavior. PTT completely dissolved in water from 240-320 °C. Optimal conditions that attained the highest yield of 90.5% TPA and 69.03% PDO were 300 °C for 15 minutes. The formation of hydrogen ions in water due to the increasing concentration of TPA catalyzes the conversion of PDO to 1,5-dioxocane52.
Eberl et al proved that enzymatic degradation is possible also for aromatic polymers. Thermobifida
fusca was isolated and found to have the properties of both a lipase and an esterase. T. fusca was able
to degrade PTT films to TA. It was also able to degrade cyclic PTT oligomers, which is an unwanted Figure 8 Chemical Structures of PTT, DMT and
by-product in the synthesis of PTT polymers. Methyl-(2-hydroxypropyl) terephthalate (MHPT) was obtained as a side product. The paper did not report on the formation of PDO53.
3.3.2. Alcoholysis
3.3.2.1. Glycolysis
The depolymerization of PTT was studied by Kim et al using different glycols as a solvent in the presence of NaOH (Figure 9). Amongst the glycols, diethylene glycol monoethyl ether (DEGMEE) afforded the fastest rate. PTT could be completely depolymerized at 190 °C for 60 minutes to obtain disodium terephthalate (which could be acidified into TPA) and PDO. This is because PTT is more soluble in DEGMEE which allowed the NaOH molecules to diffuse in between the PTT chains more easily50.
Figure 9 Alcoholysis of PTT in the presence of NaOH50 3.3.2.2. Methanolysis
Zhang et al used supercritical methanol in a batch-type autoclave reactor to recover PDO and DMT. The reaction after 10 minutes, at 320 °C and 10 MPa yielded up to 98% monomer recovery. This is due to supercritical methanol dissolving the PTT better than liquid methanol leading to the active ester linkages to be well dispersed51. Zhang also developed a method of size-exclusion
chromatography to analyze the products formed in the supercritical methanolysis of PTT. DMT, MHPT, bis(2-hydroxypropyl) terephthalate, methyl-(2-hydroxyethyl) terephthalate, BHET, and hydroxyethyl-(2-hydroxypropyl) terephthalate are the main products formed54.
3.4. Polycarbonate
Polycarbonate (PC) is a type of polyester notorious for its excellent mechanical properties such as: UV resistance, strong impact resistance, high electrical resistance and retardant of flames. PC finds applications in soft drink bottles, bulletproof windows, discs, and food packaging55. 6 million tons of
PC are produced annually56. There are many methods for the synthesis of PC, however, the most
popular one is through the polycondensation of bisphenol A (BPA) and carbonyl chloride or dimethyl carbonate (DMC)57. Though PC are a potential source of BPA, it is labelled as “other” and mixed with
many materials in the system of recycling plastics instead of getting its own label such that PET and other polymers have. Doing so may increase the interest in recycling PC56. An effective chemical
recycling process is one that provides a reactive atmosphere that can depolymerize PC while preserving the stability of BPA.
3.4.1. Hydrolysis
The hydrolysis of PC is difficult as it is insoluble in water. Often high temperatures, high pressures and long reaction times are necessary. Hence, many different types of strategies are being employed. Watanabe et al compared the use of high pressure and high temperature steam as the reaction medium to using water in the liquid phase at 300 °C. In these conitions, BPA has been known to further breakdown into phenol and isopropyl phenol (Figure 10). Isopropyl phenol further decomposes into acetone and phenol. In steam, the PC was completely depolymerized into phenol and acetone as expected, however, Watanabe decided to focus on the BPA stability and only reported BPA decomposition. It was seen that after 5 minutes a yield of 80% of BPA was obtained. Steam is favorable as the amount of water needed to perform the reaction is dramatically reduced. This avoids the large amounts of energy necessary to remove excess water60.
Jung et al performed the hydrolysis of PC in THF as a co-solvent since the use of co-solvents had previously been reported as increasing the monomer yield due to increased solubility. The reaction was performed both in acidic and basic conditions in water. Basic conditions depolymerized the PC more readily61. Deirram, performed the hydrolysis of PC under microwave irradiation with various
co-solvents. THF or 1,4-dioxane were used as co-solvents in a solution of NaOH and water. A yield of 94% BPA was obtained in 12.5 minutes at 110 °C with the use of THF62. Liu et al did a similar reaction
without the use of microwave and the same yield of 94% was achieved in 8 hours at 100 °C. Liu also determined that the amount of water and NaOH played an important role in the reaction, therefore, the ideal mass ratio that obtained the maximum yield was PC:water = 1.5:1 and PC:NaOH = 10:158.
Huang et al studied the effects of additives on the rate of hydrolysis and the yield of BPA. Through kinetic studies they found that the flame retardant decabromodiphenyl ether increased the rate of hydrolysis by lowering the activation energy while the plasticizer di-n-octyl phthalate decreased the rate of hydrolysis. Both additives, however, decreased the yield of BPA63.
3.4.2. Alcoholysis
3.4.2.1. Glycolysis
Hydrolysis of PC can only recover BPA, therefore, other forms of chemical recycling are important. Kim examined the glycolysis of PC using EG in the absence of a catalyst and toxic solvents to develop a greener process. A BPA yield of 95.6% was achieved at 220 °C for 85 minutes at a ratio of EG/PC of 464.
Oku et al looked at alkali catalyzed glycolysis with 10mol% NaOH. The products generated consists of 42% monohydroxyethyl ether of BPA (MHE-BPA), 11% bishydroxyethyl ether of BPA (BHE-BPA), and
42% BPA. This is because ethylene carbonate (EC) Figure 11 Glycolysis of PC64
is formed as an intermediate in the glycolysis of PC, this EC reacts further with BPA to form MHE/BHE-BPA65. In non-catalyzed glycolysis the EC no longer reacts with BPA and but instead
decomposes into linear carbonates and creates CO264,65. Adding 1.6 mol of EC into the reaction
mixture produced only BHE-BPA65.
3.4.2.2. Methanolysis
Methanolysis, like hydrolysis, is challenging due to the insolubility of PC in methanol. Therefore, traditional methanolysis catalysts cannot be reused and lead to equipment corrosion as a result of the high pressures and temperatures needed. Piñero et al aimed to develop a continuous process of methanolysis that could be scaled up using methanol in supercritical or near supercritical conditions as a solvent/reagent and, as a catalyst NaOH. Total depolymerization of PC took place at 120-140 °C with 1.5-2 kg m-3 of NaOH in methanol. A yield of 35% of DMC was reported and was even lower at
low pressures and high water concentrations. BPA was obtained in 80-90% yield demonstrating that a continuous process can be used in the recovery of BPA. Crystallization in water afforded pure BPA crystals66. Jie et al also reported using supercritical ethanol to recover monomers BPA and diethyl
carbonate (DEC) as ethanol has a lower critical point than methanol. reaction temperatures ranged from 290 °C in a high pressure autoclave batch reactor to recover 89% DEC and 90% BPA after 50 minutes59.
Liu tested the potential of several ionic liquids. [Bmim][Cl] performed the best giving a yield of 95% of both DMC and BPA at 105 °C for 2.5 hours with the weight ratio of 2:3:2 PC:methanol:[Bmim][Cl]55.
Later on Liu synthesized a new IL [Bmim][Ac] which had better catalytic activity than [Bmim][Cl]. The monomer yield was obtained in the same time at 90 °C instead of 105 °C. The reusability of the IL was tested and no significant decrease in activity was experienced57.
Similar to hydrolysis, the use of co-solvents is also a technique in the methanolysis of PC. Hu et al used a mixture of methanol and toluene in a 1:1 ratio with a catalytic amount of NaOH to depolymerize PC. 96% BPA crystals and 100% DMC in solution were produced after 70 minutes at 60 °C67. Liu also used co-solvents in the alkali catalyzed methanolysis. N-methyl-2-pyrrolidone,
1,4-dioxane, THF or 1,2-dichloroethane were utilized in a solution with NaOH and methanol. THF gave the highest yield of 95% BPA and 81% DMC. The solvent ratio again proved to be an important parameter and the ideal mass ratio was PC:methanol = 1:1, PC:NaOH = 50:1 for 35 minutes at 40 °C58.
Generally, high amounts of catalyst loadings are required to recover BPA efficiently. Do et al reported the use of 1,5,7-triazabicyclo [4.4.0]-dec-5-ene (TBD) in various alcohols under mild conditions with reduced catalyst loading (Figure 12). Methanol as a solvent provided the best yield with >98% of both DMC and BPA. Furthermore, TBD could also depolymerize PC using DMC as a solvent, reducing the need for more auxiliary solvents56.
Figure 12 Alcoholysis of PC with a TBD catalyst as reported by Do et al56
3.4.3. Aminolysis
Hata carried out the aminolysis of PC to produce 1,3-dimethyl-2-imidazolidinone (DMI) and BPA both at >94% yield at 100 °C. N,N’-dimethyl-1,2-diaminoethane was used at the aminolysis agent in dioxane both with sodium carbonate as a catalyst. The reaction could also be carried out in DMI reducing the need for other solvents while keeping high yield of 91% for both monomers68.
3.4.4. Ammonolysis
Hatakeyama performed aminolysis of PC using a dilute aqueous ammonia solution in a semi batch reactor at 160 °C at 10 MPa. The reaction was performed twice, one with 0.6 mol/kg of ammonia and one with 0.6 mol/kg of NaOH. Results showed that ammonia obtained a yield of 93.8% BPA in 80 minutes while the NaOH only obtained a yield of 20% in 120 minutes. Kinetic studies showed that ammonia acted as a nucleophilic reagent69.
3.5. PET copolyesters
3.5.1. Poly-ethylene Isosorbide
Terephthalate
Isosorbide can be prepared from glucose in two steps. The first step is glucose
hydrogenation followed by sorbitol dehydrogenation70. It is industrially
produced from starch8. Inserting a rigid
monomer like isosorbide in the polymer backbone is advantageous for increasing the glass transition temperature (Tg) of the
polymer71. This makes PEIT useful for many
different applications and to fill the demand for inexpensive bottles with a high Tg that are able to
withstand being hot filled and pressurized without being distorted70. Poly-ethylene Isosorbide
Terephthalate (PEIT) is synthesized through the bulk polycondensation of DMT or TPA, EG, and isosorbide (Figure 15)8.
3.5.1.1. Enzymatic Degradation
Boisart et al patented a process wherein PEIT is chemically recycled through enzymatic degradation. The enzyme is derived from a recombinant microorganism, or an organism whose genome had been modified. The PEIT is placed in contact with the culture medium containing the recombinant microorganism, a carbon source (such as glucose), and a nitrogen source (such as ammonium chloride) at temperatures between 25–50 °C for 5 to 72 hours. This method may be used to simultaneously to depolymerize a mix of two polymers using two different enzymes from recombinant microorganisms9.
A process wherein the polymer is amorphized in an extruder to decrease the crystallinity of the polymer before depolymerizing was described. The amorphization is performed by subjecting the polymer to temperatures above the melting temperature then subjecting to below glass transition temperature and shear stress. The depolymerization step is performed using a depolymerase72.
Up to now there have not been enzymes to degrade polymers in pellet or film form, only in emulsion form. Li et al isolated a new polypeptide that has PEIT degrading ability from strains of
Micromonospora sp or a recombinant microorganism that has been modified to produce the
polypeptide. Furthermore, an auxiliary polypeptide was identified that did not degrade PEIT, but instead decreases the temperature that the degrading enzyme is active in. Based on the excellent Figure 13 Ammonolysis of PC and its products120
properties, this technology may be applied industrially. The depolymerization was performed at 37 °C for 24 to 72 hours73.
Alvarez et al patented a similar process except using polypeptides from the
Actinomadura sp. An auxiliary polypeptide
was not used in the process. The enzyme could be used on films and pellets at 45 °C74.
Topham patented a process wherein the amino acid sequence of an esterase is modified. This was done by substituting some amino acid sequences at specific positions. The depolymerization is performed at 60 – 70 °C75.
3.5.2. Eastman Copolyesters
Eastman Chemical Company is one of the world leaders in material production. It is an American company whose main industry is in the synthesis of fibers, chemicals and plastics76. In 2018, their $10 billion revenue
was largely due to the sale of plastics and
other polymeric material77. In 2007, Eastman developed a new class of copolyesters called Eastman
Tritan™ Copolyesters featuring properties such as good toughness, easy processability, and improved chemical and heat resistance. It can be used in a wide array of applications such as helmets, bottles, appliances and in the automotive industry78. Eastman Tritan™ Copolyesters are synthesized
from three monomers, DMT, 1,4-cyclohecanedimethanol, and 2,2,4,4-tetramethyl-1,3-cyclobutanediol. Ratios vary depend on the desired application79. Eastman Chemical Company have
several patents in the recycling of polyesters. They may be used for a wide array of polyesters, including PET and Tritan.
3.5.2.1. Alcoholysis
3.5.2.1.1. Glycolysis
A process of glycolysis was patented by Eastman Chemical Company. The process includes exposing the polyester to a glycol in a molar ratio of 1:5 glycol:dicarboxylic acid units at temperatures of 150– 300 °C at pressures ranging from 0.5 to 3 bars. An agitated reactor vessel is used. The reaction produces an upper layer of low-density contaminants and a lower layer of oligomers from the polyester which are removed in separate streams. It is particularly useful for other contaminants such as non-polyester polymers, aluminum, sand, glue, and other chemicals7.
3.5.2.1.2. Methanolysis
Eastman Chemical Company patented a process to chemically recycle polyesters using superheated alkyl alcohols (methanol, ethanol etc.) to a mixture of the recyclable polyester dissolved in its component monomers or oligomers to produce a melt. Several processes patented by Eastman chemical company apply this technique of depolymerization80–82. However, they differ in the type of
reactor used in the process. Pell et al make use of a counter-current reactor80. Gamble et al (1994)
use a jacketed reactor with agitator81. Gamble et al (1996) employ a staged column reactor82. The
patent of Pell et al also describes a process of hydrolyzing DMT with a ratio between 0.19:1 to 6:1 water to DMT to produce TPA which can be crystallized from the reactor80.
Another process eliminates the need to return products that have not been completely depolymerized (so called half-esters) back into the reactor. The polyester is depolymerized with the use of supercritical alkyl alcohols to obtain a mixture of half-esters and the component monomers. This mixture is subjected to transesterification after removing impurities to obtain a low-weight polyester that can be used as a feedstock either in TPA or DMT manufacturing processes. The advantage of this process is that it requires less equipment83.
3.6. Biobased Polyesters
3.6.1. Poly(L-lactic acid)
Poly(L-lactic acid) (PLA) is a biobased, biodegradable polymer that has recently become an attractive, economically viable alternative to petroleum-based plastics84. It is made from the
ring-opening polymerization of lactide or the condensation of L-lactic acid, both of which can be procured from the fermentation of sugars85. Both routes, however, produce the same lactic acid oligomer as an
intermediate86. Furthermore, its good processability allows it to be easily injection molded and
extruded into various products87. It has found uses in applications such as bottles, trays, and films for
food packaging due to it thermal and mechanical properties88. Its biodegradability and non-toxicity
allows PLA to be a material of interest for scaffolds, threads, and micro and nanoparticles for biomedical applications89. For these applications, 217 000 tons of PLA was produced in 201890. With
the rapid growth in the market for bioplastics, an issue that arises in the development of a technologically viable recovery system that is efficient and economical that does not interfere with the current recycling systems set in place. Using renewable feedstock does not provide the necessary sustainability benefits if the material cannot be recycled87.
3.6.1.1. Hydrolysis
Faisal used high temperature and high pressure water to recover L-lactic acid from PLA at subcritical conditions. The highest yield of 92.5% L-lactic acid was obtained at a temperature of 250 °C for 10-20 mins. Lower temperatures produced succinic acid and higher temperatures resulted in a racemic mixture of L-lactic acid an D-lactic acid along with the formation of propionic and acetic acid as by products88. High-temperature and high-pressure water was also used by Tsuji et al to hydrolyze PLA
in the melt. The maximum yield attained of L-lactic acid was 90% at 250 °C for 10–20 minutes. The hydrolysis activation energy of PLLA decreased from 20 kcal mol-1 to 12.2 kcal mol-1 in the melt than
as a solid84.
Lazzari et al studied the degradation mechanisms in basic and acidic conditions. In acidic conditions, hydrolysis is catalyzed by presence of protons to cleave the ester bonds, lactic acid was the main product formed. At a higher pH, a backbiting mechanism dominates and leads to the formation of lactide89.
To combat the use of high temperatures, Okamoto optimized the catalytic depolymerization of PLA using the readily available, environmentally benign clay catalyst montmorillonite K10 (MK10). MK10 is a layered structure that consists of negatively charged aluminosilicates containing a catalytic proton and bound water molecules in between and on the surface of the layers. The reaction at 100 °C in toluene for 6 hours produced a high yield of a lactic acid oligomers and L-lactic acid monomers that could be readily polymerized into high molecular weight PLA. Adding ethanol into the reaction
mixture accelerated the depolymerization. The MK10 could be used up to 5 times with no decrease in activity86.
A similar catalyst, montmorillonite K5 (MK5), was studied by Tsuneizumi et al to degrade typical PLA blends, PLA/PE and PLA/PBS. The PLA in the PLA/PE blend was depolymerized in a toluene solution at 100 °C for 1 hour forming the lactic acid oligomer. This had no effect on PE which was recovered through reprecipitation. PBS and PLA were separated based on their solubility differences in toluene. PBA was degraded with lipase and PLA was again degraded with MK5. The oligomers formed could be repolymerized into high molecular weight PLA91.
Cozach et al patented a process wherein PLA is ground, dissolved in a lactic ester, and then subjected to hydrolysis. By
dissolving PLA in lactic ester, solid impurities, for instance other polymers that do not dissolve, are removed, therefore a polymer blend containing PLA is also viable for this process. Hydrolysis is carried out at temperatures ranging between 120 – 140 °C at a pressure between atmospheric pressure and 10 bar to obtain almost 100% yield of lactic acid92.
3.6.1.2. Enzymatic Degradation
Lipase has been reported to form cyclic oligomers, hence, Takahashi made use of lipase to depolymerize PLA, PDLA, and PDLLA in organic solvents. Degradation was affected by the solvent type and the duration and temperature of the reaction. PDLLA was readily degraded in a 3/7 mixture of chloroform/hexane at 60 °C. Meanwhile, PLA needed a higher temperature of 100 °C in o-xylene with small amounts of water93.
Jarerat et al found that the bacteria Amycolaptopsis orientalis could be induced into an enzyme that exhibited a significant PLA degrading activity in a liquid basal medium that contained proteins, peptides or amino acids94. The A. orientalis could be economically produced by batch cultivation in a
liquid basal medium with silk fibroin powder. It acted as a stereospecific enzyme and prevented the formation of D-lactic acid. Once impurities were removed, the oligomers and L-lactic acid could be readily repolymerized into PLA95. Though these methods take advantage of low temperatures, dilute
lactic acid solutions are obtained that contain enzyme impurities. Hence, a substantial amount of energy is still needed to concentrate the solutions and remove impurities95.
3.6.1.3. Alcoholysis
3.6.1.3.1. Methanolysis
Due the insolubility of PLA in alcohols, often harsh reaction conditions are required in the alcoholysis of PLA. To contest this, Song et al investigated the use of ILs. Methyl lactate could be produced in up to 92.5% yield in a 5:1:0.02 methanol:PLA:[Bmim][acetate] ratio for 3 hours at 115 °C. Methyl lactate
could be easily purified through distillation96. Hirao et al performed alcoholysis under microwave
irradiation in ethanol and butanol. Butanol is typically used in esterification to purify lactic acid as it forms a split-phase with water for easy separation. Kinetic studies showed that the activation energies remained the same in conventional vs microwave heating, however the k0 values were much
higher under microwave irradiation. Furthermore, the rate determining step was determined to be the nucleophilic attack by the alcohol, therefore, ethanol had a faster higher k0 value due to the
hydroxyl group’s mass ratio to the whole molecule (Figure 17)97.
Figure 17 Mechanism of the alcoholysis of PLA97
Coszach et al also patented a process similar to the hydrolysis described earlier. PLA was first dissolved in a lactic acid ester, this is advantageous as the dissolution could be conducted at higher temperatures than the dissolution of PLA in alcohols as the esters generally have higher boiling points. Following this the alcoholysis was performed at 120–140 °C between atmospheric pressure and 1 MPa using a transesterification catalyst (Lewis acid) with an alcohol with the number of carbons corresponding to the carbons on the lactic acid ester. A yield of almost 100% is attained98.
Major concerns arising with the growth of PLA products is that it will disrupt the recycling methods currently in place, for instance, PLA and PET bottles can only be distinguished using near infrared sensors. The mixing of these polymers will decrease the efficiency of recycling techniques. Hence, Sanchez et al investigated the chemical recycling of a mix of PET and PLA. PLA was first depolymerized into methyl lactate with a 70% yield in methanol at 65 °C for 15 hours in the presence of zinc acetate. The depolymerization of PLA in a mixture with PET decreased the monomer yield by 5%. The yield for the same reaction in ethanol was lower (21%). PET was not depolymerized under these conditions but was instead removed through filtration and subjected to glycolysis to obtain BHET99.
3.6.2. Poly-(R-3-alkanoate)
Poly-(R-3-alkanoate) (PHA) are produced by various microorganisms in nature as a reserve carbon and energy storage when placed in environmental conditions lacking nutrients and other carbon sources100,101. PHAs, in the beta position, contain an asymmetric carbon making them optically active
polymers, and, in nature only contain the R enantiomer 101. Figure 18 shows the general structure of
PHAs, the R group typically contains up to C15 hydrocarbon chains or just hydrogen102. The most
common PHA is poly(3-hydroxybutyrate) (PHB)100,103.
PHAs are biopolyesters that are highly stereoregular and crystalline103. PHAs are promising
biodegradable plastics as they are readily available through a one-step fermentation process. They also originate from renewable resources104. Typically from beet sugar in the EU102. Furthermore, they
have physical properties that allow them to behave as thermoplastics or elastomers by varying the composition of the R group. However, their commercialization is limited by low yields and weak
mechanical properties (brittle). To combat this, often PHAs are copolymerized to yield copolymers with improved properties105. PHAs comprise 1.4% of the global plastic production106.
PHAs have been chemically recycled through thermal degradation methods and pyrolysis, however, this technique yields crotonic acid (CA) oligomers containing an unsaturated and a carboxylic acid chain end, and cyclic oligomers. CA cannot be used as a feedstock to synthesize polyesters, however, it can be used as a comonomer, particularly with vinyl acetate, in the industrial production of paints, coatings, adhesives, binders, ceramics, and agrochemicals104,107. 3-hydroxybutytric acid (3HB), the
monomer for PHB synthesis, is produced through hydrolysis100.
Biological processes by both microbial and enzymatic activities are currently considered as the sustainable chemical recycling method for green plastics. This includes composting, the enzymatic transformation of polyesters into useful materials, and the chemical recycling via polymerization intermediates.104. Furthermore, thermal degradation produces a product range with a high
polydispersity index108. Hence, the chemical recycling of PHAs will be discussed further.
Figure 18 Chemical structure of a) PHA and b) PHB102
3.6.2.1. Hydrolysis
PHB granules produced by bacteria are composed of a solid shell consisting of lamellar crystals and a non-crystalline core. Lauzier examined the degradation mechanism of acid hydrolysis to obtain PHB oligomers. In the first stage of degradation, random scission reorganizes the granules into lamellar morphology and in the second distinguishable weight loss occurs. A strong acid (3.0M HCl) and a temperature of 104.5 °C significantly reduces the Mn in the first stage with slow weight loss
(20-25%) proving instant penetration of the protonated water molecules into the granule shell. The small weight loss is a results of the competition between solubilization and recrystallization. More appreciable weight loss was observed after 6 hours, entering the second stage of degradation. PHB proved to be stable in room temperature even in the presence of strong acid109.
Yu et al studied the mechanism and kinetics for the acid and base catalyzed depolymerization of PHB. Alkaline hydrolysis showed a parallel formation of 3HB and CA, however, acid hydrolysis showed that more CA was formed over longer periods of time (2% 3HB and 90% CA after 14 hours) indicating that CA was formed from the dehydration of 3HB in acid. PHB was more susceptible to hydrolysis under basic conditions. In dilute acid conditions, the bond hydrolysis could be re-esterified as protons are catalysts for such a reaction. Adding small amounts of alcohols blocks the re-esterification as the hydroxyl anion lowers the ester bond breakage energy barrier103. Yang et al
experimented on the use of different green solvents (water, methanol and ethanol) to achieve high degradation rates in the alkaline-catalyzed hydrolysis to produce PHB monomers or oligomers. Alkaline methanol provided the best degradation and completely depolymerized PHB into 3HB, 3-methoxybutanoic acid (3MB), and CA. Prolonged reaction time afforded a higher 3HB yield as 3MB reacted with methanol to form it. The rates were higher than that of thermal degradation 100.
3.6.2.2. Enzymatic Degradation
Abe et al examined the effect of stereoregularity on the enzymatic degradation of PHB with a PHB-depolymerase from the bacteria Alcaligenes faecalis. It was found that the enzyme had a high affinity towards isotactic PHB polymers. The products of degradation were monomers, dimers, trimers, and tetramers of 3HB110. Kaihara et al examined the potential of lipases from Candida Antartica to
transform poly[R-3-hydroxybutyrate)-co-12%(R-3-hydroxyhexanoate)] and poly[R-3-hydroxybutyrate)-co-12%(R-3-hydroxyvalerate) into oligomers. A high yield of oligomers were obtained in diluted organic solvents at 70 °C. When repolymerized, the concentrated oligomer solution achieved the same polyester. Lipases showed a faster degradation rate than PHB-depolymerase. The polymers were of too low molecular weight for practical applications, therefore, more research is needed for this technology to be viable104.
Figure 19 Synthesis and depolymerization routes of PHB104
Poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P(3HB-co-4HB)) is another common PHA copolymer. Rodriguez et al used two lipases (Lipase AK and Lipopan Conc BG) to convert high molecular weight P(3HB-co-4HB) into oligomers. Lipopan Conc BG is a 1,3 regiospecific lipase, it had better activity and specificity as the original molecular weight peak disappeared after 72 hours. Both lipases did not distinguish between the ester bond of the primary or secondary alcohols 111. In nature,
only the R configuration of PHB exists, however, synthetically, both R ans S configurations of PHB can be produced. This affects the degradability as 100% S-PHB was unable to be degraded by PHB-depolymerases, however 67-77% R configuration showed faster degradation than natural PHB due to reduced crystallinity112.
3.6.2.3. Alcoholysis
3.6.2.3.1. Methanolysis
Acidic methanol was used to perform depolymerization by de Roo et al. A mixture of methyl esters were obtained at 100 °C after 86 hours. Main products included hydroxyoctanoic acid (37.7%), hydroxybutyric acid (6%), hydroxydecanoic acid (29.6%), hydroxydodecanoic acid (9.4%) and 3-hydroxytetradecanoic acid (4%). The various methyl esters could be easily separated through distillation. Subsequent saponification of the esters provided the acid homologues 101.
Song et al investigated the use of acidic functionalized ILs in methanol to depolymerize PHB. It was found that 1-methyl-3-(3-sulfopropyl)-immidazolium hydrogen sulfate ([HSO3-pmim][HSO4]) increased the rate of degradation as compared to common acids such as H2SO4 and H3PO4. Methyl
3-hydroxybutyrate was the main product formed in 83.75% yield at 140 °C for 3 hours113.
3.6.3. Poly-Butylene Succinate
Poly-butylene succinate (PBS) is synthesized in two steps, first, succinic acid (SA) and BD undergo esterification or dimethyl succinate (DMS) and BD undergo transesterification to produce low molecular weight oligomers. The oligomers can be polymerized through polycondensation to form high molecular weight polymers114. Succinic acid is a bio-based chemical that is derived from the
fermentation of sugars, it is one of the top twelve bio-based chemicals that have market potential. Furthermore, BD is easily produced by the reduction of succinic acid. Therefore, a fully bio-based PBS is easily accessible115.
Figure 20 Structure of PBS116
PBS find use in food packaging, bottles, hygiene products and films. Oligomers of PBS may also be used as polyurethane elastomer building blocks114. PBS is characterized by strong mechanical
properties and good thermal stability owing to its high degree of crystallinity. However, because of this crystallinity, PBS is slowly biodegradable116. Hence, chemical recycling techniques are necessary to
recycle PBS.
3.6.3.1. Hydrolysis
The effect of crystallinity on the alkali-catalyzed hydrolysis was studied by Cho et al by inducing morphological changes on the PBS. Degradation studies found that PBS with spherulites containing tightly packed fibrils were slower to degrade. Furthermore, they concluded that the rate of hydrolysis is faster in the amorphous region than in the crystalline region117. Hydrolysis in the solid state is
difficult for highly crystalline polymers such as PBS. Hence, Tsuji et al examined the hydrolysis with high temperature and high pressure water with PBS in the melt. A maximum yield of 80% SA and 30% BD was attained at 270 °C for 25 minutes without the presence of a catalyst. The low yield of BD was attributed to its low stability at this temperature118.
3.6.3.2. Enzymatic Degradation
Biodegradable polymers have been previously reported to be have their ester bonds cleaved by hydrolases in aqueous environments. Recombination of the cleaved bonds can then take place in conditions with limited water. The potential for PBS to exhibit the same behavior was examined by Okajima et al. PBS was degraded using lipase in a solution consisting of organic solvents containing small amounts of water. Cyclic diester oligomers were the main products of the depolymerization at 85 °C after 24 hours. Repolymerization using lipase as a catalyst in water poor conditions afforded PBS with a MW of 21 000 in 75% yield, however, the initial starting Mw was 99 000119.
Figure 21 Enzymatic degradation of PBS93
Jbilou et al aimed to develop a continuous process to chemically recycle PBS. Extractive extrusion was used with PBS in the melt, to eliminate the use of solvents in the presence of the enzyme Candida
antarctica lipase. A twin extruder showed a high rate of depolymerization due to more homogeneous
efficiency. 44% SA was produced using 10% of lipase after 30 minutes. Further optimization is necessary for extensive degradation115. Upon using enzymatic degradation on multiblock copolymers
of PBS with aliphatic diacids or diols, it was found that they degrade faster than PBS homopolymers116.
3.6.4. Poly-ethylene Furandicarboxylate
Poly-ethylene furandicarboxylate (PEF) is a biobased alternative to PET. Instead of using pure TPA, 2,5-furandicarboxylic acid (FDCA) is used with EG in the synthesis of PEF. FDCA is recognized as one of the twelve sugar-based building blocks that hold potential for the production of biobased materials by the US Department of Energy. FDCA is produced from the oxidation of 5-hydroxymethylfurfural (HMF) which can be obtained from the dehydration of fructose with an acid catalyst. HMF is not commercially available as it is unstable in the acidic conditions needed to synthesize them and further reacts to form formic and levulinic acid 14. Avantium Chemicals B.V. developed a method to
convert solid materials containing hemicellulose, cellulose and lignin through various steps of acid hydrolysis into high yields of 5-(chloromethyl)furfural. This can be used to produce HMF120.
PEF films have a higher oxygen and water vapor barrier than PET films. Furthermore, they have improved mechanical strength and are completely transparent making them useful in applications such as food packaging for products sensitive to oxygen and moisture such as meat, dairy, cereals and personal care or medical products14.
3.6.4.1. Hydrolysis
A study by Furanix Technologies found that furan dicarboxylate-containing polyesters depolymerize faster than PET. Furanix Technologies have patented a method of hydrolysis wherein PEF, that may or may not contain another diacid, including, but not limited to terephthalic acid may be depolymerized into the FDCA and diol. Before hydrolysis, PEF is dispersed in water to form a slurry. PEF is then reacted with excess water with a depolymerization catalyst at temperatures between 18 – 350 °C. Catalysts that can be used include mineral acids, metal compounds, however, the most preferred catalysts are homogeneous basic catalysts such as metal hydroxides, metal alkoxides, metal carboxylates and/or metal carbonates, though organic basic catalysts are also suitable. The reaction can be performed in a batch reactor, or, the more commercially advantageous alternative, in a continuous reactor wherein a polymer stream is continuously being fed into a reactor and a product stream is withdrawn. The monomers can be obtained through precipitation and filtration or distillation. When water and a base are used the free dicarboxylic acid can be obtained through treatment with a Bronsted acid121.
