by Julie Lombard
The extremely rapid increase in mass production of plastic from six decades ago led to 8.3 billion metric tons of plastic being produced, most of it existing as disposable products, shortly discarded as waste after a brief life-time. As traditional plastic takes over 400 years to degrade, most of this waste still exists on the planet in some form and the amount of plastic which is thrown away each year is enough to circle the Earth four times. An average UK household throws away at least 40 kg of plastic per year, and nationwide, we use 7.7 billion plastic bottles per year, which is an average of 117 bottles per person, per year. According to The Ocean Cleanup, 5 trillion pieces of plastic waste float in the world’s oceans, affecting over 1 million marine animals such as fish, turtles, mammals and rendering extinct over 700 species. The plastic waste problem has been a prompt which continues to shape the focus and efforts of the materials and consumers industries. The United Nations have been working on methods to reduce plastic growth by hand-wringing and corporate pledges but the demand for plastics grew a further 3.5% in 2019. Latest figures also demonstrate that less than 10% of plastic is recycled leaving an overall over 90% of worldwide produced plastic as waste.
Almost all aspects of daily life in the modern world involve plastics or rubber in some form or the other. The multiple functions served by plastic products in modern society are irreplaceable, and as certain features of plastics are essential for their applications. Plastics are made of polymers, which allow for a wide range of properties, thus their use in markets ranges from simple plastic bags to home commodities. A promising solution to the plastic issue is, therefore, the design of plastics which can maintain the properties offered by the material while having the ability of degrading under pseudo-natural conditions. The key to the usefulness of these materials is the ease of unharmful and uncontaminating disposal after their use. Candidates for the structural composition of these degradable plastics are biodegradable polymers.
Polymers Polymers are long chains of repeating monomer units (small organic molecules) that have different properties which make them very suitable for a number of applications in today’s world. The reaction involved in the making of polymers is called polymerisation and it consists of the addition of monomer units to result in the formation of a long macromolecule. There are different types of polymerisation reactions which depend on the monomers used, and which are performed under different conditions in the industry. The main polymers which are used in the plastic industry are illustrated in Table 1 below together with their starting monomer units and their most common applications.
Table 1: Main polymers used in the plastic industry, their monomer and their common uses.
Polymer Monomer Uses
Polypropylene Propylene Product packaging, automotive industry and textiles
Polystyrene Styrene Lightweight protecting disposable packaging, home and appliance insulation
Polyvinyl chloride Vinyl chloride Building constructions, electronics, health care appliances, wire/cable insulation
Polyethylene Ethylene films, tubes, packaging, plastic bags Urea-formaldehyde Urea and formaldehyde Wood products, coatings, textiles industry
Rubber Isoprene Tyres, elastic materials
BUNA - S / BUNA - N Butadiene, Styrene/Vinyl Cyanide Synthetic rubber
Teflon Tetra Fluoro Ethane Non-stick-cookware-plastics
Nylon-6 Caprolactam Fabric
Bakelite phenol and aldehyde Automotive components, industrial applications, electrical appliances, kitchenware products
For the purpose of this investigation, the two polymers that were found to be most relevant to the plastic problem of the 21st century were chosen: polyethylene and polypropylene.
Polyethylene (PE)
The structure of PE is made up of repeating units of the ethylene monomer (a small organic molecule consisting of 2 C and 4 H atoms) and is synthesised by organic polymer addition as illustrated by Figure 1 below.
Synthesis conditions for the reaction require very high pressures and temperatures, with the addition of an organic peroxide radical initiator. The initiator can stimulate the breaking of the double bond between the two C atoms of the ethylene monomer, which results in a free radical (very reactive intermediate species with an unpaired electron) with the ability to join the next ethylene molecule forming a chain of which length keeps increasing via the repeating propagation mechanism. As a result of this reaction, large polymer chains (macromolecules) start forming. The mechanism for the reaction involving initiation, propagation and termination steps is presented by Figure 2 below.
The resulting PE chains are not covalently joined but instead are held together in a crystalline structure by intermolecular forces. The overall structure of the chains (linearity/branching) together with the intermolecular forces acting between these results in the physical and chemical properties of the materials formed. The two most common types of PE are high density and low-density PE (HDPE and LDPE, respectively) and these have different chemical and physical properties which make them useful in a range of applications. LDPE has a lower number of side branches and therefore lower crystallinity as the packing of chains is less efficient. LDPE has a melting point of 105-115℃, good weather ability, very low water absorption and is, therefore, suitable for use in packaging, pipes and fittings. HDPE has a higher melting point of 120-140℃ thanks to the strength of intermolecular forces increasing with greater linearity of the polymer chains. HDPE is very chemically resistant to solvent and has a high tensile strength and very low water absorption. It is therefore also used in packaging as well as consumer/household goods, fibres and textiles, together with some applications in the automotive industry.
Polypropylene (PP)
The structure of PP is made up of repeating units of the monomer propylene (a small unsaturated organic molecule) by chain-growth polymerization, also using the Ziegler-Natta and Metallocene catalysts as illustrated by Figure 3 below.
Two main types of PP are homopolymers and copolymers which are further divided into subclasses. Homopolymers are made of the same type of monomer unit providing strong intermolecular forces between the PP chains, which allows these materials to have a high melting point of 160-165℃ and are lightweight and have a density of 0.904-0.908 g/cm³. They have a great strength to weight ratio and are overall stiffer and stronger than copolymers; they have good processability and impact resistance. PP copolymers are made up of two monomer species and present weaker intermolecular forces between the chains, therefore, leading to their lower melting points of 145-150℃ and a density equal to or lower than homopolymers (ex. impact copolymer 0.898-0.900 g/cm³). They are softer and have a better impact strength, are tougher and more durable. They also have high processability and impact resistance. PP is used in a wide range of applications such as food packaging, film and fibre and it is also used for injection moulded products ranging from kitchen objects to car pieces.
Thanks to the use of inexpensive Ziegler-Natta and Metallocene catalysts, the synthetic processes for PE and PP have low costs and offer efficient processability. Despite large fluctuations in the consumer price of these materials, the national average cost of PP in 2014 was 311£ per ton and the current average costs of PE is £0.38/kg which explains their extremely high demand for applications. Like most wide-application polymers used today, the properties of both PE and PP make them incredibly durable and these molecules are non-biodegradable in the environment. Due to this, recycling of these polymers is unfavourable in economic terms and unattractive for private investors, therefore highly contributing to the world's plastic waste problem. Recycling of these materials is possible: current improvements in the recycled market in the UK presented an average price for HDPE of £1220 per tonne and an average for LDPE is £1193 per tonne. Due to these costs, cheaper polymer production continues being instigated.
Biodegradable Polymers
The same properties making ‘traditional’ plastics essential to our everyday life, such as strength, stability and durability, are also those which make them prone to contributing to a worldwide environmental issue. A lot of research has been done in the attempt of solving the problem of plastic pollution in the modern world, and one of the most prominent areas of study has been that of biodegradable polymers. By chemically designing the initial monomers, the total lifetime of a plastic can be controlled and modified to the desired purpose, whether this is not to degrade at all (building and transport industry) or to degrade after a single use (such as disposable cutlery and plastic bags). Developments in technology and high consumer demand for sustainable packaging have led to the growing production of biodegradable materials which are competitive alternatives and substitutes of common plastics. Biodegradable polymers can be bio-based or fossil fuel-based and have the ability of decomposing in a controlled composting environment or even under natural environmental conditions. Some bio-based and fossil-based biodegradable polymers are presented in Table 2 below.
Table 2: Main bio- and fuel-based biodegradable polymers, their monomer and their uses.
Biopolymer Monomer Uses
Bio-based
Starch glucose Foams, films & bags, moulded items
Polyhydroxyalkanoates C13, C12, C11, and C10 monomers moulded items
Cellulose and cellulose acetates Beta Glucose, Acetate composites fibre-board
Fatty acid polymers fatty acids and glycerol composites, adhesives, compatibilizers
Lignin polymers p-hydroxyphenyl (H), guaiacyl (G) composites, adhesives, compatibilizers
and syringyl (S)
Protein polymers H,N,O,C compatibilizers films
Polylactic acid lactic acid injection, moulding, fibres
Fossil fuel-based
Polycaprolactone ε-caprolactone compostable bags, medical application (sutures and fibres), surface coatings, adhesives, stiffeners, orthopaedic splints.
Polybutylene adipate polymer of 1,4-butanediol additive for biodegradable
terephthalate and adipic acid and the polymer plastics for flexibility
of dimethyl terephthalate (DMT)
with 1,4-butanediol
Polybutylene succinate Succinic acid, Butanediol food packaging like disposable tableware, paper cups and gas barrier packaging, agricultural mulch film
Polyvinyl alcohol vinyl acetate packaging films (replace LDPE and HDPE), coatings and additives for paper and board production
Biodegradability
The chemical bonding within the functional group (side chain) of the individual monomer units of a polymer is essential in the determination of the ease of degradation of a material by biological agents. The driving force of biodegradation reactions is the use of C within the polymer chain (expressed as Cpolymer) by microorganisms in their life cycles (expressed as Cbiomass). The reactions taking place are overall quite complex, however, under aerobic conditions, they can be simplified as described by Equation 1.
The different processes of biodegradation can be grouped into three main stages (Figure 4): biodeterioration, biofragmentation, and microbial assimilation and mineralisation (Stages I-III).
During biodeterioration (Stage I), loss of the mechanical properties of the plastic material is reduced mainly by abiotic (non-biological) methods, as plastics are generally biologically un-reactive. Methodologies range from the use of physical forces to UV-light or chemical exposure. Quantification of biodeterioration is measured by the changes in elastic and tensile strength together with the change in brittleness of the plastic. Biofragmentation (Stage II) involves the attack of the now enzymatically vulnerable oligomers (shorter chains of the polymer fragments resulting from Stage I) by the work of microorganisms. Two important rate-determining factors of this process are the structure of the functional groups which determine the ease of breakdown, and the enzyme availability as dependent on the microorganisms present. Visual inspection or mass loss are used to monitor this stage. Finally microbial assimilation and mineralisation (Stage III) involves the microorganisms generating biomass and carbon dioxide/methane depending on aerobic/anaerobic conditions. Gas evolution or microorganism biomass increase can both be used as a means of measuring the rate of this final process.
Two Examples
Other than waste management, interest in developing bio-based, biodegradable (compostable) materials has also been incentivized by sustainability and economic advantages. Fossil fuel-based rely on non-renewable raw materials which have been affected by a sharp rise in global prices due to their depletion. This investigation, therefore, focuses on two examples of bio-based, biodegradable polymers, which were found to be the most promising candidates for fighting the plastic problem: Polyhydroxyalkanoates and Polylactic acid.
Polyhydroxyalkanoates (PHAs)
PHAs are a family of biobased and biodegradable natural polyesters produced by an extensive variety of microorganisms, which represent a suitable replacement for fossil fuel-based thermoplastics. The synthesis involves the accumulation of PHA granules during unbalanced growth of microbe fermentation. The general molecular structure of PHA together with an example of a short-chain (C3-C5) and medium-chain length (C6-C14) as classified by the number of monomeric units are all illustrated by Figure 5.
The most common form of PHA is poly-3-hydroxybutyrate (P3HB) synthesised from extensive chains of hydroxy fatty acid monomers. P3HB is naturally produced by bacteria and other living microorganisms from biorenewable resources such as carbohydrates and fats. P3HB is a highly crystalline, linear polyester of 3-hydroxybutyric acid and is generated as a carbon reserve in a wide variety of bacteria. It is produced industrially through fermentation of glucose by the bacterium Alcaligenes eutrophus as presented by Figure 6.
Alternatively to this route, chemical synthesis of the material is also possible via ring-opening polymerisation (ROP) of cyclic esters as catalysed by metal-based or organic catalysts.
Biodegradability and the optimal conditions for the degradation processes of PHA strongly depend on the solid-structure of the material. Degradation rate can therefore be controlled by chemical structure and solid properties of the specific type of PHA polymer synthesised. P(3HB) is classified as a semi crystalline thermoplastic (melting temp. 180°C) and the microorganism R. pickettii was reported to hydrolyze it at 37°C, with the degradation step followed by erosion of chains in the crystalline state. The rate of enzymatic erosion of P(3HB) increases with a decrease in the material’s crystallinity as reductions of the extent of order in the structure may be decreasing strength of the polymer and other mechanical properties.
Great research efforts have been dedicated to the decrease in production costs of PHAs, however the current cost of the biopolymer is estimated to be 3-4 times higher (£1.81-2.21/ld) than that of traditional plastics such as PP and PE (£ 0.48-0.70/Id), therefore still presenting an economic challenge to its use.
Polylactic acid (PLA)
PLA is a thermoplastic and long-chain polymer made through the fermentation of bacteria and derived from the lactic acid monomer. The latter is a by-product or intermediate product of plant metabolism, however, it can also be industrially produced from a number of starch or sugar-containing agricultural products, such as cereals. Lactic acid production based on microbial carbohydrate fermentation is the most chemically and economically favourable synthesis method. PLA can be produced by three different mechanisms: ring-opening polymerisation, direct polycondensation and direct polymerisation. Condensation polymerisation and ring-opening polymerisation are the two most common routes as shown by Figure 7 below.
The condensation process (A → C) is carried out at a temperature below 200 °C; above this temperature, the entropically favoured lactide monomer (B) would be generated as shown. As this is a condensation reaction, one equivalent of water is generated for every esterification step. Ring-opening polymerization (B → C) of lactide (structure B) involves the use of various metal catalysts in solution or as a suspension. The metal-catalyzed reaction tends to cause racemization of the PLA, reducing its stereoregularity compared to the starting material. As the production methods described are both relatively simple and inexpensive, PLA is one of the cheapest bioplastics, therefore providing a competitive alternative to traditional plastic materials. As a brittle material, additives or polymers are used to make PLA suitable for its uses.
References
● https://www.livescience.com/60682-polymers.html ● https://byjus.com/jee/polymers/ ● https://www.creativemechanisms.com/blog/all-about-polypropylene-pp-plastic ● https://www.livescience.com/60682-polymers.html ● https://byjus.com/jee/polymers/ https://www.scientificamerican.com/article/how-are-polymers-made ● https://www.nature.com/subjects/polymer-synthesis ● https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Map%3A_Organic_Che mistry_(Vollhardt_and_Schore)/12%3A_Reactions_to_Alkenes/12.15%3A_Synthesis _of__Polymers ● https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Structure/ polymer.htm ● https://www.bbc.co.uk/bitesize/guides/ztr7b82/revision/2 ● https://www.thebalancesmb.com/what-does-biodegradable-mean-2538213 ● http://www.pepctplastics.com/resources/connecticut-plastics-learning-center/biodegra dable-plastics/ ● https://www.sciencehistory.org/science-of-plastics ● http://polymer.chem.cmu.edu/~kmatweb/2003/Mar/02Okadabiodegradable.pdf ● https://eng.libretexts.org/Bookshelves/Materials_Science/Supplemental_Modules_(M aterials_Science)/Polymer_Chemistry/Polymer_Chemistry%3A_Chemical_Compositi on/Polymer_Chemistry%3A_Intermolecular_Forces ● https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5445709/ ● https://www.which.co.uk/reviews/recycling/article/what-are-supermarkets-doing-about -plastic ● https://aquasolutionsgib.com/583-2/ ● https://www.conserve-energy-future.com/advantages-disadvantages-uses-biodegrad able-plastics.php ● https://study.com/academy/lesson/what-is-polyethylene-properties-uses-quiz.html ● https://www.reliance-foundry.com/blog/polyethylene-plastic#gref ● https://omnexus.specialchem.com/selection-guide/polyethylene-plastic ● http://faculty.washington.edu/finlayso/Polyeth/Group_B/mech.html ● https://www.essentialchemicalindustry.org/polymers/polyethene.html ● https://plasticker.de/preise/pms_en.php?kat=Mahlgut&aog=A&show=ok&make=ok ● https://ec.europa.eu/environment/waste/studies/packaging/costsbenefitsannexes1_7. pdf ● https://chem.libretexts.org/Courses/Winona_State_University/Klein_and_Straumanis _Guided/11%3A_Radical_Reactions/11%3A5_Radical_chain_reactions ● https://www.wrap.org.uk/content/plastic ● http://www.vantageproducts.com/polypropylene.html ● https://omnexus.specialchem.com/selection-guide/polypropylene-pp-plastic ● https://www.essentialchemicalindustry.org/polymers/polypropene.html ● https://www.creativemechanisms.com/blog/all-about-polypropylene-pp-plastic
● https://guichon-valves.com/faqs/pp-polypropylene-manufacturing-process-of-pp-polyp ropylene/ ● https://en.wikipedia.org/wiki/Polypropylene ● https://www.bpf.co.uk/plastipedia/polymers/pp.aspx ● https://www.icis.com/explore/commodities/chemicals/polypropylene/ ● https://www.letsrecycle.com/prices/plastics/ ● https://www.sciencedirect.com/topics/chemistry/polyhydroxyalkanoate ● https://www.biosphereplastic.com/biodegradableplastic/uncategorized/is-pla-compost able/ ● https://bioresources.cnr.ncsu.edu/resources/polyhydroxyalkanoates-their-importanceand-future/ ● https://www.smithsonianmag.com/science-nature/corn-plastic-to-the-rescue-1264047 20/ ● https://www.sciencedirect.com/science/article/abs/pii/S1359511316300861 ● https://www.nature.com/articles/s41467-018-04734-3 ● https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5559386/ ● https://mmbr.asm.org/content/63/1/21/figures-only https://www.researchgate.net/publication/7742075_Degradation_of_P3HB_and_P3H B-co-3HV_in_biological_media https://www.researchgate.net/figure/P3HB-co-3HV-production-cost-US-per-kg-for-vari ous-3HV-co ● polymer-content-calculated_fig3_227070519 ● https://www.nature.com/articles/am201648 ● https://www.sciencedirect.com/topics/materials-science/polylactide ● https://greenliving.lovetoknow.com/Type_of_Biodegradable_Plastic ● https://www.researchgate.net/figure/Types-of-biodegradable-polymers-39_tbl2_2489 08943 ● https://www.globalplasticsheeting.com/our-blog-resource-library/bid/92169/polypropyl ene-is-it-different-from-polyethylene ● https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachm ent_data/file/817684/review-standards-for-biodegradable-plastics-IBioIC.pdf ● https://www.keeptruckeegreen.org/is-biodegradable-plastic-too-good-to-be-true/ ● https://www.nationalgeographic.com/environment/2018/11/are-bioplastics-made-from -plants-better-for-environment-ocean-plastic/ ● https://www.sciencedirect.com/topics/chemistry/biodegradability
The extremely rapid increase in mass production of plastic from six decades ago led to 8.3 billion metric tons of plastic being produced, most of it existing as disposable products, shortly discarded as waste after a brief life-time. As traditional plastic takes over 400 years to degrade, most of this waste still exists on the planet in some form and the amount of plastic which is thrown away each year is enough to circle the Earth four times. An average UK household throws away at least 40 kg of plastic per year, and nationwide, we use 7.7 billion plastic bottles per year, which is an average of 117 bottles per person, per year. According to The Ocean Cleanup, 5 trillion pieces of plastic waste float in the world’s oceans, affecting over 1 million marine animals such as fish, turtles, mammals and rendering extinct over 700 species. The plastic waste problem has been a prompt which continues to shape the focus and efforts of the materials and consumers industries. The United Nations have been working on methods to reduce plastic growth by hand-wringing and corporate pledges but the demand for plastics grew a further 3.5% in 2019. Latest figures also demonstrate that less than 10% of plastic is recycled leaving an overall over 90% of worldwide produced plastic as waste.
Almost all aspects of daily life in the modern world involve plastics or rubber in some form or the other. The multiple functions served by plastic products in modern society are irreplaceable, and as certain features of plastics are essential for their applications. Plastics are made of polymers, which allow for a wide range of properties, thus their use in markets ranges from simple plastic bags to home commodities. A promising solution to the plastic issue is, therefore, the design of plastics which can maintain the properties offered by the material while having the ability of degrading under pseudo-natural conditions. The key to the usefulness of these materials is the ease of unharmful and uncontaminating disposal after their use. Candidates for the structural composition of these degradable plastics are biodegradable polymers.
Polymers Polymers are long chains of repeating monomer units (small organic molecules) that have different properties which make them very suitable for a number of applications in today’s world. The reaction involved in the making of polymers is called polymerisation and it consists of the addition of monomer units to result in the formation of a long macromolecule. There are different types of polymerisation reactions which depend on the monomers used, and which are performed under different conditions in the industry. The main polymers which are used in the plastic industry are illustrated in Table 1 below together with their starting monomer units and their most common applications.
Table 1: Main polymers used in the plastic industry, their monomer and their common uses.
Polymer Monomer Uses
Polypropylene Propylene Product packaging, automotive industry and textiles
Polystyrene Styrene Lightweight protecting disposable packaging, home and appliance insulation
Polyvinyl chloride Vinyl chloride Building constructions, electronics, health care appliances, wire/cable insulation
Polyethylene Ethylene films, tubes, packaging, plastic bags Urea-formaldehyde Urea and formaldehyde Wood products, coatings, textiles industry
Rubber Isoprene Tyres, elastic materials
BUNA - S / BUNA - N Butadiene, Styrene/Vinyl Cyanide Synthetic rubber
Teflon Tetra Fluoro Ethane Non-stick-cookware-plastics
Nylon-6 Caprolactam Fabric
Bakelite phenol and aldehyde Automotive components, industrial applications, electrical appliances, kitchenware products
For the purpose of this investigation, the two polymers that were found to be most relevant to the plastic problem of the 21st century were chosen: polyethylene and polypropylene.
Polyethylene (PE)
The structure of PE is made up of repeating units of the ethylene monomer (a small organic molecule consisting of 2 C and 4 H atoms) and is synthesised by organic polymer addition as illustrated by Figure 1 below.
 |
| Figure 1: Synthesis of PE from the ethylene monomer units |
Synthesis conditions for the reaction require very high pressures and temperatures, with the addition of an organic peroxide radical initiator. The initiator can stimulate the breaking of the double bond between the two C atoms of the ethylene monomer, which results in a free radical (very reactive intermediate species with an unpaired electron) with the ability to join the next ethylene molecule forming a chain of which length keeps increasing via the repeating propagation mechanism. As a result of this reaction, large polymer chains (macromolecules) start forming. The mechanism for the reaction involving initiation, propagation and termination steps is presented by Figure 2 below.
 |
| Figure 2: Mechanism for the synthesis of PE from ethylene by addition polymerisation |
The resulting PE chains are not covalently joined but instead are held together in a crystalline structure by intermolecular forces. The overall structure of the chains (linearity/branching) together with the intermolecular forces acting between these results in the physical and chemical properties of the materials formed. The two most common types of PE are high density and low-density PE (HDPE and LDPE, respectively) and these have different chemical and physical properties which make them useful in a range of applications. LDPE has a lower number of side branches and therefore lower crystallinity as the packing of chains is less efficient. LDPE has a melting point of 105-115℃, good weather ability, very low water absorption and is, therefore, suitable for use in packaging, pipes and fittings. HDPE has a higher melting point of 120-140℃ thanks to the strength of intermolecular forces increasing with greater linearity of the polymer chains. HDPE is very chemically resistant to solvent and has a high tensile strength and very low water absorption. It is therefore also used in packaging as well as consumer/household goods, fibres and textiles, together with some applications in the automotive industry.
Polypropylene (PP)
The structure of PP is made up of repeating units of the monomer propylene (a small unsaturated organic molecule) by chain-growth polymerization, also using the Ziegler-Natta and Metallocene catalysts as illustrated by Figure 3 below.
 |
| Figure 3: Synthesis of PP from the propylene monomer unit. |
Two main types of PP are homopolymers and copolymers which are further divided into subclasses. Homopolymers are made of the same type of monomer unit providing strong intermolecular forces between the PP chains, which allows these materials to have a high melting point of 160-165℃ and are lightweight and have a density of 0.904-0.908 g/cm³. They have a great strength to weight ratio and are overall stiffer and stronger than copolymers; they have good processability and impact resistance. PP copolymers are made up of two monomer species and present weaker intermolecular forces between the chains, therefore, leading to their lower melting points of 145-150℃ and a density equal to or lower than homopolymers (ex. impact copolymer 0.898-0.900 g/cm³). They are softer and have a better impact strength, are tougher and more durable. They also have high processability and impact resistance. PP is used in a wide range of applications such as food packaging, film and fibre and it is also used for injection moulded products ranging from kitchen objects to car pieces.
Thanks to the use of inexpensive Ziegler-Natta and Metallocene catalysts, the synthetic processes for PE and PP have low costs and offer efficient processability. Despite large fluctuations in the consumer price of these materials, the national average cost of PP in 2014 was 311£ per ton and the current average costs of PE is £0.38/kg which explains their extremely high demand for applications. Like most wide-application polymers used today, the properties of both PE and PP make them incredibly durable and these molecules are non-biodegradable in the environment. Due to this, recycling of these polymers is unfavourable in economic terms and unattractive for private investors, therefore highly contributing to the world's plastic waste problem. Recycling of these materials is possible: current improvements in the recycled market in the UK presented an average price for HDPE of £1220 per tonne and an average for LDPE is £1193 per tonne. Due to these costs, cheaper polymer production continues being instigated.
Biodegradable Polymers
The same properties making ‘traditional’ plastics essential to our everyday life, such as strength, stability and durability, are also those which make them prone to contributing to a worldwide environmental issue. A lot of research has been done in the attempt of solving the problem of plastic pollution in the modern world, and one of the most prominent areas of study has been that of biodegradable polymers. By chemically designing the initial monomers, the total lifetime of a plastic can be controlled and modified to the desired purpose, whether this is not to degrade at all (building and transport industry) or to degrade after a single use (such as disposable cutlery and plastic bags). Developments in technology and high consumer demand for sustainable packaging have led to the growing production of biodegradable materials which are competitive alternatives and substitutes of common plastics. Biodegradable polymers can be bio-based or fossil fuel-based and have the ability of decomposing in a controlled composting environment or even under natural environmental conditions. Some bio-based and fossil-based biodegradable polymers are presented in Table 2 below.
Table 2: Main bio- and fuel-based biodegradable polymers, their monomer and their uses.
Biopolymer Monomer Uses
Bio-based
Starch glucose Foams, films & bags, moulded items
Polyhydroxyalkanoates C13, C12, C11, and C10 monomers moulded items
Cellulose and cellulose acetates Beta Glucose, Acetate composites fibre-board
Fatty acid polymers fatty acids and glycerol composites, adhesives, compatibilizers
Lignin polymers p-hydroxyphenyl (H), guaiacyl (G) composites, adhesives, compatibilizers
and syringyl (S)
Protein polymers H,N,O,C compatibilizers films
Polylactic acid lactic acid injection, moulding, fibres
Fossil fuel-based
Polycaprolactone ε-caprolactone compostable bags, medical application (sutures and fibres), surface coatings, adhesives, stiffeners, orthopaedic splints.
Polybutylene adipate polymer of 1,4-butanediol additive for biodegradable
terephthalate and adipic acid and the polymer plastics for flexibility
of dimethyl terephthalate (DMT)
with 1,4-butanediol
Polybutylene succinate Succinic acid, Butanediol food packaging like disposable tableware, paper cups and gas barrier packaging, agricultural mulch film
Polyvinyl alcohol vinyl acetate packaging films (replace LDPE and HDPE), coatings and additives for paper and board production
Biodegradability
The chemical bonding within the functional group (side chain) of the individual monomer units of a polymer is essential in the determination of the ease of degradation of a material by biological agents. The driving force of biodegradation reactions is the use of C within the polymer chain (expressed as Cpolymer) by microorganisms in their life cycles (expressed as Cbiomass). The reactions taking place are overall quite complex, however, under aerobic conditions, they can be simplified as described by Equation 1.
The different processes of biodegradation can be grouped into three main stages (Figure 4): biodeterioration, biofragmentation, and microbial assimilation and mineralisation (Stages I-III).
 |
| Figure 4: Three stages of biodegradation |
During biodeterioration (Stage I), loss of the mechanical properties of the plastic material is reduced mainly by abiotic (non-biological) methods, as plastics are generally biologically un-reactive. Methodologies range from the use of physical forces to UV-light or chemical exposure. Quantification of biodeterioration is measured by the changes in elastic and tensile strength together with the change in brittleness of the plastic. Biofragmentation (Stage II) involves the attack of the now enzymatically vulnerable oligomers (shorter chains of the polymer fragments resulting from Stage I) by the work of microorganisms. Two important rate-determining factors of this process are the structure of the functional groups which determine the ease of breakdown, and the enzyme availability as dependent on the microorganisms present. Visual inspection or mass loss are used to monitor this stage. Finally microbial assimilation and mineralisation (Stage III) involves the microorganisms generating biomass and carbon dioxide/methane depending on aerobic/anaerobic conditions. Gas evolution or microorganism biomass increase can both be used as a means of measuring the rate of this final process.
Two Examples
Other than waste management, interest in developing bio-based, biodegradable (compostable) materials has also been incentivized by sustainability and economic advantages. Fossil fuel-based rely on non-renewable raw materials which have been affected by a sharp rise in global prices due to their depletion. This investigation, therefore, focuses on two examples of bio-based, biodegradable polymers, which were found to be the most promising candidates for fighting the plastic problem: Polyhydroxyalkanoates and Polylactic acid.
Polyhydroxyalkanoates (PHAs)
PHAs are a family of biobased and biodegradable natural polyesters produced by an extensive variety of microorganisms, which represent a suitable replacement for fossil fuel-based thermoplastics. The synthesis involves the accumulation of PHA granules during unbalanced growth of microbe fermentation. The general molecular structure of PHA together with an example of a short-chain (C3-C5) and medium-chain length (C6-C14) as classified by the number of monomeric units are all illustrated by Figure 5.
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| Figure 5: short and medium chain length structure of PHA. |
The most common form of PHA is poly-3-hydroxybutyrate (P3HB) synthesised from extensive chains of hydroxy fatty acid monomers. P3HB is naturally produced by bacteria and other living microorganisms from biorenewable resources such as carbohydrates and fats. P3HB is a highly crystalline, linear polyester of 3-hydroxybutyric acid and is generated as a carbon reserve in a wide variety of bacteria. It is produced industrially through fermentation of glucose by the bacterium Alcaligenes eutrophus as presented by Figure 6.
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| Figure 6: The synthesis process for production of PHA |
Biodegradability and the optimal conditions for the degradation processes of PHA strongly depend on the solid-structure of the material. Degradation rate can therefore be controlled by chemical structure and solid properties of the specific type of PHA polymer synthesised. P(3HB) is classified as a semi crystalline thermoplastic (melting temp. 180°C) and the microorganism R. pickettii was reported to hydrolyze it at 37°C, with the degradation step followed by erosion of chains in the crystalline state. The rate of enzymatic erosion of P(3HB) increases with a decrease in the material’s crystallinity as reductions of the extent of order in the structure may be decreasing strength of the polymer and other mechanical properties.
Great research efforts have been dedicated to the decrease in production costs of PHAs, however the current cost of the biopolymer is estimated to be 3-4 times higher (£1.81-2.21/ld) than that of traditional plastics such as PP and PE (£ 0.48-0.70/Id), therefore still presenting an economic challenge to its use.
Polylactic acid (PLA)
PLA is a thermoplastic and long-chain polymer made through the fermentation of bacteria and derived from the lactic acid monomer. The latter is a by-product or intermediate product of plant metabolism, however, it can also be industrially produced from a number of starch or sugar-containing agricultural products, such as cereals. Lactic acid production based on microbial carbohydrate fermentation is the most chemically and economically favourable synthesis method. PLA can be produced by three different mechanisms: ring-opening polymerisation, direct polycondensation and direct polymerisation. Condensation polymerisation and ring-opening polymerisation are the two most common routes as shown by Figure 7 below.
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| Figure 7: polymerisation of PLA by condensation polymerisation (A → C) and ring-opening polymerisation (A → B → C). |
Despite being a degradable bioplastic, PLA is relatively hard to compost as temperature and water levels needed for this to occur are not available in the natural environment. In fact, it can take up to six to twelve months to degrade in soil compared to one to six months to degrade in commercial facilities. Decomposition of PLA needs to happen in the presence of oxygen in order to produce carbon dioxide and water (takes around 47 to 90 days: process progress images illustrated in Figure 8) as if PLA were to be found in a landfill without oxygen it would produce methane gas, which is nearly 20 times more detrimental to the environment than carbon dioxide. PLA also does not decompose in marine water, in fact, according to CalRecycle only 3% after six months decomposed. Overall, even though PLA does not decompose quickly nor efficiently in soil or seawater leading to problems when littered, it is still significantly less harmful than traditional polymers and of lower contribution to the plastic waste issue.
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| Figure 8: The biodegrading process of PLA. |
Conclusion Biodegradable polymers present a crucial area of research for effective tackling of the 21st century’s extreme issue of plastic pollution, and they involve the customisation of a material’s durability to its purpose and applications before the plastic is composted at the end of its lifetime. The replacement of traditional polymers with biodegradable ones still faces a number of economic and legislative difficulties: the production of conventional plastics is still very cheap compared to that of biodegradable materials, and limited progress has been made in terms of bans and regulations on the properties of single-use plastics (for example on the extent of recyclability). Nonetheless, consumer demand for sustainable plastics continues to rapidly grow in line with increased consumer awareness of the issue and its devastating effects on many aspects of life on our the Planet.
Optimisation of the synthesis processes of biodegradable polymers such as PLA and PHA together with the extent of their biodegradability are related to the underlying chemistry of their molecular chains, as determined by the identity of the monomeric functional groups. Refinement of our knowledge of synthetic polymer chemistry, as induced by research funding, may result in significant progress in these areas; improved design of the chemical composition and additives used in single-use plastic products may lead to cheaper polymers with controlled usable and degradation lifetimes, to enable rapid biodegradation at the end of their use.
Other than the substitution of conventional plastics with biodegradable polymers, alternative solutions to the plastic waste problem involve regulation for plastic waste management and subsidising the economic costs of collecting and recycling of existing traditional polymers. Moreover, these solutions would possibly be most effective if brought about as part of an international and worldwide effort to combat a crucial issue, threatening the life of multiple species on our Planet. This project will undoubtedly take time and motivated efforts, however, it may be one of the few ways of rectifying the issue and its negative consequences in the near future.
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