Plastic or polymer waste has become a serious problem on land and in the ocean in the last two decades. Plastic has undergone a century-long transformation from being praised as a scientific miracle to being despised as an ecological disaster. The media has focused particularly on the significant issues in Asia and the abundance of floating plastic observed in mid-oceans during the last few years while discussing ocean and river plastics. In addition, the media frequently shows us images of marine life being engulfed in waste, plastic containers, and bags. A recent study revealed a plastic bag/packaging at the bottom of the Pacific Ocean Mariana Trench, which is 11 km underneath the ocean surface [1]. Cleaning up plastic waste from coastal beaches in whatever quantity seems to have little impact. Less than 20% of plastic in the world is being recycled, which is another drawback.
The synthesis of Bakelite in 1907 followed the production of the first plastic material in 1850. When nylon was developed during World War II, it replaced silk in parachutes and ropes, sparking the beginning of the plastic revolution. In the 1950s and 1960s, new polymers and manufactured items began to take off. Although the community was aware of ecological issues in the 1960s, as with many other innovations, little attention was given to the concerns associated with plastic trash. Currently, 8 million tons of the approximately 300 million tons of plastic generated year end up in the oceans [2,3]. The amount of plastic in the oceans is predicted to surpass the mass of fish by the year 2050 if we keep polluting the waters in this way [4]. In addition, 90% of all seabirds have plastic in their guts, and 1/3 of marine species have been discovered entangled in marine waste. The major end market sector, plastic packaging, is accounted for nearly 40% of all plastic use globally.
1 Why Plastics are Important?
Plastic has become a material on which we have grown to rely, and it is responsible for many aspects of modern society.
Plastic has improved our quality of life by enabling:
- The housing of computers, cellphones, and other electronic devices;
- Insulating homes;
- Clothing and fabrics utilization;
- Most stores to carry more international and fresher goods packaged in plastic;
- Medical innovations like plastic syringes and tubing;
- Toys, of which two-thirds are made from plastic and most are used for a short period before being destroyed;
- The use of plastic in automobile dashboards, seat coverings, interior surfaces, electrical insulators, and bumpers;
- Conduit piping for gas, electric, and telephone wires and water pipes.
2 Recycling of Plastics
According to estimates, just 9% of all plastic ever produced has been recycled, 12% of which has been burned, with the other portion ending up in landfills, seas, and soils [5]. Nearly half of all plastic produced, according to estimates, has been produced since the year 2000. The plastics sector is expanding quickly. With the United States recycling fewer than 9% of its plastic manufacturing, the average global plastic recycling figure is less than 20% [5]. The term “recycle” has been used to describe a wide range of practical approaches to handling plastic. It should only be applied to plastic waste reused or processed to produce the same or different goods. In fact, degrading the polymeric substance is one of the answer to the plastic waste problem. Proper recycling is rare, and even the plastic bottles that are collected each week separately might not remain as plastic bottles after all; instead, they might be hydrolyzed and reduced to monomers before repolymerization to polyethylene terephthalate (PET), which is used to make a variety of products, including carpet and other synthetic fabric materials. The plastic that is most effectively recycled is PET. The difficulty in sorting the many forms of plastic is one of the issues with recycling plastic trash. Symbols are frequently written on plastic packaging to facilitate separation (Figure 1).
Figure 1: Different Plastics Codes
Every year, 110 billion empty bottles and other liquids are produced using the following materials: PET for water and soft drink bottles; HDPE for shampoo and other relevant containers; LDPE for milk bottles; PP for plastic bags and bottle caps; PS for plastic containers and kitchenware; and expanded PS for hot drink cups and shielded packaging [5]. Mixed plastic packaging (trays, tubs, food packaging and pots) consisting of polyurethane and PS is difficult to recycle. Before being transformed into new items like refuse carrier bags, sacks, containers, wheel bins, potted plants, meal trays, and even polyester fabric for clothes, plastic collected for recycling is first separated by polymer type.
Supermarkets generate a large portion of our plastic waste. According to reports, about 40% of all created plastics are utilized as packaging materials [5]. The thin grocery store plastic bag is, in fact, one of the emblems of the plastic waste issue.
It seems there is no other option to clear the current backlog of rubbish other than to burn the remaining waste (and maybe recover the energy) while considering how harmful the fumes are) and recycle what can be recovered. The inability to easily separate the various forms of plastic is the main cause.
The recent media attention to plastic garbage has sparked a strong desire to take action. The manufacturing, storage, distribution, use, sale, import, and transit of numerous plastic goods are prohibited, for instance, by the Indian government [6]. This rushed and hurried decision is being contested in court. Other governments have adopted a more cautious stance; in the UK, efforts are afoot to halt “avoidable” plastic trash generation by 2042 and expand the 5 cents carrier bag fee [7]. The UK also considers how taxes or fees might encourage recycling and force plastic packaging manufacturers to cover disposal costs [8]. By 2030, all plastic packaging in the European Union should be recyclable and reusable, according to the European Commission. A decrease in the manufacturing of single-use plastic and limitations on using microplastics are only two of the strategies it outlines in its “Plastic Strategy” to combat plastic waste. Only 30% of the 25 million tons of plastic generated annually in the European Union are currently recycled [9]. This percentage is 9.5% in the United States, while it has been calculated that 86% of all plastic containers and packaging are not collected or recycled internationally. PET is the most recyclable form of plastic in the US, with 19.5% of all plastic bottles being recycled. Following this are low density (10.3%) and high density (10.3%), both of which are used to make thin plastic bags (5.3%). Less than 1% of other plastic types, such as polypropylene, polystyrene (food packaging), and polyvinyl chloride (hose pipes), are recycled [10].
Another significant issue with plastic garbage has just come to light. Many affluent established and even emerging nations export their garbage to impoverished nations with ineffective or nonexistent waste management, like Turkey, India, Malaysia, and Indonesia [10]. The garbage is frequently not recycled or handled and is left in enormous open mounds. Japan, the US, and Germany are the worst export violators. The mountains of plastic garbage, some of it from the UK, that are accumulating in Malaysia and posing a health risk, especially when burned in open flames to dispose of the plastic, were recently brought to light in a TV documentary that aired on BBC on June 10, 2019. Due to these factors, China possibly prohibited the import of plastic garbage in 2018.
3 What Should be Our Role Regarding Plastics
Although it is not an ideal fodder, plastic is a crucial component of daily use. We must find solutions to the issue since there is no going back in time. Substitute all plastic products with alternatives would be extremely expensive and result in significant CO2 and other greenhouse gas emissions. It is difficult to produce new polymers that are both biodegradable and manufactured from the non-fossil feedstock. It will need specialists from various fields, including engineering, biology, chemistry, and material sciences. Perhaps this is a good place to start to reduce the quantity of plastic we use as packaging accounts for more than a third of all plastic produced.
The formation of plastic trash should be decreased by boosting the recycling of plastics, maybe by decreasing the types of plastic that are produced; combined plastic or layered plastic goods should be restricted, ensuring that plastics are retained within a circular economy. In a nutshell, we should live by the “reduce, redesign, reuse, and recycle philosophy.”
Single-use goods should be maintained to a minimum, and municipal governments should strive to recycle all plastics. Reduce the daily sales of plastic bottles, the majority of which are filled with water, as a place to start. In 2020, it was predicted that half a trillion plastic bottles would be sold worldwide [5]. In developed countries, tap water is equally beneficial for one as the costly bottled water; most are single-use plastic bottles. Consumers should carry a reusable bottle that can be filled rather than purchasing bottled water. The amount of plastic waste would significantly decrease as a result.
Packaging materials must be revised, and multilayer packaging must be abandoned to use more recycled plastic. It would include decreasing the variety of plastics produced so that only recyclable plastics could be used for packaging and producing essential commodities.
Microplastics, a new plastic issue, have lately become more prominent. This is crucial for the oceans. A recent National University of Ireland study found that microplastics in the intestines of 73% of deep-water animals in the Northwest Atlantic [11].
The notion is those producers of plastic or those who make plastic products should be in charge of collecting and recycling plastic. Since only some forms of plastic can be recycled, this will almost probably lead to a decrease in the amount of plastic produced globally and a limitation on the sorts of plastic that may be produced.
Oxo-biodegradation is a significant research priority regarding the sunlight-induced breakdown of plastic materials. In the end, oxo-biodegradable plastic degrades, but only into tiny pieces of microplastic. This would undoubtedly lessen the amount of plastic that ends up in the middle of the ocean. As one might expect, these tiny plastic pieces would eventually degrade more quickly than regular plastic, but it is not the reality. The chemical breakdown of plastics into carbon dioxide, water, and maybe methane might take years or even decades. There is no definitive conclusion on the detrimental impact of microplastic particles on animal health. The costs of producing biodegradable and compostable polymers are probably too high, and these materials often degrade extremely slowly into their chemical constituents. It is advisable to restrict the manufacturing of recyclable quality plastic due to the microplastic problem with oxo-plastics. This would entail limiting the production of plastics to a possible dozen varieties, all of which can be gathered and recycled.
Everyone might contribute in different ways to change the world:
- Always keep a bottle, cup, and reusable bag;
- Stop purchasing bottled water, fill reusable bottles with tap water, and government officials should install filling stations in public areas;
- Reuse washable, dryable, and reusable plastic bags;
- Stop using drinking straws made of plastic;
- Because frozen food is typically packaged with plastic, avoid purchasing it;
- Avoid purchasing juice that typically comes in plastic bottles and instead consumes fresh fruit and freshly squeezed juice;
- Bring recyclable containers for a picnic and school meals;
- Avoid buying or utilizing wet wipes;
- Refrain from accepting complimentary plastic “kids” toys from shops or restaurants.
Figure 2: Plastics/Polymers life cycle analysis with different recycling routes
4 Different Plastics Recycling Techniques
4.1 Mechanical Recycling
There are several stages to recycling plastic, and each one is crucial. Here’s a terrific chance to learn more about the procedure if someone wants to address the infamous plastic trash problem.
4.1.1 Collection
Sorting the waste is the first step. When customers collect like items in a single pile, this marks the beginning of the recycling process for plastics and other materials. Collecting waste is essential for every organization, whether it be a commercial or a public one.
For instance, a common practice in Finland is for individuals to bring in their empty beverage cans and bottles and receive compensation in the form of money at supermarkets and retail establishments.
4.1.2 Classification and Identification
It is time to sort out the plastics that have now arrived at the recycling plant. First, plastics are separated based on characteristics and qualities. Usually, depending on their qualities, kind, and grade, the appropriate plastic types will be selected from the steam.
The mounds of various recyclable items at the nearby recycling center must have been enormous. Every mound of plastic garbage collected from public drop-off locations should only contain plastic waste; no paper, glass, or metal should be present.
4.1.3 Cleaning
It is crucial to clean up everything now that it has been sorted and categorized. Due to the possibility of contaminants, washing is another crucial stage in the procedure. Large quantities of recycled plastics might have their quality ruined by certain contaminants.
Impurities in plastics can generally refer to a few different things. Food particles, grime, and sealants must be removed since the procedure requires clean plastics.
4.1.4 Shredding and Pelletizing
It is time to shred the recycled plastics once they have been collected, sorted, categorized, and cleaned. The plastic is broken down into several smaller pieces in a shredder at this stage of the recycling process for plastic. The process of melting and creating pellets from the little shredded bits is a crucial step before pelletizing.
Plastic bits of various sizes are shrunk to smaller ones. This enables the production of recovered plastic granulates or pellets from recycled plastics.
Unfortunately, sometimes the shredding procedure still reveals hidden contaminants. Metal is an example of such impurity. Even tiny metal particles can harm the melting equipment for processing plastic. Metal portions in polymers should be avoided for this reason. This emphasizes how crucial it is to separate the trash into the appropriate bin.
4.1.5 New Plastic Product
Finally, the procedure is finished, and bits of the shredded plastic is transformed into useable goods for various firms. Now that it is no longer a beverage bottle or another form or size, the recycled plastic may begin its new life as something else.
4.2 Chemical Recycling
Chemical plastic recycling utilizes catalysts, heat, and pressure to convert plastic waste into monomers, other chemical raw materials, or fuels [12]. The two primary categories of chemical recycling methods are those that dissolve the polymers/plastics and (ii) those that break chemical bonds in polymer chains. The typical strategy for condensation polymer types, such as polyamides (PA) and polyesters, is to dissolve the ester or amide linkages. In contrast, one must tackle the highly stable carbon-carbon bonds in polyolefins (polyethylene and polypropene).
Currently, all plastic trash is recycled mechanically, which is only effective for homogeneous and contaminant-free plastic waste, which is challenging to collect. For instance, a significant portion of the plastics in garbage from building and demolition projects, end-of-life care, and other sources cannot be recycled mechanically. Additionally, the trash from plastic packaging frequently includes composite materials and laminate constructions, whose mechanical recycling is difficult.
According to research published by the American Chemistry Council, advancing plastic recovery and recycling technologies in the US can have a roughly $10 billion economic effect [13]. According to McKinsey, using chemical recycling processes to finish the mechanical recycling of plastics can significantly raise recycling rates. Additionally, it may have a positive economic impact and aid in reviving the entire petrochemical sector [14]. The end markets must take in more than 10 Mt of recovered material to meet the European Commission’s legally mandated recycling objective for plastic packaging waste of 50 and 55% recycling rates by 2025 and 2030, respectively.
4.2.1 Depolymerization and Leaching
Synthetic polymers can be categorized in one of two ways, depending on how they were formed: (1) addition polymers (chain reaction polymers) and (2) condensation polymers. In addition, polymers are synthesized by the chain reaction of double-bonded monomers. Polyethylene (PE), Polypropylene (PP), Polyvinyl Chloride (PVC), Polystyrene (PS), and Polymethyl Methacrylate (PMMA) are prominent examples. Condensation polymers involve at least two functionally distinct molecules. These functional groups can develop a connection through a condensation process. A tiny molecule, such as water, is released while a new connection is developed. Polyesters, polyurethanes, and polyurea are a few examples of common condensation polymers.
In a depolymerization process, polymers are broken down into monomers or oligomers for later usage, such as in the polymerization process. Increased temperature or other factors, such as the presence of a hydrolytic agent, may cause the process. Which chemical procedures can be used to break down polymers into monomeric or oligomeric products depends on the chemistry of polymers. The additive polymers frequently go through heat depolymerization. Condensation polymers require conditions like hydrolysis since they do not depolymerize thermally. Several other polymers can be thermally broken down into their monomers with acceptable yields, such as PMMA (yield about 90%) and PS (yield higher than 70%).
Different factors can start polymer chain breakdown or disintegration in addition to chemical processes. Typically, processes like thermal, photochemical, mechanical, biological, radiation chemical, and chemical breakdown of polymeric materials are categorized. It is important to remember that polymer breakdown or degradation only happens randomly and reduces the characteristics or molecular weight of the polymer. However, several of these degradation beginning modalities may be used to disintegrate polymer, even resampling depolymerization carefully. Degradation is defined as weakening a material’s qualities due to structural changes, whereas decomposition is the process through which a substance loses its original form. Mixed polymers or certain chemical residues do not always inhibit or stop depolymerization processes. Organic contaminants, however, may still be present in the product molecules.
4.3 Thermochemical Recycling of Plastics Waste
4.3.1 Pyrolysis
Pyrolysis is a process in which waste polymers are heated and pressed while being oxygen-free. Plastic pyrolysis occurs typically at temperatures between 370 and 420C, and the end product is an oily combination. The polymers often break down into smaller hydrocarbon molecules without oxygen. Alkenes, unsaturated hydrocarbons, make up a higher portion of all pyrolysis products produced from waste plastic than fossil crude oils.
Different variables govern the composition of pyrolysis yields. Some of these include the make-up of the waste feedstock, reactor type, and process variables, including temperature, heating rate, pressure, and residence time. Several pyrolysis processes include catalytic cracking, thermal cracking (also known as rapid pyrolysis), hydrothermal liquefaction (HTL), microwave pyrolysis and flash pyrolysis. Differing pyrolysis parameters can produce varied ratios of hydrocarbon oils and waxes, aromatics, and olefin. The pyrolysis procedure may be adjusted to yield significant amounts of monomeric chemicals from some particular waste streams, such as PS, PMMA, and nylon (PA6) wastes, which is advantageous for refining procedures. For example, PS may be depolymerized to styrene with yields as high as 80%, and following the distillation process, a styrene monomer with a purity of 99.6 wt% can be produced.
Table 1: Major plastic types in the waste stream and their pyrolyzed products [15]
Regardless of the method or usage of catalysts, the output produced by thermal depolymerization or thermal breakdown of waste plastic materials requires at least some purification before its constituent parts may be used as fuels or raw materials. Only direct energy consumption in heaters is feasible without purification. However, the range of potential products that may be generated from pyrolytic oil obtained from waste plastics is broad and ranges from diesel and gasoline fuels to food-grade polymers after the subsequent upgrading stages.
4.3.2 Gasification
An industrial gas mixture, “synthesis gas” or “syngas”, is produced by gasifying waste plastic at temperatures between 700 and 1100 in the presence of a regulated amount of air, oxygen-enriched air, or steam. Syngas is a combination of gases having carbon monoxide (CO) and hydrogen (H2) as its primary constituents and lesser quantities of other gases like carbon dioxide (CO2) and hydrocarbons, including methane (CH4). Syngas may be cleaned up, processed, and converted into fuels or chemicals, or they can be burned to provide heat and power.
5 Challenges in Plastic Recycling
5.1 Increasing Knowledge
In reality, hundreds of different types of materials go by the label “plastic.” Due to ignorance, different plastic kinds are mixed together during production operations, making recycling considerably more challenging. This frequently results in the burning of plastics, which is a significant waste of precious resources.
5.2 Value Finding
We must determine the value of plastic recycling since virgin raw materials are occasionally less expensive. Sales and sustainability communication can both benefit from emphasizing the recycling efforts and goals for consumers and other stakeholders.
5.3 Recycling Design
Many items are made in a way that makes it challenging to separate and recycle the plastic component. For instance, multiple plastics may be blended, or the plastic might be attached or bonded with other materials like metal screws and glue. It is simpler to dismantle things into waste fractions that do not contain leftovers of other materials when these concerns are considered at the design stage. This demands a specialized understanding of the subject matter and skills.
5.4 Proper Sorting
Each form of plastic has distinct characteristics that influence its color, shape, structure, and melting point. Plastic is a complicated substance. So that it may be kept as pure as possible, it is crucial to separate plastic into several groups. Without the proper understanding, the entire batch of plastic might be squandered in this situation. For example, different polymers have particular processing and degradation temperatures. Unmelted material will remain if there are high melting polymer traces (PET) in low polymer processing temperature material (PP). In contrast, if low degradation material (PVC) traces are found in a high processing material (PET), the resulting carbon black can contaminate the whole stream.
5.5 Cost
Many expenses are involved in collecting the waste (or mixed) plastic, sorting it out, applying different mechanical and chemical processes and getting the required output. In many cases, when the process is not carried out properly, there can be an issue regarding the quality of the final product. Due to this reason, it sometimes becomes less attractive from the business point of view.
5.6 Compatibility
There are many compatibility issues when recycled plastic is used along with virgin plastics to get some cost benefits. The recycled plastics purchased from the external party have issues related to the quality consistency. Sometimes compatibilizers are utilized to overcome the loss in the final product’s physical and mechanical properties, but it demands special technical expertise with added cost.
6 Ecofriendly Plastics Alternatives
The terms “biodegradable” and “bioplastic” are commonly used interchangeably. At the same time, not all bioplastics are biodegradable [16]. Bioplastics are polymers that meet either of the following requirements: they are made from biological materials and degrade naturally [17]. Any renewable organic resource of biological origin and organic waste are included in the definition of “bio-based,” which refers to a polymer manufactured entirely or partly from biomass [18]. The term “biodegradable” describes a substance’s capacity to break down into natural substances such as carbon dioxide, water, and biomass due to microbial activity [19]. The scientific observation of a certain amount of deterioration during a specified period and under a specified set of circumstances is required [20]. Meanwhile, biodegradable plastic must follow tight regulations as it degrades in commercial composting facilities. As a result, there are three types of bioplastics: those which are both biobased and biodegradable, those that are exclusively biobased, and those that are only biodegradable. Poly (lactic acid) (PLA) [21], poly (hydroxy alkanoates) (PHAs) [22] are both bio-based and biodegradable plastics, and bio-based are poly (butylene succinate) (bio-PBS) [23], polymers made of cellulose, starch, lignin, and chitosan. Examples of bioplastics that are bio-based but not biodegradable are poly (amides) (bio-PP), poly (ethylene) (bio-PE), and poly (ethylene terephthalate) (bio-PET). Last but not least, biodegradable bioplastics made from fossil resources include poly (caprolactone) (PCL) [24], poly (butylene adipate terephthalate) (PBAT), and poly (vinyl alcohol) (PVA) [25]. Additionally, bio-based materials chemically comparable to their fossil-based counterparts, such as bio-PE, are occasionally referred to as drop-in polymers [26].
Figure 3: Synthesis route of bio-based plastics
6.1 Bio-based and Biodegradable Plastics
Biobased and biodegradable plastics are increasingly used in agricultural applications, food packaging, (food service) ware, retail bags, and fibers/nonwovens [27]. Bio-based drop-in plastics, including bio-PE and bio-PET, can be utilized in the same applications as fossil-based plastics [28]. As with fossil-based plastics, carefully selecting a bio-based and biodegradable packaging material is necessary to ensure that a complete product has a useful shelf life. Some plastic characteristics, like bio-based polymers’ low water vapor barrier, may limit one usage while enhancing another [29]. The PLA material has advantages for (breathable) vegetable and fruit packing but disadvantages in water bottles. Bio-based and biodegradable plastics must meet the same standards for food safety as fossil-based polymers. Numerous bio-based polymers have received certificates attesting to their suitability for interaction with food [30].
Figure 4: Global bio-based and bio-degradable polymer production in 2021 [31]
On the other hand, natural biodegradable polymers can be predominantly made from renewable resources or synthesized from them [35]. Due to their enormous advantages for the business, renewable-based biodegradable plastics have gained more attention since they are made from plants. Additionally, bio-based plastics can reduce our reliance on petroleum supplies and minimize environmental carbon emissions. PLA and polyhydroxyalkanoates (PHAs) are now the most highly regarded bio-based and environmentally acceptable plastic materials studied. Plant resources that are renewable yearly are used as the raw material for the manufacturing of PLA and PHA. This guarantees that, in principle, all aliphatic polyesters will be handled sustainably. Due to their biodegradability, these bio-based polymers might be converted back into CO2 and used by plants to photosynthesize [36]. Thus, synthesizing PLA and PHA may be considered a pollution-free and carbon-neutral process. The net quantity of carbon in the atmosphere stays consistent over the long term and across borders. PLA and PHA are two examples of bio-based and biodegradable polymers that are frequently referred to as eco-friendly and renewable to reduce the use of fossil fuels.
Additionally, it is predicted that these products will be used more widely and that new levels of global biodegradability will be produced for regulatory reasons. The pace at which PLA and PHA break down may be controlled by altering the characteristics of the molecules, such as their weight, distribution of monomers in order, and crystallinity. The PLA and its copolymers have been employed in the biomedical and pharmaceutical industries to synthesize reusable surgical threads and matrices designed to coordinate the drug’s administration [37].
Figure 5: Non-biodegradable and biodegradable plastics types [31]
7 Summary
The world’s inability to handle the fast-rising output of disposable plastic goods has made plastic pollution one of the most critical environmental problems. Plastic waste is most noticeable in impoverished Asian and African countries, where rubbish collection services are either ineffective or nonexistent. However, the developed world also has issues adequately collecting used plastics, particularly in nations with poor recycling rates. Because plastic waste has become so common, the United Nations is trying to get everyone to agree on a set of rules.
While preserving the long-term accessibility of crude resources, biodegradable polymers can replace conventional plastics to better the environment. When used to sterilize food and medical equipment, biodegradable plastics have shown to be quite effective. Because of the development of biodegradable plastics, many problems can be solved, and a green environment can be sustained for a very long period. The primary issues that need to be successfully resolved are the high production costs and poor mechanical, physical, and chemical performance of some biodegradable plastics, which need additional research to prevent them from competing with other environmental consequences.
8 Endnotes
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