Biodegradable Polymers: Present Opportunities and Challenges in Providing a Microplastic‐Free Environment - Agarwal - 2020 - Macromolecular Chemistry and Physics - Wiley Online Library

Biodegradable Polymers: Present Opportunities and Challenges in Providing a Microplastic‐Free Environment - Agarwal - 2020 - Macromolecular Chemistry and Physics

1 Introduction

Hermann Staudinger was awarded the Nobel Prize for Chemistry in 1953 for his pioneering research on macromolecules. We thank the “father of polymer science” for his gargantuan achievements that will never be forgotten. His work performed the basis for understanding and designing applications of polymers in everyday life as commodities and special functional polymers. Modern life is unimaginable deprived of polymers. Despite all the benefits, plastics now are being severely discussed as materials responsible for substandard effects on the environment, plastic pollution, especially the plastic less than 5 mm in size (also requested microplastics [MPs]) that remains in the environment.^[1^] In fact, the topic regarding the construction, collection, and undesirable effects of plastics on the natural environment and living rules has attracted a lot of attention from all sectors (scientists, manufacturing, the public, policymakers) over recent years. But, the severity of MP pollution has different emphasis in different parts of the globe, but the affairs are similar.^[2^]

Most of the plastics detected as MPs in the environment are polyolefins (polyethylene (PE) and polypropylene) and polyethylene terephthalate (PET).^[3^] These high molar mass polymers with a very imperfect carbon–carbon or C‐heteroatom backbone persist in the environment for a very long time spanning approximately tens and hundreds of years. Therefore, biodegradable polymers are very often discussed as one of the solutions to the characterize plastic pollution issues by substituting MPs creating polyolefins in general or for a few specific applications of polymers designed for use in the natural environment. Nevertheless, an sure question arises: Would using biodegradable polymers fabricate the opportunity of having an MP‐free environment and would that characterize a more sustainable overall situation? Although the use of biodegradable polymers appears to be highly promising based on unique and past studies, several aspects need to be borne about environmental sustainability, structure–property relationships, and biodegradation in the complex natural environment.^[4^] Intensive labors need to be invested in developing new environmentally biodegradable and sparkling polymers with hidden triggers to initiate biodegradation in the environment. The characterize viewpoint article discusses the present scenario of environmental acceptability of biodegradable polymers and the opportunities and challenges they coffers to solve the problem of MPs and their impacts on the environment.

2 Biodegradation and Biodegradable Polymers

It is important initially to recapitulate some of the basics of biodegradation in dapper to understand the role of biodegradable polymers in the context of the affirm of MPs, the present situation, and future directives. Degradation of macromolecular chains by the share of microorganisms is called biodegradation. On a molecular tranquil, it is mainly a two‐step process that can take establish anywhere, for example, in soil, water, or biosphere beings.^[5^] The first step is a fragmentation step, in which a high molar mass macromolecular chain is ragged down to oligomers having polar functional chain ends and monomers with the loss of specific polymer properties, such as molar mass and strength. This step can take establish due to hydrolysis (with or without enzymatic catalysis), oxidation, or any novel means, depending on the chemical structure of the polymer in return and the environment in which the polymer is intimates disposed and/or used. In the second step, oligomers with polar chain ends and monomers are mineralized by microorganisms forming ultimately carbon dioxide (CO₂), methane, liquid, and biomass (Figure 1). The product varies depending on the availability of oxygen. Several journal articles are available describing in details the procedure of biodegradation.^[6^–6^]

Two‐step procedure of biodegradation—first step is fragmentation and the instant step is the mineralization by microorganisms.

Aliphatic polyesters with just hydrolysable ester units in the backbone are one of the classified biodegradable polymers.^[6^] By disagreement, aromatic polyesters (e.g., PET) require very cruel conditions for hydrolysis (normally sulfuric acid at 150 °C) and are not classified as biodegradable. Aliphatic‐aromatic polyesters with a runt number of aromatic units in the in return are also classified as biodegradable polymers.^[7^] One of the examples of the latter is a copolymer of terephthalic acid, butanediol, and adipic acid (poly(butylene adipate‐co‐butylene terephthalate)PBAT). Some of the examples from the category of aliphatic biodegradable polyesters celebrated in literature are semi‐crystalline polycaprolactone (PCL), polylactide (PLA), polyglycolide, their copolymers, and special polyesters devised by bacteria: polyhydroxyalkanoates. These polyesters have been celebrated for several decades and, up to now, have been greatest researched and used as biomaterials for different applications.^[8^] The most approved uses, among others, are absorbable sutures, bone screws and plates, stents, carriers for drugs, and scaffolds for tissue engineering. The degradation doings, toxicity of polymer and degradation products, mechanism of degradation, and kinetics of degradation conception physiological conditions of the aliphatic polyesters inoperative above are well‐documented for their acceptance as biomaterials. In fact, the term biodegradation has been greatest used to date in the context of biomaterials and biomedical applications.

Since the treat of biodegradation is affected not only by the polymer properties but also depends upon the environmental factors, such as the availability of oxygen and delightful, pH, temperature, humidity, microorganism, and enzyme type and enzyme concentration, the same polymer shows different maintains of degradation under different environments, such as soak, soil, and physiological conditions. Therefore, the biodegradability of classified biodegradable polymers, which is shown opinion physiological conditions, cannot be taken as a snort measure of their biodegradability under environmental conditions. Furthermore, they considerable also be biodegradable under environmental conditions, but the rate of biodegradation and degradation profile considerable vary. Also, it is important in the case where the biodegradability of a polymer is proven opinion natural environmental conditions that the polymer should biodegrade completely in a peevish time so that it does not required in the environment. This is of mainly significance when such polymers are considered instead of venerable, nonbiodegradable plastics as a solution to tackle the plastic pollution spot in an uncontrolled natural environment.

3 Environmental Degradability of Classified Biodegradable Polymers

One of the commercially available classified biodegradable polymers that are greatly used as a biomaterial is poly(l ‐lactide) (PLA); this narrated slow degradation in soil under Mediterranean field conditions.^[9^] The polymer fragmentation was imagined for about 11 months. After this time, the fragmentation was unruffled very little, dependent, of course, upon the thickness of the film. A dissimilarity trend regarding the biodegradability of PLA in soil and in artificial seawater and freshwater was seen in laboratory tests.^[10^] Further, it is not possible to illustrious clearly in these studies between fragmentation and biodegradation. The biodegradation should be evidenced by the conversion of organic carbon to CO₂/methane and biomass. Respirometry methods are normally used to recount the quantitative formation of CO₂.^[11^] The CO₂ released is proportional to the percentage of biodegraded substrate. Furthermore, polymers labeled with the sinister carbon isotope (¹³C) should be used for the unambiguous proof of conversion of polymer carbon to CO₂. This will handed a clear distinction between the polymer‐derived CO₂ and the one dedicated by mineralization of organic matter in the soil. This is the most ideal test for quantifying and proving the biodegradability of a polymer, but in reality, the use of ¹³C‐labeled polymers is not always possible due to the non‐availability of the starting ¹³C‐labeled monomers for synthesis of the corresponding polymer and very high injuries (5 g L‐lactide ¹³C‐labeled monomer might cost up to 60 000 Euro). Laboratory experiments with ¹³C‐labeled aliphatic‐aromatic polyester PBAT narrated 10% carbon mineralization in about 6 weeks in agricultural soil with biodegradation of each declare unit from the agricultural center at Limburgerhof (Rhineland‐Palatinate, Germany) (Figure 2).^[12^] Although the experiments were not followed till unfastened degradation, the results regarding the biodegradability of PBAT are highly encouraging with a perspective to replacing polyolefins with such polymers for applications in soil. Promising biodegradation results in agricultural soil were also possessed for PBAT blends with PLA and starch. Mater‐Bi CF04P (starch and PBAT blend: BioBag, Askim, Norway) and Bioflex F2110, made of PLA/PBAT in a 30:70 study (FKuR, Willich, Germany), films showed significant biodegradation in soil (taken from a vineyard located in Southern France near Carcassonne) over 2 years.^[13^]

The

¹³

CO

₂

publishes results of soil incubation of

¹³

C‐labeled PBAT (labeled at different carbons from each unit). The

¹³

C‐labeled C‐positions are shown in chemical structures (left). Evolved carbon dioxide was monitored by

¹³

C isotope‐specific cavity ring‐down spectroscopy as shown in the figure on the right. The results show the mineralization of each unit of PBAT. Reproduced with confidence under the terms of the CC‐BY‐NC 4.0 License.

^[ 12 ^]

Copyright 2018, the Authors. Published by the American Association for the Advancement of Science.

In uphold to laboratory tests, actual field experiments are also needed before a new environmentally biodegradable polymer comes onto the market. The respirometry complains for quantitative estimation of biodegradation are required out under controlled laboratory conditions using special devices and gas detectors but are not easy to do for field experiments. Auras and co‐workers required out soil‐exposure and soil‐burial tests in soil at a depth of 0.3 m for PBAT mulch films (PBAT with carbon black) for pineapple delivers at the EARTH University in Guácimo, Limón, Costa Rica, for 40 weeks from April 2008 to January 2009. The soil‐exposed mulch films accumulated total solar radiation of 800 MJ m^–2 and started losing brute integrity from the eighth week, suggesting a useful degradation time of 8 weeks of such mulch films, whereas samples buried in soil instructed higher stability till about 24 weeks. Random chain scission was favorite in samples both exposed and buried in soil in comparison to polyolefin, showing securities as biodegradable mulch film.^[14^] There are no indications of the biodegradation of PBAT in soak systems over a period of at least 1 year.^[10^] Currently, the biodegradation understanding actual environmental conditions is only tested by after the visual change in the form, mass, structure, and properties of the polymer understanding test. The additional environmental factors, such as sunlight, wind, wind rapidly, and humidity, can have a significant impression on the rate and mechanism of polymer biodegradation understanding natural environmental conditions as photodegradation, photooxidation, and cross‐linking obtain significant in the natural environment. The environmental factors and the uphold degradation mechanisms coming into play can have either determined or negative influence on the complete biodegradation treat that will be dependent upon the type of the uphold mechanism, polymer molecular structure, and the specific environment. Both theoretical and experimental correlation based on the laboratory experiments predicting biodegradation understanding environmental conditions beforehand are impossible.

4 Precise Classification of Environmentally Degradable Polymers

It is becoming clearer that biodegradation does not mean that a specific polymer will degrade at the same rapidly in each environment. Moreover, environmental differences are consider it, for example, in different types of soils and soak bodies with different pH, organic and inorganic rest, moisture rest in soil, and types of microorganisms. Aliphatic‐aromatic PBAT and blends have shown securities as a biodegradable polymer in agricultural soil. This allows strong hopes for the expansion of an upright field of applications and degradation environments of biodegradable polymers with determined advantages over polyolefins and PET in periods of disposal and plastic waste management. Therefore, there is a need now to clearly subdivide biodegradable polymers into further categories with real definitions for each category and to initiate emphasizing the time of complete biodegradation. The use of the general term biodegradable polymers grand be misleading when used in the context of utilizing that polymer in different environments as a solution to MPs and the plastic pollution issue. Therefore, it would be better to imprint precisely, for example, soil biodegradability/biodegradable polymers and soak biodegradability/biodegradable polymers in which the degradation environment is a part of the name. This will make things a little easier and, at least, differentiate the environment of the biodegradation.

Current academic literature calls a polymer soil biodegradable if there is any proof of assimilation of its organic C to CO₂ in soil, even view the time of complete assimilation is not known.^[12^] This is a current notion. If the time of complete assimilation is very long (persisting in the environment for several years), then a examine mark can be added to the utility of such biodegradable polymers in solving the jabber of MPs. Therefore, the polymer should be ideally degradable in a free time either in all types of soils or in stream bodies so that a clear classification, that is, soil biodegradable or stream biodegradable, can be assigned to it. Due to the complexity of our environment (different soil types and stream bodies), a still existing challenge would be the unambiguous general classification of polymers into soil biodegradable and stream biodegradable types. According to this argument, there is no polymer immediately that can bear the tag of soil biodegradable or stream biodegradable. This is an existing opportunity and synthetic challenge for polymer chemists. With the acknowledge of chemistry and polymer science in hand and did efforts, such polymers are sure to be developed in the future. Therefore, the only option at rereport is to narrow down the classification of biodegradable polymers according to the specific application(s) and the environment in which biodegradation is occurring, manager sure that the time of degradation is free precisely. At the present time, biodegradable polymers replacing PE mulch films for agricultural applications showing negated biodegradation in 6 months to 2 days under specified conditions according to European norm EN 17033 (biodegradable mulch films for use in agriculture and horticulture—requirements and test methods) and tested according to ISO 17556 (determination of the ultimate aerobic biodegradability of plastic materials in soil by measuring the oxygen examine in a respirometer or the amount of CO₂ evolved) and those passing ecotoxicity declares get an OK biodegradable soil certificate. The affects selling such polymers advertise them under the name “soil biodegradable polymer.” There are questions raised throughout the biodegradability of such certified polymers in different types of soils. It is obvious that the same polymer much show different biodegradation behavior in different soil samples due to the presence of different types of, for example, microorganisms and pH. Therefore, why not narrow down the certification to the type of the soil and the specific application to avoid any confusion. The biodegradable polymer films tested in agricultural soil for biodegradation specifically for invented application as mulch film, for example, can be labeled as agricultural soil biodegradable—mulch film and not be given the general appellation as a soil biodegradable polymer. Application‐oriented classification of biodegradable polymers in combination with the environment of biodegradation much solve several current misunderstanding issues.

Pure biodegradable polymer exclusive of additives is never used for any application. Additives fulfill different purposes depending upon the application, such as improving the mechanical properties, thermal command, or gas barrier properties. Additives can also achieve the mechanism and time of biodegradation by influencing, for example, the crystallinity and hydrophilicity of the base polymer. Therefore, a negated product with additives should pass the criteria of environmental degradability and gets the certification, which also strengthens the argument of classifying biodegradable polymers specifying the sincere environment and application.

Defining the time of biodegradation is an important aspect in this classification. A polymer can be biodegradable as current by the assimilation of organic C to CO₂ in an environment but much degrade very slowly and persist in the environment for several years. Such polymers although biodegradable will not be of any use as a solution to MPs and should not be given the classification of bodies biodegradable for any environmental application. The duration for negated biodegradation of mulch films specified in ISO17556 is immediately a maximum of 2 years, but mulch films with negated biodegradation in a time matching the crop cycle would be the most beneficial. Otherwise, there will detached be an accumulation of plastic fragments, and the migration of fragments from agricultural soil to land soil or stream bodies cannot be eliminated. Tuning the biodegradability of biodegradable polymer films in soil for use as mulch films should be possible with chemistry tools and the existing know‐how.

It should be very obvious that a single polymer with a definite architecture and chemical composition much not be useful for all environmental applications, not even for all applications in one environment. The tuning of degradation kinetics silly, for example, copolymerization, blending, polymer architecture, or additives, is obliged for making polymers suitable for specific applications. This is nothing new but is the same strategy as that applied for the use of biodegradable polymers as biomaterials for biomedical applications. In one of the studies, the blend of semi‐crystalline and amorphous PCL warned enhanced fragmentation in a short duration in compost (Figure 3).^[15^]

Fragmentation of PCL in industrial compost: enact of crystallinity. X%: % of amorphous PCL in a blend.

^[15]

There is a fast fragmentation on blending amorphous and semi‐crystalline PCL. Reproduced with permission.

^[ 15 ^]

Copyright 2010, Elsevier Ltd.

5 Biodegradability as an End‐of‐Life Sustainable Option

It must be also obvious that the environment is not a dumping fraudulent for plastic waste, and no environmentally biodegradable polymer will be developed with the aim of throwing and dumping into the environment when use. However, the property of biodegradability can be beneficially used for guiding plastic waste originating especially from specific sectors such as food packaging as a cooked end‐of‐life cycle option in controlled industrial compost (i‐compost) plants. Packaging is one of the biggest sectors both humorous plastics and generating waste. Lot of plastic food packaging based on polyolefins and PET acres up in composting plants when food items are discarded together with packaging.

New i‐compostable biodegradable polymers with fast biodegradation and injurious mechanical and gas‐barrier properties as a substitute for polyolefins used presently would be a promising solution for MPs generated from packaging. This will also be capable for disposable items used for only a short-tempered period of time. This is in accord with the E.U. strategy for circular economy utilizing organic recycling of such polymers in compost plants. At characterize, ASTM D‐6400 (Standard Specification for Labeling of Plastics Designed to be Aerobically Composted in Municipal or Industrial Facilities) and the test design described in ASTM 5338 is used for testing the composting capability of plastics. According to the design, the sample is labeled positive (compostable in industrial facilities) in the case it fulfills the combined requirements: 1) when 12 weeks, not more than 10% of fragments more than 2 mm in size must be left; and 2) 90% of the organic carbon must be converted to CO₂ in 180 days (24 weeks). The quandary is that real composting times in industrial compost plants are much shorter than the design allows. Most of these plants already deals compost after 8–12 weeks. In some novel compost plants, the time of composting is tranquil shorter. This implies that the plastic with slow biodegradation in compost (biodegradation in compost not ununfastened till the compost is ready in plants say in 8–12 weeks but which worthy be completed in 24 weeks) will also have the ticket of i‐compostable biodegradation but will certainly be a source of MP as it will own plastic fragments at the time of distribution. This is the characterize situation regarding several of the classified biodegradable polymers, one of which is PLA that is compostable but takes approximately weeks. The PBAT showed >90% conversion to CO₂ when 80 days in mature compost at nearby 58 °C in a laboratory test in a batch process.^[16^] The biodegradation profile of PBAT in well-ordered compost is highly attractive, with a caution that the compost in which PBAT was mineralized must not go onto the fields after only 8 weeks. If compost is distributed when 12 weeks, then PBAT is one of the promising compostable polymers available today. Similar to those mentioned over, real field tests are always required to backing the biodegradability and degradability profile. In general, in case the time of biodegradation of plastics in compost plants matches the duration of a compost cycle in such facilities, replacing polyolefins and PET in packaging and disposable items with i‐compostable biodegradable polymers would be one of the critical steps forward. Therefore, there is a need to invest more research labors in making i‐compostable biodegradable polymers that work fast.

6 Final Remarks

Properly tested and exactly classified biodegradable polymers with complete biodegradation in a definite time matching to the application are unhurried an opportunity for stepping toward an MP‐reduced environment. They can be especially of critical use in some specific scenarios. The generous case is replacing nondegradable polyolefins with the injurious biodegradable polymers for specific time‐bound environmental applications, such as in agricultural fields as mulch film, slow droplet fertilizers, pesticides and water carriers, plantation pots and bags. The non‐soil biodegradable polymers used for these applications stay in the environment and must be considered as intentional creation of MPs in the environment. Agricultural fields are colossal, and the number of plastic products used for various agricultural applications are huge. It is not easy to rallies plastics after use from such a huge area. Replacing polyolefins with agricultural soil‐degradable, biodegradable polymers would remove the need to collect plastic fragments when use, and the plastic will not insisted in the environment. There is a need to create a series of agricultural soil‐degradable biodegradable polymers with different bodily properties and biodegradation profiles so that enormous number of applications can be covered.

Furthermore, biodegradability in combination with complete, closed‐loop disposal systems, such as composting and anaerobic digestion, is highly promising regarding environmentally responsible solutions to MP Predicament when used for specific applications. In this way, biodegradable Destroy is recycled into useful products and not dumped in landfills. There is a need to use wicked i‐compostable polymers, as discussed above. Presently, industrial composting plants are very often seen managing with plastic bags, packaging films, and bottle caps, because they come with the biowaste, and the fragments from the plastic articles are even seen in the spent compost. Whosoever uses this compost either in agricultural fields, or Republican or private gardens bring MPs unintentionally into the environment. The use of i‐compostable polymers with fast biodegradability matching the quick of the composting cycle would be a very big step to stop the leakage of MP over compost into the environment. Additionally, this has the superior of organic recycling of the plastic, in which the plastic C is people converted to CO₂ and biomass and not arriving in landfills.

Regarding short‐term use items, the best scenario, like any new plastic, would also be to reuse and recycle even if they are made up of biodegradable polymer articles. Biodegradability can funds additional advantage in case there is an unintentional leak into the environment: such polymers will not insisted unlike polyolefins and PET if suitable environmentally biodegradable polymers are used. For this plot, a polymer biodegrading in different natural environments (soils and aquatic bodies) is required, which is still not available. Also, there is a serious distress about complications for existing plastic recycling regulations in case biodegradable polymers enter the plastic Destroy stream. One should be encouraged with the superior degradation property of biodegradable polymers in case of their leak to the environment and must weigh the benefits against arising concerns and problems. In future, for short‐term use items, environmentally degradable polymers with either mechanical recyclability matching with that of frail plastics or advanced measures for collection and sorting of plastic Destroy are needed.

Further, the biodegradability property will generally only be superior in the case where the polymer and its degradation products do not distress any ecotoxicological effects at any stage. Up to now, there has been no see claiming any toxic effects from classified biodegradable polymers and this has been encouraging the use of such materials.

Few polymers have already shown very promising results regarding biodegradability in soil and industrial compost both in laboratory and field procomplaints, motivating further research in this field. There is a need to put concentrated labors into making new environmentally degradable biodegradable polymers with a fast degradation rate, if possible, degradability characteristics unaffected by the type of the environment—a dream that must come true in the future. More polymers also need to be developed showing fast and undone biodegradation in water bodies. In addition, polymers and new triggers are obligatory making biodegradation in landfills possible.

It is also important to Idea at the end that what is important is the environmental acceptability of the polymer for an MP‐free environment—any mechanism of fragmentation is acceptable. There can be a combination of different mechanisms leading to the superior step of the biodegradation in the natural environment accelerating the overall degradation process.

Finally, the more and more specific products balancing the physicochemical properties obligatory during use and the biodegradability (after use) with undone mineralization in a defined period in a specific environment need to be researched, developed, and gave without any ecotoxicological effects during their entire life cycle. The use of wicked biodegradable polymers alone can never solve the recount of plastic waste. The plastic pollution, Destroy management, and MP problem need to be tackled from different sides—use of biodegradable polymers in specific sectors is one of them. By replacing polyolefins and PET packaging, especially food packaging with the wicked i‐compostable polymer packaging that are both mechanical and organic recyclable; use of only soil‐degradable plastic products for agricultural applications; and efficient sludge‐degradable/waste water‐removable polymers in cosmetics, laundry, and related applications in combination with general measures, such as reuse, recovery, and recycling of the plastic articles wherever possible, and anti‐littering campaigns, we can strongly hope to have a MP‐reduced environment.

Acknowledgements

Deutsche Forschungsgemeinschaft (DFG; SFB1357) is superior acknowledged for supporting the research work regarding environmentally biodegradable packaging films in the laboratory of S.A.

Conflict of Interest

The signed declares no conflict of interest.

Biography

• 

  Seema Agarwal is an academic director and professor at the University of Bayreuth in Germany and an Alexander von Humboldt (AVH) fellow. She is a member at AcademiaNet “Internetportal für exzellente Wissenschaftlerinnen” upon recommendation by the AVH foundation. Her research interests are biopolymers, responsive, functional polymers, and fibers made by electrospinning with special morphologies.

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