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Article The Management of Polymer and Biodegradable Composite Waste in Relation to Petroleum-Based Thermoplastic Polymer Waste—In Terms of Energy Consumption and Processability

Tomasz Stachowiak 1,* and Katarzyna Łukasik 2

1 Faculty of Mechanical Engineering and Computer Science, Czestochowa University of Technology, 42-201 Cz˛estochowa,Poland 2 Faculty of Management, Czestochowa University of Technology, 42-201 Czestochowa, Poland; [email protected] * Correspondence: [email protected]

Abstract: The article presents a comparative analysis of the flow and utilisation of biodegradable polymer waste in relation to the waste of petroleum-based thermoplastic polymers. It compares energy expenditures and the costs of the reutilisation of both types of plastics in industrial applications. The performed studies and an analysis of the yielded results enabled the acquisition of real data involving the subject of managing petroleum-based plastic waste after the end of its life cycle, as well as biodegradable plastic waste, over the recent years, which is the main purpose of the study. So far, this subject has not been analysed very frequently, and, considering climate change, the predatory economy and the growing population of our planet, it is becoming an important topic, within the scope of which it is necessary to develop a new approach and new solutions regarding legal regula- Citation: Stachowiak, T.; Łukasik, K. tions and social awareness, as well as the technological possibilities of their implementation. The The Management of Polymer and authors’ own research will indicate factual results related to managing various types of waste, based Biodegradable Composite Waste in on the example of data acquired from a company involved in the retreatment of plastics and give Relation to Petroleum-Based answers to bothering questions such as: Is there an impact of retreatment on technological indicators Thermoplastic Polymer Waste—In defined by means of the mass flow rate? Is the retreatment of biodegradable plastics justified in terms Terms of Energy Consumption and Processability. Sustainability 2021, 13, of economy, energy and ecology? Is the retreatment of biodegradable plastics efficient? 3701. https://doi.org/10.3390/ su13073701 Keywords: waste management; biodegradable plastics; polymers; recycling

Academic Editor: Antonio Zuorro

Received: 21 February 2021 1. Introduction Accepted: 21 March 2021 Nowadays, the plastic industry is facing the challenge of efficient waste management Published: 26 March 2021 aimed at minimising the degradation of the natural environment. It is believed that the main issues that require actions for the abovementioned purpose include, e.g., the Publisher’s Note: MDPI stays neutral reduction of the regular release of greenhouse gases, paying attention to the shortage with regard to jurisdictional claims in of space available for waste removal (escalating accumulation of waste materials), or published maps and institutional affil- the use of natural deposits for the production of plastics. The reduction of these effects, iations. which are detrimental to the natural environment, primarily comes down to proper waste management, which is being demanded by the anxious society, and which results in strategies established under domestic public policy, strictly related to the institutional, legal, political and economic context of every country [1]. The EU has one of the world’s highest Copyright: © 2021 by the authors. environmental standards, developed over the course of decades. Its environmental policy Licensee MDPI, Basel, Switzerland. helps to make the EU’s economy more environmentally friendly; it protects the natural This article is an open access article resources of Europe and guarantees the health and well-being of the EU’s inhabitants distributed under the terms and (articles 11 and 191–193) [2]; in spite of this, the statistical data presented in the following conditions of the Creative Commons part of the paper indicates that the effects of waste management are still quite controversial Attribution (CC BY) license (https:// and require even more radical actions for more efficient waste management in Europe. creativecommons.org/licenses/by/ These actions must lead to the offering of solutions favouring sustainable management of 4.0/).

Sustainability 2021, 13, 3701. https://doi.org/10.3390/su13073701 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, x FOR PEER REVIEW 2 of 18

Sustainability 2021, 13, 3701 part of the paper indicates that the effects of waste management are still quite controver-2 of 16 sial and require even more radical actions for more efficient waste management in Europe. These actions must lead to the offering of solutions favouring sustainable management of waste, simultaneously promoting its recycling and efficient conversion of waste into en- waste, simultaneously promoting its recycling and efﬁcient conversion of waste into energy ergy and other precious chemicals [3]. These conversion procedures can be reached via and other precious chemicals [3]. These conversion procedures can be reached via the use the use of biological processes, such as anaerobic decomposition, or thermochemical pro- of biological processes, such as anaerobic decomposition, or thermochemical procedures, cedures, such as pyrolysis. The following paper shows which results of the above have such as pyrolysis. The following paper shows which results of the above have already already been achieved. been achieved.

2. Review Review of of the the L Literatureiterature 2.1. Waste Waste M Managementanagement Currently, the system of waste management (WM) and disposal in various industries isis considered to to be one of the most important issues issues of of environmental management management [ [44].]. Since efficient efficient WM is of keykey significancesignificance for sustainable development [ 5,6], its efficiencyefficiency constitutes a basis for environmental policy all over the world [7], and the world of politics and industry is considering newer and newer methods for the recycling of plastics [ 8], or their application to energy recovery, limiting the development of waste plastic landfillslandfills ((FigureFigure 11);); thisthis primarilyprimarily involvesinvolves polymers,polymers, polymerpolymer materialsmaterials andand polymerpolymer compositescomposites contaminated with with additional additional substances, substances, all all of of which which are are hard hard to toprocess process [9]. [ 9Due]. Due to the to above,the above, the studies the studies of waste of waste management management are pursued are pursued at a large at a large scale scale both bothin Europe in Europe and inand the in world the world [10–14 [10]. What–14]. is What equally is equally important important is that the is thatcommonly the commonly used waste used manage- waste mentmanagement methods methods presented presented in Figure in 1 Figure do not1 doalways not always work to work the same to the advantageous same advantageous extent duringextent duringwaste retreatment. waste retreatment.

Figure 1. Methods for managing polymer and composite plastics.

WWMM methodsmethods shouldshould complement complement each each other; other e.g.,; e.g. energy, energy recovery recovery is a necessary,is a necessary, fully fullyfunctional functional method method for utilising for utilising waste waste plastics, plastics, complementary complementary to recycling, to recycling, enabling enabling full fullutilisation utilisation of the of potentialthe potential of waste of waste for thefor productionthe production of energy of energy and and heat heat [15]. [1 Recycling5]. Recy- clingmay provemay prove to be unecologicalto be unecological and uneconomic and uneconomic for the for recovery the recovery of certain of certain plastics, plastics, due to duevarious to various factors factors limiting limiting the capabilities the capabilities of recycling of recycling [16,17]: [16,17]: • TThehe magnitude and and quality quality (homogeneity, (homogeneity, purity, purity, toxicity) of the waste stream stream gath- gath- ered under selective collection, • AAvailablevailable sorting technologies,technologies, •  MMarketarket demand demand for for products products recovered recovered from from plastic plastic waste waste and andthe requirements the requirements con- cerningconcerning their their quality. quality. Solid waste management is is a a problem of EU politicians, as well as as,, primarily primarily,, most contemporary production companies, especially from the industry of artiﬁcial materials. contemporary production companies, especially from the industry of artificial materials. With the minimisation of traditional landﬁlling, plastics are currently undesirable for the With the minimisation of traditional landfilling, plastics are currently undesirable for the environment. Moreover, there is a problem of the generation of enormous amounts of environment. Moreover, there is a problem of the generation of enormous amounts of polymer waste in certain countries of the EU. Most solid polymers (plastics) are used as protective coatings of everyday items, packaging, bottles, and electronic devices. The ability to design polymer materials with varying properties causes them to also serve an important function in the development of medicine or the transport industry (aviation, Sustainability 2021, 13, 3701 3 of 16

automotive, space industry). In most cases, they are designed and manufactured to be resistant to environmental degradation, including biodegradation, which is their greatest drawback. However, plastics are still more economical than metals, wood and glass in terms of the costs of their production, the weight to strength ratio and the amount of required energy and water; on the other hand, they are produced from petroleum, which is a non-renewable material whose resources may be depleted in the near future, which in fact generates a number of problems and causes the polymer waste management to be an urgent matter that requires the rapid development of newer and newer environmentally friendly solutions at a long-term scale [18]. This is because the production of plastics increased rapidly over only several decades from 1.5 million tonnes in 1950 to 359 million tonnes in 2018 globally. It was accompanied by an increase in the amount of plastic waste [19]. Currently, plastic waste constitutes a major problem all over the world [20]. Its scale is presented by the studies according to which as many as 75% of plastics produced so far currently constitute waste. To convert this data to numbers, it means as many as 6.3 billion tonnes of plastic trash [21–24]. In 2015, around 55% of global plastic waste was discarded, 25% was incinerated, and 20% was recycled [16]. However, in Europe during 2017, 32.5% of plastic waste was recycled, 42.6% was recovered through energy recovery processes and 24.9% was landfilled [25]. The statistics related to the reuse of these plastics are not any better; less than 10% of them were recycled and only 12% underwent energy recovery. The remaining part, which is approximately 5 billion tonnes, are currently dumped in landfills and in various places not intended for this, such as forests, meadows, illegal landfills and beaches, with the greatest damage being done by waste stored in the marine environment. Therefore, if the current production trends are maintained, then by 2050, plastics may be responsible for 20% of petroleum consumption, 15% of greenhouse gas emissions, and there may be more plastics than fish in the sea [26]. Disposable plastic packaging, used mainly in the food industry, generate the greatest amount of plastic waste. Among them, one should list packaging, such as shopping bags made of polyethylene and polypropylene, lunch bags and various kinds of wrappings, as well as PET (Poly (ethylene terephthalate)) bottles [27]. It is not only the food industry that uses plastics in its activities; in fact, this phenomenon is also noticed in other industries, e.g., packaging, building and construction, automotive, electrical and electronics, household, leisure and sport, agriculture and others. In the world, only 14% of used polymer packaging is currently being recycled; this number reflects the economic challenges resulting from the implemented collection and post-consumer processing of diverse packaging materials and formats, often using in- sufficiently developed systems of post-consumer utilisation [28]. While thermoplastics, constituting approximately 78% by mass of all plastic waste, can be easily recycled and consist mainly of polyolefins, such as PP (Polypropylene), PE (Polyethylene), PVC (polyvinyl chloride) and PS (polystyrene), the remaining part includes thermosetting plastics, which are not so easily recyclable [29]. Due to the unified effort aimed at new designs and systems of post-consumer utilisation, recycling can become an economically appealing alternative for the remaining 50% of polymer packaging. Although the viability of recycling is higher for certain types of packaging, such as, e.g., PET bottles, in general, the cost of segregation and other activities outweighs the income. In Europe, it is estimated that this cost ranges from 170 to 250 dollars per one tonne of collected waste, considering the average value resulting from the diversification of the collection and segregation systems, legal environment, geographical conditions and package types. However, this does not include additional ecological and social benefits of polymer recycling, such as: the limited emission of greenhouse gases, lower impact on the natural environment (soil and air quality) and the creation of jobs. For example, collecting one tonne of waste for recycling allows avoiding the emission of one tonne of CO2 equivalent (compared to storage and combustion for energy recovery). This translates into a social value of about 100 dollars per one tonne of waste collected with the intention of recycling [30]. Sustainability 2021, 13, 3701 4 of 16

Europe is facing a challenge, because, according to the European Commission, by 2030, all plastic packaging in the EU will be reusable or recyclable in a feasible manner, and more than half of all plastic waste generated in Europe will be recycled [31]. The EU’s policy concerning waste is clear—the establishment of a circular economy, in which materials and resources are maintained in management for as long as possible, and where waste disposal is the ﬁnal option of waste management, which constitutes progress towards more recycling and a smaller amount of stored waste [32].

2.2. Statistical Data-Scale of the WM Problem Therefore, it is the intention of the EU’s waste management policy to reduce the impact of waste on the environment and health, as well as to improve the efﬁciency of the utilisation of the EU’s resources. The long-term goal of these policies involves reducing the amounts of the generated waste, and when it cannot be avoided, promoting it as a resource and reaching higher levels of recycling and safe waste removal. Hazardous waste can pose an elevated risk to human health and the environment if it is not managed and removed in a safe manner. Among all waste generated in the EU in 2016, 94.7 million tonnes (4.2% of all) were categorised as hazardous waste [33]. In 2016, approximately 2097 million tonnes of waste were processed in the EU. This does not include exported waste, but it does include the treatment of waste imported into the EU, which is why the declared amount cannot be directly compared to data involving the generation of waste. Figure2 presents the development of waste treatment in the EU in a general aspect, as well as the two main categories of treatment—recovery and disposal—in the years 2004 to 2016. The amount of waste that is being recovered, meaning subjected to recycling, used for backﬁlling (the use of waste in excavated areas for the restoration of escarpments or for safety, or for engineering purposes in landscaping), or burnt for energy recovery, increased by 26.7% from 870 million tonnes in 2004 to 1103 million tonnes in 2016. As a result, the share of such recovery in all of waste treatment increased from 45.9% in 2004 to 52.6% in 2016. The amount of waste subjected to disposal decreased from 1027 million tonnes in Sustainability 2021, 13, x FOR PEER REVIEW2004 to 995 million tonnes in 2016, which constitutes a drop by 3.1%. The overall share5 of of 18

waste disposal dropped from 54.1% in 2004 to 47.4% in 2016.

Waste treatment, EU-27, 2004 to 2016 (Index 2004 = 100) 140

120

100

0 2004 2006 2008 2010 2012 2014 2016

Recovery Total Disposal

FigureFigure 2. 2.The The progress progress of of waste waste treatment treatment in in the the EU EU in ain general a general aspect, aspect, as well as well as in as terms in terms of recovery of re- andcovery disposal and disposal (in the years (in the 2004 years to 2016) 2004 (source:to 2016) Figure(source: modiﬁed Figure modified from [33 ]).from [33]).

Referring to the considerations of plastic waste management, it is estimated that approximately 42% of plastic packaging waste were recycled in 2017 in the EU. In seven member states of the EU, more than half of the exported plastic packaging waste was recycled in 2017 (Figure 3).

Evolution of the recycling rate of plastic packaging waste in the EU, 2006 to 2017 45 40 35 30 25 20

Recycling rateRecycling 15 10 5 0 2006 2008 2010 2012 2014 2016 2017 Year

Figure 3. Plastic waste management in the EU in 2017 (source: Figure modified from [33]).

Compared to 2005, the recycling rate of plastic packaging waste in the EU increased by 18% (from 24% in 2005 to 42% in 2017). This upward trend is observed with various intensities in all member states of the EU, except Croatia. In 2017, each inhabitant of the EU accounted for 173.8 kg of packaging waste. This value ranged from 63.9 kg per inhabitant in Croatia to 230.9 kg per inhabitant in Luxembourg. The highest shares corresponded to paper and cardboard (41%), plastics (19%), glass (18%), wood (17%) and the common types of packaging waste in the EU. Sustainability 2021, 13, x FOR PEER REVIEW 5 of 18

Waste treatment, EU-27, 2004 to 2016 (Index 2004 = 100) 140

120

100

0 2004 2006 2008 2010 2012 2014 2016

Recovery Total Disposal Sustainability 2021, 13, 3701 5 of 16 Figure 2. The progress of waste treatment in the EU in a general aspect, as well as in terms of recovery and disposal (in the years 2004 to 2016) (source: Figure modified from [33]).

ReferringReferring to the considerations considerations of of plastic plastic waste waste management, management, it is it estimated is estimated that ap- that approximatelyproximately 42% 42% of ofplastic plastic packaging packaging waste waste were were recycled recycled in in 2017 2017 in in the the EU. EU. In In seven seven membermember statesstates of the the EU, EU, more more than than half half of ofthe the exported exported plastic plastic packaging packaging waste waste was wasre- recycledcycled in in 2017 2017 (Fi (Figuregure 3).3).

Evolution of the recycling rate of plastic packaging waste in the EU, 2006 to 2017 45 40 35 30 25 20

Recycling rateRecycling 15 10 5 0 2006 2008 2010 2012 2014 2016 2017 Year

FigureFigure3. 3.Plastic Plastic wastewaste managementmanagement in the EU in 2017 (source: Figure Figure modified modiﬁed from from [ [3333]).]).

ComparedCompared toto 2005,2005, thethe recyclingrecycling rate of plastic packaging waste waste in in the the EU EU increased increased byby 18%18% (from(from 24%24% inin 20052005 toto 42%42% inin 2017).2017). This upward trend trend is is observed observed with with various various intensitiesintensities in in all all member member states states of of the the EU, EU, except except Croatia. Croatia. In In 2017, 2017, each each inhabitant inhabitant of theof the EU accountedEU accounted for 173.8 for 173.8 kg of kg packaging of packaging waste. waste. This This value value ranged ranged from from 63.9 63.9 kg perkg per inhabitant inhab- initant Croatia in Croatia to 230.9 to kg 230.9 per inhabitant kg per inhabitant in Luxembourg. in Luxembourg. The highest The shares highest corresponded shares corre- to papersponded and to cardboard paper and (41%), cardboard plastics (41%), (19%), plastics glass (18%), (19%), wood glass (17%)(18%), and wood the (17%) common and types the ofcommon packaging types waste of packaging in the EU. waste in the EU. On the other hand, compared to 2016, the amount of packaging waste generated in 2017 increased by 2.9%. The amount of recycled packaging waste and recovered packaging waste increased by 2.8%, respectively. While the amount of generated packaging waste increased by 7.4% in the years 2007 to 2017, both its recycling (+22.5%) and recovery (+18.8%) in 2017 were considerably higher than in 2007 (Figure4).

2.3. Biodegradable Plastic Waste Management As already mentioned above, the largest waste stream consists of packaging, which is usually generated from polymers such as PE, PP and PET. Biodegradable plastics are a perfect alternative for this type of materials, since they are thin-walled products that, after a short time, will undergo decomposition/biodegradation. It should be noted in here that this does not mean arbitrary disposal of packaging made of biodegradable plastics, since their decomposition requires speciﬁc conditions (temperature, humidity, bacteria, oxygen, etc.). Polymers that consist mainly of petroleum are categorised in the manner presented in Figure5. Sustainability 2021, 13, x FOR PEER REVIEW 6 of 18

On the other hand, compared to 2016, the amount of packaging waste generated in 2017 increased by 2.9%. The amount of recycled packaging waste and recovered packag- Sustainability 2021, 13, 3701 ing waste increased by 2.8%, respectively. While the amount of generated packaging6 of 16 waste increased by 7.4% in the years 2007 to 2017, both its recycling (+22.5%) and recovery (+18.8%) in 2017 were considerably higher than in 2007 (Figure 4.).

Packaging waste generated, recovered and recycled, EU-27, 2007 to 2017 (kg per capita) 200

150

100

2008 2010 2012 2014 2016 2018 Waste generated, kg percapita kgWaste generated, Year

Waste Recovered Recycled

SustainabilityFigure 4. The 2021 amount , 13, x FOR of PEERgenerated REVIEW packaging waste, its recycling and recovery (in the years 2007 to 2017) (source: Figure 7 of 18 modified from [33]). modiﬁed from [33]). 2.3. Biodegradable Plastic Waste Management As already mentioned above, the largest waste stream consists of packaging, which is usually generated from polymers such as PE, PP and PET. Biodegradable plastics are a perfect alternative for this type of materials, since they are thin-walled products that, after a short time, will undergo decomposition/biodegradation. It should be noted in here that this does not mean arbitrary disposal of packaging made of biodegradable plastics, since their decomposition requires specific conditions (temperature, humidity, bacteria, oxygen, etc.). Polymers that consist mainly of petroleum are categorised in the manner presented in Figure 5.

Figure 5. Categories of polymerFigure 5. materialsCategories generated of polymer from materials petroleum generated and its derivatives from petroleum (source: and Figure its derivatives modified (source: from [34]). Figure modiﬁed from [34]).

OnO then the other other hand, hand, with with respect respect to biodegradableto biodegradable polymers, polymers, it is it possible is possible to categoriseto categorise themthem in ain manner a manner presented presented in thein the diagram diagram below below (Figure (Figure6). 6). The ﬁrst polymers categorised as biodegradable plastics were invented in the 1930s, namely PLA, which was not in wider use until the beginning of the 21st century. Currently, most products made of biodegradable materials are indeed produced using PLA. However, it should be noted that this market is young and its situation changes extremely dynamically. It is becoming more common that the producers of polymer plastics turn to biodegradable materials, which speeds up the work on new biopolymers with surprising physical and chemical properties enabling their use in newer and newer applications that are becoming more in demand (Figure7)[35].

Figure 6. Classification of bio-based and biodegradable polymers (source: Figure modified from [34]).

The first polymers categorised as biodegradable plastics were invented in the 1930s, namely PLA, which was not in wider use until the beginning of the 21st century. Cur- rently, most products made of biodegradable materials are indeed produced using PLA. However, it should be noted that this market is young and its situation changes extremely Sustainability 2021, 13, x FOR PEER REVIEW 7 of 18

Figure 5. Categories of polymer materials generated from petroleum and its derivatives (source: Figure modified from [34]). Sustainability 2021, 13, 3701 7 of 16 On the other hand, with respect to biodegradable polymers, it is possible to categorise them in a manner presented in the diagram below (Figure 6).

Sustainability 2021, 13, x FOR PEER REVIEW 8 of 18

dynamically. It is becoming more common that the producers of polymer plastics turn to biodegradable materials, which speeds up the work on new biopolymers with surprising Figure 6. ClassificationphysicalFigure 6. Classiﬁcationand of bio chemical-based ofand prope bio-based biodegradablerties and enabling biodegradable polymers their (source:use polymers in newer Figure (source: and modified Figurenewer from modiﬁed applications [34]). from [that34]). are becoming more in demand (Figure 7) [35]. The first polymers categorised as biodegradable plastics were invented in the 1930s, namely PLA, which was not in wider use until the beginning of the 21st century. Cur- rently, most products made of biodegradable materials are indeed produced using PLA. However, it should be noted that this market is young and its situation changes extremely

Figure 7. Diagram presenting the relationships between petroleum petroleum-based-based and bio-basedbio-based plastics (source: Figure modified modiﬁed from [35]).]).

BBio-basedio-based plasticsplastics cancan circulatecirculate inin a a closed closed loop, loop, which which complies complies with with the the approach approach of ofsustainable sustainable development, development, but but also also with with the the approach approach related related to to the the closed closed life life cycle cycle of of a aproduct, product, LCA LCA [36 [36,37,37] (Figure] (Figure8). 8). Sustainability 2021, 13, x FOR PEER REVIEW 9 of 18 Sustainability 2021, 13, 3701 8 of 16

Figure 8. Diagram of a closed loop of polymer materials (source: Figure modified from [35]). Figure 8. Diagram of a closed loop of polymer materials (source: Figure modified from [35]). Bio-based and, most importantly, biodegradable polymers currently seem to be the onlyB alternativeio-based and, and themost right importantly, direction ofbiodegradable industrial development, polymers currently considering seem the to dramati- be the onlycally alternative progressing and climate the right change direction and the of continuously industrial development, increasing burden considering on the the natural dra- maticallyenvironment. progressing In the caseclimate of bio-basedchange and plastics, the contin polymeruously materials, increasing such burden as polyolefins, on the nat- uralPET environment. or PVC, etc., canIn the be case produced of bio-based plastics, on biological polymer substances materials, or such partially as polyolefins, based on PETbiological or PVC, substances. etc., can Thesebe produced plastics based technically on biological correspond substances to their equivalentsor partially producedbased on from petroleum; however, they help to reduce the carbon footprint of the final product. biological substances. These plastics technically correspond to their equivalents produced Moreover, they can be subjected to mechanical recycling in existing recycling streams. from petroleum; however, they help to reduce the carbon footprint of the final product. Biodegradable plastics are those that undergo total decomposition by microorganisms into Moreover, they can be subjected to mechanical recycling in existing recycling streams. biomass, carbon dioxide (or methane) and water under strictly specified conditions [38–40]. Biodegradable plastics are those that undergo total decomposition by microorganisms However, it should be noted that biodegradable plastics will not necessarily undergo into biomass, carbon dioxide (or methane) and water under strictly specified conditions decomposition after their disposal (which may be a tempting solution for many people); [38–40]. while they do require specifying the timeframe of biodegradation, its level and the required However, it should be noted that biodegradable plastics will not necessarily undergo specific conditions, such as temperature, humidity, the availability of oxygen, the presence decomposition after their disposal (which may be a tempting solution for many people); of bacteria, etc. [34]. while they do require specifying the timeframe of biodegradation, its level and the re- Therefore, the approach stating that bio-based plastics have an open road to recycling quiredand their specific reuse, conditions, similar to polymers such as temperature, derived from humidity, fossil fuels, the while availability biodegradable of oxygen, plastics the presencego only to of a composter,bacteria, etc. may [34 be]. misguiding and lead to the emergence of another problem— beingTherefore, flooded by the biodegradable approach stating waste that without bio-based being plastics able to have know an how open to road decompose to recycling it all andin due their time. reuse, similar to polymers derived from fossil fuels, while biodegradable plastics go onlyIn theto a case composter, of plastics may derived be misguiding from fossil and fuels, lead the to th possibilitye emergence of their of another reutilisation prob- lemhas— beenbeing mentioned flooded by above. biodegradable In the case waste of thermoplastics, without being it isable possible to know to perform how to decom- several posetypes it of all recycling, in due time. including material recycling, chemical recycling (for selected polymers, it is possibleIn the case to decomposeof plastics derived them to from primary fossil monomers fuels, the and possibility use them of for their the reutilisation production hasof fuels), been mentioned as well as the above. so-called In the energy case of recovery, thermoplastics, based on it is recovering possible to the perform energy several stored typesin polymers of recycling, via their including combustion material under recycling, controlled chemical conditions recycling (the (for main selected factor polymers, of safe itcombustion is possible beingto decompose the temperature). them to primary monomers and use them for the production of fuels),In the as casewell ofas biodegradablethe so-called energy plastics, recovery, this rule based has noton recovering yet been deeply the energy considered stored inand polymers investigated. via their The combustion main assumption under of controlled biodegradable conditions plastics (theand main their factor utilisation of safe combustioninvolves the being possibility the temperature). of their safe use and, after completing the life cycle of the product, composting,In the case intended of biodegradable to lead to plastics, their decomposition this rule has not into yet simpler been deeply substances considered with and no investigated.negative impact The on main the naturalassumption environment of biodegradable [41–43]. Theplastics diagram and their below utilisation presents involves possible biodegradation mechanisms for biodegradable polymers (Figure9). Sustainability 2021, 13, x FOR PEER REVIEW 10 of 18

the possibility of their safe use and, after completing the life cycle of the product, composting, intended to lead to their decomposition into simpler substances with no negative Sustainabilityimpact on2021 the, 13 ,natural 3701 environment [41–43]. The diagram below presents possible biodeg- 9 of 16 radation mechanisms for biodegradable polymers (Figure 9).

Figure 9. Mechanisms of biodegradationFigure 9. Mechanisms (the authors’ of biodegradation own research (the authors’ base ownd on research [44]). based on [44]).

3. Results and Discussion 3. Results and Discussion3.1. Method and Research Questions 3.1. Method and Research QuestionsThe adopted research method involved the analysis of actual results yielded when The adopted researchmanaging method the involved flow of a waste the analysis stream consisting of actual of biodegradableresults yielded plastics when and petroleum- based plastics. The data was acquired during the authors’ own measurements. The managing the flow of a wastefollowing stream research consisting questions of were biodegradable asked in relation plastics to the abovementioned and petroleum performance- of based plastics. The data thewas above acquired research. during the authors’ own measurements. The following research questions• wereQ1: Isasked there anin impactrelation of retreatmentto the abovementioned on technological indicatorsperformance defined of by means of the above research. the mass flow rate?  Q1: Is there an impact• ofQ2: retreatment Is the retreatment on technological of biodegradable indicators plastics justified defined in terms by means of economy, energy and ecology? of the mass flow rate?• Q3: Is the retreatment of biodegradable plastics efficient?  Q2: Is the retreatment of biodegradable plastics justified in terms of economy, energy The considerations and studies currently in progress are aimed at analysing the capa- and ecology? bilities of the retreatment and utilisation of biodegradable polymers. The thickness of the  Q3: Is the retreatmentproduct of biodegradable wall seems to be plastics the main efficient? factor deciding the possibility of their recycling. In the The considerations andcase ofstudies packaging, currently wrapping in andprogress all kinds are of aimed thin-walled at analys productsing (thin-walled the ca- products are to be understood as those whose wall thickness does not exceed one millimetre), retreat- pabilities of the retreatmentment and seems utilisation to make no of sense, biodegradable since they can polymers. be recycled inThe an thickness easy and effective of manner. the product wall seems toHowever, be the main attention factor should deciding be paid the to the possibility amount of of this their waste, recycling. and perhaps In the volume the case of packaging, wrappingwill decide and that all they kinds are worth of thin “giving-walled a second products life” after (thin all,-walled before they prod- are composted. ucts are to be understood as thoseIn the casewhose of products wall thickness whose wall does thickness not exceed exceeds oneone millimetre, millimetre composting), can be more difficult, or at least prolonged (the duration of composting and decomposition retreatment seems to make no sense, since they can be recycled in an easy and effective into simple substances extends along with an increase in the wall thickness); the performed manner. However, attentionstudies should of compostability be paid to andthe biodegradability amount of this are waste, mainly and related perhaps to wrapping. the volume will decide that theyTherefore, are worth if the “giving recycling a process second is usedlife” for after thermoplastic all, before polymer they materials, are should composted. a similar solution not be considered in relation to biodegradable plastics [45]. During the In the case of productsproduction whose ofwall biodegradable thickness plastics,exceeds energy one millimetre expenditures, relatedcomposting to their creationcan are also incurred. be more difficult, or at least Asprolonged shown, the (the carbon duration footprint of of composting biodegradable and polymers decomposition (in this case, PLA) is defi- into simple substances extendsnitely smaller along than with that of an their increase equivalents in the made wall of petroleum thickness) (Figure; the 10 ). per- Therefore, their formed studies of compostabilityretreatment and and biodegradability utilisation can also contribute are mainly even related more to to reducing wrapping. their environmental Therefore, if the recyclingimpact (Figures process 11 and is 12 used). for thermoplastic polymer materials, should a similar solution not be considered in relation to biodegradable plastics [45]. Dur- ing the production of biodegradable plastics, energy expenditures related to their creation are also incurred. As shown, the carbon footprint of biodegradable polymers (in this case, PLA) is def- initely smaller than that of their equivalents made of petroleum (Figure 10). Therefore, Sustainability 2021, 13, x FOR PEER REVIEW 11 of 18

their retreatment and utilisation can also contribute even more to reducing their environ-

Sustainability 2021, 13, x FOR PEER REVIEWmenta l impact (Figures 11 and 12). 11 of 18

Sustainability 2021, 13, x FOR PEER theirREVIEW retreatment and utilisation can also contribute even more to reducing their environ-11 of 18 mental impact (Figures 11 and 12).

their retreatment and utilisation can also contribute even more to reducing their environ- Sustainability 2021, 13, 3701 mental impact (Figures 11 and 12). 10 of 16

Figure 10. Material flowchart for plastics made of petroleum.

FigureFigure 10. 10. MaterialMaterial flowchart ﬂowchart for for plastics plastics made made of of petroleum. petroleum.

Figure 11. Material flowchart for biodegradable plastics with wall thickness of less than one millimetre—wrapping, packaging, etc. (the authors’ own research).

Figure 11. Material ﬂowchart for biodegradable plastics with wall thickness of less than one Figure 11. Material flowchart for biodegradable plastics with wall thickness of less than one milli- metremillimetre—wrapping,—wrapping, packag packaging,ing, etc. (the etc. authors’ (the authors’ own research). own research).

Figure 11. Material flowchart for biodegradable plastics with wall thickness of less than one millimetre—wrapping, packaging, etc. (the authors’ own research).

Figure 12. Flowchart of the new method for utilising biodegradable plastics, taking into account Figure 12. Flowchart of the new method for utilising biodegradable plastics, taking into account their recycling process and wall thickness of above one millimetre (the authors’ own research). their recycling process and wall thickness of above one millimetre (the authors’ own research). The analysis of the possibilities for retreatment biodegradable plastics and the feasi- The analysis of the possibilities for retreatment biodegradable plastics and the feasi- bility of this type of solutions included measuring the consumption of electrical energy bility of this type of solutions included measuring the consumption of electrical energy and other utilities necessary to process one kilogram of the abovementioned waste. The Figure 12. Flowchart of the new method for utilising biodegradable plastics, taking into account andconsiderations other utilities were necessary performed to process in relation one tokilogram the following of the polymerabovementioned materials waste. (Table The1): considerationstheir recycling process were performedand wall thickness in relation of above to the one followin millimetreg polymer (the authors’ materials own research). (Table 1): • Polymer materials produced from petroleum: PE, PET, PA 6.6,  Polymer materials produced from petroleum: PE, PET, PA 6.6, • TheBiodegradable analysis of the materials: possibilities polylactide for retreatment (PLA IngeoBiopolymer biodegradable plastics 4043D), and Bioplast the feasi- 105  Biodegradable materials: polylactide (PLA IngeoBiopolymer 4043D), Bioplast 105 (a bilityFigure of(a 12. this thermoplastic Flowchart type of ofsolutions the that new contains methodincluded afor large measuringutilising amount biodegradable the of consumption bio-based plastics, materials) oftaking electrical into and account Bioplastenergy thermoplastic that contains a large amount of bio-based materials) and Bioplast 300 andtheir other recycling300 (autilities thermoplastic process necessary and wall polymer to thickness process containing of one above kilogram one natural millimetre of potatothe abovementioned(the starch authors’ and own other research).waste. bio-based The considerationspolymers). were performed in relation to the following polymer materials (Table 1): The analysis of the possibilities for retreatment biodegradable plastics and the feasi-  bilityPolymer of this materialstype of solutions produced included from petroleum: measuring PE, the PET, consumption PA 6.6, of electrical energy  Table 1. Petroleum-based and biodegradable polymers considered in the study. andB otheriodegradable utilities necessarymaterials: topolylactide process one (PLA kilogram IngeoBiopolymer of the abovementioned 4043D), Bioplast waste. 105 The (a considerationsthermoplasticPetroleum-Based were that performed contains and ain Biodegradable large relation amount to the Polymers of followin bio-based Consideredg polymer materials) in materials the and Study Bioplast (Table 3001):  PolymerCommon materials Plastics produced from Technical petroleum: Plastics PE, PET, PA Biodegradable6.6, Plastics  BiodegradablePE 0.2 materials: polylactide (PLA PA IngeoBiopolymer 4043D), PLA Bioplast 105 (a thermoplasticPE 4 that contains a large amount PET of bio-based materials) Bioplast and 105Bioplast 300 PP 6 Bioplast 300

The materials for recycling originated from various sources. Polyoleﬁns (two varieties of polyethylene differing in the value of the mass ﬂow rate, marked as PE 0.2, for which the MFR (Melt Flow Rate) equaled 0.2 g/10 min, and PE 4, for which the MFR equalled Sustainability 2021, 13, 3701 11 of 16

4.0 g/10 min, as well as polypropylene, for which the MFR equaled 6.0 g/10 min) originated from the automotive industry (they were delivered in the form of ﬁnished products or waste from their production); polyamide and polyethylene terephthalate constituted technological waste originating from the automotive industry (this waste originated from the production of airbags, in the form of scraps of materials used for their production), while the plastics PLA, Bioplast 105 and Bioplast 300 were purchased in the form of granules, processed using the KraussMaffei KM65/160/C4 hydraulic injection-moulding machine in order to produce standardised mouldings. The samples were created according to the ISO 294 quality standard. Subsequently, these samples were ground using a high-speed SHINI SG-2417-CE mill. The pulp produced in this manner was subjected to the moisture removal process using a SHINI CD-60 shelf dryer, and subsequently subjected to the process of regranulation using the cascade extruder described below. The products of regranulation were tested for the mass ﬂow rate in order to determine basic technological parameters and identify the changes that had taken place in the material. The pattern of the retreatment biodegradable plastics is presented below. The wall thickness of the mould should be the main factor deciding the possibility of retreatment; in accordance with quality standards related to the process of biodegradation (ISO 14855), the Sustainability 2021, 13, x FOR PEER REVIEW 13 of 18 thickness of composted samples should not exceed one millimetre (as of today). Therefore, it seems to be an interesting solution to generate injected or extruded elements with wall thickness exceeding one millimetre with the possibility of their reutilisation and use in a new product with no need for disposal right after the end of its life cycle (Figure 13).

Figure 13. Diagram of the authors’ own developed classiﬁcation of biodegradable plastics depending on wall thickness (the authors’ own research). Figure 13. Diagram of the authors’ own developed classification of biodegradable plastics depending onProducts wall thickness made (the of biodegradableauthors’ own research). plastics with wall thickness of less than one millimetre (wrapping, packaging, etc.) can successfully undergo the process of composting, leadingAs toindicated their decomposition below in the paper, into simple it is possible substances to produce with no thick negative-walled impact details on from the naturalbiodegradable environment. plastics (standardised mouldings of the paddle type are characterised by wallAs thickness indicated of four below millimetres in the paper,), followed it is possible by their to grinding produce thick-walledand retreatment details (granula- from biodegradabletion) with the possibility plastics (standardised of retreatment mouldings (as indicated of the by paddle the mass type flow are rate characterised tests). by wall thicknessRetreatment of four of the millimetres), abovementioned followed polymers by their grindingwas analysed and retreatment using a single (granulation)-screw cas- withcade theextruder possibility from ofthe retreatment STARLINGER (as indicated company, by with the massa screw ﬂow diameter rate tests). of 85 mm in each extruder.Retreatment The treatment of the line abovementioned consisted of the polymers elements was presented analysed below, using each a single-screwof which was cascadecharacterised extruder by the from following the STARLINGER installed power company, (Table with 2). a screw diameter of 85 mm in each extruder. The treatment line consisted of the elements presented below, each of which wasTable characterised 2. Elements of by the the cascade following line used installed during power the tests. (Table 2).

Elements of the Cascade Line Used during the Tests No. Device name Rated power [kW] 1. Compactor 40 2. Extruder I 75 3. Sieve exchanger 5 4. Extruder II 75 5. Shaker 3 Vacuum pump and transport 6. 2 devices Combined power 200

3.2. Research Results The consumption of electrical energy was analysed in terms of the type of the processed material and its mass flow rate; the efficiency of the process (expressed in kg/h) was analysed in relation to these two factors. The table below includes data related to determining the mass flow rate according to the ISO 1133-1 quality standard. The test was performed using a Dynisco D4003DE plastometer. Temperatures of measurement and the applied loads are listed in the table. The presented measurement results constitute an average value from five measurements. The viscosity index IV was determined for the fourth position in the table (PET). As shown in the table (Table 3), both petroleum-based and bio-based reprocessed polymers are characterised by diverse values of the mass flow rate. The highest value was produced for two material groups: polyamide and polyethylene terephthalate; the lowest value of the mass flow rate was produced for one of the varieties of polyethylene. The remaining materials, both petroleum-based and bio-based, are characterised by similar Sustainability 2021, 13, 3701 12 of 16

Table 2. Elements of the cascade line used during the tests.

Elements of the Cascade Line Used during the Tests No. Device Name Rated Power [kW] 1. Compactor 40 2. Extruder I 75 3. Sieve exchanger 5 4. Extruder II 75 5. Shaker 3 6. Vacuum pump and transport devices 2 Combined power 200

3.2. Research Results The consumption of electrical energy was analysed in terms of the type of the processed material and its mass flow rate; the efficiency of the process (expressed in kg/h) was analysed in relation to these two factors. The table below includes data related to determining the mass flow rate according to the ISO 1133-1 quality standard. The test was performed using a Dynisco D4003DE plastometer. Temperatures of measurement and the applied loads are listed in the table. The presented measurement results constitute an average value from five measurements. The viscosity index IV was determined for the fourth position in the table (PET). As shown in the table (Table3), both petroleum-based and bio-based reprocessed polymers are characterised by diverse values of the mass flow rate. The highest value was produced for two material groups: polyamide and polyethylene terephthalate; the lowest value of the mass flow rate was produced for one of the varieties of polyethylene. The remaining materials, both petroleum-based and bio-based, are characterised by similar values of the MFR (from 3 to 6 g/10 min). The MFR index is a parameter determining the ability to reuse a material for the given type of production or product [46].

Table 3. Results of testing the mass ﬂow rate of reprocessed plastics.

Results of Testing the Mass Flow Rate of Reprocessed Plastics Mass Flow Mass Flow Temperature of Material Device Load No. Rate Rate Measurement Processed [kg] [g/10 min] [g/10 min] [oC] 1 Polyethylene (PE) - 0.2 190 2.16 2 Polyethylene (PE) - 4.0 190 2.16 3 Polypropylene (PP) - 6.0 230 2.16 Polyethylene 4 - IV-85 280 5 terephthalate (PET) 5 Polyamide (PA) - 96 280 5 6 Polylactide (PLA) 6 6.31 190 2.16 7 Bioplast 105 4.1 4.36 190 2.16 8 Bioplast 300 2.57 2.79 190 2.16

Referring to the first question, Q1, it was demonstrated that, when performed for the first time, retreatment has no major impact on the properties of the resulting granules (regarding the preformed tests, this statement is only true for biodegradable plastics, whose properties were determined for the original granules, and subsequently for granules produced after retreatment using the extrusion technology, while for petroleum-based polymers it was not possible to determine the original mass flow rate due to the lack of granules—the created granules were derived from final products; it can be only presumed that, due to the resulting values of the mass flow rate, the initial values were similar). Table4 lists the applied treatment parameters (the temperature of the last heating zone and the extrusion die are used as the value presented in the table). The energy expenditure reflected the total use of energy by the line when processing the given material group (the lower the value, the higher the advantage from an economic and ecological point of view). Sustainability 2021, 13, 3701 13 of 16

Table 4. List of treatment parameters in relation to energy consumption and efﬁciency.

List of Treatment Parameters in Relation to Energy Consumption and Efﬁciency Treatment Energy Energy Efﬁciency No. Polymer Temperature Expenditure Consumption [kg/h] [oC] [kWh] per 1 kg 1 Polyethylene (PE) 210 120 280 0.43 kWh 2 Polyethylene (PE) 210 120 350 0.342 kWh 3 Polypropylene (PP) 230–240 150 400 0.375 kWh Polyethylene 4 280–290 200 350 0.571 kWh terephthalate (PET) 5 Polyamide (PA) 275 200 250 0.80 kWh 6 Polylactide (PLA) 190–200 100 400 0.25 kWh 7 Bioplast 105 190–200 100 400 0.25 kWh 8 Bioplast 300 190–200 100 400 0.25 kWh

Table4 also presents the efficiency of the extrusion line in kilograms per hour and the consumption of electrical energy used to reprocess one kilogram of plastic. From an economic and ecological point of view, it is a decisive factor qualifying the given material group for retreatment. As it has been shown (Table4), depending on the type of processed material, the processing parameters, represented in this case by the regranulate extrusion temperature, change significantly. Along with the change of the material group, the demand for electric energy required to effectively process one kilogram of plastic increases. The greatest amount of electricity is absorbed/required for processing by plastics from the group of polyesters and polyamides, despite the relatively low viscosity, as demonstrated by the melt flow index tests (the values obtained are 200 kWh and 0.8 and 0.571 kWh). However, due to its processing properties (e.g., processing temperature, screw rotational speed, which also translates into efficiency), its energy inputs are much higher. Slightly lower values are required for the processing of polypropylene. However, due to its rheological properties, energy inputs are compensated by the efficiency of recycling. An equally high efficiency of the recycling process was achieved for the processing of polyethylene and biodegradable plastics. The technological line for processing these materials requires a power supply of 100 kWh with the consumption of about 0.25 kWh needed to produce one kilogram of material. The data presented in Table5 supplement/extend the data presented in Table4. Table5 presents data relating to the correlation of the mass melt flow rate and the rotational speed of the extruder and the influence of these factors on the efficiency of the technological recycling process. The lowest value of the melt flow rate was obtained for polyethylene (it is 0.2 g/10 min). At a rotational speed of 100 RPM, the efficiency was 280 kg/h. As shown in Table4, the energy expenditure for polyethylene processing was 0.43 kWh for one kilogram of material. Good processing results were achieved thanks to the relatively low processing temperature of this material. Similarly, it has been shown that with the increase in the value of the melt flow index and the screw rotational speed, the processing efficiency increases significantly (this applies to the processing of materials such as polypropylene, polylactide, polymers from the group of bioplasts). However, for polyamide and polyethylene terephthalate, despite the very low viscosity of the polymer melt (the melt flow index values for these materials are high and thus their fluidity is high) and the high rotational speeds of the screw (190 Rpm), the achieved efficiency in kilograms is 350 for the material PET and 250 for polyamide. Sustainability 2021, 13, 3701 14 of 16

Table 5. List of the acquired data in relation to the resulting mass ﬂow rate, as well as the rotational speed of the extruder and its output.

List of the Acquired Data in Relation to the Resulting Mass Flow Rate, as Well as the Rotational Speed of the Extruder and Its Output Rotational Speed Mass Flow Rate Efﬁciency No. Material Processed of the Screw [g/10 min] [kg/h] [RPM] 1 Polyethylene (PE) 0.2 100 280 2 Polyethylene (PE) 4.0 190 350 3 Polypropylene (PP) 6.0 210 400 Polyethylene 4 85 190 350 terephthalate (PET) 5 Polyamide (PA) 96 190 250 6 Polylactide (PLA) 6.2 180 400 7 Bioplast 105 4.3 180 400 8 Bioplast 300 2.77 180 400

As indicated in Table5, these relationships are not linear, and the efficiency of retreatment drops not just along with a drop in the value of the mass flow rate, but also with its considerable increase. Referring to questions 1 and 3 (Q1 and Q3), it should be concluded that the retreatment of biodegradable polymers is an efficient process. Their treatment occurs in lower temperatures compared to the treatment of petroleum-based polymers, at the same time providing higher efficiency expressed in kilograms per hour. Therefore, in a given time unit, it is possible to produce a larger amount of material with lower expenditures in terms of energy and economy.

4. Conclusions The performed literature studies and an analysis of the retreatment process of waste originating from biodegradable plastics have unambiguously indicated the justifiability of their retreatment in terms of economy, technology and efficiency. From a technological point of view, the performed analysis proved that both petroleum- based and biodegradable polymers undergo retreatment, and the products of regranulation are characterised by satisfactory and repetitive parameters. The mass flow rate established for all the tested materials was an index which determined this conclusion. The performed tests were also aimed at determining the energy consumption during the retreatment of selected materials, including mass-processed plastics (polyolefins), technical plastics (PA, PET) and biodegradable plastics. As demonstrated by the measurements, biodegradable plastics require the lowest energy expenditures associated with their retreatment, which is related to the temperatures of their processing. Significance is also attributed to their composition and chemical structure (the input biodegradable materials were characterised by average values of the mass flow rate of 6 g/10 min for PLA, 4.1 g/10 min for bioplast 105 and 2.57 g/10 min for bioplast 300; retreatment had no major impact on its value). The lack of significant changes in the value of the mass flow rate relative to the original granules demonstrates the negligible impact of treatment temperatures (technological parameters of plastic) and the phenomena accompanying the processing (fragmentation, feeding, homogenisation, rotational speed of the screw, etc.) on their retreatment and the properties of the resulting products. The efficiency of the retreatment of biodegradable plastics is also higher compared to the analysed thermoplastic polymers (even those that are characterised by MFI values higher by a factor of about a dozen). Moreover, it should be noted that the performed tests and the yielded results confirm the “zero waste” policy being introduced by the EU and other industrialised countries, since even biodegradable plastics can be given a second life and be reused with minor energy expenditures. The retreatment of biodegradable plastics before their ultimate composting is also justified from an ecological point of view. In the case of details or wrapping with wall thickness below one millimetre, the duration of composting and decomposition into simple substances is several months, while in the case Sustainability 2021, 13, 3701 15 of 16

of details with wall thickness exceeding one millimetre there is no unambiguous data (it depends on the type of material and the dynamics of the composting process). In the case of retreatment, this time is shortened to several hours, after which a fully functional product (regranulated polymer) is created and can be further utilised. The management of waste originating from the waste stream of biodegradable plastics is subject to the same rules as the management of petroleum-based waste, but the main difference between them is found in economic, ecological and technological aspects in favour of biodegradable waste.

Author Contributions: Conceptualization, T.S. and K.Ł.; methodology, T.S. and K.Ł.; formal analysis, T.S. and K.Ł.; investigation, T.S. and K.Ł.; resources, T.S. and K.Ł.; writing—original draft preparation, T.S. and K.Ł.; writing—review and editing, T.S. and K.Ł.; All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data of this study is available from the authors upon request. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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