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Review Bio- (Bio-PE), Bio- (Bio-PP) and Bio-Poly( terephthalate) (Bio-PET): Recent Developments in Bio-Based Polymers Analogous to -Derived Ones for Packaging and Applications

Valentina Siracusa 1,* and Ignazio Blanco 2

1 Department of Chemical Science (DSC), University of Catania, Viale A. Doria 6, 95125 Catania (CT), Italy 2 Department of Civil Engineering and Architecture, University of Catania an UdR-Catania Consorzio INSTM, Viale Andrea Doria 6, 95125 Catania, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-3387275526

 Received: 27 June 2020; Accepted: 18 July 2020; Published: 23 July 2020 

Abstract: In recent year, there has been increasing concern about the growing amount of waste coming from daily life. Different kinds of synthetic are currently used for an extensive range of needs, but in order to reduce the impact of petroleum-based plastics and material waste, considerable attention has been focused on “green” plastics. In this , we present a broad review on the advances in the research and development of bio-based polymers analogous to petroleum-derived ones. The main interest for the development of bio-based materials is the strong public concern about waste, and footprint. The sustainability of those polymers, for general and specific applications, is driven by the great progress in the processing technologies that refine feedstocks in order to obtain bio-based that are used as building blocks. At the same time, thanks to the industrial progress, it is possible to obtain more versatile and specific chemical structures in order to synthetize polymers with ad-hoc tailored properties and functionalities, with engineering applications that include packaging but also durable and electronic goods. In particular, three types of polymers were described in this review: Bio-polyethylene (Bio-PE), bio-polypropylene (Bio-PP) and Bio-poly(ethylene terephthalate) (Bio-PET). The recent advances in their development in terms of processing technologies, product development and applications, as well as their advantages and disadvantages, are reported.

Keywords: bio-based polymers; bio-based plastics; Bio-polyethylene (Bio-PE); bio-polypropylene (Bio-PP); Bio-poly(ethylene terephthalate) (Bio-PET); bio-polyolefins; packaging;

1. Introduction Today, the public interest in the environment, climate change and the limited resources of are driving governments, companies and scientists to find interesting and ecofriendly resource alternatives to petroleum. Polymers, that is to say plastics, that are obtained from petroleum are used for diverse and/or specific application, by tailoring their chemical structure well to the specific required performances. 85–90% are standard plastics, each one with its preferred application, but only seven types of polymers cover approximatively two thirds of the total plastics demand in all applications (packaging, building and construction, automotive, electric and electronics, others), and these are: Low Polyethylene (LDPE), Low Linear Density Polyethylene (LLDPE), High Density Polyethylene (HDPE), Polypropylene (PP), (PS), (PVC)

Polymers 2020, 12, 1641; doi:10.3390/polym12081641 www.mdpi.com/journal/polymers Polymers 2020, 12, 1641 2 of 17 and polyethylene terephthalate (PET) [1]. A general distribution of such is reported Polymersin Figure 20201:, 12, x FOR PEER REVIEW 2 of 17

Figure 1. GeneralGeneral distribution distribution and application of plastics.

Polyolefins,Polyolefins, followed by , comprise the major part of plastics, corresponding to about 45%, and are used especially for packaging applications, [1] [1].. As As still still highlight highlighteded by by Nguyen Nguyen et et al. al. [1] [1],, the depletio depletionn of 5% of petroleum and natural per year for their production, the continuous and dramatic rise in plastic consumption in view of the importance importance of considering considering the final final destination of these materialsmaterials (where(where are are the the 300 300++ billion billion kg ofkgplastic of plastic produced produced each year?),each year?), and the and interest the interest in finding in findrenewableing renewable resources resources to build ourto build indispensable our indispensable plastics must plastics be followed. must be Asfollowed. an example, As an in example, the United in theStates, United 60% States, of plastics 60% are of plasticlandfilled,s are 10% landfilled, are recycled 10% are and recycled 30% are and unaccounted 30% are unaccounted for, contributing for, contributingto terrestrial andto terrestri aquatical environmentaland aquatic environmental concern. Furthermore, concern. Further as is alsomore reported, as is byalso Nguyen reported et al.,by Nguyenthe weight et al., of the plastic weight accumulated of plastic accumulated in the ocean in will the exceed ocean will the weightexceed the of fishesweight by of 2050fishes probably, by 2050 probably,considering considering that 10 tons that of PE,10 tons PP and of PE, foamed PP and PS (floatingfoamed PS plastics) (floating are plastics) discarded are every discarded minute every into minutethe ocean. into In the this ocean. context, In this bio-based contex plasticst, bio-based could plastics offer an could important offer an contribution important tocontribution reducing the to reducinguse of and the dependence use of and on dependence limited petroleum on limited resources petroleum and resources to decreasing and theto decreasing related environmental the related environmentalimpacts. As reported impacts. by As Niaounakis reported by and Niaounakis by Shen and et al. by [2 Shen,3], bio-based et al. [2,3] plastics, bio-based can plastics be defined can be as “definedman-made as or“man man-processed-made or man organic-processed macromolecules organic macromolecules derived from biological derived from resources biological and for resources plastic and and fibre for plasticapplications and fibre”. They applications can be biodegradable”. They can be or biodegradable not, sometimes or compostablenot, sometimes and compostable can be synthesized and can from be synthesizedrenewable resources from renewable or from resources comingor from fromresources plants coming and/or from animals, plants and and/or sometime animals, they and can sometimereturn to naturethey can as return , to carbon nature dioxide, as water, inorganic , compounds inorganic and compound biomass vias and the compostingbiomass via theprocess, composting leaving process, no toxic lea andving/or no distinguishable toxic and/or distingui residuesshable [4–8], residues according [4– to8], ISOaccording and ASTM to ISO rules. and ASTMThe biodegradability rules. The biodegradability depends on the depends chemical on the structure chemical of the structure polymers of the but polymers not on the but sources not on used the sourcesfor the monomers’ used for the collection. monomers’ collection. The bio bio-based-based plastic that has been investigated up to now could be div dividedided into three principal groups [9] [9]:: 1. bio-plastics that are based on renewable resources and that are biodegradable, like plastic, 1. bio-plastics that are based on renewable resources and that are biodegradable, like starch plastic, polymers, proteins, lignin and plastics, (PLA), polyhydroxy cellulose polymers, proteins, lignin and chitosan plastics, polylactic acid (PLA), polyhydroxy alkanoates (PHAs), but also (PHBs), polyhydroxyvalerate (PHV) and alkanoates (PHAs), but also polyhydroxybutyrates (PHBs), polyhydroxyvalerate (PHV) and their their in different percentages (PHBV); this class now includes polymers such as copolymers in different percentages (PHBV); this class now includes polymers such as PVC, PE, PVC, PE, PP, PET, and (PA), named as bio-plastics because the starting PP, PET, nylon and polyamides (PA), named as bio-plastics because the starting monomers could monomers could be obtained from biological resources; be obtained from biological resources; 2. bio-plastics based on petroleum resources, which are 100% biodegradable, like (PCL), succinate (PBS), polybutylene (PBA) and its copolymers with synthetic polyesters like polybutylene adipate-terephthalate (PBAT) and polyvinyl (PVOH);

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2. bio-plastics based on petroleum resources, which are 100% biodegradable, like polycaprolactone (PCL), (PBS), polybutylene adipate (PBA) and its copolymers with synthetic polyesters like polybutylene adipate-terephthalate (PBAT) and polyvinyl alcohol (PVOH); Polymers 2020, 12, x FOR PEER REVIEW 3 of 17 3. bio-plastics obtained by using monomers coming from mixed biological and petroleum resources 3. bio-plasticslike polyesters obtained obtained by using with petroleum-derived monomers coming terephthalic from mixed acid and biological biologically and derived petroleum , resources1,4-butanediol like polyesters and 1,3-propanediol, obtained with such petroleum as polybutylene-derived terephthalic terephthalate acid (PBT), and polytrimethylene biologically derivedterephthalate ethanol, 1,4 (PTT),-butanediol polyethylene-co-isorbite and 1,3-propanediol, terephthalate such as polybutylene (PEIT), terephthalate (PUR) (PBT), and polytrimethyleneresins (thermoset terephthalate plastic). (PTT), polyethylene-co-isorbite terephthalate (PEIT), polyurethaneA summarized (PUR) personal and epoxy overview (thermoset of these plastics plastic). is reported in Figure2: A summarized personal overview of these plastics is reported in Figure 2:

Biodegradable (PCL, PBS, PBAT, PVOH)

Petroleum based

Not biodegradable (bio-PE, bio-PP, bio-PET, bio-PVC, nylon)

PLA, PHAs, PHB, PHV, PHBV

Bio-based plastics based Starch plastics (from , potato, corn), cellulosic plastics and proteineous plastics (from plant and animal proteins)

Polyesters (bio-PBT, bio-PTT, bio- PEIT) Mixed Resorces based (Biosources/petroleum) Thermosets (bio-based Epoxy, bio- based PUR)

Figure 2. Classification of bio-based plastics. Figure 2. Classification of bio-based plastics. Starting from 2000, bio-based plastics have attracted great interest thanks to the fact that in the Starting from 2000, bio-based plastics have attracted great interest thanks to the fact that in the last few years the development of new technological approaches has driven an interest in emerging last few years the development of new technological approaches has driven an interest in emerging new plastics obtained from renewable feedstocks [10–13]. Many old processes have been modified new plastics obtained from renewable feedstocks [10–13]. Many old processes have been modified and revised in order to be used to produce chemical intermediates for production, such and revised in order to be used to produce chemical intermediates for polymer production, such as, as, for example, the chemical dehydration of ethanol, which could be used to produce Bio-PE and for example, the chemical dehydration of ethanol, which could be used to produce Bio-PE and other other plastics. Despite some of the plant capacities still being small when compared with the plastics. Despite some of the plant capacities still being small when compared with the petrochemical ones, such as, for example, in the production of PHAs polymers, others are very big ones, such as, for example, in the production of PHAs polymers, others are very big in size, such as in size, such as the bio-based PE production plant. New materials with improved properties and the bio-based PE production plant. New materials with improved properties and special special functionalities could be obtained thanks to the continuous advancements and functionalities could be obtained thanks to the continuous advancements and innovations of the of the [14,15]. Today, it is even almost possible to have a bio-plastic alternative bioplastics industry [14,15]. Today, it is even almost possible to have a bio-plastic alternative to to conventional plastic material, with nearly the same properties and but with the conventional plastic material, with nearly the same properties and performance but with the great great advantage of reducing the or of being able to provide additional waste advantage of reducing the carbon footprint or of being able to provide additional management options, such as industrial composting [9,16]. Furthermore, the growing concerns about options, such as industrial composting [9,16]. Furthermore, the growing concerns about the the environmental impact of plastics and the interest to reduce the dependency on fossil fuel resources environmental impact of plastics and the interest to reduce the dependency on fossil fuel resources have increased the interest in such bio-plastics even more, with a steady increment in the number of have increased the interest in such bio-plastics even more, with a steady increment in the number of materials, applications, products, manufacturers, and end-users. Regarding the economic, materials, applications, products, manufacturers, converters and end-users. Regarding the economic, political and social development, this field is of great interest, especially for single-use plastics goods, political and social development, this field is of great interest, especially for single-use plastics goods, the object of the Single-Use Plastics Directive implementation, of the new EU Commission’s European the object of the Single-Use Plastics Directive implementation, of the new EU Commission’s European Green Deal of 2020, of the Circular Economy Action Plan and the framework for bio-based and biodegradable plastics [17–19]. A life cycle assessment (LCA) study could be a good tool to evaluate the environmental sustainability of materials and technology [20–22]. An interesting paper, presented by Chen et al. [23], reports a comparative LCA study on petroleum-based and bio-based PET within the context of the . The consumer-use phase as well as the end-of-life phase ( and/or disposal) were excluded from the study, while the production phases were

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Green Deal of 2020, of the Circular Economy Action Plan and the framework for bio-based and biodegradable plastics [17–19]. A life cycle assessment (LCA) study could be a good tool to evaluate the environmental sustainability of materials and technology [20–22]. An interesting paper, presented

Polymersby Chen 2020 et, al.12, x [23 FOR], reportsPEER REVIEW a comparative LCA study on petroleum-based and bio-based PET bottles4 of 17 within the context of the United States. The consumer-use phase as well as the end-of-life phase considered,(recycling and from/or disposal)fossil resources were excluded and from from biomass the study, feedstock while for the PET production and bio phases-PET, respectively. were considered, For thefrom polymer fossil resources production, and fossil from-based biomass ethylene feedstock glycol for (EG), PET andfossil bio-PET,-based terephthalic respectively. acid For (PTA), the polymer corn, switchgrassproduction, and fossil-based wheat ethylene raw glycolmaterials (EG), for fossil-based bio-EG, and terephthalic corn stover acid and (PTA), forest corn,residues switchgrass for bio- PTAand wheatwere selected straw raw as the materials building for bl bio-EG,ock materials. and corn 12 PET stover ands scenario forest residuess were established for bio-PTA, and were 1 kgselected of PET as bottles the building was chosen block as materials. the functional 12 PET unit bottles [23]. The scenarios researchers were found established, that biomass and 1 kgfeedstock of PET extractionbottles was and chosen processing as the functional showed unita great [23er]. Theimpact researchers in terms found of environmental that biomass feedstockemission extractionthan the correspondingand processing fossil showed processes a greater did impact, due to in the terms extra of energy environmental required emission for agricultural than the operations corresponding and duefossil to processesthe chemicals did, requested due to the as extrafertilizers energy for the required production. for agricultural The results operations are highly sensitive and due to to iso the- butanolchemicals production requested [23] as. for the production. The results are highly sensitive to iso-butanol productionConsidering [23]. the emerging interest in the synthesis of traditional polymers by using biological low-costConsidering resources theand emerging thanks to interestthe advances in the in synthesis biotechnologies, of traditional in this polymers review we by have using focused biological our attentionlow-cost on resources bio-plastics and thanksbelonging to the to the advances first category, in biotechnologies, named Bio-PE, in thisBio-PP review and weBio- havePET, focusedand we choseour attention these because on bio-plastics they are the belonging most used to polymers the first category,in the world, named especially Bio-PE, as Bio-PPpackaging and materials Bio-PET, [24]and. weAs reported chose these in the because literature, they arePE dominates the most used milk polymers packaging in and the world,cosmetic/ especially as packaging, packaging PPmaterials dominate [24s]. many As reported sub-segments in the literature, but as minor PE dominates player, and milk PET packaging dominates and the cosmetic carbonated/non/detergent- carbonatedpackaging, drinks PP dominates market [25,26] many. sub-segments but as minor player, and PET dominates the carbonated/non-carbonated drinks market [25,26]. 2. Bioplastics Market and General Application 2. Bioplastics Market and General Application As reported by European [9], the total bioplastics production capacity is set to increase from Asaround reported 2.11 bymillion European tons in Bioplastic 2019 to approximately [9], the total bioplastics 2.43 million production tons in 2024, capacity as reported is set to in increase Figure 3:from around 2.11 million tons in 2019 to approximately 2.43 million tons in 2024, as reported in Figure3:

2024

2023

2022

2021 Years 2020

2019

2018

0 200 400 600 800 1000 1200 1400 1600 tonnes of plastics production

Bio-based non-biodegradable plastics Bio-based biodegradable plastics

Figure 3. Forecast of the bioplastics production in tons from 2019 to 2024. Figure 3. Forecast of the bioplastics production in tons from 2019 to 2024. Actually, bioplastic production represents about 1% of the 360 million tons of plastic produced annuallyActually, in the bioplastic world, but production the market represents is in continuous about 1% increase of the thanks 360 mi tollion even to betterns of plastic technologies produced and annuallyemerging in fields the ofworld interest, but and the application market is in [9 ].continuous Some polymers increase showed thanks the to highest even growthbetter technologies rate, such as andPHAs emerging and bio-PP, field withs of interest great industrial and application interest [ for9]. theirSome application polymers show in evened the more highest sectors. growth Thanks rate to, suchthis, theas PHAs Bio-PP and production bio-PP, with is destined great industrial to grow byinterest a factor for of their about application six by 2024, in whileeven more for the sectors. PHAs Thankfamilys theto th growthis, the wasBio-PP estimated production to be is three destined times to higher grow inby five a factor years, of starting about six from by 20192024, [ 9while]. for the PHAs family the growth was estimated to be three times higher in five years, starting from 2019 [9]. In particular, the global production capacities of bio-based non-biodegradable plastics account for about 45% of the total, while bio-based biodegradable plastics account for about 55% of the total, as following reported in Figure 4, by material type:

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In particular, the global production capacities of bio-based non-biodegradable plastics account for about 45% of the total, while bio-based biodegradable plastics account for about 55% of the total, as following reported in Figure4, by material type: Polymers Polymers2020, 12, 2020x FOR, 12 PEER, x FOR REVIEW PEER REVIEW 5 of 17 5 of 17

21 21

14 13 13 14 12 12 12 10 9 9 10 % of production type % production of by % of production type % production of by

4 4 1 1 1 1 1 1 1 1

Bio-PP Bio-PTTBio-PP Bio-PETBio-PTT Bio-PETBio-PA Bio-PABio-PE othersBio-PE others PHA PHAPBS PBATPBS PBATPLA StarchPLA othersStarch others PolymerPolymer type type

Figure 4. Percent (%) of global bio-based plastics production, by type. Figure 4.Figure Percent 4. Percent(%) of global (%) of bio global-based bio plastics-based plasticsproduction, production, by type. by type. The largest field of application for bio-based plastics is packaging (both flexible and rigid packaging),The largestThe largest which field of accountsfield application of application for over for bio 50% for-based of bio 1.14 - plasticsbased million plastics is tons packaging of is bio-based packaging (both plastic flexible(both production flexible and rigid and (in rigid 2019), packagingfollowedpackaging), which by), textile, accountwhich consumeraccounts for overs for 50% goods, over of 50%1.14 millionof 1.14 million tons and of horticulture, tonsbio-based of bio -plasticbased automotive plasticproduction production and (in transport 2019), (in 2019), goods, followedcoatingsfollowed by textile, and by , textile,consumer consumer buildinggoods, goods,agriculture and agriculture construction, and horticulture, and electrics horticulture, automotive and electronics, automotive and transport andand otherstransport goods like, goods toys, [9]. coatingsThecoatings and percentage adhesives, and adhesives, distribution building building and is reported construction, and construction, in Figure electrics5. electrics and electronics and electronics, and others, and likeothers toys like [9 ]toys. [9]. The percentageThe percentage distribution distribution is reported is reported in Figure in Figure5. 5. others othersElectrics/ElectronicsElectrics/Electronics Building/ConstructionBuilding/Construction 1% 1% 2% 2% 4% 4% /AdhesivesCoatings/Adhesives 6% 6% FlexibleFlexible packaging packaging 31% 31% Automotive/TransportAutomotive/Transport 7% 7%

Agriculture/HorticulAgriculture/Horticul ture ture 8% 8%

ConsumerConsumer goods goods 8% 8%

Textile Textile Rigid packaging Rigid packaging 11% 11% 22% 22% Figure 5. Market segments for bio-based plastics. Figure 5.Figure Market 5. Marketsegments segments for bio- basedfor bio plastics-based .plastics .

Asia is Asiathe major is the production major production hub in hubthe world in the (45%),world followed(45%), followed by Europe by Europe (25%), North(25%), AmericaNorth America (18%) and(18%) South and America (12%). The(12%). land The used land for used the forrenewable the renewable feedstock feedstock for the forproduction the production of bio- of bio-

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Polymers 2020, 12, x FOR PEER REVIEW 6 of 17

plasticsAsia actually is the major comprises production 0.016% hubof the in thetotal world agricultural (45%), land followed, corresponding by Europe to (25%), 0.79 million North America (18%)hectares and out South of 4.8 America billion hectares. (12%). The 97% land of those used hectares for the are renewable dedicated feedstock to pasture for (3.3 the billion production of bio-plasticshectares), feed actually and food comprises (1.24 billion 0.016% hectares) of, theand totalothers agricultural such as material land, uses corresponding (106 million hectares) to 0.79 million hectaresand out of (53 4.8 million billion hectares). hectares. Despite 97% of the those growing hectares production are dedicated of bio-plastics, to pasture in 2024 (3.3 the billion land hectares), feeddedicated and food to them (1.24 will billion reac hectares),h a value of and 0.021%, others corresponding such as material to 1.00 uses million (106 hectares, million hectares)highlighting and biofuels (53even million more hectares). the absence Despite of competition the growing between production renewable of feedstock bio-plastics, used for in 2024the production the land dedicated of to themplastics will and reach that a used value for of food 0.021%, and feed corresponding [9]. to 1.00 million hectares, highlighting even more the absence3. Bio-B ofased competition Polyethylene between (Bio-PE) renewable feedstock used for the production of plastics and that used for food and feed [9]. Thanks to its great performance, PE is the largest polymer used in the world. The starting 3. Bio-Based used Polyethylene for synthesis (Bio-PE)is ethylene, which is also the for other polymers such as PVC and PS, and it is generally obtained from petroleum feedstock by . However, great interest is nowThanks being shown to its to great obtain performance, this polymer PEfrom is biological the largest resources polymer, and, used as of in now, the the world. followed The starting monomerapproach usedhas been for to synthesis synthetize is the ethylene, ethylene which monomer is also by dehydration the case for of other bio-ethanol, polymers obtained such asfrom PVC and PS, andglucose, it is generally following obtainedthe general from scheme petroleum reported in feedstock Figure 6: by distillation. However, great interest is now being shown to obtain this polymer from biological resources, and, as of now, the followed approach has been to synthetize the ethylene monomer by dehydration of bio-ethanol, obtained from , following the general scheme reported in Figure6:

others Bio-based by anaerobic by dehydration, high polymer (PVC, PER, PS, fermentatiion temperature, catalyst • Sugra cane • bio-ethanol (CH3CH2OH PUR)

• beet and CO2) • bio-ethylene (CH2=CH2 • bio-Polyethylene (bio- • strach crops and water) • Glucose (C H O ) as • azeotropic of HDPE, bio-LDPE, bio- • lignocellulosic 6 12 6 juice and () hydrous ethanol and LLDPE) vinasse

by cleaning, slicing, by distillation by shredding, milling

Figure 6. General scheme for Bio-PE production.

The glucose could be obtained from different biological feedstocks, such as sugar cane, , starch crops coming from , wheat or other grains and lignocellulosic materials. When starting from sugar cane, for example, processes like cleaning, slicing, shredding and milling are used to obtain Figure 6. General scheme for Bio-PE production. the main sugar cane juice product and the sugar cane fiber by-product (bagasse). The juice, containing 12–13%The of glucose sucrose, could is anaerobicallybe obtained from fermented different biological in order feedstocks to obtain, ethanol.such as sugar The cane, ethanol sugar is distilled inbeet, order starch to remove crops coming water, from giving maize, an wheat azeotropic or other solution grains and of hydrous lignocellulosic ethanol, materials. at 95.5 When vol.-%, and a by-productstarting from named sugar cane, vinasse. for example, The subsequent processes like polymerization cleaning, slicing, of shredding the ethylene and milling monomer are used obtained this wayto obtain is the the same main that sugar is followed cane juice when product using and ethylenethe sugar cane derived from by-product petroleum, (bagasse). and The the juice, corresponding bio-polymercontaining 12 is–13% identical of sucrose in its, is chemical, anaerobically physical, fermented mechanical in order to properties obtain ethanol. to fossil-based The ethanol PE, is also with distilled in order to remove water, giving an azeotropic solution of hydrous ethanol, at 95.5 vol.-%, regards to the mechanical recycling processes [27]. From polymerization, different types of bio-PE and a by-product named vinasse. The subsequent polymerization of the ethylene monomer obtained couldthis waybe obtained: is the same bio-HDPE that is followed with a low when degree using of ethylene short-chain derived branching from petroleum, (about seven and thebranches per 1000corresponding C atoms), bio bio-LLDPE-polymer is with identical a high in its degree chemical, of physical, short-chain mechanical branching properties and to bio-LDPE fossil-based with a high degreePE, also of with short-chain regards to branching the mechanical+ long-chain recycling processes branching [27] (about. From polymerization, 60 branches per different 1000 Ctypes atoms). For a Polymersbetterof bio understanding2020-PE , could12, x FOR be obtained:PEER of this,REVIEW bio Figure- HDPE7 shows with a thelow chemical degree of structuresshort-chain of branching such polymers: (about seven 7 of 17 branches per 1000 C atoms), bio-LLDPE with a high degree of short-chain branching and bio-LDPE with a high degree of short-chain branching + long-chain branching (about 60 branches per 1000 C atoms). For a better understanding of this, Figure 7 shows the chemical structures of such polymers:

Figure 7. General chemical structure of (a) HDPE, (b) LLDPE and (c) LDPE polymers. Figure 7. General chemical structure of (a) HDPE, (b) LLDPE and (c) LDPE polymers.

All chemical, physical and mechanical characteristics are well reported in the literature, as are their engineering applications [28]. LLDPE is obtained by copolymerization of ethylene and , or octane. For its processing, the same plant and machinery can be used [27]. Of course, bio- PE is not biodegradable. The bagasse by-product is used as a primary fuel source in the sugar mill process because its combustion produces an amount of heat that can cover the energy needs. A surplus of heat and/or electricity can also be produced, depending on the plant dimension. Vinasse by-product can instead be used as [29]. From 2010, on a commercial scale, the first companies for the production of bio-PE have been the Brazilian company [30,31] for food packaging, , personal care, automotive and toys applications (in this case in a joint venture with Brinquedos Estrela, a major Brazilian toy company), and the joint venture Dow and Crystalsev [32] for food packaging, as well as agricultural and industrial purposes. Dow is the second largest chemical manufacturer in the world, while Crystalsev is the major Brazilian ethanol producer. Their joint venture was born in order to produce bio-LLDPE. Furthermore, the chemical companies Solvay, Nova Chemicals and are in the bio-PE market, contributing to the production of about 20% of the world’s bio-based plastics production. Braskem will cover 10% of the worldwide plastic market by the year 2020 [32]. At the beginning, bio-ethylene production was not considered to be cost-competitive when compared with petrochemical-derived ethylene, but starting from 2008 the price of one of ethanol derived from sugar cane has become competitive with the price of one barrel of crude (about $115 US versus $80 US; 1 L = 0.0085 barrel). The price of 1 kg of bio-PE is about 30% higher than petrochemical PE [32]. Actually, as mentioned before, bio-ethylene derived from bio-ethanol is used for the synthesis of other polymers like PVC (used in the construction and building industry), due to severe criticism concerning impacts on the environment, human health and safety, especially in the packaging field, due to the difficulty of separating PVC from post-consumer waste. PS, epoxy resins and rubbers like ethylene propylene monomer rubber (EPDM) could be further bio-based candidates for the use of bio-based ethylene. Enriquez et al. [33] reported a study where Bio-PE (in particular Bio-HDPE) was used in blends with bio-PTT (37% bio-based) in order to obtain a with a good balance between stiffness and toughness. Bio-PE, with the trade name SHC7260, was produced by Braskem company () and provided by Competitive Green Technologies (Ontario, Canada) and was used as received. Blends were mechanically obtained by melt processing in a co-rotating twin screw extruder and then injected in a mold at 30 °C. Different PPT/Bio-PE blends were obtained (0, 20, 40, 50, 60, 80, 100 wt.% of Bio-PE). From thermal analyses via differential scanning calorimetry, immiscibility was observed due to the presence of individual melting peaks at 230 °C for PPT and 118 °C for Bio-PE, showing a poor interfacial with a decreasing stiffness [33]. Brito et al. [34] investigated the mechanical performance of PLA/Bio-PE blends, while Castro et al. [35] and Kuciel at al. [36] investigated the performance of Bio-PE after the addition of natural fibers, used to increase the stiffness of the resulting materials. In particular, Kuciel et al. explored the mechanical performance of bio-composite materials obtained with bio-PE and synthetized using ethanol coming from (Braskem, Brazil) and four different natural fillers (wood flour, ultrafine cellulose powder, chopper fibers and of mineral tuff ) at a percentage of 25%, in comparison to net bio-PE. These composites were obtained via compounding followed by injection molding. Natural fibers and particles, with a low environmental impact and low cost, were used in order to improve the mechanical strength and

Polymers 2020, 12, 1641 7 of 17

All chemical, physical and mechanical characteristics are well reported in the literature, as are their engineering applications [28]. LLDPE is obtained by copolymerization of ethylene and butene, hexane or octane. For its processing, the same plant and machinery can be used [27]. Of course, bio-PE is not biodegradable. The bagasse by-product is used as a primary fuel source in the sugar mill process because its combustion produces an amount of heat that can cover the energy needs. A surplus of heat and/or electricity can also be produced, depending on the plant dimension. Vinasse by-product can instead be used as fertilizer [29]. From 2010, on a commercial scale, the first companies for the production of bio-PE have been the Brazilian company Braskem [30,31] for food packaging, cosmetics, personal care, automotive and toys applications (in this case in a joint venture with Brinquedos Estrela, a major Brazilian toy company), and the joint venture Dow and Crystalsev [32] for food packaging, as well as agricultural and industrial purposes. Dow is the second largest chemical manufacturer in the world, while Crystalsev is the major Brazilian ethanol producer. Their joint venture was born in order to produce bio-LLDPE. Furthermore, the chemical companies Solvay, Nova Chemicals and Petrobras are in the bio-PE market, contributing to the production of about 20% of the world’s bio-based plastics production. Braskem will cover 10% of the worldwide plastic market by the year 2020 [32]. At the beginning, bio-ethylene production was not considered to be cost-competitive when compared with petrochemical-derived ethylene, but starting from 2008 the price of one barrel of ethanol derived from sugar cane has become competitive with the price of one barrel of crude oil (about $115 US versus $80 US; 1 L = 0.0085 barrel). The price of 1 kg of bio-PE is about 30% higher than petrochemical PE [32]. Actually, as mentioned before, bio-ethylene derived from bio-ethanol is used for the synthesis of other polymers like PVC (used in the construction and building industry), due to severe criticism concerning impacts on the environment, human health and safety, especially in the packaging field, due to the difficulty of separating PVC from post-consumer waste. PS, epoxy resins and rubbers like ethylene propylene diene monomer rubber (EPDM) could be further bio-based candidates for the use of bio-based ethylene. Enriquez et al. [33] reported a study where Bio-PE (in particular Bio-HDPE) was used in blends with bio-PTT (37% bio-based) in order to obtain a copolymer with a good balance between stiffness and toughness. Bio-PE, with the trade name SHC7260, was produced by Braskem company (Brazil) and provided by Competitive Green Technologies (Ontario, Canada) and was used as received. Blends were mechanically obtained by melt processing in a co-rotating twin screw extruder and then injected in a mold at 30 ◦C. Different PPT/Bio-PE blends were obtained (0, 20, 40, 50, 60, 80, 100 wt.% of Bio-PE). From thermal analyses via differential scanning calorimetry, immiscibility was observed due to the presence of individual melting peaks at 230 ◦C for PPT and 118 ◦C for Bio-PE, showing a poor interfacial adhesion with a decreasing stiffness [33]. Brito et al. [34] investigated the mechanical performance of PLA/Bio-PE blends, while Castro et al. [35] and Kuciel at al. [36] investigated the performance of Bio-PE after the addition of natural fibers, used to increase the stiffness of the resulting materials. In particular, Kuciel et al. explored the mechanical performance of bio-composite materials obtained with bio-PE and synthetized using ethanol coming from sugarcane (Braskem, Brazil) and four different natural fillers (wood flour, ultrafine cellulose powder, kenaf chopper fibers and microparticles of mineral tuff filler) at a percentage of 25%, in comparison to net bio-PE. These composites were obtained via compounding extrusion molding followed by injection molding. Natural fibers and particles, with a low environmental impact and low cost, were used in order to improve the mechanical strength and modulus [37–39], while bio-PE was chosen as a bio-based polyolefin green matrix for structural Natural Fibre Composites (NFC) and Wood Plastic Composites (WPC). The disadvantage of using such fillers for a structural application is that PE-based WPC or NFC becomes hygroscopic if in contact with moisture and a water environment, leading to dimensional changes and accelerated ageing [36]. This behavior is attributed to a water-swollen filler, as well as the degradation of the fillers and of the interfacial adhesion between Polymers 2020, 12, x FOR PEER REVIEW 8 of 17

modulus [37–39], while bio-PE was chosen as a bio-based green matrix for structural Natural Fibre Composites (NFC) and Wood Plastic Composites (WPC). The disadvantage of using such fillers for a structural application is that PE-based WPC or NFC becomes hygroscopic if in contact with moisture and a water environment, leading to dimensional changes and accelerated ageing [36]. This behavior is attributed to a water-swollen filler, as well as the degradation of the fillers and of the interfacial adhesion between the filler and the polyolefin matrix. The use of fine cellulose microparticles and tuff microparticles, which are the easiest to process, showed a higher Polymerselongation2020, 12 at, 1641break and a lower water uptake. 8 of 17

4. Bio-Based Polypropylene (Bio-PP) the filler and the polyolefin matrix. The use of fine cellulose microparticles and tuff microparticles, whichAfter are the ethylene, easiest to propylene process, showed is the a most higher important elongation organic at break building and a lower block water for uptake. poly-olefin production. PP is the second most important polymer after PE [40]. Bio-PP could be obtained from 4.biological Bio-Based resources Polypropylene by butylene (Bio-PP) dehydration of bio-isobutanol obtained from glucose and subsequent polymerization, following the general scheme reported in Figure 8: After ethylene, propylene is the most important organic building block for poly-olefin production. PP is the second most important polymer after PE [40]. Bio-PP could be obtained from biological resources by butylene dehydration of bio-isobutanol obtained from glucose and subsequent polymerization, following the general scheme reported in Figure8:

by by dehydration polymerization • Glucose •bio- • bio-PP • iso- butylene • bio- butanol propylene by intermediate not yet fully by steps commercialized

Figure 8. General scheme for Bio-PP.

With respect to the production of Bio-PE, the process followed to obtain bio-PP has been less explored, which explains why bio-PP has not yet been commercialized. As for bio-PE, the first company that produced Bio-PP on a pilot-plant scale was Braskem, but their process route is still confidential. The most promising route to obtain propyleneFigure 8. isGeneral probably scheme through for Bio ,-PP. further processed to obtain propylene monomer via Lurgi’s methanol-to-propylene (MTP) process or UOP’s methanol-to-olefins processWith [32], respect using theto the same production industrial of plants Bio-PE, as forthe petrochemicalprocess followed methanol. to obtain bio-PP has been less exploredAs reported, which by explains Chen and why Patel bio [-41PP], has another not routeyet been could commercialized. be the production As of for 1,2-propanediol bio-PE, the first or acetonecompany through that produced fermentation Bio-PP and on a furthera pilot- conversionplant scale towas 2-propanol Braskem, bybut dehydration their process to route propylene. is still confidential. The most promising route to obtain propylene is probably through methanol, further 5.processed Bio-Based to Polyethyleneobtain propylene Terephthalate monomer via (Bio-PET) Lurgi’s methanol-to-propylene (MTP) process or UOP’s methanol-to-olefins process [32], using the same industrial plants as for petrochemical methanol. Polyesters represent a large group of polymers that have the potential to be produced from As reported by Chen and Patel [41], another route could be the production of 1,2-propanediol bio-based feedstocks [42,43]. The most relevant for a shift towards bio-based chemicals are PET, or through fermentation and a further conversion to 2-propanol by dehydration to propylene. PBT, PBS, PBA and the copolymers PBAT, poly(butylene succinate-co-lactate) (PBSL), poly(butylene succinate adipate) (PBSA) and poly(butylene succinate terephthalate) (PBST), but also polyvinylacetate 5. Bio-Based Polyethylene Terephthalate (Bio-PET) (PVAc), polyacrylates, poly(trimethylene naphthalate) (PTN), poly(trimethylene isophthalate) (PTI) and thermoplasticPolyesters represent , a large group as reported of polymers by the that European have the Bioplastic potential Report to be produced [9]. Those from polymers bio- arebased produced feedstocks from [ a42 bio-based,43]. The ,most whilerelevant diacid for ora shift diester towards could bio be either-based bio-basedchemicals (like are PET, succinic PBT, or adipicPBS, acid)PBA orand petrochemical-based the copolymers PBAT, (like PTA poly(butylene and/or di-methyl succinate terephthalate,-co-lactate) (PBSL), DMT). poly(butylene succinateAmong adipate) them, PET (PBSA) is the largestand poly(butylene used polyester, succinate with physical terephthalate) and mechanical (PBST), properties but also that makepolyvinylacetate it suitable to (PVAc), be used polyacrylates, for fibers (65%) poly(trimethylene and packaging (35%) naphthalate) applications. (PTN), This poly(trimethylen last applicatione regardsisophthalate) bottles (76%),(PTI) and (11%) and polyester films (13%). elastomer It plays, as anreported important by rolethe European in the plastic Bioplastic market, but due to its poor sustainability due to a much slower degradability, it poses serious environmental issues, especially for waste treatment, when it is used for short time applications, as for example when it is employed as food packaging. It was first commercialized in 1940, especially for synthetic fibers and film applications. From 1970 onwards, it was used to produce bottles, with a continuous growing development. Since 2010, in order to realize a sustainable plastic, both starting chemicals such as (EG) and PTA and/or DMT monomers have to be obtained from biological sources [44–47]. The general scheme of synthesis is reported in Figure9: Polymers 2020, 12, x FOR PEER REVIEW 9 of 17

Report [9]. Those polymers are produced from a bio-based diol, while diacid or diester could be either bio-based (like succinic or ) or petrochemical-based (like PTA and/or di-methyl terephthalate, DMT). Among them, PET is the largest used polyester, with physical and mechanical properties that make it suitable to be used for fibers (65%) and packaging (35%) applications. This last application regards bottles (76%), containers (11%) and films (13%). It plays an important role in the plastic market, but due to its poor sustainability due to a much slower degradability, it poses serious environmental issues, especially for waste treatment, when it is used for short time applications, as for example when it is employed as food packaging. It was first commercialized in 1940, especially for synthetic fibers and film applications. From 1970 onwards, it was used to produce bottles, with a continuous growing development. Since 2010, in order to realize a sustainable plastic, both starting chemicals such as ethylene glycol (EG) and PTA and/or DMT monomers have to be obtained from biological sources [44–47]. The general scheme of synthesis is reported in Figure 9: Polymers 2020, 12, 1641 9 of 17

100% biobased by dehydration monomer • Glucose • bio- •bio-etholol ethylene • bio-EG

by fermentation by oxidation

by catalytic 100% bio-based by dehydration • Glucose • bio-ethylene conversation • bio- monomer • Bio-etholol • bio-ethylene paraxylene • bio-PTA glucolsugar

by catalytic by fermentation by oxidation conversation

Figure 9. General scheme for Bio-EG and Bio-PTA monomers production.

The aliphatic monomer Bio-EG could be synthetized from the hydrolysis of , obtained via the oxidization of bio-ethylene obtained from the fermentation of glucose, followed by dehydration [48,49]. Another route could be via , based on hydrogenolysis [32,50]. The percentage of bio-based monomers can be varied, according to the stoichiometry of the reaction, obtaining a partial bio-based polymer. In 2009, the Coca-Cola Company introduced a 30% bio-based beverage bottle, named “PlantBottle”, made of 100% bio-based EG and petroleum-derived TA [51]. Toyota Tsusho Corporation, Japane Future Polyesters, Coca-Cola—Gevo Venture, PepsiCo-Virent Venture are actually the producers of Bio-PET [45]. Another route to synthetize EG from biomass is that proposed by Salvador et al. [52], via the use of different types of microorganisms. Among them, the bio-synthesis of EG could be achieved in , in high yields, by a pentose pathway, using xylose as a substrate. As reported by them, “xylose is first transformed into xylonate by the action of a dehydrogenase; after the subsequentFigure action 9. General of a dehydratase scheme for and Bio an-EG aldolase, and Bio glycoaldehyde-PTA monomers is production obtained, which. is finally reduced to EG by a reductase”. This procedure, with a yield of 98%, could be considered a promising alternative for the synthesis of bio-EG. Other approaches, such as through glucose in Saccharomyces cerevisiae using glycolytic enzymes [53], through the synthesis of serine in an engineered pathway in E. coli [54], using synthesis gas through the Wood–Ljungdahl pathway of carbon fixation present in acetogenic bacterial species such as Moorella thermoacetica and Clostridium ljungdahlii [55], and through gaseous using a strain of E. coli [56] could be used in a similar way. Following a different route, but always starting from biomass feedstock, a bio-PTA could also be synthetized, as schematically reported in Figure8. In this case, di fferent methods could be followed: the iso-butanol method, the muconic acid method, the limonene method or the furfural method [46]. The iso-butanol method was developed by Gevo, starting from sugar, which is considered a key building block for different chemicals such as isooctane, ethyl tert-butyl ether, and other . Following this method, p- is first produced by the cyclization of two isooctane molecules, via the dehydrogenation route; second, via oxidation, the p-xylene is converted to TA. Carraher et al. and Matthiesen et al. [57,58] proposed a route starting from cis,cis-Muconic acid as the starting building block, obtained from sugar (through a combination of chemical processes and biorefining). After a sequence of cis-cis to trans-trans transition steps of muconic acid, a tetrahydro Polymers 2020, 12, 1641 10 of 17

(THTA) is obtained by ethylene addiction reactions, and, subsequently, bio-PTA is obtained by dehydrogenation. Colonna et al. [59] started the synthesis of terephthalate polyesters from limonene. As a starting building block, p-cymene was used, obtained by the chemical refining of limonene. They reached an 85% conversion from limonene by the oxidation of the iso-propyl group with in concentrated nitric acid. Several researchers considered the possibility of converting furan derivate chemicals into PTA monomer (furfural method) [60–64] via the well-known Diels–Alder . First, furfural or other furan derivatives obtained by an inedible cellulose biomass is converted to maleic anhydride by oxidization and dehydration reactions; maleic anhydride is then involved in the reaction with furan in order to obtain a Diels–Alder adduct. By the subsequent dehydration of the adduct, a is obtained, finally converted into bio-PTA through phthalic acid and dipotassium . The process is still not industrialized. Tachibana et al. [65] also measured the bio-based carbon content through a mass spectroscopy analysis, measuring the ratios of the three carbon isotopes (14C, 13C, 12C). The proposed methods are schematically shown

Polymersin Figure 2020, 1012,: x FOR PEER REVIEW 11 of 17

Figure 10. SchematicFigure 10.representationSchematic representation of the methods of theused methods to achieve used bio to- achieveTA: (a) Bio-PTA.from iso-buthanol; (b) from muconic acid; (c) from limonene; and (d) from furfural. An interesting advantage of furfural is that it can be easily converted into other chemicals used forAn the interesting synthesis of advantage bio-based of polymers, furfural such is that as PBSit can and be polyoxabicyclateseasily converted [in65to]. other chemicals used for the Anothersynthesis interestingof bio-based result polymers was, presentedsuch as PBS by and Avantium, polyoxabicyclates the developer [65]. of the bio-based furandicarboxylicAnother interesting acid (FDCA). result was Starting presented from fructose, by Avantium, they obtained the developer a hydroxymethylfurfural of the bio-based furandicarboxylicchemical molecule acid (HMF), (FDCA). which was Starting converted from to fructose dimethyl, furanethey obtained (DMF) by a hydrogenationhydroxymethylfurfural reaction. Then, after several chemical steps, p-xylene was obtained and subsequently converted to bio-PTA, chemical molecule (HMF), which was converted to dimethyl furane (DMF) by a hydrogenation as previously described. Schenk et al. [66] reported about the BioBTX pilot plant, which is used to reaction. Then, after several chemical steps, p-xylene was obtained and subsequently converted to produce aromatic molecules such as , and terephthalate by means of biomass catalytic bio-PTA, as previously described. Schenk et al. [66] reported about the BioBTX pilot plant, which is (wood and lignin-rich resources). used to produce aromatic molecules such as benzene, toluene and terephthalate by means of biomass Following research by Chen et Patel [41], the aromatic monomer terephthalic acid or dimethyl catalytic pyrolysis (wood and lignin-rich resources). terephthalate could be produced using lignin, the most abundant bio-renewable resource, whichFollowing can be foundresearch in all by vascular Chen et plants Patel and [41] which, the aromatic is the second monomer most abundant terephthalic organic acid polymer or dimethyl [67]. terephthalate ester could be produced using lignin, the most abundant bio-renewable resource, which can be found in all vascular plants and which is the second most abundant organic polymer [67]. The aromatic aldehydes and syringaldehyde could be obtained by lignin extraction in order to further obtain bio-based p-xylene and then subsequently convert it into terephthalic acid. The bio-based PET obtained this way can be processed by injection molding, and extrusion, and it is identical to petrochemical PET. Of course, it is not biodegradable, but an interesting paper, presented by Salvador et al., described a microbial degradation of PET due to the action of microbial polyester hydrolase, considered a key alternative for recycling PET [52]. Actually, the glycolysis method is advantageous for depolymerizing opaque and colored PET that cannot be recycled due to the presence of pigments, but the high energy cost associated with the high temperature required and the long reaction times needed for make this process not economic and environmentally sustainable. The resulting EG and PTA monomers could be reused in the synthesis of PET, as well as for other polymers [68]. As an alternative to chemical PET depolymerization methods, a series of enzymes coming from different microorganisms could be used for a more sustainable strategy. An interesting method to determine the amount of biogenic fraction was presented by Jou et al. [69] in their short communication on the use of Radiocarbon Accelerator Mass Spectroscopy for the isotopic fraction determination, according to ASTM-D6866 (Standard Test Methods for Determining the Biobased Content of , Liquid, and Gaseous Samples Using Radiocarbon Analysis). As reported by Montava-Jorda et al. [70], bio-based polymers, either biodegradable or non-

Polymers 2020, 12, 1641 11 of 17

The aromatic aldehydes vanillin and syringaldehyde could be obtained by lignin extraction in order to further obtain bio-based p-xylene and then subsequently convert it into terephthalic acid. The bio-based PET obtained this way can be processed by injection molding, blow molding and extrusion, and it is identical to petrochemical PET. Of course, it is not biodegradable, but an interesting paper, presented by Salvador et al., described a microbial degradation of PET due to the action of microbial polyester hydrolase, considered a key alternative for recycling PET [52]. Actually, the glycolysis method is advantageous for depolymerizing opaque and colored PET that cannot be recycled due to the presence of pigments, but the high energy cost associated with the high temperature required and the long reaction times needed for depolymerization make this process not economic and environmentally sustainable. The resulting EG and PTA monomers could be reused in the synthesis of PET, as well as for other polymers [68]. As an alternative to chemical PET depolymerization methods, a series of enzymes coming from different microorganisms could be used for a more sustainable strategy. An interesting method to determine the amount of biogenic fraction was presented by Jou et al. [69] in their short communication on the use of Radiocarbon Accelerator Mass Spectroscopy for the isotopic fraction determination, according to ASTM-D6866 (Standard Test Methods for Determining the Biobased Content of Solid, Liquid, and Gaseous Samples Using Radiocarbon Analysis). As reported by Montava-Jorda et al. [70], bio-based polymers, either biodegradable or non-biodegradable, are certified as bio-based according to international standards such as EN 16640:2015, ISO 16620-4:2016, ASTM 6866-18 and EN 16785-1:2015. In particular, EN 16640:2015, ISO 16620-4:2016 and ASTM 6866-18 measure the bio-based carbon content in a material through 14C measurements, while EN 16785-1:2015 measures the bio-based content of a material using radiocarbon and elemental analyses. An alternative approach is to replace terephthalic acid by different compounds derived from biomass, such as 2,5-furandicarboxylic acid (FDCA) for the production of polyethylene furanoate (PEF), which is not the object of this review [71–73].

6. Discussions The use of bio-feedstock in the polymer sector is of great interest. Starch, cellulose, but also resins and some polyamides have been obtained from natural resources on an industrial scale, with an estimated worldwide production of 20 Mt/year (7% of the total plastics production). In contrast, the production of bio-based materials is estimated to be less than 2% of the total plastic production, despite the industry having demonstrated a great ability to process large amounts of biomass. Nevertheless, it is difficult to estimate the amount of such biomass that is necessary to replace petrochemical polymers by bio-based ones and whether this substitution would lower the environmental footprint. From a technological point of view, the progress in the plants processes have been spectacular. Some plants still have a rather small capacity, but others are comparable to typical petrochemical plants, like Braskem’s bio-based PE plant. With the continuous growing demand for bio-based plastics, plants with a bigger capacity production will be even more commercially available. From a geographic point of view, the United States and Europe are the most important areas for bio-based plastic production, followed by the Pacific region and South America, with important investments [9]. Figure 11 shows the actual development stage of emerging bio-based polymers: Polymers 2020, 12, x FOR PEER REVIEW 12 of 17

biodegradable, are certified as bio-based according to international standards such as EN 16640:2015, ISO 16620-4:2016, ASTM 6866-18 and EN 16785-1:2015. In particular, EN 16640:2015, ISO 16620-4:2016 and ASTM 6866-18 measure the bio-based carbon content in a material through 14C measurements, while EN 16785-1:2015 measures the bio-based content of a material using radiocarbon and elemental analyses. An alternative approach is to replace terephthalic acid by different compounds derived from biomass, such as 2,5-furandicarboxylic acid (FDCA) for the production of polyethylene furanoate (PEF), which is not the object of this review [71–73].

6. Discussions The use of bio-feedstock in the polymer sector is of great interest. Starch, cellulose, but also alkyd resins and some polyamides have been obtained from natural resources on an industrial scale, with an estimated worldwide production of 20 Mt/year (7% of the total plastics production). In contrast, the production of bio-based materials is estimated to be less than 2% of the total plastic production, despite the industry having demonstrated a great ability to process large amounts of biomass. Nevertheless, it is difficult to estimate the amount of such biomass that is necessary to replace petrochemical polymers by bio-based ones and whether this substitution would lower the environmental footprint. From a technological point of view, the progress in the plants processes have been spectacular. Some plants still have a rather small capacity, but others are comparable to typical petrochemical plants, like Braskem’s bio-based PE plant. With the continuous growing demand for bio-based plastics, plants with a bigger capacity production will be even more commercially available. From a geographic point of view, the United States and Europe are the most important areas for bio-based plastic production, followed by the Asia Pacific region and South Polymers 2020, 12, 1641 12 of 17 America, with important investments [9]. Figure 11 shows the actual development stage of emerging bio-based polymers:

• Bio-PP, Bio-PA(6 and 6,6), same new starch plastics (acetylated starch) R&D • Partially Bio-PET, Partially Bio-PEIT, Partially Bio-PBS, Partially Bio-PBA, stage Partially Bio-PBSA, Partially bio-PBT

Pilot plant •Bio-, Bio-PLA heat resistant

stage •Partially Bio-PUR

•Bio-PHA, Bio-PE Medium stage •Partially Bio-epoxy , Partially Bio-PVC

•Bio-PLA, Plastic from Starch, Bio-PA(11)

Large stage • Partually Bio-PTT, Partially Bio-PA (11)

Commercial scale •Plastics from cellulose, alkyd resisns

Figure 11. DevelopmentDevelopment stages stages for for emerging emerging bio bio-based-based polymers polymers..

AsAs can be observed, the the largest largest number number of of bio bio-based-based plastics plastics are are in in the the R&D R&D and and pilot pilot plant plant stage,stage, while only few materials are are in in large large and and commercial commercial stages. stages. In In the the first first case case,, different different kind kindss of of technologicaltechnological challengeschallenges are are only possible. For For the the production production of of partial partial Bio Bio-PBS,-PBS, no no technological technological barriersbarriers are present, because the production production of of succinic succinic acid acid is is technically technically ready. ready. The The esterification esterification of of succinicsuccinic acid acid with with butanol butanol to obtainto obtain PBS PBS is performed is performed at a large at a scale large but scale via petrochemical but via petrochemical precursors. With respect to PBS, the production of partial Bio-PET is even more easy. Meanwhile, the production of

Bio-PP requires several steps that present limited knowledge, making the scale-up even more difficult and more demanding. Thus, depending on the type of plastics, between 20% and 100% of the actual volume could be replaced by bio-based alternatives, but not in the short/medium term due to economic problems related to cost production and capital availability, technical scale-up, the great amounts of bio-based feedstocks and the industrial conversion to the new plastics. It has been estimated that from 2030 onwards, the substitution of petrochemical plastics with bio-based plastics is expected to be even higher, thanks to the replacement of the monomers by chemically identical bio-based compounds or bio-based compounds with the same equivalent functionality. However, if the technology becomes ready in plants, there are still several factors that can influence the speed of commercialization: financial, technology, personnel involved, interaction with other sectors, collaboration with other plants and companies from the agroindustry chain, market concerns related to the demand for bio-based goods by retailers and producers of consumer products, and, finally, regulation factors related to fiscal policy measures and public procurement. Another important factor to take into consideration in the production of bio-based materials is the competition with food, feed and bio-fuels for the supply of raw materials. Today, starch and sugar crops are the sole feedstocks supply, but a second generation of bio-ethanol, based on lignocellulosic feedstocks, can be used as future starting chemicals in the production of bio-PE, as well as for other (like PLA) [74]. Lignocellulose is cheap, abundant and not competitive with food with respect to starch and sugar crops, but the biotechnology to transform cellulose into sugar monomers, via microorganisms, requires processing steps that are more complicated. For the next two decades, the biomass demand for bio-based plastics production will remain smaller than that demanded for human food and animal feed. Polymers 2020, 12, 1641 13 of 17

The major influencing factors for the development of bio-based plastics are the technological barriers that must be overcome in order to reach a large-scale production, the suitability of these materials to be used as bulk materials, their competitiveness in terms of cost with respect to petroleum-derived ones and the availability of raw materials for their synthesis. A very interesting paper is that of Volanti et al. [75], which reports a sustainable analysis used to estimate the environmental performance of three routes followed to obtain bio-PTA for the synthesis of bio-PET: from sweet corn, from sugar beet and maize grain, and from orange peels. The results were compared with traditional technology. The production using sugar beet and maize grain was closest to the current fossil technology, the production with sweet corn was found to be the most impactful, while the production using orange peels was the greenest one. The researchers found that the bio-routes for synthetizing PTA could only be very competitive if organic waste streams were converted into raw materials for the production of building blocks; however, if dedicated crops were used, some limitations were observed on the mitigation of climate change. From this research, it is clear that an early stage evaluation is fundamental in order to identify the benefits and the disadvantages of bio-based plastic production and commercialization. It was pointed out that an important impact is recorded when crops are used, even when compared to fossil fuels use. Meanwhile, waste is a very interesting and valuable building blocks resource, showing two advantages: first, wastes are available while biomass requires dedicated cultivation, and, second, there are economical and environmental savings that result from the non-disposal of waste itself. The potentiality of these bio-based polymers to be used in bulk applications is not a problem, thanks to the fact that they are chemically identical with their petrochemical counterparts. Within the bio-based plastics sector, PLA, bio-PE, starch plastics and bio-based epoxy resins are the four key materials. PLA and starch plastics are candidates for fast growth thanks to their production cost reduction and bulk application. Bio-based epoxy resins will only grow if the bio-based starting monomer is available and its costs are lowered. Meanwhile, the key factor for the growth of Bio-PE is related to its production costs. Bio-PP is still in its lab/pilot-plant stage due to technological barriers that include conversion technologies and downstream processing technology. When considering an optimistic future scenario, it is expected that it will leave the pilot stage and enter the commercialization stage from 2021 onwards.

7. Conclusions Bio-based plastics represent a very interesting and emerging field, and the even larger growth in the use of biomass feedstock for non-food purposes has demonstrated the technical possibility of producing these materials on a million-ton scale, substituting petrochemical plastics in meaningful quantities. Actually, the most important polymers in terms of production volumes are PLA and starch plastics, but the intended trend is to promote and increase the world-wide capacity of different types of bio-based plastics. In view of the most important and representative company announcements, bio-based PE and PHA, will also become as important as PLA and starch plastics by this year. Since 1980, interest in bio-based plastics, especially biodegradable plastics, has been driven by ever more sizeable waste management problems, but now much more attention is being focused on durable plastics as a means of reducing wastes. The interest is to take advantage of the use of biomass coming from food and agricultural wastes, choosing monomers and polymers that can be easily processed within existing structures. Ethylene and propylene are a great example of this interest. In the next decade, the interest in such bio-based durable plastics will grow much more than bio-based biodegradable plastics, partly due to the fact that the management of bio-based biodegradable plastics requires major changes and investments in the waste management infrastructure with respect to petroleum-like polymers. Nevertheless, bio-based biodegradable plastics will grow continuously, occupying their own position in the plastic market. Small and medium enterprises (SMEs), such as Novamont, Biotec, Rodemburg Biopolymers, Cereplast, Tianan and so on, as well as large chemical companies, such as Braskem and Dow, are very Polymers 2020, 12, 1641 14 of 17 active in the field of bio-based plastics. Actually, for big companies, many projects on bio-based polymers represent a relatively small share of their interests, but in a near future they will have the opportunity to convert their production in order to avoid environmental impact problems, such as CO2 emissions, non-renewable energy saving, gas reduction, and waste reduction and management. Despite the existence of numerous plastic materials with a high bio-renewability, only a small fraction of these have found a place in commercial applications. The heaviest challenges will be to reduce the high cost of production and processing, minimize agricultural land use and forests, avoid competition with food production, and, of course, reduce the environmental impacts.

Author Contributions: V.S. and I.B. were both authors and writers of this paper. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.

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