maja Čolnik biodegradable polymers, current maŠa …

Chemical Industry & Chemical Engineering Quarterly

Available on line at Association of the Chemical Engineers of Serbia AChE www.ache.org.rs/CICEQ

Chem. Ind. Chem. Eng. Q. 26 (4) 401−418 (2020) CI&CEQ

401

MAJA ČOLNIK

MAŠA KNEZ HRNČIČ MOJCA ŠKERGET

ŽELJKO KNEZ

Laboratory for Separation Processes and Product Design,

Faculty of Chemistry and Chemical Engineering, University of Maribor,

Maribor, Slovenia

REVIEW PAPER

UDC 678.7:60:502/504

BIODEGRADABLE POLYMERS, CURRENT TRENDS OF RESEARCH AND THEIR APPLICATIONS, A REVIEW

Article Highlights • An overview of the different biodegradable polymers and their properties • The possibility of production of biodegradable plastics from renewable or non-renew-

able sources • Current scientific research, applications and global production of bioplastic are con-

sidered • The properties of polymers obtained from biomass are presented in details Abstract

Biodegradable polymers have been developing rapidly in the last years and are widely used today in the fields of pharmacy, clinical biomedicine, cosmetic, medical, and packing industries, tissue engineering, agriculture and other areas. The interest in biodegradable polymers has been increasing, mainly due to rising oil prices, which is the basic feedstock of plastic derived from pet-roleum, and also due to the problem of the removal of waste plastics that accu-mulate in the environment. Biodegradable polymers have many advantages in contrast to synthetic polymers and can be decomposed in the environment to non-hazardous substances. Biodegradable polymers are classified into two classes based on their synthesis, i.e., synthetic and natural polymers. They are derived either from petroleum resources or from biological resources. The fol-lowing review presents an overview of the different biodegradable polymers and their properties, current scientific research, applications, global production of bioplastic and replacement of conventional plastic.

Keywords: biodegradable, polymers, natural origin, bioplastics.

Synthetic plastic was considered to be the mat-erial of the future in the previous century, but today it represents a major environmental problem [1]. The main disadvantages of synthetic polymers are the lengthy decomposition process and the production based on non-renewable raw materials. According to expert estimation, as much as 7% of world oil and gas supplies are used for polymers production [2]. An alternative possibility is presented by polymers that are biodegradable or made from renewable sources. Thus, the production and use of bio-based and bio- Correspondence: Ž. Knez, Laboratory for Separation Processes and Product Design, Faculty of Chemistry and Chemical Eng-ineering, University of Maribor, Smetanova 17, SI-2000 Maribor, Slovenia. E-mail: [email protected] Paper received: 10 December, 2019 Paper revised: 4 May, 2020 Paper accepted: 6 May, 2020

https://doi.org/10.2298/CICEQ191210018C

degradable polymer materials significantly increase and consequently can contribute to the decrease of environmental problems concerning waste polymer materials [3]. Research on renewable resources is focused on the use of corn, soy, sugarcane, potato, rice or wheat, and seeds rich in oil or fermentation products as raw materials to produce biopolymeric materials [4]. The use of biomass to produce biopoly-mers has a great advantage, since biomass-derived polymers are biodegradable and relatively easy to recycle [3]. One of the main disadvantages of biodeg-radable polymers obtained from renewable sources is their fast degradation rate, due to their dominant hyd-rophilic character and, in some cases, inadequate mechanical properties, particularly in wet environ-ments [5]. Despite their disadvantages, biodegradable polymers offer a wide range of advantages since many plant materials are used to make them. This means that it is no longer necessary to use chemical

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fillers contained in synthetic plastic products. More-over, during the production cycle of biodegradable plastics, energy consumption is reduced while the decomposition does not lead to unnecessary release of CO2 into the environment. The use of biodegrad-able products could reduce dependence on fossil fuels and thus contribute to a cleaner environment and more and more research is therefore being done in the field of new biodegradable polymers and their improved properties.

The aim of this article is to review the recent research on the most used biodegradable polymers, to examine their application possibilities, and the pos-sibility to decrease, or even eliminate, the usage of synthetic plastics, which have already caused major environmental problems.

BIOPLASTIC - BIODEGRADABLE POLYMERS

Bioplastic materials are a family of materials with different properties and applications that can be bio-based, biodegradable or both. Biodegradable plastics are made from renewable or non-renewable sources, which completely degrade in the environ-ment through a thermochemical process into CO2, methane, water, biomass and inorganic compounds that microorganisms can easily mineralize [6,7], while bio-based polymers contain organic carbon of renew-able feedstock and are not necessarily biodegrad-able. Moreover, bio-based polymers can be synthe-sized from naturally occurring materials as well as from natural substances that have been polymerized into high molecular weight materials by chemical and/or biological methods [8]. Bio-based materials have a similar molecular structure to petrochemical polymers. Above all, the biodegradability of a plastic

materials depends on their chemical structure and not the carbon source of the polymer [8]. According to the forecast [9,10], the global bioplastics production cap-acity is set to increase from around 2 million t in 2018 to approximately 2.6 million t in 2023 (Figure 1) [9]. Currently, bioplastics represent about 1% of 335 mil-lion tons of plastic produced annually. Bioplastics are an increasing, innovative industry that offers solutions for a sustainable plastics economy and that plays a key role in the transformation to a bio-based circular economy [9,10]. Yet, despite these advantages, the data shows that the overall growth of the global bio-plastics industry is currently being slowed down by low oil prices and a lack of political support for the bio-based economy [9,10].

Bio-based non-biodegradable plastics currently make up for around 48% (1 million t) of the global bio-plastics production capacities (Table 1) [9]. Just like for the bio-based polymers, the global production of biodegradable polymers is also increasing (Table 1). The production of PLA (polylactic acid) and PHA (polyhydroxyalkanoates) is currently growing. Reg-arding the global production capacities of bioplastic by material type, the PLA share is predicted to inc-rease from 10.3 to 13.2%, while the PHA share is pro-jected to increase from 1.4 to 5.8%. The production of other biodegradable polymers should remain the same or decrease [10].

In the following, biodegradable polymers are classified as either bio-based or petrochemical-based. The material is mostly biodegradable by nature and produced from biomass (plants, animals or microorg-anisms) such as polysaccharides (e.g., starch, cellu-lose, lignin and chitin), proteins (e.g., gelatine, casein, wheat gluten, silk and wool) and lipids (e.g., plant oils and animal fats) [11,12]. The major natural polymers

Figure 1. Global production capacities of bioplastics [10].


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are starch, cellulose and soy protein. Natural rubber as well as certain polyesters, either produced by mic-roorganisms or plants (PHA), and poly-3-hydroxybut-yrate (PHB) or synthesized from bio-derived mono-mers (PLA), fall into this category [11,12]. Petrochem-ical-based biodegradable polymers such as aliphatic polyesters (e.g., polyglycolic acid (PGA), polybutylene succinate (PBS) and polycaprolactone (PCL)), aro-matic copolyesters (e.g., polybutylene succinate tere-phthalate (PBAT)) and poly(vinyl alcohol) (PVA) are produced by synthesis from monomers derived from petrochemical refining, which possess a certain deg-ree of characteristic biodegradability (Figure 2) [11,12]. Particularly, PBS is already industrially produced from renewable resources by PTT MCC Biochem Com-pany and it is called BioPBSTM.

Table 1. Global production capacities of bioplastic by material type in 2018 and 2023 [10]

Material type 2018 (%) 2023 (%)

Biodegradable 43.2 44.5

PBAT 7.2 4.2

PBS 4.6 4.1

PLA 10.3 13.2

PHA 1.4 5.8

Starch blends 18.2 15.8

Other (biodegradable) 1.5 1.4

Bio-based/non-degradable 56.8 55.5

PET 26.6 20.5

PA 11.6 11.0

PEFa 0.0 2.9

PE 9.5 11.3

PP1 0.0 2.0

Other (biobased/non-biodegradable) 10.1 7.8 aBio-based PP and PEF are currently in development and pre-dicted to be available in commercial scale in 2023

Furthermore, the latest research on the most common biodegradable polymers, the addition of copolymers, additives or fibers, which can improve the mechanical and physical properties of polymers and possible applications of biodegradable polymers are shown in Tables 2-9. Thermoplastic starch (TPS) is still the most widely used material for the research and production of biodegradable polymers. Namely, TPS has been the first biopolymer on the market, but currently two groups of biodegradable polymers have the highest market potential.

The first is polylactic acid (PLA), produced by fermenting carbohydrates, and the second important group of biodegradable polymers are polyhydroxyalk-anoates (PHA), synthesized through bacterially guided fermentation processes. Their products are widely used in different fields.

POLYMERS OBTAINED FROM BIOMASS

Starch

Starch is the major form of carbohydrate storage in green plants and is considered the second largest biomass produced on earth. Starch is a cheap and easily available raw material, which is present in large quantities in potatoes, maize, rice and wheat [4,13]. In natural form, starch is not meltable and cannot be processed as a thermoplastic [14]. Starch granules can be thermoplasticized through a gelatinization process, where the granules are disrupted and the ordered crystalline structure is lost under the influ-ence of plasticizers, shears and heat [14]. The result-ant melt-processable starch is called thermoplastic starch (TPS) [15].

Bioplastics based on starch are suitable for the production of packaging, in agriculture, for medical

Figure 2. Classifications of biodegradable polymers [12].


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and cosmetic application [16-19]. Due to its low cost and high availability [20], starch attracts increasing attention and interest of researchers and industries worldwide. Xie et al. [21] studied a completely new biodegradable sisal fiber–starch packing composite, which exhibited good biodegradability. This compo-site is a promising replacement for expandable poly-styrene (EPS) as packing material, especially under large compression loads [21] (Table 2). In comparison to sisal fiber, which is the most used natural fiber in the world, cassava starch is also often used as matrix material. In addition, Medina-Jaramillo et al. [22] investigated the active and smart biodegradable films from cassava starch and glycerol with different natural extracts such as green tea and basil by casting. The high content of phenolic compounds in the extracts led to significant antioxidant activity of the films. The films were thermally stable up to 240 °C and deg-raded in soil in two weeks [22]. Other researcher com-bined the cassava starch with biomass or biomass extract of microalgae Heterochlorella luteoviridis and Dunaliella tertiolecta [18]. Incorporation of microalgae biomass resulted in biodegradable and highly soluble films, making it difficult to apply in moist foods, while the films which contained 2.0% of H. luteoviridis ext-ract were appropriate for application in foods with high water content [18]. These prepared biodegrad-able films with antioxidant properties can used for sal-mon packaging, as they provide protection against lipid oxidation [18]. Several researchers investigated the use of starch and chitosan in the production of biofilms. Chitosan is nontoxic, biodegradable and has antimicrobial activity [23]. Thus, chitosan as a biopoly-mer has a wide range application in drug delivery sys-tems, in the areas of biomedicine, membranes, hydro-gels, adhesives, fuel cells, as a surface conditioner, for tissue engineering, etc. [24]. It has also shown a great potential for removing pollutants from water and

wastewater [25]. Besides, chitosan is a promising material to produce food packaging materials, and in comparison to starch due to its more hydrophobic nature represents an attractive combination of price, abundance and thermoplastic behavior [23]. Further, the researchers successfully prepared mixtures of corn starch (TPS) and chitosan (TPC) by extrusion and they produced more thermally stable films with potential application in the packaging industry [23].

Rice and maize are another widespread raw materials in the manufacture of biodegradable films [26]. The biodegradable films from hydroxypropylated rice starch (HPRS) provided an increase in elongation at break, water vapor permeability, film solubility and transparency, while the tensile strength value dec-reased when the propylene oxide proportion inc-reased [26]. Lopez et al. [27] studied the mechanical properties, oxygen permeability and water vapor, as well as thermo-sealing capacity of composite films from thermo-compressed films of TPS with talc nano-particles. With the addition of talc (3 mass%) to TPC, the water vapor and oxygen permeability were red-uced [27].

Tacca leontopetaloides, also known as Poly-nesian arrowroot starch, is a wild perennial herb [28]. This starch is an important food source for many Pacific Islander cultures [28]. The potential of T. leon-topetaloides starch plasticized using glycerol and crude palm oil (CPO) was investigated by Makhtar et al. [28]. The mixture becomes highly thermally resist-ant up to 430 °C when it is plasticized with CPO. The thermal behavior of glycerol TPS was quite similar to the conventional bioplastic which supports the utiliz-ation of T. leontopetaloides starch for bioplastic deve-lopment [28]. In further study, starch was used also for other applications. Namely, the disposed pres-sure-sensitive adhesive tape widely used in daily life has been contaminating the environment and has

Table 2. Starch as biodegradable polymers, addition of copolymers, additives/fibers and their potential applications

Biodegradable polymer

Material Co-polymer Additives/fibers Application Ref.

Starch Corn starch – Sisal fiber Packing material [21]

Starch Cassava starch – Glycerol, natural extracts such as green tea

Food packaging [22]

Starch Cassava starch Biomass extract of microalgae – Salmon packaging [18]

Starch Corn starch Chitosan – Packaging [23]

Starch Native rice starch – Propylene oxide Packaging [26]

Starch Corn starch – Talc nanoparticles Packaging bags [27]

Starch Polynesian arrowroot starch

– Glycerol and crude palm oil Bioplastic development [28]

Starch Potato starch – Glycerol, monohydrate citric acid Medical tapes and biomedical electrodes

[29]


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produced vast amounts of non-degradable trash. Czech et al. [29] studied the advanced biodegradable pressure-sensitive double-coated tape containing starch carrier and water-soluble partially degradable modified pressure sensitive adhesive. They observed the excellent tack and peel adhesion of these newly constructed biodegradable self-adhesive tapes, and high thermal shear strength [29]. The complete bio-degradability of starch carrier and partial biodegrad-ability of modified acrylic PSA were confirmed [29]. This environmentally friendly technology based on the starch has great potentials for diverse applications such as the paper industry for manufacturing of eco-logical biodegradable products, the production of water-soluble biodegradable labels, medical tapes and biomedical electrodes [29].

New research has confirmed that starch is a highly effective natural source and, depending on the starch source and processing conditions, a thermo-plastic material with different properties suitable for various applications and industries can be produced.

Cellulose

Cellulose is the most widespread natural organic polymer, representing about 1.5×1012 t of total annual biomass production and is considered an almost non-exhaustive source of raw material for an increased need for environmentally friendly products [30]. The primary source of cellulose is the existing lignocellu-losic source in forests [30]. Commercial cellulose pro-duction is focused on sources such as wood and naturally very pure cotton sources [30,31]. Cellulose fibers are used to produce biodegradable packaging materials, and in cosmetic products without synthetic polymers or strong composites together with biodeg-radable plastics [32]. Cellulose derivatives that have been used in bioplastic synthesis were cellulose nano crystals (CNC) [31], nanofibrillated cellulose (NFC), cellulose acetate butyrate, cellulose acetate and bio-PE [32]. The derivative of the cellulose was mixed with other biopolymers matrices in order to improve

physical properties, or a filler, such as starch and PLA [31]. NFC has an incredible potential as reinforcement material in nanocomposites for many different uses, such as foams and adhesives. Missoum et al. [33] prepared NFC and their derivatives with using three chemical surface modification strategies. Antibacterial activities of all samples were investigated against two kinds of Gram+ bacteria (Staphylococcus aureus) and Gram- bacteria (Klebsiella pneumoniae) [33]. They also strongly enhanced the photo-catalytic antimicro-bial effect of TiO2 additive (Table 3). This study shows that it is better to use grafted NFC either alone or functionalized with TiO2 if anti-bacterial properties are desired [33].

The cellulose backbone is known to be easily biodegradable in different biodegradation conditions and environments [33]. Also, biocomposites based on natural cellulose fibers (CF) and hydroxyethyl cellu-lose (HEC) were produced in the form of green pack-aging films [34]. The effect of the different single-component plasticizers on the mechanical and dyn-amic thermomechanical properties of the films was investigated. [34]. Moreover, the softening effect of the two-component plasticizer based on deep eutectic solvents (DESs) was addressed. It was found that DESs are highly promising plasticizers for cellulose-based biocomposites with similar or even better plasticizing effects compared to conventional plasticizers [34].

In the following, cellulose was also used as mat-erial for a new type of biodegradable cigarette filters to accelerate their disappearance after disposal. About 4.5 trillion cigarette butts are discarded every year. Joly et al. [35] compared the decomposition of cellulose and plastic cigarettes filters, either intact or smoked, on the soil surface or within a composting bin over a six-month field decomposition experiment [35]. It was found that conventional plastic filters take 7.5-14 years to disappear, in the compost and on the soil surface, respectively, in contrast to cellulose fil-ters, which take 2.3-13 years to disappear, in the compost and on the soil surface, respectively. [35].

Table 3. Cellulose as biodegradable polymers, addition of copolymers, additives/fibers and their potential applications

Biodegradable polymer Material Co-polymer Additives–fibers Application Ref.

Cellulose Wood pulp – TiO2 Paper, packaging and composites [33]

Cellulose Softwood kraft pulp – Natural cellulose fibers, glycerol, propylene carbonate

and ethylene carbonate

Green packaging films [34]

Cellulose Cellulose – – Cigarette filters [35]

Cellulose Sugarcane bagasse – Hemicellulose (5%) of bagasse High value plastics [36]

Cellulose Native cellulose (cotton linter)

Chitosan – Food packaging [37]

Cellulose Empty fruit bunch Cassava starch Glycerol Plastic bags and food packaging [38]


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Sugarcane bagasse was fractionated to cellu-lose, hemicellulose and lignin by a proprietary steam explosion process followed by downstream purificat-ions [36]. The cellulose was acetylated under hetero-geneous conditions to obtain cellulose acetates. The novel feature of this study was the utilization of the hemicellulose of bagasse as an internal plasticizer [36]. The results show how lignocellulosic agricultural wastes can be utilized to produce high value plastics [36].

Wu et al. [37] studied a facile way to prepare highly transparent antimicrobial films by grafting chito-san as copolymer onto the oxidized cellulose matrix, that had a high transparent property of above 80% transmittance, excellent antioxidant activity and anti-microbial properties against Escherichia coli and Sta-phylococcus aureus [37]. Using a sausage model, it was shown that the composites exhibited better per-formance than traditional polyethylene packaging material and demonstrated good potential as food packaging materials [37].

Empty fruit bunch (EFB) is available abundantly in Indonesia as a side product of CPO production [38]. Potential cellulose amount in EFB is 11.50 mil-lion t [38]. For that reason, Isroi et al. [38] investigated bioplastic production based on cellulose from EFB. Cellulose was isolated by sodium hydroxide methods and bleached using sodium hypochlorite. Purity of obtained cellulose was 97%. For bioplastic synthesis, glycerol as plasticizer and cassava starch as matrix were used. In this research, bioplastic sheets were successfully produced by casting method. In future prospects, bioplastic from EFB cellulose can be used for plastic bags and food packaging [38].

Soy proteins

Soy is a cheap and renewable source of bio-polymers, which has a great potential to replace pet-rochemical polymers in many applications. Soy pro-tein (SP) is commercially available in three various SP concentrations: soy flour (54%), soy concentrate (65-72%) and soy isolate (90%) [39].

Molecules of SP include 20 different amino acids with strong inter- and intramolecular interact-ions. Such interactions make SP unmeltable and therefore it is impossible to process SP in the form of a thermoplastic polymer, unless a sufficient amount of plasticizer is used [40,41]. The use of a significant amount of plasticizer results in low mechanical pro-perties of SP plastics. On the other hand, when the plasticizers migrate away from the SP plastics during storage or service, the materials become very brittle [40,41]. Moreover, the hydrophilicity of SP and the plasticizers leads to low moisture resistance of SP

plastics. Blending of SP polymers with biodegradable polymers is a natural choice to overcome the said lack of SP-based polymers. The polymers used for mixing with SP are hydrophobic and therefore cannot establish a strong bond with SP and a compatibilizer such as maleic anhydride is required in the mixture [40,41]. By adding it in a smaller amount, it is possible to improve the mechanical properties, the resistance to moisture and the viability due to the increased interactions within their compounds [40,41].

Many reports have been published about bio-medical applications of pure SP or blending with other polymers or macromolecules [42-49]. SP also is used in infant formulas and in baked, meat, and dairy pro-ducts [50]. The use of SP as a film-forming agent can add value to soybeans by creating new channels for marketing SP [51].

Tulamandi et al. [52] found that with the addition of gelatin to papaya puree, the films have shown sig-nificant increase in color properties, tensile strength and seal strength [52].Whereas, with the addition of defatted SP along with gelatin to the papaya puree, the films have shown significant increase in elong-ation, water permeability, water contact angle and decrease in water solubility (Table 4).

Non-biodegradable polyvinyl alcohol (PVA) deri-ved from petroleum is the primary sizing agent due to its excellent sizing performance on polyester-contain-ing yarns, especially in increasingly prevailing high-speed weaving [53]. However, due to poor biodeg-radability, PVA causes serious environmental pollut-ion, and thus, should be substituted with more envi-ronmentally friendly polymers [53]. For this reason, Yang et al. [53] developed soy sizes from SP treated with glycerol and the biodegradable triol that was also obtained from soy. It was observed that the soy sizes had good film properties, adhesion to polyester and abrasion resistance close to PVA. In summary, the fully biodegradable soy sizes have the potential to substitute PVA for sustainable textile processing [53]. Also, Zhao et al. [54] grafted soy protein with acrylic acid and cast into biodegradable films as substitutes of non-biodegradable PVA films. Acrylic acid grafting provided SP films with biodegradability, flexibility, and adhesion to yarns substantially higher than PVA, while water solubility and abrasion resistance was similar to PVA, leading to high potential applications of the grafted SP in the fields of water soluble pack-aging films and slashing to substitute PVA [54].

Reddy et al. [55] developed completely biodeg-radable SP composites reinforced with jute fibers with used water without any chemicals as plasticizer. It was found that the developed SP composites have


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excellent flexural strength, tensile strength and tensile modulus, much higher than polypropylene (PP) com-posites, even at high relative humidity (90%) [55].

Soy protein isolate (SPI) seems to be an ade-quate resource for the manufacture of natural-based superabsorbent materials due to its hydrophilic char-acter and excellent processability when combined with a plasticizer [56]. Yu et al. [57] studied SP-based films incorporated with CNCs and Cedrus deodara pine needle extract (PNE) as potential food packaging material. When a high content of PNE was incorpor-ated in the films, the water vapor permeability was decreased due to the reduction of hydrophilic dom-ains in the film matrix. Moreover, the PNE-added films contained phenolic compounds and displayed strong antioxidant activities [57]. Carpiné et al. [58] pre-sented an alternative by incorporating virgin coconut oil (VCO, hydrophobic component with good oxidative stability) into SPI films. In addition, as natural surfact-ant, the Yucca schidigera (YS) saponin was applied. Morphological analyses indicated that the incorpor-ation of VCO or yucca extract caused notable changes in the structure of the SPI films. Therefore, this data should help to better understand the role of lipids and surfactants in protein-based films for future industrial purposes [58]. Jiménez-Rosado et al. [56] studied the SP-based bioplastic matrices which were loaded with zinc sulphate monohydrate for potential applications in horticulture. It was confirmed that sig-nificant amounts of an essential micronutrient for a plant (Zn) can be incorporated into bioplastic mat-rices, modifying water absorption, mechanical and microstructural properties [56].

For medical purposes, Tansaz et al. [59] inves-tigated the fabrication of SP isolate/nanoscale bioact-ive glass composite films by solvent casting method

as a matrix for wound-dressing applications. The effect of the addition of bioactive glass nanoparticles on blood clotting was assessed [59]. The composite films could meet the essential requirements for an appropriate wound dressing with additional favorable properties such as hemostatic capability, mechanical properties and significant cell cytocompatibility [59].

BIODEGRADABLE POLYMERS OBTAINED VIA MICROBIAL PRODUCTION

Polyhydroxyalkanoates (PHAs)

PHAs are classified as natural aliphatic biopoly-esters, synthesized by many different bacteria as intracellular carbon and energy storage materials.

PHAs have the ability to combine more than 150 monomers to produce materials with very different properties and functionalities [60,61]. Mechanical and biological compatibility can be altered by mixing, altering the surface, or by combining PHA with other polymers, enzymes or inorganic materials, allowing them a wider spectrum of use [60]. The main areas of PHA use include packaging (containers and films), coatings, pharmaceutical and medical applications (wound dressings, medical devices, orthopedic pins, stents, nerve guides and bone marrow scaffolds). Besides, PHAs are used as hardeners in cosmetic products, hygiene products, toners and adhesives, electronic issues and golf balls [62]. PHA plastics are in contrast to other types of bioplastics (e.g., PLA), UV stable, can withstand temperatures up to 180 °C and are poorly water-permeable [60].

Some specific forms of PHA are not present only in microorganisms, but are also found in plants, animals, and even in humans as a component of membrane tissues in the form of polyhydroxybutyrate

Table 4. SP as biodegradable polymers, addition of copolymers, additives/fibers and their potential applications



SP Defatted soy protein (Shakthi soyas)

Gelatin, papaya puree Glycerol Packaging [52]

SP Soy proteins – Glycerol Textile processing [53]

SP Soy proteins – Acrylic acid Packaging [54]

SP Soy proteins – Jute fiber Various applications [55]

SP Soy protein isolate Cellulose nanocrystals (CNCs) and Cedrus

deodara pine needle extract

Glycerol Active food packaging material [57]

SP Soy protein isolate – Virgin coconut oil, glycerol Industrial purposes [58]

SP Soy protein isolate – Zinc sulphate monohydrate, glycerol

Horticultural crop applications [56]

SP Soy protein isolate – Nanosized bioactive glass, glycerol

Wound-dressing applications [59]


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(PHB), which is the most representative member of this family [63]. PHB is a rare example of a hydro-phobic polymer that is biocompatible and biodegrad-able. It has a high melting point (170-180 °C) and a crystalline degree close to 80%. PHB is rigid and brittle, with the degree of fragility being dependent on recrystallization, the glass transition temperature, and the microstructure of the polymer. The longer the time of its storage, the more fragile it becomes. Thermal instability during continuous polymer processing at high temperatures and the tendency to crack and fracture limit its wider use. The combination of high crystallinity and low density of nucleation sites is ref-lected in the formation of large spherulitic crystals that influence the spread of cracks and the fragility of the material. In order to improve mechanical and thermal properties, different nucleation agents are added. Among the common procedures for the modification of the original properties of PHB, it is certainly the mixing of the basic polymer component with other polymers or with additives that is the most common. PHB can be processed using conventional techno-logical processes for the processing of plastics, which makes its use widely available [64,65].

PHAs have emerged as highly promising bio-materials both for bulk and biomedical applications

(Table 5) [66]. Wound management, coronary angio-plasty, nerve regeneration, bone tissue engineering, cardiac tissue engineering and drug delivery are some examples of biomedical applications where PHA-based materials have been explored [67]. PHB has been found to have low toxicity, since it degrades in vivo to d-3-hydroxybutyric acid, a normal cons-tituent of human blood. Applications of these poly-mers previously tested or in phase of development include controlled drug release, artificial skin, and heart valves, as well as in industrial applications such as paramedical disposables [68,69]. Subsequent applications in the packaging and cosmetic industries included the production of a wide variety of bottles, tubs, bags and wrappings.

Progress in tissue engineering is dependent on the availability of suitable biomaterials. To overcome the brittleness of poly(3-hydroxybutyrate) P(3HB), and widen its biomedical applications, plasticizing of P(3HB) with oligomeric substances of related struc-ture was investigated by Lukasiewicz et al. [66]. A biosynthesized medium-chain-length polyhydroxyalk-anoate (mcl-PHA) copolymer, the plasticizer precur-sor, was obtained using vegetable waste frying oil as the sole carbon source [66]. Addition of oligomeric mcl-PHA to P(3HB) resulted in softer and more flex-

Table 5. PHA as biodegradable polymers, addition of copolymers, additives/fibers and their potential applications


Material Co-polymer Additives–fibers Application Ref.

PHA Bacillus subtilis OK2 and Pseudomonas mendocina

CH50

P(3HB) Acetic acid Soft tissue engineering applications

[66]

PHA–PHA-g-MA–TPF PHA – Treated palm fiber Maleic anhydride

3D printing filaments [70]

PHB PHB Starch – A coating material on paper or cardboard used

for food packaging

[71]

P3HB Commercial P3HB – Maleinized linseed oil (MLO) and an epoxidized fatty acid

ester (EFAE)

Packaging [72]

PHA Gordonia polyisoprenivorans VH2, Ralstonia eutropha, Pseudomonas aeruginosa

– Poly(cis-1,4-isoprene) Recycling method for rubber waste

[73]

PHA Spirulina sp. LEB-18 Microalgae

– Sodium hypochlorite, methanol

Industrial [74]

PHA Commercial PHA NCC, chitosan Tween 80 Industrial wastewater [75]

PHA Peanut oil and Cupriavidus necator H16

– Sodium hypochlorite solution

Food packaging, biotechnology industry

[76]

PHA Bacterially synthesized – GO, LAQ Food packaging applications

[77]

PHB PHB industrial Coconut fiber in nature and coconut

fiber treated

– Packaging [78]


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ible materials based entirely on PHAs. The proposed approach for P(3HB) plasticization has the potential for the generation of more pliable biomaterials based on P(3HB) which can find application in unique soft tissue engineering applications where a balance between stiffness, tensile strength and ductility is required [66].

The biodegradability, mechanical properties and fabrication of three-dimensional (3D) printing fila-ments of composite materials made from maleic anhydride-grafted polyhydroxyalkanoate (PHA-g-MA) and coupling agent-treated palm fiber (TPF) were studied by Wu et al. [70]. TPF was successfully com-bined with PHA-g-MA by an esterification procedure and 3D printing filaments were fabricated from the composites [70]. The strong interaction of the PF with the PHA matrix in the PHA-g-MA/TPF composites led to improved tensile properties and had lower water absorption than the corresponding PHA/PF mem-branes, and the aqueous release solutions had good cell viability [70]. When incubated in soil, the biodeg-radation rate of PHA-g-MA/TPF was lower than that of PHA/ PF, while still higher than that of pure PHA. Due to their low cost and excellent characteristics, the prepared composites can be used as the biodegrad-able material of 3D printing filaments [70]. In order to improve properties and cost reduction, Godbole et al. [71] studied the compatibility of PHB with starch. The results revealed that blend films had a single glass transition temperature for all the proportions of PHB and starch tested [71]. The nature of all combinations was found to be crystalline. These blend films might also be used as a coating material on paper or card-board used for food packaging [71]. Garcia-Garcia et al. [72] explored the potential of vegetable oil‐derived plasticizers (maleinized linseed oil) and an epoxidized fatty acid ester as environmentally friendly solutions for P3HB industrial formulations with improved tough-ness [72]. The optimum balance between ductile pro-perties is achieved with low plasticizer content for both plasticizer types [72].

Andler et al. [73] studied the biosynthesis of PHA utilizing poly(cis-1,4-isoprene) as the sole car-bon source in order to find recycling methods for rub-ber waste materials and alternative carbon sources for the synthesis of PHA [73]. Costa et al. [74] demon-strated that the methods used for extracting PHAs from the cyanobacterium Spirulina sp. LEB-18 show different efficiencies in the extraction of the polymers. The use of sodium hypochlorite in the initial extraction stage increases polymer accumulation, while the use of methanol at the end of the process is important for obtaining higher purity PHAs [74]. The extraction

methods significantly influenced the molecular mass, degree of crystallinity, and monomeric composition of PHAs, showing that the extraction method is critical in obtaining polymers with desired characteristics for industrial applications [74]. The PHAs extracted from the Spirulina sp. were composed largely of 11-hydro-xyhexadecanoate monomers and hydroxytetradec-anoate, which is a scientific novelty because these building blocks are constituents of completely new polymers [74]. Further, in the study of Soon et al. [75] the nanocellulose and chitosan were incorporated into the PHA composite with the use of the Pickering emulsion-electrospinning method and the material was then tested as adsorbent for the removal of dye from industrial wastewater. The results showed that dye (Congo red) removal percentage in the case of nanocellulose addition was much better (30.9%) than in the case of chitosan addition (10.5%) [75]. In the work of Perez-Arauz et al. [76], peanut oil as a carbon source for production of PHA films was used for the first time. The novel PHA films showed positive mech-anical, physical and barrier properties so the material could be suitable for food packaging applications and in the biotechnology industry [76]. Xu et al. [77] created the well-designed multifunctional and robust PHA/GO-g-LAQ (long alkyl chain quaternary salt functionalized graphene oxide) nanocomposites with superior gas barrier, heat resistant and inherent anti-bacterial performances, which could be ideal for eco-friendly food packaging [77].

The influence of the coconut fiber used as reinf-orcement in PHB composites was studied by da Silva Moura et al. [78]. The microstructure showed a good interfacial adhesion between the PHB and coconut fiber [78].

BIODEGRADBLE POLYMERS CHEMICALLY SYNTHESIZED USING MONOMERS OBTAINED FROM AGRO-RESOURCES

Polylactic acid (PLA)

PLA is a synthetic biodegradable polyester with a monomer, lactic acid (LA), derived from natural resources. The most used raw material is maize or other industrial plants with a high starch content. In the fermentation processes of hydrocarbons, Lacto-bacillus lactic acid bacteria or Rhizopus oryzae fungi are used. The fermentation process requires the pre-sence of bacterial strains and sufficient amounts of carbon (glucose, sucrose or lactose), nitrogen (yeast extract, peptides), and mineral elements for their active functioning. The final product of bacterial syn-thesis is an optical isomer of lactic acid L- (+), which


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can be converted to polymeric form of poly-(L-lactic acid) (PLLA) of low molecular weight in subsequent polycondensation reactions. The properties of the PLA depend on its molecular structure, crystallinity, the size of the spherulitic crystals, morphology, and the level of arrangement of the polymer chains. The stereochemical structure of PLA determines its crys-talline arrangement. Depending on the selected syn-thesis, polymerization process, the final form of the PLA can be completely amorphous (with 50-93% con-tent of L-lactic acid) or semicrystalline (with >93% content of L-lactic acid) [79,80]. The glass transition temperature of the PLA homopolymer is 55 °C and the melting temperature is about 175 °C. Under con-trolled composting conditions, the material is com-pletely biodegradable and can be processed using standard plastic processing techniques. To improve process properties, lactide is often copolymerized with an enantiomer of the opposite configuration. This represents a copolymerized form of PLA, referred to poly-(DL-lactide) (PDLLA) [4,81].

The use of PLA polymers has great potential for automotive and packaging industries and medicine. It can be used in the production of flexible and water-proof films, bottles, labels and cups, bakery pack-aging and disposable dishes. In biomedicine, it is used as a binding material in tissue engineering [81]. However, its poor thermal stability, rigidity, intrinsic brittleness, low crystallization rate and high cost limit its large-scale applications [82]. The introduction of boron nitride (BN) as a reinforcement inorganic mat-erial in PLA may significantly improve their properties, making them more suitable for packaging of electro-nic products. Namely, BN has superior thermal con-ductivity and mechanical properties. The prepared PLA/BN composite material has improved mechanical properties and it is suitable for packaging in industries (Table 6) [83]. D'Amico et al. [84] prepared biodegrad-

able polymeric blends based on poly(3-hydroxybut-yrate) (PHB) and PLA by melt mixing. They con-cluded that plasticized PHB/PLA (30/70) and PHB/ /PLA (40/60) blends, may be considered as sus-tainable alternatives to current non-natural and non-biodegradable materials for food packaging films, considering their flexibility, transparency, possibilities for processing at the industrial level and compo-stability in terms of their final disposition [84]. By embedding nanochitosan in a PLA matrix using poly-ethylene glycol (PEG) as a cross linking agent and PVA as plasticizer, the composite film was developed by Fathima et al. [85]. The interactions between the PLA/NCS and PEG had a significant effect on the ten-sile strength and the heat-sealing properties. Anti-microbial properties of PLA/NCS films have been con-firmed against aerobic microorganisms and, con-sequently, the PLA/NCS composite films can be used for packaging of fresh prawn to increase their shelf life [85]. Arrieta et al. [86] prepared flexible electro-spun PLA-PHB biocomposites with chitosan or cat-echin as active film materials. Due to well-known anti-oxidant activity of both loaded components and good qualities of PLA-PHB biocomposites, these are pro-mising materials for biodegradable film applications (agricultural mulch films, films for food packaging) [86]. In a brand-new in vitro study, Abasian et al. [87] have successfully incorporated the NaX/Fe3O4 nano-particles and doxorubicin (DOX) into the PLA/chitosan nanofibers. These prepared nanofibers show excel-lent potential in local chemotherapy of carcinoma tumors. Namely, in vitro studies have shown that the maximum killing percentage of human carcinoma cells was 82% after 7 days [87].

Swaroop et al. [88] produced biofilms by reinfor-cement of magnesium oxide (MgO) nanoparticles in PLA biopolymer using the solvent casting method. PLA/MgO films exhibited superior antibacterial effi-

Table 6. PLA as biodegradable polymers, addition of copolymers, additives/fibers and their potential applications

Biodegradable polymer Material Co-polymer Additives/fibers Application Ref.

PLA, PLA–BN PLA resin – Boron nitride Packaging in industries [83]

PLA Industrial PLA PHB Tributyrin (TBL) Food packaging [84]

PLA PLA resin Nanochitosan Polyethylene glycol, polyvinyl alcohol

Packing of fresh prawn [85]

PLA Industrial PLA PHB Chitosan, catechin, acetyl-tri-n--butyl citrate (ATBC)

Agricultural, food packaging [86]

PLA Industrial PLA chitosan DOX Trifluoroacetic acid

Local chemotherapy [87]

PLA Commercially available PLA

– MgO nanoparticles Food packaging [88]

PLA Industrial PLA ABS Cardanol 3D printing [89]

MPEG-PLA Industrial MPEG-PLA – Verapamil and doxorubicin Ovarian cancer [90]


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cacy and they caused progressive damage and death of nearly 46% of E. coli bacterial culture after a 12 h treatment [88]. The produced films are transparent, capable of screening UV radiations and exhibit sup-erior antibacterial efficacy what makes them an excel-lent food packaging material [88].

Moreover, the researchers investigated the effect of cardanol-based compatibilizers, which can react by reactive extrusion with PLA and ABS (acrylo-nitrile-butadiene-styrene). The modified chains of both PLA and ABS with cardanol derivatives during reactive extrusion would intersperse between PLA and ABS, reducing the interfacial tension and imp-roving the interfacial adhesion, leading to the out-standing compatibilization of the blend [89]. For medi-cal application, Zheng et al. [90] co-encapsulated verapamil and DOX in methoxy poly(ethylene glycol)- -poly (L-lactic acid) (MPEG-PLA) nanoparticles to rev-erse the drug resistance in ovarian cancer [90]. These MPEG-PLA nanoparticles reveal high biocompatibility and superior safety. They also demonstrated the enhanced anticancer effects of verapamil and DOX loaded nanoparticles, with a sufficient evaluation of safety in vivo [90].

BIODEGRADABLE POLYMERS DERIVED FROM PETROLEUM

Polycaprolactone (PCL)

PCL is a semicrystalline aliphatic polyester, which is well known for its slow biodegradation rate and high biocompatibility. PCL is produced by ring-opening polymerization of ε-caprolactone in the pre-sence of various anionic and cationic catalysts [91]. Due to its slow biodegradation, it is suitable for pro-duction of prolonged-release delivery systems. The decommissioning takes place over two phases. First, the hydrolytic cleavage of the ester bond is carried out, and in the second phase, intracellular degrad-ation occurs into the non-toxic metabolites that are excreted directly from the body or after the metabolic change in the Krebs cycle [91]. Unlike most other bio-degradable polyesters, PCL does not locally lower the pH in the area of its degradation [91].

The main problem is its hydrophobicity, which can be solved by combining it with other natural or synthetic polymers, thus improving its mechanical properties and biocompatibility [92]. Besides, PCL has low glass transition temperature (-60 °C) and a low melting point (60 °C), but presents an ideal bio-degradable matrix for natural antimicrobial agents as it can be processed at low temperature. Moreover, the toughness of PCL is superior to that of most bio-degradable polymers. On the other hand, lower

modulus and rigidity of PCL limit its many applications [93]. High molecular weight PCL is not suitable as an oxygen barrier material. Despite many attractive pro-perties (slow degradation rate, high permeability, easy availability and good mechanical properties), the applications of PCL have also been limited by its rel-atively high cost [94,95].

Nevertheless, PCL is very common in the pro-duction of food packaging and, because of its excel-lent biocompatibility, it is explored as a structural mat-erial in tissue engineering (e.g., microstatic PCL foams are used for tissue regeneration and stem cell transplantation) [96].

PCL has been repeatedly and successfully used for the production of nanofibers [97]. Choi et al. [98] were among the first that produced nanofibers from PCL/collagen for cell regeneration of skeletal muscles [98] (Table 7). Zamani et al. [99] successfully incor-porated metronidazole benzoate (MET) in PCL nano-fibers for periodontal diseases [99]. In the work of Potrč et al. [100], PCL nanofibers were incorporated in poorly water-soluble agents such as resveratrol, carvedilol and ibuprofen, thereby enabling prolonged release from the delivery form [100]. Zupančič et al. [101] produced PCL nanofibers with built resveratrol in order to treat periodontal disease [101]. Sessini et al. [102] studied the influence of the addition of poly-ester-grafted-cellulose nanocrystals on the shape-memory properties of biodegradable PLA/PCL nano-composites [102]. Joo et al. [103] obtained biocom-patible biodegradable PU/PCL blends by blending. Such polymer blends with suitable mechanical pro-perties and shape-memory behavior can be used in surgical sutures or other medical devices [103]. Xu et al. [104] studied the feasibility of utilizing PCL as a biocompatible and biodegradable binding agent to fabricate electrospun three-dimensional (3D) scaf-folds. The obtained 3D scaffolds are soft while elastic, and they possess interconnected and hierarchically structured pores with sizes ranging from sub-microns to hundreds of microns; hence, they are morpho-logically similar to natural extracellular matrices (ECMs), thus well suited for cell functions and tissue formation [104]. These various thermoplastic poly-mers could be fabricated into 3D nanofibrous scaf-folds/structures by first making blend nanofibers with PCL, followed by processing via the thermally ind-uced (nanofiber) self-agglomeration (TISA) method, and finally being thermally stabilized [104].

Poly(butylene succinate) (PBS)

PBS is synthesized by the polycondensation reaction between succinic acid and butanediol. The


412

reaction takes place in two steps. In the first part of sintering, the esterification takes place between the diacids and the diols. Then, polycondensation takes place under high temperature conditions to form high molecular weight PBS [105].

PBS has a relatively low biodegradation rate due to its high crystallization rate and high crystal-linity. It has good thermal stability and mechanical properties. Furthermore, PBS naturally decomposes to nontoxic and harmless products such as water and CO2 [106]. To promote the physical properties, to extend the application field, and to increase the bio-degradability of PBS, numerous approaches have been used, such as physical blending, copolymer-ization, or formation of composites. Owing to the excellent processability of PBS, it can be processed using conventional polyolefin equipment in the range of 160-200 °C. Injection, extrusion or blow molding are suitable for the processing of PBS [107].

The applications of PBS are still increasing in many areas. In the field of packaging, PBS can be processed into foils, bags, or food and cosmetic pack-aging. In agriculture, PBS is used to produce mul-ching foils or delayed-release materials for pesticides and fertilizers. PBS is also increasingly used in fishing materials, forestry, construction, or other areas where processing and recycling of materials is problematic [106]. In the medical field, PBS can be used as a biodegradable drug for encapsulation systems and is also investigated for implants [108].

Production of PBS is more expensive than of synthetic polymers, hence it can be mixed with cheap natural fibers or fillings (wood). Jiang et al. [109] pre-pared a wood plastic composite, where were the CaCO3 acted as a reinforcing component, and alu-minum hypophosphite, ammonium polyphosphate and calcium hypophosphite as flame-retardant com-ponents were separately incorporated into PBS mat-rix. The investigation of mechanical and thermal pro-perties of PBS composites, can present a foundation

for the development of various application of PBS composites in different fields [109] (Table 8). Shi et al. [110] studied the preparation, characterization, and biodegradation of PBS/cellulose triacetate (CT) blends. PBS and CT were blended with different ratios using chloroform as a solvent. This finding indicated good hydrophilicity [110]. The weight loss became close to 90% after 16 h of degradation for PBS/CT10 [110]. Therefore, the blend economized the PBS material, was environment-friendly, and showed preferable solid-state properties [110].

Sun et al. [111] investigated non-isothermal crystallization of biopolyesters of PBS formed via in situ polymerization in the presence of poly(vinyl but-yral) (PVB). The effects of the PVB content and mole-cular weights on non-isothermal crystallization and mechanical properties were examined in detail [111]. Adding PVB greatly reduced spherulitic sizes and increased the peak temperature of crystallization [111]. An optimal content was observed for a minimal size of spherulites, and the PVB with a low molecular weight led to a smaller spherulitic size than its count-erpart of a high molecular weight did [111]. A mole-cular model was proposed to explain accelerated nuc-leation and spherical growth from PVB’s structural perspective. The nucleated PBS exhibited a consider-able improvement in mechanical properties [111]. The design and synthesis of a new PBS ionomer con-taining a novel bisfunctional phosphinate monomer, potassium salt of 10H-phenoxaphosphine-2,8-dicar-boxylic acid,10-hydroxy-,2,8-dihydroxyethyl ester,10- -oxide (DHPPO-K) was studied by Xu et al. [112]. PBS ionomers containing aromatic cationic groups were synthesized by condensation polymerization of succinic acid and 1,4-butanediol in the presence of DHPPO-K [112]. The crystallization properties, rheo-logical properties, dynamic mechanical properties, oxygen permeability, thermal conductivity and phys-ical properties were thoroughly investigated consider-ing their structure-property relationships [112].

Table 7. PCL as biodegradable polymers, addition of copolymers, additives/fibers and their potential applications



PCL Industrial PLC Collagen / Cell regeneration of skeletal muscles [98]

PCL Industrial PLC / Metronidazole benzoate (MET) Periodontal diseases [99]

PCL Industrial PLC / Resveratrol, carvedilol and ibuprofen

Different fields [100]

PCL Industrial PLC / Resveratrol Periodontal disease [101]

PCL Industrial PLC PLA CNC Biomedicine, food packaging [102]

PCL Industrial PLC PU / Medical devices [103]

PCL Industrial PLC Cellulose acetate (CA) / Tissue engineering [104]


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PTT Public Company Limited (PTT) and Mitsu-bishi Chemical Corporation (MCC) produce not only petroleum-based PBS but also bio-based PBS (BioPBSTM). Both plastics have the same properties and processability, the only difference is their source material. BioPBSTM is produced from succinic acid derived from natural resources (sugarcane, cassava and corn) and 1,4-butanediol. BioPBSTM products have a high heat deflection temperature point and high seal strength in heat sealing. The polymer is highly compatible with PLA. Bio-based PBSA con-tains a higher percentage of amorphous structure, is stickier and has higher adhesion strength than stan-dard PBS. Properties of BioPBSTM are similar to those of LDPE. Possible commercial applications for BioPBSTM are paper lamination, multi-layer bags, and films (plastic bags, mulch films) [113].

Qahtani et al. [114] used BioPBS and PLA for making more sustainable 3D-printed PLA/BioPBS blends. It was found that, with the blending ratio 90/10 of PLA and BioPBS, higher tensile and impact strength were achieved than by the neat PLA, while further addition of BioPBS lead to improved tough-ness of the 3D blends and increased the viscosity [114]. Furthermore, BioPBS was combined by varying contents of apple pomace (AP) [115] and grape pom-ace (GP) [116], byproducts from wine and apple juice industries, and biocomposites were fabricated by melt

extrusion-injection. Both byproducts can be utilized as fillers and used to decrease the amount of polymer in a bio-based composite blend. The results indicated that high levels of GP and AP can be successfully incorporated into a polymer matrix and improve its mechanical and thermo-mechanical properties. So, prepared BioPBS-based composites could be used for the production of disposable or single-use and sustainable biodegradable applications [115,116]. Platnieks et al. [117] prepared BioPBS/micro cellu-lose (MCC) woody-like composites, by using a melt blending of 70 wt.% of MCC processed from bleached softwood. Besides, in order to enhance dispersion and compatibility, MCC was modified by using carbo-diimide (CDI), polyhydroxy amides (PHA), alkyl ester (EST), (3-aminopropyl) trimethoxysilane (APTMS), maleic acid anhydride (MAH), and polymeric diphe-nylmethane diisocyanate (PMDI). The chemically modified MCC composites lead to very enhanced mechanical, thermo-mechanical and thermal proper-ties of the PBS/MCC composites. This composite material can be suitable for various construction applications, including profiles, decks and housing appliances [117]. Unlike the others, Cooper et al. [118] created highly porous electrospun fibers, where BioPBS were dissolved in three different solutions: chloroform, chloroform/N,N-dimethylformamide (DMF), or chloroform/ /dimethyl sulfoxide (DMSO). Prepared

Table 8. Research PBS as biodegradable polymers, addition of copolymers, additives/fibers and their potential applications



PBS Industrial PBS Decayed wood CaCO3

AHP, APP and CaHP Different fields [109]

PBS Industrial PBS Cellulose triacetate – Different fields [110]

PBS Synthesized PBS with the content of butyral group

Poly (vinyl butyral) (PVB)

Tetrabutyl titanate Biochemical engineering [111]

PBS ionomer Synthesized PBS ionomer by condensation polymerization of

succinic acid, 1,4-butanediol and DHPPO-K

– – Package material, such as film, foams

[112]

BioPBS BioPBSTM PLA – Different industries and applications

[114]

BioPBS BioPBSTM – Apple pomace Packaging [115]

BioPBS BioPBSTM

– Grape pomace Food packaging [116]

BioPBS BioPBSTM

(FZ71PB) Cellulose CDI, PHA, EST, APTMS,

MAH, PMDI Construction applications [117]

BioPBS BioPBSTM

(FZ91PM) – CHCl3

DMSO DMF

Biomedical or tissue engineering application

[118]

PBSA BioPBS™ (FD92PM)

Collagen – Agriculture applications [119]


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in this way, the electrospun BioPBS fibers have high porosity at the micro- and nanoscale and conse-quently have suitable mechanical properties for pot-ential in wound healing and soft tissue engineering [118]. In the work of Seggiani et al. [119], bio-based PBSA and raw hydrolysed collagen (HC), by-products of the tannery industry, were used to produce ther-moplastic blends. Different PBSA/HC blends with 5-20 wt.% of HC were processed by extrusion, film blowing and injection molding. The PBSA/HC-based blends with higher content of HC (10-20 wt.%) showed good mechanical properties and present promising candidates to produce biodegradable products for agricultural applications [119].

Poly(butylene-adipate-co-terephthalate) (PBAT)

PBAT is an aliphatic aromatic copolyester that can be fully degraded in a few weeks [120]. It has good mechanical properties, such as adequate tough-ness and tear resistance, but it has a low barrier char-acter to water vapor, oxygen and carbon dioxide [120].

PBAT can be used to produce fibers, textile, films and packaging materials. It has also been used for the preparation of antimicrobial films that could be used in food packaging to inhibit bacterial growth, helping to preserve food products safely [121]. In such films, PBAT serves as the bulk of the film with the antimicrobial agent being incorporated during pro-cessing. Sangroniz et al. [120] studied PBAT/PHAE blends for packaging applications. It was found that immiscible blends were obtained with very good com-patibility. The mechanical performance and the per-meability to water vapor, limonene and carbon dioxide were measured to assess the suitability of these blends for packaging applications [120] (Table 9). Tavares et al. [122] studied the PBAT-kraft lignin (KL) blend films, which were obtained by extrusion. Multi-layer composites were obtained by lamination, in which PBAT-KL blend films were bonded to a poly-ethylene layer using polyurethane adhesive [122]. PBAT-KL could be a synergetic solution for food packaging that can integrate the reuse of industrial waste, mechanical properties, flexibility, peeling res-istance, and the well-known biodegradability of PBAT [122].

In the work of Wei et al. [123], the non-leaching antimicrobial biodegradable PBAT was prepared

through reactive extrusion with PBAT and the antimic-robial thermoplastic starch (ATPS) in the presence of the coupling agent, 2,2′-(1,3-phenylene)-bis(2-oxazo-line) (PBO). The antimicrobial PBAT films with excel-lent and rapid antimicrobial activity were obtained by using a blown film extrusion system. PBAT films have a great potential to be used in the prevention of dis-ease infections in hospitals and in fields requiring rapid and highly efficient antimicrobial activity [123].

BIODEGRADABLE ALTERNATIVES TO CONVENTIONAL PLASTICS

Many bioplastics have mechanical properties equivalent to those of their conventional counterparts (e.g., PP, PS and PE) and can be processed using technologies widely used in the polymer industry (e.g., compounding, film processing and molding) [10]. Their use has been found in many short service life applications where biodegradability is a key adv-antageous feature, including consumer packaging (e.g., trays, pots, films and bottles in food packaging), convenience food disposables (e.g., cutlery/table-ware), bags (shopping, garden or domestic waste), agriculture mulch films, personal care disposals (e.g., nappies) and even golf tees [10]. Bioplastic polymers have also been used in more durable applications such as in textiles, consumer goods, automotive parts, and building and construction where the focus is on the use of renewable (bio)resources and any inherent biodegradability properties need to be sup-pressed or controlled by careful design [124].

PLA is eco-friendly, it is 100% bio-based and biodegradable, but only under certain conditions, and it is industrially compostable [125]. PLA is also a bio-compatible material and should not produce carcino-genic or toxic effects in local tissue treatment. Com-pared to the other biopolymers such as PCL, it has a better thermal processibility [125]. PLA is also relat-ively hydrophobic and has a slow degradation rate. Table 10 shows that PLA has similar mechanical pro-perties to PET polymer. Elastic modules and tensile strength are comparable to PET [125].

High molecular weight PCL polymer has mech-anical properties similar to PE (HDPE and LDPE), processing a tensile strength of 12-30 MPa and a break extension of 400-900% [3,105]. Disadvantages

Table 9. Research of PBAT as biodegradable polymers, addition of copolymers, additives/fibers and their potential applications

Biodegradable polymer Material Co-polymer Additives/fibers Application Ref.

PBAT Industrial PBAT PHAE – Packaging applications [120]

PBAT Industrial PBAT Kraft lignin – Food packaging [122]

PBAT Industrial PBAT ATPS, PHGH PBO Prevention of disease infection in the hospital [123]


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of PCL are very low glass transition temperature (-60 °C) and low melting point (60-65 °C), which prohibits its application at elevated temperatures [3,105]. Therefore, PCL is often blended with other polymers, such as PP, PC, polyethylene oxide (PEO) and starch, to produce composites with desired properties [89].

Due to the plastic-like properties and biodegrad-ability of PHAs, these polymers present a potential replacement for non-degradable PE, PP [126]. PHB homopolymers possess several physical properties, e.g., glass transition temperature (15 °C), crystallinity (80%), and tensile strength (40 MPa), similar to those of PP. However, PHB is significantly more brittle than PP (strain at break 6 vs. 400%) [123]. PBS is bio-degradable and currently mostly fossil-based but could in theory be 100% bio-based. Its mechanical pro-perties resemble those of PE and PP, while its melt-ing point is between 90-120 °C, and glass transition temperature around -45 to -10 °C [3,127,128]. PBS has good processability, better than that of PLA [105]. Furthermore, the properties of BioPBSTM are similar to those of LDPE [113]. PBAT is also currently mostly fossil-based. Its commercial name is Ecoflex®, pre-pared by BASF (Germany). Ecoflex® F Blend C1200 has properties similar to PE-LD because of its high molecular weight and its long-chain branched mole-cular structure [105].

Cellulose and starch are alternatives for PP and PS. Cellulose acetate (CA), which is the most import-ant cellulose derivative, has tensile strength compar-able to PS [129]. Starch blends are completely bio-degradable and 25-100% bio-based, when starch is added to one or several biodegradable polymers [129].

CONCLUSION

More than its origin, the chemical structure of the biopolymer is the key factor that determines its biodegradability. Indeed, the future outlook for dev-elopment in the field of biopolymer materials is pro-mising; the future of each polymer is dependent on its competitiveness but also society’s ability to pay for it. Biodegradable plastics made from renewable res-ources can retain all the benefits of petroleum-based plastic without the negative environmental impact. The discovery and implementation of plastic made from natural resources is an enormous leap into the future for different branches of industry. In order to successfully replace biodegradable polymers with current plastics, cooperation is needed not only with industry and academics, but also with various dis-ciplines such as chemistry, engineering, material sci-ence, biogeochemistry and climate science. This will require plenty of time and a key multidisciplinary dev-elopment will be needed.

Acknowledgments

The authors would like to acknowledge the Slovenian Research Agency (ARRS) for financing the research within Programme P2-0046.

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Table 10. Comparison of the properties of biodegradable and synthetic polymers

Polymer Glass transition (°C)

Tg Melting temperature (°C)

Tm Tensile strength

MPa Strain at break

% Flexural modulus

MPa

PHB [105] 15 175 40 6 3500-4000

PLA [89,125] 55 165 59 7 5000

BioPBS [113] -22 115 30 170 630

PBS [105,109] -32 114 57 700 656

LDPE [105] -120 110 35 176 400

HDPE [105] -120 129 29 1070 650

PP [105] 5 163 44 1370 800

PBAT (Ecoflex) [105] - 110-115 32-36 580-800 -

PVC [130] 82 100-260 20-76 150-400 1300-4690

PET [131,132] 73,5 250 80 36 2410

PCL [3,105] -60 60-65 23 400-900 -

PS [133] 100 240 30-60 1.5-2.3 3200


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MAJA ČOLNIK MAŠA KNEZ HRNČIČ

MOJCA ŠKERGET ŽELJKO KNEZ

Laboratory for Separation Processes and Product Design, Faculty of

Chemistry and Chemical Engineering, University of Maribor, Maribor, Slovenia

PREGLEDNI RAD

BIORAZGRADIVI POLIMERI, TRENUTNI TRENDOVI ISTRAŽIVANJA I NJIHOVA PRIMENA: PREGLED

Biorazgradivi polimeri su se brzo razvijali poslednjih godina i danas se široko koriste u farmaciji, kliničkoj biomedicini, kozmetici, medicini, industriji pakovanja, inženjerstvu tkiva, poljoprivredi i drugim oblastima. Interes za biorazgradivim polimerima raste, uglavnom zbog porasta cena nafte, koja je osnovna sirovina za dobijanje plastike na bazi nafte, kao i zbog problema uklanjanja otpadne plastike koja se akumulira u životnoj sredini. Biorazgradivi polimeri imaju mnogo prednosti u odnosu na sintetičks polimere i mogu se razgraditi u životnoj sredini do bezopasnih materija. Biorazgradivi polimeri se klasifikuju u dve klase na osnovu njihove sinteze, tj. sintetičke i prirodne polimere. Dobi-jaju se iz naftnih ili bioloških resursa. Ovaj pregledni rad predstavlja pregled različitih biorazgradivih polimera i njihovih svojstava, trenutna naučna istraživanja, primene, globalnu proizvodnju bioplastike i zamenu konvencionalne plastike.

Ključne reči: biorazgradivi, polimeri, prirodno poreklo, bioplastika.

maja Čolnik biodegradable polymers, current maŠa …

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