Polish J. of Environ. Stud. Vol. 19, No. 2 (2010), 255-266

Review and Biodegradable – a Review

Katarzyna Leja*, Grażyna Lewandowicz

Department of Biotechnology and Food Microbiology, Poznań University of Life Sciences, Wojska Polskiego 48, 60-627 Poznań, Poland

Received: 3 June 2009 Accepted: 2 November 2009

Abstract

Synthetic polymers are important in many branches of industry, for example in the packaging industry. However, they have an undesirable influence on the environment and cause problems with waste deposition and utilization. Thus, there is a tendency to substitute such polymers with polymers that undergo biodegrad- able processes. Increasing interest in applying polymers based on natural materials such as has been observed. This review describes biodegradation processes of xenobiotics such as aromatic compounds, (PVA, , , and nylon), and polymer blends (Starch/Polyethylene, Starch/, and Starch/PVA). Moreover, this review includes information about biodegradable polymers such as mixtures of synthetic polymers and substances that are easy digestible by (chemically modified starch, starch-polymer composites, thermoplastic starch, and biodegradable packing materials), synthetic materials with groups susceptible to hydrolytic microbial attack (), and biopolyesters (poly-β-hydrox- yalkanoates). Production of this kind of material and introducing it to the market is important for the natural environmental. It may result in decreasing the volume of waste dumps.

Keywords: biodegradation, biodegradable polymers, xenobiotics

Introduction pollution of groundwater and surface . Synthetic poly- mers are recognized as major solid waste environmental Developments in science and technology, especially pollutants. Another problem is disposal of agricultural plas- over the last two decades, have increased the amount of tic wastes. Since 1990, the industry has invested $1 synthetic polymers produced worldwide each year. Each billion to support increased recycling, and to educate com- year approximately 140 million tonnes of synthetic poly- munities. Landfilling is the most common method for dis- mers are produced [1]. In the United States, synthetic poly- posing of municipal solid waste. Many synthetic polymers, mers are estimated to be approximately 20% of the volume resistant to chemical and physical degradation, are pro- of municipal solid waste. A similar situation has been duced and utilized. They present disposal problems when observed in Germany. In Australia, most household waste their usefulness ceases. Especially for agricultural plastic ends up in municipal sites, estimated to be 25% of wastes, an alternative method of disposal is biodegradation. total waste by weight [2]. The presence of these substances Biodegradation concerns specially designed so-called in the environment brings about important problems, biodegradable polymers [3]. Increasing amounts of syn- including a challenge to wastewater treatment plants and thetic polymers produced results in increasing interest in polymer biodegradation. The recent incorporation of bio- *e-mail: [email protected] logical waste treatment (i.e., composting and biogasifica- 256 Leja K., Lewandowicz G. tion) in an integrated approach to solid waste management most important factors affecting growth. has resulted in a growing commercial interest in the devel- Also important are sources of carbon and nitrogen, and pH. opment of biodegradable materials for consumer products Fungi active in the biodegradation process are [4, 5]. On the market are a number of materials known as Sporotrichum, Talaromyces, Phanerochaete, Ganoderma, biodegradable plastics (i.e., starch-based materials, cellu- Thermoascus, Thielavia, Paecilomyces, Thermomyces, lose-derived polymers, bacterial polyesters and a range of Geotrichum, Cladosporium, Phlebia, Trametes, Candida, synthetic polymers). The main problem associated with Penicillium, Chaetomium, and Aerobasidium [8, 10-12]. designing biodegradable polymers is the optimization of The biodegradation process can be divided into (1) aer- their chemical, physical and/or mechanical properties, as obic and (2) anaerobic degradation (Fig. 1). well as their biodegradability [5, 6]. Aerobic biodegradation:

Polymer + O2 -> CO2 + H2O + + residue(s) (1) Biodegradation Anaerobic biodegradation:

There is a world-wide research effort to develop Polymer -> CO2 + CH4 + H2O + biomass+ residue(s) (2) biodegradable polymers as a waste management option for polymers in the environment. Biodegradation (i.e. biotic If is present, aerobic biodegradation occurs and degradation) is a chemical degradation of materials (i.e. carbon dioxide is produced. If there is no oxygen, an anaer- polymers) provoked by the action of microorganisms such obic degradation occurs and is produced instead of as bacteria, fungi and algae. The most common definition carbon dioxide [3, 13, 14]. When conversion of biodegrad- of a biodegradable polymer is “a degradable polymer able materials or biomass to gases (like carbon dioxide, wherein the primary degradation mechanism is through the methane, and nitrogen compounds), water, salts, action of metabolism by microorganisms.” Biodegradation and residual biomass occurs, this process is called - is considered a type of degradation involving biological ization. Mineralization is complete when all the biodegrad- activity. Biodegradation is expected to be the major mech- able materials or biomass are consumed and all the carbon anism of loss for most chemicals released into the environ- is converted to carbon dioxide [3]. ment. This process refers to the degradation and assimila- Biodegradable materials have the proven capability to tion of polymers by living microorganisms to produce decompose in the most common environment where the degradation products. The most important organisms in material is disposed, within one year, through natural bio- biodegradation are fungi, bacteria and algae [7]. Natural logical processes into non-toxic carbonaceous soil, water or polymers (i.e., , , nucleic acids) are carbon dioxide [5]. The chemical structure (responsible for degraded in biological systems by oxidation and functional group stability, reactivity, hydrophylicity and [3]. Biodegradable materials degrade into biomass, carbon swelling behaviour) is the most important factor affecting dioxide and methane. In the case of synthetic polymers, the biodegradability of polymeric materials. Other impor- microbial utilization of its carbon backbone as a carbon tant factors are inter alia, physical and physico-mechanical source is required [2]. properties, e.g., molecular weight, porosity, elasticity and Bacteria important in the biodegradation process include, morphology (crystalline, amorphous) [15, 16]. inter alia, Bacillus (capable of producing thick-walled endospores that are resistant to heat, radiation and chemical disinfection), Pseudomonas, Klebsiella, Actinomycetes, Xenobiotics Biodegradation Nocardia, Streptomyces, Thermoactinomycetes, Micromonospora, Mycobacterium, Rhodococcus, Xenobiotics are man-made compounds, frequently Flavobacterium, Comamonas, Escherichia, Azotobacter halogenated hydrocarbons, that are notoriously difficult for and Alcaligenes (some of them can accumulate polymer up microbes to breakdown in the environment. Biodegradation to 90% of their dry mass) [2, 7-9]. Temperature is one of the of synthetic materials is complicated. Saturated alkanes are more susceptible to aerobic bacterial attack than unsaturat- ed aliphatic hydrocarbons. Moreover, alkanes with a long carbon chains and straight structures are more prone to aer- obic biodegradation. The most common aerobic pathway for alkane degradation is oxidation of the terminal methyl group into a carboxylic acid through an intermedi- ate, and eventually complete mineralization through β-oxi- dation [5, 17-19]. Over the last two decades the increasing attention of xenobiotic biodegradation under anaerobic conditions such as in groundwater, sediment, landfill, sludge digesters and bioreactors, has been observed. Generally, anaerobic bacte- Fig. 1. Schema of under aerobic and ria able to degrade xenobiotics are present in various anaer- anaerobic conditions [87]. obic habitats, inter alia sediments, waterladen soils, reticu- Polymer Biodegradation and Biodegradable Polymers... 257 lo-ruminal contents, gastrointestinal contents, sludge Numerous studies have documented that aromatic com- digesters, feedlot wastes, groundwater, and landfill sites pounds can be degraded under nitrate-reducing, iron-reduc- [20]. Anaerobes use natural organics such as proteins, car- ing, sulfate-reducing, and methanogenic conditions. bohydrates, and many others as carbon and energy sources. Although biodegradation of aromatic compounds has been D. oleovorans, G. metallireducens, D. acetonicum, shown to occur under anaerobic conditions, information on Acidovorax, Bordetella, Pseudomonas, Sphingomonas, the extent of this activity is limited [22]. Variovorax, Veillonella alkalescens, Desulfovibrio spp., Aerobic degradation of aromatic compounds involves Desulfuromonas michiganensis, and Desulfitobacterium their oxidation by molecular oxygen. Oxidation leads to the halogenans are the major groups of anaerobic microorgan- production of intermediates that enter central metabolic isms involved in biodegradation of xenobiotic compounds. pathways including the Krebs Cycle and β-oxidation [23- Pure cultures of this above-mentioned bacteria must be iso- 25]. Microorganisms use oxygen (during aerobic respira- lated under strict anaerobic conditions (sulfate-reducing tion) to hydroxylate the benzene ring (Fig. 2), resulting in and methanogenic) [19]. subsequent fission of the ring. Mono- and di-oxygenase incorporate one or two atoms of oxygen, respec- Biodegradation of Aromatic Compounds tively, into the ring [26]. The major reactions (catalyzed by di-oxygenases) is the cleavage of the aromatic double bond Aromatic compounds are ubiquitous in nature. located between two hydroxylated carbon atoms (ortho Although there is some debate as to their origin in the pathway), or adjacent to a hydroxylated carbon atom (meta environment, it is generally accepted that most are not of pathway), or in an indole ring [27]. biosynthetic origin but are derived from the pyrolysis of In the benzene aerobic biodegradation process, three organic compounds. Indeed, next to glucosyl residues, the intermediates are present: catechol, protocatechuate, and benzene ring is the most widely distributed unit of chem- gentisic acid. These compounds are broken down by simi- ical structure in nature. Benzene, ethylbenzene, toluene, lar pathways to simple acids and aldehydes that are readily styrene, and the xylenes are among the 50 largest-volume used for cell synthesis and energy [28]. Polycyclic aromat- industrial chemicals produced, with production figures of ic compounds (e.g. toluene, xylenes, naphthalene and eth- the order of millions of tons per year. These mentioned ylbenzene) are degraded by similar mechanisms as ben- compounds are widely used as fuels and industrial sol- zene. Di-oxygenaze initiates an attack and it produces a vents. Moreover, they and polynuclear aromatic com- dihydrodiol. Then, the dihydrodiol is converted to a cate- pounds provide the starting materials for the production of chol-like compound (Fig. 3) that can undergo ortho or meta pharmaceuticals, polymers, agrochemicals, and more ring fission. di-oxygenaze is indispensable to aer- [21]. obic naphthalene transformation [25, 29, 30].

Fig. 2. Aerobic benzen biodegradation [25].

Fig. 3. Aerobic naphthalene biodegradation [25]. 258 Leja K., Lewandowicz G.

Some non-oxygenated aromatics, such as toluene, ben- that they do not readily break down in the environment and zene, ethylbenzen, and xylenes, can be transformed under therefore can litter the natural environment [32]. Using anaerobic conditions by denitrifying bacteria, in the pres- biodegradable and compostable plastics can reduce the ence of alternative electron acceptors. In the absence of amount of plastics in . The use of biodegradable oxygen, substituted aromatic compounds are more easily polymers is increasing at a rate of 30% per year in some degraded than non-substituted compounds. The addition of markets worldwide. Composting is a promising waste man- a substituent group onto the benzene ring makes possible an agement option for degradable plastics because this process alternative degradation method, including side-chain group is designed to degrade wastes [33, 34]. attack (Fig. 4) [21, 25]. The first documented report for Several organizations are involved in setting standards benzene and toluene degradation under anaerobic condi- for biodegradable and compostable plastics: the U.S. tions appeared in 1980, concerning studies of biodegrada- Composting Council (USCC), the American Certification tion in microcosms containing samples from the site of the System of Biodegradable Products Institute (BPI), the Amoco Cadiz oil spill. In 1984 and 1985, field evidence for Environment & Plastics Industry Council, the American anaerobic degradation was reported. Laboratory studies Society for Testing and Materials (ASTM), the European demonstrated that many different electron acceptors could Committee for Standardization (CEN), Japan’s GreenPla replace oxygen in anaerobic degradation. Certain aromatic program, and the British Plastics Federation. The restric- compounds have been shown to be degraded under denitri- tions from these organizations help create biodegradable fying conditions, methanogenic conditions in microcosms, and compostable products that meet the increasing world- and by iron-reducing organisms. However, the existence of wide demand for more environmentally friendly plastics microbial communities capable of degrading aromatic [34]. materials with sulfate as the electron acceptor has not yet Biodegradable polymers decompos into carbon diox- been proven. Some scientists have suggested that sulfate- ide, methane, water, inorganic compounds or biomass by reducing bacteria were involved in the biodegradation, the actions of microorganisms [34]. Biodegradable poly- although definitive proof to this effect has not been pre- mers used for producing biodegradable agricultural films sented [31]. include inter alia, copolyesters, poly(vinyl)alcohol, poly(vinyl)chloride, acylated starch-plastic, modified Biodegradation of Plastics starch and vegetable oil [3].

Increasing interest in plastic biodegradation is observed PVA Biodegradation because of environmental pollution. Plastics are composed mainly of carbon, hydrogen, nitrogen, oxygen, chlorine and The chemical structure of poly(vinyl) alcohol (PVA) is bromine, and are used in automobile production, space composed mainly of head-to-tail 1,3-diol units. However, exploration, irrigation, agriculture, health and other indus- only 1-2% of head-to-head 1,2-diol units exist in PVA. The tries [11, 32]. Generally, 2-3 million tons of plastics are 1,2-diol content influences some properties of PVA, inter used each year in agricultural applications. The Statewide alia its biodegradability. Factors affecting PVA biodegrad- Waste Characterization Study reported that approximately ability include the degree of polymerization (DP), the 350,000 tonns of rigid plastic packaging containers were degree of saponification (hydrolysis; DS), tacticity of the disposed of in California during 2003, which represents main chain, ethylene content, and 1,2-glycol content. In approximately 1% by weight of the overall waste stream. general, DP and DS did not have a significant influence on Plastic trash bags comprised 1% and plastic film, compris- the biodegradation of PVA [35]. ing 2.3% of the waste stream. The commercial sector gen- Poly(vinyl)alcohol historically has been produced erated about 50% of the waste, the residential sector 30% of industrially by the hydrolysis of poly(vinylacetate). As the the waste, and the self-hauled sector 20%. In 2003, plastics vinyl alcohol it cannot exist due to its tautomer- contributed to 12% by weight of the waste stream for com- ization into acetoaldehyde. PVA was discovered in 1924, mercial waste, 9.5% of the waste from residential waste, when a PVA solution was obtained by saponifying and 3.9% of the waste stream in self-hauled waste. Plastics poly(vinyl ) with caustic soda solution. PVA has such are inert, that is resistant to biodegradation. But generally, physical properties as viscosity, film forming, emulsifying, the main environmental disadvantage of plastic materials is dispersing power, adhesive strength, tensile strength, and

Fig. 4. Aerobic toluene biodegradation: side group attack [25]. Polymer Biodegradation and Biodegradable Polymers... 259 flexibility. PVA is resistant to water, oil, grease, and solvent. Poly(ε-Caprolactone) It was widely used, especially in fabric and paper sizing, fiber coating, adhesives, emulsion polymerization, films for PCL is a synthetic linear polyester with almost 50% packing and farming, and the production of crystallinity. It is biologically degradable and consists of 6- poly(vinylbutyral). The production of PVA peaked as the hydroxyhexanoates. The environmental degradation of volume of synthetic, water-soluble polymers in the world PCL is affected by the actions of bacteria that are widely amounted to about 1,250 kt in 2007. Asia (especially China distributed in the ecosystem [38]. Recent studies have and Japan), Western Europe, and the USA aspire to be the shown that some filamentous fungi and yeasts also can greatest producers and consumers of poly(vinyl)alcohol. hydrolyze PCL to water-soluble products. Scientists have About 76% of worldwide PVA production is in Asia [35]. reported that Pullularia pullulans can efficiently degrade a Poly(vinyl)alcohol is a vinyl polymer in which the main lower molecular weight PCL film (Mw=1,250), but the chains are joined by only carbon-carbon links. The history extent of decomposition of a PCL film with Mw above of PVA biodegradation extends back more than 70 years, 15,000 by the fungus was negligible. PCL with higher Mws since the first report of degradation by Fusarium lini B [35, (25,000 and 35,000) is degraded by Penicillium sp. strain 36]. Recently, it was reported that 55 species of microor- and a yeast. However, increasing the Mw of the polyester ganisms (including bacteria, fungi, yeast and mould) par- reduced its degradability. One of the most important factors ticipate in degradation of poly(vinyl)alcohol. Scientists influencing PCL biodegradation are the PCL degrading have isolated the Pseudomonas bacteria from soil bacteri- microorganisms that produce different types of PCL hydro- um growing on PVA as the source of carbon. Pseudomonas lases, even though the biodegradability of a polymer is is the main PVA degrader. This bacterium produces and related to its Mw. PCL could be hydrolyze by esterases and secretes an enzyme that degrades PVA. This enzyme was lipases [39]. Synthetic polyesters such as PCL, isolated, purified and characterized. Polivinyl alcohol dehy- poly(ethylene dicarboxylic acids), and poly(butylene dicar- drogenase (PVADH) from Pseudomonas ssp. 113P3 cat- boxylic acids) could be degraded by lipases of R. delemar alyzed an oxidation reaction of PVA to produce beta-dike- and Rhizopus arrhizus. During the degradation process in a tone structure on PVA. The degradation mechanism includ- biotic environment, the amorphous fraction of PCL is ed two steps: the conversion of the 1,3-glycol structure of degraded before the degradation of the crystalline fraction. two successive repeating units to a β-diketone by a random Ester-lingages are easy to hydrolyze [2]. PCL is not sus- oxidative dehydrogenation reaction or oxidation of one ceptible to bacterial and fungal PHB depolymerases [36, hydroxyl group yielding monoketone structures. This 39-41]. Some of the fungal phytopathogens (Fusarium process was catalyzed by an extracellular secondary alcohol spp.) produce cutinase that degrade a cutin polyester (the oxidase enzyme. And the second step was a reaction that structural component of a plant cuticle) [36]. broke the carbon-carbon bond and converted one of the ketone groups to a carboxylic group. This results in chain Poly(L-Lactide) scission (Fig. 5) [3]. But according to the products produced by the first step of PVA degradation, there are two possible PLA is a biocompatible thermoplastic with a melting pathways for this second step: either hydrolysis of β-dike- temperature of 175ºC and a glass transition temperature of tone structures of oxidized PVA (oxiPVA) by a β-diketone 60ºC. It is synthesized by the polymerization of L-lactic hydrolase (oxiPVA hydrolase) or the aldolase reaction acid. PLA can be hydrolyzed by the lipase from R. delemar involving the monoketone structures of oxiPVA [35]. and the proteinase K from Tritirachium album and also by the polyester depolymerase from Polyesters Comamonas acidovorans [42]. PLA is more resistant to microbial attack in the environment than other microbial In polyesters, are bonded by ester linkages. and synthetic polyesters [36, 43, 44]. Enzymes that degrade this polymer are ubiquitous in living organisms (e.g., Thermomonosfora Fusca and Aliphatic Polyalkylene Dicarboxylic Acids Streptomyces albus) [1, 37], so it may be degraded in sev- eral biotic environments. The most important factors affect- A variety of aliphatic polyesters such as PEA, PES, ing biodegradability are molecular mass and crystallinity adipate (PPA), PBA, PBS, and polyhexylene [38]. carbonate have been widely produced commercially.

Fig. 5. Degradation mechanism of polyvinyl alcohol: (a) enzyme-catalyzed oxidation of the 1,3-glycol structure to a b-diketone; (b) chain scission of the b-diketone. 260 Leja K., Lewandowicz G.

Biodegradation of these polymers is measured by microbial has been given to its biodegradation. The very poor growth on an agar plate containing the respective emulsified biodegradability of nylon in comparison with aliphatic polymer substrate [45, 36], or by enzymatic degradation tests polyester is assumed to be due to its strong intermolecular using fungal hydrolases (e.g., lipase, PEA depolymerase, and cohesive force caused by hydrogen bonds between molec- PHB depolymerase). These polyesters can also be hydrolyzed ular chains of nylon [48]. by several filamentous fungi such as Aspergillus flavus, A. Nylon is a synthetic polyamide with repeating niger, A. versicolor, Aureobasidium pullulans, P. funiculosum, groups (-CONH-) in its backbone. Some forms of nylons and Chaetomium globosum. However, increasing the Mw and have been shown to biodegrade by fungi and bacteria. hydrocarbon content in the polymers consisting of dicar- Bacterium Geobacillus thermocatenulatus is used to biode- boxylic acid units reduces their biodegradability [36]. grade nylon 12 and nylon 66. Some scientists have shown that bacterial degradation of nylon 12 is associated with the Polyethylene (PE) enzymatic hydrolysis of amine bonds, which is accompa- nied by the formation of 12-amino dodecanoic acid. Also, Polyethylene is widely used for various one-trip appli- they have proposed that further oxidation would result in cations like food packaging, retail industry uses and agri- the degradation of 12-1mino dodecanoic acid into carboxyl cultural uses. These applications lead to a large quantity of and other degradation products (Fig. 6) [11, 49]. plastic waste, causing serious environmental problems [46]. PE is the most problematic plastic that is resistant to Biodegradation of Polymer Blends microbial attack. Polyethylene subjected to 26 days of arti- ficial UV irradiation before being buried in soil evolved The degradation of the more readily biodegradable less than 0.5% carbon by weight after 10 years. Without component controls the rate of degradation of polymer prior irradiation, less than 0.2% carbon dioxide was pro- blends [2]. duced. Similarly, a polyethylene sheet that had been kept in contact with moist soil for a period of 12 years showed no evidence of biodeterioration. However, some studies demonstrated partial biodegradation of polyethylene after shorter periods of time. It showed that UV photooxidation, thermal oxidation or chemical oxidation with nitric acid of polyethylene prior to its exposure to a biotic environment did enhance biodegradation [47].

Polyethylene is a synthetic polymer with –CH2-CH2 repeating units in the polymer backbone. This polymer is resistant to biodegradation, which results from highly stable C-C and C-H covalent bonds and high molecular weight. The mechanism of biodegradability of polyethylene includes alteration by adding a carbonyl group (C=O) in the polymer backbone. The altered polyethylene molecule undergoes biotic oxidation. In the process of biodegradation, PE mole- cules containing carbonyl groups first get converted to alco- hol by the monooxygenase enzyme. After that, alcohol is oxi- dized to aldehyde by the alcohol dehydrogenase enzyme. Next, aldehyde dehydrogenase converts aldehyde to the fatty acid. This fatty acid undergoes β-oxidation inside cells [11]. Biotic factors that may bring about polyethylene biodegrada- tion included inter alia bacteria, fungi, biosurfactants pro- duced by microbes to attach on PE surface, biofilm growth on PE surface, uptake of shorter chain PE polymers mem- branes, and assimilation of such short chains via β-oxidation pathway inside cells using intracellular enzymes. And abiot- ic factors are: sunlight and photooxidation, the addition of carbonyl radicals into -CH2-CH2- backbone due to photoox- idation and propagation of Norrish type I and II degradation, diffusion of O2 into PE crystals, and PE chain scission [11].

Nylon Fig. 6. Proposed mechanism of nylon biodegradation by fungus peroxidase. Methylene group adjacent to the nitrogen atom in Nylons are produced in large quantities as fibers and the nylon polymer backbone is first oxidized by the enzyme plastics all over the world. Nylon is one of the most impor- fungus peroxidase. The -CH- radical undergoes a stepwise oxi- tant synthetic polymers. However, nowadays little attention dation process releasing various degradation products [11]. Polymer Biodegradation and Biodegradable Polymers... 261

Synthetic materials (e.g., polyolefins) are not degraded microorganisms and their populations are the main factors by microorganisms in the environment, which contribute influencing the degree of degradation. Enzymes capable of to their long life. Therefore, recycling and degradation of cleaving the ester bonds of poly-(hydroxyalkanoate) are plastics is an important issue for environmental and eco- poly-(hydroxybutyrate) depolymerases and lipases [2, 58, nomic reasons. Blending has become an economical and 59]. It is well-known that the addition of starch filler versatile route to obtain polymers with a wide range of improves the rate of degradation of polyesters [60]. desirable properties. Partially biodegradable polymers Mani and Bhattacharya have reported that biodegrad- obtained by blending biodegradable and non-biodegrad- able blends of starch and aliphatic polyester give excellent able commercial polymers can effectively reduce the vol- properties when small amounts of compatibilizers (anhy- ume of plastic waste by partial degradation. They are more dride functionalized polyesters) are added. The tensile useful than completely biodegradable polymers due to the strengths are comparable to that of synthetic polyester, even economic advantages and superior properties imparted by at a starch level of 70% by weight. The elongation is dras- the commercial polymer used as a blending component tically reduced as the percentage of starch is increased [50]. (2001) [61]. Starch, an omnipresent bio-material, is one of the most abundant and inexpensive sources Starch/PVA Blends which have the unique characteristic of biodegradability and easily dissolve in water. Blends of synthetic poly- PVA is compatible with starch and blends are expected mers and have been extensively studied since to have good film properties. If both components are these blends can be prepared so they are biodegradable. biodegradable in various microbial environments, PVA and Biodegradable polymer films are made inter alia from starch blends are biodegradable. The hydrophilic nature of low density polyethylene (LDPE), rice starch and potato PVA enhances compatibility with starch, making it suitable starch [51]. for the preparation of polymer blends. The use of PVA with starch improves the mechanical properties of the blends Starch/Polyethylene Blends [62]. Starch/polyvinyl alcohol blends are one of the most popular biodegradable plastics, and are widely used in With the development of the plastics industry, packaging and agricultural applications [51]. Although the starch/polyethylene plastics with good degradability and processing and mechanical properties of starch/PVA blends compatibility are increasingly desired. The dry native have been investigated extensively, there is only a limited starch was directly mixed in the polyethylene matrix with number of publications about it [2, 63]. blends used for many products [52]. Starch is a highly hydrophilic macromolecule. It is often used as the degradable additive in the preparation of Biodegradable Polymers biodegradable polyethylene film. Polyethylene is resistant to microbial breakdown. The possibility of biodegradation In Poland, synthetic materials constitute 3 to 10% of the is connected with molecular size (the higher the molecular mass of waste materials and every year it rises 3%. It results size, the smaller the possibility of biodegradation). The in increasing the surface of waste dumps. The solution to great difference between starch and polyethylene in their the problem may be biodegradable polymers. properties results in poor compatibility of starch/ Biodegradable materials may be divided into three groups: polyethylene blends [52]. the mixtures of synthetic polymers and substances which The biodegradation of polyethylene is affected by pre- are easy digestible by microorganisms (inter alia, modified liminary irridation in the ultraviolet range, the presence of natural polymers, natural polymers such as starch and cel- photodegradative enhancers, its morphology and surface lulose), synthetic materials with groups that are susceptible area, additives and antioxidants [2, 53, 54]. The degradation to hydrolytic microbial attack (for example, polycaprolac- of the carbon-carbon backbone may be enhanced by the tone) and the biopolyesters from bacterial sources. The addition of readily biodegradable compounds, such as most popular and important biodegradable polymers are starch, to a low-density polyethylene matrix [55]. The rate aliphatic polyesters [e.g., polylactide, poly(ε-caprolactone), of biodegradation of starch-filled polyethylene depends on polyethylene oxide, poly(3-hydroxybutyrate), polyglycolic starch content and is sensitive to environmental conditions acid] and thermoplastic proteins [64]. The major advan- [2, 56]. The most important trigger for the biodegradation tages of biodegradable plastics are that they can be com- of polyethylene is oxidation of impurities such as fats and posted with organic wastes and returned to enrich the soil, oils [55]. The biodegradation rate-limiting component is the their use will not only reduce injuries to wild animals oxygen concentration at the surface of the films [2]. caused by dumping of conventional plastics, but will also lessen the labor cost for the removal of plastic wastes in the Starch/Polyester Blends environment because they are degraded naturally, their decomposition will help increase the longevity and stabili- Blends of starch and polyester are completely ty of landfills by reducing the volume of garbage, they biodegradable when each component in the blend is could be recycled to useful monomers and by biodegradable, as well as compostable [40]. The type of microbial and enzymatic treatment [65]. 262 Leja K., Lewandowicz G.

Mixtures of Synthetic Polymers and Substances Thermoplastic starch is processed like synthetic plastics that are Easy Digestible by Microorganisms through extrusion and injection units. Unfortunately, it is a very hydrophilic product. Some authors tried to modify the Chemically Modified Starch starch structure, e.g. by acetylation, to reduce the hydrophilic character of the chains [73, 74]. Native starch films are hydrophilic and have poor mechanical strength. They cannot be used for packaging Biodegradable Packing Materials applications [66]. The addition of amylopectin to the films A wide range of materials are used for packaging appli- adversely effects their flexibility, tensile strength and elon- cations including metal, glass, wood, paper or pulp-based gation to a breaking point [67]. In the case of altered starch materials, plastics or combination of more than one materi- physical and chemical properties, modifications are used. als as composites. They are applied in three broad cate- The glucopyranose units of starch molecules have free gories of packaging: primary packaging (in contact with the hydroxyl groums at C-2, C-3 and most C-6 carbon atoms. goods and taken home by consumers), secondary packag- Derivatization of the glucosidic hydroxyl group can be car- ing (covers the larger packaging, i.e., boxes, used to carry ried out using mild conditions. The nature of the attached quantities of primary packaged goods), tertiary packaging group and the degree of substitution (number of substitu- (used to assist transport of large quantities of goods, i.e., tions per D-glucopyranosyl structural unit) are the main wooden pallets and plastic wrapping). Secondary and ter- factors that influence the properties of modified starch. tiary packaging materials have less material variation and Commercial starches have a degree of substitution less then they are relatively easier to collect and sort for recycling 0.2, while a fully substituted starch has a degree of substi- purposes. Primary packaging materials are dispersed into tution of 3 [68]. Derivatives can be prepared as starch households and also they are largely mixed, contaminated . This is usually done by an acetylation reaction in and often damaged and thus pose problems in recycling or which active chloride in a pyridine solvent used. The ester reuse of the materials [75]. group acts as an internal plasticizer to increase the fatty acid Over 67 million tonnes of packaging waste is generated chain length, which results in greater internal plasticization annually in the EU, comprising about one-third of all [2]. municipal solid waste. In the UK, 3.2 million tons of house- Starch-Polymer Composites hold waste produced annually are packaging, which equates to over 12% of the total household waste produced. Starch is often used as a biodegradation additive. The In developed countries, food packaging represents 60% of major role of starch has been found to provide higher oxy- all packaging. This is due primarily to strict food packaging gen permeability as it is consumed by microorganisms [69]. regulations and also the drive to enhance appearance and so Starch components are more rapidly degraded by increase sales. In the UK, the proportion of food that is unfit microorganisms than the synthetic polymers in the blends. for consumption before it reaches the consumer is 2%, Biodegradable packaging materials may be divided into two whereas in developing countries, where packaging is not as types. The first type includes starch-filled and the second widespread, this loss can be in excess of 40% [75-78]. type includes starch-based polymers. The starch content of Starch is the most commonly used natural polymer in starch-filled polymers is normally less than 15% by weight. biodegradable packing production (it is abundant, inexpen- In traditional plastics, particularly in poliolefins such as sive, readily available and biodegradable in many environ- starch/high density polyethylene, starch is used as natural ments). Often, blends of starch and polyethylene are used to filler. These films biodeteriorate on exposure to microbial produce biodegradable bags [79]. environments. [2, 54, 70]. Starch-based polymers contain relatively higher percentages of starch (more than 40%), mixed with synthetic components. It makes them attractive The Synthetic Materials with Groups Susceptible for biodegradability and several blends have been prepared to Hydrolytic Microbial Attack and tested for packaging applications. The range of mechan- ical properties of these polymers falls short of the required Polycaprolactone properties, but improvements have been made by blending, co-polymerization, grafting and cross-linking [2, 71]. Polycaprolactone (PCL) is a semi crystalline aliphatic polyester derived from a ring opening polymerization of Thermoplastic Starch caprolactone. It has a relatively low melting point of 60ºC. It is completely biodegradable by microorganisms (includ- Starch is often use to make biodegradable polymers but ing some phytopathogens) in marine, sewage sludge, soil is difficult to process thermoplastically. It results from lim- and compost ecosystem. Many phytopathogens secrete cuti- ited internal mobility of the polyanhydroglucose chain nase (serine hydrolase) that degrades cutin, the structural which prevents thermal transition in the chemical stable polymer of the plant cuticle. Due to its biodegradability and temperature range, so plasticizers are used to enable melt- biocompatibility, PCL has attracted much attention in the ing. Water and glycerol are the most common plasticizers medical field mainly as controlled release drug carriers [80- [2, 72]. 82]. Polymer Biodegradation and Biodegradable Polymers... 263

Polycaprolactone is degraded in at least two stages. The poly(hydroxyalkanoate)s (PHAs) that occur widely in first stage involves non enzymatic hydrolytic ester cleavage, nature. They are produced by a wide variety of micro- autocatalyzed by carbon end groups of the polymer chains. organisms as an internal carbon and energy storage, as part When the molecular weight is reduced to 5,000, the second of their survival mechanism. Up to now, over 90 different stage starts with a slowing down of the rate of chain scission types of PHA consisting of various monomers have been and the beginning of weight loss because of the diffusion of reported and the number is still increasing [86]. Bacterially oligomeric species from the bulk. The polymer becomes synthesized PHAs can be produced from a variety of prone to fragmentation and either enzymatic surface erosion renewable resources, and are truly biodegradable and high- or phagocytosis contributes to the absorption process [41]. ly biocompatible thermoplastic materials. Therefore, PHAs are expected to contribute to the construction of an envi- The Biopolyesters ronmentally sustainable society. Some PHAs behave simi- larly to conventional plastics such as polyethylene and polypropylene, while others are elastomeric. Thus, blend- ing offers much scope for expanding their range of applica- Polyhydroxyalkanoates (PHAs) are a diverse family of tions. The most representative member of this family is intracellular biopolymers synthesized by a wide range of poly(3-hydroxybutyrate) (PHB). Blending of polymers is microorganisms under conditions of nutrient stress. PHAs an effective alternative to acquiring new materials with play an important role in encystment and sporulation [82]. desired properties. PHA occurs in blends with such poly- Polyhydroxyalkanoates (PHAs) are synthesized as cellular mers as hydroxybutyrate (HB), 3-hydroxyvalerate (3HV), and energy storage materials. They are usually accumulat- 3-hydroxypropionate (3HP), 3-hydroxyhexanoate (3HH), ed as a result of a nutrient limitation. PHAs have similar and 4-hydroxybutyrate (4HB) [86]. properties to conventional plastics and are completely biodegradable [83]. PHAs have potential as biodegradable Blends of Poly(d,l)Lactide Family replacements for conventional bulk commodity plastics and therefore promote sustainable development [82]. One of the PLA are produced from renewable resources such as PHAs is (PHB), which is accumulat- starch, are biodegradable and compostable, and they have ed in various bacteria including Alcaligenes eutrophus and very low or no and high mechanical performance, Azotobacter vinelandii [83]. comparable to those of commercial polymers. However, the thermal stability of PLAs is generally not sufficiently high Poly-β-Hydroxyalkanoates enough for them to be used as an alternative in many com- mercial polymer applications. The various PLA blends Biopolyesters are natural macromolecules, from bacter- were studied to improve their thermal properties. A stereo- ial sources. Though they are relatively expensive, an complex is formed from enantiomeric PLAs, poly(L-lactic increasing interest in using them is observed. It results from acid) (PLLA) and poly(D-lactide) (i.e., poly(D-lactic acid) their environmentally friendly nature in terms of manufac- (PDLA)) due to the strong interaction between PLLA and ture, use and disposal. In 1994, 500 tons of biopolyesters PDLA chains. The stereocomplexed PLLA/PDLA blend were produced. The simplest and most common poly-β- has a melting temperature (Tm) (220-230ºC) ca. 50ºC high- hydroxyalkanoate is poly- β-hydroxybutyrate (PHB). PHB er than those of pure PLLA and PDLA (170-180ºC), and is synthesized as a storage material by a large number of can retain a non-zero strength in the temperature range up resting bacteria. The amount of PHB in bacteria normally is to Tm. Moreover, the PLLA/PDLA blend has a higher 1 up to 30% of their dry weight. Under controlled fermenta- hydrolysis resistance compared with that of the pure PLLA tion conditions, some Azotobacter and Alcaligenes ssp. can and PDLA, even when it is amorphous-made, due again to accumulate polymer up to 90% of their dry mass [2, 84]. the strong interaction between PLLA and PDLA chains. PHB has two limitations in its use: a narrow process- PLLA is usually hard and brittle, which hinders its usage in ability window, and a relatively low-impact resistance due medical applications, i.e. orthopedic and dental surgery. to its high degree of crystallinity. These drawbacks have Poly DL-lactic acid (PDLLA) can degrade quickly due to hampered the utilization of PHB as a common plastic. its amorphous structure, thus the degradation time of Blends of PHB with other polymers can offer opportunities PLLA/PDLLA blends can be controlled through various to extend and explore their many useful and interesting blending ratios [86]. properties and to modify its undesirable properties. For example, PHB has been blended with poly(ethylene oxide), Conclusion poly(vinyl butyral), poly(vinyl acetate), poly(vinylphenol), cellulose acetate butyrate, chitin and [85]. This review has covered the major concerns about the natural and synthetic polymers, their types, uses and Aliphatic Polyester Blends degradability. Technical and economical problems connect- Poly(Hydroxyalkanoate) ed with materials recycling has induced scientists to search for new materials that may be organically recycled. 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