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Int J Mol Sci. September 2009; 10 (9): 3722-3742.
Plastic is a generic name given to various high molecular weight polymers, which can be degraded by various methods. However, given their abundance in the environment and their specificity of attacking plastics, the biodegradation of plastics by microorganisms and enzymes appears to be the most efficient process. When plastics are used as substrates for microorganisms, the assessment of their biodegradability should not be based solely on their chemical structure, but also on their physical properties (melting point, glass transition temperature, crystallinity, storage module, etc.). In this review, microbial and enzymatic biodegradation of plastics and some factors that affect their biodegradability are discussed.
Keywords: aliphatic polyesters, bioplastics, biodegradability, enzymatic degradation, microbial degradation
With the advancement of technology and the increase of the world's population, plastic materials have found many applications in all aspects of life and industries. However, most conventional plastics such as polyethylene, polypropylene, polystyrene, polyvinyl chloride and polyethylene terephthalate are non-biodegradable and their increasing accumulation in the environment is a threat to the environment. planet. To overcome all these problems, some steps have been taken. The first strategy was to produce plastics with a high degree of degradability.
The term "bioplastic" is used in a confusing way. In our understanding, however, bioplastics are either biodegradable plastics (that is to say., made from fossil fuels) or biosourced plastics (that is to say., synthetic plastics from biomass or renewable resources). The relationship between biodegradable plastics and biosourced plastics is illustrated in. Polycaprolactone (PCL) and polybutylene succinate (PBS) are petroleum-based, but can be degraded by micro-organisms. On the other hand, blends of poly (hydroxybutyrate) (PHB), poly (lactide) (PLA) and starch are produced from biomass or renewable resources and are therefore biodegradable. Although polyethylene (PE) and nylon 11 (NY11) can be produced from biomass or renewable resources, they are not biodegradable. Acetylcellulose (AcC) is biodegradable or non-biodegradable, depending on the degree of acetylation. Acc with low acetylation can be degraded, whereas those with high substitution rates are non-biodegradable.
Biodegradable plastics are considered by many to be a promising solution to this problem because they are environmentally friendly. They can be derived from renewable raw materials, thus reducing greenhouse gas emissions. For example, polyhydroxyalkanoates (PHA) and lactic acid (raw materials for PLA) can be produced by fermentative biotechnological processes using agricultural products and microorganisms (1-3). Biodegradable plastics offer many benefits, including increased soil fertility, low build-up of bulky plastics in the environment (which will inevitably minimize wildlife injury), and reduced waste management costs. In addition, biodegradable plastics can be recycled to useful metabolites (monomers and oligomers) by microorganisms and enzymes. A second strategy involves the degradation of certain plastics derived from petroleum by biological processes. A typical example can be seen in the case of some aliphatic polyesters such as PCL and PBS which can be degraded with enzymes and microorganisms (4-6). Studies have also shown that polycarbonates (especially aliphatic types) have some degree of biodegradability (7).
Third, bold attempts are underway to recycle non-biodegradable plastics. For example, polystyrene (used in the manufacture of some spoons, plates, cups and some disposable packaging materials) can be recycled and used as a filler for other plastics.
Before generalizing the applications of biodegradable plastics, it is important to evaluate and understand the mechanisms involved as well as the microorganisms associated with biodegradation. Regarding the microbial and enzymatic degradation of plastics, we will discuss two aspects: one is based on the microbial characteristics (enzyme) and the other on the characteristics of plastics. Microbial characteristics (enzymes) involve the distribution and types of microorganisms, as well as their growth conditions (such as pH, temperature, moisture content, oxygen, nutrients, etc.) and types of enzymes (intracellular and extracellular enzymes, exogenous types of endogenous cleavage). An interesting question is: "What are the characteristics of plastics that can effectively promote the biodegradability of plastics?" We have generally focused on the chemical structure of polymers with respect to the biodegradability of water-soluble polymer materials.
When evaluating the biodegradability of solid polymers, in addition to their chemical properties, it should also be noted their physical properties as aggregates of polymers. In other words, we should consider not only the first order structures, but also the higher order structures of polymers that play an important role in the biodegradation process. In addition, it should be mentioned that the surface conditions (surface, hydrophilic, hydrophobic properties) of plastics generally influence the biodegradation mechanism of plastics. In this review, we will discuss the biodegradation of plastics by both microbial and enzymatic processes and several factors that govern their biodegradability.
2. Biodiversity and presence of microorganisms degrading polymers
Biodiversity and the presence of microorganisms degrading polymers vary according to the environment (soil, sea, compost, activated sludge, etc.). It is necessary to study the distribution and the population of the micro-organisms degrading the polymers in various ecosystems. As a general rule, the adhesion of microorganisms to the surface of plastics, followed by colonization of the exposed surface, is the main mechanism involved in microbial degradation of plastics. Enzymatic degradation of plastics by hydrolysis is a two-step process: the enzyme binds to the polymer substrate and then catalyzes hydrolytic cleavage. The polymers are degraded into oligomers, dimers and monomers of low molecular weight and finally mineralized into CO2 and H2O.
The clear zone method with agar plates is a widely used technique for screening for degradation agents of polymers and for evaluating the potential for degradation of different microorganisms in polymer. Microorganisms are seeded in agar plates containing emulsified polymers and the presence of polymer degrading microorganisms can be confirmed by the formation of clear halogen zones around the colonies. This occurs when the polymeric degrading microorganisms excrete extracellular enzymes that diffuse through the agar and degrade the polymer into water-soluble materials. Using this technique, it has been confirmed that PHB, polypropiolactone (PPL) and PCL degrading agents are widely distributed in different environments (8-10). The majority of strains capable of degrading PHB belong to different taxa such as Gram-positive and Gram-negative bacteria. Streptomyces and mushrooms (9). It has been reported that 39 bacterial strains of the classes Firmicutes and proteobacteria may degrade PHB, PCL and PBS, but not PLA (10). Only a few microorganisms degrading PLA have been isolated and identified. The population of aliphatic microorganisms degrading polymers in different ecosystems was found to be in the following order: PHB = PCL> PBS> PLA (8,11).
3. Factors influencing the biodegradability of plastics
The properties of plastics are associated with their biodegradability. The chemical and physical properties of plastics influence the biodegradation mechanism. Surface conditions (surface, hydrophilic and hydrophobic properties), first order structures (chemical structure, molecular weight and molecular weight distribution) and high structures (glass transition temperature, melting temperature, modulus of elasticity crystallinity and crystal structure)) of polymers play an important role in biodegradation processes.
In general, side chain polyesters are less assimilated than those without side chains (4). Molecular weight is also important for biodegradability because it determines many of the physical properties of the polymer. The increase in the molecular weight of the polymer decreases its degradability. PCLs of higher molecular weight (Mn> 4000) were degraded slowly by Rhizopus delemar lipase (endo-cleavage type) than that with low Mn (12). In addition, the morphology of the polymers greatly affects their biodegradation rates. The degree of crystallinity is a decisive factor in biodegradability, since the enzymes primarily attack the amorphous domains of a polymer. The molecules of the amorphous region are packed, which makes it more susceptible to degradation. The crystalline portion of the polymers is stronger than the amorphous region. The degradation rate of PLA decreases with increasing polymer crystallinity (13,14). As indicated in, the melting temperature (Tm) of the polyesters has a significant effect on the enzymatic degradation of the polymers. The higher the Tm, the lower the biodegradation of the polymer (12,15,16). In general, Tm is represented by the following formula:
where ΔH was the change of enthalpy melt and ΔS was the change of entropy melt. It is well known that the interactions between the polymer chains mainly affect the value of ΔH and that the internal rotation energies corresponding to the rigidity (flexibility) of the polymer molecule remarkably affect the value of ΔS.
The chemical structures of aliphatic polyester, polycarbonate, polyurethane and polyamides, as well as their (Tm) are listed in. Aliphatic polyesters (ester bond (-CO-O-)) and polycarbonates (carbonate bond (-O-CO-O-)) are two typical plastic polymers that have high potential for use as biodegradable plastics. because of their sensitivity to lipolytic properties. enzymes and microbial degradation. Compared to aliphatic polyesters and polycarbonates, aliphatic polyurethanes and polyamides (nylon) have higher Tm values. The high values (Tm) of polyurethane and polyamide (nylon) are due to the high ΔH value due to the presence of hydrogen bonds between the urethane bond-based (-NH-CO-O-) polymer chains and the amide bond (- NH-CO-) respectively.
|Last name||Chemical structure||Tm (° C)|
|Polyester||-O- (CH2)6-O-CO- (CH)2)4-CO-||60|
|polycarbonate||-O- (CH2)4-O-CO-O- (CH2)4-O-CO-||65|
|polyurethane||-NH- (CH2)6-NH-CO-O- (CH2)4-O-CO-||180|
|Polyamide||-NH- (CH2)6-NH-CO- (CH)2)6-CO-||240|
|Polyamide||-NH- (CH2)6-NH-CO- (CH)2)4-CO-||265|
In contrast, the high Tm of the aromatic polyester is caused by the low value of ΔS with increased stiffness (decreased flexibility) of the polymer molecule based on an aromatic ring.
4. Aliphatic polyesters derived from fossil resources
4.1. Poly (ethylene adipate) (PEA)
PEA ((-OCH2CH2OOC (CH2)4CO-) n) is a polyurethane prepolymer. It is often mixed with other polyesters to obtain desirable specific properties such as soft segments. PEA degrading microorganisms were screened and isolated using PEA (Mn 3000) as the sole source of carbon. Among the isolated microorganisms degrading PEA, Penicillium sp. strain 14-3 exhibited the strongest activity. PEA was degraded in 120 h at high cell concentrations. This strain can degrade not only PEA but also aliphatic polyesters such as poly (ethylene succinate) (PES), PBS and polybutyl adipate (PBA) (17). The enzyme responsible for the degradation of PEA has been purified and is considered as a kind of lipase with high specificity for the substrate. The purified enzyme has a molecular weight of 25 kDa and could degrade various types of aliphatic polyesters, such as poly (β-propiolactone) (PPL) and poly (-caprolactone) (PCL), but not poly (dl3-methylpropiolactone) or poly (dl3-hydroxybutyrate) (5). This enzyme can also hydrolyze vegetable oils, triglycerides and methyl esters of fatty acids. Since the purified enzyme of Penicillium sp. strain 14-3 has properties similar to those of lipase, some commercially available lipases and esterases were used to confirm whether they were capable of degrading PEA. The results showed that the lipases of R. arrizus, R. delemar, Achromobacter sp. and Candida cylindracea and esterase of pork liver showed activities on PEA and PCL (6).
4.2. Poly (Ca-Caprolactone) (PCL)
PCL ((-OCH2CH2CH2CH2CH2CO-) n) is a biodegradable partially crystalline synthetic polyester having a low melting point (60 ° C) and a glass transition temperature (Tg) of -60 ° C. It is prepared by ring-opening polymerization of caprolactone. PCL has been shown to be degraded by the action of aerobic and anaerobic microorganisms widely distributed in various ecosystems. In addition, the degradation of high molecular weight PCL was investigated using Penicillium sp. strain 26-1 (ATCC 36507) isolated from the soil. PCL is almost completely degraded in 12 days. This strain can also assimilate unsaturated aliphatic and alicyclic polyesters, but not aromatic polyesters (4). Thermotolerant microorganism degrading the PCL identified as Aspergillus sp. strain ST-01 was isolated from the soil. PCL was completely degraded by this strain after 6 days of incubation at 50 ° C (18). PCL and PHB were degraded under anaerobic conditions by new species of genetically engineered microorganisms Clostridium (19).
PCL can be degraded by lipases and esterases (6). The degradation rate of PCL depends on its molecular weight and degree of crystallinity. Enzymatic degradation of PCL by Aspergillus flavus and Penicillium funiculosum showed that a faster degradation was observed in the amorphous region (20).
The biodegradability of PCL can be increased by copolymerization with aliphatic polyesters (21,22). In general, the copolymers have lower crystallinity and Tm than homopolymers and are therefore more likely to degrade.
The susceptibility of the PCL films prepared at different quenching temperatures (-78, 0, 25, 50 ° C) by R. arrhizus the lipase was evaluated at 30 ° C. Large spherulites formed on a PCL film soaked at 50 ° C, but no spherulite was observed on a PCL film soaked at -78 ° C. X-ray diffraction pattern of PCL films quenched at 25 ° C and 50 ° C indicated the growth of PCL crystals in the c-axis direction (crystalline unit thickness), but the crystallinity did not increase. not as much (about 40%) with rising quenching temperature. The sensitivity of the PCL film soaked at -78 ° C was the highest and decreased with increasing quenching temperature. It has been confirmed that the size of spherulites is an important factor for the biodegradation of PCL. In addition, the retention module of the PCL film samples increased with increasing quenching temperature. The PCL film soaked at 50 ° C had the highest storage module, while a low storage modulus was observed on the PCL film tempered at -195 ° C. The PCL film storage module also increased with the increase of the draw ratio. In addition, the sensitivity of PCL movies to R. arrhizus lipase decreased with increasing rate of attracted As the polyester preservation modulus can be determined over a wide range (from the temperature below the surface temperature up to the surface temperature), we can predict the rate of enzymatic degradation of the polyesters using the value of the polyester storage modulus at 30 ° C (typical evaluation temperature). biodegradability) (23).
4.3. Poly (β-propiolactone) PPL
PPL ((-OCH2CH2CO-) n) is a biodegradable chemosynthetic aliphatic polyester having good mechanical properties. The structural units of this polyester are similar to PHB and PCL, thus it can be degraded by both the PHB depolymerase and the lipase (6,24,25). Many PPL-degrading microorganisms are widely distributed in a variety of environments and the majority of these microorganisms belong to Bacillus sp. (26). PPL-degrading microorganisms were isolated in different ecosystems and on 13 isolates, nine of these strains were identified Acidovorax sp. Variovorax paradoxus, Sphingomonas paucimobilis. PHB has also been degraded by these isolates (27). R. delemar can also degrade the PPL (6). In addition, a new depolymerase of PHB from a thermophilic Streptomyces sp. was also able to degrade the PPL (28).
4.4. Poly (butylene succinate) (PBS) and poly (ethylene succinate) (PES)
PBS ((-O (CH2)4OOC (CH2)2CO-) n) and PES ((-O (CH2)2OOC (CH2)2CO-) n) are synthetic aliphatic polyesters having high melting points of 112 to 114 ° C and 103 to 106 ° C, respectively. They are synthesized from dicarboxylic acids (for example., succinic and adipic acids) and glycols (for example., ethylene glycol and 1,4-butanediol) (29). Their mechanical properties are comparable to those of polypropylene and low density polyethylene (LDPE).
PBS degrading microorganisms are widely distributed in the environment, but their ratio to the total number of microorganisms is lower than that of PCL degraders. The degradation of PBS by Amycolatopsis sp. HT-6 has been studied and the results have shown that this strain can degrade not only PBS but also PHB and PCL (30). Several thermophilic actinomycetes of the Japanese microorganism culture (JCM) were examined for their ability to degrade PBS. Microbispora rosea, Excellospora japonica and E. viridilutea clear zone formed on agar plates containing emulsified PBS. Mr Rosea was able to degrade 50% (w / v) of the PBS film after eight days of culture in a liquid medium (31).
PES is another chemically synthetic aliphatic polyester that is prepared either by ring-opening polymerization of succinic anhydride with ethylene oxide, or by succinic acid polycondensation and by Ethylene glycol (32). In contrast to microbial polyesters that may degrade in a variety of environments, degradability of PES has been found to be highly dependent on environmental factors (33). In addition, the environmental distribution of PES-degrading microorganisms is limited compared to micro-organisms degrading PHBs and PCLs. A thermophilic Bacillus sp. The TT96, a PES degrader, has been isolated from the ground. This bacterium can also form clear areas on the PCL and PBS plates but not on the PHB (34). A number of SEP degrading mesophilic microorganisms have been isolated from aquatic environments and soils. Phylogenetic analysis revealed that the isolates belong to the genera. Bacillus and Paenibacillus. Of the isolates, strain KT102 that is related to Bacillus Pumilus was chosen because it could degrade PES film at the fastest rate among isolates. This strain can degrade PES, PCL and olive oil, but not PBS, PHB and PLA (35). In addition, several fungi were isolated in various ecosystems and isolates formed clear areas around the colony on agar plates containing PES. A strain NKCM1003 belonging to Aspergillus clavatus has been selected and it can degrade the PES film at a speed of 21μg / cm2/ h. (36). Comparative studies on the biodegradability of three poly (alkylene succinates) (PES, PBS and poly (propylene succinate) (PPS)) of the same molecular weight were studied with the help of R. delemar lipase. PPS with low Tm (43-52 ° C) had the highest rate of biodegradation followed by PES, due to the lower crystallinity of PPS compared to PES and PBS (37).
4.5 Aliphatic-Aromatic Copolyesters (AAC)
Due to the limited properties of many types of biodegradable aliphatic polyester that are important for many applications, we have attempted to combine the biodegradability of aliphatic polyesters with the good material properties of aromatic polyesters.
It has been reported that AAC, which consisted of PCL and aromatic polyester such as poly (ethylene terephthalate) (PET), poly (butylene terephthalate) (PBT) and poly (isophthalate) ethylene) (PEIP) was hydrolysed by R. delemar lipase (15). The susceptibility of these AACs to hydrolysis by R. delemar lipase decreased rapidly with increasing aromatic polyester content. The AAC lipase susceptibility (consisting of PCL and PEIP, the latter being used as a low Tm aromatic polyester (103 ° C)), was superior to that of other AACs. It has been hypothesized that the stiffness of the aromatic ring in the AAC chains influences their biodegradability with this lipase.
Another synthetic AAC containing adipic acid and terephthalic acid can also be attacked by microorganisms (38). Kleeberg et al. evaluated the biodegradation of AAC synthesized from 1,4-butanediol, adipic acid and terephthalic acid. Thermobifida fusca (previously known as Thermomonospora fusca) isolated from compost, showed degradation rates 20 times higher than those usually observed in a routine composting test (39). A thermophilic hydrolase of Thermobifida fusca It's proven inducible not only by the AAC but also by the esters. This enzyme has been classified as serine hydrolase with a great similarity with triacylglycerol lipase Streptomyces albus G and triacylglycerol acylhydrolase Streptomyces sp. M11 (40).
5. Aliphatic polyesters from renewable resources
5.1. Poly (3-hydroxybutyrate) (PHB)
PHB ((-O (CH3) CHCH2CO-) n) is a natural polymer produced by many bacteria to store carbon and energy. This polymer has attracted the interest of research and commercial interests around the world because it can be synthesized from low-cost renewable raw materials and the polymerizations operate under mild processing conditions with minimal impact on l & # 39; environment. In addition, it can be biodegraded in aerobic and anaerobic environments, without formation of toxic products. Chowdhury reported for the first time the PHB degrading microorganisms from Bacillus, Pseudomonas and Streptomyces species (41). Since then, several aerobic and anaerobic microorganisms degrading PHB have been isolated from soil (Pseudomonas lemoigne, Comamonas sp. Acidovorax faecalis, Aspergillus fumigatus and Variovorax paradoxus), activated and anaerobic sludge (Alcaligenes faecalis, Pseudomonas, Illyobacter delafieldi), seawater and lakewater (Testosterone of Comamonas, Pseudomonas stutzeri) (42). The percentage of micro-organisms degrading PHB in the environment was estimated to be between 0.5 and 9.6% of the total number of colonies (10). The majority of PHB degrading microorganisms were isolated at ambient or mesophilic temperatures and very few of them were able to degrade PHB at a higher temperature. Tokiwa et al. stressed that high temperature composting is one of the most promising technologies for the recycling of biodegradable plastics and thermophilic microorganisms that can degrade polymers, which play an important role in the composting process (43). Thus, microorganisms capable of degrading various types of polyesters at elevated temperatures are of interest. A thermophilic Streptomyces sp. isolated from the soil can degrade not only PHB but also PES, PBS and poly (oligo (tetramethylene succinate) -co- (tetramethylene carbonate)) (PBS / C). This actinomycete has a higher PHB degradation activity than thermotolerant and thermophilic agents. Streptomyces strains of culture collections (44). A thermotolerant Aspergillus sp. was able to degrade 90% of the PHB film after five days of culture at 50 ° C (18). In addition, several thermophilic degrading actinomycetes have been isolated from different ecosystems. Of the 341 strains, 31 isolates were degrading PHB, PCL and PES and these isolates were identified as belonging to the genus. Actinomadura, Microbispora, Streptomyces, Thermoactinomyces and Saccharomonospora (45).
5.2. Poly (lactic acid) (PLA)
PLA ((-O (CH3) CHCO-) n) is a biodegradable and biocompatible thermoplastic that can be produced by fermentation from renewable resources. It can also be synthesized either by lactic acid condensation polymerization or by ring opening polymerization of lactide in the presence of a catalyst. This polymer exists in the form of three stereoisomers: poly (l-lactide) (l-PLA), poly (re-lactide) (re-PLA) and poly (dl-lactide) (dl-Pla). The manufacture of PLA from lactic acid was initiated by Carothers in 1932 (46).
Des études écologiques sur l’abondance de microorganismes dégradant le PLA dans différents environnements ont confirmé que les agents de dégradation du PLA ne sont pas largement distribués et qu’ils sont donc moins sensibles aux attaques microbiennes que d’autres polymères microbiens et synthétiques aliphatiques (10,11,34). La dégradation du PLA dans le sol est lente et il faut beaucoup de temps pour que la dégradation commence (47,48).
Dégradation microbienne du PLA en utilisant Amycolatopsis sp. a été signalé pour la première fois par Pranamuda et al. (11). Depuis lors, plusieurs études portant sur la dégradation microbienne et enzymatique de la PLA ont été publiées (49). De nombreuses souches de genre Amycolatopsis and Saccharotrix ont pu dégrader à la fois le PLA et la fibroïne de soie. Les principaux composants d'acides aminés de la fibroïne de soie sont l-alanine et glycine et il existe une similitude entre la position stéréochimique du carbone chiral de lunité d'acide lactique de PLA et lunité -alanine dans la fibroïne de soie. La fibroïne de la soie est l’un des analogues naturels du poly (l-lactide), ainsi, les microorganismes dégradant le PLA pourraient probablement identifier le l-lactate unité comme un analogue de lunité -alanine dans la fibroïne de soie. Plusieurs matériaux protéiniques tels que la fibroïne de soie, l'élastine, la gélatine et certains peptides et acides aminés stimulent la production d'enzymes à partir de micro-organismes dégradant la PLA (50–54).
Williams (55) a étudié la dégradation enzymatique du PLA en utilisant la protéinase K, la bromélaïne et la pronase. Parmi ces enzymes, la protéinase K de Album de Tritirachium était le plus efficace pour la dégradation du PLA. La protéinase K et d’autres sérine protéases sont capables de dégrader l-PLA et dl-PLA mais pas re-PLA. En outre, la protéinase K hydrolyse préférentiellement la partie amorphe de l-PLA et la vitesse de dégradation diminue avec l'augmentation de la partie cristalline (56,57). Fukuzaki et al. ont rapporté que la dégradation des oligomères de PLA était accélérée par plusieurs enzymes de type estérase, notamment Rhizopus delemar lipase (58). La PLA dépolymérase purifiée de Amycolatopsis sp. était également capable de dégrader la caséine, la fibroïne de soie, le sucre (Ala)3–pNA mais pas PCL, PHB et Suc- (Gly)3–pNA (50). Leurs études ont montré que la PLA dépolymérase était une sorte de protéase et non une lipase.
Il a été rapporté que l’a-chymotrypsine peut dégrader le PLA et le PEA avec une activité plus faible sur le poly (butylène succinate-co-adipate) (PBS / A). De plus, plusieurs sérine protéases telles que la trypsine, l’élastase, la subtilisine ont été capables de s’hydrolyser. l-PLA (59).
6. mélanges de polymères
6.1. Mélanges de polyester avec d'autres polymères
Le mélange de polymères biodégradables est une approche permettant de réduire le coût global du matériau et de modifier les propriétés et les taux de dégradation souhaités. Comparé à la méthode de copolymérisation, le mélange peut être un moyen beaucoup plus facile et plus rapide d’atteindre les propriétés souhaitées. Plus important encore, grâce au mélange, d’autres polymères moins coûteux pourraient être incorporés les uns aux autres. La miscibilité des mélanges est l’un des facteurs les plus importants qui affectent les propriétés finales du polymère. Certains des avantages de la production de mélanges miscibles sont les suivants: morphologie monophasée et reproductibilité des propriétés mécaniques. Cependant, la formation de mélanges miscibles, en particulier avec des polymères non biodégradables, peut ralentir ou même empêcher la dégradation des composants biodégradables.
Iwamoto et al. mis au point des mélanges de plastiques en combinant le PCL avec des plastiques classiques tels que le polyéthylène basse densité (LDPE), le polypropylène (PP), le polystyrène (PS), le nylon 6 (NY), le poly (éthylène téréphtalate) (PET) et le PHB, et évalué leur dégradabilité enzymatique . Les mélanges de PCL et de LDPE, de PCL et de PP ont conservé la haute biodégradabilité du PCL. En revanche, la dégradabilité de la partie PCL dans les mélanges PCL et PS, PCL et PET, PCL et PHB a remarquablement diminué. Dans le cas de mélanges de PCL et de NY ou de PS, la biodégradabilité du PCL n'a pas tellement changé. En général, il semble que plus la miscibilité du PCL et des plastiques conventionnels est élevée, plus la dégradation du PCL sur leurs mélanges est difficile. R. arrhizus lipase (60). En outre, il a été constaté que les dégradabilités des mélanges PCL / LDPE (61) et PCL / PP (62) par la lipase pouvaient être contrôlées, en fonction de la structure de leur phase.
Différents mélanges de PHB ont été réalisés avec des polymères et des polysaccharides biodégradables et non biodégradables. La miscibilité, la morphologie et la biodégradabilité des mélanges de PHB avec PCL, PBA et poly (acétate de vinyle) (PVAc) ont été étudiés. Les mélanges PHB / PCL et PHB / PBA n'étaient pas miscibles à l'état amorphe tandis que les mélanges PHB / PVAc étaient miscibles. La dégradation enzymatique de ces mélanges a été réalisée à l'aide de PHB dépolymérase à partir de Alcaligenes feacalis T1. Les résultats ont montré que la perte de poids des mélanges diminuait linéairement avec l’augmentation de la quantité de PBA, PVAc ou PCL (63).
Koyama et Doi ont étudié la miscibilité, la morphologie et la biodégradabilité du mélange PHB / PLA. Les sphérulites des mélanges ont diminué avec l'augmentation de la teneur en PLA et le taux d'érosion de surface enzymatique a également diminué avec l'augmentation de la teneur en PLA dans le mélange. Il était évident que les mélanges de polymères contenant du PHB présentaient généralement des propriétés et une biodégradabilité améliorées par rapport au PHB pur (64).
Différents mélanges de l-PLA / PCL (75/25, 50/50, 25/75) ont été préparés et une dégradation enzymatique a été observée à l'aide de la protéinase K ou Pseudomonas lipase. La protéinase K était capable de dégrader le domaine amorphe du PLA mais pas la partie cristalline de l-PLA ou PCL. Au contraire, Pseudomonas la lipase peut dégrader à la fois la partie amorphe et cristalline de la PCL, mais pas l-PLA (65).
6.2. Mélanges de Polyesters et d'Amidon
Blends of synthetic polymers and starch offer cost performance benefits because starch is renewable, cheap and available year-round. In this case the starch blended can be in the form of granules or gelatinized starch or even starch which has been modified chemically to a thermoplastic. It is generally known that blends of PCL and granular starch exhibit a high degree of biodegradation (61).
Takagi et al. developed PCL/gelatinized starch blends using corn starch acetates and evaluated their biodegradabilities by an enzyme, α-amylase. Their biodegradabilities rapidly decreased with an increase in PCL content (66).
The feasibility of producing PCL/granular starch blends using different starch from cassava, sago and corn was reported by Pranamuda et al. Both tensile strength and elongation of the blends decreased as the starch content increased. The blends were not good in tensile strength but were relatively good in elongation. Continuous phase dispersion of the starch in the PCL was observed in the films using SEM. Degradation of PCL/starch blends showed that as the starch content increased, the polymer blends became more biodegradable using lipase. This could be attributed to the increase in the surface area of PCL after blending with starch, thereby rendering it more susceptible to biodegradation (67).
Noomhorm et al. developed PCL/tapioca starch (granular and gelatinized) blends using poly(dioxolane), a poly(ethylene oxide-alt-methylene oxide), as compatibilizer. Their biodegradabilities by α-amylase increased as the starch content increased, but were independent of the dispersion of starch in the PCL matrix (68).
PLA and starch are good candidates for polymer blends because both are biodegradable and derivable from renewable resources. Starch can improve the biodegradability and lower the cost while PLA can control the mechanical properties of the blend. However, starch is a hydrophilic material, which does not interact well with hydrophobic polyesters resulting to unfavorable qualities of the blends. In line with this, several approaches have been proposed and developed to overcome the problem of incompatibility of starch and synthetic polymer blends (69,70). Very good interfacial adhesions of PLA/starch blends were achieved by grafting PLA using maleic anhydride (MA) (71). Jang et al. investigated the interfacial adhesion between PLA and starch using MA and maleated thermoplastic starch (MATPS). Scanning electron microscopy (SEM) showed that MA is a good compatibilizer and PLA/starch blends had increased crystallinity. On the other hand, MATPS is not effective for PLA/starch blends. PLA/starch blends which were compatibilized with MA showed higher biodegradability than ordinary PLA/starch blends at the same PLA ratio (72).
The properties and biodegradability of PBS/A and corn starch (5%–30% w/w) blends were investigated by Ratto et al. Results showed that the tensile strength decreased with increase in starch content. Soil burial test showed that the rate of biodegradation increased significantly when the starch content was increased to 20%. It was confirmed that the molecular weight of PBS/A decreased after soil burial indicating that biodegradation was enhanced by the presence of starch (73).
Aliphatic polycarbonates are known to have greater resistance to hydrolysis than aliphatic polyesters. Imai et al. first reported the biodegradation of poly(ethylene carbonate) (PEC) implanted in dog tissue and suggested that pronase treatment might be effective in diminishing the PEC mass (74). Kawaguchi et al. reported that PEC was degraded enzymatically in the peritoneal cavity of rats, but not with poly(propylene carbonate) (PPC) (75).
The distribution of PEC (Mn 50,000)-degrading microorganisms seems to be limited, although PPC (Mn 50,000) appears to be non-biodegradable. The percentage of PEC-degrading microorganisms among total colonies ranged from 0.2% to 5.7% (76).
Suyama et al. isolated poly(hexamethylene carbonate) (PHC, Mn 2000)-degrading microorganisms which were phylogenetically diverse. Roseateles depolymerans 61A formed di(6-hydroxyhexyl) carbonate and adipic acid from PHC, and di(4-hydroxybutyl)-carbonate and succinic acid from poly(butylene carbonate) (PBC, Mn 2,000) (77). Pranamuda et al. found that Amycolatopsis sp. HT-6 degraded high molecular-weight poly(butylene carbonate) (PBC, Mn 37,000). In a liquid culture containing 150 mg of PBC film, 83 mg of film was degraded after seven days cultivation (78).
Suyama and Tokiwa reported that a cholesterol esterase from Candida cylindracea, lipoprotein lipase from Pseudomonas sp., and lipase from C. cylindracea, Chromobacterium viscosus, porcine pancreas, Pseudomonas sp., and R. arrhizus degraded PBC (Mn 2000). Lipase and lipoprotein lipase from Pseudomonas sp. could also degrade high molecular-weight PBC (Mn 30,000). Lipoprotein lipase from Pseudomonas sp produced 1,4-butanediol, CO2 and di(4-hydroxybutyl) carbonate from PBC (79).
8. Polyurethanes (PU)
PU have various applications such as in the manufacture of plastic foams, cushions, rubber goods, synthetic leathers, adhesives, paints and fibers. There are two types of polyurethanes, that is, the ester type and the ether type. Most commercial polyurethane products are composed of soft segments derived from the polymer-diol, e.g., PCL-diol, polyethylene glycol, poly-tetramethylene glycol, and hard segments from the diisocyanate, e.g., 1,6-hexamethylene-diisocyanate (HDI), diphenylmethane-4,4’-diisocyanate (MDI), tolylene-2,4-diisocyanate (TDI), and diols such as ethylene glycol and butanediol.
Darby and Kaplan reported that polyester-type polyurethanes (ES-PU) were more susceptible to fungal attack than polyether-type polyurethanes (ET-PU) (80). Tokiwa et al. found that R. delemar lipase and hog pancreatic lipase can hydrolyze the ES-PU composed of MDI, PCL-diol (Mn 2,000) and 1,4-tetramethylenediol (molar ratio 2:1:1). The amount of degradation products obtained from the ES-PU film with hog pancreatic lipase was approximately half of that produced by R. delemar lipase (53% degradation of the original ES-PU film) after 24 h reaction. Hydrolysis rates of ES-PU containing either MDI or TDI were lower than that of ES-PU containing HDI. Thus, it was suggested that the rigidity of ES-PU molecules based on the aromatic rings, rather than the hydrogen bonds among the ES-PU chains, would influence their biodegradability by R. delemar lipase (21).
Crabbe et al. reported on the degradation of an ES-PU and the secretion of an enzyme-like factor with esterase properties, by Curvularia senegalensis, a fungus isolated from soil (81). Subsequently, Nakajima-Kambe et al. showed that Comamonas acidovorans strain TB-35 was able to degrade ES-PU made from poly(diethylene adipate) (Mn 2,500 and 2,690) and TDI. A purified ES-PU-degrading enzyme from C. acidovorans TB-35, a type of esterase, hydrolyzed the ES-PU and released diethylene glycol and adipic acid (82).
Santerre et al. (83) and Wang et al. (84) reported that cholesterol esterase from bovine pancreas degraded ES-PU synthesized from TDI, PCL-diol (Mn 1,250) and ethylenediamine, and released the hard-segment components.
However, it seems that no microbe can degrade polyurethane completely, and therefore, it is difficult to clarify the fate of residues after degradation of ES-PU by both microorganisms and enzymes. Furthermore, it is difficult to determine whether ET-PU itself was degraded by microbes to any significant extent.
9. Polyamide (Nylon)
9.1. Nylon 6
Polyamide (nylon) has excellent mechanical and thermal properties, good chemical resistance and low permeability to gases, but it is known to be resistant to degradation in the natural environment. The poor biodegradability of nylon in comparison with aliphatic polyesters is probably due to its strong interchain interactions caused by the hydrogen bonds between molecular chains of nylon. Some microorganisms such as Flavobacterium sp. (85) and Pseudomonas sp. (NK87) (86) have been reported to degrade oligomers of nylon 6, but they cannot degrade nylon 6 polymers. Moreover, some white rot fungal strains were reported to degrade nylon 66 through oxidation processes (87).
9.2. Nylon 4
It has been reported that nylon 4 was degraded in the soil (88) and in the activated sludge (89). The results confirmed that Nylon 4 is readily degradable in the environment. Furthermore, the biodegradability of nylon 4 and nylon 6 blends was investigated in compost and activated sludge. The nylon 4 in the blend was completely degraded in 4 months while nylon 6 was not degraded (90). Recently, Yamano et al. was able to isolate polyamide 4 degrading microorganisms (ND-10 and ND-11) from activated sludge. The strains were identified as Pseudomonas sp. The supernatant from the culture broth of strain ND-11 degraded completely the emulsified nylon 4 in 24 h and produced γ-aminobutyric acid (GABA) as degradation product (91).
Generally speaking, degradation of polyamides is still unclear. Thus further investigations on the pathways of degradation are necessary.
9.3. Copolyamide-Esters (CPAE)
In order to improve the properties of biodegradable aliphatic polyesters for various fields of applications and to find the reason why industrialized aliphatic polyamides (nylon) are not biodegradable, CPAE were synthesized by the amide-ester interchange reaction between PCL and various nylons. The susceptibility of CPEA to hydrolysis by R. delemar lipase decreased with shortening of the nylon blocks in CPAE chains and with increasing nylon content. The simple blends of PCL and nylon retained high biodegradability of PCL. Thus it was assumed that the amount and distribution of hydrogen bonds, based on the amide bonds, in the CPAE chains influenced their biodegradability by this lipase (92).
Komatsu et al. synthesized CPAE from ɛ-caprolactam with ɛ-caprolactone or δ-valerolactone by ring-opening copolymerization using Na catalyst under reduced pressure, and examined their degradation by R. arrhizus lipase. Most CPAEs were degraded by the lipase. The biodegradability decreased with an increase in Tm of CPAEs as a result of an increase in the amide bond contents (93).
It would be very important that various types of interaction among macromolecular chains, which are related to Tm and storage modulus, are taken into consideration when designing the biodegradable solid polymers.
10. Polyethylene (PE)
PE is a stable polymer, and consists of long chains of ethylene monomers. PE cannot be easily degraded with microorganisms. However, it was reported that lower molecular weight PE oligomers (MW = 600–800) was partially degraded by Acinetobacter sp. 351 upon dispersion, while high molecular weight PE could not be degraded (94).
Furthermore, the biodegradability of low density PE/starch blends was enhanced with compatibilizer (95). Biodegradability of PE can also be improved by blending it with biodegradable additives, photo-initiators or copolymerization (96,97). The initial concept of blending PE with starch was established in UK to produce paper-like PE bag. A few years later, the idea to blend PE with starch and photoinitiators was conceived in the US as a way of saving petroleum, though its biodegradability was also taken into account.
Environmental degradation of PE proceeds by synergistic action of photo-and thermo-oxidative degradation and biological activity (i.e., microorganisms). When PE is subjected to thermo- and photo-oxidization, various products such as alkanes, alkenes, ketones, aldehydes, alcohols, carboxylic acid, keto-acids, dicarboxylic acids, lactones and esters are released. Blending of PE with additives generally enhances auto-oxidation, reduces the molecular weight of the polymer and then makes it easier for microorganisms to degrade the low molecular weight materials. It is worthy to note that despite all these attempts to enhance the biodegradation of PE blends, the biodegradability with microorganisms on the PE part of the blends is still very low.
11. Polypropylene (PP)
PP is a thermoplastic which is commonly used for plastic moldings, stationary folders, packaging materials, plastic tubs, non-absorbable sutures, diapers etc. PP can be degraded when it is exposed to ultraviolet radiation from sunlight. Furthermore, at high temperatures, PP is oxidized. The possibility of degrading PP with microorganisms has been investigated (98).
12. Polystyrene (PS)
PS is a synthetic hydrophobic polymer with high molecular weight. PS is recyclable but not biodegradable. Although it was reported that PS film was biodegraded with an Actinomycete strain, the degree of biodegradation was very low (99). At room temperature, PS exists in solid state. When it is heated above its glass transition temperature, it flows and then turns back to solid upon cooling. PS being a transparent hard plastics is commonly used as disposable cutleries, cups, plastic models, packing and insulation materials.
13. Conclusions and Future Prospects
Biodegradable plastic is an innovative means of solving the plastic disposal problem from the standpoint of development of new materials. In general, plastics are water-insoluble, thermo-elastic polymeric materials. Biodegradability of plastics is affected by both their chemical and physical properties. Beside the covalent forces of polymer molecules, various kinds of weak forces (i.e., hydrogen bond forces, van der Waals forces, coulombic forces, etc.) among macromolecular chains affect not only the formation of polymer aggregates, but also the structure and physical properties and function (reactivity) of the polymer aggregates. The biodegradation mechanisms of plastics as shown in this review can be applied to biomass that are composed of polymeric materials (i.e., cellulose, hemicellulose, lignin, chitin, silk fibroin, etc.).
Furthermore, knowledge about the biodegradation mechanisms of plastics would be useful for studies on protein conformational diseases that are associated with aggregation, deposition and crystallization of abnormal proteins such as Alzheimer’s disease and bovine spongiform encephalopathy (BSE). Proteinase K and l-PLA-degrading enzyme from Amycolatopsis sp. can degrade both l-PLA and silk fibroin. It is well known that proteinase K can degrade prion protein, of which mis-folded form of it is resistant to proteinase K and is implicated in BSE in cattle. Polyesters (i.e., l-PLA, re-PLA, PCL, PHB) can be used as a model for abnormal protein, because it is easy to change their high order structures by quenching and elongation, etc. and to evaluate their rate of enzymatic degradation.
Lipolytic enzymes such as lipase and esterase can hydrolyze not only fatty acid esters and triglycerides, but also aliphatic polyesters. We can understand that lipolytic enzyme has an important role in the degradation of natural aliphatic polyesters such as cutin, suberin and esteroid in the natural environment and animal digestive tract. However, it is not certain whether human body produces any aliphatic polyesters or not.
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