There are some plastics that are processed to avoid the strong carbon bonds formed in the heating process, but they will not last as long as the mainstream plastics, and will start to break down much sooner. Consider the source of your drink:. Plastic bottles can be recycled:. Why are plastics considered non-biodegradable? Environmental Science. Jul 31, Plants,, animal bodies all consumed by some form of bacteria and get decomposed.
Explanation: But plastic materials can not be consumed by such earth bacteria. Jul 30, Explanation: Plastic is made from petroleum, a product of oil, using heat and a catalyst to change the propylene into polypropylene, a substance not found in nature. This actinomycete has higher PHB-degrading activity than thermotolerant and thermophilic Streptomyces strains from culture collections [ 44 ]. A thermotolerant Aspergillus sp. Furthermore, several thermophilic polyester degrading actinomycetes were isolated from different ecosystems.
It can also be synthesized either by condensation polymerization of lactic acid or by ring opening polymerization of lactide in the presence of a catalyst. The manufacture of PLA from lactic acid was pioneered by Carothers in [ 46 ]. Ecological studies on the abundance of PLA-degrading microorganisms in different environments have confirmed that PLA-degraders are not widely distributed, and thus it is less susceptible to microbial attack compared to other microbial and synthetic aliphatic polymers [ 10 , 11 , 34 ].
The degradation of PLA in soil is slow and that takes a long time for degradation to start [ 47 , 48 ]. Microbial degradation of PLA using Amycolatopsis sp. Since then, a number of research studies dealing with microbial and enzymatic degradation of PLA have been published [ 49 ].
The main amino acid constituents of silk fibroin are l -alanine and glycine and there is a similarity between the stereochemical position of the chiral carbon of l -lactic acid unit of PLA and l -alanine unit in the silk fibroin. Silk fibroin is one of the natural analogues of poly l -lactide , thus, the PLA degrading microorganisms may probably identify the l -lactate unit as an analogue of l -alanine unit in silk fibroin.
Several proteinous materials such as silk fibroin, elastin, gelatin and some peptides and amino acids were found to stimulate the production of enzymes from PLA-degrading microorganisms [ 50 — 54 ]. Williams [ 55 ] investigated the enzymatic degradation of PLA using proteinase K, bromelain and pronase. Among these enzymes, proteinase K from Tritirachium album was the most effective for PLA degradation. Furthermore, proteinase K preferentially hydrolyzes the amorphous part of l -PLA and the rate of degradation decreases with an increase in the crystalline part [ 56 , 57 ].
Fukuzaki et al. The purified PLA depolymerase from Amycolatopsi s sp. Their studies showed that PLA depolymerase was a kind of protease and not a lipase. Moreover, several serine proteases such as trypsin, elastase, subtilisin were able to hydrolyze l -PLA [ 59 ]. The blending of biodegradable polymers is one approach of reducing the overall cost of the material and modifying the desired properties and degradation rates. Compared to copolymerization method, blending may be a much easier and faster way to achieve the desired properties.
More importantly, through blending, other less expensive polymers could be incorporated with one another. Miscibility of the blends is one of the most important factors affecting the final polymer properties. Some of the advantages of producing miscible blends are: single phase morphology and reproducibility of the mechanical properties.
However formation of miscible blends especially with non-biodegradable polymers can slow down or even inhibit the degradation of the biodegradable components. Iwamoto et al.
In general, it seems that the higher the miscibility of PCL and conventional plastics, the harder the degradation of PCL on their blends by R. Different blends of PHB have been performed with biodegradable and non-biodegradable polymers and polysaccharides. Enzymatic degradation of these blends was carried out using PHB depolymerase from Alcaligenes feacalis T1.
The spherulites of the blends decreased with an increase in the content of the PLA and the rate of enzymatic surface erosion also decreased with increasing PLA content in the blend.
It was evident that polymer blends containing PHB usually showed improved properties and biodegradability when compared with pure PHB [ 64 ]. 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. Their biodegradabilities rapidly decreased with an increase in PCL content [ 66 ]. 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. 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. 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 ].
Jang et al. Results showed that the tensile strength decreased with increase in starch content. Aliphatic polycarbonates are known to have greater resistance to hydrolysis than aliphatic polyesters. Imai et al. Kawaguchi et al. The percentage of PEC-degrading microorganisms among total colonies ranged from 0. Suyama et al. 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, [ 77 ].
Pranamuda et al. In a liquid culture containing 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.
Lipase and lipoprotein lipase from Pseudomonas sp. 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. The amount of degradation products obtained from the ES-PU film with hog pancreatic lipase was approximately half of that produced by R.
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. Crabbe et al. Subsequently, Nakajima-Kambe et al. Santerre et al. 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. 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. 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 ]. 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.
The strains were identified as Pseudomonas sp. Generally speaking, degradation of polyamides is still unclear. Thus further investigations on the pathways of degradation are necessary. 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.
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. 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.
PE is a stable polymer, and consists of long chains of ethylene monomers. PE cannot be easily degraded with microorganisms. Biodegradability of PE can also be improved by blending it with biodegradable additives, photo-initiators or copolymerization [ 96 , 97 ]. 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. 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.
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 ]. 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.
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. The biodegradation mechanisms of plastics as shown in this review can be applied to biomass that are composed of polymeric materials i.
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. 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. National Center for Biotechnology Information , U. Int J Mol Sci.
Published online Aug Calabia , 2 Charles U. Ugwu , 1 and Seiichi Aiba 2. Find articles by Yutaka Tokiwa. Buenaventurada P. Find articles by Buenaventurada P. Charles U. Find articles by Charles U. Find articles by Seiichi Aiba. Author information Article notes Copyright and License information Disclaimer. This article has been cited by other articles in PMC. Abstract Plastic is a broad name given to different polymers with high molecular weight, which can be degraded by various processes.
Keywords: aliphatic polyesters, bio-based plastics, biodegradability, enzymatic degradation, microbial degradation. Introduction With the advances in technology and the increase in the global population, plastic materials have found wide applications in every aspect of life and industries. Open in a separate window. Figure 1. Bio-plastics comprised of biodegradable plastics and bio-based plastics.
Biodiversity and Occurrence of Polymer-Degrading Microorganisms Biodiversity and occurrence of polymer-degrading microorganisms vary depending on the environment, such as soil, sea, compost, activated sludge, etc. To test how different kinds of plastic bag fare in different environments, Imogen Napper at the University of Plymouth collected carrier bags with various claims about biodegradability, and put them in three different natural environments over a period of three years: buried in soil, left in the sea, and hung up in the open air.
She tested bags labelled as biodegradable, compostable, and oxo-biodegradable, as well as conventional high density polyethylene HDPE bags. The European Commission has recently recommended a ban on oxo-biodegradable plastics , because of fears that they break down into microplastics. In soil it remained intact for two years, but disintegrated when the researchers loaded it with shopping.
After nine months in the open air, all of the bags had disintegrated or were beginning to come apart, mostly breaking down into microplastics. When plastics break down in the sea they become microplastics — which Napper argues is more problematic Credit: Getty Images.
One company investigating how its own products break down in a marine environment is Novamont, producer of Mater-Bi — a starch-based plastic used in the compostable carrier bags launched by the Co-op this year.
A report released by the company and conducted in partnership with Hydra, a German marine research institute, and the University of Siena, Italy, says that the product fully biodegrades in seawater on a timescale of between four months and a year, leaving no toxic residues.
Not necessarily. These plastics might not solve our marine plastic pollution problem, but they are well suited to tackling another big environmental problem: food waste. The biggest potential area of impact for compostable plastics is in food service.
From coffee cups to sandwich packaging to takeaway containers, putting food in compostable plastics means that — in an ideal world, at least — the plastic and any food waste still stuck to it can be composted together. Farm workers in Mexico harvest white nopal, the juice from which can be used to manufacture bioplastics Credit: Getty Images. By reducing the amount of traditional plastics that contaminate food waste, we can at least ensure that some of that wasted food is eventually used as compost, rather than ending up in landfill or incineration.
But since , a new European biodegradability standard for these mulches means that farmers can buy plastic that they can plough back into the field safe in the knowledge that it will break down and not harm the soil. Industry, too, is beginning to use bio-lubricants used to keep machines running smoothly rather than fossil fuel-based ones. If they spill, and all machine oils eventually spill, then they will not damage the environment.
0コメント