They cleave the chain into smaller fragments hydroxy acids , either monomers or oligomers, used as sources of carbon and energy  -. The chains must be previously cleaved by extracellular depolymerases, then soluble monomers and oligomers are introduced and metabolized within the cells.
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Unlike the aquatic environment, the soil environment makes it difficult to transport the enzymes secreted by microorganisms over long distances to the substrate [ 59 ]. Biodegradation under aerobic conditions results mostly CO 2 , H 2 O and biomass, whereas, in anaerobic conditions, it results mainly, in addition to the above components, CH 4 methane. PHB is abiotically degraded by hydrolysis, with random cleavage of the ester linkages, especially at high temperatures above o C.
It is also biotically degraded by many genera of bacteria, archaea and fungi, with biofilm formation. The rate of biodegradation with PHA depolymerase is 10 2 3 faster than that of hydrolytic degradation.
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The degradation of the amorphous regions is faster than that of the crystalline regions. In the PHBV copolymer, the crystallinity maintained constant, the addition of 3-hydroxy-valerate decreases the hydrolysis rate, as well as the enzymatic degradation rate. PCL is a biodegradable polyester obtained from raw materials originating from petroleum, through ring opening polymerization of the lactone with suitable catalysts Figure 2.
It has good resistance to water and organic solvents. PCL is a polymer stable against abiotic hydrolysis, which occurs slowly with molecular weight decrease. Its melting temperature is low, as its viscosity, facilitating its thermal processing. PCL may present spherulitic structure.
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It is a soft and flexible polymer, that may be used in blends with other biodegradable polymers, such as starch. A major global manufacturer is Solvay Capa, 5, t per year. Some applications are foamed food trays, bags, bioabsorbable medical items, replacement of gypsum in the treatment of broken bones, etc. PCL may be degraded by many microorganisms, including bacteria and fungi, that are spread by soils and water bodies [ 56 ]. However, an initial stage of abiotic hydrolysis appears to be necessary [ 61 ]. The rates of hydrolysis and biodegradation depend on molecular weight and crystallinity [ 40 ].
Pronounced biodegradation occurs with molecular weights below about 5, g-mol Abiotic and biotic degradation take place preferentially in the amorphous phase. Enzymes from the two major classes of excreted esterases - lipases and cutinases - are able to degrade PCL and its blends [ 62 ].
Biodegradation causes surface erosion, without reduction of molecular weight [ 54 ]. PLA is an aliphatic polyester, derived from renewable resources, e. It is a polymer produced from lactic acid Figure 2 , which is obtained from the fermentation of various carbohydrate species: glucose, maltose and dextrose from corn or potato starch; sucrose from beet or sugar cane; and lactose from cheese whey [ 63 ].
The lactic acid monomer may be obtained by fermenting carbohydrate crops such as corn, sugar cane, cassava, wheat and barley, being eventually converted to lactide by means of a combined process of oligomerization and cyclization, with the use of catalysts.
Mitsui used a solvent based process to remove water azeotropically in the condensation polymerization process. Neste has obtained high molecular weight PLLA i. All the others use the dimer lactide process, with lactide ring opening polymerization. In the process using lactides, the additional step of dimerization of lactic acid increases production costs, but improves the control of molecular weight and end groups of the final polymer [ 38 ].
Through the stereochemical control of lactic acid ratio of D- and L- optical isomers , one can vary the crystallinity of PLA and also rate of crystallization, transparency, physical properties and even the biodegradation rate. DL-PLA is used when it is important to have a homogeneous dispersion of the active species in the single-phase matrix, such as in devices for controlled release of drugs in the same manner that PLAGA copolymers.
L-PLA is preferred for applications where mechanical strength and toughness are required, such as in sutures and orthopedic appliances. The mechanical properties are somewhat higher than those of polyolefins in general. PLA is a hard material, similar in hardness to acrylics as methyl methacrylate.
Because of its hardness, PLA fractures along the edges, resulting in a product that cannot be used. To overcome these limitations, PLA must be compounded with other materials to adjust the hardness [ 65 ]. The low glass transition temperature see Table 2 is the reason for the limited resistance of PLA to heat, making PLA inadequate for hot drink cups, for example. PLA is suitable for frozen food or for packages stored at ambient temperatures. It is a polymer with consolidated use in the medical area, due to its biocompatibility and biodegradability in the human body.
PLA-based resins may be modified to adapt to many applications, from disposable food-service items to sheet extrusion, and coating for paper [ 40 ]. The abiotic degradation of PLA takes place in two stages: a diffusion of water through the amorphous phase, degrading that phase; and b hydrolysis of crystalline domains, from the surface to the center [ 61 ]. The ester linkages are broken randomly. A semicrystalline material such as poly L-lactate presents a hydrolysis rate much lower than that from an amorphous material, such as poly D,L-lactate , with half-lives of, respectively, one or a few years, and a few weeks.
The hydrolysis is self-catalyzed by the acidity of the resulting carboxylic groups [ 66 ]. PLA can not directly be degraded by microorganisms, but requires first abiotic hydrolytic degradation, so that the microorganisms mainly bacteria and fungi, which form biofilm can metabolize the lactic acid and its oligomers dissolved in water. Abiotic hydrolysis takes place at temperatures above the glass transition temperature, i. Thus PLA is fully biodegradable in composting conditions of municipal waste plants, although it may need a few months to several years to be degraded under conditions of home composting, soil or oceans [ 35 , 63 , 67 ].
Furthermore, the PLA degrading microorganisms are not widespread in the environment [ 20 , 61 , 67 ].
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The polymer passes the tests of compostability, provided that the thickness of the parts do not exceed around mm. The extracellular enzymatic degradation consists of two steps: a the enzyme is adsorbed on the polymer surface, through its binding site; and b ester bonds are cleaved through the catalytic site of the enzyme [ 61 ].
The polymer chain ends are attacked preferentially. The biodegradation rate is a function of the crystallinity and the content of L-monomers [ 68 ]. Some enzymes proteases that may degrade PLA are proteinase K, pronase and bromelain. It is the simplest linear polyester, consisting only of a methylenic group between the ester linkages.
It may be synthesized in a way quite similar to that of PLA, by the ring opening polymerization of glycolide, that is the cyclic dimer of glycolic acid. Glycolate is copolymerized with lactate in order to obtain a copolymer with appropriate stiffness and elongation capacity known as PLAGA. The biodegradation of the PGA is usually faster than that of PLA, although an initial stage of abiotic hydrolysis appears to be necessary, since the polymer has a phase in the crystalline state and another in the amorphous glassy state [ 61 ].
PGA and its copolymers with lactic acid have very important medical applications: body absorbable sutures; ligaments reestablishment, through resorbable plates and screws; drugs of controlled release; grafting of arteries; etc. Although the homopolymers PGA and D-PLA are not biodegradable, copolymers of glycolic acid and D-lactic acid, which may still contain L-lactic acid, are usually biodegradable by lipase enzymes [ 67 ]. The degradation of PGA seems to follow the same steps of PLA: diffusion of water into the amorphous region, with degradation and erosion; hydrolytic attack of the crystalline region; and biodegradation of monomers and oligomers dissolved in water.
The aliphatic-aromatic polyesters have petrochemical origin, and are generally produced through traditional polycondensation reactions. They consist of aliphatic chain segments residues of 1,4-butanediol, and of adipic or succinic acid , which provide flexibility, toughness, and biodegradability and aromatic segments residues of terephthalic acid and 1,4-butanediol , which impart mechanical strength and rigidity. PET, an aromatic polyester, decomposes very slowly in recalcitrant aromatic oligomers [ 69 ].
The degradation of the aliphatic-aromatic polymers may be oxidative, hydrolytic and enzymatic. The hydrolytic degradation occurs in presence of water, and is self-catalyzed by the acidity of the carboxylic acids, being more intense inside the part. The enzymatic hydrolysis uses non-specific enzymes, such as hydrolases and lipases, produced by an enormous variety of organisms, especially the mycelium-forming microorganisms fungi and actinomycetes [ 69 ].
The amorphous regions are degraded preferentially over the crystalline regions, both chemically and biologically.
Interestingly, there is an inverse relationship between the melting temperature of the polyester and its rate of biodegradation, indicating that the crystalline characteristics are a very important factor in its biodegradability [ 69 , 70 ]. The polyesters that behave like elastomers at the degradation temperature, undergo enzymatic degradation from the moment in which they are placed in the disposal environment, showing surface erosion.
The polyesters that behave like glass at the degradation temperature are enzymatically degraded only at the end of the degradation process, from the by-products of the preliminary abiotic degradation.
Although the aliphatic-aromatic polyesters present high degradability in industrial composting, their rate of degradation in soil and water bodies is much lower, and their degradability under anaerobic conditions is even lower [ 69 ]. Some applications are the same typical for LDPE: transparent blown films, mulch films for agriculture, films for package and bags, and also blown bottles, filaments, injection moulded and thermoformed products [ 69 ].
Poly butylene adipate-co-terephthalate PBAT : Some of its main applications are films mulch, containers, bags , filaments, thermoformed and injection moulded products, and blown bottles. It may also be mixed with thermoplastic starch [ 69 , 71 ]. Poly butylene succinate-co-terephthalate PBST : Some of its main applications are blown films, filaments, blown and injection moulded containers, thermoformable cups and trays, paper coating, etc.
PBST has good mechanical properties, reasonable processing and biodegrades slowly [ 69 , 72 ]. There is not a standard test of biodegradability of universal validity. For the hydro-biodegradable and inherently biodegradable polymers, it is common the use of patterns for testing compostability. These are standards for biodegradation in the special conditions found in industrial composting, that require short timescales and rapid CO 2 emissions.
There are also standards for biodegradation in soils and aquatic environments but they are less used. Finally, the third group of biodegradable polymers consists of oxo-biodegradable materials, that is, those that need to undergo the chemical process of oxidation combination with oxygen, which leads to breakage of the molecules before they become biodegradable. In general, all the traditional widely used plastic materials are oxo-biodegradable, that is, over time undergo oxidative degradation, what leads to the breakdown of their molecules into smaller fragments, highly oxygenated, capable of being biodegraded.
However the time scale to complete degradation and biodegradation is too long: it takes several decades [ 25 , 26 , 73 ].
To accelerate this process, organic salts such as stearates of transition metals such as iron, manganese and cobalt are added, so reducing the time required for degradation and biodegradation to some years [ 74 ]. Such additives are known as pro-oxidants or pro-degrading. Until now, these salts have shown no toxicity to animals, plants or microorganisms, being rather micronutrients to them. To this group belong lignin, lignocellulose, natural rubber and polybutadiene without the need of pro-oxidant additives as well as traditional plastics, such as polyethylene, polypropylene, polystyrene and PET, all these formulated with pro-oxidant additives.
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