Why cant polyalkenes biodegrade

Starch is usually obtained from corn, potato and cassava, although it may be obtained from other sources as well. From the viewpoint of the life-cycle analysis, products from plant or food wastes are highly favored. In order to prepare thermoplastic starch, the crystal structure of starch has to be destroyed, either by mechanical working, pressure or heat, or by addition of plasticizers, such as water and glycerin.

The gelation process occurs in two steps. The destructuring of starch is defined as the melting and the disorganization of the molecular structure of the starch granules and subsequent molecular dispersion in water. A problem that can also be an advantage is the high hygroscopicity and solubility in water [ 34 ]. To improve processability, mechanical properties and moisture resistance while maintaining biodegradability, starch may be mixed with aliphatic or aliphatic-aromatic polyesters, such as polycaprolactone PCL , polylactic acid PLA and poly butylene adipate-co-terephthalate PBAT , suffering complexing with these polymers [ 33 , 35 ].

The complex formed by amylose with the complexing agent is usually crystalline, characterized by an amylose single helix around the complexing polymer [ 32 ]. Amylopectin does not interact with the complexing polymer, remaining in the amorphous phase. Starch can also be blended with polyvinyl alcohol PVOH , for the production of foamed products, such as trays for food. Starch esters reinforced with natural fibers exhibit properties similar to those of polystyrene PS.

Starch is biodegraded by amylases in a huge variety of different environments. The biodegradation of starch results from the enzymatic attack of the glycosidic linkages, reducing the chain size and producing mono-, di- and oligosaccharides, easily metabolized by biochemical pathways.

Some manufacturers still mix low levels of starch with polyethylene. The rapid biodegradation of the former increases the available surface of the latter to degradation.

Possibly, some of the starch degrading microorganisms may also help with the slow biodegradation of polyethylene [ 36 ]. The starch has stabilized PE regarding thermo-oxidation, but has promoted its photo-oxidation. Among the applications for starch, are films for packaging, shopping bags, garbage bags, mulch films, disposable diapers, foams, foamed trays for food, injection moulded products, blown bottles and flasks, filaments, etc.. The foaming process involves melting or softening the polymer and blending it with a foaming agent, typically pentane or carbon dioxide.

It is used mainly for polystyrene PS. It is the main component of plants, with natural production per year estimated at 7. In wood, the cellulose fibrils are joined together by lignin, which is a resin binder. Each glycosidic unit has three hydroxyl groups, that promote strong interactions by hydrogen bonds. The spatial structure allows molecules crystallize in a horizontal plane, forming fibers. As a consequence, cellulose is sparingly soluble and not processable by thermal and mechanical action, i.

Cellulose is a rigid material, whose fibers may be used to reinforce other materials. It presents a small elongation capacity.

In order to become thermoplastic, it is necessary that about two of the three hydroxyl groups of the glycosidic units be reacted.

Cellulose is biodegraded by the extracellular cellulase enzyme complex, that is induced in most microorganisms [ 39 ]. A manufacturer of cellulose based films is Innovia NatureFlex products. Cellulose acetate is a thermoplastic derivative obtained by partial esterification of the hydroxyl groups with acetic acid or anhydride.

With an average degree of substitution of up to 2. The main applications of cellulose are: timber, furniture and fuel; textiles such as cotton; paper, membranes, and explosives. Important cellulose derivatives are cellulose acetate and cellulose acetate butyrate thermoplastic esters ; ethyl, hydroxyethyl and hydroxypropyl cellulose. Chitin and chitosan: The amount of naturally synthesized chitin is estimated at about 1 billion tons per year, produced by fungi, arthropods, molluscs and some plants.

Chitosan is the wholly or partly deacetylated chitin. Because it is biocompatible and biodegradable, it has several biomedical applications: cosmetics, personal care, diet food, treatment for thermal burns, sutures, artificial skin, control of drug delivery, etc.. Chitosan is biodegraded by enzymatic hydrolysis. The higher the degree of acetylation, the lower the crystallinity and the higher the rate of biodegradation obtained [ 38 , 41 ].

Pectin: is a polysaccharide obtained from citrus peel and remains of apples or other fruits used to obtain juice. It is a bonding material of plant cells. It is used as thickener, gel-forming agent, emulsion stabilizer, dietary fiber that is not digested by humans, etc.. Xanthan: is a polysaccharide obtained from fermentation of glucose or sucrose by the bacteria Xanthomonas. It is used as food emulsifier additive, rheological modifier thickener of oils and cosmetics with the bentonite , as a stabilizer of aqueous gels, etc.

Pullulan: is a polysaccharide obtained from the fermentation of starch by the yeast Aurobasidium pullulans. It is edible and tasteless. It is used as edible films for food packaging with high oxygen permeability, oral care products, adhesives, thickeners, stabilizers, etc.. They may bind to certain ions e. Alginates rich in G blocks bind to a larger number of ions, producing more rigid and resistant gels, whereas alginates rich in M blocks are more flexible and enable higher diffusion rates of solutes through the gel.

Alginates may be produced from natural sources brown algae , by extraction and purification by sterile filtration. But they may also be produced by fermentation of various microorganisms. Chemically modified alginates are also synthesized: by esterification with propylene oxide for beers and salad dressings , alkylation with alcohols drug delivery systems , alkylation or allylation of binder groups in order to obtain photo-crosslinkable gels.

There are several methods to fabricate globules, microcapsules, fibers, films and membranes. Alginates are susceptible to degradation: by cleavage of glycosidic linkages through hydrolysis in acidic or basic environments, by free radical oxidation, and by enzymatic degradation. Hemicellulose is a polysaccharide consisting of around monomer units of different sugars, such as xylose highest contents , mannose, galactose, rhamnose and arabinose, statistically distributed in the chain, which is branched.

As a consequence, the material is amorphous, and has low mechanical and hydrolysis resistance. It is easily hydrolyzed by many hemicellulases enzymes from bacteria and fungi. Hemicelluloses are embedded in the cell walls of plants, bond with pectin another carbohydrate to cellulose to form a network of cross-linked fibres.

Lignin is a complex and heterogeneous cross-linked polymer, containing aromatic rings, C-C bonds, phenolic hydroxyls, and ether groups, with molar mass higher than 10 4 g mol It is formed in chemical association with cellulose, giving lignocellulose, in the cell walls of plants. Thus lignin is not a polysaccharide, but a complex substance consisting of aromatic structures with alkoxy and hydrocarbon substituents that link the basic aromatic unit into a macromolecular structure through carbon-carbon and carbon-oxygen bonds.

It is not heterogeneous both in chemical composition and molar mass. Lignocellulose is strong and tough, and provides physical, chemical and biological protection to the plant. Lignin is resistant to peroxidation see oxo-biodegradable polymers , as a result of the presence of many antioxidant-active phenolic groups, which act as protective agents against abiotic peroxidation and biological attack by peroxidase enzymes [ 43 - 45 ]. It is also present in some algae. After the cellulose, it is the second most abundant organic polymer on earth, with about 50 million tons being industrially produced annually.

It is the main humus-forming component, which provides nutrients and electric charges to the soils. Humus is slowly biodegradable by oxidase and peroxidase enzymes, produced especially by fungi.

Although basidiomycetous white-rot fungi and related litter-decomposing fungi are the most efficient degraders of lignin, mixed cultures of fungi, actinomycetes, and bacteria in soil and compost can also mineralize lignin [ 46 ].

The main industrial use of lignin is still the power generation, as biofuel. A biodegradable material based on lignin, obtained as a byproduct from the manufacture of paper, mixed with vegetable fibers, is manufactured by Tecnaro under the trade name of Arboform [ 47 ].

Its mechanical properties show high rigidity and low deformability. The amide linkages are readily degraded by enzymes, particularly proteases. Soy proteins have been used for edible films and even automotive parts, but proteins have not been consolidated as a thermoplastic of worldwide use [ 48 ]. Polyamino acids with free carboxylic groups, such as polyaspartic acid and polyglutamic acid, are excellent candidates for use as water soluble biodegradable polymers [ 40 ]. In addition to thermoplastics, other possible applications of proteins are in coatings, adhesives, surfactants and gelatin capsules for pharmaceutical uses.

Natural rubber is poly 1,4-cis-isoprene , naturally synthesized by the rubber tree, Hevea brasiliensis , present in its milky sap or latex. It is also synthesized industrially by polyaddition of isoprene. It partially crystallizes when stretched. Due to the long sequences of double bonds one per monomeric unit , this rubber has a high reactivity with oxygen, undergoing the peroxidation reactions, yellowing very quickly, and being biodegraded at a relatively high rate [ 1 ].

The hydrogen atoms attached to carbon atoms at the alpha position relative to the unsaturations are more reactive, or more labile [ 21 ].

Among the main applications are tires and tubes. Polyvinyl alcohol is a biodegradable polymer obtained by partial or complete hydrolysis of polyvinyl acetate PVA, of petrochemical origin to remove acetate groups Figure 2.

The vinyl alcohol monomer almost exclusively exists as the tautomeric form acetaldehyde, which does not polymerize [ 49 ]. PVOH is the only water soluble biodegradable polymer, whose main chain consists only of carbon atoms.

Solubility and biodegradability are imparted by the hydroxyl groups, that are capable of establishing hydrogen bonds with water. The partial hydrolysis leaves acetate residues, that allow PVOH solubility in cold water, and decrease the biodegradability. Even being an atactic polymer, with non-organized space distribution of the hydroxyl groups in the main chain, PVOH shows crystallinity, because the hydroxyl groups are small enough to accommodate within the crystal, not hindering it [ 49 ].

Annual production exceeds 1 million tons. Some major applications are: thickener in paint industry; paper coating, hair sprays; shampoos; adhesives; biodegradable products for the feminine hygiene; diapers bottoms; water soluble packaging films detergents, disinfectants, scouring powder, pesticides, fertilizer, laundry, etc.

It is believed that the PVOH degrading microorganisms are not spread throughout the environment, and that they are predominantly bacteria and fungi yeasts and moulds [ 50 ]. Before the start of biodegradation, a period of acclimatization may be required.

Acclimatization natural conditions and acclimation laboratory conditions are the adjustment process of an organism or a colony to an environmental change, normally occurring in short periods of time days or weeks. The biodegradation mechanism consists of a random cleavage of 1,3-diketones, that are formed by the enzymatic oxidation of the secondary hydroxyls [ 51 ].

EVOH is a copolymer of ethylene and vinyl alcohol, obtained from ethylene and vinyl acetate, followed by hydrolysis. It is used as an oxygen barrier film in multilayer films for packaging. Its high cost limits its applications as a biodegradable material. The second group of biodegradable polymers is formed by hydro-biodegradable materials, i.

Therefore, the decomposition process of the polymer occurs in two stages: first, the molecules break up into small fragments by hydrolysis; and second, these fragments are biodegraded by microorganisms. In both stages, the presence of water is essential, both to chemically fragment the molecules, and to be consumed by microorganisms, that need much water in their cells. To this group belong the aliphatic i. PCL can also be biodegraded directly by enzymes produced by microorganisms, without the initial stage of hydrolysis [ 24 ].

Polyesters are polymers in which the bonds between the monomers occur via ester groups. There are many types of natural esters, and their degrading enzymes - the esterases - are present everywhere, together with the living organisms. The ester bonds are generally easy to hydrolyze [ 20 ].

The group of biodegradable polyesters mainly consists of: a linear aliphatic i. Polyhydroxyalkanoates PHAs are polyesters of several hydroxyalkanoates that are synthesized by many microorganisms as a carbon and energy storage material. The hydroxyalkanoates can be synthesized from natural substances such as sucrose e.

Precisely for this reason, this material is rapidly biodegraded in various environmental conditions by many different organisms.

The molecular weight generally varies from 50, to 2,, g mol The monomers are all optical isomers R, the only ones capable of being hydrolyzed by depolymerases, in the isotactic form [ 53 ].

PHAs polymers and copolymers are semicrystalline, with the molecules conformed in helices in the crystalline lamellae, which form spherulites. The native intracellular granules of PHAs with 0. PHAs are attacked by intracellular PHAs- depolymerases enzymes, but not extracellular depolymerases [ 54 , 55 ]. PHB is a biological storage material that is used by archaea, bacteria and fungi as feed source. There are more than 75 bacterial genera capable to synthesize PHAs, that are also produced by archaea, fungi, plants and animals, in the soils and aquatic bodies.

In addition to the well known 3-hydroxybutyrate, more than monomeric constituents have already been identified [ 53 , 56 ]. The PHB synthesis can occur, for example, as follows: the bacteria are inoculated in a small batch reactor, along with sucrose, other nutrients and water, pH is adjusted and the temperature is raised.

The growing colony is transferred to successively larger reactors. After the initial growth of bacteria, the competition period starts, with bacterial storage of PHB in the cytoplasm. The molecular weight increases continuously up to reactor cooling and addition of solvent, what will cause cell lysis and dissolve PHB, which is then purified and dried. The powder obtained in the extraction process is transformed into pellets in an extruder. At the same time nucleating agents and plasticizers are added to improve processability and mechanical properties.

There are also successful attempts to develop genetically modified plants to produce PHAs [ 40 ], but the products obtained are very expensive, not being accepted by the market.

Blend-stabilizing copolymers, with an intermediate chemical structure, may be obtained by separated synthesis, by transesterification or by the action of peroxides on the two components. A significant product is the copolymer poly 3-hydroxybutyrate-cohydroxy-valerate , or PHBV Figure 2 , which presents lower crystallinity and rigidity than PHB, increasing the flexibility and the elongation capacity [ 57 ].

Some applications are: tubes for seedlings, injection and blown moulded containers, and films [for example, obtained with PHBH, or poly 3-hydroxybutyrate-cohydroxy-hexanoate ]. For medical applications, the price of PHAs is already acceptable, although it is still too high for the commodity market, such as for packaging. Some important aspects to be improved in PHB are: strong degradation during processing PHB undergoes -elimination reactions, which cleave molecules and form chains with terminal unsaturation [ 60 ].

This is a consequence of the high crystallinity and the large spherulites formed, since the crystals nucleate slowly but grow fast. The PHAs may undergo simultaneously hydrolytic, oxidative and enzymatic degradation.

The PHAs degrading microorganisms are widely distributed in the environment. Just as bacteria and archaea, fungi are also excellent decomposers [ 59 ]. In addition to their high degradative potential, many fungi have remarkable capacity to expand on the substrate surface, surrounding it with their hyphae, which release extracellular enzymes close enough to achieve the substrate [ 59 ].

PHAs are biodegradable in windrow composting, soil or marine sediments. The enzymes which are involved in the degradation of PHAs are depolymerases, hydrolases which may be intra- or extracellular and endo- or exoenzymes The enzymes may be classified as intra- or extracellular according their action inside or outside the cell, and also as endo- or exoenzymes, according their action inside or at the end of the substrate molecule.

They are usually induced enzymes whose expression is repressed in the presence of other carbon sources such as glucose and organic acids. 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. 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 ]. Incineration of polymers releases a lot of heat energy, which can be used to generate electricity. However, there are problems with incineration. Carbon dioxide is produced, which adds to global warming. Toxic gases are also produced, unless the polymers are incinerated at high temperatures. Recycling polymers is a good idea, but not always practical.

Many polymers are mixed with other materials and paints or dyes and separation is difficult and expensive. Many polymers now carry labels to indicate how they should be recycled. High-Tech 24 , 10—11 Meier, M. Plant oil renewable resources as green alternatives in polymer science. Goldbach, V. Long-chain aliphatic polyesters from plant oils for injection molding, film extrusion and electrospinning. This paper reveals how plant oils can be converted by means of highly selective catalysis to produce polyesters with properties that mimic polyethylene.

Witt, T. Liu, C. Biomacromolecules 12 , — Gross, R. Enzyme-catalysis breathes new life into polyester condensation polymerizations. Trends Biotechnol. Large-ring lactones from plant oils. Pepels, M. Mimicking linear low-density polyethylenes using modified polymacrolactones.

Macromolecules 48 , — Peng, Y. Ring-opening metathesis polymerization of a naturally derived macrocyclic glycolipid. Macromolecules 46 , — Roesle, P. Synthetic polyester from algae oil. Cordier, P. Self-healing and thermoreversible rubber from supramolecular assembly. Nature , — Montarnal, D. Silica-like malleable materials from permanent organic networks.

Altuna, F. Self-healable polymer networks based on the cross-linking of epoxidised soybean oil by an aqueous citric acid solution. Chen, G. Plastics derived from biological sources: present and future: a technical and environmental review. This review provides a techno—environmental assessment of bio-based polymers and monomers. Galbis, J. Synthetic polymers from sugar-based monomers.

Bozell, J. Auras, R. An overview of polylactides as packaging materials. Inkinen, S. Abdel-Rahman, M. Recent advances in lactic acid production by microbial fermentation processes. Dusselier, M. Shape-selective zeolite catalysis for bioplastics production. Science , 78—80 Lactic acid as a platform chemical in the biobased economy: the role of chemocatalysis. Ikada, Y. Stereocomplex formation between enantiomeric poly lactides. Macromolecules 20 , — Groot, W. Life cycle assessment of the manufacture of lactide and PLA biopolymers from sugarcane in Thailand.

Life Cycle Assess. Corbion Purac successfully develops PLA resin from second generation feedstocks. Poly hydroxyalkanoates — a fifth class of physiologically important organic biopolymers. Eerhart, A. This paper describes a life-cycle assessment that compares the outputs associated with petrochemical-derived PET and biomass-derived PEF.

Burgess, S. Chain mobility, thermal, and mechanical properties of poly ethylene furanoate compared to poly ethylene terephthalate. Macromolecules 47 , — Jong, E. Delidovich, I. Alternative monomers based on lignocellulose and their use for polymer production. PubMed Google Scholar. Alternating copolymerization of epoxides and cyclic anhydrides: an improved route to aliphatic polyesters.

Longo, J. Poly propylene succinate : a new polymer stereocomplex. Hong, M. Nature Chem. Myers, D. Polymer synthesis: to react the impossible ring. Xiong, M. Scalable production of mechanically tunable block polymers from sugar. Natl Acad. USA , — Juntaro, J. Creating hierarchical structures in renewable composites by attaching bacterial cellulose onto sisal fibers.

Braun, B. Supra-molecular ecobionanocomposites based on polylactide and cellulosic nanowhiskers: synthesis and properties. Goffin, A.

From interfacial ring-opening polymerization to melt processing of cellulose nanowhisker-filled polylactide-based nanocomposites. Biodegradable polymers from renewable sources: rheological characterization of hemicellulose-based hydrogels. Biomacromolecules 6 , — Jung, Y.

High-performance green flexible electronics based on biodegradable cellulose nanofibril paper. ADS Google Scholar. Haba, O. Macromolecules 38 , — Mikami, K. Polycarbonates derived from glucose via an organocatalytic approach. Upton, B. Strategies for the conversion of lignin to high-value polymeric materials: review and perspective.

Rahimi, A. Formic-acid-induced depolymerization of oxidized lignin to aromatics. Wang, X.