New Age Biodegradable and Compostable Plastics

 

Rummi Devi Saini

Department of Chemistry, SMDRSD College, Pathankot 145001, India.

*CorrespondingAuthorE-mail:rummisaini@yahoo.com

Recent trends in biodegradable polymers call attention to significant improvements as far as unique design strategies to offer advanced plastics with equally good execution. Though there are a few deficiencies in terms of either technology or cost of production particularly on account of utilizations in environmental pollution. Hence, there is a need to have a new perspective on the design, properties and utilization of these polymers with a vision to develop new strategies for future developments. The paper reviews the present state of biodegradable plastics and discusses the salient features of the design and properties of biodegradable polymers.Exceptional stress is given to the issues and prospects of (I) approaches grasped to make non-biodegradable engineered polymers, for instance, polyethylene, biodegradable and (ii) biodegradable polymers and copolymers created utilizing renewable resources especially poly(lactic acid) based polymers and copolymers which are developing as the desired biodegradable materials for future.

KEY WORDS:

 

 

 

INTRODUCTION:

The driving force behind the development of biodegradable polymers is the disposal of synthetic plastics, as they are resistant to degradation. With the development of biodegradable materials, the industry must discover their unique applications. Material utilization and final method of biodegradation are dependent on the composition and procedure used for their preparation.An integrated waste management system is also required in order to efficiently use, recycle, and dispose of biopolymer materials ¹. In addition to developing biodegradable plastics, other factors such as reducing the consumption of sources, reuse of existing materials and recycling of discarded materials must also be considered.

 

Polymer materials are solid, non-metallic compounds with high molecular weights. They are made of repeating macromolecules, and have different characteristics depending upon their composition. Every macromolecule which is repeated in a polymeric material is known as a mer unit. A single mer is known as a monomer, while repeating mer units are called as polymers. A diversity of materials both renewable and non-renewable is utilized as feedstock resources for current plastic materials. Non-biodegradable plastics that are prepared from non-renewable feedstocks are normally petroleum-based, and fortified by glass or carbon². Renewable source feedstock comprises microbial-developed polymers and those obtained from starch and its derivatives. It is probably strengthened with materials such as natural fibres, from plants for example flax, jute, hemp, and other cellulose ³.

 

As biodegradable plastics can be formed from inexhaustible feedstock, thus they consequently they diminish greenhouse gas emissions and are considered as environmentally-friendly. For instance, polyhydroxyalkanoates (PHA) and lactic acid (raw materials for polylactic acid, PLA) might be obtained by fermentative biotechnological procedures by means of agricultural products and microorganisms.^4-6 With the utilization of biodegradable plastics, there is low aggregation of bulky plastic materials in the environment which in turn reduces the cost of waste management. Besides, biodegradable plastics may be reprocessed to useful metabolites by microorganisms and enzymes. Biological processes may also be used for the degradation of some petroleum based plastics. Aliphatic polyester, for example, PCL and PBS are believed to encounter degradation with microorganisms and enzymes.^7-9 Studies have demonstrated that aliphatic polycarbonates have some level of biodegradability.¹⁰

 

DEFINITION OF BIODEGRADABLE PLASTICS:

Biodegradable plastics have been characterized by The American Society for Testing of Materials (ASTM) and the International Standards Organization (ISO) as those plastics which experience a critical change in their substance structure under particular environmental conditions.¹¹ Because of these progressions they have lost some of the physical and mechanical properties. Naturally occurring microorganisms such as bacteria, fungi and algae usually degrade biodegradable plastics into smaller and safe substances. Biodegradable plastics can be categorized as oxidatively degradable, photodegradable, hydrolytically degradable, or those which might be composted. Ordinarily, the adherence of microorganisms on the surface of plastics followed by the control of the uncovered surface is the primary procedure occurring in the microbial breakdown of plastics. The enzymatic degradation of plastics by hydrolysis happens by a two-step process. In the initial step, the enzyme binds to the polymer substrate and then catalyses a hydrolytic cleavage of the polymer. The polymers breakdown into low atomic weight oligomers, dimers and monomers and then finally to CO₂ and H₂O. The degradation capability of different microorganisms towards a polymer is normally evaluated by utilizing a clear zone methodology with agar plates. Agar plates having mixed polymers are infused with microorganisms. These polymer-degrading microorganisms discharge extracellular enzymesthat diffuse through the agar and after that breakdown the polymer into water solvent substances. With the help of this procedure, it was found that polypropiolactone (PPL), poly (hydroxybutyrate) (PHB) and Polycaprolactone (PCL) degraders are widely distributed in a variety of environments.^12-14 Most of the strains which can degrade PHB belong to different classes, such as Gram-negative and Gram-positive bacteria, Streptomyces and fungi.¹³ Nearly 39 bacterial strains of the classes Proteobacteria and Firmicutes have been found to degrade PHB, PCL, and PBS, but not PLA¹⁴. Just a few microorganisms have been separated and recognized which can degrade PLA. In the different environments the number of aliphatic polymer-degrading microorganisms has been observed to be in the following order: PHB = PCL > PBS > PLA

 

STRUCTURE:

Biodegradable polymers involve ester, amide, or ether bonds. The biodegradable polymers can be divided into two broad groups on the basis of their structure and mode of synthesis. One of these groups is agro-polymers, i.e. those obtained from biomass. The other comprises of bio polyesters, which are obtained from microorganisms or artificially produced using either naturally or synthetic monomers.¹⁶

 

BIODEGRADABLE POLYMERSORGANIZATIONBASED ON STRUCTURE AND OCCURRENCE:

Examples of agro-polymers are polysaccharides, e.g. starches in potatoes or wood, and proteins, and animal based whey or gluten obtained from plants. Polysaccharides comprise of glycosidic bonds, which take a hemiacetal of a saccharide and bind it to a alcohol along with dehydration. Proteins are consisting of amino acids, which have various functional groups. These amino acids come together again through condensation reactions to form peptide bonds, which are consisting of amide functional groups. Examples of bio polyesters include polyhydroxybutyrate and polylactic acid ¹⁷.

 

SYNTHESIS:

Polyesters are the most prevalent and most studied class among biodegradable polymers. Various methods used for synthesis of polyesters include direct condensation of alcohols with acids, ring opening polymerisation (ROP), and metal-catalyzed polymerization reactions. A major weakness in the step wise polymerization by condensation of an acid and an alcohol is the need to remove water from the system in order to drive the equilibrium reaction in the forward direction. This can prompt harsh reaction conditions with prolonged reaction time, bringing about a wide dispersity. A large variety of starting materials can be used to synthesize polyesters, and nature of every monomer type provides the polymer chain with diverse characteristic properties. The ROP of cyclic dimeric glycolic or lactic acid provide α-hydroxy acids those then polymerize to form poly-(α-esters). Polymerization of polyesters can be initiated by a number of organo metallic initiators comprising tin, zinc, and aluminium complexes. The most widely accepted is tin (II) octanoate and has been acknowledged as a food additive by the U.S. FDA, yet there are still a few reservations about utilizing the tin catalysts in the synthesis of biodegradable polymers for biomedical purposes. The synthesis of poly (β-esters) and poly (γ-esters) can be done by comparable ROP or condensation procedures as with poly (γ-esters). Advancement of metal free process that involves the utilization of bacterial or enzymatic catalysis in polyester arrangement is likewise being investigated upon.¹⁸ These reactions have the advantage of usually being regioselective and stereospecific however endure disadvantage because of the high costs of bacteria and enzymes, long response times, and formation of low molecular weight products.

 

 

Ways of formation of polyester utilizing lactic acid.

i) Condensation of lactic acid into dimeric lactide followed by ring-opening polymerization to form polylactic acid.

 

ii) Direct condensation of lactic acid, signifying the need to consistently expel water from the system in order to drive the reaction in the forward direction.

 

Other than polyesters, other classes of polymers are also of interest. Polyanhydrides are dynamic area of research in drug delivery because they only degrade from the surface and so are able to release the drug they carry at a constant rate. Polyanhydrides may be prepared by a number of methods which are also used in the preparation of other polymers, involving condensation, dehydrochlorination, dehydrative coupling, and ROP. Polyurethanes and polyester amides are used as biomaterials. Polyurethanes were initially exploited for their biocompatibility, durability, resilience, but now being examined for their biodegradability. Polyurethanes are normally synthesized using a diisocyanate, a diol, and a polymer chain extender. The initial reaction is carried out between the diisocyanate and the diol, with the diisocyanate in excess to ensure that the ends of the new polymer chain are isocyanate groups. This polymer then becomes capable to be treated with either a diol or a diamine to give urethane or urethane-urea end groups, respectively. The properties of the polymer are influenced by the choice of terminal groups. Moreover, the use of vegetable oil and biomass in the development of polyurethanes and in addition the transformation of polyurethanes to polyols, is a dynamic area of research.¹⁹

 

 

Synthesis of polyurethane from diisocyanate and a diol.

To cap this polymer, chain extenders of either diols or diamines can be added in order to modify the properties.

 

BIODEGRADATION METHODS:

The breakdown of polymer materials can take place by microbial action, photo degradation, or chemical degradation. As the end products of degradation of all these three methods are stable and are found in nature so they are categorized under biodegradation. Numerous biopolymers may be decomposed when dumped in landfills, composts, or soil. The materials will break down, only if the requisite microorganisms are available. Ordinary soil bacteria and water are usually sufficient for the microbial degraded plastics.¹² Polymers which depend on naturally grown resources such as starch or flax fibre are susceptible to degradation by microorganisms. The material might or might not break down more quickly under aerobic conditions, subject to the conditions utilized, and the microorganisms required. In the case of materials where starch is added as an additional substance to a traditional plastic matrix, the polymer is in contact with the dust and/or water is degraded by the action of microorganisms. The microbes attack the polymer, assimilate the starch, after which a porous, sponge like structure with a huge interfacial area, and lower structural strength is left behind. When the starch constituent has been exhausted, the polymer lattice starts to get degraded by an enzymatic attack. Every reaction brings about scission of a molecule, progressively diminishing the weight of the polymer matrix until the entire material has been digested. Another way to deal with microbial degradation of biopolymers includes developing of microorganisms for the particular function of processing polymer materials.¹³ This is a more demanding process that in the long run costs more, and avoids the utilization of inexhaustible resources as biopolymer feedstock. The microorganisms required are designed so as to attack and breakdown petroleum based plastics. In spite of the fact that this method diminishes the quantity of waste, it does not help in the conservation of non-renewable resources. Photodegradable polymers experience degradation by the action of sunlight. In several cases, polymers are broken down to small pieces when attacked photo chemically.¹⁴ Microbial degradation must occur later in order to achieve true biodegradation. The polymers found to be most susceptible to photo degradation are polyolefin, a type of petroleum-based conventional plastic. Projected approach for further development of photodegradable biopolymers comprises addition of substances which increase photochemical reactions e.g. addition of benzophenone amends the composition of the polymers to contain more UV absorbing groups (e.g. carbonyl), and synthesizing new polymers with groups which are light sensitive to light. The biopolymers those both and photo degradation and microbial breakdown , find their applications in the use of disposable mulches and crop frost covers. Some biodegradable polymer materials are found to dissolve rapidly in particular chemical aqueous solutions. As specified before, Environmental Polymer’s product ‘Depart’ is soluble in hot water.¹⁵ once the polymer is dissolved, the solution contains polyvinyl alcohol and glycerol. Like several photodegradable plastics, the total biodegradation of their aqueous solution takes place later, through microbial digestion. The requisite microorganisms are easily found in wastewater treatment plants. Procter & Gamble has developed a product named Nodax PBHB, which is similar to Depart. As Nodax is soluble in alkaline solution, so when it is in contact with a solution having high PH, the material undergoes a fast structural breakdown. Biopolymer materials which break down on in aqueous solutions are appropriate to be used for the disposal and transport of biohazards and medical wastes. Industrial washing machines are designed so as to dissolve and wash away the aqueous solutions to support microbial degradation

 

Factors on which Biodegradability of Plastics Depends:

The biodegradability of plastics depends upon their properties. The mechanism of biodegradation is affected by both the physical and chemical properties of plastics. The properties such as surface area, hydrophilic and hydrophobic character, the chemical structure, molecular weight, glass transition temperature and melting point, elasticity and crystal structure of polymers play important role in the biodegradation processes.

 

Usually, polyesters with side chains experience degradation less easily than those without side chains ⁷. Molecular weight plays a significant role in determining the biodegradability of polymers as it determines many physical properties of the polymers. In general, biodegradability the polymer decreases withincreasing the molecular weight of the polymer. Besides, the rates of biodegradation of polymers are also greatly affected by the morphology of polymers. The degree of crystallinity is also a key factor affecting the biodegradability of polymers because the action of enzymes mostly occurs at the amorphous areas of a polymer. This is because the molecules in the amorphous part of polymer are loosely packed and make it more susceptible to degradation. However, the crystalline part of the polymer which has closer packing of the molecules is more resistant to degradation than the amorphous region. The studies have shown that the rate of degradation of PLA decreases with an increase in crystallinity of the polymer. ^{21, 22} The melting temperature (Tm) of polymers also has a large effect on the enzymatic degradation of polymers. The higher the melting point of the polymer, the lower is the biodegradation of the polymer. ^{20, 23, 24}

 

Tm = ΔH/ΔS

 

Where ΔH is the enthalpy change on melting and ΔS is the entropy change on melting.

 

The aliphatic polyesters [have ester bond (-CO-O-)] and polycarbonates [have carbonate bond (-O-CO-O-)] are the two plastic polymers which show high prospective to be usedas biodegradable plastics, due to their susceptibilities to lipolytic enzymes and microbial degradation. Whereas aliphatic polyurethane and polyamides (nylon) are less prone to biodegradation as compared to aliphatic polyesters and polycarbonates, as they have higher Tm values which results from their large ΔH values because of the presence of hydrogen bonds in the polymer chains, the presence of the urethane bond (-NH-CO-O-) and the amide bond (-NH-CO-) in polyurethane and polyamides (nylon) respectively.

 

The rigidity of the polymer molecule increases due to the presence of an aromatic ring which results in the small ΔS value so polymer has the high Tm and hence low biodegradability of aromatic polyester.

 

ALIPHATIC POLYESTERS FROM AGRO-RESOURCES:

Poly (3-Hydroxybutyrate) (PHB):

PHB, [-O(CH3)CHCH2CO-]_n is a natural polymer formed by several bacteria as a resources to storecarbon and energy. This polymer has attracted interest worldwide because it can be produced from renewable low-cost feedstocks and the process of polymerizations can be performed under mildconditions without causing muchimpact on environment. Additionally, it experiences biodegradation in both aerobic and anaerobic conditions, without forming any harmful degradation products. A few aerobic and anaerobic PHB-degrading microorganisms have been segregated from soil, for example, Pseudomonas lemoigne, Acidovorax faecalis, Aspergillus fumigatus, Comamonas sp. and Variovorax paradoxus, from activated and anaerobic slime, for example, Illyobacter delafieldi, Alcaligenes faecalis, Pseudomonas, and from seawater and lake water, for example, Pseudomonas stutzeri, Comamonas testosterone.^33,34 The level of PHB-degrading microorganisms in the environment has been found to be 0.5-9.6% .¹⁰ Majority of the PHB-degrading microorganisms has been found to be equipped for degrading PHB at moderate temperatures and just a couple of them are fit for degrading PHB at higher temperature.

 

Tokiwa et al. proposed that as composting at high temperature is the most encouraging innovation for recycling biodegradable plastics so the thermophilic microorganisms which can degrade polymers are important in the composting process.³⁵ A thermophilic Streptomyces sp. isolated from soil can degrade PHB, PES, PBS and poly[oligo(tetramethylene succinate)-co-(tetramethylenecarbonate)]. A thermotolerant Aspergillus sp. has been found to degrade 90% of PHB film after five days cultivation at 50 °C. ²⁵

 

Polylactic Acid (PLA): PLA, [-O(CH3)CHCO-]_n Polylactic acid is a linear aliphatic polyester which is a biodegradable and biocompatible thermoplastic that can be produced by fermentation from renewable resources. It can likewise be formed by condensation polymerization of lactic acid or from lactide by its ring opening polymerization in the presence of a catalyst. The production of PLA from lactic acid was established by Carothers in 1932.³⁶ Lactic acid is also formed via starch fermentation, as a co-product of corn wet milling. The ester linkages present in PLA are sensitive to chemical hydrolysis as well as enzymatic chain cleavage. PLA is commonly mixed with starch to increase its biodegradability and make it cost effective.

 

In any case, the starch-PLA mix has to some degree of fragility which represents a noteworthy disadvantage in a large number of its applications./ This drawback can be overcome by utilizing a variety of low molecular weight plasticisers, for example, sorbitol, glycerol and triethyl citrate. The PLA-degraders have not been observed to be extensively dispersed as shown by biological investigations on the populations in PLA-degrading microorganisms in various conditions and in this way PLA is less inclined to microbial attack with respect to other microbial and manufactured aliphatic polymers.^{22, 23, 31} However a few strains of genus Saccharotrix and Amycolatopsis are able to degrade PLA.

 

Williams ³⁷ examined the enzymatic degradation of PLA by bromelain, proteinase K and pronase enzymes. Proteinase K from Tritirachium collection has been observed to be the best for PLA degradation, among these enzymes. Numerous esterase-type enzymes, particularly Rhizopus delemar lipase have been observed to accelerate the degradation of PLA oligomers by Fukuzaki et al. ³⁸

 

PLA are mostly utilized as thermoformed substances, for example,containers, drink mugs, take-away food plate and planter boxes. Due to its good rigidity characteristics, the material has potential to replace polystyrene and PET in some of their applications.

 

 

Polyhydroxyalkanoates (PHA) Polyesters:

Aliphatic polyesters, Polyhydroxyalkanoates (PHAs) are obtained naturally by means of a microbial procedure on sugar-based medium and act as carbon and energy storing material in microscopic organisms. Hence PHAs are a group of intracellular biopolymers made by numerous microscopic organisms as intracellular carbon and energy storing granules with the polymer aggregating in the microorganisms' cells in the course of development. PHAs are largely manufactured from renewable resources by fermentation.^{39, 40} They were the principal biodegradable polyesters to be utilized as a part of plastics. The polyhydroxybutyrate (PHB) and polyhydroxyvalerate (PHV) are the two key individuals from the PHA family. Aliphatic polyesters, for example, PHAs and homopolymers and copolymers of hydroxybutyric acid and hydroxyvaleric acid, have been found to be rapidly biodegradable. PHA can be degraded by basic hydrolysis of the ester bond even without enzymes to catalyze the hydrolysis i.e. by abiotic degradation. /However, the enzymes if introduced degrade the remaining products till complete mineralization, in the course of the biodegradation procedure. Many organizations make bacterial PHA. For instance, PHB Industry in Brazil produces PHB and PHBV with 45 % crystallinity, from sugar stick molasses ⁴² followed by many big companies for example, P&G, DSM and so on. PHAs are considered as biodegradable and thus reasonable for utilizing as bundling material. PHAs are additionally considered as biocompatible and accordingly can be utilized for biomedical applications, for example, sedate exemplification, tissue building and so on. The generation of PHA can possibly supplant engineered non-degradable polymers in different applications⁴³: bundling, agribusiness, recreation, fast-food, cleanliness and in addition medication and biomedical because of its biocompatible nature. ^{41, 44}

 

ALIPHATIC POLYESTERS FROM FOSSIL RESOURCES:

PBS (Polybutylenesuccinate) and PES (Polyethylenesuccinate) Polyesters:

PBS, [-O (CH₂)₄OOC(CH₂)2CO-]n and PES, [-O(CH₂)₂OOC(CH₂)₂CO-]n are synthetic aliphaticpolyesters having high melting points of 112-114 °C and 103-106°C, respectively. They may be prepared by the reaction of dicarboxylic acids for example, adipic and succinic acid with glycols for example ethylene glycol and 1,4-butanediol.²⁹ PBS is biodegradable and biodegrade by a hydrolysis mechanism. Hydrolysis takes place at the ester linkages leading to the formation of low polymer which get further degraded by micro-organisms due to their lower molecular weight. A leading manufacturer of PBS polymers, SK Chemicals (Korea), has reported that a 40 micron thick film of PBS when kept in the garden soil for one month, undergoes 50% degradation. Although PBS degrading microorganisms are found to be broadly distributed in the environment, but they are present in the lower ratio to the total microorganisms than found for PCL-degrading microorganisms. The Amycolatopsissp. HT-6 are found to breakdown PBS, PHB and PCL.³⁰Microbisporarosea, Excellospora japonica and E. Viridilutea have been observed to form a clear zone on agar plates holding emulsified PBS. M. Rosea is capable of degrading 50% of PBS film after cultivation for eight days in liquid medium. ³¹

 

Various PES-degrading microorganisms were separated from soil and aquatic environments and have been recognized to associated with the genera Bacillus and Paenibacillus. Among the separates, strain KT102 which belongs to Bacillus pumilus could degrade PES film at the highest rate. This strain can degrade PES, PCL but not PHB, PBS and PLA ³⁵. Some fungi separated from a variety of ecosystems formed clear zones around the colony on agar plates containing PES.

 

PBS has mechanical properties as good as polypropylene and low-density polyethylene and hence can be valuable to a range of applications by traditional melt processing techniques. PBS is generally mixed with other substances, for example, adipate copolymers and starch to make its applications cost-effective. Some PBS and PBS-A biodegradable plastics are also commercially available. These polyesters might be utilized as mulch film, bags, packaging film, and flushable hygiene products.

 

Polycaprolactone:

(PCL): Polycaprolactone (PCL) [-OCH₂CH₂CH₂CH₂CH₂CO-]n is a biodegradable synthetic aliphatic polyester made by the ring-opening polymerization of caprolactone using metal alkoxides, for example, aluminum isopropoxide, tin octoate. Melting point of PCL is low, between 58-60°C. It has low viscosity and is easy to process. ^{18, 19, 43}

 

PCL has been observed to be degraded by the action of aerobic and anaerobic microorganisms which are broadly distributed in various environments. The PCL was found to be nearly totally degraded in 12 days, when the degradation of high molecular weight PCL was studied using Penicilliumsp. strain 26-1 (ATCC 36507) isolated from soil. This strain has likewise been seen to degrade unsaturated aliphatic and alicyclic polyesters however it doesn't degrade aromatic polyesters.⁷ PCL has been observed to be totally degraded by thermo tolerant PCL-degrading microorganism recognized as, Aspergillus sp. strain ST-01, segregated from soil following 6 days incubation at 50 °C. ²⁵

 

PCL can likewise be degraded by chemicals, for example, esterases and lipases.⁹ The rate of degradation of PCL relies upon its molecular weight and level of crystallinity. PCL undergoes enzymatic degradation faster in the amorphous region by Aspergillus flavusand Penicilliumfuniculosum.²⁶ The biodegradability of PCL might be enhanced by copolymerization with aliphatic polyesters since copolymers have been found to have lower Tm and lower crystallinity when compared with homopolymers, and thus are more inclined to degradation.^{27, 28} Tokiwa et al. have examined the hydrolysis of PCL and biodegradation by fungi.⁴⁵ They have revealed that PCL can be readily degraded by enzymes. The marine biodegradation of PCL has been examined by Janiket. al. (1988) and they have observed that the PCL in seawater was totally degraded in two months, though in salt water it had lost just 20% of its weight. Subsequently it demonstrates that the enzymes in the seawater support to accelerate the biodegradation of PCL and other biodegradable plastics.

 

PCL is broadly utilized as a PVC strong plasticizer or as polyols for polyurethane applications. It additionally finds a few applications because of its biodegradable character in areas, for example, biomedicine, for instance controlled release of medications and clean environment, for instance delicate compostable pakaging materials.

 

ALIPHATIC-AROMATIC COPOLYESTERS (AAC):

Aliphatic-aromatic (AAC) copolyesters have the combined benefit of biodegradable properties of aliphatic polyesters with the strength and execution properties of aromatic polyesters. This category of biodegradable plastics give totally biodegradable plastics having properties as good as to those of commonly used polymers, for example, polyethylene. TPS is regularly mixed with AACs to reduce their cost. AAC utilize nearly same starting materials as for plastics and fossil fuels.

 

In spite of the fact that AACs are formed from non-renewable energy sources, they are biodegradable and compostable. ACCs can totally biodegrade to carbon dioxide, water and biomass. In an active microbial environment the polymer becomes invisible to the naked eye usually, within 12 weeks. Besides the characteristic biodegradability of the polymer itself, the degree and speed of biodegradation rely upon a number of environmental factors, for example, temperature, surface area, moisture and the methods of its formation of the formation.

 

 AAC comprising of PCL and aromatic polyester, for example, poly(ethylene terephthalate) (PET), Poly (butylene terephthalate) (PBT), and poly(ethylene isophthalate) (PEIP) have been found to be hydrolysed by R. delemar lipase. ²³ The vulnerability to hydrolysis of these AAC's by R.delemar lipase decrease rapidly with increase in aromatic polyester content. The rigid nature of the aromatic ring in the AAC chains was accepted to impact their biodegradability with the lipase. Another synthetic AAC containing adipic acid and terephthalic acid can likewise be attacked by microorganisms, e.g. Thermobifidafusca as reported by Kleeberget al. ³⁸

 

The two primary kinds of commercial AAC plastics are Ecoflex™ created by BASF and Eastar Bio™ made by Eastman. Different grades of plastic have been designed to coordinate its particular application by controlling its chain branching and chain lengthening. AACs have almost all characteristic properties which are required for the cling film such as transparency, flexibility and anti-fogging performance thus this material has vast prospective to be utilized in commercial food wrap for fruit and vegetables.Its compostable nature adds much more benefit to its applications.

 

BLEND PLASTICS:

Blends of Polyester with Other Polymers:

The biodegradable polymers are mixed with other polymers in order to decrease the overall cost of the material and alter the required properties and rates of degradation. Mixing is a appreciably simpler and quick approach to achieve the desirable properties relative to copolymerization process.

 

Iwamoto et al. formed blend plastics by mixing PCL with traditional plastics, for example, low density polypropylene (PP), polyethylene (LDPE), polystyrene (PS), poly(ethyleneterephthalate) (PET), PHB and nylon 6 (NY) and evaluated their enzymatic degradabilities. It was found that the higher the miscibility of PCL and traditional plastics, the more difficult is the degradation of PCL in their blends by R. arrhizuslipase.^{46, 47}

 

With biodegradable and non-biodegradable polymers and polysaccharides, a variety of blends of PHB have been prepared. The studies of enzymatic degradation of these blends utilizing PHB depolymerase from AlcaligenesfeacalisT1 demonstrated that the weight reduction of the blends decreased directly with increase in the content of PBA, PVAc or PCL. ⁴⁷

 

Koyama and Doi evaluated the different properties and biodegradability of PHB/PLA blend. The polymer blends with PHB have been found to commonly show enhanced properties and biodegradability relative to pure PHB. ⁴⁸

 

Blends of starch and polyesters:

As starch is renewable, cheaper and is available throughout the year so blending of starch with synthetic polymers gives cost and performance benefits. Studies have shown that blends of PCL and granular starch show greater degree of biodegradation.^47,49 PLA and starch both are biodegradable and are derived from renewable resources so favoured for preparing polymer blends. In their blends, PLA can control the mechanical properties of the blend while starch enhances the biodegradability and brings down the cost of the polymer.⁵⁴ Ratto et al. evaluated the properties and biodegradability of PBS/An and corn starch (5%-30% w/w) blends and established that the rigidity decreased with increase in starch content. With 20% rise in starch content, the rate of biodegradation has been seen to rise appreciably using soil burialtest.⁵⁰

 

APPLICATIONS OF BIODEGRADABLE PLASTICS:

Research and development work is just a piece of the work that is done with a specific aim to acclimate the utilization of biodegradable polymer material. The design of such materials generally starts with a theoretical application. It is required to substitute a current material, or to supplement one. Areas where applications for biopolymers have been presented include drug, packaging, agriculture, and the automotive industry.⁵¹ Many materials that have been produced and marketed are helpful in more than one of these classes. Biopolymers that may be employed in packaging continue to receive more consideration than those designated for any other application. All levels of government, predominantly in China and Germany, are endorsing the widespread use of biodegradable packaging materials in order to lessen the volume of inert materials currently being disposed in landfills, inhabiting scarce available space. The 41% of plastics are estimated to be utilized in packaging, and that almost half of that volume is employed for packaging food products. The application of biopolymers in packaging is due to its renewable and biodegradable characteristics. ⁵² The starch material is treated by an acetylation process, chemical treatments, and post-extrusion steaming. Mechanical properties of the material are adequate, and true biodegradability is attained.⁵³ The biopolymer materials suited for packaging are often used in agricultural products. Ecoflex, generally is used in both areas. A thin Ecoflex film may be used to cover Young plants which are predominantly vulnerable to frost., This film can be buried back into the soil, at the end of the growing season where it is degraded by the suitable microorganisms.⁵⁹ It is established that the yield of spring wheat rises with the use of a clear plastic mulch to cover wheat seeds immediately after seeding for less than 40 days. After that the plastic films commence to degrade in average soil conditions.^{54, 57} The medical world is constantly changing, and therefore the materials employed by it also require persistent adjustments .The biopolymers used in medical fields must be compatible with the tissue they are found in, and may or may not be anticipated to break down after a given time period.^55,58 It is reported that researchers working in tissue engineering are attempting to develop organs from polymeric materials, which are suitable for transplantation into humans. The plastics would require injections with development factors in order to persuade cell and vascular growth in the new organ. Work accomplished in this area includes the advancement of biopolymers with adhesion sites that act as cell hosts in giving shapes that resemble different organs.

 

CONCLUSION:

The sectors of automotives, agriculture, medicine, and packaging all need environment friendly plastics and polymers. Since the level of biodegradation may be tailored to specific needs, each industry is able to generate its own ideal material. The different methods of biodegradation are likewise a key benefits of such materials, since methods of degradation might be changed to industry determinations. Biodegradable plastic is an inventive method for settling the plastic disposal issue from the perspective of development of new materials. Environmental responsibility is continually expanding in significance to the both consumer and industry. For the individuals who produce biodegradable plastic materials, this is a major benefit. Biopolymers confine carbon dioxide discharges during production, and degrade to organic matter after disposal. Though synthetic plastics are a more economically attainable choice than biodegradable ones, an enlarged accessibility of biodegradable plastics will allow numerous customers to pick them based on their environmentally responsible disposal. The procedures which hold the greatest potential for further development of biopolymer materials are those which utilize renewable feedstock. Biodegradable plastics containing starch as well as cellulose fibres seem to be most likely to encounter persistent development in utilization. Microbial developed plastics are scientifically stable and an original thought, however the foundation required to economically grow their utilization is as yet extreme, and difficult to develop. Time is of the substance for biodegradable polymer development, as society's present perspectives on ecological obligation influence this a perfect time for further development of bioplastics.

 

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Received on 07.02.2018         Modified on 18.02.2018

Accepted on 26.02.2018         © AJRC All right reserved