Dr.Mrs.G.D.SHAH ( Ph.D. Chemical Engg. M.E. POLYMER TECH.)
Dr.Mrs.G.D.SHAH ( Ph.D. Chemical Engg. M.E. POLYMER TECH.)
I/C HEAD OF PLASTICS ENGG.DEPT.
GOVT. POLYTECHNIC, AHMEDABAD GUJARAT, INDIA.
Biodegradable polymers can be classified into natural polymers, synthetic polymers and modified natural polymers. Various biodegradable polymers are described with respect to their composition, structure, properties and applicability.
Today polymer industry is facing environmental problems. Most of the polymer does not degrade under standard environmental conditions. Yet it would be interesting to know about various polymers that degrade in biological environment. Also by increasing their use the environmental problems can be resolved to a certain extent.
CLASSIFICATION OF BIODEGRADABLE POLYMERS:
Biodegradable polymers can be divided into three broad classifications:
1. Natural polymers
2. Synthetic polymers
3. Modified Natural polymers
These classes may be further subdivided as:
1. Natural polymers or biopolymers
2. Synthetic polymers (a) Carbon chain backbone
(b) Heteroatom chain backbones
3. Modified Natural polymers
(a) Blends and grafts
(b) Chemically modified
NATURAL POLYMERS OR BIOPOLYMERS:
They are produced naturally by all living organisms. They are environmentally acceptable degradable polymers. Most common natural polymers are:
1. Polysaccharides starch and cellulose
2. Polyesters polyhydroxyalkanoates
3. Proteins silk and poly (2-Glutamic acid)
4. Hydrocarbons natural rubbers
They are largely limited to starch and cellulose derivatives for practical applications either in plastics or as a water-soluble polymers. Both these polymers are composed of D-Glycopyranoside repeating units to very high molecular weight, thousands of units. They differ in that starch is poly (1,4-2-D-Glucopyranoside) and cellulose is poly (1,4-B-D-Glucopyranoside). This difference in structure control biodegradation rates and properties of the polymers. Complex carbohydrates like microbially produced xanthan, curdlan, pullulan, hylauranic acid alginates, carageenan and guar are accepted biodegradable polymers. Xanthan is the prominent microbial polysaccharide and finds use in the food industry. It is also used as a thickener in many industrial applications.
They must be used as found in nature because they are not soluble or fusible without decomposition. They are widely used as fibbers e.g. wool, silk, gelatin (collagen) which is used as an encapsulate in the pharmaceutical and food industry. The structure of proteins is an extended chain of amino acid joined through amide linkages that are readily degraded by enzymes, particularly protease. Recent activity in poly (r-glatanic acid) with control of stereo chemistry. The inclusion of manganese ions may be important for future development in biodegradable water-soluble polymers with carboxyl functionality.
They are produced by many bacteria as intracellular reserve material for use as a food source during periods of environmental stress. They are biodegradable, can be processed as plastic materials, are produced from renewable resources and can be produced by many bacteria in a range of composition. The thermoplastic polymers vary from soft elastomer to rigid brittle plastics in accordance with the structure of the pendent side-chain of the polyester. All the polyesters are 100% optically pure and are 100% isotactic. Longer side chain polyesters are produced by variety of bacteria, usually as copolymers and with low crystallinity, low melting points and low Tg. These polyesters are elastomeric and have excellent toughness and strength. They are inherently biodegradable. But as the chain length increases the rate of biodegradation is greatly reduced.
Synthetic polymers do not have natural origin, the plethora of enzymes available in nature for degrading natural polymers are not useful for synthetic polymers. Researches are carried out for synthetic polymeric structure that can be biodegraded. Some guidelines based on polymer structure, polymer physical properties and environmental conditions at the exposure sites have emerged for polymer structure. Following generalization can be made:
1. A higher hydrophilic/hydrophobic ration is better for degradation.
2. Carbon chain polymers are unlikely to biodegrade.
3. Chain branching is deleterious to biodegradation.
4. Condensation polymerization is more unlikely to biodegrade.
5. Lower molecular weight polymers are more susceptible to biodegradation.
6. Crystallinity reduces biodegradability.
Favorable polymer physical properties include water solubility and sample purity. Environmental conditions to be considered in evaluation o biodegradability are temperature, pH, moisture, oxygen, nutrients, suitable microbial population, concentration and test duration.
POLYMERS WITH CARBON BACKBONES:
These polymers may be represented by and is considered derivatives of PE. Where ‘n’ is the degree of polymerization and ‘R’ is the functional group. The functional groups and the molecular weight of the polymer control their properties that vary in hydrophobicity, solubility characteristics, Tg and crystalinity.
Fungal and bacterial growth tests indicate that polyethylene and other high molecular weight carbon chain polymers do not support growth. Anomalous results were observed when plasticizers or low molecular weight impurities were added. It was also found that branching of hydrocarbon chains limits biodegradation. There is an increase in biodegradation with lower molecular weight because of the transportation of polymer across cell wall is more likely at lower molecular weight or it may be the mechanism of biodegradation or because of random or chain end cleavage prior to entering the cell. The slow biodegradation process in PE can be accelerated with surfactants or an oxidation process.
Their biodegradation requires an oxidation process and most of the biodegradable vinyl polymers contain an easily oxidisable functional group. To improve biodegradability of vinyl polymers, catalysts are added to promote their oxidation or photodegradation or both. Incorporation of photosensitive groups like ketones into the polymers is also attempted.
It is the most readily biodegradable of vinyl polymers. It is readily degraded in wastewater activated sludge. The microbial degradation of PVAL has been studied. Its enzymatic degradation by secondary alcohol peroxide isolated from soil bacteria of the pseudomonas strain is also studied. The initial step involves the enzymatic oxidation of the secondary alcohol groups in PVAL to ketone groups. Hydrolysis of ketone goups results in chain cleavage. Other bacterial strain, such as flavobacterium and azetobacter were effective in degrading PVAL.
Controlled chemical oxidation of PVAL into PEK. It has similar structure to the intermediate formed as PVAL is biodegraded. PEK is found more susceptible to hydrolysis and biodegradation than PVAL. it is the polymeric form of acetoacetone, hence it forms metal chelate. Its water solubility, reactivity and biodegradability make it potentially useful material in biomedical, agricultural and water treatment areas. By using dyes as model it was found that PEK and PCL blends are excellent controlled release systems.
PVAL is obtained from hydrolysis of polyvinyl acetate. Controlled hydrolysis of PVAc followed by oxidation provides degradation material having a wide range of properties and degradability. PVAc undergoes slow biodegradation in soil burial tests. The weight loss increases with the acetate content.
Polyalkyl acrylates and polycyanoacrylates generally resist biodegradation. Weight loss in soil burial test is reported for copolymers of ethylene and propylene with acrylic acid, acrylonitirde and acrylamide poly alkyl 2-cyanoacrylates. Polymethyl 2- cyanoacrylate is the most degradable among the esters. The degradability decreases as the alkyl size increases. Poly iso beetyl 2- cyanoacrylate nano particles is degraded in two enzyme free media a pH 7 and pH12 in the presence of rat liver microsomes.Poly 2-hydroxy ethyl methacrylate is generally cross-linked with a small amount of ethylene dimethacrylate. It swells in water to form a hydrogel and is widely used in biomedical area due to its good biocompatibility.
HETEROATOM CHAIN BACKBONE POLYMERS:
Their linkages are quite frequently found in nature and they are more likely to biodegrade than hydrocarbon based polymers. They include polyesters, polyamides, polyethers, polyacetates and other condensation polymers.
Aliphatic polyesters are the most easily biodegraded synthetic polymers known.
TABLE: Biodegradable polyesters in commercial development
polybutylene succinate adipate
poly (L- lactic acid)
The simplest poly (a-hydroxyacid), polyglycolic acid (PGA) has been successfully used as biodegradable suture. The partially crystalline polyester is generally obtained from the polymerisation of diglycolide with a tin catalyst. Also poly (L- lactic acid) PLLA is obtained from dilactide and is used as suture. Aromatic polyesters such as PET exhibit excellent material properties, they prove to be almost totally resistant to microbial attack. Aliphatic polyesters on the other hand are readily biodegradable, but lack good mechanical properties that are critical for most applications. All polyesters degrade eventually, with hydrolysis (degradation induced by water) being the dominant mechanism. Synthetic aliphatic polyesters are synthesised from diols and dicarboxylic acids via condensation polymerisation, and are known to be completely biodegradable in soil and water. These aliphatic polyesters are, however, much more expensive and lack mechanical strength compared with conventional plastics such as polyethylene.
Polyester based polyurethanes are more susceptible to degradation than those derived from polyether diols. As the flexibility of polyurethane increases, their susceptibility towards biodegradability increases. Polyamide urethane prepared from amino alcohols are easily degraded by substilin.
Nylon 6, Nylon 6,6 generally resist microbial and enzymatic attacks but oligomers of e-amino hexanoic acid are degraded by enzymes and microorganism. The loss of tensile strength of nylon in vivo is by 25% after 89 days and 83% after 726 days. Etching of a nylon fiber after 210 days in vivo is also seen. In vivo degradation of nylon is caused by hydrolysis and proteolytic enzyme catalysis. Incorporation of methyl, hydroxy and benzyl groups into polyamide chains improves the biodegradability. Biodegradability of polyamide esters decreases by shortening polyamide blocks and increasing polyamide content.
Polyureas prepared from lysine esters with 1,6 hexane diisocyanate are readily degradable and is used in polymer drug application. The polyester urea from phenylamine containing hydrophobic benzyl group is readily hydrolyzed while unsubstituted polyester urea from glyline is not affected.
Fibre forming polyanhydrides are very susceptible to hydrolysis. Poly[bis-(p-carboxyphenoxy)methane] – PCPM was examined as matrix material for controlled release of drugs and was found to erode slightly faster in vivo than in vitro buffer. The erosion of PCPM is mostly heterogenous surface erosion which gives a near zero order released rate.
POLY [AMIDE-ENAMINE] s:
Homogeneous erosion of hydrophilic biodegradable polymer matrix system undergo progressive loosening or swelling. Hence by surface erosion, a near zero order release can be obtained if diffusion release is small. Poly [amide-enamine]s are are found susceptible to hydrolysis and biodegradation.
The polymer contains phosphorous nitrogen chains. They are readily hydrolyzed to give phosphoric acid and ammonia derivatives.
MODIFIED NATURAL POLYMERS:
Natural polymers are modified so that the environmentally acceptable polymer can be developed. Hence the modification should not interfere the biodegradation process. The modification includes blends with other natural and synthetic polymers, grafting of other polymeric composition and chemical modification to introduce some desirable functional group by oxidation or some other simple chemical reaction such as esterification or etherification.
Starch is made thermoplastic at elevated temperature in the presence of water as a plasticizer, allowing melt processing alone or in blends with other thermoplastic. Water lowers the melt transition temperature of starch so that processing can be done well below the degradation temperature.
The most important commercial application is the blending of PE with starch in presence or absebce of other additives. The starch –PE are compatibilized with:
1. Ethylene acrylic acid copolymer
2. Ethylene vinyl alcohol polymers
7. Cellulose acetate
The product based on this chemistry are characterized by incomplete biodegradation, water sensitivity.
The blending composition is limited only by number of polymers and the compatibility of the components.
1. PHA with cellulose acetate
2. PHA with PCL
3. PLA with PEG
4. Chitosan with Cellulose
5. PLA with inorganic fillers
6. PHA and aliphatic polyesters with inorganics
NATURALLY PRODUCED POLYMERS:
Polyhydroxyalkanoates (PHAs) are aliphatic polyesters naturally produced via a microbial
process on sugar-based medium, where they act as carbon and energy storage material in bacteria.They were the first biodegradable polyesters to be utilised in plastics.
The two main members of the PHA family are polyhydroxybutyrate (PHB) and polyhydroxyvalerate (PHV).Aliphatic polyesters such as PHAs, and more specifically homopolymers and copolymers of hydroxybutyric acid and hydroxyvaleric acid, have been proven to be readily biodegradable. Such polymers are actually synthesised by microbes, with the polymer accumulating in the microbes’ cells during growth. The most common commercial PHA consists of a copolymer PHB/PHV together with a plasticiser/softener and inorganic additives such as titanium dioxide and calcium carbonate. A major factor in the competition between PHAs and petroleum based plastics is in production costs. Opportunities exist however for obtaining cheaper raw materials that could reduce PHA production costs. Such raw materials include corn-steeped liquor, molasses and even activated sludge. These materials are relatively inexpensive nutrient sources for the bacteria that synthesise PHAs. The PHB homopolymer is a stiff and rather brittle polymer of high crystallinity, whose mechanical properties are not like polystyrene, though it is less brittle. PHB copolymers are preferred over general purposes as the degradation rate of PHB homopolymer is high at its normal melt processing temperature. PHB and its copolymers with PHV are melt processable semi-crystalline thermoplastics made by biological fermentation from renewable carbohydrate feedstocks. They represent the first example of a true biodegradeable thermoplastic produced via a biotechnology process. No toxic by-products are known to result from PHB or PHV.
Poly-hydroxybutyrate-co-polyhydroxyhexanoates (PHBHs) resins are one of the newest type of naturally produced biodegradable polyesters. The PHBH resin is derived from carbon sources such as sucrose, fatty acids or molasses via a fermentation process. These are ‘aliphatic-aliphatic’ copolyesters, as disctint from ‘aliphatic-aromatic’ copolyesters. Besides being completely biodegradable, they also exhibit barrier properties similar to those exhibited by ethylene vinyl alcohol (see Section 3.1.2). Procter & Gamble Co. researched the blending of these polymers to obtain the appropriate stiffness or flexibility.
1. Use of degradable polymers can be increased to protect environment.
2. The properties of degradable polymers should be compared with those of conventional synthetic polymers
3. The properties of degradable polymers that are not acceptable can be improved by adding suitable additives, changing the chemical structure, incorporating functional groups etc.
4. After modifying the degradable polymer for achieving strength, the degradability should be assessed.
5. The residue from the polymer after degradation should be assessed for its harmfulness to the environment.
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