Biodegradable polymers and plastics recycling

In short

Biodegradable polymers are macromolecules whose chains are broken down by micro-organisms (bacteria, fungi, actinomycetes) into CO₂, water, methane and biomass, usually after an initial hydrolysis or oxidation step; plastics recycling is the set of mechanical, chemical and energy routes that return conventional polymers to use. The two together form the "end-of-life" half of polymer materials, and they are now driven in India by the Plastic Waste Management Rules, 2016 (as amended, with EPR targets from 2022), the Solid Waste Management Rules, 2026 and CPCB's compostable/biodegradable certification. Exams ask for degradation mechanisms, PLA and PHA synthesis, test standards (compostability, disintegration), the resin identification codes, recycling unit operations and the EPR framework. Employers include Indian Oil, Reliance, Balrampur Chini's PLA project, Finolex, Supreme Industries, Ganesha Ecosphere, Shakti Plastic, Bisleri/Coca-Cola packaging teams and recycling start-ups, plus CPCB/SPCB and brand-owner compliance roles.

Explained

Definitions and classification

• Biodegradable: degraded by micro-organisms; defined by the extent of mineralisation (CO₂ evolved) in a specified time, medium and temperature. A polymer is not "biodegradable" in the abstract - only under a stated environment (industrial compost, soil, fresh water, marine, anaerobic digester).
• Compostable: biodegradable *plus* disintegration, no ecotoxicity and no heavy metals above limits, in composting conditions - a stricter, test-defined subset.
• Bio-based ≠ biodegradable: bio-PE and bio-PET are made from sugarcane ethanol but are chemically identical to fossil PE/PET and are not biodegradable; PCL, PBS and PBAT are petro-based but are biodegradable. The two axes are independent.
• Four quadrants: (i) bio-based + biodegradable - PLA, PHA, starch blends; (ii) bio-based + non-biodegradable - bio-PE, bio-PET, PA 11; (iii) fossil + biodegradable - PCL, PBAT, PBS, PVA; (iv) fossil + non-biodegradable - PE, PP, PS, PVC, PET.
• Oxo-degradable plastics: PE/PP with a transition-metal (Mn, Fe, Co) pro-oxidant that fragments the chain by oxidation into microplastics without proven mineralisation. India's Plastic Waste Management Rules prohibit the manufacture, import, stocking, sale and use of carry bags and products made of such "oxo-degradable" plastic - treat them as banned, not as biodegradable.
• Degradation mechanisms by chain structure: hydrolysable backbones (ester, amide, carbonate, anhydride, glycosidic) are attacked by water and enzymes; all-carbon backbones (PE, PP, PS) are not, which is why they persist.

Important biodegradable polymers

• PLA (polylactic acid/polylactide): from lactic acid produced by fermentation of corn/sugarcane/cassava starch. Two routes - (a) direct polycondensation giving low molecular weight, (b) industrial route: lactic acid → oligomer → cyclic lactide dimer → ring-opening polymerisation with Sn(II) octoate catalyst → high molecular weight PLA. Stereochemistry controls properties: PLLA is semi-crystalline (Tm ≈ 170-180 °C), PDLLA amorphous; Tg ≈ 55-60 °C, so plain PLA cups deform in hot tea. Stiff, clear, brittle (elongation 3-6%); degrades by bulk hydrolysis of ester links, fast only above its Tg - hence "industrially compostable" (58 °C), slow in soil or sea.
• PHA/PHB/PHBV: polyhydroxyalkanoates stored intracellularly as carbon reserve by bacteria (*Cupriavidus necator*, *Bacillus*) under carbon excess and nutrient limitation; recovered by solvent extraction or cell disruption. PHB is highly crystalline (Tm ≈ 175 °C), brittle, with a narrow processing window (degrades near its Tm); copolymer PHBV (with hydroxyvalerate) lowers Tm and crystallinity and improves toughness. PHAs biodegrade by extracellular depolymerase enzymes in soil, sludge and seawater - the only common class with good marine biodegradability.
• Starch and starch blends: native starch is destructurised with water/glycerol to give thermoplastic starch (TPS); very hydrophilic and weak, so blended with PBAT, PCL or PLA. Dominant in Indian compostable carry bags.
• PBAT: poly(butylene adipate-co-terephthalate), flexible, PE-like, used as the toughening partner in nearly every compostable film; the aromatic (terephthalate) units give strength, the aliphatic adipate units give biodegradability.
• PBS: poly(butylene succinate), from succinic acid + 1,4-butanediol; semi-crystalline, Tm ≈ 115 °C, better heat resistance than PLA.
• PCL: poly(ε-caprolactone), ROP of ε-caprolactone; Tm ≈ 60 °C, very low Tg (−60 °C), excellent biodegradability - used as a blend component, in adhesives and in medical devices.
• PGA, PLGA, PDO: glycolide/lactide copolyesters used in absorbable sutures and drug delivery; degradation time tuned by the lactide:glycolide ratio.
• Others: cellulose acetate (low DS), PVA (water-soluble, biodegradable by specific microbes), chitosan, PHBH, protein and alginate films.

Mechanism, stages and testing of biodegradation

• Stages: (1) biodeterioration/surface colonisation and biofilm; (2) depolymerisation - abiotic hydrolysis or oxidation plus extracellular enzymes cut chains to oligomers/monomers; (3) assimilation - small molecules pass the cell wall and enter metabolism; (4) mineralisation to CO₂ (aerobic) or CO₂ + CH₄ (anaerobic), water and biomass.
• Factors that speed it up: ester/amide links in the backbone, low crystallinity, low molecular weight, hydrophilicity, large surface-to-volume ratio (thin film), temperature near or above Tg, moisture, correct pH, nutrient-rich active microbial medium. Additives, high crystallinity, crosslinking and chain branching slow it.
• Measurement: respirometry - CO₂ evolved as a percentage of the theoretical CO₂ from the carbon content (ThCO₂), against a cellulose reference; also BOD/O₂ uptake, weight loss, molecular weight drop by GPC, tensile loss, SEM of surface erosion.
• Surface versus bulk erosion: fast-hydrolysing polymers (polyanhydrides) erode at the surface and keep their shape; PLA takes up water throughout and degrades in the bulk, losing strength before mass - important in implants.
• Indian and international criteria: IS 17088 is the Indian standard for compostable plastics (specification for compostable plastics; it follows the ISO 17088 approach - 90% mineralisation relative to the reference within 180 days, 90% disintegration through a 2 mm sieve in 12 weeks, heavy-metal limits and plant-growth ecotoxicity). IS 17899 covers biodegradable plastics. Compostable/biodegradable products in India need a CPCB certificate and must carry it; marketing a product as compostable without it is a violation. Related test methods: ISO 14855 (controlled composting), ISO 14851/14852 (aqueous), ISO 17556 (soil), ISO 14853/15985 (anaerobic), ASTM D6400, EN 13432.

Plastics recycling - routes and unit operations

• Hierarchy: reduce → reuse → primary recycling (closed-loop reprocessing of clean, known scrap, e.g. in-plant runners) → secondary/mechanical recycling (post-consumer, melt reprocessing) → tertiary/chemical recycling (back to monomer, oligomer or fuel) → quaternary (energy recovery - incineration with heat, co-processing in cement kilns, refuse-derived fuel) → landfill (last).
• Mechanical recycling line: collection → sorting (manual, plus NIR/FTIR optical sorting, eddy-current for metals, magnetic separation) → size reduction (shredder, granulator) → washing (hot caustic wash for PET to remove glue and labels) → density/float-sink separation (PP 0.90, PE 0.92-0.96 float in water; PET 1.38, PVC 1.40, PS 1.05 sink) → drying → extrusion with melt filtration and vacuum devolatilisation → pelletising → compounding with virgin resin, stabilisers, chain extenders, impact modifiers.
• Why quality drops: chain scission and oxidation at each heat history (lower molecular weight, higher MFI for PE/PP, lower intrinsic viscosity for PET), residual moisture causing hydrolysis in PET and PA, contamination, colour mixing, loss of additives, and immiscibility - most polymer pairs are thermodynamically incompatible, so blends are brittle. Remedies: compatibilisers (maleic-anhydride-grafted PP, SEBS-g-MA), chain extenders (pyromellitic dianhydride, Joncryl-type epoxy acrylics for PET), re-stabilisation, solid-state polycondensation to rebuild PET IV.
• PVC is the worst contaminant in PET recycling: it decomposes near 200 °C releasing HCl, which yellows and degrades PET; even a few hundred ppm ruins a batch. Conversely PET flakes in a PVC stream remain unmelted "crystals".
• Chemical/feedstock recycling:
  • Depolymerisation (for condensation polymers): PET glycolysis with excess ethylene glycol → BHET; methanolysis → DMT + EG; hydrolysis → terephthalic acid + EG; enzymatic depolymerisation using engineered cutinase/PETase; nylon 6 depolymerised back to caprolactam.
  • Pyrolysis of polyolefins (400-600 °C, no oxygen) → pyrolysis oil, wax and gas, upgraded to naphtha; catalytic cracking (ZSM-5, FCC catalysts) lowers temperature and narrows the product range. Gasification gives syngas. PVC and PET in the feed are harmful (HCl, benzoic acid, corrosion).
  • Dissolution/solvent purification is physical, not chemical - dissolves and re-precipitates the same polymer (PS, PP).
• Bottle-to-bottle rPET: India requires minimum recycled plastic content in rigid packaging under the EPR regime, and FSSAI permits recycled PET for food contact subject to the prescribed conditions and process validation - the hot caustic wash and solid-state polycondensation steps are what make rPET food-grade.
• Energy content: PE/PP have calorific values around 43-46 MJ/kg (comparable to fuel oil), which is why cement kilns co-process plastic waste; PVC's chlorine and PET's low calorific value make them poor fuels.
• Resin identification codes (1-7): 1 PET, 2 HDPE, 3 PVC, 4 LDPE, 5 PP, 6 PS, 7 others. 1, 2 and 5 are the readily recycled streams; multilayer and metallised films are the hardest (no single melt phase) and are mostly routed to cement co-processing or road-making.

Indian regulation and practice

• Plastic Waste Management Rules, 2016 (notified under the Environment (Protection) Act, 1986) with amendments in 2018, 2021, 2022 and later: thickness limits (plastic carry bags not less than 120 microns; non-woven PP bags by GSM), ban since 1 July 2022 on listed single-use plastic items (earbuds with plastic sticks, balloon sticks, cutlery, straws, stirrers, plates, cups, wrapping film for sweet boxes, invitation cards, cigarette packets, PVC banners below 100 microns, and others), and prohibition of oxo-degradable plastic for carry bags and products.
• EPR (Extended Producer Responsibility): the 2022 Guidelines put legally binding obligations on producers, importers and brand-owners (PIBOs) registered on the CPCB EPR portal, in four categories - Cat I rigid, Cat II flexible (single layer), Cat III multilayered, Cat IV compostable. Obligations cover collection targets, mandatory recycling targets, minimum recycled content in new packaging and end-of-life disposal, discharged through EPR certificates traded from registered recyclers/waste processors; shortfalls attract environmental compensation. Targets and percentages are stepped up year by year by amendment - cite the current year's notified figure rather than a remembered number.
• Solid Waste Management Rules, 2026 replaced the 2016 Rules from 1 April 2026 and govern segregation at source (wet/dry/domestic hazardous), the duties of waste generators and urban local bodies, and the handling of plastic in the dry stream; the Battery Waste, E-Waste and Construction and Demolition Waste rules sit alongside.
• Other instruments: the Environment (Protection) Act, 1986 and Water/Air Acts for consent to operate recycling units from the State Pollution Control Board; BIS registration and the Quality Control Orders on specified polymers and products; GST on plastic waste/scrap and on recycled granules falls under the current slab structure (5/18/40% since 22 September 2025) - verify the exact HSN rate before quoting it.
• Indian capacity and realities: India generates of the order of 4 million tonnes of plastic waste a year (CPCB annual reports; the figure is revised each year), with a high but largely informal recycling rate; PET recycling is the most organised (Ganesha Ecosphere, Reliance's R-Elan recycled polyester). Plastic waste is used in bituminous road construction (IRC-accepted practice using 6-8% shredded waste plastic by weight of bitumen for dry-process coating of aggregates) and in cement kiln co-processing.

Comparison, advantages and limitations

• Advantages of biodegradable polymers: divert organic-contaminated packaging (food wrap, agricultural mulch, bin liners) from landfill; mulch films need no retrieval; medical absorbable devices avoid a second surgery; lower fossil carbon when bio-based.
• Limitations: cost typically 1.5-4 times commodity PE/PP; lower heat and barrier performance; industrial compost conditions are rarely available in Indian cities, so they often end up in landfill where anaerobic degradation produces methane; they contaminate mechanical recycling streams (PLA looks like PET in a sorting line and ruins the melt); "biodegradable" labels encourage littering; food-versus-fuel/land concerns for crop feedstock.
• Mechanical versus chemical recycling: mechanical is cheap, low-energy and low-CO₂ but downgrades properties and needs clean, sorted single-polymer feed; chemical handles mixed, coloured and contaminated waste and restores virgin quality but is energy- and capital-intensive with lower yields.

Key points

• Biodegradation - microbial conversion of a polymer to CO₂, water, biomass (and CH₄ anaerobically); always specified with an environment and a time.
• Compostable - biodegradable + disintegrating + non-ecotoxic under compost conditions; in India certified by CPCB against IS 17088.
• Bio-based vs biodegradable - independent properties; bio-PE is bio-based but permanent, PCL/PBAT are fossil-based but biodegradable.
• Hydrolysable backbone - esters, amides, carbonates and glycosidic links degrade; all-carbon PE/PP/PS backbones do not.
• PLA - lactide ring-opening polymerisation, Tg ≈ 55-60 °C, Tm ≈ 170-180 °C, stiff and brittle, needs ~58 °C industrial compost.
• PHA/PHB - bacterially synthesised intracellular polyester; PHB brittle and highly crystalline, PHBV tougher; best marine biodegradability.
• TPS - starch plasticised with water/glycerol, hydrophilic and weak, blended with PBAT/PCL/PLA for carry bags.
• Oxo-degradable - pro-oxidant-filled PE that only fragments into microplastics; prohibited for carry bags and products in India.
• Surface vs bulk erosion - PLA degrades in bulk and loses strength before mass; polyanhydrides erode from the surface.
• Recycling hierarchy - primary (closed loop), secondary (mechanical), tertiary (chemical), quaternary (energy recovery).
• Float-sink separation - PP/PE float in water; PET, PVC and PS sink - the cheapest separation available.
• PVC contamination - releases HCl above ~200 °C and destroys PET quality; the single most damaging impurity.
• Property loss on reprocessing - chain scission raises MFI in polyolefins and lowers intrinsic viscosity in PET; chain extenders and compatibilisers mitigate it.
• Chemical recycling - PET glycolysis/methanolysis/hydrolysis and enzymatic depolymerisation; pyrolysis and gasification for polyolefins.
• Resin codes - 1 PET, 2 HDPE, 3 PVC, 4 LDPE, 5 PP, 6 PS, 7 others; multilayer films are the least recyclable.
• EPR - PIBOs registered on the CPCB portal meet collection, recycling and recycled-content targets through traded EPR certificates, with environmental compensation for shortfall.
• Single-use plastic ban - listed items prohibited from 1 July 2022; carry bags must be at least 120 microns.
• Waste plastic in roads and kilns - 6-8% shredded plastic on aggregate in the dry process; cement co-processing exploits PE/PP's 43-46 MJ/kg.

Formulas and examples

Example 1 - theoretical CO₂ and percent biodegradation of PLA. PLA repeat unit C₃H₄O₂, M = 3(12) + 4(1) + 2(16) = 36 + 4 + 32 = 72 g/mol; carbon fraction f_C = 36/72 = 0.500.

• For a 10.0 g dry PLA sample: ThCO₂ = 10.0 × 0.500 × 44/12 = 10.0 × 0.500 × 3.6667 = 18.33 g CO₂.
• A composting test (58 °C, ISO 14855) evolves 17.2 g from the vessel and 0.7 g from the blank-corrected baseline already subtracted, i.e. net 16.5 g: D = 16.5 / 18.33 × 100 = 90.0% - just meeting the 90% mineralisation criterion.

Example 2 - PLA crystallinity by DSC. A PLA film shows ΔH_m = 38.4 J/g and a cold-crystallisation exotherm ΔH_cc = 12.9 J/g.

• Net melting enthalpy = 38.4 − 12.9 = 25.5 J/g.
• X_c = 25.5 / 93 × 100 = 27.4% - a partly crystalline film; the lower the crystallinity, the faster the hydrolysis.

Example 3 - intrinsic viscosity loss and recycled-content blending in PET. Virgin bottle-grade PET has IV = 0.80 dL/g; after washing and extrusion the flake measures 0.72 dL/g.

• Loss = (0.80 − 0.72)/0.80 × 100 = 10.0% drop in IV, i.e. a fall in molecular weight that would weaken a blown bottle.
• Solid-state polycondensation raises it back to 0.78 dL/g. A preform plant must deliver 30% recycled content at 0.79 dL/g minimum. Using virgin at 0.80 and rPET at 0.78, the mass-weighted estimate is 0.30 × 0.78 + 0.70 × 0.80 = 0.234 + 0.560 = 0.794 dL/g - acceptable.
• Mass check: for a 12 g preform, recycled resin = 0.30 × 12 = 3.6 g per bottle; a line making 20,000 bottles/hour consumes 3.6 × 20,000 = 72,000 g/h = 72 kg/h of rPET.

Example 4 - EPR obligation and environmental compensation (illustrative arithmetic). A brand-owner put 5,000 t of Category I rigid plastic packaging on the market in the base year. Its notified recycling target for the obligation year is 70% (check the current notified percentage, which is stepped up annually).

• Obligation = 5,000 × 0.70 = 3,500 t to be discharged through EPR certificates from registered recyclers.
• It procures certificates for 3,050 t → shortfall = 3,500 − 3,050 = 450 t.
• If environmental compensation is levied at ₹5,000 per tonne of shortfall (rate as notified by CPCB - verify the current rate and the multiplier for repeat default), compensation = 450 × 5,000 = ₹22.5 lakh.

Example 5 - energy recovery versus mechanical recycling of a mixed stream. A city collects 100 t/day of dry plastic waste: 25 t PET, 30 t HDPE/LDPE, 20 t PP, 5 t PVC, 20 t multilayer film.

• Mechanically recyclable with good yield: PET + PE + PP = 25 + 30 + 20 = 75 t. At 85% process yield, pellets = 75 × 0.85 = 63.75 t/day.
• Residue to co-processing = (75 − 63.75) + 20 (multilayer) = 11.25 + 20 = 31.25 t/day; PVC (5 t) is kept out of both the PET melt and the kiln feed because of HCl.
• Energy from the 31.25 t at an average CV of 38 MJ/kg = 31,250 × 38 = 1,187,500 MJ ≈ 1,187.5 GJ/day (≈ 330 MWh thermal equivalent).

Where it is used

• Packaging and FMCG: compostable pouches, carry bags and cutlery (Truegreen, Ecolife and other CPCB-certified converters); rPET bottle-to-bottle lines at Reliance, Ganesha Ecosphere and bottlers supplying Coca-Cola India, Bisleri and PepsiCo.
• Agriculture and horticulture: PBAT/PLA/starch mulch films that are tilled in rather than retrieved; controlled-release fertiliser coatings; nursery pots.
• Medical and pharmaceutical: PLGA sutures, screws and pins, drug-delivery microspheres and stent coatings - manufactured by Indian device and CRO companies and tested against Indian Pharmacopoeia 2026 and the Medical Devices Rules.
• Municipal and civil engineering: urban local body dry-waste MRFs under the Solid Waste Management Rules, 2026; shredded plastic in bituminous roads; cement plants (ACC, UltraTech, Dalmia) co-processing plastic-rich RDF.
• Compliance and consultancy roles: EPR registration and certificate procurement on the CPCB portal for PIBOs, SPCB consent and annual returns, life-cycle assessment and sustainability reporting (BRSR) teams.
• Job roles: polymer process engineer in a recycling/compounding plant, QC chemist (MFI, IV, DSC, FTIR identification), R&D for compatibilised blends and compostable grades, plant operations in PET washing lines, environmental compliance officer.

Interview questions

• Q: Is every bio-based plastic biodegradable? A: No. Bio-PE and bio-PET come from plant-derived ethanol but are chemically identical to fossil grades and do not biodegrade, while fossil-based PCL, PBAT and PBS do - the feedstock and the end-of-life behaviour are independent.
• Q: Why does PLA not degrade in a home compost pit or in the sea? A: Its hydrolysis becomes fast only above its Tg of about 55-60 °C with high moisture; a home pit or seawater stays well below that, so it persists for years.
• Q: What makes a polymer chain microbially attackable? A: Hydrolysable links in the backbone (ester, amide, carbonate, glycosidic), low crystallinity, low molecular weight and hydrophilicity; all-carbon chains like PE and PP offer nothing for enzymes to cut.
• Q: Why is PHB difficult to process? A: It is highly crystalline with a melting point near 175 °C and starts thermally degrading just above it, so the processing window is narrow; copolymerising with hydroxyvalerate (PHBV) lowers Tm and widens it.
• Q: Why is PVC the worst contaminant in a PET recycling line? A: Above about 200 °C PVC releases HCl, which catalyses PET chain scission and causes yellowing, so even a few hundred ppm spoils the batch; PET and PVC are also both denser than water, so float-sink does not separate them.
• Q: How do you separate PE, PP and PET flakes cheaply? A: Float-sink in water - PP (0.90) and PE (0.92-0.96) float, PET (1.38) sinks; PP and PE are then split in a brine or alcohol bath of intermediate density, or by NIR optical sorting.
• Q: Why do recycled polyolefins have a higher melt flow index than virgin? A: Each heat and shear history causes chain scission and oxidation, lowering molecular weight and therefore viscosity, so MFI rises and impact strength falls.
• Q: What is a compatibiliser and why is it needed for mixed plastic waste? A: A graft or block copolymer (for example maleic-anhydride-grafted PP or SEBS-g-MA) that sits at the interface of two immiscible polymers, lowering interfacial tension and transferring stress - without it a PE/PP/PS blend is brittle.
• Q: How is PET chemically recycled back to bottle grade? A: By depolymerising the ester links - glycolysis with excess ethylene glycol to BHET, methanolysis to DMT + EG, or hydrolysis to terephthalic acid + EG - then purifying the monomer and repolymerising, which removes colour and contamination that mechanical recycling cannot.
• Q: What does EPR require of a brand-owner in India? A: Registration on the CPCB EPR portal and discharge of category-wise collection, recycling and recycled-content targets through EPR certificates from registered recyclers, with environmental compensation for any shortfall.
• Q: Are oxo-degradable carry bags an acceptable alternative? A: No - they only fragment into microplastics without proven mineralisation, and the Plastic Waste Management Rules prohibit oxo-degradable plastic carry bags and products in India.
• Q: Why can compostable plastics be a problem for recyclers? A: PLA is mistaken for PET by sorters and consumers; a small fraction in the PET melt causes haze, bubbles and strength loss, so compostables need a separate collection stream to be useful.
• Q: What test proves a product is compostable in India? A: Mineralisation and disintegration testing to IS 17088 (90% mineralisation relative to a cellulose reference within 180 days, 90% disintegration through a 2 mm sieve in 12 weeks) plus heavy-metal and plant ecotoxicity limits, certified by CPCB.

Common mistakes

• Treating "bio-based", "biodegradable" and "compostable" as the same word - they are three different claims with different tests.
• Calling oxo-degradable PE biodegradable, when it is fragmentation into microplastics and is prohibited for bags and products in India.
• Quoting biodegradation as a single number without the medium, temperature and duration ("90% in 180 days at 58 °C in controlled compost" is the claim; "biodegradable in 180 days" is not).
• Computing ThCO₂ from the whole sample mass instead of its carbon fraction, or forgetting the 44/12 factor and the blank correction.
• Assuming mechanical recycling restores virgin properties - it reduces molecular weight every cycle, and PET must be dried before extrusion or it hydrolyses further.
• Mixing up the resin codes (calling code 3 polypropylene) or believing a code means the item is actually recycled in practice.
• Quoting old EPR percentages, old carry-bag thicknesses or the superseded Solid Waste Management Rules, 2016 - the targets are revised by amendment and must be taken from the current notification.
• Writing "PLA = polylactic acid made by condensation" in an exam without mentioning the industrial lactide ring-opening route that gives high molecular weight.

Related topics

Commodity and engineering plastics · Polymer additives and compounding · Polymerisation kinetics and mechanisms · Polymer characterisation: DSC, TGA, GPC and FTIR · Plastics processing: extrusion, injection and blow moulding · Environmental engineering: solid waste management and EPR