PET compost degradation analysis is a laboratory evaluation that measures how polyethylene terephthalate (PET) materials behave under controlled industrial composting conditions, quantifying biodegradation, disintegration, and ecological safety. Guided by international standards such as ISO 14855, ASTM D5338, and EN 13432, this testing determines whether PET-based products can legitimately be classified as compostable or whether they persist in organic waste streams. This article explains the analytical workflow, sample types, key test parameters, acceptance criteria, and practical applications of PET compost degradation analysis.
Technical Overview
Polyethylene terephthalate (PET) is a semi-crystalline, aromatic polyester produced at a scale of tens of millions of tonnes per year for beverage bottles, food trays, films, fibers, and technical parts. Its chemical durability — the combination of an aromatic terephthalate backbone, strong intermolecular interactions, high crystallinity, and a hydrophobic surface — gives PET excellent mechanical and barrier properties, but it also makes the polymer remarkably resistant to microbial attack. As a result, conventional PET is generally regarded as non-biodegradable in composting environments, where it can persist long after the surrounding organic matter has fully stabilized.
PET compost degradation analysis is a controlled laboratory evaluation that quantifies how PET and PET-derived materials behave under industrial composting conditions. The analysis measures the ultimate aerobic biodegradation of the material by monitoring the carbon dioxide evolved as microorganisms metabolize the polymer, evaluates the physical disintegration of the sample, and confirms that the resulting compost remains free of ecotoxic effects and excessive heavy-metal contamination. Together, these data determine whether a material can legitimately be described as compostable or whether it will simply fragment and persist.
The testing framework is defined by internationally recognized standards: ISO 14855 (Parts 1 and 2) and ASTM D5338 for biodegradation, ISO 16929 and EN 14045 for disintegration, and EN 13432, EN 14995, and ASTM D6400 as the overarching compostability specifications. Beyond certification, the analysis is a powerful research tool for developers of modified copolyesters, PET blends, and enzymatically degradable materials, because kinetic curves of CO2 evolution directly reveal how formulation changes affect microbial assimilation.
Testing Samples
A wide range of PET-based materials can be evaluated, and sample preparation strongly influences the measured degradation rate. Powders and thin films expose more surface area than thick-walled parts; therefore, the physical form of the sample must always be documented alongside the results.
Virgin PET Resins and Conventional Products
This category includes pelletized PET resin, bottle-grade and film-grade materials, preforms, bottles, thermoformed trays, and monofilaments or spun fibers. These samples establish the baseline behavior of the polymer and are frequently used as negative references, since they typically mineralize only a few percent of their carbon within the standard test window.
Modified PET and Aliphatic-Aromatic Copolyesters
Formulators improve degradability by incorporating aliphatic ester units, flexible diols, or hydrolysis-prone segments into the PET backbone. Compost degradation analysis verifies whether such modifications translate into measurably faster chain scission and microbial assimilation under thermophilic conditions, rather than merely improved fragmentation.
PET Blends and Biocomposites
Blends such as PET/PBAT, PET/PLA, and PET compounded with plasticized starch, cellulose, or natural fibers are tested to determine whether the biodegradable phase is consumed, leaving behind persistent PET residues. Materials containing pro-oxidant additives (so-called oxo-degradable plastics) are also examined, because these systems fragment into microplastics without true mineralization and are increasingly restricted by regulation.
Reference and Control Materials
Every test series includes blank compost (inoculum only), a positive reference such as microcrystalline cellulose or ashless filter paper, and, where relevant, a negative reference such as polyethylene film. These controls validate inoculum activity and allow the biodegradation of the test material to be expressed both absolutely and relative to a known biodegradable substrate.
Testing Items
The scope of a PET compost degradation analysis typically combines biodegradation measurement with physical, chemical, and ecotoxicological assessments:
- Ultimate aerobic biodegradation: cumulative CO2 evolution expressed as a percentage of the theoretical CO2 (ThCO2) calculated from the carbon content of the sample.
- Biodegradation kinetics: lag phase, biodegradation phase, and plateau phase, often modeled with first-order or logistic functions.
- Disintegration degree: mass percentage of sample retained on a 2 mm sieve after defined composting periods.
- Molecular weight distribution by gel permeation chromatography (GPC) and intrinsic viscosity to detect chain scission.
- Thermal characterization by DSC and TGA, including glass transition, melting behavior, and crystallinity changes.
- Chemical structure by FTIR, tracking carbonyl index and hydroxyl or acid end-group formation associated with hydrolysis.
- Surface morphology by SEM, documenting erosion, pitting, cracking, and microbial colonization.
- Mass loss, dimensional change, and wettability by contact angle measurement.
- Heavy metals and regulated elements (As, Cd, Cr, Cu, Hg, Mo, Ni, Pb, Se, Zn) in the sample or in the final compost.
- Ecotoxicity: seed germination and plant biomass growth, with optional aquatic and earthworm assays.
- Compost quality parameters: pH, volatile solids, carbon-to-nitrogen ratio, dry solids, and inoculum maturity.
Testing Methods
The core of the analysis is a controlled composting respirometry test, complemented by disintegration trials, ecotoxicological verification, and instrumental characterization of recovered samples.
Aerobic Biodegradation by Evolved CO2 (ISO 14855-1 / ASTM D5338)
Test material is mixed with mature compost inoculum — typically sourced from the organic fraction of municipal solid waste and aged two to four months — in static, aerated vessels maintained at 58 ± 2 °C with a moisture content of approximately 50–60%. CO2-free air is passed through the vessels at a controlled flow rate, and the carbon dioxide released is quantified either continuously by infrared analysis or periodically by trapping in alkaline solution followed by titration or gravimetry. ISO 14855-2 offers a closed-vessel gravimetric alternative suited to smaller laboratory setups. The test runs until the biodegradation curve reaches a plateau or for a maximum of 180 days. Biodegradation is calculated as the net CO2 of the test vessel (after subtracting the blank) divided by the ThCO2 derived from the elemental composition of the polymer. A run is considered valid when the cellulose reference exceeds 70% biodegradation within approximately 45 days and blank respiration remains stable near the end of the test.
Disintegration Testing (ISO 16929 / EN 14045)
Sample pieces of defined geometry, commonly 25 mm × 25 mm for films, are buried in a pilot-scale compost mass and incubated under thermophilic conditions for up to 12 weeks. After recovery, the compost is dried and sieved through a 2 mm mesh; the degree of disintegration is the fraction of original dry mass that no longer remains as visible fragments above the sieve. A material passes when at least 90% of its mass disintegrates. Disintegration demonstrates fragmentation, not mineralization — many materials pass this step yet fail the biodegradation criterion.
Ecotoxicity and Compost Quality Assessment
Finished compost containing degraded sample residues is mixed with reference soil and used to grow indicator plants such as cress, barley, or ryegrass following OECD 208 or EN 13432 Annex E. Germination rate and plant biomass must reach at least 90% of the values obtained with blank compost. Heavy-metal contents are compared against the statutory limits of the applicable specification, for example under EN 13432: Zn 150, Cu 50, Ni 25, Cr 50, Pb 50, Cd 0.5, Hg 0.5, As 5, Se 0.75, and Mo 1 mg/kg dry substance.
Complementary Material Characterization
To interpret the respirometry data, samples recovered at intervals are analyzed by GPC for molecular weight decline, DSC and XRD for crystallinity evolution, FTIR for chemical changes at the ester bonds, and SEM for surface erosion and biofilm development. In mechanistic studies, these measurements distinguish abiotic hydrolysis from true enzymatic assimilation and reveal whether degradation is confined to amorphous regions while crystalline domains persist.
Acceptance Criteria for Compostability
Under EN 13432, ASTM D6400, and ISO 18606, a material may be labeled industrially compostable only when it achieves at least 90% biodegradation within 180 days (or 90% of the biodegradation reached by the cellulose reference in the same period), disintegrates by at least 90% within 12 weeks, keeps heavy metals below the prescribed limits, and produces compost without ecotoxic effects. Conventional PET fails the biodegradation criterion, typically mineralizing less than about 5–10% of its theoretical carbon during the entire test, which is why standard PET products cannot carry compostability marks. Certain modified copolyesters and blends with a high biodegradable phase content can approach or meet the criteria — precisely the question such analysis is designed to answer.
Applications
PET compost degradation analysis supports decision-making across the entire material value chain:
- Certification and labeling: substantiating claims under schemes such as Seedling and OK compost INDUSTRIAL (EN 13432), BPI (ASTM D6400), and AS 4736, which in turn gate access to organic-waste collection programs.
- Product development: ranking modified PET grades, copolyesters, blends, and additive packages according to measured mineralization kinetics.
- Regulatory compliance: demonstrating conformity with packaging and single-use plastics legislation, extended producer responsibility requirements, and advertising-claim rules for environmental statements.
- Waste management: providing compost facility operators with evidence of whether PET-based items will break down or contaminate screening and refining operations.
- Research and academia: generating publication-grade biodegradation curves, kinetic parameters, and structure–degradability correlations.
- Benchmarking and quality control: verifying lot-to-lot consistency of degradable formulations and comparing candidate materials before commercial launch.
FAQ
Does PET biodegrade in compost?
No. Conventional PET is a persistent, semi-crystalline aromatic polyester that resists enzymatic hydrolysis at rates relevant to composting. In standardized 180-day tests it typically mineralizes only a small fraction of its carbon, far below the 90% threshold required for a compostable designation.
How long does a PET compost degradation analysis take?
The biodegradation test runs until the CO2 evolution curve reaches a plateau, up to a maximum of 180 days under ISO 14855-1 or ASTM D5338. Disintegration testing adds about 12 weeks, and plant ecotoxicity assays require roughly two additional weeks of growth time, so a complete evaluation usually spans six to seven months including reporting.
Which standards govern the analysis?
Biodegradation is measured according to ISO 14855-1 or ISO 14855-2 and ASTM D5338; disintegration according to ISO 16929 or EN 14045; ecotoxicity according to OECD 208 and EN 13432 Annex E; and overall compostability is judged against EN 13432, EN 14995, ASTM D6400, and ISO 18606.
What is the difference between biodegradation and disintegration?
Disintegration is the physical breakdown of a sample into fragments smaller than 2 mm, while biodegradation is the biochemical conversion of polymer carbon into CO2, water, and microbial biomass. A material can disintegrate completely yet fail to biodegrade — the behavior characteristic of oxo-fragmentable plastics — which is why both endpoints are tested independently.
Can any PET-based material pass compostability testing?
Some can. Heavily modified aliphatic-aromatic copolyesters and blends in which PET is a minor phase may reach the required 90% biodegradation, provided the persistent aromatic fraction is low enough. Each formulation must be verified by testing; predictions based on composition alone are not accepted for certification.
What sample information and quantities are required?
Testing laboratories generally need 100–500 g of material (powder, film, or finished product), the elemental composition or carbon content for the ThCO2 calculation, the intended disposal environment, and, if available, density and additive information. The cellulose reference and blank compost are supplied by the laboratory.
How are results reported?
A final report presents the cumulative biodegradation curve, the maximum percentage of ThCO2 achieved within the test period, kinetic parameters, the disintegration percentage, photographs and microscopy of recovered residues, heavy-metal data, ecotoxicity results, and a clear pass/fail statement against the referenced specification.