Plastic compost degradation testing is a critical laboratory process for evaluating whether plastic materials can biodegrade safely and effectively under controlled composting conditions. This comprehensive guide covers internationally recognized testing standards such as ISO 14855, ASTM D5338, and EN 13432, along with sample requirements, testing procedures, and acceptance criteria for industrial compostability certification.
Technical Overview
Plastic compost degradation testing is a laboratory-based biodegradation assessment designed to determine the extent to which plastic materials decompose when exposed to a controlled aerobic composting environment. The test simulates the thermophilic conditions of industrial composting facilities, typically maintained at 58°C ± 2°C with defined humidity, aeration, and microbial activity levels. During the degradation process, microorganisms metabolize the carbon in the plastic material and convert it into carbon dioxide, water, biomass, and mineral salts. By continuously monitoring the evolved carbon dioxide and comparing it against the theoretical total carbon content of the sample, laboratories can calculate a precise biodegradation percentage over time.
This testing regime forms the scientific foundation for compostability claims and is a mandatory requirement for certification under major international frameworks including EN 13432, ASTM D6400, and ISO 17088. A material must demonstrate not only biodegradation but also physical disintegration and ecological safety before it can be labeled as compostable.
Testing Samples
Biodegradable Polymer Resins and Pellets
Raw polymer materials such as PLA (polylactic acid), PBAT (polybutylene adipate terephthalate), PBS (polybutylene succinate), PHA (polyhydroxyalkanoates), and starch-based blends are tested in powder or granule form to maximize surface area for microbial attack and to verify the intrinsic biodegradability of the base material.
Finished Products and Articles
Consumer and industrial products including food packaging films, coffee capsules, cutlery, shopping bags, agricultural mulch films, thermoformed containers, and injection-molded items are tested in their final commercial form, often in pieces smaller than 5 mm × 5 mm to replicate realistic composting conditions.
Additive-Modified and Composite Materials
Materials containing plasticizers, pro-oxidant additives, mineral fillers, natural fibers, or colorants must be evaluated as complete formulations, since additives can significantly influence or inhibit the biodegradation pathway of the polymer matrix.
Reference and Control Materials
Every valid test series includes a positive reference material such as cellulose microcrystalline powder, which must achieve at least 70% biodegradation within the test period to confirm compost inoculum viability, along with a blank control containing only inoculated compost to establish baseline respiration.
Testing Items
A complete compostability evaluation comprises four mandatory testing modules, each addressing a distinct aspect of material behavior in a composting environment.
- Ultimate Aerobic Biodegradation: Determination of the degree of biodegradation by measuring CO2 evolution, expressed as a percentage of the theoretical CO2 (ThCO2) that the sample carbon could produce. Acceptance typically requires ≥90% within 180 days under industrial composting conditions.
- Disintegration Test: Evaluation of physical fragmentation by placing test specimens in compost within mesh bags and determining the mass of fragments larger than 2 mm after a defined composting period. The requirement is ≥90% disintegration, meaning no more than 10% of the original dry mass may remain on a 2 mm sieve.
- Ecotoxicity Testing: Assessment of the finished compost containing degraded material through plant germination and biomass growth tests using species such as cress, barley, or ryegrass. Germination rate and plant biomass must each achieve ≥90% relative to blank compost controls.
- Chemical Characterization and Heavy Metals: Measurement of volatile solids (dry solids ≥50%, volatile solids ≥50% of dry mass) and quantification of regulated heavy metals including zinc, copper, nickel, cadmium, lead, mercury, chromium, molybdenum, arsenic, selenium, and fluorine against specified threshold limits.
Testing Methods
Aerobic Biodegradation by CO2 Evolution
The core method follows ISO 14855-1, ISO 14855-2, or ASTM D5338, employing a static or dynamic respirometric system. Test material is mixed with mature, inoculated compost in sealed reactors held at 58°C with moisture content between 50% and 60%. Air free of CO2 is passed through the reactors at a controlled flow rate, and the evolved CO2 is captured in alkaline absorption solutions (NaOH or Ba(OH)2) and quantified by titration, or measured directly by infrared gas analysis. The cumulative CO2 is compared against the theoretical value calculated from the sample's elemental carbon content, yielding the biodegradation curve over 45 to 180 days.
Disintegration Under Compost Conditions
Conducted per ISO 16929 or ISO 20200, specimens are placed in fine-mesh bags and buried within a compost mass maintained at 58°C. After 12 weeks (ISO 16929) or the equivalent accelerated cycle (ISO 20200, using synthetic compost at 58°C), the material is sieved through a 2 mm screen and residual fragments are dried and weighed to calculate the disintegration percentage.
Alternative Biodegradation Screening Methods
Aqueous biodegradation tests such as ISO 14851 (oxygen demand measurement) and ISO 14852 (CO2 evolution in aqueous medium) may be used as preliminary screening tools, while ISO 17556 addresses biodegradation in soil for applications such as agricultural mulch films. These methods provide complementary data when compost-specific conditions are not the sole end-use scenario.
Ecotoxicity and Plant Growth Assessment
Following the disintegration phase, the resulting compost is blended with a reference soil substrate and seeded with indicator plants. Germination counts and dry biomass yields are statistically compared against compost from a blank control run, in accordance with the ecotoxicity clauses of EN 13432 and OECD 208 guidance.
Elemental and Instrumental Analysis
Heavy metals are quantified by ICP-OES or ICP-MS following acid digestion, carbon and hydrogen content are determined by elemental analysis to calculate ThCO2, and volatile solids are measured by ignition at 550°C in a muffle furnace.
Applications
Plastic compost degradation testing serves a wide range of regulatory, commercial, and research purposes across the plastics value chain.
- Certification Support: Generating the technical data file required for seedling, seedling-mark, and OK Compost conformity marks based on EN 13432, EN 14995, ASTM D6400, and AS 4736 requirements.
- Regulatory Compliance: Demonstrating conformance with single-use plastic directives, packaging waste regulations, and national compostability labeling laws in the European Union, North America, Australia, and China.
- Product Development: Comparing biodegradation kinetics of new formulations, optimizing polymer blends, and validating the impact of additives, fillers, and processing conditions on compostability.
- Quality Control: Verifying batch-to-batch consistency of biodegradable resin production and incoming material verification for converters and brand owners.
- Scientific Research: Supporting academic studies on polymer-microbe interactions, degradation mechanisms, and environmental fate modeling of bio-based materials.
- Waste Management Planning: Providing compost facility operators with validated data on whether specific products can be accepted in organic waste streams without contaminating the final compost.
FAQ
What biodegradation percentage is required to pass the test?
Under EN 13432 and ASTM D6400, a material must achieve at least 90% biodegradation of the theoretical CO2 within 180 days under industrial composting conditions, or reach a level equivalent to the cellulose positive reference (at least 90% of the reference value). The 90% threshold accounts for the fact that a portion of carbon is assimilated into microbial biomass rather than mineralized to CO2.
How long does a complete compostability test take?
The biodegradation phase typically runs 90 to 180 days, the disintegration phase requires approximately 12 weeks, and ecotoxicity testing with plant growth cycles adds another 3 to 4 weeks. Including chemical characterization and reporting, a full evaluation generally requires 6 to 9 months when conducted sequentially, though parallel execution of independent modules can shorten the overall timeline.
What is the difference between biodegradable and compostable?
Biodegradable describes any material capable of being broken down by microorganisms, without specifying rate or conditions. Compostable is a stricter, legally defined claim requiring proven biodegradation, disintegration, and ecotoxicity performance under specified composting conditions. All certified compostable plastics are biodegradable, but not all biodegradable plastics are compostable.
Does the test represent home composting conditions?
No. Standard tests such as ISO 14855 and ASTM D5338 simulate industrial composting at 58°C, which maintains thermophilic microbial activity. Home composting occurs at lower, fluctuating temperatures (ambient to 30°C), so products certified as industrially compostable may not degrade in a backyard compost pile. Separate standards and certification schemes address home compostability.
Why must cellulose and blank controls be included?
The cellulose positive control verifies that the compost inoculum is biologically active and capable of sustained degradation; if cellulose fails to reach 70% biodegradation, the test is invalidated. The blank control measures the background respiration of the compost itself, which must be subtracted from all sample measurements to isolate the CO2 attributable to the test material.
Can oxo-degradable plastics pass compostability testing?
Generally not. Oxo-degradable plastics fragment through abiotic oxidation rather than true microbial metabolism, and they typically fail the biodegradation requirement because they do not convert carbon to CO2 at the required rate. Their fragments may also fail the disintegration and ecotoxicity criteria, which is why such materials are excluded from most compostability certification schemes.