Degradation Test Under Controlled Composting Conditions: Standards, Methods, and Applications

The degradation test under controlled composting conditions is the internationally recognized laboratory method for measuring the ultimate aerobic biodegradability of plastic materials, expressed as the percentage of evolved carbon dioxide relative to the theoretical carbon dioxide content of the sample. Conducted in accordance with ISO 14855, ASTM D5338, EN ISO 14855, and equivalent national standards, this test provides the core scientific evidence required for industrial compostability claims, product certification, and regulatory compliance. This article explains the test principle, sample requirements, key testing items, procedures, acceptance criteria, and typical applications of the method.

Technical Overview

The degradation test under controlled composting conditions is a respirometric laboratory method used to determine the ultimate aerobic biodegradability of plastic materials. The test simulates an intensive industrial composting environment, typically maintained at a thermophilic temperature of 58 °C ± 2 °C, in which a mature compost inoculum metabolizes the organic carbon of the test material into carbon dioxide, water, and microbial biomass. By continuously quantifying the carbon dioxide evolved and comparing it with the theoretical carbon dioxide (ThCO₂) calculated from the elemental composition of the material, the test yields a quantitative biodegradation percentage that forms the scientific foundation of every industrial compostability claim.

The method is standardized internationally and is technically harmonized across the following standards organizations:

• ISO 14855-1 — Determination of the ultimate aerobic biodegradability of plastic materials under controlled composting conditions, method by analysis of evolved carbon dioxide (general method)
• ISO 14855-2 — Gravimetric measurement of carbon dioxide evolved in a laboratory-scale test
• ASTM D5338 — Standard test method for determining aerobic biodegradation of plastic materials under controlled composting conditions
• EN ISO 14855 — European adoption of the ISO method
• GB/T 19277.1 — Chinese national standard, technically equivalent to ISO 14855-1
• JIS K 6953 — Japanese industrial standard corresponding to ISO 14855

Because the method is internationally aligned, a single well-designed test campaign performed in accredited third-party laboratories can support compostability documentation across multiple regulatory and certification frameworks.

Testing Samples

Biodegradable Polymer Resins, Pellets, and Masterbatches

Raw materials are the most common test subjects because they allow formulation-level screening before product development. Samples are typically cryogenically ground or milled to a powder with a particle size below 250 μm to maximize surface area and ensure reproducible degradation kinetics. Frequently tested materials include polylactic acid (PLA), polybutylene adipate terephthalate (PBAT), polybutylene succinate (PBS/PBSA), polyhydroxyalkanoates (PHA/PHB), polycaprolactone (PCL), poly(propylene carbonate), polyglycolic acid, starch-based compounds, and their blends.

Finished Products and Packaging

End products are tested to substantiate compostability claims for certification and market access. Typical articles include compostable shopping and organic-waste bags, food service ware, cutlery, cups, lids, straws, coffee capsules, rigid and flexible packaging, thermoformed trays, agricultural mulch films, textile fibers, and nonwoven materials. Products are usually cut into small pieces or reduced in size according to the requirements of the applicable product standard.

Additives, Fillers, and Reference Materials

Compound components such as plasticizers, mineral fillers, nucleating agents, impact modifiers, and pigments are evaluated as part of the finished formulation. Each test series also includes mandatory control specimens: mature compost inoculum alone as a blank, and cellulose as the biodegradable positive reference. An inert, non-biodegradable polymer such as polyethylene may be included as an optional negative control.

Testing Items

A complete controlled-composting degradation study typically comprises the following measurement items:

• Ultimate degree of aerobic biodegradation, expressed as a percentage of theoretical carbon dioxide
• Cumulative and rate-based carbon dioxide evolution curves for the test material, blank, and reference
• Biodegradation kinetics compared in parallel with the cellulose positive reference
• Time required to reach the biodegradation plateau and the 90% threshold
• Volatile solids and ash content of the inoculum and residual solids
• Moisture content, pH, and carbon-to-nitrogen ratio monitoring throughout the exposure period
• Visual and mass-based assessment of residual fragments at test conclusion, where applicable

Testing Methods

Test Principle

The test is performed in sealed compost bioreactors, which may be glass columns or stainless-steel vessels ranging from approximately two liters to several hundred liters in capacity. Each vessel is filled with mature compost inoculum, typically three to six months old and derived from the organic fraction of municipal solid waste. The test material is homogeneously mixed into the inoculum, and carbon-dioxide-free, humidified air is passed through the vessels at a constant controlled flow rate. The carbon dioxide generated by microbial respiration is captured in alkaline absorption traps and quantified by acid-base titration, infrared gas analysis, or gravimetric determination of the mass gain of absorption columns as described in ISO 14855-2. Background respiration from the blank vessels is subtracted from the test vessels to isolate the contribution of the sample.

Controlled Test Conditions

• Temperature maintained at 58 °C ± 2 °C, representing thermophilic industrial composting
• Moisture content of the solid matrix held at approximately 50% to 55% of dry mass, with incoming air humidified
• Constant air flow rate verified by calibrated flow meters to ensure adequate oxygen supply and consistent carbon dioxide transport
• Minimum triplicate vessels for the test material, the blank, and the cellulose reference
• Test duration continued until the biodegradation curve reaches a plateau, for a maximum of 180 days
• Regular verification of pH, moisture, and inoculum activity throughout the exposure period

Calculation and Validity Criteria

The biodegradation percentage is calculated as the net cumulative carbon dioxide evolved by the test vessel minus the blank, divided by the theoretical carbon dioxide of the test material and multiplied by 100. The theoretical carbon dioxide is derived from the carbon content of the sample determined by elemental analysis. The test is considered valid only if the cellulose reference reaches more than 70% biodegradation within 45 days and the blank carbon dioxide production remains within the range specified by the standard, for example 50 to 150 mg CO₂ per gram of volatile solids during the initial ten days. Under certification frameworks such as EN 13432, ASTM D6400, and ISO 17088, a material is regarded as satisfactorily biodegradable when it achieves at least 90% biodegradation within 180 days, either in absolute terms or relative to the cellulose reference.

Related and Companion Methods

The controlled-composting test is frequently combined with complementary standards to complete a full compostability assessment. Disintegration is evaluated separately at pilot scale in accordance with ISO 16929 or EN 14045. Aqueous aerobic biodegradation can be assessed under ISO 14852 or ISO 14851, and biodegradation in soil under ISO 17556. Ecotoxicity of the resulting compost is commonly examined through terrestrial plant growth tests such as OECD Guideline 208.

Applications

Results from the controlled-composting degradation test are used across the entire value chain of biodegradable materials:

• Certification support under EN 13432, ASTM D6400, ISO 17088, and ISO 18606, where the laboratory biodegradation result is the central technical requirement
• Research and development screening of PLA, PBAT, PBS, PHA, starch blends, and novel bio-based polymers, enabling formulation comparison and additive optimization
• Regulatory compliance for packaging and single-use plastic regulations in major markets, supported by testing performed in accredited third-party laboratories
• Substantiation of compostability claims for product labeling, green procurement programs, and e-commerce listings
• Quality control for biodegradable resin producers and converters verifying batch-to-batch consistency
• Academic research and life cycle assessment studies requiring experimentally determined biodegradation kinetics

For manufacturers targeting multiple regions, aligning the test program with ISO 14855 and ASTM D5338 from the outset minimizes duplication and accelerates certification timelines.

FAQ

Q: How long does the degradation test under controlled composting conditions take?

A: The standard exposure period runs until the biodegradation curve reaches a plateau, with a maximum duration of 180 days. In practice, well-disintegrating materials such as cellulose often plateau within 45 to 90 days, while more crystalline polymers may require the full test window.

Q: What is the pass criterion for biodegradation?

A: Under EN 13432, ASTM D6400, and ISO 17088, the material must achieve at least 90% biodegradation within 180 days, either in absolute terms or relative to the cellulose reference.

Q: What sample form and quantity are required?

A: ISO 14855-1 specifies a powder with particle size below 250 μm for resin-type samples, while finished products are typically cut into small pieces per the applicable product standard. The required sample mass scales with vessel size, generally ranging from tens of grams in laboratory-scale reactors to several kilograms in larger compost vessels.

Q: Is a high biodegradation result alone sufficient to claim compostability?

A: No. Complete industrial compostability certification also requires passing disintegration testing per ISO 16929, compliance with heavy metals and regulated substances limits, and ecotoxicity evaluation of the resulting compost.

Q: How does this test differ from home composting assessments?

A: The controlled-composting test operates at 58 °C to represent industrial facilities. Home composting occurs at ambient temperatures of roughly 20 to 30 °C, so claims for home compostability require separate testing under the relevant national or regional specifications for lower-temperature biodegradation.

Q: Can the test distinguish between biodegradation and mere fragmentation?

A: Yes. Because the method quantifies evolved carbon dioxide, it measures true mineralization of the polymer carbon rather than physical breakdown into invisible fragments, making it the most rigorous indicator of ultimate aerobic biodegradability.