Evaluation Framework for Marine Biodegradable Materials
The evaluation system for marine biodegradable materials consists of two stages: laboratory tests and field tests in real marine environments. Laboratory tests determine in a short period whether a material has biodegradability, evaluating microbial degradation using indicators such as BOD and CO₂ generation. In contrast, field tests are positioned as experiments to confirm the disintegration behavior of materials under conditions close to natural environments.
ISO16636 is one of these field tests and was established as a field test method to evaluate disintegration in aquatic environments by immersing samples in water. Samples are installed at a depth of 1.5–3 m below the water surface, and disintegration behavior is evaluated using indicators such as weight loss and film thickness reduction. This test does not prove biodegradation itself but is positioned as a test to confirm disintegration behavior under natural environmental conditions.
Structure of Marine Biodegradable Materials
Materials with marine biodegradability share common chemical structures.
Representative examples
・PHA
・PHBH
・PCL
・Cellulose
These materials contain hydrolyzable bonds such as ester bonds, and molecular chains are cleaved in water. The resulting low-molecular-weight components are then degraded through microbial metabolism and eventually mineralized into carbon dioxide and other substances.
This structure differs significantly from many petroleum-based plastics. Research on marine biodegradable materials is a field in which material design and evaluation standards are developed simultaneously.
Structural Difference with Rubber Materials
Rubber materials are in many cases polymers with carbon chains as their main backbone.
Typical examples
・Natural rubber (polyisoprene)
・SBR
・BR
These do not contain ester bonds and therefore do not follow the same degradation pathway as marine biodegradable plastics. In addition, rubber used in tires forms a three-dimensional structure through sulfur crosslinking, which provides high stability against microbial degradation.
Tire materials also include the following components.
・Carbon black
・Zinc oxide
・Vulcanization accelerators
・Antioxidants
Considering these compounding components as well, the design philosophy differs significantly from that of marine biodegradable materials.
Disintegration and Degradation
In ISO16636 testing, the target of evaluation is disintegration rather than biodegradation. Disintegration refers to the phenomenon in which a material fragments and its mass decreases, and it does not necessarily indicate microbial degradation.
In natural environments, the following factors act simultaneously.
・Microbial activity
・Oxidation
・Water flow
・Abrasion
・Physical breakage
Therefore, field tests evaluate material disintegration behavior using indicators such as weight loss and film thickness reduction.
Relationship with TRWP
This evaluation framework is structurally connected to the issue of tire wear particles.
TRWP (Tire and Road Wear Particles) are particles generated by friction between tires and road surfaces and are reported to enter rivers and marine environments.
Generation process of TRWP
Tire
→ Wear
→ Particle formation
→ Drainage system
→ River
→ Ocean
The issue here is how TRWP changes in the environment.
Research on marine biodegradable materials has established testing systems to evaluate how materials disintegrate in marine environments. In contrast, research on tire materials has mainly focused on the generation volume and toxicity of wear particles.
Connecting the two leads to the following question.
Tire particles that reach the marine environment
・Do they disintegrate
・Do they persist
・Are they microbially degraded
Field tests such as ISO16636 are organized as frameworks to observe material disintegration behavior in natural environments. In research on tire wear particles as well, methodologies to evaluate physical changes and degradation behavior in the environment may become increasingly important.
Position of Material Design
Research on marine biodegradable materials is a field where molecular material design and evaluation standards develop simultaneously.
In contrast, tire material design prioritizes durability and mechanical properties.
Material design
Marine biodegradable materials
Polymers containing degradable bonds
Tire materials
Crosslinked polymers designed for long-term durability
Due to this structural difference, the possibility of designing tire rubber as a marine biodegradable material is currently limited.
However, frameworks to evaluate disintegration behavior and particle transformation in the environment exist as a common issue between marine biodegradation research and wear particle research.