Title of Research:
22-D-01 Elucidation of the mechanism and rate of microplastic formation contributing to risk
assessment and preparation of standard microplastics
Principal Investigator:
Kuroda Shinichi (Graduate School of Science and Engineering, Gunma University)
29-1 Honmachi, Ota City, Gunma Prefecture 373-0057(tel) 0276-50-2434 (e-mail) skuroda@gunma-u.ac .jp
Collaborators:
Kuriyama Takashi (Graduate School of Organic Materials and Systems Science, YamagataUniversity), Igarashi Toshio (Research Division for Science and Technology, Kanazawa University),
Hiejima Yusuke (Research Division for Science and Technology, Kanazawa University), Tokumitsu
Katsuhisa (Materials Science Division, Faculty of Engineering, University of Shiga Prefecture) ,Ohtani Hajime (Graduate School of Engineering, Nagoya Institute of Technology), Kouzai Hiroaki(Department of Science and Engineering, Kanto Gakuin University), Kawai Takahiko (Graduate
School of Science and Engineering, Gunma University)
Summary of Research:
MP collected in the lower reaches of the Arakawa River were sorted and observed. The MP
pellets were ball-milled with glass beads. The results showed that as the size of the MPs
decreased, they became more spherical in shape. All MPs that were surface peeled off in the ball
milling test were less than 1 mm in diameter, indicating that the MPs changed from flaky MPs toparticle-like MPs. This is interpreted to indicate that the formation mechanism of MPs is similar tothat of rocks converted to gravel by river transport (i.e., collapse by mechanical action).
The iPP sheets were exposed to high temperatures, and the results showed that the thermallydeteriorated sheets had localized bleaching and yellowing in the peripheral areas, while a healthy
area remained in the central part. In the bleached areas, cracks were formed due to shrinkage
caused by chemical crystallization and propagated in one direction, forming a columnar structure.
In the yellowing area, chemical crystallization progressed further and some of the cracks collapsed
to form fine powder of about 0.2 mm in diameter.
Dumbbell pieces punched from PET bottles, PE sheets, and PP sheets were exposed to axenon weather meter, and the results showed that photo-oxidation degradation progressedsignificantly in PP, with a marked decrease in molecular weight and mechanical properties, andcracks appeared and developed. The molecular weight of PET also decreased, and cracks
appeared on the surface in a mosaic-like pattern, and some of the cracks appeared to be peelingoff and collapsing.
Based on the above findings, it is considered that the collapse and exfoliation of the"weathering layer" generated by the cracks that occur with the degradation of plastics are deeply
related to the generation of MP. The formation mechanism of this "weathering layer" can beexplained by a pattern formation in which shrinkage propagates, as is the case with dry cracks.
The aerosols were classified and collected for analysis by pyrolysis GC/MS, and the pyrolysis
GC/MS allowed the identification and quantification of atmospheric MPs without any specialpretreatment. The fact that no pretreatment is required is also important to prevent contamination.
On the other hand, the use of a blade-type jet mill with many blades in a swirling stream ofcompressed air, which is an improvement over conventional jet mills, has made it possible toreduce various plastics to near-spherical particles of approximately 10 μm in diameter.
Furthermore, UV irradiation of atomized plastics in a mixed solvent of acetone and alcohol
enabled efficient surface oxidation.
Timeline: March 2022-.
Topics: Online presentation at the 2022 LRI Research Report Meeting
Publications: Presentations: (Only the PI's international presentation)
1) Shinichi Kuroda，”Elucidation of the formation mechanism of microplastics”， Microplastics
Advance Research and Innovation Initiative (MARII) Workshop on “Advancements and steps
towards a holistic, quantitative risk assessment on microplastics”, 12 – 13 October 2022 (Online).