The problem the researchers set out to solve is both familiar and severe. Expanded polystyrene, or EPS, buoys dominate marine aquaculture because they are cheap and buoyant, but their structure is inherently fragile. Ultraviolet radiation, wave action, and wind easily fragment them into particles smaller than five millimeters, the accepted definition of microplastics. In South Korea alone, more than twenty million EPS buoys are estimated to be in use on fish farms, and a 2023 report from the Ministry of Oceans and Fisheries found that buoy debris accounted for over forty percent of the plastic waste collected along the country’s coastlines. That debris has been detected not just on the ocean surface but along shorelines and in deep-sea sediments, where it is ingested by plankton, crustaceans, fish, and shellfish before entering the human food chain.

While Korea has already moved to restrict new EPS buoy installations in certain regions and has certified alternatives such as expanded polyolefin and expanded polypropylene, none of these plastic-based substitutes eliminates microplastic generation altogether. The researchers from the Korea Institute of Ceramic Engineering and Technology and Dongduk Women’s University took a fundamentally different approach. Rather than searching for a friendlier plastic, they turned to an inorganic material system in which microplastics simply cannot form, drawing on the same manufacturing principles used to produce foamed glass bricks.

The core of the new technology is the porous ceramic bead, sometimes called cellular or multicellular glass. Each bead is an inorganic material with a glassy matrix enclosing countless independent closed pores separated by thin glass walls. Because these pores trap air inside sealed cells, the beads float, much like the individual foam beads of an EPS buoy. To make them, the team selected soda-lime glass, the inexpensive material used in ordinary flat windows, as the base. Recycled waste glass was crushed and pulverized in an attrition mill without any washing or chemical pretreatment, then mixed with a binder and foaming agents before being shaped into spherical green granules on a disk-type pelletizer.

Choosing the right binder proved critical. The team tested four candidates: polyvinyl alcohol, basalt powder, kaolin, and water glass, or sodium silicate. Beads made with polyvinyl alcohol and water glass showed superior sphericity and lower densities, ranging from 0.23 to 0.28 grams per cubic centimeter, while kaolin and basalt produced irregular, partially fused shapes with densities up to 0.35 grams per cubic centimeter. Polyvinyl alcohol decomposes completely during foaming and leaves no residue, but water glass offered an added advantage: its water content evaporates during heating while the remaining silica vitrifies into the glass matrix, actually enhancing foaming behavior. Combined with easy handling, thermal stability, and low cost, this made water glass the optimal choice, requiring roughly twenty percent by weight in the mixture.

Particle size and temperature emerged as the twin levers controlling density. In the foaming process, fine glass particles partially fuse near their softening temperature, encapsulating the foaming agents whose decomposition or reaction generates gas that expands within the softened glass to form closed-cell pores. Smaller particles produce thinner pore walls and therefore lighter beads, and the experiments confirmed a dramatic effect. When glass powder was ground to below 25 micrometers, the foamed beads reached a bulk density of approximately 0.2 grams per cubic centimeter at foaming temperatures between 800 and 850 degrees Celsius. Particles larger than 45 micrometers were deemed unsuitable for buoy applications. The researchers paired petroleum coke carbon with calcium carbonate as a composite foaming agent, finding that 0.3 percent carbon and 0.5 percent calcium carbonate delivered the lowest density, while excess carbon caused over-foaming and structural collapse.

Preheating proved equally important. Working in a rotary kiln divided into preheating, foaming, and stabilization zones, the team found that a preheating temperature of 600 degrees Celsius produced the lowest-density beads. Below that threshold the foaming reactions were insufficiently activated, while above it premature softening and partial coalescence of glass particles increased density. After foaming, the beads were stabilized at 500 to 550 degrees Celsius and slowly cooled to 40 degrees to relieve thermal stress. To keep the molten, expanding beads from fusing together inside the kiln, the researchers added release agents such as clay, kaolin, or cement, each meeting a demanding set of criteria: melting points above that of the glass, chemical inertness, good flowability, and cost-effectiveness. With these agents in place, the finished beads showed smooth surfaces and densities between 0.19 and 0.21 grams per cubic centimeter.

Cross-sectional micrographs confirmed that the beads’ interiors consist of uniformly distributed, independent closed-cell pores, the architecture responsible for buoyancy. The beads were then consolidated into full buoys using white Portland cement as an inorganic binder, a route the team chose over sintering-based interfacial fusion for its simplicity and mechanical stability. Spherical beads two to five millimeters in diameter were mixed with cement, packed into molds, compacted, and dried for roughly nine to ten hours before demolding. The resulting buoys retained the spherical bead structure and the low density needed to float, and prototypes in a variety of shapes floated stably for more than sixty days without noticeable degradation.

Performance testing against Korean eco-friendly buoy certification standards underscored the technology’s practical promise. Two 40-liter cylindrical buoys, weighing 10.04 and 11.14 kilograms respectively, were verified by the Korea Conformity Laboratories to deliver buoyancies of 16.38 and 14.05 kilograms-force. Because water can partially fill the voids between beads, the researchers distinguished dry-basis and wet-basis weights; even accounting for the wet weight of about 18 kilograms, the buoys exceeded the certified buoyancy threshold. Critically, in the standard impact test, in which a 10-kilogram pendulum hammer is dropped from two meters, neither buoy fractured or ruptured, showing only shallow surface indentations. Because the individual beads are impermeable and the interconnected structure blocks water ingress, even damaged buoys retain their buoyancy, unlike hollow polymer floats that fail catastrophically when cracked.

The researchers, funded by the Korea Institute of Marine Science and Technology Promotion through the Ministry of Oceans and Fisheries, argue that with minor design optimization these ceramic buoys could satisfy existing certification requirements and become a next-generation, microplastic-free alternative to EPS systems. Because the beads are made from waste glass and bound with inorganic cement, the approach simultaneously recycles a major waste stream and eliminates a major microplastic source at its origin. If scaled, glass-turned-ceramic buoys could offer coastal aquaculture a permanent, durable answer to one of the ocean’s most visible pollution problems.

Subject of Research: Development of eco-friendly porous ceramic buoys made from waste glass to replace expanded polystyrene buoys and mitigate marine microplastic pollution

Article Title: Fabrication of porous ceramic buoys (Ⅰ): preparation of porous ceramic beads for buoy manufacturing

Article References: Park, S.-M., Hong, Y., Oh, C., Kim, J., & Lee, C.-T. (2025). Fabrication of porous ceramic buoys (Ⅰ): preparation of porous ceramic beads for buoy manufacturing. Advances in Industrial and Engineering Chemistry, 1(1), Article 38. https://doi.org/10.1007/s44405-025-00035-8

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00035-8

Keywords: porous ceramic buoys, microplastics, soda-lime glass, foamed glass beads, buoyancy, marine pollution, expanded polystyrene, recycled glass, rotary kiln foaming, eco-friendly materials, aquaculture, ceramic beads