Microplastics are often discussed mainly as particles: their persistence, their distribution through soil and water, and the possible physical or toxicological effects of ingestion by organisms. This focus is important, but it can overlook another relevant pathway—the release of additives originally incorporated into the plastic or rubber matrix.

Many materials contain antioxidants, pigments, plasticizers, ultraviolet stabilizers, flame retardants, and processing aids. Some are chemically bound, but many are only physically embedded in the polymer. While an article remains intact, durable, and stable, matrix inclusion may substantially limit exposure. In such cases, low bioavailability can be a reasonable assumption, particularly where the article has little realistic potential to fragment during use or after disposal.

For these low-fragmentation applications, long service life, controlled use, maintenance, and proper recycling can help keep additives contained within the matrix and prevent uncontrolled release. Durable articles that remain in managed material cycles are therefore different from products that are routinely abraded, weathered or dispersed into the environment. Recycling can extend the useful life of the material, but it should preserve containment and avoid converting a large article into a more exposed form without adequate controls.

The situation changes when the material is likely to undergo microplastification: progressive weathering, abrasion, cracking and fragmentation into microplastic-sized particles.

As particle size decreases, the surface-area-to-volume ratio increases, and the distance an additive must travel to reach the surface becomes much shorter. Weathering may also create cracks, pores, and oxidized regions that increase accessibility. Microplastics may therefore act not only as particles, but also as continuing sources of additives and transformation products.

The 6PPD case in tire rubber illustrates this principle. Tire wear produces small rubber particles, and 6PPD can become accessible at the surface and transform into 6PPD-quinone, a substance associated with acute toxicity in sensitive fish species.

A precautionary assessment should therefore distinguish between two broad situations. Where an article has a low probability of microplastification and remains within a durable, controlled, and recyclable use cycle, matrix inclusion and limited bioavailability may remain reasonable assumptions. Where fragmentation, abrasion, or uncontrolled environmental release is foreseeable, those assumptions are insufficient on their own.

In higher-risk cases, the assessment should also consider additive diffusion, leaching, particle-size reduction, weathering, transformation products, and long-term exposure in water, soil, and sediment.

The key regulatory point is that the behavior of the intact article does not always reflect the behavior of the same material after aging, recycling, abrasion, or fragmentation. Where microplastification is plausible, the environmental effects of additives in the matrix should be included in the risk assessment rather than treated as permanently contained.

The full scientific analysis, including the diffusion model, the influence of particle size and crystallinity, and the 6PPD case study, is available in the article Microplastification as an Accelerator of Additive Release from Plastics. The article provides the detailed assumptions, calculations, limitations, and references supporting the precautionary approach outlined above.