A new Views & Comments article published in Engineering highlights that additives in plastics represent a major, understudied frontier in recycling science, with substantial knowledge gaps regarding their behavior, accumulation, and impacts across circular life cycles. Authored by Ali Gooneie and Kim Ragaert from Maastricht University, the article defines additives as chemical substances intentionally added to virgin or recycled plastics to modify end-use properties, following European standard EN 17615:2022, while excluding inorganic fillers and separately applied components such as inks and tie layers. The authors note that production and degradation of these additives can generate non-intentionally added substances (NIAS), which are relevant but not classified as additives themselves.

While extensive research exists on recycling monomaterials and binary polymer blends, far less attention has focused on minor components like additives, whose fate during real-world recycling remains poorly understood. The article points out that functional additives supporting stability, flexibility, color, and flame resistance create extensive chemical diversity across polymers and applications. These substances accumulate systematically throughout the plastic life cycle, with residues from synthesis, degraded fragments from processing, and further breakdown during the use phase leading to complex mixtures of reactive chemical species in post-consumer waste. Collection and sorting streamline polymer types but do not resolve additive diversity within the same polymer category, and some additives may leach during washing before recycling.

Mainstream recycling technologies each face distinct challenges with additives. Mechanical recycling, though energy-efficient, cannot remove or neutralize additives, leading to gradual accumulation of unwanted compounds; dilution with virgin material helps mitigate this issue. Chemical recycling breaks down polymers into monomers or feedstocks and can separate additives via purification, yet processes such as pyrolysis are sensitive to certain elements and compounds that may disrupt reactions, damage equipment, or form toxic byproducts. Emerging solvent-based recycling selectively dissolves polymers to separate additives and contaminants, potentially recovering valuable additives, but faces constraints around solvent selection, separation complexity, and environmental and economic performance.

Analytical limitations further hinder progress, as many additives occur at low concentrations and are hard to detect with existing offline and in-line methods, restricting tracing, regulatory compliance, and process design. The authors also cite limited transparency around additive formulations, often protected as proprietary information, alongside legacy additives and NIAS that raise health and environmental concerns, including leaching into ecosystems and food chains.

The article outlines priority research directions, including mapping additive flows and interactions across recycling technologies, identifying degradation mechanisms and mutual interference between different additive classes, and developing faster, more accurate detection supported by data science and artificial intelligence. The authors emphasize that simply supplementing recycled plastics with extra additives is not sustainable without systematic study of cascade effects, and greater transparency across the value chain is essential to improve material safety, performance, and circularity in plastic recycling.

The paper “Additives: The Next Frontier in Recycling Research,” is authored by Ali Gooneie, Kim Ragaert. Full text of the open access paper: https://doi.org/10.1016/j.eng.2025.12.022. For more information about Engineering, visit the website at https://www.sciencedirect.com/journal/engineering.