A weight of evidence analytical approach: Understanding chemical composition of synthetic polymers to aid in the interpretation of biodegradation results

In 2023, the European Commission published a ground-breaking restriction on the use of synthetic insoluble polymer microparticles (i.e., microplastics) that included fragrance microcapsules, despite its low contribution to the microplastic pool. Setting the bar for microplastic regulations, the restriction included a first-of-their-kind exemption for microparticles that met specified rules for proving biodegradability. Among the exemption to this restriction called for the polymer and all components of the polymer “blend” to be biodegradable, under specific testing methods. However, the restriction did not provide clear guidance to distinguish this “blend.” In the present study, examples of fragrance microcapsules were used to introduce a rigorous purification method and a suite of analytical techniques such as FT-IR, SEC, GC, spectroscopy, amino acid (LC) and renewable carbon analysis (MS) in a weight of evidence (WOE) approach as means to provide clarity and differentiation of a “blend” vs a “non-blend”. Biodegradation studies followed the OECD 310 specified by the European Commission for concluding polymer biodegradability. In one example, the fragrance microcapsule prepared through interfacial polymerization of polyisocyanate and gelatin, with 83 % biodegradation of the purified wall, was concluded as a “blend”; distinct layers of biodegradable biopolymer shell and a non-biodegradable polyurea shell. In another example, a new prepolymer process was developed between the polyisocyanate and gelatin to form a “non-blend” co-polymer, despite having similar starting materials, biodegradation results and morphologies versus the first microcapsule. This study highlights the necessity of the WOE for validating polymer composition to avoid false positive biodegradability conclusions. This approach may be extended to assessing the structure of any complex, insoluble polymeric material.