Integrating a Chemicals Perspective into the Global Plastic Treaty

Driven by the growing concern about plastic pollution, countries have agreed to establish a global plastic treaty addressing the full life cycle of plastics. However, while plastics are complex materials consisting of mixtures of chemicals such as additives, processing aids, and nonintentionally added substances, it is at risk that the chemical aspects of plastics may be overlooked in the forthcoming treaty. This is highly concerning because a large variety of over 10,000 chemical substances may have been used in plastic production, and many of them are known to be hazardous to human health and the environment. In this Global Perspective, we further highlight an additional, generally overlooked, but critical aspect that many chemicals in plastics hamper the technological solutions envisioned to solve some of the major plastic issues: mechanical recycling, waste-to-energy, chemical recycling, biobased plastics, biodegradable plastics, and durable plastics. Building on existing success stories, we outline three concrete recommendations on how the chemical aspects can be integrated into the global plastic treaty to ensure its effectiveness: (1) reducing the complexity of chemicals in plastics, (2) ensuring the transparency of chemicals in plastics, and (3) aligning the right incentives for a systematic transition.

S1.2 Formation of dark colors from mixing plastics with different additives/pigments, reducing the aesthetics of secondary plastics. Although simultaneous use of sulphur-containing additives with cadmium or lead compounds (say, for example, thiotin or antimony mercaptide stabilisers in conjunction with lead or cadmium ones) is avoided in formulating practice, it can occur in the processing of PVC scrap of different or uncertain origins. Where fresh stabiliser is to be added to a batch of scrap material of unknown composition to 'post-stabilise' it for processing and service, or where two or more such batches are to be mixed in processing, the possibility of cross-staining should be checked beforehand (even for black material). (p. 155) • The Sustainable Packaging Coalition. n.d. Design for Recycled Content Guide. https://sustainablepackaging.org/projects/design-for-recycled-content-guide/ Color consistency and color matching are common challenges in using recycled plastic, since brands tend to implement very stringent color requirements for packaging. Unlike virgin plastics, which initially do not contain pigments, recycled plastics are derived from mixtures of materials that may contain a wide range of pigmentation. Brands interested in using recycled plastics must manage their expectations and commit to finding ways to work with the color variations present in recycled resins.
White or lightly colored recycled plastics may take on an off-white color. Clear recycled plastics may take on a somewhat yellowed appearance due to the reheating process, or a cloudy appearance due to contamination in the recycled feedstock. Natural, white or lightly colored recycled plastics can be adjusted by adding colorants to match brand colors, however, their new color may appear less vibrant than virgin material colored with the same colorant. Mixed-color streams of recovered plastics can typically only be recycled into dark, opague colors.
While these challenges are more pronounced with higher levels of recycled content, there are numerous examples of plastic packaging containing upwards of 30% recycled content with no or negligible aesthetic deficiencies. That percentage can be considered a general threshold above which aesthetic challenges should be expected to be more noticeable.

S1.3 Formation of toxic by-products from chemical additives during recycling
• Budin C, Petrlik J, Strakova J, Hamm S, Beeler B, Behnisch P, Besselink H, van der Burg B, Brouwer A. 2020. Detection of high PBDD/Fs levels and dioxin-like activity in toys using a combination of GC-HRMS, rat-based and human-based DR CALUX® reporter gene assays.

S1.4
The presence of diverse additives can reduce the compatibility of different plastic waste streams with the same polymer type.
• Mixing PET trays in the PET bottle recycling reduces the quality of recycled PET.
o European Commission. 2019. A circular economy for plastics -insights from research and innovation to inform policy and funding decisions. https://op.europa.eu/en/publication-detail/-/publication/33251cf9-3b0b-11e9-8d04-01aa75ed71a1/language-en/format-PDF/source-87705298 For example, clear PET bottles end up in the same material stream as clear PET trays, where the latter are more diverse through differences due to additives and the formation process. As a result, during the grinding steps of the recycling processes, bottles will be shredded into homogenous scraps while trays will tend to produce smaller scraps, more heterogeneous parts, and more dust which might not be efficiently recycled. (p. 115) S1.5 Concern about contamination of secondary plastics by legacy hazardous additives has resulted in regulations in some parts of the world limiting certain waste plastics from recycling.
• Waste materials containing persistent organic pollutants such as polybrominated diphenyl ethers (PBDEs) and hexabromocyclododecane above allowed concentration limits shall not be directly re-used or recycled within the EU, according to the POP Directive (EU 2019/1021 In carrying out such a disposal or recovery, any substance listed in Annex IV may be isolated from the waste, provided that this substance is subsequently disposed of in accordance with the first subparagraph. 3. Disposal or recovery operations that may lead to recovery, recycling, reclamation or re-use on their own of the substances listed in Annex IV shall be prohibited. o The treatment and disposal of the bottom ash and fly ash of municipal solid waste incineration is an un-ignorable problem. The current results showed that there were a certain amount of microplastics and heavy metals in the fly ash and bottom ash generated from the incineration of municipal solid waste, polluting the surrounding soil environment. Macroplastics and microplastics were all found in the samples, and the content of microplastics in fly ash, bottom ash and soil was 23, 171, and 86 items/kg dw, respectively. The abundance of microplastics in bottom ash was significantly greater than that in around soils. The total proportion of (micro)plastics with different particle sizes in bottom ash, fly ash and soil were: < 0.5 (41.6%, 45.6%, 37.1%), 0.5-1 (27.2%, 30.6%, 25.9%), 1-2 (12.6%, 12.4%, 12.7%), 2-5 (10.2%, 10.2%, 15.7%), and > 5 mm (8.4%, 1.1%, 8.7%). The proportion of microplastics with smaller particle size was higher, and it was easier to diffuse and migrate into the surrounding environment. refuse-derived fuel during the start-up, which was 5-folds higher than that during steadystate conditions. The high concentrations during start-up were attributed to poor combustion conditions that led to PBDD/F formation from BFRs. The researchers also published a similar study on PBDE emissions and found that the average concentration during start-up was 38.8 ng Nm −3 (also 5-folds higher than during steady-state conditions) (Wyrzykowska-Ceradini et al., 2011b). The predominant congeners in the raw flue gas were those present in commercial deca-, octa-, and penta-BDEs, suggesting that these compounds desorb from the waste and get emitted without complete destruction.
o This study found that start-up processes could contribute to at least 27%, 55%, and 2% PBDD/F, PBB, and PBDE emissions of an entire year of MSWI operations.

S2.2
Waste-to-fuel: in addition to contaminated exhaust and residues, the presence of many metals and halogenated chemicals may result in lower quality of the end fuel products, and thus, pose problems on the incinerators and other thermal facilities using the fuels.
• Chiang H, Lin K. 2014. Exhaust constituent emission factors of printed circuit board pyrolysis processes and its exhaust control. o In this work, steam cracking of two post-consumer plastic waste pyrolysis oils blended with fossil naphtha was performed in a continuous bench-scale unit without prior treatment. Product yields and radiant coil coke formation were benchmarked to fossil naphtha as an industrial feedstock. Additionally, the plastic waste pyrolysis oils were thoroughly characterized. Analyses included two dimensional gas chromatography coupled to a flame ionization detector for the detailed hydrocarbon composition as well as specific analyses for heteroatoms, halogens and metals. It was found that both pyrolysis oils are rich in olefins (∼48 wt%) and that the main impurities are nitrogen, oxygen, chlorine, bromine, aluminum, calcium and sodium.
o Steam cracking of the plastic waste derived feedstocks led to ethylene yields of ∼23 wt% at a coil outlet temperature of 820 °C and ∼28 wt% at 850 °C, exceeding the ethylene yield of pure naphtha at both conditions (∼22 wt% and ∼27 wt%, respectively). High amounts of heavy products were formed when steam cracking both pyrolysis oils, respectively. Furthermore, a substantial coking tendency was observed for the more contaminated pyrolysis oil, indicating that next to unsaturated hydrocarbons, contaminants are a strong driver for coke formation.
o Both coke formation and fouling have been put in relation with heteroatoms and metal contaminants present in the post-consumer plastic waste pyrolysis oils. Consequently, using plastic waste derived feedstocks for industrial steam crackers poses opportunities and risks. On the one hand, these feedstocks are attractive as they help to close the material loop. On the other hand, use of these feedstocks may lead to operational issues because of increased coke formation and fouling. Therefore, the "unknowns" in plastic waste pyrolysis oils and their individual influence on industrial steam crackers must be further investigated.   o Although the individual compounds will be specific to the material, conventional as well as bio-based and biodegradable plastics can contain all these chemical categories.

S4.1 Similar levels and complexity of chemicals found in bio-based plastics compared to petroleumbased plastics -leaching of (toxic) chemicals in observed in migration experiments
Additives are particularly relevant for polymers extracted from natural resources, such as starch and cellulose, or from microorganisms, such as PLA, because of their limited physical properties, such as thermal resistance and barrier properties. o By performing both extraction experiments, as well as migration experiments simulating realistic use conditions, Zimmermann and coworkers detected the presence and release of a wide range of chemicals, several of which induced in vitro toxicity, both from petroleum-based and from bio-based plastics (e.g., PLA).

S4.2
Chemical additives added to bio-based plastics based polylactic acid (PLA) to achieve durability for certain applications o As with petroleum-based plastics, the additive make-up of a bio-based plastic will depend on the desired application. Polylactic acid (PLA) is one of the most prominent bio-based plastics, used in a wide range of applications, including in biodegradable applications. However, the brittleness of PLA, its sensitivity to humidity and susceptibility to weathering, requires the addition of fillers or chemical additives to enhance its durability for application requiring a higher longevity.

S4.3
The production of food-contact plastics based on high levels of plant fibers involves the use of several indispensable additives that may pose high risks to food safety.  o To date, excessive migration of hazardous substances (such as melamine) has been reported in some products made of PPCs, and the safety and applicability of PPCs as food contact materials need to be further studied.
o The surface of plant fiber is rich in hydroxyl and carbonyl, which makes plant fiber hydrophilic [24]. However, synthetic resins are mostly nonpolar structures resulting in poor compatibility between the two phases when they are blended with plant fibers [26,33,34], manifested by peeling between two phases, material strength decrease, and poor processability [7]. To increase the compatibility and improve the performance of PPCs, it is usually necessary to introduce proper functional groups for the surface modification of plant fiber to reduce the hydrophilicity, or use additives such as plasticizers and compatibilizers in the compounding process [4,6].
o Surface Modification of Plant Fiber.
Silane is a commonly used surface treatment agent. Cellulose can be treated with silanol aqueous solution, or silane coupling agents [47]. Olive husk flour [28], and a bamboo cellulose nanowhisker [47] treated with different silanes were all found to disperse more evenly in composites. Furthermore, the interfacial compatibility of PPCs was enhanced, and the mechanical properties and thermal stability properties were improved to varying degrees. However, an excessive amount of silanes would lead to its self-condensation reaction, which would cause insufficient silylation reaction and a lower grafting degree of the functional group [47].
Besides silane, other substances can also be used for the surface modification of plant fibers. Pyrrole can be oxidized and polymerized on the surface of bamboo fiber, and the resulting polypyrrole can improve the compatibility between bamboo fiber and PLA, and thus improve the mechanical properties and thermal stability of composites [48].
Alkali treatment is also a commonly used surface treatment method for plant fibers.
Alkaline alkylation reaction occurs on the treated fiber surface, which is beneficial to blending with synthetic resins [7]. However, alkali treatment may reduce the inherent strength of plant fibers [47,49]. In a study, palm fiber (  o Additives, with relatively low molecular weight and high reactivity, are easier to migrate and may have higher safety risks. Long-term exposure of maleic anhydride, which is commonly used in PPCs, will cause certain damage to the respiratory system, digestive system, and kidney [71,72]. Many countries and regions have also set a migration limit for this substance [73,74]. The migration of phthalates as a plasticizer, benzophenonone (BP) and 4-methylbenzophenonone (4MBP), which may be photoinitiators from photocured printing inks or adhesives, were also found in plant fiber-based materials [67,75].
o In addition, the persistent organic contaminants perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) that are refractory with long half-lives and have accumulation effects in organisms, can be used in plant fiber-based materials as surfactants for water-proof and oil-proof functions [76]. Relevant studies have shown that such substances may have reproductive and developmental toxicity and are related to cancer and thyroid diseases [77,78]. Thus, the possibility of perfluorinated or polyfluorinated substances migration should also be considered to avoid associated risks.

S5. Supporting Details on the Impacts of Chemical Additives on Biodegradable Plastics
S5.1 Biodegradable plastics (petroleum-or biobased) only fully biodegrade (i.e., mineralize) in industrial composting facilities, but partial disintegration and/or slow degradation in the natural environment may lead to formation of microplastics and/or release of chemical additives. o The process of biodegradation differs strongly between different environments (mostly dependent on temperature and presence of biodegrading microorganisms), with the most aggressive environment typically being compost, then soil, followed by fresh and marine water and finally landfill. Most standards (like EN 13432) for compostable or biodegradable plastics only refer to industrial compositing and are not applicable to natural environments.
• Rosenboom J-G, Langer R, Traverso G. 2022. Bioplastics for a circular economy. Nature Reviews Materials 7, 117-137. https://doi.org/10.1038/s41578-021-00407-8 o Of note is also the special case of oxo-degradable plastics, which are formulated with additives that render otherwise persistent plastics degradable, but leave behind poorly degradable microplastic fragments (their use is therefore restricted in many countries).

S6. Supporting Details on the Impacts of Chemical Additives on Durable Plastics
S6.1 Migration, transformation and adsorption of chemicals from plastic water bottles during dishwashing.
• Tisler S and Christensen JH 2022. Non-target screening for the identification of migrating compounds from reusable plastic bottles into drinking water. o In this study, we investigated the migration of FCMs from plastic bottles into drinking (tap) water over 24h at room temperature. We detected > 400 plastic related compounds as well as > 3500 dishwasher related compounds. The study shows the importance of considering special cleaning steps for plastic bottles. Generally, dishwasher related compounds were found to adsorb more to plastic than glass, and especially the more non-polar compounds were difficult to remove even by additional water flushing afterwards. Furthermore, the dishwashing process enhanced the migration of plasticizers, antioxidants, and photoinitiators into the drinking water. Therefore, the highest predicted toxic hazard was calculated for the used plastic bottles, that had been refilled directly after the dishwasher, without further flushing. However, 15 out of the 20 highest peaks in the new bottles were assigned to oligomers of plasticizers, as well as aromatic amines which could potentially originate from slip agents or antioxidants. These compounds showed continuous leaching, even after flushing. To our knowledge, no toxicity data are available for these compounds. The identification of DEET in the plastic bottles also raised S19 the question whether the ubiquitous detection of DEET from its use as an insect repellant is correct, or if it has another source.

S6.2
Migration of additives from plastics accelerates product embrittlement and reduces durability. o Additives are often essential to maintain plastic product performance (and safety).
Through migration of additives out of durable plastics over time embrittlement and failure of plastic products is accelerated. Migration influenced by ambient chemical and physical conditions (and therefore depends on the type and location of application).

S6.3
Increased stability of plastics through the protective function of specific chemical additives can have unwanted side-effects.
• Tian Z et al. 2020. A ubiquitous tire rubber-derived chemical induces acute mortality in coho salmon. Science 371, 185-189. https://doi.org/10.1126/science.abd6951 o The tire rubber additive N-(1,3-dimethylbutyl)-N′-phenyl-p-phenylenediamine (6PPD) is designed to protect the tire rubber against ozone-mediated oxidation. However, 6PPD transforms to 6PPD-quinone (and related transformation products) upon deploying its protective antioxidative function, a chemical that is now known to be highly toxic to aquatic species, such as coho salmon.