sustainability Article Plastic Bottle Cap Recycling—Characterization of Recyclate Composition and Opportunities for Design for Circularity Markus Gall 1,* , Andrea Schweighuber 2, Wolfgang Buchberger 2 and Reinhold W. Lang 1 1 Institute of Polymeric Materials and Testing, Johannes Kepler University Linz, Altenberger Strasse 69, 4040 Linz, Austria; [email protected] 2 Institute for Analytical Chemistry, Johannes Kepler University Linz, Altenberger Strasse 69, 4040 Linz, Austria; [email protected] (A.S.); [email protected] (W.B.) * Correspondence: [email protected]; Tel.: +43-732-2468-6626 Received: 17 November 2020; Accepted: 10 December 2020; Published: 11 December 2020 Abstract: In line with efforts to create a circular economy of plastics, recent EU legislation is strengthening plastic bottle recycling by ambitious separate collection targets and mandatory recycled content obligations. Furthermore, explicit design requirements on the caps of bottles and composite beverage packaging have been introduced. These caps are typically made of polyethylene or polypropylene and often contain additives such as slip agents and anti-statics. Commercially available bottle cap recyclates (BCRs) as well as specifically formulated model compounds were analyzed in terms of composition by means of infrared spectroscopy, differential scanning calorimetry, and high-performance liquid chromatography. Their composition was found to be heterogeneous due to polyolefin cross-contamination, directly reflecting the diversity of cap materials present in the market. Slip agent legacy additives originating from the initial use phase were found and quantified in both commercial and model cap recyclates. This highlights the opportunity for redesigning plastic bottle caps not only in response to regulatory requirements, but to pursue a more comprehensive strategy of product design for circularity. By including considerations of polymer resin and additive choice in cap manufacturing, more homogeneous waste streams could be derived from plastic bottle cap recycling, enabling recycling into more demanding and valuable applications. Keywords: circular economy; design for circularity; EU policy; legacy additives; mechanical recycling; plastic bottle cap; polyethylene; polypropylene; single-use plastics directive; slip agent 1. Introduction 1.1. Changing Policies and Regulatory Requirements Affecting Plastic Bottles and Their Caps The concept of a circular economy of plastics is quickly gathering momentum [1–4] as our understanding of national [5,6], regional [7], and global [8,9] plastics flows and stocks, as well as the associated environmental burdens [10,11], is evolving. Plastic packaging in particular [3,5] is in the focus when it comes to addressing the transition from linearity to circularity. Packaging is both the main application field for single-use plastics [12] and the biggest group among different post-consumer plastic waste fractions [5,13]. Plastic bottles made of polyethylene terephthalate (PET) are a good example of well-defined post-consumer plastic waste streams that are suitable for high-quality mechanical recycling [14]. The recycling schemes and processes for PET bottles have evolved to very high standards that allow for post-consumer recyclates to be reintroduced into new food contact applications [15]. Year by year, humans produce several hundred billion plastic bottles [16]. As long as appropriate collection Sustainability 2020, 12, 10378; doi:10.3390/su122410378 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 10378 2 of 21 Sustainability 2020, 12, x FOR PEER REVIEW 2 of 21 schemeshumans are inproduce place, theseseveral bottles hundred can b beillion closed-loop plastic bottles recycled [16]. into As newlong bottlesas appropriate (bottle-to-bottle) collection [15]. Besidesschemes that, bottlesare in place, may these be open-loop bottles can recycledbe closed-loop into therecycled textile into industry new bottles (bottle-to-fiber) (bottle-to-bottle) [17 [15].] or into thermoformingBesides that, sheet bottles products may be open-loop (bottle-to-sheet) recycled [into18]. the Therefore, textile industry while (bottle-to-fiber) a number of recycling [17] or into paths exist forthermoforming PET bottles, sheet there products seems to(bottle-to-sheet) be no high-value [18]. reutilizationTherefore, while path a number for the bottleof recycling caps, whichpaths are separatedexist for from PET the bottles, PET duringthere seems recycling to be no (see high-val Figureue1 ).reutilization path for the bottle caps, which are separated from the PET during recycling (see Figure 1). FigureFigure 1. The 1. plasticThe plastic bottle, bottle, its characteristics its characteristics with with relevance relevance to recycling,to recycling, and and potential potential recycling recycling paths paths (own illustration). (own illustration). The caps, however, represent a growing waste stream and too often they find their way into the The caps, however, represent a growing waste stream and too often they find their way into the environment [19]. Growth forecasts for major packed beverage sectors such as bottled water [19] and environmentspecifically [19 for]. Growthplastic bottle forecasts caps for[20,21] major have packed been vital beverage in pre-COVID-19 sectors such times as bottled and it waterseems [19] and specificallyreasonable to for assume plastic that bottle the material caps [ 20volu,21mes] have are set been to further vital in increase pre-COVID-19 [22,23]. times and it seems reasonableMost to assume recent EU that legislation the material on single-use volumes plastics are set tolaid further down increasein Directive [22 (EU),23]. 2019/904 [24] is Mostgoing recentto target EU this legislation segment by on demanding single-use plasticsthat caps laid and downlids of inboth Directive beverage (EU) bottles 2019 and/904 composite [24] is going to targetbeverage this segment packaging by demandingwill be subject that capsto produc and lidst requirements of both beverage (Article bottles 6), andextended composite producer beverage packagingresponsibility will be subject(Article to 8 product paragraph requirements 2), and awar (Articleeness 6),raising extended measures producer (Article responsibility 10) in future. (Article 8 paragraphAdditionally, 2), and there awareness will be a raisingrequirement measures for separate (Article collection 10) in future.for recycling Additionally, (Article 9) there of plastic will be a requirementbeverage for bottles separate including collection their caps for recyclingand lids. The (Article separate 9) of collection plastic beverage target for bottlesbeverage including bottles is their 77% by 2025 and 90% by 2029 [24]. The foreseen product requirements demand that caps and lids caps and lids. The separate collection target for beverage bottles is 77% by 2025 and 90% by 2029 [24]. remain attached to the container during usage in order to avoid littering. All bottle cap manufacturers The foreseenare hence product confronted requirements with the need demand to redesign that caps their and caps, lids if remainthey are attachedto be marketed to the containerin the EU. duringIn usageview in order of the to juxtaposition avoid littering. of more All stringent bottle cap requirem manufacturersents on material are hence circularity confronted on the one with hand the and need to redesignquite their impactful caps, ifproduct they are design to be marketedrequirements in thefor EU.caps In on view the ofother the hand, juxtaposition attention of needs more to stringent be requirementsdevoted to on understanding material circularity the current on the system. one hand Ideally, and harvesting quite impactful the potential product benefits design of increased requirements for capsbottle on cap the otherrecycling hand, will attention become needsa reality to bein devotedthe near tofuture understanding by making clever the current adjustments system. that Ideally, harvestingcombine the legislatively potential required benefits changes of increased with a bottleviewpoint cap of recycling design for will circularity. become a reality in the near future byThe making literature clever provides adjustments a number that of combine examples legislatively of reuse andrequired repurposing changes approaches with afor viewpoint plastic of designbottles for circularity. and their components. Bottles with their caps on have been investigated for their suitability as alternative building materials in the construction sector [25–27]. Post-consumer plastic bottle caps The literature provides a number of examples of reuse and repurposing approaches for plastic have been suggested to function as core elements in aluminum-plastics composite sandwich bottlesstructures and their [28]. components. Other small- Bottles and large-scale with their repu capsrposing on have strategies been investigated included the for handcrafting their suitability of as alternativemolecular building models materials for chemistry in the teaching construction purposes sector out [of25 used–27]. caps Post-consumer [29,30] and the plastic utilization bottle of caps caps have been suggestedas an alternative to function combustion as core fuel elements to low-quality in aluminum-plastics lignite [31], respectively. composite sandwich structures [28]. Other small- and large-scale repurposing strategies included the handcrafting of molecular models for chemistry teaching purposes out of used caps [29,30] and the utilization of caps as an alternative