Recycling's environmental credit in life cycle assessment (LCA) is governed by the technical substitutability coefficient - the amount of virgin material that a unit of recycled ('secondary') material actually displaces. Rigamonti et al. (2020) proposed guidelines for computing these coefficients from one representative technical property per material-application pair and derived sixteen coefficients for ten secondary materials, while acknowledging that the estimation methodology is not settled: several technical properties usually determine functionality jointly, and coefficients for materials passing through multiple recycling loops - where the material entering each new loop is a mixture of virgin and previously recycled material - were not quantified. A 2023 review of 49 papers states that no universally agreed substitutability approach exists, a 2025 test-based study of multi-cycle recycling of high-density polyethylene (HDPE), polyethylene terephthalate (PET), poly(lactic acid) (PLA) and polyhydroxybutyrate (PHB) - a scope that excludes polypropylene - explicitly calls for a harmonized testing method for quality degradation across polymers, and no work to date couples multi-loop mixture dynamics to substitutability coefficients. The problem asks for a settled, validated estimation methodology - general property-selection and aggregation rules for multi-property applications, and loop-dependent coefficients for virgin/previously-recycled mixtures, demonstrated at least for polypropylene - together with the coefficients it yields, the extension of coverage beyond the sixteen published coefficients, and their propagation into waste-management LCA.

Background

Life cycle assessment (LCA) is the standard framework for quantifying the environmental impacts of products and systems from raw-material extraction to end of life. When a waste-management system recovers materials for recycling, LCA typically credits the secondary material with avoiding production of an equivalent amount of virgin material. The strength of that credit is governed by the substitution ratio - here called the technical substitutability coefficient - defined as the amount of virgin material functionally displaced per unit of secondary material. Many studies assume a 1:1 ratio, which is generally incorrect: recycling usually alters material properties, so one kilogram of recyclate (the recovered secondary material itself) displaces less than one kilogram of virgin material, and the coefficient depends on the application in which the recyclate is used. Recycling is called closed-loop when the material returns to its original application and open-loop when it is used in a different one; the substitutability coefficient is the LCA quantity that makes this distinction quantitative. Reviews of the literature up to 2020 found a wide variety of mutually inconsistent calculation procedures for these coefficients, sometimes under different terminology. Rigamonti, Taelman, Huysveld, Sfez, Ragaert and Dewulf (2020) therefore proposed guidelines for calculating technical substitutability coefficients from measured technical characteristics: identify the key function of the secondary material's target application, select a single representative technical property characterizing that function, and compute the coefficient as the ratio of that property in the secondary material to that in the displaced virgin material. Applying this to two case studies - minerals recovered from municipal solid waste incineration bottom ash (the residue of waste incineration, used for instance in construction) and mechanically recycled plastics - they derived sixteen coefficients for ten secondary materials. They also summarized an older cycle-count approach that discounts quality by the number of recycling loops a material can withstand, RQ=1−1/(nrc+1)RQ = 1 - 1/(n_{rc}+1), where nrcn_{rc} is the number of recycling loops the material survives (Rigamonti, Grosso and Sunseri 2009, following the ISO/TR 14049 technical report, which gives worked examples of allocation in life cycle assessment). The authors themselves flagged what remains unresolved. First, the one-property-per-application choice is a simplification made for feasibility: in most applications several properties (mechanical, processing, optical) jointly determine whether a recyclate can substitute virgin material, and there is no settled rule for selecting or combining them across applications. Second, the plastic case study derived coefficients for recyclate displacing virgin polypropylene (PP) within a single loop, but across a succession of recycling loops the material entering each new cycle is itself a mixture of virgin and previously recycled PP; the cumulative effect of repeated loops on mechanical performance - and hence the coefficient applicable in later loops - was not quantified. Third, the sixteen published coefficients are explicitly not exhaustive, and the bottom-ash coefficient depends strongly on the composition of the incinerated waste, so it requires re-derivation rather than transfer to other incineration contexts.

Problem Statement

Rigamonti et al. (2020) left the estimation methodology for technical substitutability coefficients of secondary materials in waste-management LCA unsettled. The open problem is to establish a settled, validated method for computing these coefficients - one that a qualified LCA practitioner can apply reproducibly - which resolves, within a single coherent methodology: (1) a general rule for selecting and aggregating the several technical properties (mechanical, processing, optical) that jointly determine the functionality of the secondary material in its target application, replacing the source framework's ad hoc choice of one representative property per material-application pair; (2) the quantification of substitutability under multiple recycling loops, in which the material entering each successive cycle is a mixture of virgin and previously recycled material - measuring and modelling the cumulative effect of repeated loops on the technical properties so that loop-dependent coefficients are derived rather than assumed constant, with polypropylene, the source's flagship unresolved case, as the first target (no test-based multi-loop coefficient currently exists for it); and (3) demonstrated transferability, by applying the method to extend coefficient coverage beyond the sixteen published values - which must include at least one of: a further secondary material, or bottom ash from an incineration context whose waste composition differs from the source case study. For this purpose a method counts as settled and validated when it states a reproducible procedure in general form, is demonstrated against measured technical properties rather than assumed values, and yields explicit coefficient values; the demonstrations required here are the polypropylene multi-loop case together with the extension beyond the sixteen published coefficients. The problem also asks how the resulting coefficients propagate into the substitution accounting of a waste-management LCA.

References

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