Complementary roles for mechanical and solvent-based recycling in low-carbon, circular polypropylene

Significance Polypropylene is a relatively low-cost polymer with useful material properties, making it one of the most widely produced plastics. Unfortunately, repeated mechanical recycling of polypropylene degrades its properties, performance, and aesthetics, so recycling infrastructure for polypropylene is underdeveloped and it often ends up in landfills. Solvent-assisted recycling processes like dissolution have emerged, offering near virgin-quality recycled polypropylene and the promise of greater circularity. To clarify the sustainability of circular polypropylene, we offer a detailed life-cycle evaluation of mechanical recycling, dissolution-based recycling, and virgin polypropylene production. We find that while dissolution-based recycling offers modest greenhouse gas savings relative to virgin polypropylene, it serves as an important upgrading step to broaden markets served by recycled polypropylene and displace demand for virgin resin.


Virgin Polypropylene Production
Virgin polypropylene production involves two main processes: propylene monomer synthesis and conversion to polypropylene (PP).Propylene production happens alongside petroleum refining and the production of several other products, including other olefins like ethylene.In fact, propylene is often considered a byproduct of ethylene production (1).The main production process is thermal cracking or steam pyrolysis of fossil feedstocks (2).Propane is cracked to produce propylene and ethane is cracked to make ethylene.The "cracking" refers to the breaking of C-H bonds to allow for double bonds.
The process starts with feeding saturated hydrocarbons and steam to a hydrocracker where they are heated to ~1000ºC (2).In the case of olefin production, lighter phase feedstocks like LPG or light naphtha are preferable (1).To stop the reaction after sufficient heating, the cracked products are cooled with heat exchangers generating high pressure steam.The gas stream goes through a centrifugal compressor to remove fuel oil and then undergoes hydrogen sulfide removal.Lastly, fractional distillation separates the reaction products.
Propylene is converted into PP via free-radical polymerization usually with Ziegler-Natta (Z-N) or metallocene catalysts (1,2).There are multiple processes used by industry to make polypropylene including gas-phase polymerization and solution or liquid-phase polymerization (2).In gas-phase polymerization, propylene vapor is mixed with the catalyst in a fluidized bed reactor (Figure 1) (2,3).The reactor is typically kept at 80-90ºC with a pressure of 90-25 atm.Any gaseous propylene that does not react is added back to the feed stream.The result of the reaction is solid PP which is then dried and pelletized.Liquid-phase polymerization is conceptually similar except the feedstock propylene is liquid and the reactor is tubular.
We use mass and energy flow data for PP production, inclusive of both propylene synthesis and conversion to polymer, from a 2011 ACC (American Chemistry Council) report for analysis (2).

Plastic Waste Sorting
Our analysis begins with plastic waste sorting at an MRF.The efficacy and associated benefits of most recycling processes are highly dependent on feedstock composition and require waste materials to be sorted for reclamation (4).Initial sorting occurs at MRFs, where recyclable waste is separated by material type (e.g., plastics, fibers, metals, glass).Generally, MRFs also separate plastics by polymer type using optical sorters and near infrared (NIR) technology.Most MRFs in the U.S. today primarily target and bale PET and high-density polyethylene (HDPE), with a particular focus on bottles, during sorting (5,6).Other polymers, including PP, can also be targeted for selective recovery using the same NIR technology.However, PP is more commonly baled with other non-PET, non-HDPE plastics in mixed bales.These mixed bales are typically called #3-7 bales referring to the resin identification codes for polyvinyl chloride (#3), low-density polyethylene (#4), PP (#5), polystyrene (#6), and other plastics (#7).While #1 and #2 plastics, PET and HDPE respectively, are typically targeted for separate recovery, sorting efficiencies at MRFs are imperfect and some of these materials may end up in Mixed #3-7 bales.Mixed #3-7 bales include rigid product forms but exclude film plastics, which are harder to separate and recycle.
To represent current practices across most of the U.S., we assume the initial input to any PP recycling process is a mixed #3-7 bale (Figure S1) rather than a PP-specific waste bale from an MRF.Non-target (non-PP) materials are then separated out by float-sink separation and routed for disposal.If a wider variety of polymer types, including PP, become attractive candidates for recycling, MRFs may add new "lines" to recover these materials separately.Alternatively, so-called secondary MRFs may be constructed to take in #3-7 bales and further separate the material (7).Neither of these developments will dramatically impact the energy footprint of recycling.The energy footprint of sorting at MRFs (4.7-7.8 kWh of electricity per tonne of waste throughput) is small compared to the thermal and electrical energy needed during mechanical or advanced recycling processes (4).Because only 3.7% of current MRF throughput are #3-7 plastics (8), we assume the MRF energy and GHG impacts that can be allocated to #3-7 bales are negligible.

Feedstock Type
The assumed input for all of our recycling scenarios is a mixed #3-7 bale, the most typical bale containing PP available from MRFs.We initially assumed the bale composition reported by APR (The Association of Plastic Recyclers) in a 2017 presentation (9), which was derived from a 2015 APR report which goes into further detail on MRF bale compositions (10).This included an "Other" category of 11% but instead of having unknowns, we assumed the maximum allowable contamination of paper (2%), metals (assumed to be aluminum) (2%), and liquids (assumed to be water) (1%) as given by the APR's model bale specifications (11).The remaining unknown component (a total of 6%) was distributed evenly to PET, HDPE and PP.The final composition used for this study is presented in Figure S1 below.In reality, bales will vary considerably from MRF to MRF.Furthermore, sorting operations and subsequently, the composition of outgoing MRF bales, are constantly evolving with changing market conditions; as the price of different materials and polymers change, MRFs modify operational configurations and bring in new equipment to recover valuable, salable material.

Feedstock Transportation
We assume that on average, transportation of material from site of sorting/separation (i.e.MRFs) to PP recyclers (mechanical or otherwise) includes 0.66 tonne-km of truck transport and 0.25 tonne-km of rail transport per kg of incoming material to recyclers (12).We assume flatbed trucking for truck transport.

Limitations of mechanical recyclate
PP can only undergo traditional mechanical recycling a few times before irrevocably degrading to unusable materials in circular manufacturing.PP has methyl groups on every other carbon atom, making it prone to chain scission during mechanical grinding and high shear rates during melt processing (13).In contrast, polyethylene (PE), the most common plastic, can withstand over 30 recycling cycles (13,14).Repeated mechanical recycling of PP results in reduced molecular weight, increased crystallinity, reduced impact resistance, and increased opacity (15)(16)(17)(18)(19).Because of this physical degradation and difficult-to-separate impurities in plastic waste streams, the material produced from mechanical recycling is usually of lower quality than virgin resin (20).Mechanically recycled PP has inferior physicochemical properties than virgin PP.For most applications, PP recyclate must be blended with virgin polymer to ensure sufficient material performance (15,(21)(22)(23).For some uses including automotive applications and food-safe packaging, which have stricter thermal, chemical, and mechanical property requirements for safety reasons, PP recyclate can be deemed entirely unsuitable (20,22,24,25).

Substitution factors for mechanical recyclate
Some studies account for the imperfect substitution between mechanically recycled and virgin PP by using a substitution factor (Figure S2).For example, 1 kg of mechanically recycled PP can be treated as functionally equivalent to 0.7-1.0kg of virgin PP (26)(27)(28)(29)(30)(31)(32).Some studies may use terms like "substitution factor" or "substitution ratio" in reference to technical recovery efficiency of a recycling process (e.g., mass maintained versus lost during recycling process) (27,33,34), market substitution factor (e.g., additional material required to make a given product with lower-grade recyclates versus virgin-grade polymer) (26,27,31,33,34), value-corrected market substitution factor or displacement rate (e.g., price-based ratio describing market uptake of recyclate) (30,34,35), and blending limits (e.g., when blending with virgin material, the maximum allowable recycled content to avoid excessive quality loss) (23) (Figure S2).For the purposes of conducting a life-cycle assessment, clearly defining an appropriate functional unit (i.e., virgin material displaced per unit of waste plastic versus per unit of recyclate) is critical.

Figure S2. Substituting Virgin PP with Recycled PP
This figure depicts how recycled material can substitute virgin material use and the limits to perfect substitution for mechanical recyclates.Note that both recycling processes have mass losses which are not depicted in this figure; only loss to quality is included.
This study does not apply a substitution factor because these values are uncertain and product-specific.There are no currently available, robust estimates for how much virgin resin production has actually been offset by recyclates (36).While technical process efficiencies are well understood and can be measured directly for a particular recycling method, market substitution factors and blending limits are under-reported and variable depending on application and product type (37).Furthermore, the amount of recycled PP that the market could absorb is likely still much greater than the quantity available today.Until the industry-wide capacity for blending recycled PP has been reached for all product and application types, recycled PP can offset virgin material use on a 1:1 basis unless increased material use is also required (Figure S2).

Energy Consumption Results
The energy consumption results, normalized in units of kWh per tonne, are presented in Figure S4 and Table S1 below.Our modeling efforts in SuperPro Designer yield full material and energy balance data for mechanical recycling processes.These results are only used in the LCA of solvent-assisted recycling to model pretreatment to PP dissolution; they are not used in the LCA of PP mechanical recycling because real-life facility-scale data is available in literature (12).

LCA Model: Calculations
The basic math upon which the LCA model is built is provided below: ( − ) =  N = total number of unit processes  = NxN input-output matrix with life-cycle inventories for each unit process (non-zero data listed in Table S4)  = NxN identity matrix  = N length vector w/ direct requirements for scenario analysis  = N length vector w/ life-cycle requirements for scenario analysis  = N length vector w/ emission factors for each unit process (emission factors by pollutant type are provided in Table S3)

LCA Model: Emission Factors
All values are in units of kg of pollutant (given by column name) per unit indicated in the unit process name.These are not necessarily life-cycle emission factors and only definitely include direct emission impacts (in several cases, this means fugitive emissions associated with fuel combustion); full life-cycle impacts must be assessed through the model using this data along with IO data from Table S4.When possible, GHG emission factors are separated by pollutant type; in cases where this is not possible, total GHG impact in CO2 equivalence is given by the CO2 column while the CH4 and N2O columns are marked with zeros.

Table S4. LCA Model: Input-Output Matrix Relationships
Our LCA model uses a physical units-based input-output matrix that is with life-cycle inventories for each unit process/product included.The relevant non-zero values are included in this table.Each unit process/product is listed with a unit.The value indicates the amount of the upstream/downstream requirement in its listed unit required to make 1 unit of the primary unit product/process.If any unit processes from Table S3 is not included in this table, there are no appreciable upstream/downstream impacts for that parameter.

Unit Process
Upstream

Sensitivity Analysis
To capture uncertainty and variability in process yield, energy consumption, and transportation impacts for the PP recycling scenarios, we conduct 10,000 Monte Carlo simulations using triangular probability distributions as given by Table S5.For all parameters, except for energy consumption for mechanical recycling, the mode is equivalent to the original model value.
Because the original energy use data for mechanical recycling is not broken down by pretreatment and extrusion (12), we opted to use the SuperPro Designer results for the modes instead of the original model data.The total energy consumption for mechanical recycling used in the original LCA falls within the aggregated ranges in Table S5.For mechanical processes, we use specific minimum and maximum values from SuperPro Designer and Larrain et al.
(2021) (53).For all other parameter probability distributions, the maximum and minimum values are equally spaced from the mode.Table S8.Emission Factors Forecast for PP Production and Recycling Emission (Data for Figure 4) The units for all values are kg CO2eq per tonne of PP output.

Figure S4 .
Figure S4.Mechanical Recycling Energy Consumption by Unit Process

LCA Results: Tabulated Data Table S7. Life-Cycle Greenhouse Gas Impacts from Virgin Production and Recycling (Data for Figure 2)
Preprocessing for solvent-assisted recycling includes much of what is considered part of the main process for mechanical recycling.This includes electricity and natural gas use.For mechanical recycling, preprocessing includes washing agents.Units for all values are kg CO2eq per tonne PP produced. *