Thiocracking of Multi-Materials: High-Strength Composites from Post-Consumer Food Packaging Jars

Abstract

A significant waste material threatening sustainability efforts are post-consumer food packaging goods. These ubiquitous multi-materials comprise chemically disparate components and are thus challenging targets for recycling. Herein, we undertake a proof-of-principle study in which we use a single-stage method to convert post-consumer multi-material food packaging (post-consumer peanut butter jars) to a high compressive strength composite (PBJS₉₀). This is accomplished by thiocracking the ground jar pulp (10 wt. %) with elemental sulfur (90 wt. %) at 320 °C for 2 h. This is the first application of thiocracking to such mixed-material post-consumer goods. Composite synthesis proceeded with 100% atom economy, a low E factor of 0.02, and negative global warming potential of −0.099 kg CO₂e/kg. Furthermore, the compressive strength of PBJS₉₀ (37.7 MPa) is over twice that required for Portland cement building foundations. The simplicity of composite synthesis using a lower temperature/shorter heating time than needed for mineral cements, and exclusive use of waste materials as precursors are ecologically beneficial and represent an important proof-of-principle approach to using thiocracking as a strategy for upcycling multi-materials to useful composites.

1. Introduction

Despite strides in chemical recycling methods for various types of plastics, the quantity of plastic waste deposited in landfills or incinerated far exceeds the quantity recycled [1,2]. The amount of landfilled plastic waste is projected to grow at an exponentially faster rate than its recycling and incineration [3,4,5], and it is already estimated that 91% of plastic waste is deposited as litter in the environment or landfilled, rather than recycled [6,7,8,9]. Plastics exposed to the elements in the environment can shed microplastics, fibers, and other pollutants into the atmosphere, water systems, and surrounding areas, leading to detrimental effects on the environment, ecological systems, and human health [10,11,12,13,14,15,16,17,18,19,20,21,22]. There is a dire need to ameliorate these detrimental effects by finding improved methods for chemically recycling various plastics [23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41]. Many single-use packaging items in circulation today are multi-materials comprised of various plastics, paper, and other materials, often in multi-layer assemblies fixed together by adhesives that are not easy to physically separate into their chemically distinct components [42,43,44,45,46,47]. Familiar examples include plastic jars or bottles having paper or plastic labels. The separation process is further complicated in many cases by contamination of packaging components with unused goods such as food waste in discarded jars and bottles. Mixed-composition waste thus poses a colossal challenge for chemical recycling, in particular due to the divergent chemical reactivity of each component. In order to create a quintessential green economy, it is necessary to design recycling methods that can address various plastics and materials in a single process. Thiocracking is a promising approach in this regard. Thiocracking is a general term for the breakdown of organic polymeric materials by heating them in the presence of elemental sulfur [48,49,50,51]. Because elemental sulfur is a byproduct of the oil refining process stockpiled at waste sites [52,53], its utilization further contributes positively to sustainability and environmental remediation goals [54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122]. Thiocracking has proven effective in chemical recycling/upcycling of a wide range of synthetic plastics and biopolymers including polyesters [49,123,124,125], polyacrylics [50], polycarbonates [51], lignin [126,127,128], cellulose [129], starch [130,131], and raw lignocellulosic biomass [132,133]. The broad applicability of thiocracking is attributable to the homolytic ring-opening of sulfur and self-polymerization to form polymeric sulfur diradicals at >159 °C. These radical-terminated sulfur catenates react with both organic radicals and radical-reactive species that form when organics are heated, allowing diverse reactivity with a range of functional groups (Figure 1). Thiocracking is thus an attractive method for the potential single-stage, one-pot, atom-economic conversion of mixed-material waste streams.

The objective of the current study is to demonstrate that thiocracking is a viable method for upcycling challenging multi-materials that (1) are difficult to physically separate into their individual components, (2) comprise challenging mixtures of plastic, paper, and food waste, and (3) are produced in high volume. Post-consumer peanut butter jars meet all of these criteria. Peanut butter is popular globally, with consumption worldwide projected to increase by 36% between 2023 and 2028, and over 612 million jars of peanut butter are already produced annually in the United States alone [134]. Other than removing the lid, it is difficult to physically separate any of the other components of a post-consumer peanut butter jar: the induction seal and label are attached with adhesive and residual peanut butter sticks tenaciously to the sides of the jar, making it a prototypically challenging multi-material for the current study (Figure 2).

Previous investigations into the preparation of composites via reaction of elemental sulfur with triglycerides, PET, polysaccharides, or combinations thereof (vide infra) suggested that a more complicated mixture of such chemical components may give similar high-strength composites. For these reasons, a peanut butter jar was selected as a food waste container having triglycerides (representative of fat in various food wastes) and cellulose (to represent polysaccharides present in many food wastes), and PET as a high-volume plastic waste.

Herein, we report the thiocracking of a post-consumer peanut butter jar material with elemental sulfur to yield the composite PBJS₉₀, which has 2.2 times the compressive strength required for residential building foundations. Composite PBJS₉₀ is prepared entirely from waste materials with 100% atom economy, a remarkably low E factor [135] of 0.02, and a negative global warming potential of −0.099 kg CO₂e/kg.

2. Materials and Methods

2.1. Instrumentation and Calculations

Proton NMR spectra were acquired on a Bruker NEO-300 MHz (Bruker, Billerica, MA, USA) at room temperature and data processed with MestReNova x64-14.3-30573 software. All spectra reported were calibrated to the residual solvent peak from deuterated chloroform. Fourier transform infrared spectra were obtained using a Shimadzu IR Affinity-1S (Shimadzu Corporation, Columbia, MD, USA) instrument with an ATR attachment operating over 400–4000 cm^–1 at ambient temperature. UV-Vis data were collected on an Agilent Technologies Cary 60 UV-Vis (Agilent Technologies, Inc., Santa Clara, CA, USA) using Simple Reads software (Cary WinUV Scan Application Version 5.1.0.1016). SEM and EDX were acquired on a Schottky Field Emission Scanning Electron Microscope SU5000 (Hitachi High-Tech, Tokyo, Japan) operating in variable pressure mode with an accelerating voltage of 15 keV.

Thermogravimetric analysis (TGA) data were recorded on a TA SDT Q600 (TA Instruments, New Castle, DE, USA) instrument over the range 25 to 800 °C, with a heating rate of 10 °C·min^–1 under a flow of N₂ (20 mL min^–1). DSC data were acquired (Mettler Toledo DSC 3 STARe System, Mettler Toledo, Columbus, OH, USA) over the range −60 to 140 °C with a heating rate of 10 °C·min^–1 under a flow of N₂ (200 mL·min^–1). Each DSC measurement was carried out over three heat–cool cycles. For percentage crystallinity calculations, T_m, ΔH_m and ΔH_cc, the data were taken from the third heat/cool cycles. Melting enthalpies and cold crystallization enthalpies were calculated using DSC data. The reduction in the percentage crystallinity of the composite PBJS₉₀ with respect to sulfur was calculated using the following equation.

$$∆{\chi }_c=1-\left\{{\displaystyle \frac{∆H_{m\left(PBJ{S}_{90}\right)}-∆H_{cc\left(PBJ{S}_{90}\right)}}{∆H_{m\left(S\right)}-∆H_{cc\left(S\right)}}}\right\}\times 100\%$$

where the variables are defined as follows:

• $∆{\chi }_c$ Change in percentage crystallinity with respect to sulfur
• $∆H_{m\left(PBJ{S}_{90}\right)}$ Melting enthalpy of composite materials (PBJS₉₀)
• $∆H_{cc\left(PBJ{S}_{90}\right)}$ Cold crystallization enthalpy of composite materials
• $∆H_{m\left(S\right)}$ Melting enthalpy of sulfur
• $∆H_{cc\left(S\right)}$ Cold crystallization enthalpy of sulfur

Compressive strength analysis was performed on a Mark-10 ES30 (Mark-10 Corporation, Copiague, NY, USA) test stand equipped with a M3-200 force gauge (1 kN maximum force with ±1 N resolution) with an applied force rate of 3–4 N·s^–1. Compression cylinders were cast from silicone resin molds (Smooth-On Oomoo^® 25 tin-cure, Oomoo Corp, Richmond, BC, Canada) with diameters of approximately 6 mm and heights of approximately 10 mm. Samples were manually sanded to ensure uniform dimensions and measured with a digital caliper with ±0.01 mm resolution. Compressional analysis was performed in triplicate, and the results were averaged. Flexural strength analysis was performed using a Mettler Toledo DMA 1 STARe System (Mettler Toledo, Columbus, OH, USA) in single cantilever mode. The samples were cast from silicone resin molds (Smooth-On Oomoo^® 25 tin-curve, Oomoo Corp, Richmond, BC, Canada). The sample dimensions were approximately 1.5 mm × 15 mm × 23 mm. Flexural analysis was performed in triplicate and the results were averaged. The clamping force was 1 cN·m.

2.2. Preparation of Jar for Thiocracking

Whereas the jar (without the lid) eventually used for thiocracking was ground without separating the components, another identical jar was first analyzed to assess its components and their contribution to the jar. A post-consumer peanut butter jar was thus physically deconstructed and the peanut butter residue was recovered from the jar by rinsing with hexanes followed by removal of hexanes by rotary evaporation under reduced pressure. The mass of each component was recorded to allow calculation of each component’s contribution to the overall mass (

Table 1. Mass breakdown of each component of PBJ and the primary constituents of each component.

Component	Mass	% of Upcycled Mass	Primary Chemical Components
Jar Without Lid	38.800 g	100%	Muti-Material (breakdown below)
Induction Seal	0.265 g	0.68%	Aluminum, cellulose, wax, polymer(s)
Adhesive	0.020 g	0.05%	Acrylic/rubber polymer(s)
Label	1.473 g	3.79%	Cellulose
Residual Peanut Butter	15.482 g	39.9%	Triglycerides
Jar Body	21.560 g	55.6%	Poly(ethylene terephthalate)

). Each individual component was analyzed by FTIR spectroscopy (Supplementary Materials Figures S4–S9), TGA (Supplementary Materials Figures S14–S17), and DSC (Supplementary Materials Figures S19–S22). An identical post-consumer jar of peanut butter was taken without the lid as the organic component for use in the thiocracking process. The difference in mass between the two identical jars was assumed to be the difference in peanut butter residue remaining in the jar. The jar for thiocracking was placed in a blender and processed for several minutes to produce a coarse aggregate. The mixture was then added in small batches to a coffee grinder to produce the fine aggregate (PBJ) used as the organic component in thiocracking.

2.3. Synthesis

CAUTION: Heating elemental sulfur with organics can result in the formation of H₂S or other gases. Such gases can be toxic, foul-smelling, and corrosive. The temperature must be carefully controlled to prevent thermal spikes, contributing to the potential for H₂S or other gas evolution. Rapid stirring shortened heating times, and very slow addition of reagents can help avoid unforeseen temperature spikes.

2.3.1. Preparation of PBJS₉₀

To a Parr bomb reactor were added 13.5 g (0.053 mol) elemental sulfur and 1.50 g PBJ. The reactor was heated to 320 °C and allowed to run for 2 h before cooling to room temperature and the composite was removed from the reactor, giving the composite as a black matte solid in quantitative yield. Upon completion of this reaction, no mass was observed, indicating this reaction proceeded with 100% atom economy. Two batches of PBJS₉₀ were prepared and metrics were identical within statistical error.

2.3.2. Heating of PBJ in the Absence of Sulfur to Give hPBJ

In a glovebox under an atmosphere of dry N₂(g) was added approximately 5 g of PBJ to the Parr bomb reactor. The reactor was heated to 320 °C for 2 h before cooling to room temperature to yield 3.10 g of hPBJ as a non-remeltable, heterogeneous solid comprising a mixture of light- and dark-colored particles.

2.3.3. Depolymerization of PBJS₉₀

In a glovebox under an atmosphere of dry N₂(g) was added 100 mg PBJS₉₀ and 175 mg LiAlH₄ to a glass vial equipped with a magnetic stir bar. The solid mixture was suspended in 7 mL anhydrous toluene and sealed with a rubber septum. The reaction was stirred for 24 h at room temperature. At the conclusion of the reaction time, the reaction vessel was removed from the glovebox and placed in an ice bath under a flow of N₂ gas. The reaction was slowly quenched with 5% (v/v) HCl:ethanol until no more evolution of hydrogen gas was observed. The solution was washed with HCl(aq) (pH = 5) three times. The organic layer was separated, and the solvent removed by rotary evaporation under reduced pressure, yielding 23 mg of the yellow solid d-PBJS₉₀.

2.4. Determination of Dark Sulfur Content

To determine the dark sulfur content, a modified literature method for quantification by UV-vis spectroscopy in ethyl acetate was employed [136]. To a 250 mL volumetric flask was added 6.7 mg PBJS₉₀ (weighed with a microbalance) and approximately 230 mL ethyl acetate. The mixture was allowed to stir for 30 min after which the solution was made up to the mark of 250 mL with ethyl acetate. A 3 mL aliquot of this solution was transferred to a cuvette and 3 mL pure ethyl acetate was transferred to a separate cuvette to serve as a blank. Data were collected at 275 nm and the dark sulfur content calculated from a calibration curve having the equation y = 36.124x + 0.012 (R² = 0.9967), where y is the absorbance and x is the concentration of sulfur in mg/mL.

2.5. Mechanical Strength Analysis

Cylinders with diameters of approximately 6 mm and heights of approximately 10 mm, appropriate for compressive strength measurements, were prepared by melting the composite at 160 °C, then slowly and carefully poured into molds and allowed to solidify. Samples were stored at room temperature for 4 d prior to strength measurements. The samples were sanded to remove flack and measured with a digital caliper with a ±0.01 mm resolution.

3. Results and Discussion

3.1. Design and Preliminary Analysis of Multi-Material

In this study, the overarching goal was to assess the extent to which a simple, single-stage thiocracking process could be applied to a representative post-consumer mixed waste packaging item comprising waste plastic, paper, adhesives/seals and food waste. As a test item for this proof-of-principle study, a post-consumer jar (total weight = 47.247 g) that when purchased by the consumer was filled with peanut butter was selected. The jar contained residual peanut butter waste that had been left by the consumer. All components were adhered to the main jar body other than the lid, which was physically removed and was not used for the subsequent steps of the study. Although an entire jar (without the lid) was used for the thiocracking procedure, another identical jar was separated so that its individual components could be analyzed and the percentage composition by mass of each component quantified (Table 1).

The jar had contained 510 g of peanut butter when purchased, and the post-consumer jar contained 15.482 g of residual peanut butter (3.0% of purchased product). The residual peanut butter, primarily composed of triglycerides, and the jar body, composed of poly(ethylene terephthalate) (PET), were determined to be the two majority components (>95 wt. % of thiocracked organics) of the jar. The minority components (<5 wt. % of thiocracked organic mass) were the label (primarily cellulose), the induction seal (aluminum foil, wax, polymer coating, and pulp board), and the adhesive (acrylic and rubber polymers), collectively accounting for <5% of the thiocracked mass. Preliminary analysis of each component of the jar was carried out using infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, and, for soluble components, proton nuclear magnetic resonance (¹H NMR) spectrometry. These analyses further validate the identity of each component and provide points of comparison for the derivative composite discussed below. The data for each component from each of these techniques are provided in the Supplementary Materials File as Figures S1, S4–S9, S14–S17 and S19–S22.

Olefins found in the peanut butter residue triglycerides are expected to undergo inverse vulcanization (addition of sulfur across C–C π bonds, as shown in Figure 1a) during thiocracking [93,137,138,139,140,141]. The total olefin content contributed by the residue was thus quantified by ¹H NMR spectrometry with 2,3,4,5,6-pentafluorobenzaldehyde added as an internal standard (Figure S2). Calculations based on the ratio of integration of the alkene region (4.5–5.5 ppm) versus the aldehydic proton resonance on the internal standard (10.3 ppm) indicated an olefin content of 4.3 mmol∙g^–1 for the peanut butter residue alone and an olefin content of 1.7 mmol∙g^–1 for the jar used in thiocracking.

3.2. Reactivity of Individual Components with Elemental Sulfur

We have undertaken several studies to understand the reactivity of the primary individual jar components with elemental sulfur under thiocracking conditions. Triglycerides, the primary constituent of the peanut butter residue, undergo inverse vulcanization to give composites as previously reported (Figure 1a) [142]. At the temperatures used for thiocracking, triglycerides also undergo transesterification reactions with both PET [125] and cellulose (the primary constituent of paper) [132,133]. In the case of PET, this transesterification was demonstrated to lead to its depolymerization into oligomers of fatty acid-functionalized terephthalate derivatives [125]. Cellulose derivatized with olefins, as by transesterification with triglycerides in the current case, also reacts with sulfur to give composites formed via crosslinking of olefins by sulfur [130,143]. Analogous tandem depolymerization/transesterification/vulcanization of the mixed-material jar components in the current case is likewise responsible for forming homogeneous composites comprising structures shown in Figure 1b,c.

3.3. Thiocracking and Chemical Analysis of Composite PBJS₉₀

The post-consumer peanut butter jar without the polypropylene lid was converted into a mixture by coarse grinding in an industrial blender followed by comminution in a coffee grinder to give the finely ground pulp (PBJ, Figure 3a) used in the thiocracking process. Composite PBJS₉₀ was then prepared by heating PBJ (10 wt. %) with elemental sulfur (90 wt. %) at 320 °C in a stainless-steel autoclave for 2 h (Figure 3b). This process yielded a black, glassy material of low viscosity which remained shiny once cooled, that was readily shaped by pouring the molten material into a silicone mold and allowing it to cool to room temperature (Figure 3b). In contrast, when ground PBJ is heated under the same conditions, the product is a brittle, heterogeneous solid comprising a mixture of light- and dark-colored particles. This solid is not remeltable and cannot readily be shaped.

High sulfur-content materials (HSMs) can be depolymerized by their reaction with LiAlH₄, which leads to breakage of S–S bonds and consequent conversion of the S–C crosslink points to thiols. PBJS₉₀ was depolymerized in this way to give d-PBJS₉₀. Because only a single thiol sulfur remains in d-PBJS₉₀ where each polysulfur crosslinking chain (–S_x– in Figure 1) had been in PBJS₉₀, the majority of the 90 wt. % sulfur is removed, thus improving the spectroscopic signal-to-noise ratio for characterization of organics in the material, as was readily demonstrated by IR spectra for PBJS₉₀ and d-PBJS₉₀ (Figures S3, S11 and S13 in Supplementary Materials). Infrared spectra for PBJS₉₀ and d-PBJS₉₀ showed evidence of the anticipated S–C bond formation by a broad S–C stretch at 728 cm^–1 in PBJS₉₀ and d-PBJS₉₀. Further evidence of microscale homogeneity was obtained by imaging a film of PBJS₉₀ by scanning electron microscopy with elemental mapping by energy dispersive X-ray (SEM/EDX). These images showed an even distribution of carbon, oxygen, and sulfur throughout the material, and no evidence of gross phase separation throughout the surface of the material on a 100 μm scale (Figure 3c).

Although no distinct small molecules were identified from the complex mixture resulting from the thiocracking of the mixed waste in the current study, several prior studies were undertaken to understand the chemistry of the components under these reaction conditions. For example, we have previously reported detailed studies on the reactions of elemental sulfur with (1) triglycerides [142,144], (2) polysaccharides (cellulose and starch) [129,131], (3) lignocellulosic biomass [132,133], (4) mixtures of peanut oil with polysaccharides (starch or cellulose) [145], and (5) PET with triglycerides [123,125]. These studies revealed that (1) triglycerides undergo transesterification with polysaccharides and PET, (2) sulfur adds across the pi bonds in triglyceride-derived olefins. All three classes of components—triglycerides, polysaccharides, and PET—are present in the organic monomer feed for PBJS90, leading to the formulation of the chemical structure as shown in Figure 4.

The sulfur in composites like PBJS₉₀ is usually present as crosslinking polysulfur catenates (–S_x– in Figure 1) that are covalently attached to the organic molecules and some physically entrapped oligosulfur species known as “dark sulfur” [146]. The presence of larger quantities of dark sulfur generally leads to poorer mechanical strength in high sulfur content composites. To further validate the chemical composition of PBJS₉₀, the amount of dark sulfur in PBJS₉₀ was quantified by UV-Vis spectroscopy by the published method [136]. This analysis revealed a dark sulfur content of only 14 wt. % in PBJS₉₀. Despite the high PET content in PBJS₉₀, its dark sulfur content is more similar to sulfur composites made from triglyceride mixtures (e.g., CanBG₉₀ made from 90 wt. % sulfur, 5 wt. % brown grease and 5 wt. % canola oil, and having 14% dark sulfur content) [144] than to composites made from PET and sulfur, for example the 82% dark sulfur content in SPG (90 wt. % sulfur with 10 wt. % geraniol/PET derivative) [49] or 88% dark sulfur content in mPES (90 wt. % sulfur with 10 wt. % oleyl-derivatized PET) [123]. The low dark sulfur content in PBJS₉₀ predictably trends with its high compressive strength, as discussed in the following section.

3.4. Thermal and Mechanical Properties

Thermogravimetric analysis (TGA and DTGA,

Table 2. Thermal and morphological properties of mixed composition waste and sulfur composite PBJS₉₀ with comparison to elemental sulfur.

Material	Td [a] °C	Tm [b] °C	Tg [c] °C	Cold Crystal. Peaks/°C	ΔHm J/g	ΔHcc J/g	Percent Crystallinity [d]	Dark Sulfur (%) [e]
PBJS90	218	117	NA	36	27	–5	29	14
S8	229	118	NA	NA	44.8	NA	100	0

^[a] The temperature at which the 5% mass loss was observed. ^[b] The temperature at the peak maximum of the endothermic melting from the third heating cycle. ^[c] Glass transition temperature. ^[d] The reduction in percentage crystallinity of each sample was calculated with respect to sulfur (normalized to 100%). ^[e] Percentage of extractable sulfur calculated from UV-vis data according to a modified literature procedure [136].

and Figures S14–S18 in the Supplementary Materials) showed a single T_d (here defined as the temperature at which 5% mass loss is observed) at 218 °C in PBJS₉₀. This value is slightly lower than the T_d observed for cyclo-S₈ (229 °C) and is attributable to sublimation of sulfur—from both dark sulfur and decomposing polysulfur crosslinking chains—out of the material. TGA thermograms for PBJ components (Figures S14–S18 in the Supplementary Materials) revealed higher T_d values than that observed in PBJS₉₀. No additional thermal features attributable to unreacted individual jar components (PET, cellulose, etc.) were observable, providing further evidence of the established transesterification/vulcanization pathways expected for the triglyceride, cellulose, and PET components comprising >99% of the organic content of PBJS₉₀ (Figure 1).

Thermomorphological changes in the composite PBJS₉₀ were also evaluated by differential scanning calorimetry (DSC, Table 2 and Figures S19–S23). DSC thermograms also showed a cold crystallization peak at 36 °C and a melting feature at 117 °C for PBJS₉₀. The percentage crystallinity calculated from melting and cold crystallization enthalpies of PBJS₉₀ was determined to be 29%, indicating there are some crystalline regions present but that the material is primarily composed of amorphous materials, consistent with the presence of amorphous –S_x– crosslinking chains.

Cylinders for compressive strength analysis were prepared by melting PBJS₉₀ at 180 °C and pouring the material into molds. Samples were allowed to sit for 96 h prior to testing, following the convention for high sulfur-content materials with 90 wt. % sulfur content for compressive strength analysis [129]. The compressive strength of PBJS₉₀ was found to be 37.7 ± 2.9 MPa (

Table 3. Mechanical properties of PBJS₉₀ compared to other high sulfur-content materials, materials containing recycled plastics, and commercially available building materials.

Sample	Compressive Strength (MPa)	After Acid (MPa)	Strength Retained (%)	Compressive Modulus	Flexural Strength (MPa)	Flexural Modulus
PBJS90	37.7 ± 2.9	35.4 ± 4.5	94%	74 ± 5	5.64 ± 0.32	631 ± 20.8
APS95 [a]	35.7	ND [h]	ND	ND	4.8	690
SPG [b]	23.1	ND	ND	ND	4.7	ND
mPES [c]	26.9	ND	ND	ND	7.7	ND
Brick 1 [d]	11.2	ND	ND	ND	ND	ND
Brick 2 [e]	16.4	ND	ND	ND	2.75	ND
Brick 3 [f]	9.0	ND	ND	ND	ND	ND
C62 Brick [g]	8.6	ND	ND	ND	ND	ND
OPC	17	ND	ND	ND	3.7	580

^[a] Composite of allylated peanut shells (5 wt. %) and sulfur (95 wt. %) [132] ^[b] Composite of PET (5 wt. %), geraniol (5 wt. %), sulfur (90 wt. %) [49] ^[c] Composite of glycolyzed PET (10 wt. %) and sulfur (90 wt. %) [123] ^[d] Brick with HDPE from physical recycling via melting and compression [147] ^[e] Brick with 85% fine PVC aggregate in cement [148] ^[f] Brick of OPC with 10% mixed plastic waste aggregate, cement optimized [149] ^[g] Brick classification C62 for building brick with negligible weathering. ^[h] ND = not determined in the reported paper.

, Figure S25, stress strain plots in Figure S24 of the Supplementary Materials), over 200% stronger than that required of OPC for use in residential building foundations. PBJS₉₀ exhibits exceptional compressive strengths, exceeding that of other high sulfur-content materials comprised of similar materials such as APS₉₅ (35.7 MPa), SPG (23.1 MPa), and mPES (26.9 MPa). PBJS₉₀ also exceeds the mechanical properties of other cement materials containing plastics or plastic aggregates (Table 3).

Whereas mineral-based legacy building materials show significant degradation upon exposure to acidic conditions, previous studies on high sulfur-content materials demonstrate that they often show impressive resistance to corrosion by acids. To investigate the corrosion resistance of PBJS₉₀, its compressive strength was remeasured following submersion in 0.5 M H₂SO₄ for 24 h. After this acid challenge, the compressive strength was determined to be 35.4 ± 4.5 MPa, indicating that PBJS₉₀ retained 94% of its strength under conditions that will completely dissolve OPC. PBJS₉₀ also displays an exceptionally low 0.14% uptake of water following 24 h of submersion (cf. 28% for OPC) [129]. Low water uptake is another important metric for weathering resistance, as seasonal freezing and thawing cause expansion–contraction cycling of absorbed water that leads to a crack formation and is a primary mechanism for OPC failure in temperate climates [150].

The flexural strength of PBJS₉₀ was determined through dynamic mechanical analysis in a single cantilever mode. Rectangular prisms were prepared by pouring material into a mold and subsequent manual sanding to a thickness of 1.4 mm. PBJS₉₀ was found to have a flexural strength of 5.64 ± 0.32 MPa, and a flexural modulus of 631 ± 20.8. This falls within the range of other high sulfur-content materials (Table 3), and exceeds that of average flexural strength values observed by OPC.

3.5. Environmental and Sustainability Impact Analysis

If the goal is to replace OPC with PBJS₉₀, some assessment of the environmental impact of the two materials will be insightful. The E factor [135] is a common metric for estimating the relative environmental impact of a material in which the focus is on the amount of waste produced versus the amount of useful product obtained. The E factor is equal to the mass of waste produced divided by the mass of useful product produced, such that a lower E factor indicates lower relative waste generation. The E factors for commercial bulk chemicals range from <1 to over 50 [151,152]. If we consider the inputs to making PBJS₉₀ to be sulfur and the jar with its lid, we calculate the E factor (using masses for one whole jar with the lid as delineated in Table 1) as:

$$E factor={\displaystyle \frac{waste}{useful product}}={\displaystyle \frac{lid}{\left(lidless jar+sulfur\right)}}={\displaystyle \frac{8.45 \mathrm{g}}{\left(38.8 \mathrm{g}+349 \mathrm{g}\right)}}=0.02$$

The E factor for PBJS₉₀ is nearly two orders of magnitude lower than that of OPC (1.4) [151]. The conversion of the lidless jar and sulfur to PBJS₉₀ proceeds with 100% atom economy (no lost mass within error), so the only waste of the process is the lid. Since the lid was already discarded as waste by the consumer of the peanut butter, the net waste to the environment is approximately zero.

A potential shortcoming of the E factor and atom economy metrics is that they focus on the mass/atom balance and do not account for energy consumption or net carbon dioxide emission. For this purpose, it is instructive to determine the global warming potential, an estimate of kilograms of CO₂ emitted per kilogram of useful material made (kg CO₂e/kg). In the current case, a low global warming potential of −0.099 kg CO₂e/kg was calculated for PBJS₉₀, making its preparation slightly carbon-offsetting (estimates, assumptions, and inputs for these calculations are provided in the Supplementary Materials). This compares to a much higher value of 1.0 kg CO₂e/kg for OPC. Not only is the global warming potential of OPC quite high, but OPC is also the most-produced synthetic good by mass, such that its manufacture is responsible for ~8% of all anthropogenic CO₂ production, similar to or exceeding the amount produced by noncommercial automobile transportation [153,154,155,156,157]. These data emphasize the need for waste-derived cements or geopolymer cements [158,159,160] to replace OPC as elements of the built environment as a significant measure against runaway CO₂ production in the future.

The work described herein contributes positively to achieving several United Nations Sustainable Development Goals (SDGs). The work described herein advances SDG 12: Responsible Consumption and Production by demonstrating a single-stage method for converting complex, multi-material post-consumer packaging into high-strength composites, thereby potentially reducing waste and promoting more sustainable material lifecycles. The resultant composites have a negative global warming potential so if these composites could be used as alternatives to traditional building materials, this would contribute to reduced carbon emissions, directly supporting SDG 13: Climate Action. The utilization of waste-derived inputs underscores the relevance to SDG 9: Industry, Innovation, and Infrastructure. Furthermore, the ecological benefits, including the reduction in microplastic pollution, align with SDG 14: Life Below Water and SDG 15: Life on Land. New technologies come with their own challenges, and the environmental impact of the composites will need to be thoroughly explored before their widespread use, but this proof-of-concept study holds promise for a more sustainable future.

4. Conclusions

With the quantity of single-use packaging components being deposited into landfills rising exponentially faster than recycling, it is crucial to develop versatile methods to address the challenges currently present in recycling mixed-composition waste. The proof-of-principle study reported herein demonstrates the first utility of thiocracking for recycling consumer waste products of mixed composition through its reaction with fossil fuel refining byproduct sulfur. The resulting composite, PBJS₉₀, was prepared with a low E factor, 100% atom economy, and a negative global warming potential of −0.099 kg CO₂e/kg. The composite showed a compressive strength greater than 200% of that required for building foundations and flexural strengths also exceeding that of ordinary Portland cement, a material with a high global warming potential of 1.0 kg CO₂e/kg. The process and materials described herein may support UN SDGs 9 and 12–15 in humanity’s quest for a more sustainable future. The attractive properties of PBJS₉₀ suggest its potential as a greener alternative to legacy mineral-based structural materials, pending, of course, results of long-term environmental impact and weathering studies. Nonetheless, this proof-of-principle study holds promise for the use of thiocracking to effectively recycle mixed waste comprised of various plastics, food remnants, greases, cellulose-based materials, and other contaminants to durable structural goods.