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Article

Exploring New Applications of Municipal Solid Waste

by
Harrison Appiah
1,
Ezra Bar-Ziv
2,
Jordan L. Klinger
3 and
Armando G. McDonald
1,*
1
Department of Forest, Rangeland and Fire Sciences, University of Idaho, Moscow, ID 83844, USA
2
Department of Mechanical Engineering, Michigan Technological University, Houghton, MI 49931, USA
3
Idaho National Laboratory, Idaho Falls, ID 83415, USA
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(8), 3719; https://doi.org/10.3390/su17083719
Submission received: 24 March 2025 / Revised: 18 April 2025 / Accepted: 18 April 2025 / Published: 20 April 2025
Browse Figures
Versions Notes

Abstract

This study aimed to (i) characterize municipal solid waste (MSW) sourced from Utah and Michigan transfer stations and (ii) upcycle, produce, and evaluate composites derived from this MSW. Composition analysis showed that the MSW was composed of a variety of commodity plastics, paper/cardboard, and inorganic materials. Detailed chemical analysis for lignin, cellulose, hemicellulose, and lipids was performed. The plastics identified were mainly polyethylene, polypropylene, polystyrene, and poly (ethylene terephthalate). The compoundability of the MSW was assessed by torque rheometry. Composites were prepared by compounding the MSW in an extruder. A composite flexural strength of 29 MPa and a modulus of 1.0 GPa was achieved. The thermal properties of the composites were also determined. The melt flow behavior of the MSW composites at 190 °C was comparable to wood plastic composite formulations.

1. Introduction

Municipal solid waste (MSW) poses a significant challenge in contemporary society, reflecting the consumptive habits of urban and industrial populations worldwide [1,2]. In particular, the presence of plastics such as polyethylene (PE), polypropylene (PP), polystyrene (PS), and poly (ethylene terephthalate) (PET) (PET) (PET) is significant, as these materials are notoriously difficult to recycle due to their chemical properties, size, and widespread use in consumer products. The United States alone produces a staggering 263 million tonnes of waste annually, with a recycling rate of less than 10% [3]. Such figures underscore the urgency of addressing MSW as a priority issue, not only for environmental preservation, but also for safeguarding public health and well-being [4]. Conventional disposal methods like landfilling and incineration, which were once relied upon as convenient solutions, now stand in contradiction to sustainability and circular economic goals [5]. These approaches are increasingly being scrutinized due to their environmental impacts, including concerns about land use, greenhouse gas emissions, and potential harm to human health [6]. As population burgeons and urbanization intensifies, the pressure on waste management systems escalates, necessitating innovative approaches that transcend the limitations of traditional methods [7].
Converting MSW into products will mitigate the strain on conventional waste management infrastructure but also create avenues for sustainable resource utilization and upcycling [8]. Understanding the makeup of MSW is important to better implement strategies for sorting classes of materials and recovering value. MSW is made up of (i) organic materials, such as food scraps, paper/cardboard, fabric, and yard trimmings, (ii) polymeric materials, such as high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), and numerous other polymers, and (iii) metals and (iv) inorganic materials (concrete, ceramics, glass, etc.) [9,10]. The heterogeneity of MSW demands tailored solutions that recognize and accommodate regional variations in waste composition and generation patterns [11,12]. The biodegradable components (food, paper, yard trimmings, etc.), while inherently recyclable through composting, pose challenges due to their tendency for rapid decomposition, which can lead to foul odors, methane and carbon dioxide emissions, and leachate production if not properly managed [13]. However, harnessing fractions of MSW presents an opportunity for resource recovery, such as soil enrichment, biochemical extraction, and bioenergy generation from the organic portion, while the plastic components can be used in composites, thereby closing the energy/carbon loop and mitigating the impact on the environment [14].
The emergence of plastic composites from MSW represents a promising avenue for both waste diversion and resource recovery [15]. Plastic composites, often composed of a blend of recycled plastics, unrecyclable plastics (e.g., multi-layer pouches), and other materials such as wood fibers, paper, or glass, offer a potential solution to the challenges posed by plastic waste in the environment [16]. Wood–plastic composites (WPCs) can be used in a wide range of applications, such as construction materials automotive components, furniture, and consumer goods [17,18]. Their resistance to rot, decay, and insect damage makes them particularly appealing for outdoor applications. It has been shown that mixed paper and mixed plastic waste can be compounded into pellets and used as solid fuel [19]. Xu and coworkers showed that composite materials can be produced from mixed paper and plastic waste by extrusion, with a flexural modulus of 1.1 GPa [20]. Due to the varied compositions of MSW, it is important to understand these compositions and their properties before any valorization strategies can be drawn. Current developments in MSW valorization have shown promise for addressing some environmental issues and creating products of additional value. For example, Saleem et al. developed a process that uses municipal plastic waste to create superhydrophobic films, which find applications in electronics, anti-icing, and as matrixes for electromagnetic composites [21]. Another creative strategy used by Yaqoob et al. turned MSW into precursors to produce carbon-based compound fibers by combining chemical and mechanical recycling methods, which may open new opportunities for high-value applications of waste materials [22]. Understanding the intricate interactions between waste content, processing techniques, and end-product quality is becoming more and more important as research in this area develops to provide efficient and long-lasting MSW valorization solutions. The characterization of this waste is central to understanding how it might be integrated into composite materials.
This study aimed to characterize and upcycle MSW, sourced from material recovery facilities in Utah and Michigan, into composite materials and explore other potential applications. Proximate and detailed chemical analyses were used to characterize the MSW samples. This study also includes an analysis of the organic materials present in MSW, such as lignin, cellulose, hemicellulose, and lipids, which may also be harnessed in composite production, particularly for their potential to enhance the mechanical and thermal properties of the final product. MSW was compounded, by extrusion, into composite materials. The mechanical, thermal, and rheological properties of the composite materials were determined to evaluate their potential applications.

2. Materials and Methods

2.1. Materials

All chemicals used were of analytical grade and purchased from Fisher Scientific (Hampton, NH, USA). Specific chemicals included α-cellulose sourced from Acros Organics (Waltham, MA, USA), and glacial acetic acid, perchloric acid, dichloromethane (CH2Cl2), sulfuric acid, acetone, and ethanol obtained from Fisher Scientific (Hampton, NH, USA). Sugar standards (D-(+)-glucose, D-(+)-mannose, D-(+)-galactose, D-(-)-xylose, D-(+)-arabinose) and fatty acid standards were sourced from Alfa Aesar (Ward Hill, MA, USA).
The MSW was received from waste recovery management facilities in Salt Lake City, Utah (UT), and Ann Arbor, Michigan (MI), as part of an Idaho National Laboratory project. The MSW was retrieved from each facility’s feed intake, and all metal pieces were separated as detailed in [23]. The metal-free MSW was primarily reduced using a Jordan Reduction Solutions Knife mill (Birmingham, AL, USA) with a 75 mm screen to obtain a volume of 4.5 m3. Fractional analysis of the material was manually completed to determine the major constituents from 15 grabs. The material was further milled using a Forest Concepts rotary shear crumbler (Auburn, WA, USA) with a 6 mm rotor head to <6 mm and then reprocessed using a 3 mm rotor head to <3 mm. The MSW crumb (<3 mm) was thoroughly blended in a Colorado mixer to obtain MSW samples for this study.

2.2. Analysis of MSW

Moisture content (MC) was determined using an HB 43-S Mettler-Toledo moisture analyzer (Columbus, OH, USA). Ash content was measured in triplicate at 600 °C for 16 h in a muffle furnace following ASTM D1102 [24]. Volatile matter (VM) was measured in triplicate at 950 °C for 7 min in a muffle furnace following ASTM E872 [25]. Fixed carbon (FC) was determined from VM minus ash content. The calorific value of the dry MSW was determined by bomb calorimetry (Parr Instruments model 1261, Moline, IL, USA) according to ASTM D5865 [26] and calibrated with benzoic acid. FTIR spectra were obtained on (i) randomly selected pieces of MSW crumb from a sample batch of MSW and (ii) four random pieces of extruded MSW and spectra averaged using a Thermo-Nicolet iS5 spectrometer (Waltham, MA, USA) with an attenuated total reflection (ATR) accessory (iD5, ZnSe ATR crystal). The spectra of the samples were collected in the 4000 and 650 cm−1 range at 4 cm−1 spectral resolution after 64 scans, with a total acquisition time of around 60 s for each spectrum. The spectra were analyzed with the Omnic v9.3 program (Thermo-Nicolet, Waltham, MA, USA). The identity of the selected MSW crumb pieces was determined by matching them with the FTIR spectral library (Thermo-Nicolet, Waltham, MA, USA) and authentic polymer samples. The hydroxyl (HI), carbonyl (CI), and cellulose (CeI) indices were calculated as a ratio of the band absorbance at 3340 cm−1, 1721 cm−1, and 1035 cm−1, respectively, to the -CH2- band at 2918 cm−1 [27]. X-ray diffraction (XRD) was performed on milled MSW crumb using a Siemens D5000 diffractometer (Billerica, MA, USA) in reflection mode with Cu-Kα radiation (λ = 0.154 nm) from 2θ = 2 to 80° in 0.05° steps. The peaks in the X-ray diffractograms were identified to specific materials by searching the RRUFF database [28].
The MSW was analyzed for CH2Cl2 soluble extractives, lignin, and carbohydrate content. MSW samples (4–5 g) were Soxhlet-extracted with CH2Cl2 (150 mL) overnight, and the extract was concentrated to dryness to determine the yield gravimetrically according to ASTM D1108 [29]. The CH2Cl2 extracts were analyzed by gas chromatography–mass spectrometry (GCMS) using an ISQ-Trace1300 (ThermoScientific, Waltham, MA, USA) system ZB-5 (30 m × 0.25 mm, Phenomenex, Torrance, CA, USA) capillary column under a temperature gradient of 40 °C (1 min) to 280 °C (at 5 °C·min−1) to determine their fatty acid methyl ester derivatives (FAME), as described by [30]. The extractive-free MSW was subjected to primary (72% H2SO4, 2 mL, for 60 min) and secondary (4% H2SO4, 117 kPa and 121 °C for 30 min) hydrolysis to determine Klason lignin (KL) content gravimetrically according to ASTM D 1106 [31]. Acid soluble lignin (ASL) was determined by absorbance at 205 nm of the filtered hydrolysate (made up to 250 mL) using an extinction coefficient (ε) of 110 L·g−1·cm−1 (Genesys 50, ThermoScientific, Waltham, MA, USA) [32]. Carbohydrate analysis was performed on the filtered hydrolysate (5 mL) according to ASTM E 1758 [33]. The monosaccharides were quantified by HPLC (two Rezex RPM columns, 7.8 mm × 300 mm, Phenomenex, Torrance, CA, USA) at 85 °C via elution with water (0.5 mL·min−1) using differential refractive index detection (Waters model 2414, Milford, MA, USA).
Total lignin content was also determined using the acetyl bromide method (LAB) [34]. Extractive-free MSW (5 mg) was incubated in acetyl bromide in acetic acid (25%, 5 mL) and 70% perchloric acid (0.2 mL) at 70 °C for 60 min. The mixture was transferred to a 100 mL volumetric flask containing 2M NaOH (10 mL) and acetic acid (25 mL). The absorbance was measured at 280 nm (Genesys 50, ThermoScientific, Waltham, MA, USA), and the lignin content was determined using an ε of 20.09 Lg−1cm−1. The total carbohydrate content of the extractive-free MSW was determined using a modified phenol–sulfuric acid method [35]. Next, 10 mg of sample and cellulose standard (2–10 mg, Sigmacell 101, St. Louis, MO, USA) was incubated in 77% H2SO4 (0.1 mL) for 5 min; then, 5% aqueous phenol solution (1 mL) was added, immediately followed by concentrated H2SO4 (5 mL). After 30 min, the absorbance at 490 nm was recorded.

2.3. MSW Composites and Testing

The compounding behavior of MSW was performed on a Haake Polylab mixer/torque rheometer (Waltham, MA, USA) with a 60 mL Roller rotor 600 mixing head, fill capacity of 70%, temperature of 155 °C, and a speed of 60 rpm. MSW was compounded and extruded into a rod (9 mm dia) using a co-rotating twin-screw extruder (Leistritz (Allendale, NJ, USA), 18 mm dia, 720 mm long, 200 rpm, barrel temperature 160 °C). The extruded rod was milled (<6 mm) using a plastic granulator (Sterling BP608, New Berlin, WI, USA). Samples of loose MSW crumb and compounded MSW (14 g) were compression-molded, in quadruplicate, into discs using a 75 mm Ø pellet die at 150 °C with 9 tonnes of applied load for 10 min, then cooled to room temperature. The discs were sawn into 12 mm wide × 3 mm thick strips on a hobby table saw (Proxxon model KS 115, Föhren, Germany) for flexural testing. Flexural three-point bending tests were performed according to ASTM D790 [36], using an Instron 1132–5500 universal testing machine with a 5 kN load cell, support span of 48 mm, and crosshead speed of 1.2 mm·min−1.
Parallel-plate dynamic rheology experiments were performed on compounded MSW molded discs (2 mm × 25 mm Ø) using a Bohlin CVO 100 N rheometer (East Brunswick, NJ, USA) equipped with an extended temperature unit at 190 °C and 0.1% strain from 0.01 Hz to 100 Hz. Complex viscosity (η*), elastic modulus (G’), tan δ, and viscous modulus (G”) were measured [37,38].
Thermogravimetric analysis on MSW samples (5–8 mg) was performed on a Perkin Elmer TGA-7 (Shelton, CT, USA) instrument from 30 to 800 °C at a heating rate of 20 °C·min−1 under N2 (30 mL·min−1). Differential scanning calorimetry (DSC) was used to obtain the melt temperature (Tm), glass transition temperature (Tg), degree of crystallinity (Xc), and crystallization temperature (Tc) of the MSW samples (5–6 mg) on a Perkin Elmer DSC-7 (Shelton, CT, USA) instrument. The samples were heated from 25 to 280 °C at 10 °C·min−1, cooled from 280 to 25 °C at −10 °C·min−1, and reheated to 280 °C at 10 °C·min−1. The enthalpy of fusion for PET, PE, and PP were 140, 293, and 207 J·g−1, respectively [39].
Water absorption tests were performed following ASTM D570 [40] by submerging the compounded MSW in a deionized water bath maintained at 25 °C for varying time intervals. After immersion for 24 h and other time intervals, the specimens were carefully removed from the water bath. Any surface water on the specimens was carefully removed using a paper towel and samples were weighed to determine weight gain.

3. Results

3.1. Characterization of MSW

The grab analysis of the raw MSW before shredding gave an estimate of paper and plastic contents. UT MSW contained 41% plastic and 59% paper/cardboard, while MI MSW contained 56% plastic and 44% paper/cardboard. FTIR spectral analysis of the crumbed (<3 mm) MSW (200 pieces) was performed to determine the identity of plastic and other materials present, and the plastic composition is shown in Figure 1a. The plastics identified were mainly polyethylene (PE), cellulose/paper, PP, and PS, with minor amounts of polybutadiene (PBD), polyethylene terephthalate (PET), and polyurethanes. Zinchik et al. found PE, PET, PP, polyamide-nylon, PVC, and other materials in mixed plastic/paper waste [27]. Differences in plastic composition between MI and UT MSW were expected due to their geographical locations. Calcium carbonate and talc (aluminosilicate) were also detected in the MSW and probably associated with paper fillers/coatings. The analysis of the spectra will be discussed later.
The MSW was analyzed by XRD to determine the minerals present (Figure 1b). Calcite (CaCO3) at 2θ = 29.3°and cristobalite (SiO2) at 2θ = 22° were identified in both the UT and MI MSW samples [RRUFF database]. A minor amount of aluminosilicate (kaolinite) at 2θ = 12.4° [RRUFF database] was detected in MI MSW. In addition, peaks assigned to lignocellulosic biomass were also observed at 2θ = 18° (amorphous lignin, cellulose, and hemicellulose) and 29° (crystalline cellulose (200) lattice) [41].
Proximate analysis was performed on the MSW samples, and the results are given in Table 1. The MSW samples had an ash content of 10–11%, lower than the reported values of 21% [23]. FC values of 9–12% were observed in the range of the MSW literature values of 5–10% [23,42]. The VM values were 77–80% and within the range of reported values for MSW (69–81%) [42]. The calorific value for the MSW was 27–30 kJ·g−1, higher than that reported by [23] at 21.5 kJ·g−1 and [42] at 22.2 kJ·g−1.
The CH2Cl2 extractive (lipid) content of the two MSW samples was around 9% (Table 1). Stylianou reported lipid values of 2–6%, while Abdullah obtained 11% [43,44]. The lipid composition, representing FAME derivatives, showed that the MSW extracts contained myristic, palmitic, linoleic, oleic, and stearic acids (Table 1). Bekier detected a range of saturated (C15 to C22) and unsaturated (C16 to C20) fatty acids in MSW and composted MSW [45].
Lignin content was determined gravimetrically by the Klason method. The UT MSW gave a high KL value of 38% and the collected KL sample contained glass and sand particles, which explains the unexpectedly high value. Price et al. observed high lignin values of 25% in fresh landfill material [46]. The MI MSW sample had a KL of 15%, which is in the range (6–18%) of other reports [44]. To eliminate the variability in KL determination due to insoluble inorganic material, lignin content was also determined by the lignin acetyl-bromide (LAB) method. This approach gave LAB values of 29–31% for MSW, which were still high and could be attributed to chromophoric groups present other than lignin [43].
The carbohydrate composition of MSW was determined by HPLC analysis. The main neutral sugars in the MSW samples were glucan or cellulose (23–34%) and xylan (3–7%) hemicellulose, together with minor amounts of galactan and mannan (Table 1). Abdullah obtained glucan values of 28% in MSW. A modified phenol–sulfuric acid method was also employed to determine total carbohydrate content at values of 48–55% in the MSW (Table 1) [44]. The literature values report lower total carbohydrate levels in MSW: from 35 to 43% [44,46]. The total carbohydrate values for MI MSW by the two methods were comparable. However, for the UT MSW, very different values were obtained from the two methods (28% and 55%), and this cannot be explained.

3.2. Compounding of MSW

The MSW material was evaluated for its compoundability to form a composite material by torque rheometry at 155 °C (Figure 1c). The analysis focused on two key indicators of compounding efficiency: maximum torque and final torque after 15 min of processing. Both UT and MI MSW samples reached a similar maximum torque value of 71 Nm. However, the UT MSW achieved this peak significantly faster, at 1.3 min into processing, compared to the MI MSW, which required 2.9 min to reach the same maximum torque value. These peak torque values were higher than those reported for 60% wood–40% polyethylene composites, at around 35 Nm [47]. The torque at 15 min (steady state) was around 32 Nm. The low plastic and high fiber contents of the MSW contributed to the high torque values and time required to fully compound the mix. Compounding increased the temperature of the melt to 204 °C and 200 °C at 8 min for the MI and UT MSW, respectively. This temperature rise has also been seen in wood–plastic composite experiments [47]. The melt temperature then decreased gradually to 193–196 °C at 15 min. This temperature profile is significant because it directly affects the crystallization behavior of the matrix, influencing the final mechanical properties of the composite. The initial higher temperatures promote better fiber–matrix integration, while the subsequent cooling phase allows for controlled crystallization, resulting in improved dimensional stability of the final product.
Since the MSW could be successfully compounded in a torque rheometer, compounding using a twin-screw extruder was employed to produce homogeneous composite materials for evaluation. This compounding approach has been used to form homogeneous extruded mixed plastic/paper pellets as a replacement for coal [27].

3.3. Characterization of Extruded MSW Composites

The compounded composites were evaluated by a combination of FTIR spectroscopy, thermal analysis, flexural strength, and rheometry. FTIR spectroscopy was performed on the compounded MSW samples and compared to the averaged spectra of 10 g of MSW (Figure 2). For all spectra, a broad O-H stretching band was observed, centered at 3330 cm−1, which indicates the presence of a hydroxyl group of cellulosic material. The dominant C-H stretching vibrations at 2917 and 2851 cm−1 were attributed to the methylene (-CH2-) groups of PE and PP [48]. Also, a C-H stretching band attributed to a methyl group in PP was seen at 2952 cm−1. Bands at 1462, 1376, and 1159 cm−1 were, respectively, assigned to the C-H stretching bands of the -CH2, -CH3, and CH groups and were distinctive of PP [48]. The existence of carbonyl groups (C=O) represented by a band at 1720 cm−1 indicates the presence of an ester that is assigned to PET and/or hemicellulose [46]. The band at 1100–1000 cm−1 was assigned to the C-O stretching of hemicellulose and cellulose [49]. The aromatic band at 1507 cm−1 was assigned to lignin [49]. Cis-(728 cm−1) and trans-vinylene (975 cm−1) bands were seen in all the samples [48].
The relative amounts of hydroxyl, carbonyl, and cellulose to PE in the MSW samples were analyzed by calculating HI, CI, and CeI indices, respectively. The compounded UT MSW material had high HI, CI, and CeI values of 0.34, 0.24, and 0.56, respectively, as compared to the compounded UT MSW with values of 0.13, 0.21, and 0.27. This shows that the UT MSW had a higher cellulosic content than the MI MSW material, supported by compositional analysis (Table 1). The average UT and MI MSW spectra gave HI, CI, and CeI values, respectively, of 0.15–0.16, 0.12–0.14, and 0.25–0.31.
DSC was performed to determine the melt (Tm), glass transition (Tg), and crystallization (Tc) temperatures, as well as the degree of crystallinity (Xc) of the polymers in the MSW. Figure 3 shows the DSC thermograms (cooling and second heat cycles) of the MI and UT compounded composites. A summary of the thermal properties of the compounded MSW is given in Table 2. Three melting peaks (Tm1, Tm2, and Tm3) were observed in the MSW samples around 125–131 °C, 158 °C, and 245–246 °C, assigned to PE, PP, and PET, respectively. A similar thermogram of mixed plastic waste was obtained by Kazemi et al., showing mainly PE and PP melting peaks [50]. A Tg was observed at 89 °C for UT and 94 °C for MI, and both were assigned to PS [48,51]. On the cooling cycle, two Tcs were seen at 209–214 °C and 111–115 °C, associated with PET and PE (plus PP), respectively. The Xc values of PE, PP, and PET were determined from the second heating curve and reported based on the original mass of the MSW sample. The MI sample showed higher Xc values than UT due to its higher plastic content. These findings show that the composite is composed of a mix of PE, PP, PET, and PS, and these have a wide range of Tm and Tg temperatures. Extrusion was carried out at 160 °C to avoid fiber degradation, and under these conditions, PE, PP, and PS would be in a molten state; however, PET would be solid. Mixes of plastic generally have poor mechanical properties due to their incompatibility, and improvements can be achieved by the addition of a compatibilizer and/or extenders.
The melt flow characteristics of the MSW composites were determined by dynamic rheometry at 190 °C. The rheological data obtained (flow curves) show a decrease in complex viscosity (η*) with shear rate (frequency) consistent with shear thinning (pseudoplastic) behavior (Figure 4). This reduction in viscosity at high shear rates occurs due to the disentanglement of the polymer chains. Notably, the flow curves of the MSW were comparable to mixed plastic–paper waste [37] and wood–plastic composites [52]. For comparative purposes, the η* of MI-MSW, at 1 Hz, was about an order of magnitude higher than that of HDPE, and this is attributable to the fiber/paper content (Table 3). The differences in η* (at 1 Hz) between MI-MSW (42.6 kPa.s) and UT-MSW (27.8 kPa.s) are likely due to compositional differences. To assess the rheological behavior, the rheological flow-curve data were fitted to the power–law model for non-Newtonian fluids [53]. The model values of consistency coefficient (k), flow behavior index (n), and correlation coefficient (R2) are given in Table 3. The fitted models showed good fits, with R2 > 0.97. The low n value (0.115) for UT-MSW is consistent with a higher fiber/paper content than MI-MSW (n = 0.242), and this has been observed in wood–plastic composites [53,54]. The MSW-based composites exhibited shear-thinning behavior with a power law index of 31.0 and 45.1 kPa.s for UT and MI, respectively, falling within the processing window of a 60% wt. wood–fiber polymer composite at 498 kPa.s and a flow index behavior (n) of 0.246 [55].
The thermal degradation behavior of the compounded MSW was determined by TGA (Figure 5a). The compounded MSWs were shown to have five transitions, as determined from the first-derivative TGA (DTG) thermograms (Figure 5b) at 73 °C (water loss), 361–365 °C (paper–biomass degradation), 510–513 °C (PE/PP/PET degradation), 625–626 °C (unknown), and 726–733 °C (silicone rubber and/or CaCO3 degradation). A summary of the degradation temperature onsets (Tonset) and DTG maxima (DTGm) peaks for the compounded MSW samples and reference compounds (HDPE, PP, PET, Nylon 6,6, cellulose, kraft paper, silicone rubber, and CaCO3) are given in Table 4.
Mechanical properties are critical indicators of a material’s performance and suitability for different applications. The three-point flexural strength (FS) and modulus (FM) values were determined for compression-molded MSW crumb and compounded MSW samples, and the results are given in Table 5. The compounded MI MSW had the highest FS at 29 MPa, in the range for HDPE (14–48 MPa) [56], while the UT compound was 24% lower. The higher FS for the MI sample could be attributable to its higher plastic content. The compression-molded MSW crumb composites had a significantly lower FS than the corresponding compounded MSW composites. The compounded UT MSW was shown to have the highest FM at 1.0 GPa, comparable to that found for mixed plastic/paper waste by Ewurum et al. [37] and PP (1.1–1.7 GPa) [57], while the MI compound had a value of just 0.81 GPa. The higher FM for the UT compound could be due to its higher cellulosic content. For comparison, the FS and FM of a commercial WPC were, respectively, 22 MPa and 2.8 GPa, and for particleboard were 15 MPa and 2.8 [58,59]. The FS and FM of HDPE-based WPCs with coupling agents were considerably higher at 45 MPa and 2.3 GPa, respectively [60]. The FM for the compression-molded MSW was low at 0.2 GPa. Due to the poor compatibility between the mixed plastics and wood fiber, the addition of coupling agents (such as maleated polyethylene/polypropylene and glycidyl methacrylated polyolefin) and chain extenders incorporating long-chain branching (e.g., dicumyl peroxide and polymer–epoxypropyl–methacrylate formulation) and grafting (dicumyl peroxide) would most likely improve the properties of the composite [37].
The water absorption tests were performed over 21 days on compounded MSW and compression-molded MSW crumb (Figure 6). The compounded composites showed less water absorption (7–12%) than the compression-molded crumb samples (17–35%) on day 21. The UT MSW samples (e.g., 12% for compounded composite) had higher water absorption values than the MI MSW samples (e.g., 7% for the compounded composite), and this is most likely attributable to UT MSW’s higher pulp fiber content. For all samples, the water absorption increased rapidly in the first two days of the immersion test and then gradually increased with time. This trend was shown to exhibit Fick’s diffusion behavior [61]. These water absorption plots were like those of WPCs, and the water uptake was proportional to wood/natural fiber content, with plastic having essentially no water uptake [62]. Clemons also observed higher moisture uptake in compression-molded WPCs versus extruded WPCs [63].

4. Technical Challenges and Future Direction of MSW Valorization

MSW is inherently heterogeneous, comprising various materials with different chemical, physical, and biological properties. This heterogeneity poses great challenges to the valorization of MSW, making it difficult to effectively process and convert waste into valuable products. Limitations such as economic feasibility and the complexity of waste composition hinder the effective separation, processing, and recycling of materials [64]. Additionally, the regulatory landscape and public awareness of the benefits and possibilities of waste valorization remain underdeveloped in many regions, further impeding widespread adoption in OECD countries [65]. However, the field of valorization has seen significant advancements in recent years, largely driven by the urgent need for sustainable waste management solutions and the growing demand for renewable energy sources. Despite these developments, obstacles remain that must be addressed before transforming laboratory results into products of commercial importance. Sharma et al. emphasized the necessity for a comprehensive strategy for waste management by highlighting the need for life-cycle assessments in assessing the true environmental and economic implications of various MSW valorization options [66]. While several technologies have been proposed and developed, their adoption and efficacy vary widely across the global waste management landscape. Source separation of waste with material recovery, if implemented at the point of generation by municipalities, can significantly increase recycling rates while reducing the contamination that often plagues mixed-waste streams, creating a clean feedstock for composite applications. For mixed-plastic biowaste, the anaerobic digestion of source-separated organic material stands as the optimal treatment method, offering multiple benefits including renewable energy production through biogas, the creation of nutrient-rich digestate for agricultural applications, and substantial reductions in greenhouse gas emissions compared to landfilling [67,68]. It has been shown that contaminations in MSW remain a significant challenge in producing high-quality composites. Contaminants can significantly impact the thermal and mechanical properties of the resulting composite [69]. It has been shown by [70] that MSW that contains a considerable amount of contaminants results in reduced tensile strength and rheological properties compared to less-contaminated samples [70]. With batch sizes of 5 kg, this laboratory-scale investigation produced consistent results. However, industrial-scale production would require processing volumes several orders of magnitude larger, potentially introducing issues of heterogeneity and inconsistent mixing. These can be easily overcome by waste size reduction to a crumb, followed by pre-blending prior to compounding/extrusion. Hopewell pointed out that the intrinsic diversity of waste streams makes it especially difficult to ensure the uniform dispersion of MSW components in large-scale extruders [71]. Although Ragaert observed challenges with temperature control for large-scale recycling operations, which can have an impact on the final composite characteristics, our laboratory setup maintained constant processing temperatures (±5 °C) [70].

5. Conclusions

This study successfully characterized MSW from Utah and Michigan transfer stations. The MSW contained a mixture of commodity plastics, paper, and some inorganic materials. The melt flow properties were comparable to high-loading wood–plastic composites. Compounding homogenized the MSW mix to form a uniform product with reasonable flexural strength and modulus. Thermal analysis showed the presence of three melt temperatures associated with PE, PP, and PET, and one Tg for PS. Improvements in the composite properties could be made by adding coupling agents or cross-linking to increase the interfacial bonding between the various plastics and fibers. MSW-based composites have potential in non-structural applications, such as pavers, biodegradable pots, road construction, and garden fences.

Author Contributions

A.G.M.: Conceptualization, Methodology, Review and Editing; H.A.: Data Curation, Writing—Original Draft Preparation; J.L.K.: Visualization, Investigation; E.B.-Z.: Writing—Review and Editing. All authors have read and agreed to the published version of the manuscript.

Funding

Department of Energy contract number DE-AC07-05ID14517 through the Battelle Energy Alliance blanket master contract number 154756.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

We would like to acknowledge (i) Hui Li for running the torque rheometry experiments, (ii) the University of Idaho, College of Natural Resources, for support in the purchase of the FTIR spectrometer, and (iii) the M.J. Murdock Charitable Trust for their support in the purchase of the twin-screw extruder.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
ASTMAmerican Society of Standards and Testing
HDPEHigh-density polyethylene
LDPELow-density polyethylene
GCMSGas chromatography–mass spectrometry
HPLCHigh-pressure liquid chromatography
FTIRFourier-transform infrared spectroscopy
MPaMega Pascal
GPaGiga Pascal
PETPoly (ethylene terephthalate)
PBDPolybutadiene
XRDX-ray diffraction
MSWMunicipal solid waste
LABLignin acetyl-bromide
PSPolystyrene
PEPolyethylene
MCMoisture content
VMVolatile matter
FCFixed carbon
MIMichigan
UTUtah
KLKlasson lignin
HIHydroxyl index
CICarbonyl index
TmMelt temperature
TgGlass transition
XcDegree of crystallinity
PPPolypropylene

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Figure 1. (a) Relative abundance of types of plastic materials in UT and MI crumbed MSW determined by FTIR spectroscopy; (b) X-ray diffractograms of UT and MI MSW; and (c) torque rheometry curves for compounded UT and MI MSW.
Figure 1. (a) Relative abundance of types of plastic materials in UT and MI crumbed MSW determined by FTIR spectroscopy; (b) X-ray diffractograms of UT and MI MSW; and (c) torque rheometry curves for compounded UT and MI MSW.
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Figure 2. FTIR-ATR spectra of compounded and spectral averaged MSW from UT and MI.
Figure 2. FTIR-ATR spectra of compounded and spectral averaged MSW from UT and MI.
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Figure 3. DSC thermograms of compounded UT and MI MSW: (a) the 1st cooling cycle; (b) the 2nd heating cycle.
Figure 3. DSC thermograms of compounded UT and MI MSW: (a) the 1st cooling cycle; (b) the 2nd heating cycle.
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Figure 4. Flow curves (η* vs. shear frequency) of compounded MI and UT MSW and HDPE at 190 °C, and water absorption behavior of compounded and compression-molded MSW composites.
Figure 4. Flow curves (η* vs. shear frequency) of compounded MI and UT MSW and HDPE at 190 °C, and water absorption behavior of compounded and compression-molded MSW composites.
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Figure 5. (a) TGA and (b) DTG thermograms of UT and MI MSW.
Figure 5. (a) TGA and (b) DTG thermograms of UT and MI MSW.
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Figure 6. Water absorption behavior of compounded and compression-molded MSW composites.
Figure 6. Water absorption behavior of compounded and compression-molded MSW composites.
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Table 1. Proximate and chemical compositional analysis of MSW.
Table 1. Proximate and chemical compositional analysis of MSW.
UT MSWMI MSW
Fixed carbon (%)12.2 ± 1.29.3 ± 1.2
Volatile matter (%)77.2 ± 0.580.3 ± 0.8
Ash (%)10.6 ± 1.210.4 ± 1.1
Calorific value (kJ/g)27.0 ± 0.729.6 ± 0.2
CH2Cl2 extractives (%)8.6 ± 0.59.0 ± 0.5
Carbohydrates by phenol–sulfuric acid method (%) 54.6 ± 5.047.7 ± 3.5
Lignin (LAB) (%)30.7 ± 2.728.8 ± 0.9
KL (%)38.2 ± 0.714.6 ± 0.1
ASL (%)0.9 ± 0.050.5 ± 0.1
Total lignin (%)39.115.1
Glucan (%)22.5 ± 0.934.2 ± 0.6
Xylan (%)3.3 ± 0.37.2 ± 0.9
Galactan (%)0.6 ± 0.70.7 ± 0.1
Mannan (%)1.8 ± 0.34.13 ± 0.3
Total carbohydrate (%)28.246.3
Fatty acid composition
Myristic acid (µg/g MSW)2.71.8
Palmitic acid (µg/g MSW)11.810.2
Linoleic acid (µg/g MSW)3.166.3
Oleic acid (µg/g MSW)39.93.9
Stearic acid (µg/g MSW)55.65.2
Table 2. The thermal properties of compounded MSW samples determined by DSC.
Table 2. The thermal properties of compounded MSW samples determined by DSC.
Tm1Tm2Tm3Xc PE in MSWXc PP in MSWXc PET in MSWTgTC1TC2
(°C)(°C)(°C)(%)(%)(%)(°C)(°C)(°C)
UT124.6158.4249.52.31.00.4389.3214.6111.5
MI132.7160.8247.810.82.71.094.1209.1115.3
Table 3. Rheological data for extruded MSW at 190 °C.
Table 3. Rheological data for extruded MSW at 190 °C.
Formulationη* (Pa.s)Power Law Fit Model
1 HzEquationK (Pa.s)nR2
UT-MSW27,800y = 30971x−0.88530,9710.1150.992
MI-MSW42,580y = 45070x−0.75845,0700.2420.976
HDPE4465y = 4247x−0.54242470.4580.979
Table 4. Thermal degradation onset (Tonset) and DTG maxima temperatures for MSW and reference samples.
Table 4. Thermal degradation onset (Tonset) and DTG maxima temperatures for MSW and reference samples.
Tonset1 (°C)Tonset2 (°C)Tonset3 (°C)Tonset4 (°C)Tonset5 (°C)DTGm1 (°C)DTGm2 (°C)DTGm3 (°C)DTGm4 (°C)DTGm5 (°C)Residual (%)
UT482914845906887136151062573320.8
MI483184816016897336551362672617.3
Cellulose 360 385
PET 440 470
Nylon 66 443 473
PP 469 498
HDPE 499 523
Silicone 473 679 560 727
CaCO3 716 778
Table 5. Flexural strength (FS) and modulus (FM) values of compression-molded crumb and compounded MSW composites.
Table 5. Flexural strength (FS) and modulus (FM) values of compression-molded crumb and compounded MSW composites.
Flexural Strength (MPa)Flexural Modulus (GPa)
MI—Compression-molded18.3 ± 0.30.200 ± 0.001
UT—Compression-molded10.7 ± 1.30.140 ± 0.017
MI—Extruded29.9 ± 0.90.814 ± 0.015
UT—Extruded23.8 ± 0.11.01 ± 0.01
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Appiah, H.; Bar-Ziv, E.; Klinger, J.L.; McDonald, A.G. Exploring New Applications of Municipal Solid Waste. Sustainability 2025, 17, 3719. https://doi.org/10.3390/su17083719

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Appiah H, Bar-Ziv E, Klinger JL, McDonald AG. Exploring New Applications of Municipal Solid Waste. Sustainability. 2025; 17(8):3719. https://doi.org/10.3390/su17083719

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Appiah, Harrison, Ezra Bar-Ziv, Jordan L. Klinger, and Armando G. McDonald. 2025. "Exploring New Applications of Municipal Solid Waste" Sustainability 17, no. 8: 3719. https://doi.org/10.3390/su17083719

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Appiah, H., Bar-Ziv, E., Klinger, J. L., & McDonald, A. G. (2025). Exploring New Applications of Municipal Solid Waste. Sustainability, 17(8), 3719. https://doi.org/10.3390/su17083719

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