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Article

Blends of Sustainable Polymers and Waste Soy Biomass

by
Shawn Martey
1,†,
Brooklyn Hayden
1,†,
Kalsoom Jan
2,
Kerry Candlen
1,
Jo Ann Ratto
1,
Robina Hogan
3 and
Wan-Ting Chen
1,*
1
Department of Plastics Engineering, University of Massachusetts Lowell, Lowell, MA 01854, USA
2
Department of Chemistry, University of Massachusetts Lowell, Lowell, MA 01854, USA
3
Soybean Tech, LLC, Bald Head Island, NC 28461, USA
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Sustainability 2025, 17(11), 5122; https://doi.org/10.3390/su17115122
Submission received: 2 April 2025 / Revised: 19 May 2025 / Accepted: 21 May 2025 / Published: 3 June 2025
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Abstract

:
Sustainable polymers have attracted interest due to their ability to biodegrade under specific conditions in soil, compost, and the marine environment; however, they have comparatively lower mechanical properties, limiting their widespread use. This study explores the effect of incorporating waste soy biomass into sustainable polymers (including biodegradable and biobased) on the thermal and mechanical properties of the resultant blends. The dispersion of the waste soy biomass in the polymer matrix is also investigated in relation to particle size (17 µm vs. 1000 µm). Fine waste soy biomass did not significantly affect the melting temperature of the polymers (polyhydroxyalkanoates, polybutylene adipate terephthalate, polybutylene adipate terephthalate/poly(lactic) acid, and biobased linear low-density polyethylene) used in this study, but their enthalpy of fusion decreased after soy was melt-blended with the polymers. The tensile modulus of the polymers filled with fine waste soy biomass powder (17 µm) was enhanced when melt-blended as compared to unfilled polymers. Additionally, it was found that fine waste soy powder (17 µm) increased the tensile modulus of the polymer blends without significantly affecting processability, while coarse waste soy meal (1000 µm) generally reduced elongation at break due to poor dispersion and stress concentration; however, this effect was less pronounced in PHA blends, where improved compatibility was observed.

Graphical Abstract

1. Introduction

Biodegradable polymers have become materials of interest in response to the overwhelming accumulation of plastic waste [1]. Examples of these plastics include polylactic acid (PLA), polyhydroxyalkanoate (PHA), polybutylene adipate terephthalate (PBAT), cellulose, and starch [2]. Biodegradable plastics decompose into carbon dioxide, biomass, and water under appropriate conditions and environments that influence mineralization, such as temperature, aeration, humidity, pH, and microorganisms [3,4]. The use of biodegradable polymers is limited due to their performance when compared to petroleum-derived polymers obtained from fossil fuels [5]. Regardless, these plastics have found success in various applications such as packaging, coating film, food containers, compost bags, and mulch film [1].
Several different fillers have been incorporated into polymers to enhance their mechanical, barrier, and thermal properties. These fillers can be categorized as organic and inorganic materials [6]. Amongst the natural organic fillers, soy is used to enhance polymers’ mechanical properties and reduce their cost because it is inexpensive and readily available [7]. Candlen et al. [8] studied the effects of soy waste on poly (butylene adipate-co-terephthalate) and polylactic acid (PBAT/PLA) mulch film applications. The soy-filled PBAT/PLA films were compared to unfilled PBAT/PLA. Accelerated degradation of the soy-filled polymers was observed, but there was a decrease in the tensile modulus. Balla et al. [9] used soybean hull-derived fibers to reinforce thermoplastic copolyester (TPC), increasing the toughness of TPC by 30%.
Apart from the thermodynamic interactions between the polymer and the filler [10], the filler characteristics significantly affect the mechanical properties of the polymer blend. Important filler characteristics include particle size, specific surface area, and particle shape [11]. Fillers with large particles led to compromised mechanical properties of the polymer blend, while fillers with smaller sizes tend to aggregate in the polymer’s matrix. Aggregation of the fillers leads to less homogeneity, which increases the rigidity of the polymer but acts as a crack initiation site during impact, thereby reducing the impact strength of the polymer blend [11,12]. However, when the smaller-sized fillers successfully disperse uniformly in the polymer matrix, there is an improvement in the mechanical properties of the polymer blend [13]. The reinforcing effect of fillers has also been proven to affect the modulus of polymer blends [11].
In the food processing industry, there is a common practice of disposing of byproducts, which leads to economic losses. There are efforts to find various uses for these byproducts [14]. Some applications include using porous carbon materials for energy storage [15,16], cotton leaf and hull for electrochemical applications [17], defatted cottonseed meal enriched with crude fiber and protein for carbon quantum dot composited porous carbon for energy storage, which is crucial for high-performance supercapacitors [18], and soy protein isolate to improve the biodegradation of polymer composites [8,19].
Given the wide availability of soy processing byproducts in the U.S.—where soy is the second most produced crop—numerous studies have explored the valorization of soybean processing residues, including hulls, meal, and fines generated during protein extraction [20,21]. Additionally, waste soy biomass can be obtained after oil extraction. This type of waste is generated in large quantities in biodiesel manufacturing and is cheap and readily available [22]. These waste soy biomass materials are typically underutilized but possess high potential as sustainable polymer fillers [23].
The feasibility of introducing soybean biomass into polymer systems has been supported by several studies. Koshy et al. (2015) and Abdul Khalil et al. (2019) review how soy protein- and starch-based materials can be blended with synthetic and biopolymers to improve mechanical strength, reduce cost, and enhance biodegradability [6,24]. Furthermore, Candlen et al. (2022) demonstrated that soy-filled PBAT/PLA blends showed accelerated degradation behavior, albeit with trade-offs in mechanical performance [8]. These outcomes highlight the dual role of waste soy biomass as both a functional filler and a contributor to end-of-life material performance. In short, waste soy biomass has been incorporated into polymers for various applications, including promoting biodegradation, reducing the carbon footprint, and improving the mechanical properties of the polymer systems [25,26,27]. However, there are few studies systematically evaluating how particle size and the types of waste soy biomass may affect the dispersion, crystallinity, and mechanical performance of soy-filled polymer blends. To address this research gap, this study will investigate the effects of the types of waste soy biomass and varying sizes of soy on the properties of the soy-incorporated polymers.
This study presents a novel approach by incorporating two distinct types of waste soy biomass—fine soy powder (~17 µm) and coarse soy particulates (~1000 µm)—into the biodegradable polymers (PHA, PBAT, and a PBAT/PLA blend) and biobased linear low-density polyethylene (B-LLDPE). Both waste soy biomass types, fine soy powder (~17 µm) and soy meal (~1000 µm), are residual byproducts from the production of soybean protein isolate. By evaluating both particle size and mixing methods (dry mixing vs. melt processing), this work offers new insights into how soy-based agricultural waste can be used to tailor the thermal and/or mechanical properties of biodegradable and biobased polymer systems. The results contribute to the advancement of sustainable materials and open new pathways for the valorization of biomass in polymer applications.

2. Materials and Methods

2.1. Materials

Polyhydroxyalkanoate (PHA, Lot 3308, Metabolix, Cambridge, MA, USA), polybutylene adipate terephthalate (PBAT, Nurel Inzea F09E, Zaragoza, Spain), polybutylene adipate terephthalate/poly(lactic) acid (PBAT/PLA blend, Ecovio®, BASF, Florham Park, NJ, USA), and biobased linear low-density polyethylene (B-LLDPE, Braskem, I’m Green®, Philadelphia, PA, USA) were used. Notably, the PBAT used contains 10–25% of starch. It is also acknowledged that the results obtained in this study are not necessarily a generalization of all the polymers listed. The results obtained in this study are only specific to the polymers used in this study, as the mass ratio of polymeric components, the degree of copolymerization, and other characteristics (e.g., additives used) may affect the properties of polymers. However, this study may serve as a guide or reference for others performing relevant experiments.
Two types of waste soy biomass, SuperbTM (hereafter referred to as waste soy powder, supplied by Archer Daniels Midland (ADM), Decatur, IL, USA) and Hi-Pro Solvent Extracted Soy Meal (hereafter referred to as waste soy meal, donated by Indiana Soybean Alliance, Indianapolis, IN, USA), were used in this study. Waste soy powder was a byproduct generated during soybean protein isolate manufacturing processes, while waste soy meal was a byproduct of soybean oil extraction (from dehulled, defatted soy flakes). The biochemical compositions of waste soy powder and waste soy meal are summarized in Table 1.
To reduce the particle size, soy powder was further processed by Glen Mills Inc. (Clifton, NJ, USA) using a jet mill (Model 0304, Jet-O-Mizer, Telford, PA, USA) with heated grinding air. Soy meal with an average particle size of 1000 microns (i.e., 1 mm) was used as supplied to evaluate if it would be a valuable additive for polymer compounding and melt processing. The particle size distributions of all soy samples were analyzed by a particle size distribution analyzer (Coulter LS230, Brea, CA, USA), and the results are presented in Table S1 in the Supplementary Materials. For the subsequent polymer processing investigation, waste soy powder with the smallest average particle size (17 microns) and waste soy meal with the largest average particle size (1000 microns) were selected to study the effect of particle size on polymer blends’ properties. No dispersant was used in the following polymer processing study.

2.2. Processing of Soy and Polymers

2.2.1. Compounding

Prior to compounding, the waste soy biomass and polymers were dried for 3 h. Drying temperatures for B-LLDPE, PHA, PBAT/PLA, and PBAT polymer blends were 110 °C, 110 °C, 80 °C, and 80 °C, respectively. The soy (10 wt.%) was compounded with various polymers using a conical twin-screw micro-compounder (XploreTM HT-15, Sittard, The Netherlands) with a screw speed of 50 rpm. B-LLDPE, PHA, PBAT/PLA, and PBAT polymer blends were processed at 200 °C, 150 °C, 150 °C, and 135 °C, respectively, at a screw speed of 50 rpm and a residence time of 30 s in the micro-compounder. In general, polymer blend samples were processed approximately 20 °C above the melting temperature of the polymer. This was to ensure that the polymers were melted and processable. The residence time was selected to ensure proper mixing during the extrusion process. This is supported by the literature on processing polymer blends with soy biomass [8]. Polymer blends were also prepared by dry blending and compared with the melt-blended samples. Prior to processing the blends, all polymers were dried for 3 h.

2.2.2. Injection Molding Methods

The tensile bars were made using a micro-injection molding machine (IM12, XploreTM, Sittard, The Netherlands). Table 2 summarizes the micro-injection conditions used for all the polymer blends. The pressure used for all the polymer blends was 9 bar for fill pressure (for 9 s), 8 bar for pack pressure (for 8 s), and 8 bar for hold pressure (for 8 s).

2.3. Characterization Methods

2.3.1. Thermogravimetric Analysis (TGA)

To determine the onset degradation temperature of the polymers and the soy samples, TGA was performed using a thermogravimetric analyzer (Mettler Toledo TGA 2 STARe System, Columbus, OH, USA) in a nitrogen atmosphere. A weight range of 3–6 mg of the polymers was used and heated from 50 °C to 700 °C at a rate of 10 °C/min. This test was performed to estimate the maximum processing temperature of the samples.

2.3.2. Differential Scanning Calorimetry (DSC)

A differential scanning calorimeter (Mettler Toledo DSC 3 + STARe System, Columbus, OH, USA) was used to estimate the melting temperature and enthalpy of fusion of the polymers and the polymer blends. The melting temperature is helpful in estimating the processing temperature of the samples. 3 to 10 mg of the samples were heated at 10 °C/min to 200 °C and cooled at 10 °C/min to −50 °C and then heated at 10 °C/min to 200 °C.

2.3.3. Scanning Electron Microscopy (SEM)

The morphology of the polymer blends was investigated using scanning electron microscopy (SEM, JSM-6390, JEOL, Tokyo, Japan). Samples were fractured in liquid nitrogen and gold-coated for 120 s using a vacuum sputter coater (Denton vacuum desk IV, Denton Vacuum, LLC, Moorestown, NJ, USA).

2.3.4. Tensile Test Using ASTM D638

The mechanical properties of the polymer blends were tested using a tensile testing machine (Instron® 5966, Norwood, MA, USA) according to the ASTM D638 Standard [28] Test Method for Tensile Properties of Plastics. Before tensile testing, all the tensile bars were conditioned at 23 °C for 40 h following the ASTM D618 Standard [29] Practice for Conditioning Plastics for Testing, equilibrating the bars to 50% humidity. Five specimens of each sample were tested.

3. Results

3.1. Melt Processing

3.1.1. Micro-Compounding of the Polymer Blends with Soy Powder

The processing steps for micro-compounding the polymers with the waste soy biomass powder are documented in a video format (see Figure S1 of the Supplementary Materials). As shown in Figure S1, soy powder and B-LLDPE, PHA, PBAT/PLA, and PBAT polymers were melt-processable by micro-compounding. Notably, it is worth pointing out that the polymer blend made of PHA and soy powder was rubberier than the other three polymer blends with soy powder. The PHA sample used may be a blend of amorphous PHA, a semi-crystalline PHA, and a crystalline poly-(3-hydroxybutyrate) (P3HB) homopolymer [30]. Most likely, the amorphous PHA provides a rubbery flexibility, while the semi-crystalline PHA contributes to some degree of rigidity, and the crystalline P3HB homopolymer adds significant structural strength and crystallinity to the overall material, allowing for tailored properties depending on the blend ratio of each component [31,32].
TGA was performed to determine the degradation temperatures of the polymers, waste soy powder, waste soy meal, and soy-filled polymer blends. This analysis was essential to ensure that processing temperatures remained below the thermal degradation limits of the materials, particularly for waste soy biomass samples. Figure 1 shows the TGA graph of the polymers before and after compounding, while the onset degradation temperature and percentage residue of each polymer are shown in Table 3. It was also found that the polymers had some amount of inorganic residue, which may have been added to the polymer by the manufacturer to improve the mechanical properties. The degradation temperature of soy powder and soy meal was approximately 255 °C and 225 °C, respectively (see Figure S2 and Table S2 in the Supplementary Materials for more details). The difference in the degradation temperatures between waste soy biomass powder and waste soy biomass meal may be due to their biochemical composition (see Table 1 for more details), as suggested by [8,33]. However, a more in-depth investigation regarding the detailed biochemical compositions of soy powder and soy meal would be needed to elucidate this thermal degradation difference, which is beyond the scope of this study. Additionally, there was no significant change in the decomposition temperature of the composite after waste soy biomass was blended with the polymer.
DSC was carried out to identify the melting temperature of the polymers, which can be used as a reference temperature for processing each polymer in the micro-compounder. The melting temperature was determined from the second heating cycle (see Figure S2 in the Supplementary Materials for more details), and Table 4 shows the melting peak of each polymer (without soy waste biomass). Notably, two distinct peaks were observed for PHA samples, which may be a result of the rearrangement of the crystal morphology of PHA [35]. Moreover, PHA may include blends of different types of PHA, such as a semi-crystalline PHA and a crystalline poly-(3-hydroxybutyrate) (P3HB) homopolymer, which could also lead to two melting temperatures [31]. Two peaks were observed for PBAT/PLA blends because of the presence of the two constituent polymers in the blend.

3.1.2. Injection Molding

In order to understand the mechanical properties of the polymer blends (i.e., soy waste biomass-containing composites) created, tensile bars were molded via micro-injection molding, and tensile testing was performed (see Figure S3 in the Supplementary Materials). Of all the polymers tested, B-LLDPE showed the best processability. The melting temperature of B-LLDPE obtained from DSC analysis can be directly used to set up the barrel temperature without adjustment. A similar observation also applies to the soy-filled B-LLDPE. In terms of the quality of the tensile bar, there were no notable changes in the tensile bars when the material was switched from B-LLDPE to the soy-filled B-LLDPE polymer blend. Similarly, the PHA polymer was processable during the micro-injection molding process. However, it was noticed that the tensile bars created from PHA were very brittle. Removing PHA tensile bars from the mold requires extra caution to avoid breaking. Not much change was seen in the micro-injection molding process when the soy powder was added to PHA. PBAT/PLA polymers also showed no issues during micro-injection molding, but short shots were encountered when the soy was added into PBAT/PLA. On the other hand, it was difficult to injection mold PBAT, as short shots were a common issue. Eventually, the barrel temperatures had to be increased to 165 °C to prevent short shots. Soy-filled PBAT used the same temperature (165 °C) during micro-injection molding to avoid the potential short shot issues and maintain consistency.

3.2. Effects of Waste Soy Biomass Soy Powder on the Thermal Properties of the Polymer

The thermal properties of the samples were observed using DSC. The main goal was to investigate the change in thermal properties of the polymer blends based on the type of mixing that was performed. In this study, two types of mixing, dry blending (via physical mixing) and melt blending (via extrusion), were used. The raw DSC data of polymer blends with 10 wt.% soy (i.e., soy waste biomass-containing composites) using these two types of mixing are summarized in Figures S4–S7 in the Supplementary Materials. Figure 2 shows the melting temperatures of the polymer blends prepared by dry blending and melt blending, while Figure S8 presents the DSC thermogram of soy powder. As Figure S8 shows, waste soy biomass powder melted within a temperature range of 60–180 °C with a very broad melting peak (likely due to the complex protein and starch structures) [36], and the melting temperature occurred around 110 °C. The observed melting characteristics were similar to those reported in [37,38].
As Figure 2 shows, regardless of the mode of mixing, there was no notable change in the melting temperatures of the polymer samples when soy powder was added. No new peaks were identified from the polymer blends (i.e., soy waste biomass-containing composites), indicating that soy powder had good compatibility with the four polymers used in this study. In other words, the polymer blends with 10 wt.% soy powder were homogeneous at the macroscopic level, with similar melting temperatures reported. Similar findings were also reported when blending soy protein isolate powder with poly(vinyl alcohol) [37]. For PHA, two melting peaks were observed, likely because several types of PHAs are present. The PBAT/PLA blend showed two melting peaks because of the presence of both polymers in the blend [31].
Figure 2 shows the enthalpy of fusion of the samples. All melt-blended samples (i.e., soy waste biomass-containing composites) showed lower enthalpy of fusion compared to the unfilled and dry-blended samples. When melt blending polymer samples with waste soy biomass, the enthalpy of fusion decreased because the waste soy biomass (obtained from soybean protein isolate manufacturing processes) may disrupt the crystalline structure of the polymer, leading to decreased energy requirements for melting since intermolecular forces are reduced [39]. In addition, dilution effects may play some role here. Since waste soy biomass, which may be composed of protein and starch [8], is generally amorphous, its addition effectively dilutes the amount of crystalline polymer per unit mass. As reflected in the lower enthalpy of fusion shown with dry-blended and melt-blended samples (i.e., soy waste biomass-containing composites), it is also suggested that this dilution could lower the total enthalpy of fusion (due to less crystalline material that requires energy to transition from solid to liquid). Similar findings were also reported when melt blending soy protein isolate powder and polyethylene oxide for nanofiber application [40]. Moreover, the soy powder may interact with the polymer through mechanisms such as hydrogen bonding or electrostatic interactions between polymer chains and soy macromolecules. [40,41]. These interactions can immobilize certain segments of the polymer chains, preventing them from participating in the crystallization process. In summary, the lower enthalpy of fusion in the polymer blends (i.e., soy waste biomass-containing composites) is primarily due to the reduction in crystallinity caused by the presence of soy powder. The interference with the polymer chain alignment, the dilution of crystalline content, and specific interactions between soy powder and the polymer chains all contribute to this effect.

3.3. Mechanical Properties of Polymer Blends

Tensile bars created from biobased or biodegradable polymers with 10 wt.% waste soy biomass powder (i.e., soy waste biomass-containing composites) via micro-injection molding are shown in Figure S9 in the Supplementary Materials. The mechanical properties of the injection-molded tensile bars are depicted in Figure 3. Figure 3a,b show the tensile modulus and elongation at break of unfilled and soy-filled PBAT/PLA and PBAT, respectively. Waste soy biomass powder improved the stiffness of both polymers since it acts as a filler. However, the elongation at break of the samples was negatively affected by the waste soy biomass. This low elongation at break indicates the presence of the aggregation of the waste soy biomass powder, resulting in strain hardening effects [42].
The waste soy biomass powder was compared to the waste soy biomass meal filler with B-LLDPE and PHA using the same processing parameters (see Figures S10 and S11 in the Supplementary Materials). Figure 4 shows the effects of waste soy biomass powder and waste soy biomass meal on B-LLDPE. Waste soy biomass meal-filled B-LLDPE showed a slightly higher modulus, whereas the elongation at break of the polymer blend made of B-LLDPE and waste soy biomass meal decreased significantly. This is expected as the waste soy biomass meal sample has a much larger particle size, which likely led to agglomeration in the polymer blend samples.
The soy meal sample was incorporated to evaluate if it would be an effective additive for polymer processing. Figure 5 compares the tensile modulus and elongation at break of PHA samples filled with 10 wt.% soy powder and soy meal. Due to the aggregation of soy meal in the PHA polymer blend, there was an increase in the stiffness of the polymer blend made of PHA and soy meal. The elongation at break of soy meal-filled PHA samples also increased, showing compatibility between the soy meal sample and PHA.

3.4. Dispersion of Various Types of Soy Biomass in the Polymer Matrix via SEM Analysis

The dispersion of fillers in a polymer matrix has the tendency to affect the properties of the polymer [10]. SEM was used to investigate the state of dispersion of soy powder and soy meal in the polymer matrices. Figure 6 shows the morphology of the blends, where a comparatively better dispersion of the soy powder is evident in the B-LLDPE sample (Figure 6a), as opposed to agglomeration of soy meal in Figure 6b. Large voids are seen for both samples containing soy powder and soy meal, possibly due to the incompatibility of soy biomass with B-LLDPE. Figure 6c,d show the morphology of PHA filled with 10% soy powder and soy meal, respectively. These figures show a more uniform morphology compared to B-LLDPE polymer blends, suggesting that PHA is more compatible with soy powder and soy meal than B-LLDPE.
However, it is acknowledged that the SEM analysis in our study offered limited insight into the specific effects of soy particle size (fine powder vs. coarse meal) on the dispersion or interaction within B-LLDPE and PHA polymer matrices. This could be because SEM primarily provides surface morphology; it reveals filler dispersion, agglomeration, and voids at the fractured surface. However, it does not provide detailed quantitative data on interfacial adhesion or crystallinity changes, which are more directly affected by particle size. Moreover, as mentioned, PHA showed better compatibility with both waste soy powder and waste soy meal. This compatibility may lead to more uniform dispersion regardless of particle size, masking any size-dependent effects in the SEM images. In the case of B-LLDPE, poor interfacial compatibility may dominate morphology, overshadowing differences due to particle size. The presence of voids and agglomerates in both powder- and meal-filled samples suggests that compatibility issues, not particle size, were the main factor influencing morphology. In summary, SEM provided a useful overview of filler dispersion and compatibility, but due to its surface-based, qualitative nature and limitations in resolving complex interactions, it was not sufficient to highlight the nuanced effects of soy particle size in B-LLDPE and PHA blends. Complementary techniques like transmission electron microscopy (TEM), X-ray microtomography, or additional dyeing on the image analysis may further elucidate these effects.

4. Conclusions

This study demonstrates a novel and sustainable approach to enhancing biodegradable and biobased polymers by incorporating two types of waste soy biomass—fine soy powder (17 µm) and coarse soy meal (1000 µm)—into polymer matrices such as PHA, PBAT, PBAT/PLA, and B-LLDPE. Through a combination of dry mixing and melt processing techniques, we systematically evaluated how soy particle size and processing method affect the thermal behavior, mechanical properties, and dispersion of the resulting polymer blends. While waste soy biomass incorporation had minimal impact on melting temperature, melt blending consistently reduced the enthalpy of fusion, suggesting interactions between the soy filler and polymer matrix that hinder crystallinity. Fine soy powder (17 µm) generally enhanced the tensile modulus without significantly compromising processability, while coarse soy meal (1000 µm) affected elongation at break due to poorer dispersion and stress concentration, except in PHA blends where improved compatibility was observed.
The project not only optimized polymer/waste soy biomass formulations but also developed crucial processing procedures, protocols, and laboratory techniques essential for working with soy additives. Through the melt processing of polymers with and without soy, processing parameters were fine-tuned for each unique formulation, leading to the production of high-quality injection-molded parts containing soy. This research has meaningful implications for both the soy industry as well as the sustainable polymer sector. Additionally, this study confirms that inexpensive soy additives, like those used here, can be successfully incorporated into bioplastics for injection-molded parts without compromising desirable mechanical properties, opening up potential applications in agriculture, medical devices, and food packaging.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/su17115122/s1: Figure S1. TGA of soy powder (with an average particle size of 17 μm) and soy meal (with an average particle size of 1000 μm); Figure S2: DSC of polymers used in this study; Figure S3: Micro-compounding of polymers and 10 wt.% waste soy biomass powder samples: (a) LLDPE, (b) PHA, (c) PBAT/PLA, and (d) PBAT; Figure S4: Raw DSC data of polymer blends with 10 wt.% soy (i.e., soy waste biomass-containing composites): (a) B-LLDPE, (b) B-LLDPE + 10% waste soy biomass powder, and (c) B-LLDPE + 10% waste soy biomass powder (melt-blended by micro-compounder); Figure S5: Raw DSC data of polymer blends with 10 wt.% soy (i.e., soy waste biomass-containing composites): (a) PHA, (b) PHA + 10% waste soy biomass powder, and (c) PHA + 10% waste soy biomass powder (melt-blended by micro-compounder); Figure S6: Raw DSC data of polymer blends with 10 wt.% soy (i.e., soy waste biomass-containing composites): (a) PBAT/PLA, (b) PBAT/PLA + 10% waste soy biomass powder, and (c) PBAT/PLA + 10% waste soy biomass powder (melt-blended by micro-compounder); Figure S7: Raw DSC data of polymer blends with 10 wt.% soy (i.e., soy waste biomass-containing composites): (a) PBAT, (b) PBAT + 10% waste soy biomass powder, and (c) PBAT + 10% waste soy biomass powder (melt-blended by micro-compounder); Figure S8: DSC thermogram of soy powder; Figure S9: Tensile bars created from polymer blends with 10 wt.% waste soy biomass powder via micro-injection molding; Figure S10: Pellets made of B-LLDPE and 10% waste soy biomass meal (left) and waste soy biomass powder (right); Figure S11: Tensile bars made of 10 wt.% waste soy biomass meal and (a) LLDPE and (b) PHA; Table S1: Particle size distribution of soy powder and soy meal biomass; Table S2: Onset degradation temperatures of waste soy biomass powder (with an average particle size of 17 μm) and waste soy biomass meal (with an average particle size of 1000 μm).

Author Contributions

Conceptualization, S.M., B.H., J.A.R., R.H. and W.-T.C.; methodology, S.M., B.H., J.A.R. and W.-T.C.; software, S.M., B.H. and K.J.; validation, S.M., B.H., K.J. and K.C.; formal analysis, S.M. and B.H; investigation, S.M., B.H., J.A.R. and W.-T.C.; resources, J.A.R., R.H. and W.-T.C.; data curation, S.M., B.H., K.J. and K.C.; writing—original draft preparation, S.M.; writing—review and editing, S.M., B.H., K.J., K.C., J.A.R., R.H. and W.-T.C.; visualization, S.M., B.H., K.J., K.C. and W.-T.C.; supervision, J.A.R., R.H. and W.-T.C.; project administration, J.A.R., R.H. and W.-T.C.; funding acquisition, J.A.R., R.H. and W.-T.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Indiana Soybean Alliance (ISA). The authors also would like to thank the University of Massachusetts Lowell for providing start-up funds.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Acknowledgments

We would like to thank the personnel of the Department of Plastics Engineering for their continued help with this project through discussion, training, equipment, and more. We also would like to thank ADM and the Indiana Soybean Alliance for providing soy samples and Berry Global for providing PBAT samples. In addition, we are grateful for Raj Krishnaswamy’s discussion about PHA polymer blends.

Conflicts of Interest

Author Robina Hogan is employed by Soybean Tech, LLC. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. TGA of polymers (a) before and (b) after compounding.
Figure 1. TGA of polymers (a) before and (b) after compounding.
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Figure 2. Thermal properties of the samples: (a) the melting temperature and (b) the enthalpy of fusion of the samples. * the first melting peak and enthalpy of fusion of the PHA samples; ** the second melting peak and enthalpy of fusion of the PHA samples.
Figure 2. Thermal properties of the samples: (a) the melting temperature and (b) the enthalpy of fusion of the samples. * the first melting peak and enthalpy of fusion of the PHA samples; ** the second melting peak and enthalpy of fusion of the PHA samples.
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Figure 3. (a) Tensile modulus and (b) Elongation at break of the polymers of unfilled and soy powder-filled PBAT/PLA and PBAT samples.
Figure 3. (a) Tensile modulus and (b) Elongation at break of the polymers of unfilled and soy powder-filled PBAT/PLA and PBAT samples.
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Figure 4. (a) Tensile modulus and (b) Elongation at break of the polymers of unfilled, soy powder-filled, and soy meal-filled B-LLDPE.
Figure 4. (a) Tensile modulus and (b) Elongation at break of the polymers of unfilled, soy powder-filled, and soy meal-filled B-LLDPE.
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Figure 5. (a) Tensile modulus and (b) Elongation at break of the polymers of unfilled, soy powder-filled, and soy meal-filled PHA.
Figure 5. (a) Tensile modulus and (b) Elongation at break of the polymers of unfilled, soy powder-filled, and soy meal-filled PHA.
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Figure 6. SEM images of (a) B-LLDPE filled with 10% soy powder, (b) B-LLDPE filled with 10% soy meal, (c) PHA filled with 10% soy powder, and (d) PHA filled with 10% soy meal.
Figure 6. SEM images of (a) B-LLDPE filled with 10% soy powder, (b) B-LLDPE filled with 10% soy meal, (c) PHA filled with 10% soy powder, and (d) PHA filled with 10% soy meal.
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Table 1. Biochemical compositions of soy powder and soy meal (dry matter basis) used in this study.
Table 1. Biochemical compositions of soy powder and soy meal (dry matter basis) used in this study.
SampleSuperbTM Waste Soy Powder aHi-Pro Solvent Extracted Waste Soy Meal b
Crude Protein38.3%45.0%
Crude Fat0.35%1.0%
Crude Fiber6.24%3.8%
Others c55.1%50.2%
a Data were adapted from Candlen et al. [8]. b Data were provided by the Indiana Soybean Alliance (Indianapolis, IN, USA). c Other components may include starch, sugar, soluble fibers, minerals (i.e., ash), etc., which require more analysis that is out of the scope of this study.
Table 2. Micro-injection processing conditions of polymers.
Table 2. Micro-injection processing conditions of polymers.
SampleBarrel Temperature (°C)Mold Temperature (°C)
B-LLDPE20040
PHA15040
PBAT/PLA15040
PBAT16540
Table 3. Onset decomposition temperature and percent residue of samples.
Table 3. Onset decomposition temperature and percent residue of samples.
SampleOnset Degradation Temperature (°C)Percent Residue (%)
PBAT282.8 and 387.37.0
PBAT + 10% soy waste280.9 and 376.88.6
PBAT/PLA366.321.0
PBAT/PLA + 10% soy waste362.715.3
B-LLDPE464.90.5
B-LLDPE + 10% soy waste453.50.0
PHA279.013.9
PHA + 10% soy waste255.411.0
Note: PBAT used in this study contains 10–25% starch, which may start its degradation temperatures at about 280 °C [34].
Table 4. Melting temperatures of the polymers (without soy waste biomass).
Table 4. Melting temperatures of the polymers (without soy waste biomass).
SampleMelting Temperature (°C)
PBAT119.4
PBAT/PLA125.8 and 146.6
B-LLDPE125.0
PHA155.4 and 166.7
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MDPI and ACS Style

Martey, S.; Hayden, B.; Jan, K.; Candlen, K.; Ratto, J.A.; Hogan, R.; Chen, W.-T. Blends of Sustainable Polymers and Waste Soy Biomass. Sustainability 2025, 17, 5122. https://doi.org/10.3390/su17115122

AMA Style

Martey S, Hayden B, Jan K, Candlen K, Ratto JA, Hogan R, Chen W-T. Blends of Sustainable Polymers and Waste Soy Biomass. Sustainability. 2025; 17(11):5122. https://doi.org/10.3390/su17115122

Chicago/Turabian Style

Martey, Shawn, Brooklyn Hayden, Kalsoom Jan, Kerry Candlen, Jo Ann Ratto, Robina Hogan, and Wan-Ting Chen. 2025. "Blends of Sustainable Polymers and Waste Soy Biomass" Sustainability 17, no. 11: 5122. https://doi.org/10.3390/su17115122

APA Style

Martey, S., Hayden, B., Jan, K., Candlen, K., Ratto, J. A., Hogan, R., & Chen, W.-T. (2025). Blends of Sustainable Polymers and Waste Soy Biomass. Sustainability, 17(11), 5122. https://doi.org/10.3390/su17115122

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