2023 Journal Publication
Abstract: In a previous study, we demonstrated the efficient depolymerization of polyethylene terephthalate (PET) through glycolysis using antimony (III) oxide, a commonly used catalyst in PET synthesis. In the present research, we introduce a novel approach involving the synthesis of a magnetic bifunctional ionic liquid, Fe3O4@PMIM.SbBr4, containing only 2.2 wt% of antimony. The aim is to reduce the required antimony dosage for the reaction and enable its facile recovery and reuse. By employing this catalyst in PET chemical recycling through glycolysis to generate bis (2-hydroxyethyl) terephthalate (BHET) monomer, we achieved 100% PET conversion with a 96.4% yield and selectivity for BHET. This outcome was obtained using a catalyst loading of 6.0 wt% at 200 °C and 0.6 bar in a high-pressure reactor. We explored the impact of catalyst loading on BHET yield and conducted a comparative assessment of the Fe3O4@PMIM.SbBr4 catalyst against antimony (III) bromide, and another synthesized unsupported antimony-containing ionic liquid. Our results revealed the superior catalytic activity of the magnetic ionic liquid catalyst in PET glycolysis. The utilization of this catalyst offers promising potential for PET glycolysis due to its effortless separation using an external magnet, ability to produce highly pure BHET, and recyclability for repetitive use.
Abstract: We report here the production of higher-order oligomers from the glycolysis of poly(ethylene terephthalate) (PET) by using microwave irradiation in a controlled fashion, instead of its fully glycolyzed product, bis(2-hydroxyethyl)terephthalate (BHET). We show that different catalysts can generate either BHET as the ultimate glycolysis product or higher oligomers of PET under microwave irradiation. Depolymerization of waste PET with an average degree of polymerization (DP) of 417 from water bottles was performed in the presence of 0.25 wt % antimony(III) oxide (Sb2O3) as the catalyst at 240 °C and 400 W microwave power, resulting in an oligomer yield of 96.7% with an average DP of 37. Under these conditions, the conversion of PET to oligomers reached 100% in only 5 min at 240 °C (with a 10 min ramping time) and with a ethylene glycol to PET weight ratio of 2.5. In comparison, under the same reaction conditions, 0.04 wt % of zinc acetate (Zn(OAc)2), a well-known catalyst for PET glycolysis, produces only the BHET monomer in 96.3% yield. Our results demonstrated that by using Sb2O3, the same catalyst that is used extensively for PET synthesis from BHET, under microwave irradiation, the PET glycolysis can be controlled to produce higher PET oligomers as an alternative for a complete chemical depolymerization to the BHET monomer. These oligomers are more suitable for being used as additives for many applications and to produce high-quality second-generation products, including regenerated PET.
Abstract: The recycled high-density polyethylene (rHDPE) feedstock containing calcium carbonate (CaCO3) mineral was obtained from post-consumer single-use grocery bags. This feedstock is transformed into new composite materials with improved mechanical properties using a high-shear compounding process. The rHDPE/CaCO3 feedstock is injected into the high-shear polymer mixer and blended with acrylonitrile-butadiene copolymer or nitrile rubber (up to 10 wt%) and crosslinked with dicumyl peroxide. The resultant materials exhibit high mechanical performance, with a maximum tensile strength of 20.3 MPa, stiffness of 1262 MPa, and impact strength of 63 kJ/m2. These mechanical properties are profoundly higher than those of neat rHDPE feedstock. Notably, the post-consumer plastic bags contain a high amount of CaCO3 mineral of approximately 30 wt% (13 vol%), impacting the materials' mechanical properties with additives such as nitrile rubber. In addition, the materials' melt-rheology, viscoelastic, and phase morphology are analyzed. The melt-rheological characteristics indicate that the materials' complex viscosity and storage modulus are frequency-dependent, demonstrating the thermoplastic nature of the composite materials and the possibility of thermomechanical recyclability of the materials. However, treating rHDPE/nitrile rubber blends with peroxide creates resistance to the melt-flow of the materials.
Abstract: Plastics were developed to change our world for the better. However, plastic pollution has become a serious global environmental crisis. Thermoplastic polyesters and polyolefins are among the most abundant plastic waste. This work presents an in-depth non-isothermal crystallization kinetics analysis of recycled post-consumer poly(ethylene terephthalate) (rPET) and recycled polypropylene (rPP) blends prepared through reactive compounding. The effect of pyromellitic dianhydride (PMDA) on crystallization kinetics and phase morphology of rPET/rPP blends was investigated by differential scanning calorimetry (DSC) and microscopy techniques. DSC results showed that increasing rPP content accelerated rPET crystallization while reducing crystallinity, which indicates the nucleation effect of the rPP phase in blends. Further, it was found that the incorporation of PMDA increased the degree of crystallinity during non-isothermal crystallization, even though the rate of crystallinity decreased slightly due to its restriction effects. The non-isothermal crystallization kinetics was analyzed based on the theoretical models developed by Jeziorny, Ozawa, Mo, and Tobin. The activation energy of the crystallization process derived from Kissinger, Takhor, and Augis–Bennett models was found to increase in rPET/rPP blends with increasing PMDA due to hindered dynamics of the system. Rheological measurements revealed that rPET melt viscosity is remarkably increased in the presence of PMDA and reactive blending with rPP relevant for processing. Moreover, nanomechanical mapping of the rPP phase dispersed in the rPET matrix demonstrated the broadening of the interfacial domains after reactive blending due to the branching effect of PMDA. Findings from this study are essential for the recycling/upcycling thermoplastics through non-isothermal fabrication processes, such as extrusion and injection molding, to mitigate the lack of sorting options.
Abstract: Quercetin, one of the major natural flavonoids, has demonstrated great pharmacological potential as an antioxidant and in overcoming drug resistance. However, its low aqueous solubility and poor stability limit its potential applications. Previous studies suggest that the formation of quercetin-metal complexes could increase quercetin stability and biological activity. In this paper, we systematically investigated the formation of quercetin-iron complex nanoparticles by varying the ligand-to-metal ratios with the goal of increasing the aqueous solubility and stability of quercetin. It was found that quercetin-iron complex nanoparticles could be reproducibly synthesized with several ligand-to-iron ratios at room temperature. The UV-Vis spectra of the nanoparticles indicated that nanoparticle formation greatly increased the stability and solubility of quercetin. Compared to free quercetin, the quercetin-iron complex nanoparticles exhibited enhanced antioxidant activities and elongated effects. Our preliminary cellular evaluation suggests that these nanoparticles had minimal cytotoxicity and could effectively block the efflux pump of cells, indicating their potential for cancer treatment.
Abstract: There is a growing interest in sustainable materials based on recycled or natural feedstocks for the circular economy. Ideally, thermoplastic materials are anticipated to show appropriate melt-flow behavior for thermo-mechanical recycling, good melt stability, and mechanical performance. Thermoplastic polyolefins such as polyethylenes are used as single-use packaging materials and generate environmental concerns. This article reports the melt-processing and characterization of composite materials based on recycled high-density polyethylene (rHDPE) and naturally abundant calcium carbonate (CaCO3) mineral at various mass ratios such as 70/30, 50/50, and 30/70. Oxidized polyethylene (OPE) is used as an additive, with 20 wt% replacement of rHDPE, to improve the materials' mechanical performance and thermoplastic behavior. Various characteristics such as tensile, viscoelastic, melt-flow, phase morphology, and thermomechanical recyclability of the materials are analyzed. It reveals that composites with high CaCO3 content compatibilized with OPE show improved tensile strength and modulus. For example, the composite based on 10/70/20 (wt%) of rHDPE/CaCO3/OPE shows an average tensile strength of 27.4 MPa and Young's modulus of 4.5 GPa, in contrast to the rHDPE sample, which shows an average tensile strength of 12.4 MPa and Young's modulus of 967 MPa. Notably, adding OPE decreases the melt-viscosity of the rHDPE/CaCO3 materials, thus improving the composites' melt-flow and thermoplastic behavior. This study indicates the possibility of developing natural CaCO3-rich polymer composites with thermoplastic behavior for real-life applications with superior mechanical and melt-flow properties suitable for thermomechanical recycling.
Abstract: Recycled polypropylene (rPP) often contains a small amount of polyethylene (PE), which frequently compromises the performance of the material and needs to be monitored. In the current work, near-infrared (NIR) and Raman spectrometries were investigated to establish convenient and accurate methods to analyze PE content in rPP. Various spectrum pretreatment methods, including multivariate scattering correction (MSC), standard normal variate transformation (SNV), baseline correction, normalization, smoothing, and first derivative, were tested to reduce noise and improve spectrum quality for partial least squares (PLS) regression. Forward and backward interval partial least squares (FiPLS/BiPLS) methods were employed to optimize spectral range selection. The best NIR model had an R2 of 0.9971 and a root-mean-square error of prediction (RMSEP) of 0.2761 PE wt% in independent validation, and the best Raman model achieved an R2 of 0.9945 and an RMSEP of 0.3818 wt%. The models were further validated with rPP samples. The best Raman model obtained an R2 of 0.9954 and an RMSEP of 0.2404 wt% on non-colored rPP, but it failed to be applied to colored rPP. The best NIR model had a slightly weaker performance on non-colored rPP (R2: 0.9246; RMSEP: 0.9710 wt%), but it was successfully applied to grey commercial rPP samples. This work would help quality assurance of recycled PP materials.
Abstract: Crystallization kinetics of an isotactic homopolymer polypropylene (HPP), an impact copolymer polypropylene (ICPP), and their composites were studied in this work. The Avrami–Jeziorny and Mo models successfully described the crystallization process. When the ICPP content increased, the crystallization rate first increased and then decreased with the highest crystallization rate at the ICPP content of 60 mass%. The nucleation activity kept increasing with the rise of the ICPP content, demonstrating that the rubber phase in the ICPP acted as a nucleating agent and prompted the nucleation process. The decrease in crystallization rate when the ICPP content was higher than 60 mass% might be caused by the decrease in chain mobility and the increase in crystal–crystal interactions. When the ICPP content exceeded 60 mass%, the crystallization activation energy increased evidently, indicating lower polymer chain mobility. Meanwhile, the Avrami exponent, n, decreased, suggesting limited crystal growth space and higher crystal–crystal interactions. The nucleation activity showed high correlations to the mechanical and thermal properties of the materials. The Avrami exponent also had relatively high correlations to these properties. The results improved the understanding of the crystallization behaviors of the HPP–ICPP composites and helped predict their potential mechanical behavior changes.
Abstract: Gold nanodisks (AuNDs) were successfully synthesized by a green biochemical method in aqueous American cockroach (Periplaneta americana) extract (AACE) solution. The high content of water-soluble bio-organic compounds such as hydrophilic proteins and polypeptides in the AACE solution acted as reducing and stabilizing agents for AuND formation due to the interaction with Au 3+ ion precursors. The size (the diameter and the thickness) of the synthesized AuNDs was highly dependent on the concentration (%, m/v) of the AACE solution and was inversely proportional. The average diameter and the thickness of the majority of the AuNDs decreased from 198 to 21 nm and from 48 to 4.3 nm respectively as AACE concentration increases from 0.40 to 1.5%. The method developed in this work provides a convenient, safe, eco-friendly, and cost-efficient mean to produce size-controlled AuNDs.
2022 Journal Publication
Abstract: In this study polyethylene terephthalate (PET) waste from drinking water bottles was converted to bis(2-hydroxyethyl) terephthalate (BHET) in the presence of antimony (III) oxide (Sb2O3) as the catalyst and excess ethylene glycol (EG). The yield of BHET, using 0.5 wt% of the catalyst, under 200°C and 2.1 bar was 97.5%, achieved in only 1 h. The crude of the reaction and the purified monomer was characterized by differential scanning calorimetry, infrared spectroscopy, high pressure liquid chromatography, thermogravimetric analysis, and nuclear magnetic resonance. The influences of reaction parameters, such as the catalyst loading, reaction time, EG:PET weight ratio, and reaction pressure on the yield of BHET was investigated. In addition, the activity of this catalyst was compared to zinc acetate, antimony (III) acetate, and antimony (V) oxide, demonstrating the highest catalytic activity of Sb2O3 for PET glycolysis. Since Sb2O3 is one of the catalysts for the industrial synthesis of PET, the results from this work can open up a venue for the synthesis of repolymerized PET from waste PET in the same glycolysis reactor.
Abstract: Medical personal protective equipment (PPE) made from nonwoven thermoplastic fibers has been intensively used, resulting in a large amount of biohazardous waste. Sterilization is indispensable before recycling medical waste. The aim of this work is to evaluate the effects of the decontamination treatments and help properly recycle the PPE materials. The study investigated the effects of three disinfection treatments (NaClO, H2O2, and autoclave) on chemical composition, molecular weight, thermal properties, crystallinity, crystallization kinetics, and mechanical tension of three types of PPE (Gown #1, Gown #2, and Wrap) made of isotactic polypropylene fibers. The chemical compositions of the materials were not evidently affected by any of the treatments. However, the Mw of the polymers decreased about 2-7% after the treatments, although the changes were not statistically significant. The treatments barely affected the melting and crystallization temperatures and the maximum force at break, but they tended to elevate the thermal degradation temperatures. Although the treatments did not notably influence the crystallinities, crystallization rates and crystal growths were altered based on the Avrami model regression. Since the detected changes would not significantly affect polymer processing, the treated materials were suitable for recycling. Meanwhile, evident differences in the three types of raw materials were recorded. Their initial properties fluctuated notably, and they often behaved differently during the treatments, which could affect recycling operation. Recyclers should test and sort the raw materials to assure product quality. The results in this study provide fundamental data for recycling medical PPE to reduce its environmental footprint.
Abstract: Many grades of homopolymer polypropylene (HPP) and impact copolymer PP (ICPP) with a wide range of mechanical properties have been developed for a variety of applications in different industrial sectors. Management of this wide range of materials is a challenge for material suppliers and manufacturers and product developers. This research was to provide insights for managing material supplies through formulating PP with specific mechanical properties using melt compounding of ICPP and HPP. ICPP and HPP were compounded with an internal mixer at different ratios and then the mixtures were injection molded into specimens for characterization. The mechanical behaviors, fracture surfaces, and thermal properties of the mixtures were then characterized. The fracture surface results indicated that the morphologies of the rubber particles in ICPP changed after compounding with HPP, leading to different mechanical and thermal behaviors of the mixtures. Notched and unnotched impact strengths increased linearly with increasing ICPP contents. The crystallization peak temperatures increased linearly with increasing ICPP contents while the degrees of crystallinity of the mixtures decreased linearly. The thermal compounding process and the original material properties mainly determine the final mixture behaviors, and the mixture properties can be predicted based on the weight ratios of the two components.
Abstract: Thermoplastic elastomers are considered the fastest-growing elastomers in recent years because of their thermomechanical recyclability, in contrast to traditional thermoset rubbers. Polyolefins such as low-density polyethylene (LDPE) show low mechanical properties, particularly poor elongation when compared with an elastomer or rubber. In this study, LDPE resin is converted to highly ductile rubber-like materials with high elongation and low modulus properties on blending with polyisoprene rubber (IR), followed by treating with dicumyl peroxide as a curing agent and organosolv lignin as an additive. The technique of high shear melt-mixing, in conjunction with vulcanization or crosslinking using organic peroxide, is used to develop hybrid materials based on the LDPE/IR blend at a 70/30 mass ratio, where LDPE is replaced partly with lignin. Various characteristics such as tensile, viscoelasticity, melt flow, crystallinity, and phase morphology of the materials are analyzed. As expected, vulcanization with peroxide can improve the mechanical performance of the LDPE/IR blends, which is further improved with the application of lignin (2 to 5 wt. %), particularly tensile strain is profoundly increased. For example, the average values of the tensile strength, the modulus, and the ultimate elongation of neat LDPE resin are 7.8 MPa, 177 MPa, and 62%, respectively, and those of LDPE/IR/lignin/DCP 65/30/05/2 are 8.1 MPa, 95 MPa, and 238%, respectively. It indicates that the application of lignin/DCP has a profound effect on improving the ductility and elastomeric characteristics of the materials; thus, this material can have the potential to replace traditional rubber products.
2021 Journal Publication
Abstract: Polyethylene (PE) and polycarbonate (PC) are not thermodynamically miscible, and they tend to be phase-separated during melt-processing, because of significant incompatibility in structures and properties. Interestingly, minerals such as CaCO3 are used in PE-based supermarket bags for improving surface characteristics. In this study, recycled low-density polyethylene (LDPE) and PC resins are melt-blended at 50/50 wt%, without and with oxidized polyethylene (OPE) as a compatibilizer. A parallel experiment is conducted with 50/50 blends of virgin LDPE and PC resins. Various characteristics such as tensile, viscoelastic, melt-flow, and phase morphology of the materials are analyzed. The 50/50 blend of recycled LDPE/PC blend shows an average tensile strength of 15 MPa, which increases to 24 MPa on the addition of 5 wt% OPE, where total recycled content is 95 wt% including 15 wt% CaCO3. Notably, the presence of CaCO3-based filler in recycled LDPE has a profound effect in improving the mechanical performance of the materials.
Abstract: Petro-derived commodity thermoplastics are relatively inexpensive, lightweight, and non-biodegradable materials, which can be readily molded at high temperatures into a range of products. The manufacturing of such thermoplastic resins and products has increased dramatically over the last 70 years. The plastics based on polyolefins, polystyrenes, and polyesters occupy the largest share (80%) of the world’s plastic markets. However, the disposal of waste plastics has created considerable environmental concerns. Therefore, environment protection agencies and plastic manufacturers are constantly seeking appropriate techniques for recycling or upcycling waste plastics into new products. In recent years, the recycling rates are approximately 9 to 15% out of total plastics produced annually in the United States and Europe, which are predominantly limited to individual plastic fractions such as HDPE and PET. The recycling process is associated with various expensive and time-consuming sorting techniques, which are not economically attractive to the recycling industries. A significant issue is when multiple plastics are blended, they often are not chemically compatible, resulting in phase separation and inferior-quality materials. Therefore, it is important to apply certain performance modifiers during the re-extrusion of waste thermoplastics, which include compatibilizers, coupling agents, impact modifiers, and many others. There are different additives and techniques available, suitable to improve the mechanical and other associated properties of polymer blends and composites, and these can also be used for improving the performance of recycled thermoplastics. This review article summarizes various chemical additives and approaches, which can be used in thermomechanical upcycling of waste thermoplastics to new materials with superior mechanical performance via improving interfacial adhesion or phase homogeneity of polymer blends.
Abstract: Polyimide films are widely applied in harsh environments because of their outstanding performance. High-quality polyimide films are often manufactured through a two-step process. The complicated procedure results in different properties on the two sides, i.e., the air side and cast side of the films, and the quality of products from different manufacturers varies notably. In the present work, polyimide films with two thicknesses (1 and 2 mm) from four manufacturers were investigated. Atomic force microscope and FT-IR spectrometer were employed to monitor morphology, roughness, nanomechanical properties, and corresponding relative imidization degree on the two sides of each film. Statistical tools were applied to analyze the data. T-test suggests that the two sides of the same film were significantly different in roughness, DMT modulus, and relative imidization degree (p < 0.05). The roughness on the air side was consistently smaller than that of the cast side. ANOVA was used to compare differences among the manufacturers. Manufacturer B provided the smoothest films with the highest DMT moduli and imidization degrees. A positive correlation was found between the DMT modulus and imidization degree (r = 0.7330). Nanostructure and nanomechanical properties could affect the quality of the film. Striped morphology and adhesion were found on the cast side of the 2-mm film from manufacturer D, which compromised the film tension in the direction perpendicular to the strips. Investigations of morphology and mechanical properties of polyimide film at the nanoscale would help us better characterize the film, assure its quality, and select suitable film and side for proper applications.
2020 Journal Publication