Center for Materials and Manufacturing Sciences Publications | Troy University



2022 Journal Publication

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.

2022 Presentation




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.

2021 Presentation

Abstract: Fusion deposition modeling (FDM), as a powerful and versatile 3D printing technique, has become more and more popular and affordable in recent years. A FDM printer melts a filament consisting of thermoplastic polymer(s) with or without reinforcements and extrudes it through a fine nozzle to build a desired shape layer by layer. During the process, the filament is heated to a high temperature, which may cause thermal degradation of the material. Since thermal degradation is a complicated process often involving free radicals, it could generate a wide range of degradation products. Unpleasant odors are often released during printing. Even worse, toxic and carcinogenic volatiles might be produced, raising health concerns of users. In this work, thermal degradations of three types of 3D printing filaments, i.e., Nylon A (Nylon 6 with 20 vol% continuous carbon fibers), Nylon B (Nylon 6 with 9.2 vol% chopped carbon fibers), and PPS (polyphenylene sulfide with 8 vol% chopped recycled carbon fibers), were investigated. TGA analyses suggested that there were 2.1% and 3.5% weight losses for the Nylon A and B filaments, respectively, before they were heated to their printing temperature of 270°C in air. The PPS filament was relatively stable. Only less than 0.1% weight loss was observed until it reached its operating temperature of 305°C. A solid-phase microextraction (SPME) coupled with gas chromatography-mass spectrometry (GC-MS) method was employed to collect and identify the volatile degradation products. Pyridine, butyrolactone, 2-nonanone and nonanal were detected for both nylon filaments. Among them, pyridine presented in a large amount, which has a nauseating odor and has been confirmed as an animal carcinogen. Nylon A also released 1-(acetyloxy)-2-propanone, pentanoic acid, benzonitrile and 2-octanone, while Nylon B produced octanal, decanal and caprolactam. The difference between the two nylon filaments might be caused by different additives in them. The degradation products of the PPS filaments include Butyrolactone, phenol, 1-methyl-2-pyrrolidinone, acetophenone, 1-octanol, p-cresol and nonanal. Many of them are toxic with unpleasant smells, and p-cresol is a possible human carcinogen. This study helps us understand the potential health concerns related to 3D printing.

2020 Journal Publication

Abstract: Although recycled plastics provide a low-cost and environmentally friendly alternative for many applications, their desirability is significantly limited by the presence of unpleasant odors from volatile organic compounds (VOCs). In this work, a headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography-mass spectrometry (GC-MS) method was optimized to analyze volatile compounds from an odorous recycled plastic resin which was roughly composed of 85-90% polypropylene (PP) and 15-10% high-density polyethylene (HDPE). A large variety of aliphatic hydrocarbons and 13 additive residues were detected. Statistical tools were employed to screen the VOCs and successfully identified three components, i.e., 2,4-dimethyl-heptane, 4-methyl-octane and octamethylcyclotetrasiloxane (D4), which were significantly related to the odor intensity of the recycled plastic resin (p-values < 0.05). 2,4-Dimethyl-heptane has a strong, pungent plastic smell, which is very similar to the odor of the recycled resin.  It is identified as a major source of the odor. Past relevant research has not been able to establish a direct link between an odorous compound and the undesirable odor of recycled plastic until now. 4-Methyl-octane was highly corelated to 2,4-dimethyl-heptane and somewhat contributed to the odor. D4 does not have an odor, but it may serve as an indicator of some odorous residues from personal care products. 

2020 Presentation

Abstract: Polyimide film is used for many advanced applications such as high-temperature adhesives, mechanical stress buffers, and insulation techniques due to its durability, strength, heat resistance, and dimensional stability. However, a complicated manufacturing process may result in different properties on the two sides (air and cast) of the film, affecting performance. Polyimide films with a 2 mm thickness were obtained from four manufacturers (A, B, C, and D) and investigated by an atomic force microscope (AFM). A 2 by 2-micrometer area was scanned at random locations five times on each side of the films to obtain their representative nanostructures. The AFM images suggest different morphology features on different sides. There often were small round bumps on the air side for all manufacturers, which could be from dust on the surface during the imidization process. Alternatively, scratches, round dents, and aggregates were likely to be on the cast side, possibly caused by the heating surface used in the manufacture. The roughness of both sides of the films (Rq and Ra values) was calculated from the AFM images. The t-tests confirmed the cast side was rougher than the air side for all manufacturers. ANOVA tests show that film from Manufacture D had higher roughness than other manufactures on both the air and cast sides. Manufacture B provided the smoothest film. Its roughness was lower than all manufacturers on the air side and lower than manufacturers C and D on the cast side.
Abstract: Although recycled plastics provide a low-cost and environmentally friendly alternative for a wide range of applications, several factors significantly limit their desirability. Most notably among these is thermal degradation of polymers during recycling process, which may compromise their mechanical performance and cause unpleasant odor. Previous studies mainly focus on polymer decomposition mechanisms and kinetics under high temperatures when quick reactions occur. However, polymer degradation behavior under processing temperatures for plastic recycling is essentially different, which attracts the most interest of the industry. In this work, degradations of low-density polyethylene (LDPE), high-density polyethylene (HDPE) and polypropylene (PP) was investigated under a typical recycling processing temperature (200°C). A headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography-mass spectrometry (GC-MS) method was employed to detect representative volatile degradation products and monitor the degradation processes. The results show that each plastic had a unique volatile compound profile, their thermal stabilities varied notably, and they released different degradation products. LDPE had the lowest thermal stability among the three and gave out large amounts of n-alkanes with 12-16 carbons only after 20 min of heating. Accelerated degradation of PP was detected after 30 min of heating when it started to release 4-methyl-2-heptanone, heptadecane and octadecane. For HDPE, 1-tetradecene and 1-hexadecene were quickly released after 20 min of heating, but accelerated degradation was only observed when the amounts of 1-heptadecene and 1-nonadecene increased significantly after 40 min of heating. The volatile compound analysis method developed in this work is very sensitive to polymer degradation and could be used to distinguish plastic types. The results of this research may help the plastic recycling industry optimize processing conditions and improve product quality. 
Abstract: Polyethylene terephthalate (PET) accounts for 10.2% of the plastic produced worldwide and is nearly exclusively used for single use bottle packaging. Over the past few decades, recycling PET has become a major initiative around the world as result of increased pollution levels and climate change. Improvements in the reprocessing of recycled polyethylene terephthalate (rPET) for product applications will in turn diminish the demand for virgin PET and ultimately have a favorable impact on our environment. However, it has been challenging to reprocess the rPET because of its thermal degradation at elevated temperatures. The focus of this study is to address this issue and understand the effect that reprocessing has on the structure and the mechanical properties of rPET with different reprocessing conditions. The goal of this project is to develop an innovative fast processing method for compression molding rPET. Samples will be prepared from the molded rPET for thermal and mechanical testing, including differential scanning calorimetry (DSC), flexural, and impact testing.
Abstract: This paper will look at compounding recycled discontinuous carbon fiber with recycled LDPE pellets via extrusion compression molding. A novel in house metering unit will be used to compound recycled carbon fiber with recycled LDPE pellets. Differential Scanning Calorimetry (DCS) and Fourier Transform Infrared Spectroscopy (FTIR) will be conducted on the recycled LDPE to ascertain its thermal characteristics. Mechanical characterization will be conducted on the panels produced and optical Microscopy (SEM) images of the fractured surface will be done to analyze the composite failure mode. Successful demonstration of this process can be utilized to further evaluate the sustainability of thermoplastics via recycling.
Abstract: Although recycled plastics provide a low-cost and environmentally friendly alternative for a wide range of applications, there are several factors that significantly limit its desirability. Most notably among these is thermal degradation of polymers during their recycling process, which may compromise their mechanical performance and cause unpleasant odor. In this work, a headspace solid-phase microextraction (HS-SPME) coupled with gas chromatography-mass spectrometry (GC-MS) method was employed to help understand thermal degradation of low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP) under a typical processing temperature (200°C) for plastic recycling. Volatile compounds from the three plastics before and after heating were monitored. The results show that each plastic has a unique volatile compound profile. They had notably differences in thermal stability and released different degradation products. LDPE had t he lowest thermal stability among the three materials studied in this work and gave out a lot of volatile compounds after 20 min of heating. Accelerated degradation of PP was detected after 30-min heating, while HDPE was still relatively stable after heated for 40 min. The volatile compound analysis carried out in this work could be used as an effective way to identify the type of plastics and evaluate their extent of thermal degradation. The results of this research may aid the plastic recycling industry in improving product quality.

2019 Presentation