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2021 Journal Publication

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 timeconsuming 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