2022 Journal Publication
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