Yongyan Pang

A new strategy for preparation of long-chain branched polypropylene via reactive extrusion with supercritical CO2 designed for an improved foaming approach

Abstract A novel strategy was designed for the preparation of a long-chain branched polypropylene (PP) with improved foamability via reactive extrusion in the presence of supercritical CO2 (scCO2). Benzoyl peroxide was used as a radical initiator and trimethylolpropane triacrylate (TMPTA) was applied as a polyfunctional reactive monomer during extrusion. Fourier transform infrared spectroscopy and high temperature GPC confirmed that TMPTA was grafted onto PP chains, and the presence of scCO2 promoted the grafting and branching reactions, and hindered polymer degradation. A possible mechanism was proposed to explain the effect of scCO2 on the branching reactions. In addition, rheological behavior of pure PP and modified PP samples was studied to investigate the effect of long chain branching of PP on the melt viscosity and strength, and foaming behavior was studied to confirm the subsequent effect on its foamability. It was found that the long chain branching increased the melt viscosity and strength of modified PP samples, which favored the foamability, and that the foaming windows were expanded in the presence of scCO2. Thus, it provided an advanced foaming approach via preparation of long-chain branched PP through reactive extrusion with scCO2 both working as the reactive medium and the foaming agent.

Application of supercritical CO2 as the processing medium to tune impact fracture behavior of polypropylene/poly(ethylene-co-octene) blends

The objective of the present work is to study the effectiveness of the application of scCO2 as the processing medium to tune the fracture behavior of polypropylene/poly(ethylene-co-octene) (PP/POE) blends. Three types of POE were selected to blend with PP to test the feasibility of this approach. A tandem extrusion system was employed for the thermal blending of PP/POE blends, and an integrated slit die rheometer was used to measure the viscosity of pure PP and POE without or with scCO2 for viscosity ratio calculation. Scanning electron microscopy (SEM) was applied to investigate the phase dispersion and also the fractured morphologies after impact tests. The notched Izod impact tests were carried out at different temperatures to confirm the effectiveness of application of scCO2 on the alteration of the fracture behavior of PP/POE blends. It was found that PP has a higher affinity for scCO2 than POE does, and that consequently, for POE originally with a lower apparent viscosity than PP, the phase dispersion of the PP/POE blends was improved upon injection of scCO2, and thus the fracture behavior alteration from brittle to tough was observed at suitable temperatures. In comparison, for POE originally with a higher apparent viscosity than PP, the phase domains of the PP/POE blends were increased in size upon injection of scCO2, and resultantly, the fracture mode transition from tough to brittle was observed at suitable temperatures. Hence, this study confirmed the effectiveness of application of scCO2 to tune the fracture behavior of PP/POE blends, which is of great significance from both fundamental and industrial perspectives.

Mechanism Study on Char Formation of Zinc Acetylacetonate on ABS Resin

The mechanism of char formation effect of zinc acetylacetonate (Zn(acac)2) on acrylonitrile-butadiene-styrene copolymer (ABS) was studied. Thermal gravimetric analysis (TGA) was used to study the mass loss and char yield of ABS composites. In situ temperature-dependent Fourier transform infrared spectroscopy (FTIR) was used to characterize the chemical change during thermal decomposition. Roman spectroscopy and scanning electron microscopy (SEM) were applied to characterize the structure and morphology of the char after combustion. Results showed that the presence of Zn(acac)2 not only slowed down thermal decomposition of the ABS composites, but also increased the charred residue. A more compact and denser char layer with higher graphitization degree was formed for ABS composites with Zn(acac)2. To study the char formation mechanism of Zn(acac)2 on ABS, thermal decomposition was analyzed for the composites of Zn(acac)2 with PB, PS and SAN, respectively. Also, the chemical structure change was investigated for Zn(acac)2 during thermal decomposition. Based on these results, it was deduced that the increase of char yield of ABS composites was probably attributed to the interaction between the units of acrylonitrile in ABS and zinc acetate, produced during the thermal decomposition of Zn(acac)2. A proposed mechanism for crosslinking and the subsequent char formation was presented.

A new approach designed for improving flame retardancy of intumescent polypropylene via continuous extrusion with supercritical CO2

The objective of this study was to explore a new approach to enhance the flame retardancy of polypropylene/intumescent flame retardant (PP/IFR) composites through improving IFR dispersion via continuous extrusion with supercritical carbon dioxide (scCO2) as the processing medium. The IFR used in this study consisted of ammonium polyphosphate (APP) and pentaerythritol (PER) at a weight ratio of 3 : 1. Scanning electron microscopy (SEM) was applied to study the dispersion of IFR in the PP composites prepared in the presence of different contents of scCO2. Thermogravimetric analysis (TGA) was carried out to study the effect of IFR dispersion on the thermal stability and the yield of char residues of the PP composites. A plastic smoke generation tester was applied to study the specific optical density under a thermal irradiance. SEM was then used to study the morphologies of the intumescent char formed after the smoke testing. Limiting oxygen index (LOI) and vertical burning test (UL-94) ratings were evaluated to study the effect of IFR dispersion on the flame retardancy of the PP composites. It was found that scCO2 as the processing medium could significantly improve IFR dispersion and thus increase the efficiency of the flame retardant and enhance the flame retardancy of the PP composites. This new approach could allow PP composites to pass the UL-94 V-0 rating at a lower IFR content. Possible mechanisms were proposed for the scCO2-aided IFR dispersion and the improved flame retardancy of the PP composites.

Correlating Nanoparticle Dispersion to Surface Mechanical Properties of TiO2/Polymer Composites

The objective of this study is to characterize the nanoparticle dispersion and to investigate its effect on the surface mechanical properties of nanoparticle-polymer systems. Two types of TiO 2 nanoparticles were chosen to mix in two polymeric matrices: solvent-borne acrylic urethane (AU) and water-borne butyl-acrylic styrene latex (latex) coatings. Nanoparticle dispersion was characterized using laser scanning confocal microscopy. Overall, Particle A (P A , without surface treatment) dispersed better than Particle B (P B , organic treatment) in both systems. The AU-P A system exhibited the best dispersion of the four systems, however P B forms big clusters in both of the matrices. Surface mechanical properties, such as surface modulus at micron and sub-micron length scales were determined from depth sensing indentation equipped with a pyramidal tip or a conical tip. The surface mechanical properties were strongly affected by the dispersion of nanoparticle clusters, and a good correlation was found between dispersion of nanoparticle clusters near surface and the modulus-depth mapping using a pyramid tip.