Guangjian He

Introduction of a long-chain branching structure by ultraviolet-induced reactive extrusion to improve cell morphology and processing properties of polylactide foam

In this paper, long-chain branched polylactide (LCB-PLA) prepared by UV-induced reaction extrusion with trimethylolpropane triacrylate (TMPTA) was foamed by supercritical carbon dioxide (scCO2), and the effect of the long-chain branching structure on the cell morphologies of PLA foams was investigated. The LCB-PLA displayed higher complex viscosity, melting point and crystal nucleation potential under scCO2, and these factors could influence the foaming behavior of PLA which was proved by the different cell morphologies of samples foamed after various saturation times. The advantage of LCB-PLA on foaming was remarkable at high temperature and high pressure. LCB-PLA with more than 0.5% TMPTA showed nano-cells while the other samples showed micro-cells at 142 °C under 12 MPa, and the samples displayed elliptic cells with horizontal semimajor axis in linear PLA and circular cells or oval cells with vertical semimajor axis in LCB-PLA with increasing temperature. The improved cell morphology with reduced coalescence, no collapse and uniform cell distribution was also shown in LCB-PLA under higher pressure. All these results were due to the increasing matrix strength and higher crystal nucleation potential of LCB-PLA. The findings indicate that LCB-PLA possesses better foaming behavior at high temperature and high pressure. The wide foaming processing window of LCB-PLA would benefit the high temperature and high pressure foaming of PLA such as bead foaming and continuous extrusion foaming, thus broadening its application.

Impact of rapid ozone degradation on the structure and properties of polypropylene using a reactive extrusion process

In this work, rapid ozone degradation of polypropylene (PP) was developed for the aim of rheology control using a reactive extrusion process. Experiments were carried out in a co-rotating intermeshed twin-screw extruder with varied polymer throughput and reaction temperature. Ozone was introduced into the extruder to rapidly oxidize molten PP in just several seconds period. The oxidized PP was characterized through melt flow index (MFI), rheological measurement, differential scanning calorimetry (DSC), and Fourier transform infrared (FTIR) spectroscopy tests. The influence of reactive temperature and polymer throughput on the degradation reaction was studied. It was noted that molten PP could be fast and successfully degraded during this reactive extrusion process. The oxidized PP had higher MFI than that of the origin PP resin, indicating the decrease of molecular weight of PP. Carbonyl groups were formed on the PP molecular chains. This rapid oxidization process has higher reaction efficiency than the ozone degradation of PP in solid state and no harmful byproduct would be generated from this ozonizing reaction.

Comparison Study on CO2-Promoted Crystallization and Melting Behavior of Polypropylene: Homopolymer and Copolymers

Three types of polypropylene, namely propylene homopolymer (HPP), block copolymer of propylene with ethylene (CPP-B) and random copolymer of propylene with ethylene (CPP-R), were melted and isothermally crystallized in a self-designed vessel under supercritical carbon dioxide (Sc-CO2) atmosphere. The melting behavior and crystalline forms of crystallized samples were investigated using differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD). The results showed that the presence of Sc-CO2 could improve the crystallinity for all three polypropylenes, and the promoting effect was more obivious with increasing CO2 pressure. In addition, it was observed that γ-crystals could be obtained in the CPP-B and CPP-R samples crystallized under Sc-CO2, while no γ-crystals were formed in HPP under the given conditions. The relative content of γ-crystals obtained in CPP-R samples was much higher than that of CPP-B, and 100% γ-phase could be formed in the CPP-R sample when subjected to 14 MPa Sc-CO2.

In situ ozonolysis of polypropylene during extrusion to produce long-chain branches with the aid of TMPTA

The introduction of long-chain branches (LCBs) in polypropylene (PP) during the extrusion process is normally induced by peroxide chemicals which are known to lead to the formation of secondary products in the resin. Here we report a novel synthesis method to prepare LCB-PP via in situ ozonolysis during reactive extrusion in the presence of a multifunctional agent. Depending on the fast ozonation of molten PP molecules, free radicals can be generated in PP macromolecules and induce chain scission reactions during extrusion. LCB structures of PP could be formed when a multifunctional agent, trimethylolpropane triacrylate (TMPTA), was added into the PP matrix during extrusion. The effect of reaction temperature, polymer flow rate and TMPTA concentration on the LCB-PP structures is discussed. Molecular parameters of LCB-PP were detected by MALS SEC technique. Various rheological plots including elongation rheological properties were applied to distinguish the LCB structures in PP samples. Without residues and by-products of peroxide in the final polymer resin, this synthesis method of LCB-PP has highly efficient and easily adjustable merits.

Research on properties of carbon black/polypropylene composites by dynamic injection molding

Polymer composites filled with conductive carbon black (CB) are gaining popularity for electromagnetic shielding applications. Dynamic injection molding method was adopted to study the influences of vibration force field on electrical properties of polypropylene/CB composites. The results showed that the percolation phenomenon of conductivity of composites occurred at 15wt% and the calculated SE was positive correlated with the variation trend of conductivity. The calculated SE of composite was more than 30dB at a CB concentration of 30wt%, which could obtain good shielding effects. The result could offer optimum vibration parameters for producing electromagnetic shielding composites by respectively changing the amplitudes and frequencies of the vibration force field.