Papers by Author: Ya Dong He

Abstract: Carbon dioxide (CO₂) has been reported as an interesting substitute of banned ozone-depleting blowing agents, such as HCFC and HFC etc., for low-density polystyrene (PS) foam production, but it is difficult to industrialize due to its low solubility in PS matrix; therefore, high pressure is always needed in order to obtain the required gas concentrations for low density foam. Mixtures of blowing agents might be a practical way to make foam processing easy to control. In this paper, the foaming behaviors of PS-CO₂ by using water or ethanol as co-blowing agent were investigated. The performances of foams obtained by PS-CO₂, PS-CO₂-water and PS-CO₂-ethanol systems were tested respectively. It was found that cell size increased owing to the existence of co-blowing agent; in particular, the expansion ratio of PS foam obtained by CO₂-ethanol was 1.3 times greater than that by CO₂. At the same time, cell density as well as apparent density decreased with temperature increasing, while cell size showed the opposite. Cell size and apparent density, rather than cell density, decreased with pressure. These results were explained by the solution behavior of each of blowing agent.

Abstract: Effects of injection molding parameters on the residual fiber length and the mechanical properties of LFRT-PP composites, which were prepared by self-designed impregnating molding, were investigated by orthogonal experimental design method. The results showed: the length of residual glass fiber increased with the length of LFRT pellets and decreased with screw speed. The former had more remarkable impact. The mechanical properties of composites were greatly affected by the temperature of barrel. The lower temperature was benefit for obtaining the higher properties. The length of residual glass fiber was not directly corresponding with the mechanical properties. To obtain the highest mechanical properties of composites, the optimal parameter levels were: the length of pellets 10mm, the temperature of barrel 220°C, the screw speed 75 rpm and the injection pressure 80 MPa.

Abstract: In view of the complexity of twin screw and melt pump tandem extrusion systems, the identification of the dynamic characteristics of these systems was performed by using MATLAB and state subspace method. Three input one output model was obtained and the controllability and observability of the system was identified by MATLAB. The fitting degree of simulated and actual melt pressure curve increased with increasing the output variables, which indicated that the tandem extrusion system was an interrelated coupled nonlinear system.

Abstract: The fiber brakeage in compounding glass fiber-reinforced nylon-66 with different screw assemblies of the co-rotating twin screw extruder was studied, and comparative analysis of five different screw assemblies was made. The result shows that proper screw assembly could improve the residual fiber length and the mechanical property of the composite effectively

Abstract: Polybenzoxazine was one of the most widely employed matrix for advanced composites, due to their low viscosity, good dimensional stability, high glass transition temperature (Tg) and wide molecular design flexibility. To obtain high perfomance resin matrix, a fundamental understanding of the formation of crosslinking network structure and the relationship between structure and properties was essential. Therefore, the blends of benzoxazine precursor with different functionality were designed to achieve various network molecular architectures, and the effects of inhomogeneous structure of polybenzoxazine on mechanical and thermal properties were investigated. The bifunctional benzoxazine precursor (BA-a) based on bisphenol-A, formaldehyde and aniline, and the monofunctional benzoxazine monomer (Ph-a) based on phenol, formaldehyde and aniline were synthesized respectively. The blends of BA-a and Ph-a, in which the mole ratio was 1:0, 2:1, 1:1 and 1:2, repectively, were thermally cured through ring-opening reaction to obtain polybenzoxazines with various network structures. The fracture surface morphology of various polybenzoxazines was observed by atomic force microscopy (AFM). The hard phase with highly crosslinking density was dispersed in the soft phase with slightly crosslinking density, which led to the generation of inhomogeneous structure of polybenzoxazine. Dynamic mechanical thermal analysis (DMTA) of carbon fiber reinforced polybenzoxazine showed two glass transition temperatures (Tg), which corresponded to the soft phase and hard phase, respectively.With increasing the mole ratio of Ph-a, the increase of hard phase resulted in the enahncement of flexural modulus of polybenzoxazine, whereas the tensile and flexural strength of polybenzoxazine decreased due to the reduction of the crosslinking density of soft phase. Derivative thermogravimetric (DTG) analysis exhibited three major degradation steps, which characterized the decomposition, weight-loss and charring, respectively. Thermogravimetric analysis (TGA) showed that the onset degradation temperature and char yield at 850 oC increased with the increase of Ph-a mole ratio, indicating higher thermal stability and lower decomposition rate, which was attributed to the increase of hard phase with highly crosslinking density.

Abstract: Though theoretical analysis and experimental verification, a direct method that measuring melt pressure fluctuation at the nozzle to reflect melt viscosity fluctuation in injection molding process was developed. Experiments for four injection molding screws and the analysis for the relationships between the melt viscosity homogeneity and screws configuration, melt temperature homogeneity, repeating weight precision and mechanical properties of the injection molding parts were conducted. At last, the four injection screws were evaluated based on melt viscosity homogeneity method.

Abstract: In this paper, IMP software was used to simulate different injection molding screws. And by the comparison between the experiment results and the simulation results, the software was verified accurate and reliable, then we can use the software to simulate the plasticizing process of some screws which are different in metering depth or metering pitch. By analyzing the pressure curve along the screws, the relationship between the metering depth and the pressure-developing capability, and the relationship between the metering pitch and the pressure-developing capability were obtained. The results showed that the smaller depth or pitch, the stronger pressure-developing capability.

Abstract: Blending PPO and POE-g-MAH assisted by SCCO2 has been investigated. The in-line rheological behavior of PPO and POE-g-MAH containing SCCO2 were studied with the slit die and self made in line rheological testing system. The effect of SCCO2 on the reduction of shear viscosity of PPO is more obvious than that of POE-g-MAH. The shear viscosity of PPO reduced 36.7% with addition 4wt% CO2. Morphology of blends was inspected by SEM. The sizes of POE-g-MAH disperse phase decreased with the contents of CO2 increasing. As a result, the impact strength of blend is improved from 43.5KJ/m2 to 100.9KJ/m2 due to introduce 4wt% SCCO2 for PPO/POE-g-MAH (85/15) blends.