This paper reports an investigation of asynchronous flow marks on the surface of injection molded parts and short shots made from two different blends of polypropylene and ethylene-propylene random copolymer elastomers. Flow marks were observed on the surface with both blends; the spatial frequency of flow marks on the surface was greater in the blend B1, which also exhibited a greater contrast between the surface regions. The same blend was distinctly faster in the linear viscoelastic tests of shear creep recovery and shear viscosity growth. The degree of contrast between the flow-mark regions and the out-offlow-mark regions was examined with a detailed analysis of SEM micrographs of the surface regions as well as the near wall regions from short shots. This revealed that the dispersed phase was highly stretched to cylindrical strands in the glossy surface regions of both blends and retracted in the dull regions to different extents in the two cases. A comparison of the particle size distributions and aspect ratio distributions in different regions established that rapid retraction of the suspended elastomer phase was the dominant cause of changes in particle shape between surface regions. Nonlinear shear creep and creep recovery curves of the two elastomer components showed that at a time of 1 s, the fractional strain recovery of the elastomer in B1 was much higher than that of the elastomer in B2. Hence, the nonlinear elastic recovery of the elastomer phase at short times is an important factor in flow mark formation with blends of polypropylene and olefinic elastomers.
This work describes in detail the kinetic model for the cure of an epoxy-anhydride thermoset matrix resin system. The cure kinetics in both nonisothermal and isothermal modes has been characterized using differential scanning calorimetry. The Sestak-Berggren two-parameter autocatalytic model was used to describe the nonisothermal cure behavior of the resin satisfactorily. The isothermal cure data was fitted with Kamal's four-parameter autocatalytic model, coupled with a diffusion factor. These characterization data will form material property inputs for a multiscale modeling framework for the estimation of cure-induced residual stresses in thick thermoset matrix composites.
The thickness of the melt film and the temperature profiles within the melt film in the weld zone are key process variables governing the development of weldzone microstructures and the resulting development of weld strengths, during vibration welding of thermoplastics. The mathematical model described in this report is aimed at investigating the role of the rheology of the melt-specifically the magnitude and shear-rate as well as temperature dependence of the melt viscosity-in governing the process variables such as the molten film thickness and the viscosities, stresses, and the temperatures within the melt film during vibration welding. The analysis is focused on the steady-state penetration phase (phase III) of vibration welding. The coupled steady-state momentum balance and heat transfer within the melt film, formulated using the Cross-WLF (Williams-Landel-Ferry) relationship for viscosity, are solved in an iterative finite element framework. The model has been implemented for two different polymers displaying significant differences in viscosities and shear thinning behaviors. An attempt has been made to correlate the trends in the estimated melt film variables with the experimentally measured weld quality. FIG. 2. Geometry employed for analysis of steady state flow and heat transfer in the vibration welding melt film (grey colored portion), and the boundary conditions for (a) momentum balance and (b) energy balance.
In this review, we survey the state of the art on polymeric foams incorporating nano-scale fillers. Particular focus of the review is on foams from polyolefinic nanocomposite formulations incorporating a wide variety of fillers. The nano-scale additives can influence the foam structure and properties in two ways: Firstly, they can act as composite reinforcement to enhance the mechanical properties and functionality of the matrix polymer; and secondly, they can act as foaming-processing aids through modification of the rheological, thermal and crystallization properties of the matrix as well as serving as heterogeneous nucleation sites. Through a combination of these influences, and using advanced processing techniques it is possible to achieve nanocomposite foams that have higher cell density, and more uniform cell size or controlled cell-size distribution. Such controlled foam morphologies, in turn, can yield better specific mechanical properties resulting in more effective light-weighting solutions. Further, the nano-scale additives can impart additional desired functionality resulting in multi-functional foams. In this article, we provide an overview of the mechanical, thermal and a few other relevant functional properties – such as piezoelectric sensitivity, acoustics, and filtration efficiency – of foams prepared using nanocomposite formulations, along with the processing considerations for achieving high quality foams using such materials.
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