Volume 20, Issue 1

Based on torque measurement via a device specially developed by our group, the shearing torque of the mixture of UHMWPAN with swelling agent or solvent during shearing process at various temperatures could be recorded as torque-temperature diagram in the swelling and dissolution process. The temperature where the torque of the mixture began to rise as showed in the torque-temperature diagram was considered as key parameter. UHMWPAN could be swollen fully and the homogeneous solution could be easily prepared if the mixture was heated progressively and was kept for a certain period of time at the every constant temperature. In the case of UHMWPAN (content ≥ 5 %), the parameters of the swelling and dissolution process were finally obtained.

Rheology of PVC plastisol is usually conducted with steady state flow. It is inadequate, because plastisol is viscoelastic, so that deformational experiment is also required. In the viscoelastic measurements, when either frequency or amplitude is increased, dynamic viscosity decreases first (pseudo-plastic), reaching a minimum and then, increases (dilatant). It is because the shear rate during the oscillation is a product of frequency and amplitude. The pseudo-plastic behavior is explained with stress-induced phase separation into an immobilized layer of packed particles and mobile phase of reduced particle concentration. The viscosity minimum corresponds to the yield point of the immobilized layer. The yielding obeys Coulomb's criterion. After yielding the immobilized layer dilates with the further increase of shear rate. Two different modes of fracture are observed; one without yielding occurs at rather low shear stress, slightly higher than the cohesive strength of the immobilized layer. After yielding and dilation, the fracture occurs when shear stress becomes higher than normal stress. Before yielding the deformation of the immobilized layer is very small and accompanied with small fraction of particles moving over the neighboring particles. After yielding, deformation involves entire particles of the immobilized layer.

In injection molding, the injection pressure is often limited by the capacity of injection machines. Therefore, for large-volume part, multiple gates are needed so that the maximum flow path can be shortened, with a corresponding decrease in injection pressure. Many components produced by injection molding today require more than one gate. As multiple gates affect resin flow in a mold interactively, their locations might not be easily determined. For a mold with two gates, a pressure gradient search scheme (PGSS) is proposed to optimize their locations in this investigation. A search direction associated with pressure gradient is developed based on the physical nature of the mold-filling process. An alternative scheme based on genetic algorithm is also employed for comparison. Through minimizing the objective function of injection pressure, the optimum gates can be located to achieve balanced flow. Examples indicate that the proposed pressure gradient search scheme is more efficient than genetic algorithm for two gate location optimization.

In this paper, molten high density polyethylene (HDPE) was compounded with four kinds of high performance organic fibers: two types of aramid (KF29 and KF49), liquid crystalline polymer (LCP) and poly(vinyl alcohol) (VF), differing in their chemical structure and fiber lengths. From the SEM pictures, it is observed that shape and size of these organic fibers maintained almost the same even after cutting in pellets, following the mixing process. VF/HDPE and LCP/HDPE systems show generally lower rates of increase of both storage modulus and dynamic viscosity with fiber content than KF/HDPE composites. Comparison of these functions at the fixed fiber content has shown that the most effective parameter, affecting the viscoelastic behaviour of organic fiber filled systems, seems to be their rigidity/flexibility in the molten state. The influence of fiber rigidity/flexibility becomes gradually lower with the increase of both strain amplitude and angular frequency. The parameters of the equations describing relationship between relative values of viscoelastic functions and fiber content were found to be largely dependent on fiber content. Such finding remarkably differs from behaviour of short inorganic fiber filled systems, where these variables maintained constant values.

The control of microstructure development in injection molding had attracted several important studies in the recent years, being well established that the thermo-mechanical environment imposed to the polymer melt depends on the material properties, molding geometry and processing conditions. An entirely new design of an injection mold RCEM (Rotation, Compression and Expansion Mold) was developed and implemented, allowing for a wide variation of the filling and packing conditions in a flat disk geometry. One of the cavity walls (the ejection side) can rotate or move with a linear movement during the injection and holding stages. This enables several filling sequences and the variation of the cavity thickness. Two electric servomotors are use to directly drive those movements. Two temperatures and pressure sensors allow for the on-line process monitoring. Several examples of the potential uses of the RCEM are shown for the cases of amorphous and semicrystalline polymers. Results obtained with short-fiber reinforced polypropylene are also presented.

The melting and simultaneous biaxial stretching behavior of three Ziegler-Natta isotactic polypropylene resins of varying isotacticity was studied. DSC melting experiments showed that the crystallinity and melt stability of the crystalline phase depend strongly on tacticity and on the degree of undermelting, ΔT = T m – T, where T m is the melting temperature. The level of biaxial yield stress depends on the fraction of residual “unmolten” crystallinity and the crystallite size. For specimens with same thermal history, the post-yield deformation shows strong dependence on the degree of undermelting, ΔT. The deformation homogeneity and the resulting film thickness may be related to the thermal mobility of the chains and the tie chain and entanglement densities.

The viscosity during the earlier stages of the gelation process of plastisols formulated with 15 different commercial plasticizers has been obtained. Plastisols prepared were rotomoulded and mouldings obtained were characterized according to their wall thickness distribution. This parameter strongly depends on the minimum viscosity and on the rate of viscosity change during gelation (slope of the curves during the viscosity increase), and also on the temperature where no more flow of the plastisol is observed (gelation temperature). These variables are conditioned by the characteristics of the plasticizer and PVC employed, among other variables. The effect of plate and arm velocity and rotation rate on the wall thickness distribution was also studied. A simple model has been proposed to correlate the initial stage of the gelation of PVC plastisols formulated with the phthalate plasticizers. The suggested model assumes that the plastisol is a suspension constituted by a continuous phase (initially pure plasticizer) and a discontinuous phase constituted by PVC particles. It has been considered that the viscosity of the plastisol is influenced by the average mass molecular weight of the continuous phase and the temperature. The quantity of PVC dissolved in the continuous phase has been calculated by using a generic n-order kinetic equation for PVC dissolution rate. Additionally, a swelling process of the PVC particle by the plasticizer has also been considered and its effect on the viscosity of the suspension has also been accounted for. The model proposed provides a very good fitting of the sharp viscosity increase observed and is useful to predict the evolution of the viscosity of the parts in a rotomoulding process.

The drawability of as-spun polypropylene (PP) filaments was studied using a statistical approach including an experimental design and statistical analysis. The use of a statistical experimental design allows all possible interactions as well as main factors to be evaluated. As a result, one two-factor interaction (between winding speed and metering pump speed, WS * MPS) and two main factors (winding speed WS and metering pump speed MPS) were identified as factors having statistical significance on drawability of as-spun PP filaments with risk levels (P values) less than 0.005. The drawability can be improved by controlling the structure of the as-spun filaments, that is a paracrystalline structure combined with a very low overall orientation (birefringence <3.2 × 10 –3 ). Application of two-stage drawing also increases the drawability compared with one-stage drawing.

In this paper we studied the melt fracture behaviour in capillary flow of a number of polyethylenes produced by various technologies. The critical shear rates for the onset of both sharkskin and gross melt fractures were found to correlate with the high-rate extensional flow behaviour of the polymers. These findings were found to mechanistically support the generally accepted observations of melt fracture occurring at the exit (sharkskin) and entrance (gross) regions of the capillary die. In addition, it was found that boron nitride (BN) behaves as an energy dissipater that suppresses the rapid increase of extensional stress associated with gross melt fracture. This enables BN to act as an effective processing aid in postponing gross melt fracture in the extrusion of polyethylenes.

Measurements were performed to characterize the development of the cooling jets produced by a typical dual-lip air ring used in the blown film manufacturing process. The distribution of the pressure on the bubble surface and the velocity field in the region above the forming cone were measured for a rigid model bubble with a blow-up ratio of 2.5. The measurements indicated that the development of the flow below the top of the forming cone in the air ring could be divided into two regions separated by local maxima in the static and fluctuating pressure. These maxima seemed to be associated with the location where the lower jet attached to the bubble surface. There was a region of negative gauge pressure on the bubble surface below the forming cone that should act to stabilize the bubble against the forming cone in practical applications. Above the forming cone, the lower jet was initially decelerated and then entrained into the upper jet. The upper jet then attached to the bubble surface and finally the flow recovered to a wall jet along the bubble surface. The profiles of the static and fluctuating pressure in the first and third regions above the forming cone collapsed reasonably well when they were scaled using the velocity of the lower jet and the velocity of the upper jet, respectively. The magnitude of the local maxima in the static and fluctuating pressure where the upper jet attached to the bubble depended on the upper jet velocity and the ratio of the upper and lower jet velocities.

Nonlinear growth behavior was observed in two crystallization regimes, depending on the temperature. Non-linearity may be explained by the reduction of the concentration of crystallizable ethylene sequences (CES) in the melt phase. In the two regimes, the concentration of uncrystallizable ethylene sequences (UCES) increases, as the crystallization time increases, because UCES are continuously excluded from the crystal lattice into the melt phase. An empirical equation is proposed to describe the melting temperature of the crystal stem with the maximum possible length, T C, n * m , in nonlinear growth processes, assuming that the diffusion layer is negligible. A modified form of the Hoffman-Lauritzen equation (MHL) describes well the crystallization growth kinetics of LLDPE spherulites in the non-linear growth region.

This paper presents an application of the Continuous Sensitivity Equation Method (CSEM) for the optimization of the injection molding process and its three-dimensional (3D) simulation by the finite element method. Finding the proper combination of process parameters such as injection speed, and melt and mold temperatures is critical to achieving a part that minimizes warpage and has the desired mechanical properties. Very often a successful design in injection molding comes at the end of a long trial and error process. Design Sensitivity Analysis (DSA) can help manufacturers improve their designs and can produce substantial savings in terms of both time and money. This work explores the ability of sensitivity analysis to predict the effects of design parameters on the performance of an injection molding process. The paper presents results of a 3D finite element solution of the filling stage of the injection molding process. Sensitivities of the solution with respect to different process parameters are computed using the continuous sensitivity equation method. Solutions are shown for the non-isothermal filling of a rectangular plate with a polymer melt behaving as a non-Newtonian fluid. The paper presents the equations for the sensitivity of the velocity, pressure and temperature and their solution by a finite element method. Sensitivities of the solution with respect to the injection speed, the melt and mold temperatures and to the heat transfer coefficient at the cavity/mold interface are shown.

Three kinds of phosphate-containing maleimide(PMI) monomers, namely diethyl (4-(N-maleimido) phenyl) phosphate (4P) diphenyl (4-(N-maleimido) phenyl) phosphate (PP) and diethyl (2,6-dibromo-4-(N-maleimido)phenyl) phosphate (BrP), were synthesized and blended with a phenol-novolac type of epoxy so as to study their potential as reactive flame retardants. Thermal reaction of these blending systems had been discussed. Mechanical and thermal properties of blends had also been analysed. It was expected that with an increase in the PMI content in the blending system, the Michael-addition reaction would take place more and easily. Increasing the PMI contents reduced the storage modulus, the Tg and the tensile strength, however good thermal stability was maintained. The flame retardancy of all the phosphate-containing epoxy systems was promoted significantly by increasing the PMI content. Besides, the PMI with halogen atom may combine the flame retarding mechanism of gas-phase and solid-phase to promote their flame retardancy.

In the market, a film which is adapted to environmental and barrier problems and easy tear property is desired. A previous report discussed easy tear PA6 film. This report discusses the film having not only easy tear, but also high barrier properties. A biaxially oriented PA6/MXD6 blend film produced by double bubble tubular film process does possess gas barrier. So a multilayer film having high gas barrier layer and keeping easy tear cut has been studied. High gas barrier layer of high content MXD 6 in the center layer is added to PA6/MXD6 blend film in the outer layers. When MXD6 blending ratio of PA6/MXD6 blending film layer is 30 %, the property of easy straight line cut is obtained. It is found that the plate-like cylinder structure was formed in the observation of TEM for PA6/MXD6 blend film. The multilayer biaxially oriented film composed of PA6/MXD6 blend layer and MXD6 high content layer keeps high strength, easy tear property and high gas barrier. Namely, the material and production technology of a PA6 film could solve environmental and barrier problems.