Volume 21, Issue 2

Temperature and pressure dependencies of shear and elongational viscosities were examined using rotational and capillary rheometers. Two different batches of the same polymer grade have shown that the molecular structure diversion significantly influences the magnitudes of pressure coefficients, which vary more than the temperature coefficients. The pressure effect on viscosity notably depends on the amount of long-chain branching in polymer. Further, the paper shows that pVT data analysis via the Simha-Somcynsky equation of state can be employed to reveal differences in temperature and pressure viscosity dependencies through the free volume fraction.

The processing conditions of injection molding have a complex influence on the morphology and on the mechanical properties of a semicrystalline polymer. Therefore, to establish correlations between processing conditions, morphology and mechanical properties constitutes a difficult task; finding these correlations is one of the goals of a materials engineer. The purpose of this work was to study the influence of the injection molding conditions on the morphology and mechanical properties of poly(p-phenylene sulphide), PPS, using artificial neural networks, ANNs. First, a statistical analysis was done to find the more influential processing parameters that affect the morphology and mechanical properties of the PPS. Second, ANNs were applied to establish correlations between processing conditions, morphology and properties. It was found that the variables with the highest influence on the morphology and the mechanical properties were the injection and mold temperatures (T inj and T mold , respectively), as they showed a straight relationship with the crystallinity index of the injection molded part. Three different ANNs were built to predict the correlations. The ANN-1 predicted the crystallinity gradient along the thickness of the injection molded part from T mold , T inj , and flow rate, Q; the ANN-2 predicted the elastic and flexural modulus, E, and the yield stress from the crystallinity gradient, while the ANN-3 predicted the mechanical properties directly from the processing conditions. All ANNs were built with only fifteen experimental data and were trained with the group cross-validation method, GCV and with a training-test set method. Both methods showed similar and excellent performance. Thus, it can be concluded that ANNs can be used as a powerful tool in the learning of these complex correlations.

When describing the flow in single-screw machines in mathematical terms, the screw is generally assumed to be standing still inside a rotating barrel. This study looks into whether and, if so, how far this principle of kinetic reversal falsifies the calculation result. It is performed both on an analytical basis and with the aid of comparative FEA simulations, taking the general case of a non-isothermal flow. It is shown that both kinematic boundary conditions lead to absolutely identical calculation results.

Blends of high density polyethylene and polystyrene were produced via ribbon extrusion to study the effect of post-extrusion conditions on the ribbon final dimensions and morphology. An experimental setup was designed to measure the stretching force as a function of draw ratio and water contact distance for blends of 3, 6 and 9% PS in HDPE. Numerical simulations of the non-isothermal process using Matlab were in good agreement with the experimental data only when both viscous and elastic effects are included. The results show that both draw ratio and water contact distance have a definite influence on the final dimensions of the ribbon and the stretching force. It was found that deformation of the dispersed phase increases with draw ratio, but decreases with water contact distance.

This study examines the effects of average surface roughness and composition of the die wall surface on the critical shear stress for the onset of instabilities in die extrusion. Increasing the average surface roughness of the die wall from 0.1 to 15m produced an increase of the critical shear stress for the onset of flow instabilities by as much as 20% during the extrusion of HDPE. The surface of the extrudates obtained with dies having an average surface roughness greater than 5lm showed lines and grooves along the direction of extrusion. Chrome-plated and nickel-plated dies showed an average 28% increase in the critical shear stress as compared to conventional steel dies. The chemical composition of uncoated steel dies wall surfaces, including the presence of iron oxide, did not affect the critical shear stress for the onset of sharkskin significantly regardless of the type of resin used.

A large series of experiments was performed using an automated hot-gas welding apparatus to investigate the effect of welding parameters (welding temperature, welding gas flow rate, welding speed, welding force) on the structure of the heat affected zone (HAZ) in polypropylene sheets. Based on optical micrographic studies the HAZ can be divided into three well distinguishable parts: the cool, plastic and flow zone of the raw material. A strong correlation was found among the width of the HAZ and the macroscopic size of the welded seam and its strength. It was demonstrated that (under the given boundary conditions) the strength of the welded joint can be estimated from the external geometrical size of the welded seam well.

Demanding mixing tasks are usually solved through the use of tightly intermeshing, co-rotating twin screw extruders. Since both the barrel and the screw are of a modular design, the extruder can be optimised for a given task. In the framework of this investigation, the screw-element geometry for tightly intermeshing co-rotating twin screw extruders is optimised in order to achieve specific aims. The threaded elements have been optimised for different materials and operating conditions. The investigation set out to achieve a screw-element geometry that would generate the maximum pressure gradient while, at the same time, ensuring that the temperature increase and the power consumption were kept to a minimum. The pressure profile, the temperature progression and the power consumption have been calculated on the basis of one-dimensional models of the type used in the SIGMA process simulation software. The geometrical data of the individual elements was varied with the aid of non-linear algorithms, and the quality of an individual geometry was assessed on the basis of quality functions. Subsequent to the theoretical optimisation, experimental analyses were performed for purposes of verifying the optimisation method used.

The rheological and processing behavior in capillary extrusion of several poly(methyl methacrylate) (PMMA) resins was studied. The rheological characterization included: (i) frequency sweep experiments at various temperatures and application of the time-temperature superposition to obtain the master curves from which the activation energy of flow was found to be independent of molecular weight, (ii) extensional measurements using the Sentmanat Extensional Rheometer (SER) where it was found that poly(methyl methacrylate) resins exhibit strain hardening effects only at high strain rates. The capillary extrusion experiments were performed for three poly(methyl methacrylate) resins using additives in order to identify suitable processing aids for PMMA resins. First it was found that poly(methyl methacrylate) polymers exhibit spiral/helical type of distortions at a critical shear stress value of about 0.35 ± 0.03 MPa, independent of temperature and molecular weight. “Traditional” processing aids used mainly in the extrusion of polyolefins and some other commercial polymers were found ineffective in eliminating instabilities in the case of poly(methyl methacrylate) processing. On the other hand, mixing of poly(methyl methacrylate) with a proprietary blend of synthetic resins and fatty glycerides with modified organic fatty acids, MoldWiz INT-35UDH, was able to reduce the extrusion pressure and postpone the onset of gross melt fracture to higher shear rates. Finally and most importantly, the addition of different polyethylenes (LLDPE, LDPE and HDPE) resulted into a significant pressure reduction along with significant postponement of gross melt fracture to higher shear rates.

The influence of solvent evaporation rate, in the dry-casting process, on crystallinity and morphology of crystallizable polymeric membrane was studied. Poly-l-lactic acid (PLLA) was taken as an example. The membranes were characterized by scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). By controlling the solvent evaporation rate, samples with various crystallinities were prepared. From higher to lower crystallinity the membranes exhibit three types of surface morphology in sequence: granule, stripe and smooth.

A partially miscible polymer PMMA/SAN blend is used in two component injection molding to produce a sandwich structure with a heterogeneous skin layer. Two different strategies are explored to obtain the desired structure either via sequential injection molding or via the use of the fountain flow. Both cases are studied with scanning electron microscopy to visualize the structure in the skin layer. In addition, numerical modelling examples are presented to improve understanding of the morphologies that have been created with injection molding.

Polymer processing has been limited by the lack of direct flow and pressure control of the polymer melt at multiple points in space and time. Improved axial and radial valve designs are discussed that require negligible actuation force to control the flow of the pressurized melt. The forces resulting from pressure loads and shear stresses are first analyzed for an axial valve pin. Subsequently, a radial valve design is implemented and experimentally characterized using neat polycarbonate. A sigmoidal response surface is fit to the experimental data and found to very well model the observed pressure drop as a function of flow rate and valve pin position. The juncture loss at the valve port is then characterized by estimating and removing the pressure drops in the circular flow segments of the valve. Analysis indicated that the juncture loss is inversely proportional to the exposed area, or vesica piscis, formed between the circular flow channel in the valve body and the flow port on the moving valve pin. While applicable to many different polymer processing operations, the validated models are used to show the dynamic valve pin position as a function of the desired flow rate and desired cavity pressure in the hot runner of an injection mold. Finally, the impact of the designs on lower power, more compact mechanical and control system designs are discussed.

The influence of drawing and temperature on the optical and structural properties of monofilament polypropylene sutures (PP) were investigated using interferometry. Two techniques have been used in this study: 1) the polarizing interference microscope, 2) multiple-beam Fizeau interferometry. The polarizing interference microscope is used to study the variation of refractive indices and birefringence of PP sutures at different draw ratios and stresses. The stress-strain diagram of PP sutures was studied. The resulting data was used to calculate the average work per chain, Poisson's ratio, strain optical coefficient, Young's modulus and optical orientation function. Multiple-beam Fizeau interferometry was used to study the temperature effect on optical properties of PP sutures with temperature ranging from 19 to 40°C. Relationships between the optical parameters at different temperatures are given. Calculation of the density, crystallinity, dielectric constant and dielectric susceptibility at different temperatures are done. The results obtained, clarify the effect of drawing and temperature on the structure of PP sutures. Microinterferograms are given for illustration.

When compounding engineering thermoplastics with glass fibers for reinforcement purposes, a tremendous amount of polymer-containing waste is generated. However, the polymer of interest is usually commingled with the polymer used to purge the extruder. Reclaiming valuable polymer from this waste is hindered by the incompatibility with the purging material since this leads to a blend system that has poor mechanical properties and low economic value. For this reason, this potentially valuable post-industrial polymer-waste is normally land filled. The current research is directed at the characterization and reuse of a post-industrial PA66 filled with glass fibers; this was separated from LDPE purge material and its impact resistance was modified through rubber toughening. Styrene-Ethylene-Butylene-Styrene and Ethylene-Propylene elastomers grafted with maleic anhydride (SEBS-g-MA and EP-g-MA respectively) were used as the toughening agents in this study due to their compatibility with PA. Izod impact strength, tensile strength and modulus, flexural strength, and elongation at break were used to characterize mechanical properties of the rubber-toughened recycled glass-filled PA66. The morphology of the fracture surfaces of the composites was examined by Scanning Electron Microscopy (SEM). It was found that both impact strength and elongation at break increase with increasing rubber content while flexural strength and tensile strength suffer upon incorporating rubber; however, by adding moderate amounts of rubber to the recycled glass-fiber-reinforced PA66 a balance between stiffness and toughness of the material could be obtained. For example, the addition of 10 wt.% of SEBS-g-MA to the recycled PA66 with 23.62 wt.% glass fiber loading resulted in a 27% and 151% increase in tensile strength and impact strength respectively of the blends when compared with neat PA66. This suggests that compounding both glass fibers and elastomers with PA66 is a useful strategy to recycle PA66, especially for reuse in under-the-hood automotive applications.

Metal injection molding (MIM) is similar to plastic molding in many respects, but MIM compounds (metal powders with polymer binders) are more susceptible to thermally induced flow instability because of their higher thermal diffusivity. The flow patterns for a 17-4PH MIM compound were observed and simulated for mold filling through a diaphragm gate over a range of filling times and melt-mold interface temperatures. Simulation predicted the observed free annular jet and internal voids in the molded part and also predicted that initial contact with the outside wall of the gate would eliminate the jet, thereby reducing voids and surface defects. Parts made using a mold with a thicker gate verified these predictions. For combinations of operating conditions and mold geometry that gave large thermally induced viscosity gradients, both observation and simulation showed unstable, asymmetric flow. In these cases, flow slowed and stopped in one region of the gate and accelerated in other regions. When the flow was inherently unstable, simulations predicted an exponential growth in maximum temperature differences at symmetric locations in the mold gate. Based on 34 experimental observations and 102 simulations, a boundary was established between regions of stable and unstable flow in terms of the dimensionless Graetz number Gz (ratio of heat conduction time to fill time) and B, a dimensionless ratio indicating the sensitivity of viscosity to temperature differences in the mold. To establish a common basis for comparison of simulation and experiment, the melt-mold interface temperature was estimated using a heat transfer coefficient, which was a fixed value for experiment and a parameter for simulation.

Since the past few decades, environmental issues have become a serious concern in society. It is a requisite for the industry to minimize waste, in order to protect the environment from being polluted. The term waste here comprises processing waste and post-consumer waste. The amount of processing waste should not be underestimated since it might consist of approximately 10% of the total production. The recycling of EPDM rubber is an interesting topic, especially if considered in connection with the continuous market growth of EPDM. This paper is a summary of efforts that have been taken so far on this subject. Studies done on EPDM recycling include reusing EPDM as filler in new rubber products and chemical, thermal, and mechanical devulcanization of EPDM in order to achieve a material that resembles the virgin one and could be revulcanized. Such studies on EPDM recycling are discussed extensively and in the conceptual frame of more general rubber recycling processes.