Volume 11, Issue 1

This paper provides a scientific treatment for mixing of rubber with reinforcing fillers. The primary attention is given to the viscoelastic nature of rubber. It is addressed to the practitioners of rubber processing, who have hands-on experience but lack familiarity with viscoelasticity. In the first part we provide a sketch of the major events: (1) polymerization, (2) resulting molecular architecture, (3) mechanical property of gum rubber, (4) mill processability, (5) processing with the internal mixer, and (6) compounding rubber with carbon black. The focal point of this paper is to relate the mill processability to the elongation and failure behavior of gum rubber. Viscous and elastic contributions to this relation are described. This relation provides the starting point for introducing scientific treatment for rubber mixing. Mechanisms of incorporation and dispersion of carbon black are also discussed. The presentation follows in an order of description on the behavior of gum rubber, that of carbon black, the interaction between rubber and carbon black, and the interaction between the material and the internal mixer. In the second part a method of constructing viscoelastic master-curves from the tensile stress-strain data is presented. Such a master curve serves as a useful means of characterizing gum rubber in relating polymerization, resulting molecular architecture and processability. The milling and mixing of rubber with filler are achieved with fracture; the consequence of fracture is a generation of supermolecular flow units. The mechanism of fracture involves the pull out of chain ends through entanglements. The high concentration of the chain ends on the surface gives a tacky appearance to mixed compounds.

Differences in processability between linear low denisty polyethylene (LLDPE) and low density polyethylene (LDPE) as well as for their blends subjected to extensional flow were assessed in terms of melt strength and elastic parameters from shear flow such as storage modulus (G′), and first normal stress difference (N 11 – N 22 ). The results revealed similar synergistic effects for these parameters in the LDPE rich blends. A DSC test carried out on these LLDPE/LDPE blends when fast cooled disclosed the existence of two phases in the molten blends for the same composition range where the melt strength synergism was observed. The results are discussed in terms of the influence of the different molecular structures of the blend polymers and their miscibility on the measured melt strength. A mechanism is suggested to explain the observed synergistic phenomenon.

A comparative experimental study of blending polyethylene and polystyrene in a modular self wiping co-rotating and intermeshing counter-rotating twin screw extruder is described with similar modular screw configurations. Comparisons are made using different screw configurations possessing different numbers and placement of kneading disc blocks. Generally the intermeshing counter-rotating mode melts and mixes more rapidly than the co-rotating mode. The phase morphology in each machine seems to similarly from mixtures of pellets to polyethylene lamellae surrounding polystyrene pellets to a co-continuous morphology. Kneading disc blocks greatly enhance the rate of melting and mixing and are most effective when placed near the feed port rather than near the die. At 160°C, where the polystyrene has a much higher viscosity a co-continuous morphology develops. At 260°C where the polystyrene has a lower viscosity first a co-continuous and then a disperse phase morphology evolves.

Contrary to the case for the melt conveying zone, no adequate description exists as yet for the solids conveying zone of a co-rotating twin-screw extruder. A physico-mathematical model of the solids conveying zone, however, is a prerequisite for determining the location of the plasticizing zone and for establishing the optimum screw configuration that goes with a selected point of operation. This paper presents a physico-mathematical model to describe the processes taking place in the solids conveying zone. The conveying zone is divided into a partially-filled feed zone and a completely filled compression zone. Force and torque balances are drawn up for three different volume elements and, with the solutions obtained for these, it is possible to calculate the maximum conveyable throughput, the pressure profile over the length of the solid material accumulations and the drive power that is required or the solids conveying zone.

The high-speed spinning process of poly(ethylene terephthalate) (PET) was modified by introducing a liquid isothermal bath (LIB) which was kept at a temperature of 155°C and placed at a distance of 150 cm or 200 cm below the spinneret. The structure and mechanical properties of the as-spun fibers thus produced in the take-up speed range of 1 to 7 km/min were characterized. The results showed that the introduction of the LIB significantly influences the structural development of the as-spun fibers. The fibers produced using the LIB in the spinline showed significantly higher birefringence and crystallinity compared to the fibers produced using a standard high-speed spinning process. A semi-quantitative numerical simulation was performed with the incorporation of the LIB in the spinline to understand the mechanism of fiber structure formation during the modified high-speed spinning process of PET. The results showed that the solidification stress is considerably high when the solidification was assumed to occur at a position close to the exit of the LIB. The enhancement of molecular orientation was attributed to the increased frictional stress induced by the liquid layer whereas the spinline crystallization was found to be influenced strongly by the temperature of the liquid.

Die design is a daily concern in the manufacturing of extruded profiles. There are very few available Computer Aided Design (CAD) tools for profile die designers and considerable time and resources must be devoted to trial and error design procedures. This paper examines the applicability of the flow analysis network (FAN) to the design of profile extrusion dies. The FAN method is found to provide useful design information at a fraction of the computing cost required for 2.5 and 3D finite element methods. Experimental flow distribution obtained by extruding PVC compounds and polyethylene through simple profile dies are compared to model predictions. The numerical and experimental results provide evidence about the significant contribution of transverse flow to the flow distribution at die exit.

Polymer mold filling is a non-Newtonian, non-isothermal, non-steady process with free surface. It is well known that the fountain flow effect in the flow front region and the solidified layer near the cold mold wall have significant effect on the mold filling process and thus the mechanical properties of the molded parts. In this study, a non-isothermal, non-steady mold filling process including the solidified layer as well as the fountain flow effect was analyzed numerically using the finite element method. Shear rate dependency in the viscosity was taken into account by employing the Cross model. Single-step algorithm was used for the numerical integration in time and the flow front was advanced by the explicit scheme. The resulting system of algebraic equations was solved using the Picard iteration technique. In order to verify the validity of the numerical scheme, energy balance and mass conservation were checked. It was found that the energy and mass balances were satisfied within reasonable accuracies. The importance of the non-steady term was assessed by analyses done in a similar manner by assuming that the process is quasi-steady. Two results thus obtained were compared. The quasi-steady assumption was found to be insufficient in describing the inherent non-steady effect during mold filling.

Amorphous and crystalline polymers were injected into different molds of disks, with different applied injection rates. The pressures and local volume flow rates were measured for different injection molding conditions. These measures show a difference between the average volume flow rate measured during the total filling time and the nominal applied volume flow rate. Moreover the local volume flow rate at the polymer front increases between the mold entrance and the end of the cavity. Effects of different parameters of the process (polymer, geometry of the part, injection molding conditions) on the evolution of these volume flow rates were examined. Interpretation of the observed phenomena neads to take into account the effect of two characteristics of the polymers which are generally neglected: the compressibility and the associated bulk viscosity.

Surface defects in the injection molding of acrylonitrile-styrene-acrylate (ASA) weatherable polymers, such as “chevrons”, “non-uniform haze” and “step defect “, are examined. A criterion is proposed based on a flow instability mechanism which suggests that recoverable shear strain, shear stress, and coefficient of friction are the key factors controlling the formation of surface defects. The criterion is supported by various experiments.

This work is concerned with the injection molding of pellets of polypropylene (PP) containing pregenerated microfibrils of thermotropic liquid crystal polymers (TLCPs), referred to as microcomposites. The TLCPs used are HX6000 and Vectra A950. The microcomposites are produced by drawing strands of PP and TLCPs generated by means of a novel mixing technique and pelletizing the strands. The work was undertaken in an effort to improve on the properties observed for in situ composites in which the TLCP fibrils are generated during mold filling. In situ composites usually exhibit highly anisotropic mechanical properties and the properties do not reflect the full reinforcing potential of the TLCP fibers. Factors considered include the effect of in situ composite strand properties on the properties of the injection molded composite, the melt temperature used in injection molding, TLCP concentration, and the melt temperature of the TLCP. It is found that when processed at appropriate temperatures, the TLCP microfibrils are maintained through injection molding, and the composites show a good balance between machine and transverse direction properties (with ratios of the machine and transverse direction moduli ranging from 1.1 toto 1.35). The use of HX6000 rather than Vectra A allows for higher processing temperatures to be used during injection molding. The moduli of the injection molded composites increase with both increasing TLCP composition and in situ composite strand moduli. At 10 wt.-% TLCP the moduli of the injection molded composites approach the predictions of composite theory while at 20 and 30 wt.-% TLCP negative deviations from the theoretical moduli are seen. This is believed to be the result of a loss of fiber aspect ratio from fiber breakage and agglomeration. It is believed that with future studies on the effect of shear and elongational stresses during processing on the aspect ratio of the fibers a means of overcoming this problem may be found. The modulus of a 20 wt.-% HX6000 composite is similar to that of a 20 wt.-% glass composite (3.21 GPa versus 3.66 GPa), but the tensile strength of the HX6000 composite is about 28 % lower than that of the glass composite. It is expected that as the modulus and strength of the reinforcing TLCP fibrils are improved the properties of the injection molded microcomposites should exceed those of glass reinforced composites.