Volume 30, Issue 1

When certain critical conditions are exceeded, the flow of a polymer melt becomes unstable, giving rise to particular phenomena that are the subject of this review. These instabilities are a vital industrial problem, as they are the key to productivity of extrusion processes. Indeed, their onset leads to unacceptable products and is thus the upper limit of the processing conditions. They are also a fascinating scientific problem because, although studied and discussed for more than half a century, they are still not fully understood and their mechanisms are still sometimes controversial. In the present literature review, we present an overview of the studies carried out for fifty years, as well as our own opinion on the underlying mechanisms.

In this work, Halloysite Nanotube (HNT) was synthesized, using a modified method, from kaolin clay. The scanning electron microscopy (SEM) of the synthesized HNT revealed that many of the nanotubes were tubular; however, spherical and semi-spherical, platy and semi rolled structures were also observed. Fire retardant EPDM compounds composed of typical flame retardants (FR), deca-bromo diphenyl oxide (DBDPO), antimony oxide (Sb 2 O 3 ), talc and carbon black were prepared. Three elastomer compounds, with 5, 10 and 20 wt.% of synthesized HNT, and one more compound with 10 wt.% of synthesized HNT and 10 wt.% of conventional flame retardants were prepared and formed by a two roll mill. A TEM micrograph was presented to prove the well dispersion of HNT in polymer matrix. The flammability, thermal stability and tensile strength of the compounds were compared to that of typical fire retarded EPDM. The results showed that the typical flame retardants diminish the tensile strength and elongation at break of EPDM elastomers; in addition, these compounds showed the lowest thermal stability among the typical formulations. The extent of flammability of the compounds was characterized using the cone calorimetry technique. The compound containing 5 wt.% HNT showed the highest tensile strength, the least flammability and the highest thermal stability compared to the other compounds. This was related to the good dispersion and also on the tubular structure of the synthesized nanotubes. The peak of heat release rate (PHRR) and total heat release (THR) as well as the amount of hazardous gas emission for the sample having the highest mechanical response was smaller than those with conventional micro-filler flame retardants.

Laser transmission welding involves joining of transmitting and absorbing thermoplastic polymer components. Absorption is normally controlled by addition of small amounts of carbon black (CB). In thermal modelling of the process, the laser-light absorption coefficient is an important parameter that controls the rate and distribution of heat generation in the absorbing component. This paper introduces a novel experimental method to conveniently and accurately measure the absorption coefficient in the absorbing component. In this method, a laser beam scans rapidly over the surface of the CB-filled polymer. The laser power resulting in the onset of surface melting can be obtained and used to calculate the absorption coefficient based on an analytical model presented in this paper. To validate the proposed method, the experimental results were compared with those obtained by the spectrophotometer and the power-meter methods using unreinforced and glass-fiber reinforced nylon 6. The results indicate that there is a linear relationship between the absorption coefficient and carbon black level for these two polymers. The influence of glass fibers and laser scan speed on the measurement are also discussed.

In this study, a styrene/isoprene/styrene (SIS) triblock copolymer was synthesized in an intermeshing, co-rotating, twin-screw extruder. N-butyl lithium was used as the initiator. The styrene and isoprene monomer as well as 1,2-dibromoethane were sequentially added into the extruder. The weight content of isoprene in this copolymer was above 60%. Gel permeation chromatography (GPC) results showed that the polystyrene in the copolymer chains was mainly comprised of only one long block. Dynamic mechanical analysis (DMA) and transmission electron microscopy (TEM) showed that the polystyrene phase transferred from larger numbers of small microdomains to a continuous aggregation after the coupling reaction. As a result, the tensile stress and ultimate elongation was significantly improved, similar to the commercial product. Traditionally, SIS has only been synthesized by solution polymerization. The present work offers the capacity to synthesize it with very little or no solvent. This method fits the environmentally friendly trend.

The aim of this study was to evaluate various optimization strategies to be used with computational fluid dynamics (CFD), in order to optimize the geometry for a polymer die to meet the dual objectives of uniform flow at the exit with minimal pressure drop. 3D finite element simulations using the numerically optimized geometry predict a more uniform flow than simulations using the baseline geometry. However, some of the initial optimized results yielded die geometries that would be impractical to fabricate. Additional constraints were placed on the allowable geometric parameters and solution space to push the optimization towards more feasible optimal solutions. Thus we have shown how CFD-based optimization methods, coupled with knowledge of die design technology and fabrication techniques can lead to the design of better performing polymer dies.

This study adopted the novel magnetic roller soft mold embossing technique of the soft mold preparation method, embossing, combined with the existing gas-assisted UV-curable technique, to develop an innovative magnetic-assisted continuous roller platform. Using the advantages of uniform and accurate control of applied pressure of the magnetic disk, this study used the magnetic roller soft mold embossing and UV-curable technique for replicating microstructure patterns, resulting in a more mature micro-nano embossing technology and application. The experimental results showed that the new magnetic roller soft mold preparation method can accurately cast and replicate micro-nano structures of Polydimethylsiloxane (PDMS) composite magnetic iron powder, and the magnetic disk can impose uniform pressure to effectively implement complete filling during the rolling process. This innovative process provides a simple and rapid molding mechanism, which significantly improves the microstructure pattern replication technique.

Most studies on environmental stress cracking resistance (ESCR) of polymers have focused on performing mechanical tests in the solid state. In this work, investigations in the melt state were carried out, to identify potential melt indicators for a relative measure of ESCR. A normalized characteristic relaxation time (λ N ), obtained from dynamic shear experiments, was established as a valid measure of ESCR of linear low density polyethylene, where a large content of short chain branches is present. In addition, the melt strain hardening coefficient (MSHC), obtained from extensional rheological methodologies, was found to be another potential and reliable indicator of ESCR. An inverse correlation between MSHC and ESCR was observed, indicating an inverse relationship between ESCR and chain extensibility in the melt. Furthermore, a new factor called “melt hardening stiffness (mHS)” was developed from the slope of a stress vs. Hencky strain obtained from a Sentmanat extensional rheometer. mHS was found to be another promising indicator of ESCR obtained from rheological studies conducted in this work.

Alumina particles is a ceramic powder used as a dispersed phase or filler to reinforce the mechanical properties and improve thermal properties of natural rubber foams via the vulcanization process at curing temperature 150°C with a two-roll mill. The amount of alumina added in natural rubber foams were varied from 0 to 60 phr on 100 phr of natural rubber in a sulfur curing system. The sodium bicarbonate (NaHCO 3 ) and N,N′-dinitrosopentamethylenetetramine (DNPT) were used as foaming agents. The sponge rubber composite foams were characterized for the microstructure, phase formation, thermal property, particle size distribution, and mechanical property by scanning electron microscope (SEM), X-ray diffraction (XRD), thermogravimetric analyzer (TGA), particle size analyzer, and compression set testing. The fabricated sponge rubber composite foams have distinctive characteristics such as light weight and good mechanical and thermal properties suitable for energy absorption such as thermal, noise, and vibration.

The effects of additive styrene/ethylene-butylene/styrene copolymer grafted with maleic anhydride (SEBS-g-MA) were investigated on the rheology, morphology and mechanical properties of a polyethylene terephthalate (PET)/high density polyethylene (HDPE) blend. The ratio of the two components was changed in small increments to track phase inversion. The rheology measurements show that SEBS-g-MA acts differently on HDPE and PET, as different morphologies are formed due to viscosity ratio change. With the help of electron microscopy various phases after extrusion and after injection molding were revealed and identified. Because of the high viscosity of HDPE the co-continuous morphology was immediately formed when PET reached 30 vol%. The range of the co-continuous structure of the blend was wider when SEBS-g-MA was added, and the elongation at break also improved as additive content increased, without a significant strength decrease. The divergence of the mechanical properties from the theoretical value, i.e. the value determined by the mixing rule, can be explained by the changing phase structure.

The aim of this paper is to determine the reason for the large warpage of an injection molded chunky part made of glass fiber reinforced polyamide 6. State of the art commercial 3D injection molding simulation software was used to predict the warpage of the part. While the simulation predicted a product virtually free of warpage the molded parts showed a large deformation. Several molding experiments and X-ray computed tomography (μCT) scans of the moldings have been performed to figure out the reason for this. The μCT scans revealed voids in the thick walls of the part. It was found that these defects were the main cause of the unreliable simulation results. Therefore it is concluded that voids can have a major influence on the reliability of numerical warpage predictions. This paper shows a simplified method to determine the zones were voids will appear and shows the comparison with μCT-data.

Experimental studies have been carried out for single screw extrusion of PP/wood flour composite of different concentrations of wood flour. A “screw pull-out” technique was used for investigation of the material transport, melting and screw filling. The effect of the screw speed was investigated. It was concluded that WPC extrusion is substantially different from thermoplastics extrusion. Material transport and melting are strongly dependent on the WPC composition, and operating conditions. The contiguous solids melting mechanism CSM is not observed for WPC of high WF content; however, it may be observed for WPC of lower WF content, up to 50% WF. Several steps of the process are distinguished: material compaction in the solid conveying section close to the active flight and then spreading to the rest of the material, melting which is strongly dependent on the material composition, and shear or plug flow with slipping at the walls in the melt conveying section.

This paper gives a personal view on the state of art in relation to the modelling of polymer melt processing. The paper briefly reviews both industrial, laboratory and modelling developments over the last forty years and highlights the key aspects now required for realistic modelling of polymer melt processing. The paper summarizes elements relating to the numerical simulation of specific and general polymer processes and also provides topical examples of the application of numerical modelling to certain commercial processes. The paper concludes with identifying areas of polymer processing that still remain a challenge in relation to accurate prediction.

Plastics can build-up on the die lip during extrusion. This phenomenon is called die drool and can be costly for plastics producers, requiring periodic shutdowns for die cleaning. Die drool has been attributed to the cohesive fracture of the melt into a drool layer and a bulk layer. The bulk layer slips on the drool layer after fracture, resulting in heating at the slip interface, called slip heating. The heat generated through slip heating can contribute to polymer thermal degradation, and to the die drool degradation. The impact of slip heating on die temperature rise has been investigated by neglecting viscous dissipation (Gilbert and Giacomin, 2014). This work considers viscous dissipation and its importance to slip heating. We find that viscous dissipation and slip heating contribute equally to the melt temperature rise, and we conclude with a worked example showing the importance of these two heating sources during polymer processing. We also develop two sufficient conditions for the accurate use of our results, Pé ≤ 1 and ∂Θ/∂ς ≪ ∂ 2 Θ/∂ζ 2 .