SPE Library

Foaming of low density poly(ethylene-co-octene) resins by injection molding is the result of various reactions occurring during the process. This includes simultaneous decomposition of the chemical blowing agent and cross-linking of the polymer matrix during curing in the mold, followed by foaming after mold opening. Dynamic rheology as well as elongational viscosity were investigated for compounds prepared from resins with different MFIs and various cross-linking agent levels, and these results were linked to the morphology and density of the corresponding foams. Rheological requirements were finally defined quantitatively for this set of conditions.

The development of the electrical properties of composites as a function of the degree of mixedness of graphite distributed into a plasticized thermoplastic elastomer (Kraton with mineral oil plasticizer) is investigated. A wide-angle x-ray diffraction (WAXD) based quantitative phase analysis method was used to characterize the variations of the concentrations of the elastomer and the graphite particles around their mean values as a function of mixing time in an intensive batch mixer. Increasing the specific energy input during the mixing process results in a more homogeneous spatial distribution of graphite in the elastomer. The increasing specific energy input alters the rheology of the composite suggesting that significant structural changes do occur. Indeed the degree of mixedness of the graphite in the matrix is quantitatively determined to be improved, generally resulting in better coating of the individual graphite particles. This improved coating effectiveness in turn results in a decrease of the volume conductivity of the composite.

In this study, the rheological characterization in extension and in shear of melts of reactively extruded low-density and linear low-density polyethylenes was carried out. The polyethylenes were extruded with peroxide, in one case, and with peroxide and diethylmaleate, in the other, using a co-rotating twin-screw extruder. A capillary rheometer with cylindrical and converging dies was employed to obtain the shear and extensional viscosities at 200 °C. The Cogswell, Binding and Mackay-Astarita models were used to estimate the extensional viscosity of the materials. An increase in both the shear and extensional viscosities was obtained due to the presence of long branches produced by the modification processes.

Extensional viscosity of several polypropylene polymers and their blends was measured and compared to the relative foamability of thermoplastic elastomers (TPEs) containing these polymers. Both linear isotactic polypropylenes and branched polypropylenes were considered for this work. The extensional viscosity is measured using an RME Extensional Rheometer at various strain rates. The TPE foam is characterized based on density and cell size distribution. A small amount of branched polymer substituted for linear polymer was found to lead to improved foamability of the TPE. Results of this study will aid in the understanding of the foaming process in TPEs.

The continuing trend toward high output extrusion has contributed greatly to the growth of rigid vinyl applications, particularly siding and window profiles. The development of large diameter twin-screw extruders, advances in die design, and improvements in extruder downstream efficiencies are among the factors that have enabled PVC to achieve a dominant share in the building and construction market.The PVC formulation plays a key role in achieving productivity improvements. Considering how susceptible PVC is to thermal degradation, it is important to choose formulation ingredients that facilitate the processing and flow of the PVC melt at the high shear conditions generated from high throughput. Typically, acrylic impact modifiers are used in siding and window formulations to improve ductility, weatherability, and, to some degree, appearance. In developing new acrylic impact modifiers, emphasis is generally placed on making changes to the polymeric structure in order to improve impact efficiency in PVC. However, the polymer's rheology characteristics must be considered. The typical acrylic polymeric additive, whether it be impact modifier or processing aid, contributes to the melt viscosity of the PVC formulation. High viscosity produces pressure build-up which, in turn, raises melt temperatures high enough to degrade the PVC. To show that both high extrusion rates and improved impact efficiency can be attained with acrylic impact modified PVC formulations without excessive viscosity, pressure or temperature increases is the intent of this paper.

The curing and fracture mechanical performance of resin combinations containing a bisphenol-A type vinylester (VE) and various epoxy (EP) systems were studied in the entire composition range. VE was crosslinked by free-radical induced copolymerization with styrene, whereas the EPs of aliphatic (Al-EP), cycloaliphatic (CAl-EP) and aromatic (Ar-EP) nature were hardened by an aliphatic diamine compound. Curing and chemorheology of the resin compositions were assessed by differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FT-IR) and plate/plate rheometry. It was found that the curing of VE is faster than that of the EPs and thus it is the controlling parameter of the resulting morphology. Dynamic-mechanical thermal analysis (DMTA) showed a broad mechanical loss peak between the glass transitions (Tg) of the pure resins at least in a given composition range (25/75..75/25). It was supposed that the related materials possess an interpenetrating network (IPN) type structure. The fracture energy (Gc) of the VE+Ar-EP combinations followed the additive rule. On the other hand, combining VE with Al-EP and CAl-EP in the composition range 40/60... 60/40 yielded synergism in Gc the values of which reached 6 to 10 kJ/m2 . The toughness improvement was explained by the enhanced shear deformability of the network which was confirmed by fractographic results.

In this work the mechanical, magnetic and rheological properties are analyzed. The influence of different magnetic powders onto a polyethylene was studied. The magnetic characterization of isotropic plastic bonded magnets, based on strontium ferrite (SrFe12O19) and Nd2Fe14B onto polyethylene, as a function of composition was analyzed in a magnetometer at room temperature. The rheological properties were studied in a capillary rheometer; it was found that viscosity increased as the magnetic powder concentration increased in the composite.

The cooling phase of the extrusion blow molding process has a large influence on the cycle time of the process as well as on the properties and quality of the molded products. A better understanding of the heat transfer mechanisms occurring during the cooling phase will help in the optimization of both mold cooling channels and operating conditions. A continuous extrusion blow molding machine and a rectangular bottle (motor oil type) mold were used to produce bottles. A high density polyethylene (HDPE) and a metallocene polyethylene (mPE) having different rheological properties were tested. Melt and mold temperatures, cooling time, inflating pressure and die gap were varied systematically. An infrared (IR) camera was used to measure the temperature distribution of the plastic part just after mold opening as well as after part ejection. The wall thickness and dimensions of the bottles of the finished parts were measured in order to determine the shrinkage and warpage. Finally, the temperature fingerprints were used to explain what happens during the cooling phase and correlated with the final part characteristics.

The rheological behavior of Polypropylene (PP) modified with Styrene-Butadiene-Styrene copolymer (SBS), within the composition range of 10-40 wt % SBS content, was studied based on blend ratio, vulcanization agents (sulphur, peroxide) and curing times. The rubber phase was statically cured and blends with PP were mixed in a twin screw extruder. Results indicate all unvulcanized systems show an increasing melt viscosity on SBS content. Regarding vulcanizing agents, sulphur shows no variation on melt viscosity neither with shear strain nor with curing time, while peroxide shows significant changes on melt viscosity at low shear strain ranges when varying curing times from 2 to 6 min.

A two-dimensional simulation and experimental study of the flow-rate controlled co-injection molding was carried out. A skin polymer was injected first and then both skin and core polymers were injected simultaneously into a center-gated disk cavity through a two-channel nozzle to obtain an encapsulated sandwich structure. The physical modelling and simulation were developed based on the Hele-Shaw approximation and kinematics of interface to describe the two-phase flow and the interface development. The effects of rheological properties and processing conditions on the material distribution, penetration behavior and breakthrough phenomena were investigated. The predicted and measured results were found to be in good agreement with each other.

A new rheological index for long chain branching (LCB), grheo, is introduced. The method is implemented for isotactic polypropylene (iPP) melts.Using statistical analysis, the average molecular weight (MW) and molecular weight distribution (MWD) values of a large number of linear iPPs (including metallocene and iPP blends) as well as iPPs with LCB are correlated with the crossover point coordinates and the four parameters from the Carreau-Yasuda equation. This allows us to define (independently of MWD and MW) grheo, whose value is one for a linear iPP and decreases with increasing LCB content.

The object of this work is to investigate the foaming characteristics of Metallocene-catalysed polyethylenes for rotational moulding. This paper reports on the results of ongoing experimental investigations in which rheological and thermal parameters are related to the polymer structure and mechanical properties of metallocene polyethylene foams. Through adjustments to moulding conditions, the significant processing and physical material parameters, which optimise metallocene polyethylene foam structure, have been identified. The results obtained from equivalent conventional grades of polyethylene (PE) are used as a basis for comparison.

Linear viscoelastic results are presented for several polyethylenes which each exhibit, to varying degree, an increased zero shear viscosity (?0) relative to that observed for a linear polyethylene with the same weight average molecular weight (Mw). This is a well-recognized rheological signature of the presence of long chain branching (LCB). Examination of the small amplitude oscillatory shear data for the branched polyethylenes clearly reveals the presence of two separate relaxations. We examine the utility of considering these polyethylenes as blends of branched and linear species. A unique power-law behavior is observed for the dynamic viscosity in the intermediate frequency region bounded by the distinct relaxations of the linear and long chain branched components. Characterizing these rheological features appears to be a key element in formulating an understanding of the processability of polyethylenes which possess entangled branches.

The recent advent and commercialization of technology using single site, constrained geometry catalyst has made possible the introduction of unique ethylene/a-olefin elastomers with novel molecular architecture. These advances in elastomer technology have resulted in differentiated materials capable of impact modifying polypropylene polymers thereby offering new TPO blends with enhanced properties. This paper will explore high flow polypropylene blends modified with this distinct class of elastomers and will discuss the influence of elastomer comonomer choice, molecular weight and crystallinity along with discussions on the effect of dispersion, morphology and rheology.

The mechanical, thermal and rheological behavior and the morphology of a Polypropylene (PP)/high density polyethylene (HDPE) blend and of the PP/HDPE/Woodflour composite, both modified with peroxide, were evaluated. A decrease in the apparent viscosity of the blends with the increase in the content of peroxide was found. None of the peroxide modified samples showed significant variations in the melt and crystallization temperatures. The blend modified with 0.04 phr of peroxide showed the highest Young's modulus of all. The woodflour produced a significant increase in the viscosity and the Young's modulus of the composite.

Binary polymer blends with well-defined initial structure were prepared by Computer Numerical Controlling (CNC) machining, photolithography and micro-embossing. Using the methods, we designed the size and distribution of the dispersed phase and the composition of the blends. Compatibilizer can also be easily placed at the interface of the two components during sample preparation. With the micro-fabricated samples, the dynamics of phase inversion and the morphology evolution of binary polymer blends were studied in simple shear flow under isothermal conditions. The effects of interfacial tension, viscosity rate, blend composition, and shear rate on rheology and morphology evolution were investigated.

Polypropylene (PP) nanocomposites were prepared by two steps: a predispersed organoclay masterbatch was first prepared by using a twin screw extruder; the masterbatch was then letdown into base PP by using a single screw extruder. The effect of single screw mixing type on organoclay dispersion and nanocomposite properties was evaluated. The results indicated that the composites obtained from the masterbatch letdown with a single screw extruder showed better dispersion and better mechanical properties than the composite obtained from the direct compounding with twin screw extruder. Furthermore, the mechanical properties of these composites from masterbatch single screw letdown process is as good as the composite obtained from masterbatch letdown with a twin screw extruder. A rheological study also shows PP nanocomposite has the same flow characteristics as neat PP, indicating the new technology can drop in the current machine set up, without adding additional cost to end users.

The current organic peroxides most commonly used as polymerization initiators are either monofunctional or difunctional. In this work we present the performance characteristics of a new commercial organic peroxide that contains four peroxide groups. Using batch lab-scale, continuous micro-pilot experiments, and simulation we demonstrate several features of the tetrafunctional initiators in styrene polymerization. Principal advantages are 1) increases in molecular weight compared to standard initiators or thermal polymerization, 2) greater than a 20% improvement in production rate with no loss in molecular weight, and 3) the ability to introduce long-chain branching into the resin to improve rheological and processing characteristics.

In the rheology of complex fluids, the central question concerns the relationship of molecular organisation and dynamics to macroscopic constitutive behaviour. The key to addressing this issue lies in the use of spectroscopic techniques which are capable of accessing information at the molecular level during deformational flow. Examples include optical birefringence and dichroism measurements, neutron and X-ray scattering, and most recently, nuclear magnetic resonance. Through terms in the spin Hamiltonian which are sensitive to order and dynamics, NMR spectroscopy gives us access to the molecules. Through the use of magnetic field gradients, NMR microscopy and velocimetry gives us access to the mechanics. As will be shown, these two approaches are highly complementary. Furthermore, amalgamation of spectroscopic and gradient methods can be used to localise NMR so that we can select any part of the flow field for analysis.We have used 2H NMR quadrupole interaction spectroscopy to measure the deformation of PDMS melts under shear in a Couette cell. The signals were acquired from a per-deuterated benzene probe molecule that provides a motionally-averaged sampling of the entire segmental ensemble. The dependence on shear rate of the SXX (velocity) and SYY (velocity gradient), SZZ (vorticity) and SXY (shear) elements of the segmental alignment tensor has been measured, as well as the angular dependence of the deuterium quadrupole splitting at fixed shear rate. We show that the data agree quite well with the Doi-Edwards theory but significantly better when convected-constraint-release effects are included.

Nanocomposites comprised of montmorillonite clays and maleic anhydride grafted polyethylene (PE-g-MA) were prepared by melt blending. The presence of maleic anhydride (MA) derived groups grafted in PE promoted strong interaction between polymer matrix and clay, leading to complete exfoliation and dispersion of clay platelets. The non-isothermal crystallization behaviour, rheology and mechanical properties of the nanocomposites were investigated. The non-isothermal crystallization of the PE phase was strongly accelerated by the presence of clay. The viscous and elastic properties increased significantly with increasing clay content. The tensile strength of nanocomposites was improved as compared to pure PE and PE/clay mixtures, but elongation at break decreased considerably.