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During uniaxial deformation of amorphous PEN poly (ethylene 2,6 naphthalate) films, necking is developed even at rubbery temperatures above the glass transition temperature. To elucidate the structural changes occurring during neck formation and further strain hardening, a real time spectral birefringence technique together with a true stress / true strain measurement technique is applied. These techniques are able to track the changes in birefringence and stress levels as the film is being deformed. The results obtained by these two techniques permit determination of the stress optical constant and the limits at which it starts deviating from the linear behavior as well as large deformation behavior. Further investigation by X-ray and DSC measurements helps to understand and clarify the structure developed during the deformation process of the material.
Polytetrafluoroethylene (PTFE) is a remarkable material having a high melting temperature, high chemical resistance, low frictional and dielectric coefficients, etc. Due to its high melting point, PTFE cannot be manufactured by using the conventional polymer process, such as the injection molding, extrusion and blow molding, etc. In this research, PTFE powder–lubricant mixture were carefully prepared and followed by a series of techniques including paste extrusion, rolling and calendering. Effect of rolling behavior on laminated PTFE film was investigated. PTFE were laminated with combination layers of different initial rolling orientations varying from 0 degree, 45 degree to 90 degree in consequent layers. It was found that the rolling direction in each laminate layer affects the pore size of the final extended PTFE film significantly. It was also found that laminate with different combination in rolling direction would influence the characteristics of paste flow and the associated rolling pressure. The deformation on PTFE film with 0 degree laminate was serious. The final pores extend in both vertical and parallel to the rolling direction for the laminated film with 90 degree rolling direction difference in each layer. The laminated PTFE film with laminate 45 degree achieves most uniform distribution of pores.
An industry demand for plastic parts that have to withstand tibological stress is to optimize the part performance with respect to friction and wear behavior. By varying the processing conditions during molding formation, the morphology of the part’s surface can be affected significantly. This is essential for the lifetime, e.g. of a plastic gear of bearing. Therefore, an optimum can only be achieved, when the processing is carried out at appropriate conditions. Polyamide-66 as well as two polyoxymethylenes were employed as model materials. The effect of the morphology settings on the tribological behavior of plastic specimens in sliding contact was investigated, and the relationships between tribological and mechanical parameters and the morphology as well as the crystallinity were described.
This paper summarizes our extensive investigation on the low cycle (up to Nf = 5x104, where Nf is the number of cycles to failure) fatigue behavior of short glass-fiber reinforced poly(ethylene, terephthalate), or PET, thermoplastic. The modes of fatigue test include tensiontension, compression-compression, four-point bending (flexural) -- all at frequency f = 1-3 Hz, and flexural fatigue at f = 30 Hz (ASTM D-671). All tests were stresscontrolled with stress ratio R = Smin/Smax = 0.1, except for flexural fatigue at f = 30 Hz where stress ratio R = -1. The fracture surfaces of tested specimens were analyzed using scanning electronic microscopy (SEM).The results from this investigation provide comprehensive, up-to-date information and recommendations concerning methods for fatigue testing of injection molded specimens and models, prediction and optimization of low cycle fatigue properties that play a key role in determining a highly stressed plastic parts life and enduse performance, pre-selection of PET based plastic for various industrial applications.
Microcellular foams have largely been explored for thin – sheet applications, with thickness on the order of 1 mm. In this study, the basic batch microcellular process is scaled up to produce thick flat sheets, in the 3 – 15 mm range, from a number of thermoplastics such as PMMA, PS and ABS. It is shown that an unfoamed integral skin of desired thickness can be produced, making it possible to create sandwich structures with a microcellular core and a solid skin. It is hoped that these materials will open up the use of microcellular foams in load bearing applications, and as novel materials for construction.
This paper presents “cube” cell modeling development to investigate PE foam compression modulus as a function of material distribution between strut and cell wall, and open cell contents. The model shows that compression has a stronger dependency on cell wall stretch than strut strength. As a result, open cell becomes a critical factor for compression strength. PE foam samples with various degrees of open cell were made. Compressive strength is measured. Modeling results show greatly improved agreement to experimental data by considering the stretch of cell walls and bending of struts parallel to compressive force.
Nano-clays were used to assist the production of polystyrene microcellular foams. Polystyrene was first compounded with nano-clays and then foamed. The nanoclay was shown to be an effective nucleating agent and caused a reduction in cell size and an increase in cell density. Nanocomposite foams exhibit higher tensile modulus and better fire retardance. The nucleation effect of nano-clay expands the operating windows for extruding microcellular foams. The influence of nano-clays on the sorption of CO2 and the viscosity of polymer melts are also discussed.
The effect of temperature and type of physical foaming agent (HCFC-142b, n-pentane and carbon dioxide) on shear viscosity has been investigated for various types of polyolefin resins (PP, LLDPE, and HDPE). The viscosity changes have been monitored using a commercial on-line process control rheometer mounted on a twin-screw extruder.A plasticization index, based on the respective molecular weights of the foaming agent and the repeat unit of the polymer, is proposed. Comparison with an amorphous polymer, namely polystyrene, is also made, for mixtures using the same physical foaming agents.
The conditions that induce the phase separation and the bubble nucleation for the thermoplastic foam extrusion process in which physical foaming agents (PFA) are involved are obviously linked to the solubility parameters: temperature, PFA content, and pressure. However, it has been reported that flow or shear can significantly modify these degassing conditions. An inline detection method based on ultrasonic sensors was used to investigate the influence of the shear on the foaming conditions of polystyrene/ HFC134a mixtures, for PS resins of various melt flow rates. An increase of the degassing pressure at low melt temperature was observed for high viscosity resins. Deviation from solubility data has been attributed to the combined effects of elongational and shears stresses.
Polymer blends such as result from recycling of postconsumer plastics often have poor mechanical properties. Microcellular foams have been shown to have the potential to improve properties, and permit higher value uses of mixed polymer streams. In this study, the effects of microcellular batch processing conditions (foaming time and temperature) and HDPE/PP blend compositions on the cell morphology (the average cell size and cell-population density) and impact strength were studied. Optical microscopy was used to investigate the miscibility and crystalline morphology of the HDPE/PP blends. Neat HDPE and PP did not foam well at any processing conditions. Blending facilitated the formation of microcellular structures in polyolefins due to the poorly bonded interfaces of immiscible HDPE/PP blends, which favored cell nucleation. The experimental results indicated that well-developed microcellular structures are produced in HDPE/PP blends at ratios of 50:50 and 30:70. Improvement in impact strength was associated with well-developed microcellular morphology.
A new family of random propylene-ethylene copolymers ranging from 0 to 19 mole % ethylene were produced by The Dow Chemical Company using INSITE™ technology including a new catalyst. These copolymers exhibit relatively narrow molecular weight distributions and unique micro-molecular structures. Based on the combined observations from melting behavior, dynamic mechanical response, morphology, and tensile deformation, a classification scheme with four distinct categories is proposed. Type IV consists of copolymers with less than 3 mole % ethylene and crystallinity greater than 48 wt. %. The morphology is characterized by large space-filling spherulites. Type IV materials exhibit thermoplastic behavior. With increasing ethylene content, the neck becomes more diffuse. These copolymers are of Type III and Type II. With 3 to 7 mole % ethylene, Type III has a crystallinity between 48 and 33 wt. %. The spherulites are sheaf-like with some non-space-filling regions. Type II materials have a comonomer content between 7 and 15 mole % ethylene. They span a range of crystallinity from 18 to 33 wt. %. The structure is characterized by non-impinging axialites. Type I materials have more than 15 mole % ethylene and less than 18 wt % crystallinity. They exhibit elastomeric behavior with high recovery. The crystalline morphology consists of embryonic axialites. The utilities and potential applications of these new copolymers will also be discussed in this paper.
A series of ethylene-containing propylene copolymers (P/E) have been synthesized with up to 19 mol % ethylene. Tapping mode atomic force microscopy (AFM), wide angle X-ray diffraction (WAXD), optical microscopy (OM), differential thermal analysis (DSC) and stress-strain behavior were employed to characterize the solid state structure. Based on the combined experimental observations, a classification scheme with four distinct categories is proposed. Type IV copolymers (0-3 mol % ethylene and crystallinity greater than 48 wt. %) are comprised of space-filling ?-mixed spherulites that are primarily ?-phase crystals. Spherulites of Type III copolymers (3-7 mol % ethylene and 48-33 wt % crystallinity) are sheaf-like and non-space-filling. Unimpinged regions are comprised of an epitaxially crystallized cross-hatched network. Type III copolymers have both ? and ?-phase crystals. An increase in ?-to-? ratio suggests that spherulites are primarily ?-phase whereas the cross-hatched network is a mixture of ? and ?-phase. Type II copolymers (7-15 mol % ethylene and 33-18 wt % crystallinity) are characterized by axialites surrounded by a loosely woven cross-hatched network. Type I copolymers (more than 15 mole % ethylene and less than 18 wt % crystallinity) contain occasional assemblies of radial lamellae epitaxially grown off of a single lamella. Type IV materials exhibit thermoplastic behavior. With increasing ethylene content, the neck becomes more diffuse. Type I copolymers exhibit elastomeric behavior with uniform deformation and high recovery.
The elastic properties of new high comonomer propylene-ethylene copolymers are investigated. Although these materials show some degree of permanent set upon initial tensile deformation, the materials created as a result of a tensile “conditioning” process exhibit very high recovery during subsequent tensile testing. The stress-strain behavior of the conditioned materials is characterized by low modulus and reversible deformation to high strain. The data fit well with a two-parameter crosslink model. An increase in density is observed during the conditioning process suggesting melting and recrystallization. Wide angle x-ray scattering shows that the conditioning process increases orientation, and converts all ?-crystals into ?-crystals. Atomic force microscopy reveals that the conditioning process produces a fibrous network structure.
In this work several ethylene polymerizations were carried out with the metallocene system Et(Flu)2ZrCl2/MAO in the presence of different unsaturated substances. The reactions were performed at a temperature of 50°C and the concentration of the unsaturated substances, such as styrene, isoprene, indene, acrylonitrile and cyclopentene was varied. Some polymerizations with isoprene and indene showed, respectively, higher and lower activities compared to ethylene homopolymerization. The copolymerizations with styrene and cyclopentene at low comonomer concentration presented almost the same activity and with acrylonitrile did not show any activity. Another result was that the copolymers, especially with styrene, presented their melting temperature lower than that of polyethylene, which indicates that the substances did incorporate into the polymeric chain.
Linear low density polyethylenes were synthesized employing (nBu-Cp)2ZrCl2/methylaluminoxane catalyst in homogeneous and zeolite supported systems. The comonomer effect in homogeneous catalyst was observed as well as a slight negative comonomer effect in the supported system at higher comonomer content. The slight decrease in activity between 50°C and 80°C observed for the homogeneous system was attributed to the activation of the active centers at higher temperature and low MAO concentration. The increase on the comonomer content in the copolymer caused a reduction on the melting temperature and crystallinity and also multimodal melting endotherms. These multimodal Tm’s occurred due to the heterogeneous intrachain composition. The reactive ratio was calculated and according to r1 and r2 values we can expect long ethylene sequence lengths.
The kinetics of the in situ polymerization of methyl methacrylate (MMA) in the presence of benzoyl peroxide (BPO) initiator under supercritical fluid CO2 (sCO2) was studied by high pressure DSC. The influences of the reaction medium, initiator content, and reaction temperature on the polymerization rate were studied. The results indicated that the formation of PMMA follows a first order reaction mechanism. The polymerization rate in the presence and absence of sCO2 were found to be similar but the exothermic profiles were different, with the polymerization under sCO2 having a lower profile. Increasing the amount of BPO initiator accelerated the rate of PMMA formation and reduced the induction time. The molecular weight of PMMA produced under sCO2 was 34% higher than that obtained in air.
The effect of screw design on dynamic dissolution and melt sealing of HFC-134a in PS during foam twin-screw extrusion has been investigated. Ultrasound velocity and viscosity monitoring in the polymer/blowing agent solution showed that HFC-134a dissolution is sensitive to screw design. The screw configuration also played an important role in the creation of an efficient melt seal. Correlation between blowing agent pressure and concentration in the extruder were shown to obey Henry’s law and were similar to pressure – solubility data obtained off-line in the quiescent state.
Polystyrene was in-situ polymerized with a range of montmorillonite layered silicate (MLS) concentrations, and subsequently compression molded into thin laminates. The laminates were foamed in a batch supercritical CO2 chamber at various temperatures and pressures from 60°- 85°C and 7.6-12MPa. The resulting foams were analyzed by scanning electron microscopy to determine effect of MLS on cellular morphology. Differential Scanning Calorimetry was used to determine the impact of nanocomposite microstructure on glass transition of the foamed polymer.
This paper elucidates the effects of cell density on the volume expansion behavior of polypropylene (PP) foams blown with butane in extrusion. The cell density was controlled by varying the talc content, and foam expansion was observed at a fixed blowing-agent content while varying the melt and die temperatures. As observed in our previous studies, the curve of the final expansion ratio of PP foam versus temperature showed a typical mountain shape for each talc content, indicating the two governing expansion mechanisms which are gas loss and stiffening of melt. As the talc content increased, the expansion curve skewed towards the lower temperature, which showed that the expanded foams of high talc content were more susceptible to gas loss. In order to analyze this change, the early-stage expansion behavior of extruded PP foams was investigated using a CCD camera. The expansion-profile images captured at the die exit show that the expansion rate of extruded foams was observed to be faster at a high talc content because of the reduced diffusion distance of gas molecules to the nearest stabilized cell. The higher growth rate promoted the formation of an initial hump in the expansion profile which is known to be detrimental to large expansion. In order to decrease the expansion rate and thereby to remove the initial hump, the temperature had to be further decreased, and consequently, the optimum temperature to maximize expansion decreased at a higher talc content. On the other hand, the increased cell density at a high talc content increased the number of cell layers in the cross section of the extruded foam, and thereby the gas loss was localized to the cells on the surface which acted favorable for the final expansion ratio to a certain degree.
Employing a modified injection-molding technology, where the mold is opened a short stroke after injection of the polymer melt, it is possible to manufacture plastic parts with low overall density and an integral foam structure. For this processing approach, thermoplastic materials together with chemical foaming agents are used. The resulting plates are a real lightweight design with compact skin layers and a foamed core fraction. A significant gain in stiffness can be observed. The lightweight potential of the design was investigated using mechanical testing methods and the stiffness enhancement was modeled using several models for multi-layer arrangements.
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