Resolution of Crystalline Phases in Polymorphic Gel-spun Ultra-high Molecular Weight Polyethylene Fibers Using Restrained Differential Scanning Calorimetry and X-ray Diffraction PDF Download
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Author: Amos Wampler
Publisher: ProQuest
ISBN: 9780549756972
Category : Calorimetry
Languages : en
Pages :
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Book Description
Characterizing highly oriented ultra-high molecular weight polyethylene (UHMWPE) fibers can present a challenge at or near melt temperatures. The fibers tend to shrink, creating difficulties during testing, both from excessive noise and from the possible alteration of morphology. Perturbations arise during standard differential scanning calorimetry (DSC) and limit the use of an easily accessible laboratory technique. Restraining the fibers during testing, however, suppresses these perturbations and permits the clear determination of melting events as the fiber is subjected to increasing temperatures. This technique is compared to x-ray diffraction (XRD) as a second measure of crystallinity at ambient temperature, removing the concern of shrinking and reorientation during the test.
Author: Amos Wampler
Publisher: ProQuest
ISBN: 9780549756972
Category : Calorimetry
Languages : en
Pages :
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Book Description
Characterizing highly oriented ultra-high molecular weight polyethylene (UHMWPE) fibers can present a challenge at or near melt temperatures. The fibers tend to shrink, creating difficulties during testing, both from excessive noise and from the possible alteration of morphology. Perturbations arise during standard differential scanning calorimetry (DSC) and limit the use of an easily accessible laboratory technique. Restraining the fibers during testing, however, suppresses these perturbations and permits the clear determination of melting events as the fiber is subjected to increasing temperatures. This technique is compared to x-ray diffraction (XRD) as a second measure of crystallinity at ambient temperature, removing the concern of shrinking and reorientation during the test.
Author: Galip Yilmaz
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
In this work, the processing of a unique ultra-high molecular weight polyethylene (UHMWPE) polymer, in both powder and pellet form, using both regular and special injection molding techniques, was investigated in an effort to mass produce this high-grade specialty polymer. The goals of this study were to (i) improve the processability of UHMWPE, (ii) enhance the mechanical performance and overall quality of the molded parts, and (iii) develop an in-depth understanding of how the supercritical fluid (SCF), machine configuration, mold compression, mold insulating techniques, and process conditions affect the flow behavior and final quality of the injection molded UHMWPE. In the first study, two common atmospheric gases in their supercritical states-namely, nitrogen (scN2) and carbon dioxide (scCO2)-were used as processing aids in a special full-shot, high-pressure microcellular injection molding (MIM) process for processing UHMWPE pellets. The mechanical properties in terms of tensile strength, Young's modulus, and elongation-at break of the SCF-loaded samples were examined. The thermal and rheological properties of regular and SCF-loaded samples were also analyzed using differential scanning calorimetry (DSC) and parallel-plate rheometry, respectively. It was found that the processing of UHMWPE with both gases effectively reduced the thermal degradation of the material and the injection pressure, compared to regular injection molding, while still retaining the mechanical properties of the resin. In the second part, a follow-up study was conducted on conventional injection molding (IM), along with the special full-shot, high-pressure microcellular injection molding (MIM) using UHMWPE in pellet form. A relatively complicated and thin-walled mold design was used to produce box-shaped parts with varying wall thickness. Although different processing settings were tested in order to eliminate persistent short shot issues, only high-pressure MIM processing was able to fill parts completely. Furthermore, not only did high-pressure MIM processing effectively promote the processability of UHMWPE, it also reduced the very high injection pressure requirement and the high part shrinkage issues associated with the IM samples. In the third study, UHMWPE powder was processed using injection molding (IM) and injection-compression molding (ICM). The processing parameters of feeding the powders were optimized to ensure proper dosage and to avoid damaging UHMWPE's molecular structure. Dynamic mechanical analysis (DMA) and Fourier-transform infrared spectroscopy (FTIR) tests confirmed that the thermal and oxidative degradation of the material was minimized but crosslinking was induced during molding. Tensile tests and impact tests showed that the ICM samples were superior to the IM samples. A delamination skin layer was formed on the IM sample surfaces, while it was absent in the ICM samples, thus suggesting two different flow behaviors between IM and ICM during the packing phase. The delamination layer defect was the subject of the fourth study as one of the main challenges of UHMWPE molding. The delamination layer hampers UHMWPE's two key properties: wear resistance and impact strength. A mold insulation method was employed to eliminate the formation of the delamination layer. The working principle of the method was to reduce the cooling rate and the shear stress of the polymer while improving polymer chain "interdiffusion" across the entangled chain bundles during the injection filling stage via a low thermal conductivity mold coating (e.g., epoxy coating). This method yielded molded parts free of delamination by delaying skin cooling during filling and packing. Therefore, it produced parts with enhanced mechanical properties, excellent impact strength, and improved surface quality.
Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Book Description
The drawing behavior of Ultrahigh Molecular Weight Polyethylene (UHMWPE) fibers in supercritical CO2 (scCO2) is compared to that in air at different temperatures. Temperature substantially influences the drawing properties in air, while in scCO2 a constant draw stress and tensile strength are observed. Differential Scanning Calorimetry (DSC) shows an apparent development of a hexagonal phase along with significant improvements in crystallinity of air-drawn samples with increasing temperature. The existence of this phase is not confirmed by WIDE ANGLE X-RAY SCATTERING (WAXS) showing that air-drawn samples crystallize in an internally constrained manner. In contrast, scCO2 allows crystals to grow without constraints through a possible crystal-crystal transformation, increasing the processing temperature to 110 deg. C.
Author: Kesavan Ravi
Publisher:
ISBN:
Category :
Languages : en
Pages : 198
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Book Description
Recent developments showed polymer coatings to be feasible by cold spray (CS) technique on different surfaces. This is especially important for Ultra-High Molecular Weight Polyethylene (UHMWPE) which cannot be classically processed. But the mechanisms behind coating formation was not largely understood. The thesis presents a mechanistic understanding of high strain rate impact behavior of Ultra-High Molecular Weight Polyethylene and the mechanism of coating formation during CS. The coating formation is first broken down into two major categories: 1. Interaction of UHMWPE with Al substrate (impacting particle-substrate interaction) during a high-speed impact and interaction of UHMWPE with already deposited UHMWPE particles (impacting particle-deposited particles) leading to a buildup in the coating. First stage of coating formation was understood from a technique developed for this work called Isolated Particle Deposition (IPD). In the experimental IPD process, effects of gas temperature and FNA content were calibrated empirically by depositing UHMWPE particles in an isolated manner on an Al substrate. The Deposition efficiency increased with gas temperature and FNA content. The use of an ultrafast video-camera helped to determine the particle velocity, and theoretical calculations helped to evaluate the temperature of UHMWPE particles before and during the impact process. Mechanical response of UHMWPE at different temperatures were understood by calculating elastic strain energy of UHMWPE which decreased with increasing material temperature and increased with the strain rate. Rebound of UHMWPE particles on Al surface depended upon whether UHMWPE particles after impact furnished a contact area with an interfacial bond stronger than elastic strain energy of the particle. External contributions like H-bonds on the FNA surface provide sufficiently strong extra bonds at the contact surface to increase the window of deposition at higher temperatures, which was otherwise very low. Second stage of coating formation was understood from the mechanism of welding of UHMWPE grains at different interfacial loading conditions and at varying FNA contents. The morphological and mechanical characterization showed that when UHMWPE was processed under high loading conditions (using classical sintering technique), FNA particles reinforced the UHMWPE interface. On the contrary, when UHMWPE was processed under low loading conditions, FNA particles weakened the interface. Last to be discussed in the thesis is the strain rate effect of UHMWPE using Split-Hopkinson Pressure Bar (SHPB) experiments, in order to approach comparable conditions to what happens during particle impacts. This part of the study discussed in detail the effects a high strain-rate compression has on UHMWPE by analyzing its stress-strain curves, with and without FNA. Thus, the mechanical response data with the inclusion 0%, 4% and 10% FNA to UHMWPE is also presented and discussed.
