Molecular Weight Effect on the High Pressure Crystallization of Polyethylene

Yasuniwa, Munehisa; Tsubakihara, Shinsuke; Nakafuku, Chitoshi

doi:10.1295/polymj.20.1075

Cited by 12 publications

(4 citation statements)

References 13 publications

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“…Yasuniwa et al reported that, although the melting point of the PE grades increased linearly with the molecular weight, above 10 6 g/mol, the change was insignificant [42]. The results in our study agree with this finding.…”

Section: Resultssupporting

confidence: 92%

Improved Processability and the Processing-Structure-Properties Relationship of Ultra-High Molecular Weight Polyethylene via Supercritical Nitrogen and Carbon Dioxide in Injection Molding

2017

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Abstract:The processability of injection molding ultra-high molecular weight polyethylene (UHMWPE) was improved by introducing supercritical nitrogen (scN 2 ) or supercritical carbon dioxide (scCO 2 ) into the polymer melt, which decreased its viscosity and injection pressure while reducing the risk of degradation. When using the special full-shot option of microcellular injection molding (MIM), it was found that the required injection pressure decreased by up to 30% and 35% when scCO 2 and scN 2 were used, respectively. The mechanical properties in terms of tensile strength, Young's modulus, and elongation-at-break of the supercritical fluid (SCF)-loaded samples were examined. The thermal and rheological properties of regular and SCF-loaded samples were analyzed using differential scanning calorimetry (DSC) and parallel-plate rheometry, respectively. The results showed that the temperature dependence of UHMWPE was very low, suggesting that increasing the processing temperature is not a viable method for reducing injection pressure or improving processability. Moreover, the use of scN 2 and scCO 2 with UHMWPE and MIM retained the high molecular weight, and thus the mechanical properties, of the polymer, while regular injection molding led to signs of degradation.

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Section: Resultssupporting

confidence: 92%

Improved Processability and the Processing-Structure-Properties Relationship of Ultra-High Molecular Weight Polyethylene via Supercritical Nitrogen and Carbon Dioxide in Injection Molding

2017

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“…From electron microscopic observation, it was previously confirmed that the bulk sample is composed of large ECCs of the order of µm in thickness. 16 Therefore, all samples consist of ECCs. However, the constraining force applied to the samples differs in each case.…”

Section: Resultsmentioning

confidence: 99%

Hexagonal Phase of Polyethylene Fibers under High Pressure

Tsubakihara

Nakamura

Yasuniwa

1991

Polym J

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ABSTRACT:The hexagonal phase of a constrained ultra-drawn polyethylene (PE) fiber (UDPF) was studied in detail. The melting process of the constrained UDPF was examined at atmospheric pressure by DSC and X-ray measurement. The hexagonal phase appears prior to melting in the constrained UDPF. The phase diagram showing the pressure change of the hexagonal phase was determined by high pressure DT A up to about 600 MPa. The temperature range of the hexagonal phase increases with pressure and coincides with that of so-called "high pressure phase" (HPP) of it above about 300 MPa. That is, the hexagonal phase of the constrained UDPF is the same as HPP. The phase diagrams of three kinds of PE samples, two constrained UDPFs and a bulk sample composed of extended-chain crystals, determined by high pressure DT A, were compared with one another. The constraining force applied to these PE samples considerably raises the melting temperature of HPP, while it has almost no effect on the transition temperature from orthorhombic phase to HPP. By constraining force, HPP of UDPF appears even in the low pressure region that includes atmospheric pressure.

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“…The influence of pressure on the morphology of polyethylene crystals has been of special interest, and studies have been reported up to ∼800 MPa. − These studies have reported that the polyethylene crystals that are formed under pressure either from melt or from solutions may display different morphologies and/or display multiple melting transitions. The two common explanations for the multiple peaks that are given in the literature are based on the notions that (a) under high pressure, metastable crystal structures form which then undergo rearrangements during heating in differential thermal analysis (DTA) or differential scanning calorimetry (DSC) analysis, or that (b) folded-chain crystals undergo recrystallization.…”

Section: Introductionmentioning

confidence: 99%

“…Yasuniwa et. al . investigated the high-pressure crystallization of polyethylene samples of different molecular weights (in the range from 4 000 to 2.5 million) in a high- pressure DTA at pressures up to 590 MPa and temperatures up to 430 K. The DSC curves for samples with molecular weights of 4 000, 10 000, and 67 000 showed three peaks; the highest peak was associated with the thickest lamellae that is representative of the extended-chain crystals.…”

Section: Introductionmentioning

confidence: 99%

Phase Boundaries and Crystallization of Polyethylene in n-Pentane and n-Pentane + Carbon Dioxide Fluid Mixtures

Upper

Zhang

Beckel

et al. 2006

Ind. Eng. Chem. Res.

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The liquid−liquid and fluid−solid phase boundaries and the densities of 5 wt % solutions of polyethylene in n-pentane and in n-pentane (85 wt %) + carbon dioxide (15 wt %) fluid mixtures were determined over a temperature range from ∼360 to 400 K at pressures up to 52 MPa. The measurements were conducted using a variable-volume view cell equipped with sensing elements to monitor the changes in the internal volume and in the transmitted light intensity as the pressure or the temperature of the system is changed. Polyethylene crystals were formed at selected temperatures below the fluid−solid phase boundary along a series of selected constant-pressure paths. They were analyzed by scanning electron microscopy (SEM) and by differential scanning calorimetry (DSC) at ambient conditions for differences in morphological features and thermal properties. Depending on the temperature, the pressure, and the crystallization time, crystallinity levels changed from 65 to 80 %. The crystals that are produced from these high-pressure solutions all showed multiple melting transitions. The majority of the DSC scans at 10 K/min heating rate show a small melting peak at ∼395 K and two additional, more-distinct peaks in the temperature range from 399 to 403 K. The multiple transitions were, however, observed to collapse to a single melting peak at 404 K in the second heating scans. Microscopic evaluations show that the morphologies are prevailingly dominated by ellipsoid-shaped folded-lamellar units 10−20 μm in longer dimension that agglomerate. Unique, long (50−100 μm) strands of stacked lamellar structures appear to form at ∼40 MPa.

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Molecular Weight Effect on the High Pressure Crystallization of Polyethylene

Cited by 12 publications

References 13 publications

Improved Processability and the Processing-Structure-Properties Relationship of Ultra-High Molecular Weight Polyethylene via Supercritical Nitrogen and Carbon Dioxide in Injection Molding

Improved Processability and the Processing-Structure-Properties Relationship of Ultra-High Molecular Weight Polyethylene via Supercritical Nitrogen and Carbon Dioxide in Injection Molding

Hexagonal Phase of Polyethylene Fibers under High Pressure

Phase Boundaries and Crystallization of Polyethylene in n-Pentane and n-Pentane + Carbon Dioxide Fluid Mixtures

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