Supercritical CO2 processing and annealing of polytetrafluoroethylene (PTFE) and modified PTFE for enhancement of crystallinity and creep resistance

Sun, Hongliu; Cooke, Robert S.; Bates, W. Douglas; Wynne, Kenneth J.

doi:10.1016/j.polymer.2005.05.134

Cited by 22 publications

(22 citation statements)

References 25 publications

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“…The creep curves produced at all the test temperatures exhibited typical creep behavior for semicrystalline materials subjected to constant stress, which falls into three regimes: (1) instantaneous elastic response; (2) retarded viscoelastic response; and (3) permanent flow response 17. As expected, at low temperatures (up to 40°C), the creep strain increased with an increase in liquid uptake [Figure 6(a,b)].…”

Section: Resultssupporting

confidence: 60%

“…It is well known that the degree of crystallinity significantly affects the mechanical properties of polymeric materials. Earlier research has confirmed that the strength of semicrystalline polymers, including creep resistance, increases with an increase in the crystallinity ratio 17, 24. Therefore, the improvement in creep resistance of high‐liquid content samples [Figure 6(c)] and in the stiffness of the material at high temperatures [Figure 4(b)] can be attributed to a significant increase in the degree of crystallinity of PLA‐based specimens, as in indicated in Table I.…”

Section: Resultsmentioning

confidence: 67%

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Creep behavior of PLA‐based biodegradable plastic exposed to a hydrocarbon liquid

Widiastuti

Sbarski

Masood

2012

J of Applied Polymer Sci

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This study explores the effect of hydrocarbon liquid on creep behavior of polylactic acid (PLA)-based plastic. Evolution of the mechanical properties of the material was investigated experimentally by measurement of creep under tensile load. Tensile creep behavior was studied with a constant load over a temperature range from 30 to 50 C using specimens containing different levels of liquid. It was shown that the hydrocarbon liquid diffusion obeys the Fickian law of diffusion. The viscoelastic properties vary with temperature and these properties dramatically decrease above the glass transition temperature (T g ). Significant decreases in modulus and in the peak of tan d were observed with an increase in liquid concentration at low temperatures. In contrast, at high temperatures, drier material recorded lower storage modulus. However, only small changes of T g were recorded. Dependence of compliance on temperature was observed in the creep test at all levels of liquid content. With respect to drier samples, it was noted that the high liquid content material had a lower rate of increasing creep strain with temperature. Therefore, at elevated temperatures, higher creep strain of dry specimens was observed compared to those with a higher liquid content. The improvement of creep resistance and stiffening of material at high temperatures can be attributed to the significant increase of crystallinity fraction induced by liquid absorption. Understanding the effect of liquid diffusion in conjunction with temperature provides useful information for assessment of the potential use of this biodegradable plastic in load-bearing applications exposed to an organic liquid.

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Section: Resultssupporting

confidence: 60%

Section: Resultsmentioning

confidence: 67%

Creep behavior of PLA‐based biodegradable plastic exposed to a hydrocarbon liquid

Widiastuti

Sbarski

Masood

2012

J of Applied Polymer Sci

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“…In this regard, the crack surface displacement mode plays an important role, that is, the formation of fibrils is more pronounced in mode-I than mode-II (shear) 14 . The creep behavior of PTFE are also affected by the degree of crystallinity 15 , along with the . All of the above-mentioned properties are relevant for the design of PTFE components and parts, which emphasizes the importance of the accurate estimation of the crystallinity degree of PTFE as a function of the heat cycle imposed during sintering.…”

mentioning

confidence: 99%

“…The fact that PTFE processing is complex and costly, along with the significant dependence of the mechanical properties on the degree of crystallinity, has motivated researchers to carry out studies on the characterization and computational modeling of cold pressing 16,17 and sintering 3,18,19 . In several studies reported in the literature differential scanning calorimetry (DSC) was used to investigate the melting [3][4][5]7,16,[20][21][22][23] and crystallization [3][4][5]15,16,[19][20][21][22][23][24][25][26] of PTFE. Lau and co-workers 24 used DSC to measure the heat capacity and phase-change enthalpies of two sets of PTFE samples with different degrees of crystallinity in the temperature interval of -103 o C to 427 o C. The authors do not mention any processing involving pressing or sintering before the analysis, suggesting that as-received powder granules were used.…”

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confidence: 99%

Influence of Surface Crystalline Structures on DSC Analysis of PTFE

et al. 2017

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The physical and mechanical properties of polytetrafluorethylene (PTFE) are greatly dependent on the degree of crystallinity and this is extremely important for the modeling of PTFE processing which is complex and costly. Differential scanning calorimetry (DSC) is one of the most important techniques for the determination of the degree of crystallinity and powder granules of the sample are generally used in the analysis. This procedure provides samples with a high surface-to-volume ratio, resulting in the formation of a considerable number of surface crystalline structures, called warts, along with the bulk crystallization, as shown by scanning electron microscopy. The presence of warts has a significant effect on the PTFE melting enthalpy and thus hinders the correct estimation of the degree of crystallinity of industrial PTFE parts, in which bulk crystallization prevails. In this study, we propose a procedure which does not lead to the formation of warts in the DSC sample and thus allows a more accurate determination of the melting enthalpy (or the degree of crystallinity) of industrial PTFE parts. We demonstrate that samples must be extracted from the core of dense (well-pressed) parts previously sintered in an oven, and the use of powder granules and/or sintering in DSC is not recommended.

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“…To estimate the contribution of ballistic heating in crystallinity enhancement, the untreated PTFE specimens are furnace annealed at temperatures below and above the melting point for various durations, but no significant changes in the crystallinity are observed. It is a well established fact that the crystallinity of PTFE increases with annealing only if the specimens are heated for elongated periods, and are subsequently allowed to cool down at very slow rates 40–43. From these observations, one can assume that in our case, which has a resemblance with quenching after being thermally treated, the contribution of ion beam heating towards crystallinity enhancement is minimal.…”

Section: Resultsmentioning

confidence: 62%

Enhanced Crystallinity of PTFE by Ion Irradiation in a Dense Plasma Focus

Sadiq

Ahmad

Shafiq

et al. 2007

Plasma Processes & Polymers

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Nitrogen‐ion beam pulses emitted from a low‐energy (1.45 kJ) Mather‐type plasma focus device are used for the surface modification of PTFE polymer specimens. The specimens, placed at a fixed position, are implanted using different number of pulses. Raman spectroscopy and XRD are employed to probe the structural changes incurred during the ion implantation. Both techniques indicate that the crystalline order in the specimens increases with increasing the irradiation dose. The crystallinity degree of the irradiated specimens, as measured from the XRD data, is found to enhance monotonically from 40% to about 55%. Possible crystallinity enhancement mechanism of irradiated PTFE specimens via chain scission is discussed.

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Supercritical CO2 processing and annealing of polytetrafluoroethylene (PTFE) and modified PTFE for enhancement of crystallinity and creep resistance

Cited by 22 publications

References 25 publications

Creep behavior of PLA‐based biodegradable plastic exposed to a hydrocarbon liquid

Creep behavior of PLA‐based biodegradable plastic exposed to a hydrocarbon liquid

Influence of Surface Crystalline Structures on DSC Analysis of PTFE

Enhanced Crystallinity of PTFE by Ion Irradiation in a Dense Plasma Focus

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