Experimental Identification of the Deformation Mechanisms During Sintering of Cold Compacted Polytetrafluoroethylene Powders (original) (raw)
Related papers
Polymer, 2020
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Simulation of PTFE sintering: Thermal stresses and deformation behavior
Polymer Engineering and Science, 2004
A finite element model has been used to study the sintering process of Polytetrafluoroethylene (PTFE) cylinders in order to predict residual thermal stresses; both solid (rods) and hollow (billets) blocks were studied. The simulation of the process has been performed considering three separate stages: thermal, deformation and stress analysis. For each stage relevant material properties have been determined experimentally. In particular, the deformation behavior of PTFE has been thoroughly investigated by means of thermo-mechanical analysis (TMA). It has been shown that experimental results can be explained considering deformation recovery and orientation effects. Predictions of the model have been compared with experimental measurements performed on real PTFE sintered cylinders. Temperature and deformation distributions determined with the model agree well with experimental data. Fair agreement between predicted and experimentally measured residual stresses is obtained and the influence of cylinder size and applied cooling rate on residual stresses is correctly predicted.
The properties of poly(tetrafluoroethylene) (PTFE) in compression
Samples of DuPont 7A and 7C Teflon (PTFE, poly(tetrafluoroethylene)) were tested in compression at strain-rates between 10K4 and 1 sK1 and temperatures between K198 and 200 8C. Additionally, using a Split-Hopkinson pressure bar, a temperature compression series was undertaken between K100 and 150 8C at a strain rate of 3200 sK1. To investigate the small-strain response, strain gauges were used to measure axial and transverse strain allowing the Poisson ratio to be quantified. As expected, the mechanical properties were found to be strongly affected by strain-rate and temperature. Moduli were found by several methods and the trend, with respect to temperature, lends weight to the suggestion that the glass-transition temperature of PTFE is zK100 8C. The physical properties of the sintered PTFE were measured and the crystallinities measured by several techniques. q 2004 Elsevier Ltd. All rights reserved.
Thermal properties of drawn polytetrafluoroethylene
Journal of Applied Polymer Science, 1989
Polytetrafluoroethylene as polymerized (virgin) was drawn, sintered, and annealed. By thermal analysis the changes in crystal type and crystallinity and melting kinetics was analyzed. I t is shown that this analysis permits control and optimization of the various processes. In addition, a new, high melting polytetrafluoroethylene (654 K) is described. It is assumed to be metastable due to strain in the surrounding amorphous fraction.
Nonisothermal Crystallization of Polytetrafluoroethylene in a Wide Range of Cooling Rates
The Journal of Physical Chemistry B, 2013
Compared to other semicrystalline polymers, PTFE demonstrates a very fast crystallization process on cooling. This study explores for the first time the nonisothermal PTFE ultrafast crystallization under tremendously fast cooling rates (up to 800 000 K•s −1) achieved by using fast scanning calorimetry (FSC) and ultra-fast scanning calorimetry (UFSC). Regular DSC was also used to get crystallization at slower rates. The data obtained on a wide range of cooling rates (over 8 orders of magnitudes) help to get new knowledge about crystallization kinetics of PTFE. Both FSC and UFSC data show that it is impossible to bypass the crystallization and thus to reach a metastable glassy state even for the fastest cooling rate employed (800 000 K•s −1). The crystals formed under such conditions are slightly less stable than those produced under slower cooling rates, as reflected by a shift of the melting peak to lower temperature. The difference in crystal morphologies was confirmed by SEM observations. The variation of the effective activation energy (E α) with the relative extent of crystallization reveals that PTFE crystallization follows a transition from regime II to regime III around 315−312°C. Corroborated temperature dependences of E α obtained respectively for crystallizations under slow and fast cooling rates were combined and fitted to the theoretical dependence of the growth rate derived from the Hoffman−Lauritzen theory.
Kinetics of the 20°C phase transformation in polytetrafluoroethylene
Journal of Polymer Science Part A-2: Polymer Physics, 1972
The rate at which PTFE transforms from the triclinic to the hexagonal phase has been measured under isothermal conditions. Samples in a series of decreasing density and crystallinity show increased isothermal rates of transformation. Observed kinetic data are interpreted on the basis of a modified Avrami-Johnson-Mehl treatment. A model for the transformation in which planes of helix-hand reversal propagate through the lattice is shown to fit the experimental results. The transformation rate is observed to be proportional to the total (001) surface area in the polytetrafluoroethylene specimens, suggesting that nucleation of the transformation takes place at grain boundaries.
Effect of thermal aging on the crystal structural characteristics of poly(tetra fluoro ethylene)
Polymer Engineering & Science, 2007
The residual effects of cumulative thermal aging on the crystal structural characteristics of the fluoro carbon poly(tetra fluoro ethylene) (PTFE) have been studied by X-ray diffraction methods. The initial hexagonal arrangement of the PTFE chains in a 15 7 helical conformation is left unaffected by the exposures to temperatures (T), up to and beyond its melting point, T m . The unit cell registers a residual anisotropic volume expansion. The anisotropy arises from the enhanced enlargement of the basal plane dimension a compared with the axial dimension c. Conformational changes contributing to the observed increase in the chain length have been examined. Enhancement of residual crystallinity of samples aged at T's < T m suggests that the selective thermal aging could be used as an effective tool to improve the initial crystallinity of commercially available PTFE. The activation energy for 50% enhancement in initial crystallinity has been estimated as 53.9 kJ mol À1 . Aging at 4008C, a temperature above T m , is accompanied by markedly different features viz., deterioration in crystallinity and other structural characteristics. The overall behavior of thermally aged PTFE bears a marked similarity to many polyamides. POLYM.