A structural investigation of the sintering of raw polytetrafluoroethylene

Kostromina, S.V.; Zubov, Yu.A.; Shirina, N.G.; Tomashpol'skii, Yu. Ya.

doi:10.1016/0032-3950(90)90033-3

Cited by 5 publications

(5 citation statements)

References 12 publications

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“…10°C min −1 (though at different cooling rates). As reported by many authors [9, 26, 27], the endothermic peak, at 344°C, associated to the first melting of the original crystalline phase present in the as‐received material strongly differs from the peaks, observed at about 327°C and, associated to the melting of the crystalline phase formed after the first or any subsequent melting. Besides, as it is well known from PTFE specialists, the area below these peaks proves that the crystalline weight fraction in the as‐received material is much higher than the crystalline weight fraction in the sintered material.…”

Section: Results and Analysismentioning

confidence: 68%

Experimental identification of the deformation mechanisms during sintering of cold compacted polytetrafluoroethylene powders

Canto

Schmitt

Carvalho

et al. 2011

Polymer Engineering & Sci

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The influence of the cooling rate after melting on the crystalline weight fraction of pure poly(tetrafluoroethylene) (PTFE) was derived from differential scanning calorimetry tests. Specimens made of pure or filled PTFE were manufactured by isostatic or constrained uniaxial cold pressing, up to different values of the porosity. Dilatometry tests were carried out on these specimens while imposing various heating–cooling treatments. The deformations recorded during these tests strongly depend on the mode and level of compaction. The original procedures presented, herein, made it possible to identify that these macroscopic deformations, which are strongly anisotropic for uniaxially pressed specimens, result from the combination of different independent mechanisms. The mechanisms that were distinguished are reversible thermal expansion, void closure, crystalline to amorphous and vice versa transformations, and recovery, which were not observed for isostatically pressed specimens. It was also shown that, after being subjected to a standard sintering treatment, anisotropically pressed specimens still exhibit anisotropic crystallization strains. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers

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Section: Results and Analysismentioning

confidence: 68%

Experimental identification of the deformation mechanisms during sintering of cold compacted polytetrafluoroethylene powders

Canto

Schmitt

Carvalho

et al. 2011

Polymer Engineering & Sci

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“…The crystallinity of N-0-0, R-360-4 and F-360-4 is 78.0, 52.5 and 24.2%, respectively. PTFE is qualified as a semi-crystalline material, 10 and its molecules align in the composition of folded-ribbons forming lamellae crystal, [26][27][28] therefore, the obtained results could be interpreted from an angle of PTFE crystal. Starkweather 29 suggested that the transformation from extended chain crystal (ECC) in virgin PTFE into folded chain crystal (FCC) in molten PTFE (which is kinetically more accessible) could result in a lower melting point.…”

Section: Effect Of Sintering Mannermentioning

confidence: 82%

“…32 Following the rise of T S , the chains were more entangled with each other and crystallites became more randomized in orientation that enabled an amorphous phase formation during sintering, particularly above melting point of virgin membrane. 27,33 Formation of amorphous phase will surely decrease the crystallinity degree, so as a result, F-340-4 had dramatically higher value than other samples as shown in Figure 8(b).…”

Section: Effect Of Sintering Temperature and Durationmentioning

confidence: 97%

Sintering process investigation during polytetrafluoroethylene hollow fibre membrane fabrication by extrusion method

Jia

Kang

Zou

et al. 2017

High Performance Polymers

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In this study, the effect of sintering conditions including manner, temperature and duration on properties of polytetrafluoroethylene (PTFE) hollow fibre membrane fabricated by extrusion method was intensively investigated. Different from un-sintered and relaxed sintered, the fixed sintered PTFE hollow fibre membrane was observed to generate a uniform ‘fibril–node’ porous structure and a main crystal transformation to folded chain crystal with smaller size. Consequently, it was found that for fixed setting sintering, both temperature increase from 340°C to 400°C and duration prolongation obviously improved pore size, ethanol permeation performance and mechanical strength. Additionally, the test results revealed that the membrane sintered below virgin melting point (350°C) had a noticeable higher porosity but poorer ethanol permeation performance that could be primarily attributed to increased ratio of closed pore. The sintering condition exhibited evident influence on PTFE hollow fibre membrane thermal stability, though it showed no alteration to the thermal decomposition of PTFE. The obtained PTFE hollow fibre membrane was tested to evaluate their vacuum membrane distillation (VMD) performances. It was found that PTFE membrane from lower sintering temperature delivered a better salt rejection; on the other hand, the permeate flux was improved by increased vacuum pressure during VMD operation.

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“…Isso inviabiliza processos convencionais de extrusão e injeção no estado fundido, de forma que um dos principais métodos de transformação deste material é a prensagem a frio (tipicamente a 35 MPa), seguida de sinterização sob pressão atmosférica [1] . Após a polimerização, o PTFE apresenta grau de cristalinidade entre 89 e 98% [2,3] e ponto de fusão de aproximadamente 342 ○ C. Após passar por uma primeira fusão, durante a sinterização, o ponto de fusão do PTFE cai para valores ao redor de 327 ○ C [4,5] . De acordo com Kostromina et al [4] , tal alteração estaria relacionada a uma redução na perfeição da fase cristalina após a primeira fusão.…”

Section: Strabelli P G Et Al -Influência De Variáveis De Sinterizunclassified

“…Após a polimerização, o PTFE apresenta grau de cristalinidade entre 89 e 98% [2,3] e ponto de fusão de aproximadamente 342 ○ C. Após passar por uma primeira fusão, durante a sinterização, o ponto de fusão do PTFE cai para valores ao redor de 327 ○ C [4,5] . De acordo com Kostromina et al [4] , tal alteração estaria relacionada a uma redução na perfeição da fase cristalina após a primeira fusão. O grau de cristalinidade também é diminuído após a primeira fusão, ficando entre 30% e 70%.…”

Section: Strabelli P G Et Al -Influência De Variáveis De Sinterizunclassified

Influência de variáveis de sinterização na microestrutura de peças de PTFE moldadas por prensagem isostática

et al. 2014

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Resumo: O politetrafluoretileno (PTFE) é utilizado em extensa gama de aplicações críticas devido à sua excelente resistência química e térmica, baixa energia superficial e propriedades tribológicas. Devido à sua elevada viscosidade no estado fundido, o PTFE não pode ser transformado pelos métodos tradicionais de extrusão e injeção, sendo o principal método de transformação deste polímero a prensagem sob temperatura ambiente, seguida de sinterização a temperaturas acima do ponto de fusão. O tempo de sinterização é definido de acordo com as dimensões da peça fabricada, podendo variar de poucas horas até vários dias para peças de grande volume. Avaliações sobre a influência dos extremos de tempo e temperatura de sinterização, atualmente adotados na indústria, sobre a microestrutura cristalina do PTFE, são escassas na literatura científica, sendo o principal objetivo deste estudo. Placas em PTFE foram sinterizadas variando-se a temperatura entre 360 ○ C e 390 ○ C e o tempo entre 10 e 10.000 min. Calorimetria exploratória diferencial (DSC), medidas de perda de massa e de densidade e microscopia eletrônica de varredura (MEV) foram utilizadas. Os resultados das medidas de perda de massa indicaram que a degradação do PTFE aumenta com o tempo e temperatura de sinterização. Análises das entalpias de fusão e medidas de densidade apontam indiretamente a redução na massa molar e aumento no grau de cristalinidade com o aumento do tempo e temperatura de sinterização. As análises em MEV possibilitaram a observação direta da microestrutura cristalina, indicando uma tendência de aumento da largura das lamelas com o tempo e temperatura de sinterização. Os resultados obtidos podem auxiliar no controle da microestrutura do PTFE durante o processamento, o que é bastante útil para a fabricação de peças em PTFE com desempenho otimizado. Palavras-chave: PTFE, sinterização, degradação térmica, cristalinidade. Effects of Sintering Variables on the Microstructure of Isostatically Pressed PTFE PartsAbstract: Polytetrafluorethylene (PTFE) is used in a wide range of critical applications because of its outstanding chemical and thermal resistance, low surface energy and special tribological properties. Because of its high melt viscosity, PTFE cannot be processed by traditional extrusion and injection methods. Cold pressing followed by sintering above its melting temperature is the main processing technique. The sintering time is defined according to the dimensions of the manufactured part and may last from a few hours to several days for large parts. Studies of the influence of sintering time and temperature on the PTFE crystalline microstructure are scarce in the scientific literature, being the main objective of this study. PTFE plates were isostatically pressed and sintered at temperatures from 360 ○ C and 390 ○ C for periods of times between 10 e 10,000 min. Differential Scanning Calorimetry (DSC), measurements of mass loss and density and Scanning Electron Microscopy (SEM) were performed. The results of mass loss indicated that degradatio...

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A structural investigation of the sintering of raw polytetrafluoroethylene

Cited by 5 publications

References 12 publications

Experimental identification of the deformation mechanisms during sintering of cold compacted polytetrafluoroethylene powders

Experimental identification of the deformation mechanisms during sintering of cold compacted polytetrafluoroethylene powders

Sintering process investigation during polytetrafluoroethylene hollow fibre membrane fabrication by extrusion method

Influência de variáveis de sinterização na microestrutura de peças de PTFE moldadas por prensagem isostática

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