Uwe Lappan

Changes in the chemical structure of polytetrafluoroethylene induced by electron beam irradiation in the molten state

Abstract Polytetrafluoroethylene (PTFE) was exposed to electron beam radiation at elevated temperature above the melting point under nitrogen atmosphere and in vacuum for comparison. Fourier-transform infrared (FTIR) spectroscopy was used to study the changes in the chemical structure. The irradiation under nitrogen atmosphere leads to the same structures as described recently for PTFE irradiated in vacuum. Trifluoromethyl branches and double bond structures were detected. The concentrations of terminal and internal double bonds are higher after irradiation under nitrogen than in vacuum. Annealing experiments have shown that the thermal oxidative stability of the radiation-modified PTFE is reduced compared to unirradiated PTFE. The reason are the formation of unstable structures such as double bonds.

Modification of fluoropolymers by means of electron beam irradiation

Abstract High molecular weight polytetrafluoroethylene (PTFE) is transformed to free-flowing micropowder by treatment with electron beams. In case of irradiation in presence of air carboxylic acid fluoride groups are incorporated which rapidly hydrolyze to carboxylic groups in the surface-near regions due to atmospheric humidity. These polar groups reduce the hydrophobic and oleophobic properties so much that homogeneous compounding with other materials becomes possible. In addition to PTFE, copolymers of tetrafluoroethylene with hexafluoropropylene (FEP) and perfluoropropylvinylether (PFA) were modified. In case of identical irradiation conditions, the concentration of carboxylic groups is much higher in FEP and PFA than in PTFE, which is due to the lower crystallinity of the copolymers. Electron beam irradiation of PTFE was performed in vacuum at elevated temperature above the melting point. The changes in the chemical structure were studied. The concentration of CF3 branches was found to be much higher as compared to room temperature irradiation. In a practical test PTFE micropowders functionalized by electron irradiation were compounded with epoxy resins, with polyoximethylene and with polyamides. Such compounds are characterized by very good frictional and wearing behaviour in dry-running tests.

Radiation-induced branching and crosslinking of poly(tetrafluoroethylene) (PTFE)

Abstract The effect of electron beams on poly(tetrafluoroethylene) (PTFE) at elevated temperatures above the melting point on oxygen-free conditions has been studied using differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), Fourier-transform infrared (FTIR) spectroscopy, thermo-gravimetric analysis (TGA) and tensile test. The investigations have shown that the chemical structure and several properties of PTFE are greatly altered by the irradiation. DSC and WAXS indicate that the crystallinity of the PTFE irradiated with high doses is reduced. CF3 side groups and branched structures are assumed to hinder the crystallization. TGA has shown that the thermal stability of the radiation-modified PTFE is considerably lower than that of unirradiated PTFE.

Electron beam irradiation of polytetrafluoroethylene in vacuum at elevated temperature: An infrared spectroscopic study

Electron beam irradiation of polytetrafluoroethylene (PTFE) has been performed in vacuum both at elevated temperature above the melting point of PTFE and at room temperature for comparison. The changes in the chemical structure were studied by Fourier-transform infrared (FTIR) spectroscopy. The formation of double bonds in PTFE was confirmed as a result of irradiation in vacuum. Moreover, trifluoromethyl (CF3) branches were detected. The concentration of CF3 branches was found to be much higher in irradiation at elevated temperature than in room temperature irradiation. The CF3 branches are assumed to cause the reduced crystallinity indicated by the transparency of PTFE specimens irradiated in the molten state.

Modification of polytetrafluoroethylene by electron beam irradiation in various atmospheres

Abstract The changes in the chemical structure of polytetrafluoroethylene (PTFE) induced by electron beam irradiation in vacuum and in ammonia gas atmosphere have been investigated by means of Fourier-transform infrared (FTIR) spectroscopy. Double bonds and trifluoromethyl branches are formed by irradiation in vacuum. The CF3 branching is enhanced distinctly by irradiation at elevated temperature above the melting point of PTFE. After irradiation in ammonia gas atmosphere ammonium fluoride salt was detected in the irradiated fluoropolymers.

Thermal stability of electron beam‐irradiated polytetrafluoroethylene

Polytetrafluoroethylene (PTFE) was subjected to 1 MeV electron beam irradiation in air. The thermal stability and the degradation fragments of the irradiated polymer were studied in dependence on the radiation dose up to 4 MGy by thermogravimetric analysis coupled with mass spectrometry. The TGA results confirm the known decrease in the thermal stability of irradiated PTFE with increasing radiation dose. At the thermal degradation, CO2, HF, and fluorocarbon fragments are evolved from the irradiated samples. CO2 and HF are formed by decomposition of peroxy radicals up to 250°C. In addition, low molecular weight fluorocarbons are desorbed from the irradiated PTFE. At temperatures above 300°C, CO2 is formed by the decarboxylation of radiation-induced COOH groups inside the PTFE.

Identification of new chemical structures in poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether) irradiated in vacuum at different temperatures

Abstract Electron beam irradiation of perfluorinated copolymer poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether) (PFA) was carried out in vacuum at different temperatures between room temperature and temperature above the melting point of PFA. Changes of chemical structure were studied by 19F solid-state NMR and IR spectroscopy. Chain scissions and branching reactions occur simultaneously, which are indicated by formation of CF3, COF and COOH end groups, CF branching points and CF3 side groups. Branching density based on concentration of CF groups increases with increasing temperature. Formation of cross-links is concluded for PFA irradiated in molten state. Perfluoropropyl vinyl ether (PPVE) units are found to be more sensitive to radiation than tetrafluoroethylene units. COF and COOH groups are assumed to result from PPVE degradation.

Insertion of sulfur-containing functional groups into polytetrafluoroethylene (PTFE) by low pressure plasma treatment

Abstract As a first step for the surface modification of PTFE films (100 μm), low pressure plasma treatment with hydrogen (H2) as process gas was carried out in order to create active sites for the subsequently introduced sulfonating agent. The simultaneous insertion of sulfur and oxygen could be reached by using, as a second modification step, either a plasma treatment with SO2 or by immersing the pretreated samples in fuming sulfuric acid (SO3, oleum). The contact angle of the treated samples decreases from 110° (original) to 53° (H2/SO2 plasma) and 74° (H2 plasma/SO3) respectively. The sample ‘H2/SO2 plasma’ shows only at the outer surface layer oxygen and sulfur traces as shown by ESCA, while no evidence for the presence of functional groups in inner layers could be observed by ATR-FTIR spectroscopy. Otherwise, the sample ‘H2 plasma/SO3’ yielded exactly the contrary results: the observed bands in the IR spectra assigned to associated OH and to SO−3 (at 3000–3700 cm−1 and 1056 cm−1 respectively) are very strong but the O/C and S/C ratios given by ESCA were not high enough. We conclude from the different depth of surface penetration of both ESCA and IR spectroscopy that these two modification methods are able to insert functional groups into the polymer surface at different surface layers.

Fractionated crystallization in blends of functionalized poly(tetrafluoroethylene) and polyamide

Blends of poly(tetrafuoroethylene)/polyamide (PTFE/PA) were prepared to combine the good processing properties of PA with the excellent sliding properties of PTFE. For the compatibilizing of the immiscible components the chemical reaction of functional groups of modified PTFE (micro powder produced by electron irradiation in air) and polar PA during a reactive extrusion process was used. The parameter influencing the efficiency of the in-situ reaction between both components were varied. The crystallization and melting behaviour of the different blends was investigated by DSC. In dependence on the degree of compatibilization the phenomenon of fractionated crystallization of the dispersed PTFE component was observed. In this way a qualitative characterization of the dispersity of PTFE in dependence on the functionality of the components and the processing conditions is possible, and therefore an estimation of the efficiency of the in-situ reaction.

SULFONATION OF FLUOROPOLYMERS INDUCED BY ELECTRON BEAM IRRADIATION

Abstract Electron beam irradiation experiments on PTFE films immersed in water were carried out at atmospheric conditions. The samples irradiated in water were thereafter sulfonated in fuming sulfuric acid (SO3). All irradiated materials were characterized by recording IR spectra (ATR and Transmission). As expected, COF and COOH (free and associated) groups could be created on the polymer surface due to the irradiation process. Evidence of the presence of sulfur-containing functional groups in the sample irradiated in water and immersed in SO3 could be observed.