PTFE powder

The first sample is a reactive poly(tetrafluoroethylene)/polyamide 6 (PTFE/ PA) blend [43]. When mixing PTFE micro-powder and PA in an extruder at about 280°C, relatively large PTFE particles occur in the final product because of immiscibility. By irradiation with electrons in air reactive groups in the PTFE powder are formed. These functionalised particles react with the molten PA in the extruder, and graft copolymers are formed, improving the compatibility of the components. At the same time a decrease in PTFE particle size proportional to the irradiation dose can be observed, and a PTFE/PA compound with better properties is produced. 

Ofher diffusion layer approaches can also be found in the literature. Chen-Yang et al. [81] made DLs for PEMFCs out of carbon black and unsintered PTFE comprising PTFE powder resin in a colloidal dispersion. The mixture of fhese materials was then heated and compressed at temperature between 75 and 85°C under a low pressure (70-80 kg/cm ). After this, the DLs were obtained by heating the mixture once more at 130°C for around 2-3 hours. Evenfually, fhe amount of resin had a direct influence on determining the properties of fhe DL. The fuel cell performance of this novel DL was shown to be around a half of that for a CFP standard DL. Flowever, because the manufacturing process of these carbon black/PTFE DLs is inexpensive, they can still be considered as potential candidates. 

Yu et al. [139] developed a dry-deposition technique for coating the MPL onto a diffusion layer. This method consisted of forcing a mixture of carbon and PTFE powder through a mesh with the help of a vacuum pump located underneath the DL material. Once the mixture passed through the mesh, it was deposited on the surface of fhe substrate (still with the help of the vacuum pump). After this, the DL, with the MPL, was sintered at 350°C in order to melt the PTFL particles and bind all the particles together. Once the thermal stage was completed, the MPL was subjected to a rolling step in order to adjust the total thickness of the layer (MPL and DL). 

The modified supported powder electrodes used in the experiments hitherto described on the anodic activity of CoTAA are out of the question for practical application in fuel cells, as they do not have sufficient mechanical stability and their ohmic resistance is very high (about 1—2 ohm). For these reasons, compact electrodes with CoTAA were prepared by pressing or rolling a mixture of CoTAA, activated carbon, polyethylene, and PTFE powders in a metal gauze. The electrodes prepared in this way show different activities depending on the composition and the sintering conditions. Electrodes prepared under optimal conditions can be loaded up to about 40 mA/cm2 at a potential of 350 mV at 70 °C in 3 M HCOOH, with relatively good catalyst utilization (about 5 A/g) and adequate stability. 

Electron Modification of PTFE Powder and Sample Preparation.262... 

PTFE powder was modified with irradiation doses of 20, 100, 200, 300, 400, and 500 kGy using the electron accelerator ELV-2 from Budker Institute of Nuclear Physics, Novosibirsk, Russia, installed at the Leibniz Institute of Polymer Research Dresden. Figure 8 shows the schematics of the ELV-2. 

Figure 9 shows the processing parameters and steps involved in the modification of PTFE powder with 100 kGy dose. The electron treatment was carried out... 

Fig. 9 Processing parameters and number of steps involved in the electron modification of PTFE powder with a 100 kGy dose...

An optimum processing time of 180 min is required for a complete cycle of 100 kGy dose. A shutdown time of at least 8 h was necessary after every 100 kGy addition to allow sufficient diffusion of oxygen in the PTFE powder. The total time for the whole process from 20 to 500 kGy was approximately 50 h, including the 8 h shutdown intervals after every 100 kGy addition. To achieve 500 kGy, the doses were added to the PTFE powder in 100 kGy steps. These treatment parameters were chosen in order to avoid excess temperature rise, which might favor deactivation of the radical formation, as well as to control agglomerate size and chemical structure via absorbed dose. Further information on the electron accelerator (ELV-2) facility can be found in [11]. 

Rubbers and PTFE powder were first premixed in an internal mixer for 5 min at a temperature of 100°C and at a rotor speed of 50 rpm. Figure 10 shows two different crosslinking routes, i.e., thermally or using electron irradiation. In the case of EPDM, crosslinking was performed thermally and also with electron irradiation. 

Fig. II Electron spin resonance (ESR) spectra of PTFE powder after irradiation...

The particle size distribution suggests that nonirradiated PTFE powder has a broad particle size distribution compared to 500 kGy-irradiated PTFE powder. The nonirradiated PTFE powder shows a characteristic bimodal distribution compared to the unimodal distribution of 500 kGy-irradiated PTFE powder (Fig. 14). This specific bimodal distribution clearly signifies that nonirradiated PTFE powder is mainly composed of bigger agglomerates that tend to reagglomerate. By contrast,... 

Fig. 12 Infrared spectra (1,900-1,700 cm-1) of PTFE powder after modification...

Fig. 13 Mean agglomerate size of PTFE powder as a function of irradiation dose...

Figure 16 shows result of contact angle measurements on PTFE powders having different irradiation doses in comparison to nonmodified (0 kGy-irradiated) PTFE powder. The horizontal line indicates the contact angle of a typical commercial... 

The effects of electron irradiation on the properties of EPDM along with the optimization of PTFE loading for desired properties have been investigated. For preliminary investigations, PTFE powder L100X was modified with irradiation doses of 20, 100, and 500 kGy and then incorporated into EPDM at loadings of... 

It should be noted that AM is only a measure of an apparent crosslink density of compounds. It is beyond the scope of the present work to investigate in detail the effective crosslinking (physical and chemical). However, for a qualitative assessment it can be concluded that the apparent crosslink density decreases or is influenced by the E-beam irradiation of PTFE powder. PTFE500kGy-EPDM composites show much lower AM and hence lower apparent crosslink densities. It can be inferred that the state of cure and crosslinking efficiency are strongly dependent on irradiation dose. Table 3 shows the optimum curing time (f90, time required to reach 90% of the AM) as a function of PTFE loading and irradiation dose for different PTFE-filled EPDM composites. 

No clear and visible interphase can be seen between the two incompatible polymers. Slightly light and dark regions around irradiated PTFE powder are an... 

Figure 20 shows the thermal traces of (a) nonirradiated and 500 kGy-irradiated PTFE powder and (b) the corresponding PTFE0kGy-EPDM and PTFE500kGy-EPDM composites. The crystallization peak of 500 kGy-irradiated PTFE powder shifts to a lower temperature of about 303.5°C. Also, the crystallization onset occurred at lower temperature and continued down to approximately 290°C. These distinct variations in 500 kGy-irradiated in comparison to nonirradiated PTFE powder is due to the E-beam treatment process, which caused degradation of 500 kGy-irradiated PTFE powder. The molecular weight decreases due to chain scission and leads to PTFE macromolecules of different chain lengths. As a result, the crystallization peak occurs at lower temperatures and the crystallization process continues until much lower temperatures in comparison to nonirradiated PTFE powder. 

Fig. 20 DSC cooling scans of the (a) nonirradiated and 500 kGy-irradiated PTFE powders and (b) their corresponding PTFE-EPDM vulcanizates...

Figure 21 shows the tensile strength at break as a function of PTFE loading for different PTFE-EPDM composites containing E-beam-irradiated or nonirradiated PTFE powder, in comparison with the EPDM gum. 

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