Polytetra-fluoroethylene membranes

The final principles of back-pulse filter technology are the nature and properties of the GORE-TEX membrane. The membrane is composed of expanded polytetra-fluoroethylene, or e-PTFE. The membrane traces its roots to the invention of e-PTFE by Robert W. Gore in 1969. Since that time, e-PTFE has found application in many areas including medical devices, electronics, fabrics and fuel cells to name a few. In the filtration area, e-PTFE is used in the form of a membrane to capture and remove particles from both gaseous and liquid streams. 

The heart of the SLM extraction procedure is the SLM unit, in which analyte extraction, preconcentration, and sample cleanup take place in one single step. The design of the SLM unit can be engineered according to the intended use of the unit. For instance, if an SLM unit is needed for an automated and flowing sample extraction, it can be manufactured from two blocks of polytetra-fluoroethylene (PTFE), poly vinylidine difluoride (PVDF), or titanium. However, for nonautomated, nonflowing SLM extraction, a short piece of porous HF membrane is used. 

Other techniques for membrane formation include stretching the polymeric film, commonly polytetra-fluoroethylene (PTFE), while it is still in a flexible state and then annealing the membrane to lock in and strengthen the pores in the stretched membrane. The stretching process results into a distinctive membrane structure of PTFE nodes, which are interconnected by fibrils (Fig. 5). 

Polymeric materials for MF membranes cover a very wide range from relatively hydrophilic to very hydrophobic materials. Typical hydrophilic materials are polysulfone (PS), poly ether sulfone (PES), cellulose (CE) and cellulose acetate (CA), polyamide (PA), polyimide (PI), polyetherimide (PEI), and polycarbonate (PC). Typical hydrophobic materials are polyethylene (PE), polypropylene (PP), polytetra-fluoroethylene (PTFE, Teflon), and polyvinylidene fluoride (PYDF). 

During sintering, a powder of particles of a given size is pressurized at elevated temperatures in a preformed shape so that the interface between the particles disappears. Microfiltration membranes can thus be obtained from PTFE (polytetra-fluoroethylene), PE (polyethylene), PP (polypropylene), metals, ceramics, graphite and glass, with pore sizes depending on the particle size and the particle-size distribution. Porosities up to 80% for metals and 10-20% for polymeric membranes can be reached with pore sizes varying between 0.1 and 10 pm. Most of these materials have excellent solvent and thermal stability. 

As explained in a previous section, a membrane electrode assembly (MEA) consists of the polymer membrane that is sandwiched between an anode and a cathode electrode, respectively. The electrodes are composed of a conductive carbon network that supports a catalyst on a gas diffusion layer. An additive, such as polytetra-fluoroethylene (PTEE), helps bind the Pt/C catalyst to the gas diffusion layer. At the anode, the catalyst facihtates the oxidation of hydrogen into its constituent electrons and protons. As the protons are passed through the acid-doped membrane to the cathode, the electrons are passed through an external circuit, thereby creating electricity. Einally, the electrons and protons react with oxygen at the cathode electrode to form water as the final reaction product. 

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