Expanded PTFE

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Expanded PTFE (EPTFE) is one of the more innovative variations in the processing and application of PTFE in recent times.

Discovered in the 1970s, the EPTFE process is unique in that it does not require the use of soluble fillers, foaming agents or chemical additives. The product itself is chemically identical to PTFE - except that it constitutes billions of small pores within the structure of an article of PTFE. This gives it new mechanical properties and results in significant material savings.

EPTFE is used to make lightweight, waterproof and breathable fabrics, micro-porous membranes, medical tubes and implants, microwave carriers, industrial sealants and high-tensile fabrics and cords.

The material exhibits excellent dielectric properties and also a drastic reduction in creep which is a weakness of standard PTFE.

Process

The expansion process begins with paste extrusion of fine powder PTFE using typical lubricants or mineral spirits. The lubricant is completely removed by heat - similar unsintered PTFE tape or PTFE tubing. The lubricant-free extrudate, which can be in the shape of a rod, tube, or tape, is the feed for the expansion process.

The expansion process requires heating the unsintered PTFE anywhere from 35-320°C while keeping it restrained in a device capable of stretching it at high rates. The stretch rate can vary from 10% per second to 40,000% per second.

The stretched part is heated to a temperature above 330°C while being held in a restraining device to prevent its shrinkage. After this heat treatment, called "amorphous locking", for a period of time the expanded part is cooled and removed. The optimum temperature for this process is 350-370°C for anywhere between a few seconds to an hour.

Higher stretch rates and temperatures produce a more uniform matrix.

Expansion can be done by uniaxial or biaxial stretching of PTFE. The fibrils are reported to be wide and thin in cross section with a maximum width of 0.1 μm and a minimum width of one or two molecular diameter in the range of 0.005-0.01 μm. The nodes vary in size from less than a micron to 400 μm, depending on the conditions of the expansion.

A sintered PTFE part has a density of about 2.15 g/cm3 and an unsintered unexpanded part has a density of 1.5 g/cm3. The density of an expanded part can be as low as <0.1 g/cm3, with a porosity of 96%. Density and porosity have a linear relationship. Pore size is quite small, less than 1 μm, up to 90% porosity; larger size pores (1-6 μm) contribute to driving the porosity above 95%.

The heat-treated expanded web has a higher permeability than normally sintered PTFE to gases and liquids due to its porous structure. It can also act as semi-permeable membrane by allowing the wetting liquids through while being impermeable to the non-wetting fluids. For example, a gas-saturated membrane in contact with the gas and water will allow gas through and keep water out as long as the pressure of water does not exceed the water entry pressure. Research indicates that above 90% web porosity, air permeability increases drastically while water entry pressure of the web decreases to a fairly small value. There is a range where porosity can be selected to balance the air permeability and water impermeability, which is useful to applications such as clothing.

The table presents a comparison of the properties of expanded and full density (unexpanded) EPTFE. Crystallinity of the amorphously locked PTFE is about 95%, which is significantly above the highest commercially attainable value with unexpanded parts. The most striking improvement is in the tensile strength, which is orders of magnitude over the full density material and has opened new applications for PTFE. Tensile strength of expanded PTFE is calculated for the matrix by multiplying the measured value of tensile strength by the ratio of the densities of the full to expanded PTFE. Flex life and maximum service temperature of expanded PTFE are both higher than those of the full density material.