- Poly Fluoro Ltd
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Case Study - Expanded PTFE (ePTFE) Tubing
Among the most challenging processes to master within the polymer space is that of manufacturing ePTFE (expanded PTFE) tubes. ePTFE tubes combine the complexity of making standard PTFE tubes with the complexity of making expanded PTFE. Both PTFE tube extrusion and expanded PTFE manufacture are challenging to make on their own thanks to the peculiarities of PTFE as a material. Combining them only compounds the difficulties.
PTFE tube manufacture
The process of paste extrusion involves mixing a PTFE fine powder with an extrusion aid (lubricant) and then passing it though a die to achieve the final shape. The issue here is that because PTFE does not melt (or more specifically, has no melt flow) it needs to be extruded at room temperature and then passed through a heating system to cure it into its final form. The challenge is working with a dry powder, which when subjected to the high pressures of the extrusion press, starts to behave more like a fluid, but can still not be controlled easily, meaning that dimensional variations, non-concentricity, and material properties can all change depending on various factors that cannot be controlled once the extrusion begins.
Standard mono-axial ePTFE manufacture also starts with extrusion. However, since the end product is usually a tape, the extrusion itself is not as challenging as making a PTFE tube. The extrudate is then passed through a stretching device, which adds heat and force to pull the tape into its final – marshmallow-like- form that allows it to be such an effective sealing element.
One key issue with mono-axial ePTFE tape is that it is prone to splitting. Since the extrusion force only acts in the longitudinal direction, laterally the tapes tend to be weak and spit easily when torn apart sideways. This is a property that can be addressed pre-stretching, but it involves a lot of mechanical manipulation of the material.
Expanded PTFE tube
The same process to make PTFE tubes forms the beginning of the ePTFE tube process. However, unlike ePTFE gasket tape – which has a solid form that can be easily handled – the tube profile is very weak. The slightest pressure on the tube in this raw form will cause it to collapse, after which the tube is effectively useless. Careful handling is needed to ensure that the tube in this ‘green state’ holds its form until the stretching process begins. However, the stretching is itself the bigger issue. Stretching ePTFE involves gripping the tape tightly so that it can be pulled through the starching machine. However, the extruded tube cannot be gripped at all, as even squeezing it lightly between one’s fingers will cause it to collapse.
At Poly Fluoro, we have devised a number of ways to mitigate this problem. Extensive R&D went into understanding what the extruded tube would be able to withstand mechanically and building the right equipment to ensure that the tube passes through the stretching process without collapsing. In this regard, the final properties of our expanded PTFE (ePTFE) tube were the following:
Non-splitting – by creating the right kind of forces on the tube, the final product gains strength in the lateral direction and the tube no longer splits when torn sideways
Porous – like all expanded PTFE, the tubes gain a special kind of porosity, making the tube walls impervious to liquids and dust, but permeable to gases and vapours
Non-kinking – unlike regular PTFE tube, which is prone to kindling when the bending radius is breached, ePTFE tubes do not kink and will allow themselves to be bent and positioned as required
As a chemically inert, corrosion resistant material capable of taking high temperatures, PTFE tubes are highly sought after. However, with the addition of expansion, the tube takes on a whole new dimension and becomes invaluable in applications ranging from fluid control, to electrical insulation, to medical devices and grafts.
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