PTFE Myths Busted
PTFE can’t be moulded
PTFE does not flow when heated above its melting point so it can’t be melted and injected into a mould to produce the desired end-shape, this is because PTFE attains what is referred to as a gel state - where the material retains its shape. While in gel state, PTFE is soft, but still not completely pliable - making it very difficult to handle.
PTFE has to be cold moulded followed by sintering. This process involves filling a die cavity - usually a simple shape (inner-outer diameter) - and compressing the powder using a hydraulic press. Before using the powder, it must be conditioned above 19°C. Pressures range from 175-1250 KG/cm2.
Due to the high bulk density of PTFE, the resin is compressed to a third of the volume it occupies in the die. So to produce a tube at 50mm height, you would need to fill the die to 150mm. Once compressed, the PTFE is then left to dwell for anywhere between a few hours to a day (depending on the size), before being placed into a sintering oven where the heat finally exceeds the melting point of PTFE, ranging from 360°C to 380°C. At this point the granules melt and fuse together and coalesce to form the final product. The sintering temperature is held for a period of time to allow fusion, coalescence and void elimination to proceed and maximize properties.
PTFE can’t be recycled
Because PTFE is not processed using traditional thermoplastic methods such as injection moulding or extrusion it can generate a large amounts of waste during production. Due to a tendency to shrinkage, billets are generally produced 3% larger than required and most final parts need machining.
PTFE scrap can’t be incinerated as during the process highly-corrosive vapours are released. Waste that contains organic materials will in future all have to be recycled or destroyed in such a way that the materials are destroyed or irreversibly converted into non-hazardous substances. Although Virgin PTFE can now be chemically recycled to base elements, the process is currently too expensive.
Usually, thermoplastics lend themselves easily to scrap recovery. The scrap is either ground back into granules and can be re-melted and used in injection moulding, or it has some basic scrap value, for example road builders sometimes add plastic waste scrap into the tar mixture where it melts and adds some strength. Since PTFE does not melt, it does not lend itself to either of these processes. The only way of recycling PTFE scrap is to convert it into micro-powders, this only applies to virgin PTFE, filled grade scrap can’t be recycled.
There are two methods to recycle PTFE off cuts and machine waste, the first involves the remnants being cleaned and ground so that it becomes reusable like virgin PTFE itself. Alternatively, after preparatory steps such as sorting, cleaning and grinding, the scrap PTFE polymer is degraded to approximately 1% of the original degree of polymerisation by the irradiation using electron beams, gamma rays or thermo-mechanical degradation.
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