May 8, 2017 at 2:34 am #40010
We have looked at these before from a theoretical standpoint. Some of the properties remain unaffected (or hindered) by the addition of fillers, while others are impacted positively. It would be sufficient to say that compounded grades have to have at least, if not higher temperature resistance than China PTFE Sheet – as else they would not survive the sintering process – which happens at 350-400 °C
In most cases, this is only reduced by the addition of fillers – as PTFE in its virgin form shows exceptional electrical resistance. We have yet to come across an application where a filled grade of PTFE is used for purely insulation purposes
Being a soft material, the addition of fillers can greatly increase the hardness. This is especially sought after in PTFE components – where the softness of the material can lead to deformation in the long run, affecting the overall assembly within which the component is used.
Like with dielectric strength, the dynamic coefficient is usually hindered with the addition of fillers. However, because virgin PTFE exhibits significant creep – there is a case for filled grades in applications with minimum movement where a low static coefficient of friction is required.
The PV value of a compound is the product of the unit load P (MPa) on the projected area and the surface velocity V. The PV of PTFE is usually enhanced by the addition of fillers.
In general, the addition of fillers to PTFE resins improves wear resistance but reduces abrasive resistance by providing discontinuities in the PTFE resin which can be entered by sharp practices that may tear the material.
Unfilled PTFE does not absorb water. Filled PTFE compounds absorb small amounts of moisture. Since PTFE resin and fillers are not hygroscopic, any moisture picked up simply fills the voids. Extent of pickup is so small that the dimensional stability is essentially unaltered.
Again – given PTFE is unmatched amongst other materials in its ability to remain intern to chemicals, adding fillers can only reduce this property. However, it does depend ultimately on the application and whether there is a requirement for such a high level of inertness. Typically however, for applications needing this property (medical, labwares etc.) – virgin PTFE remains the preferred choice.
Now that we have looked at each of the properties, let’s look at some of the standard compounds and see how each compound alters the characteristics.
This is the most universally used PTFE filler and is normally mixed in either 15% or 25% ratios. Glass is itself highly resistant to chemicals and also exhibits very good dielectric properties; add to this the added mechanical properties and creep resistance that it provides and it’s not difficult to see why it is so sought after.
Glass fiber also offers improved wear resistance, but reduces the coefficient of friction. Furthermore, it imposes a higher wear rate on the tools while machining– making it a slightly more expensive material to machine. For the same reason, it is very difficult to ‘skive’ glass filled PTFE tapes to thicknesses of under 0.25mm – as the wear induced on the skiving blade renders the blade dull before a significant length can be skived. PTFE Rod: http://www.liantuoptfe.com/product/
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