We have spent a significant amount of time looking at PTFE as a material, comparing it to other materials and analyzing its uses based on the various properties it exhibits.
However, our focus has been purely on “virgin” PTFE – namely, PTFE in its pure form. In this form, PTFE takes on an opaque-white hue, is best describes as a soft-waxy material and is smooth to the touch.
Compounding PTFE refers to the mixture of PTFE with additives, which would both add and remove certain characteristics from virgin PTFE. It is essentially a mixture of PTFE with other substances – done for the purpose of enhancing one or more of the characteristics of PTFE, so that the compounded material would be a better overall fit to a given application.
Before we delve into the various compounds, let’s look briefly at the process behind PTFE compounds.
In truth – most of the large resin manufacturers (DuPont, Daikin, Solvay etc.) have focused on manufacturing virgin resins and left the compounding to smaller companies – who buy the virgin resin and use it in making their compounds. Although the compounder is very much the owner of the product’s quality – it must be mentioned that the input resin does have a huge impact on the final quality of the compounded grade.
For example – we had procured a large quantity of PTFE+Bronze resin from a Chinese company, only to find that when we moulded large pieces from the resin (in excess of 15-20 Kgs per piece) – the pieces would crack during sintering. When we took it up with the supplier, it became apparent that the base resin was of a poor quality, and unsuitable for large pieces.
The compounding process is usually a proprietary technology of the compounder. However, technical literature will point to one of two ways to compound resin:
1. – Physical blending – a physical process, done in an industrial blender where PTFE and the additives are added in the required proportion. The blended powder is then sifted through a mesh to separate the mixture from ‘lumps’ of PTFE that tend to form during blending. The process is repeated until all lumps are suitable removed.
Blending like this is a tedious process, and requires much iteration. Even when care has been taken, small lumps may still remain which will result in patches of white (assuming the blend is pigmented) on the final product.
– Chemical blending – this is more expensive, but also ensures fewer iterations and more uniformity in the blend. A range of chemicals is available for this process – but the basic principal is to have a liquid aid with a lower surface energy than PTFE. This will allow the pigment to flow in between the PTFE molecules so that even the lumps are suitably coated with the pigment.
However, the finer aspects of compounding are usually learnt only through experience and remain a technology that compounders would not part with easily (understandably!).
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 PTFE – 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.
Graphite is generally used in compounds destined for chemical and mechanical service. Graphite reduces initial wear and provides general strengthening characteristics to the composition. Also, graphite compounds generally display high load carrying capabilities in high-speed rubbing contact applications and exhibits the highest hardness of any of the compounds.
Of all the compounds, we have found Carbon-Graphite to wear out tools the fastest. The same tool that might give 200-300 components if done in virgin PTFE, will only give 15-20 components in Carbon-Graphite.
Bronze is usually mixed in a 40% or 60% ratio. Bronze compounds have higher hardness, lower wear, higher comprehensive strength, better dimensional stability, higher thermal conductivity, lower creep and cold flow than most other compounds.
However, test data shows that bronze compounds are not suited to many electrical applications or to those that involve corrosive service environments.
MoS2 adds substantially to the hardness, stiffness and wear resistance of PTFE resins. It reduces starting friction and has little effect on PTFE ‘s electrical and chemical properties. Generally, only small amounts of molybdenum disulfide are used, most often in conjunction with complementary fillers (usually bronze or glass).
In addition to the above fillers, we have used fillers of ceramic, stainless steel and ekonol. Many branded compounds of PTFE continue to exist (eg: Rulon, Turcite etc) – but a comparison of properties shows that there is little difference between the branded compounds and one of the regular grades.
Ultimately, choosing a compounded grade is a question of application – asking which property needs to be enhanced and which can be foregone (or compromised on). In most mechanical applications, it becomes a trade-off between higher mechanical properties (hardness, wear resistance, creep) and lower coefficient of friction.
It should be noted than very rarely does cost play a huge decider in choosing a compounded grade. While historically, bronze has been most expensive, followed by glass and then carbon-graphite (virgin PTFE has usually been priced around the same level as carbon-graphite) – their properties are so different that the end user rarely sees them as substitutes.