PTFE offers a range of excellent properties but it does come with one disadvantage, due to its flexibility it can creep and become deformed under heavy loads or high pressure.
This is where adding fillers to the compound can be beneficial, offering enhanced mechanical properties including strength and stiffness.
With all fillers there is a trade-off in the sense that some properties are enhanced while others are reduced. The purpose of fillers therefore is to enhance those properties which are important in the application to the detriment of those properties which are irrelevant. As a baseline, let’s start with the properties of standard virgin PTFE:
- Very high melting point
- Very high chemical resistance
- Wide range of operating temperatures
- Very low friction
- High electrical resistivity
- High flexibility
- Very hydrophobic
- Almost non-porous
- Very high tendency to creep
- Poor abrasion resistance
- Poor radiation resistance
The most common fillers and their benefits to PTFE
Glass Fibres
One of the commonly used fillers in PTFE. Addition of glass fibres decreases both tensile yield strength and tensile ultimate strength. However, hardness, compression properties and wear resistance increases. Glass fibres reduce flexibility and creep. In tribological applications, glass filled PTFE abrades the mating component. Not a great material where strong alkali’s are used.
Non Porous Additive (NPA)
As PTFE is a non-stick material, in-organic fibrous fillers (Like Glass fibres) have very poor fibre-polymer interface leading to porosity. Addition of NPA decreases porosity and permeation characteristics.
Carbons and Graphite’s
Typically, it is solid carbon by-product from oil refining process. Carbon increases compression strength and abrasion resistance. Reduces flexibility, creep and electrical resistance. In comparison to glass fibre, carbons are less abrasive to mating part. Carbons are available in different forms: Petroleum Cokes, Pitch Cokes, Hard Carbons, Fibrous forms and every form has different effect to the compound. Soft carbon is less abrasive to mating surface and good choice of material for non-lubricated applications. Carbon fibre is less abrasive still lubricated with water to exhibit very good wear characteristics. Carbon is not good in oxidising environments (acids, bleaches etc)
Graphites are part of carbon family and added to PTFE to improve electrical, thermal and tribological properties. Graphites improves surface finish and reduces friction but does not greatly improve wear resistance. Graphite is often used as a secondary or tertiary filler with other types of carbon and glass. Graphite’s come in different form: Flaky graphite, amorphous graphite, synthetic graphite and vein graphite’s etc. Every form imparts different properties to the compound.
Bronze
Most common metallic filler used with PTFE and typically used with in higher filler content (40-60%). As it is a metallic material, density of blended compound is 1.5 – 2x denser than unfilled PTFE. Bronze is an alloy of copper and tin, hence, it is it is attacked by chemicals. Some Bronze filled PTFEs oxidises during processing temperatures. Special antioxidant alloy grades are available where oxidation is not permissible.
There are many types of bonzes available based on shapes, composition, and particle sizes. However, functionally remains close to each other. Bronze filler significantly improves resistance to deformation under high pressure and reduces creep imparting great extrusion resistance characteristics to PTFE. As it is metallic filler, thermal conductivity of this compound is more than double than unfilled PTFE.
Stainless Steel
Increases strength, temperature resistance and wear resistance. Often used in food industry in aqueous applications for low speed and static seals.
Molybdenum Disulphide (‘Moly’ – MOS2)
MOS2 is a naturally occurring solid lubricant widely compounded with polymers to reduce friction. MOS2 exist in the form of layers and weak interlayer attraction (or repulsion) imparts lubrication characteristics to PTFE. MOS2 is a heavier filler (i.e 50% higher density than that of PTFE) and increases hardness of PTFE. Typically used as a secondary filler as it is used in combination with other filler such as glass fibre or bronze. Improves slip and hardness as well as compressive strength and wear resistance.
Polyimide (PIs)
PIs are class of aromatic heterocyclic amorphous polymers with very high temperature resistance. These polyimides are different to Polyetherimides (PEI’s) as latter ones are melt-processable and PIs are non-melt processable materials. Addition of PIs to PTFE doesn’t majorly affect frictional/slip properties whereas wear resistance increases significantly.
Typically, PIs or PI variants possess higher glass transition temperature (Tg) in the range of 320-400°C allowing hardness to be retained at higher operating temperatures imparting wear resistance to PTFE. With hardness not being too high allows no abrasion to soft counterfaces like Aluminium. Good for dry-running non-lubricated applications. As PI’s are amorphous materials, PI filled PTFE is not resistant to alcohol or oxidising agents and posses hygroscopic nature.
Wollastonite (aka ‘mineral’)
Wollastonite is a naturally occurring mineral (calcium metasilicate) with the chemical formula of CaSiO3. Wollastonite’s are similar to glass fibre but has low abrasion on mating counterface. It is used as primary and secondary filler depending on application requirement.
High Temperature Aromatic Polyester (Ekonol)
Ekonol is a polymeric filler from LCP (Liquid Crystalline Polymer) family. Ekonol type materials are not melt-processable materials as degradation and processing temperatures are very close with very high melt viscosity. Generally, Ekonol is used as a filler to PTFE’s and other polymers. Ekonol imparts good wear resistance without much compromise on friction. Does not abrade soft counterfaces. Typically used in high-speed rotating radial seals.
Polyphenylene Sulfine (PPS)
High performance polymeric filler and economical alternative to Ekonol in certain applications. Often used as a secondary additive to improve binding of PTFE to other fillers. PPS has melting point close to 280°C leading to limitation on high service temperatures.
Polyetheretherketone (PEEK)
High performance aromatic polymeric material with melting point around 340°C. When blended with PTFE, hardness and pressure resistance are increased tremendously.
Aluminium Oxide (Al₂O₃) and Calcium Fluoride (CaF2)
Alumina or Aluminium (III) oxide are naturally occurring mineral. Typical applications of Alumina are abrasive materials, glass manufacturing and refractories. Alumina is an excellent electric insulator and filled with PTFE to achieve excellent dielectric properties for high voltage insulation applications. As alumina is has very high abrasive characteristics, machining of the end components are to be avoided.
CaF2 is an inorganic compound and occurs as fluorspar- main source for Hydrogen Fluoride. CaF2 has good chemical resistance, temperature resistance and adds great dielectric properties to PTFE. Typical applications include switches and high-voltage insulator.
Barium Sulphate (BaSO4)
Naturally occurring metal sulphate mineral filler. These are high density fillers typically used with PTFE for manufacturing chemical resistant gaskets for aggressive environments or to achieve X-ray opaque characteristics.
Nickel
Nickel Filled PTFEs increases thermal conductivity, reduces creep, and has high pressure resistance. This material has radiation resistance/absorption properties often used in nuclear applications.
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