In spite of its remarkable properties pure, unfilled or virgin PTFE is inadequate for a number of more demanding engineering applications.In particular, its cold flow or creep behaviour precluded the use of PTFE in mechanical applications. Even at room temperature, PTFE experiences a significant deformation over time when it is subjected to a continuous load.Also, virgin PTFE has hardly any resilience and wears quickly despite its low coefficient of friction.In the 1960’s, the addition of fillers was found to improve a number of physical properties, particularly creep and wear rate. Most fillers are stable up to 400 °C, so they do not restrict use in high temperatures.Listed below are some common filler elements and their effect on the physical properties of the PTFE compound.
Glass fibre is a commonly used filler element with a positive impact on creep performance of PTFE, which is reduced at low and high temperatures. It also adds wear resistance and offers good compression strength. This additive is chemically inert except in the presence of hydrofluoric acid and strong bases. It has little effect on electrical properties.
Carbon and carbon-graphite
Carbon reduces creep, increases hardness and elevates the thermal conductivity of PTFE. Wear resistance of carbon filled compounds improves particularly in combination with graphite. Carbon is one of the most inert fillers except in oxidizing environments where glass performs better.Carbon provides some electrical conductivity to PTFE. PTFE filled with carbon is typically used in pneumatic sealing applications.Compounds with carbon in combination with graphite are the preferred materials for non-lubricated applications.
The addition of carbon fiber to PTFE changes its properties in the same way as glass fibre does. It lowers creep, increases flex and compressive modulus and raises hardness. In general, less carbon fibre than glass fibre is needed to achieve the same effect.The coefficient of thermal expansion is lowered and thermal conductivity is higher for compounds of carbon fibre filled PTFE. Wear rate decreases when carbon fibre parts are lubricated with water.Carbon fibre is chemically inert and can be used in strong bases and hydrofluoric acid.
Graphite is a crystalline modification of high-purity carbon. Graphite filled PTFE has one of the lowest coefficients of friction due to the low friction characteristics of graphite. It has excellent wear properties, particularly against soft mating surfaces and displays high load-carrying capability in high-speed contact applications. Graphite is chemically inert which enables its use in corrosive media.
Bronze is a common metallic filler. Large quantities (40 to 60 % by weight) of bronze reduce deformation under load and raise the thermal and electrical conductivity of PTFE compounds. These two characteristics are beneficial to applications where a part is subjected to load at extreme temperatures.Bronze filled PTFE offers excellent extrusion resistance characteristics. Bronze is an alloy of copper and tin and can be attacked by certain chemicals. Oxidation of the bronze can result in a discolouration of the finished part without impacting on the quality of the product.PTFE filled with bronze is widely used in hydraulic rod and piston seal applications.
Molybdenum disulfide (MoS2)
Molybdenum disulphide increases the hardness and stiffness of PTFE while decreasing friction. It has little effect on its electrical properties.Chemically, is largely non-reactive and dissolves only in strong oxidizing acids.It is normally used in small weight percentages and in combination with other fillers such as glass fibre.
Aromatic polyester as a filler element raises the high temperature resistance of PTFE and is excellent for use with soft, dynamic mating surfaces.Not recommended for sealing applications involving steam above 120 °C.
It is possible to pigment PTFE by using inorganic colour additives that withstand the sintering temperature of PTFE. Pigments do not significantly change the properties of PTFE. Combinations of pigments and other fillers are possible.Proprietary fillers and filler combinationsParker is continuously striving to search for and develop unique blends and formulations to enhance sealing performance in the most extreme applications.
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