Polytetrafluoroethylene (PTFE) is a high molecular weightpolymer, one of the most versatile plastic materials known and useful for a large range ofproducts in the high tech market sector. PTFE is produced from tetrafluoroethylene (TFE).
The most outstanding properties are:
- High thermal resistance.
- High resistance to chemical agents and solvents.
- High anti-stick properties.
- High dieletric properties.
- Low friction coefficient.
PTFE is generally considered a plastomer with highdegree of crystallinity; at 340°c it retains a veryhigh viscosity, thus requiring particular trasformation techniques for manufactoring of finished and semi-finished products.
PTFE can be used in a temperature range from -200°c to +260°c and there are various types:
- MODIFIED PTFE: The molecular polymer structure modification greatly improves properties like deformation, dieletric, clarity, heat sealability and deformability.
- FILLED PTFE: The addition of proper inorganic polymeric fillersimprovers properties like deformation, wear, thermalexpation and conductivity.
THERMAL PROPERTIES OF PTFE
The PTFE has specific thermal properties including:
- THERMAL STABILITY: PTFE is one of the most thermally stable plastic materials. It maintais most of its properties up to 260°c. Appreciate decomposition starts over 400°c.
- TRANSITION POINTS: The most important points of glass transition in the PTFE are: at 19°c there is an incrase in volume and 340°c PTFE changes from its crystalline structure to an amorphous phase ( melting point), whilw still keeping a high viscosity level.
- EXPANTION: The linear thermal expansion coefficient varies with the temperature.
- THERMAL CONDUCTIVITY: The thermal conductivity of PTFE changes whit the temperature variation, however this value remains very low, therefore, PTFE can be cosidered as a good insulator. The addition of selected fillers will change the thermal conductivity value according to specific applications.
- SPECIFIC HEAT: The specific heat can be considered constanti f the temperature range is far from the phase transition value.
BEHAVIOUR IN PRESENCE OF CHEMICALS AND WEATHERING ENVIRONMENTS
PTFE has the following behaviors towards external agents:
- RESISTANCE TO CHEMICALS: PTFE is practically inert to chemicals. It can be attacked only by the chlorine trifluoride and fluorine at hig temperatures and pressures.
- SOLVENT RESISTANCE: Ptfe is insoluble in almost all solvents at temperatures up to 300°c.
- GAS PERMEABILITY: The permeability does not depend only on the thickness an pressure, but also on the PTFE processing techniques.
PHYSICAL-MECHANICAL PROPERTIES OF PTFE
Here are some of the physical-mechanical properties of PTFE:
- TENSILE AND COMPRESSIVE PROPERTIES: These properties are to a large degree influenced by the PTFE processing conditions and the type of polymer. PTFE, however, can be used continuously up to 260°c, while still maintaining a certain plasticity at temperatures nea to absolute zero. The modification of the molecular structure of the polymer (PTFE MODIFIED) improves creep properties.
- FLEXIBILITY: PTFE is quite flexibile and does not break when submitted to stresses of 0.7 MPa according to ASTM D 790. Flexural modulus is between 350 to 650 MPa at room temperature, 2000 MPa at -80°c, 200 MPa at 100°c and 45 MPa at 260°c. Modified PTFE has an improved flex life.
- IMPACT PROPERTIES: PTFE has very high resilience properties at low temperatures.
- PLASTIC MEMORY: If a piece of ptfe is subjected to tensile or compression stressed below the yield point, it will keep some induced stress ( permanent deformation)after the end of mechanical operation. If the piece is reheted it loses all the induced strss and it returns to its original form. This property is known as “plastic memory” which can be very useful in many applications.
- HARDNESS: The hardness, measured according to the metod ASTM D 2240 has values between 50-60 shore D.
- FRICTION COEFFICIENT: PTFE has the lowest friction coefficients off all solid materials, between 0.07 static and 0.05 dynamic.
- WEAR: The wear dependes on the caracteristics of the sliding surface, the speed and loads. The wear is considerably reduced when adding suitable fillers to the PTFe ( SEE FILLED PTFE).
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