lavorazione ptfe

Polytetrafluoroethylene (PTFE) is an ultra-high molecular weight polymer consisting of fluorine and carbon atoms, making it one of the most versatile plastics for uses that other materials are not. PTFE is produced from the monomer tetrafluoroethylene (TFE).

Distinctive features are:

  • High thermal resistance.
  • High resistance to chemicals and solvents.
  • Highly anti-adhesive.
  • High dielectric characteristics.
  • Low coefficient of friction.
  • Non-toxicity.

PTFE is a high-crystallinity plastomer: at 340 °C, it retains a high viscosity, so special processing techniques are required for the production of finished and semi-finished ptfe parts.

It can be used in a temperature range of -200°c to +260°c and there are various types:

  • MODIFIED PTFE: modifying the molecular structure of the polymer improves its deformation, dielectric, transparency, weldability and hot formability characteristics.
  • FILLED PTFE: The addition of appropriate inorganic and polymer fillers improves its deformation, wear, expansion and conductivity characteristics.

THE THERMAL PROPERTIES OF PTFE

Ptfe has specific thermal properties including:

  • THERMAL STABILITY: PTFE is one of the most thermally stable plastics. At 260°c, PTFE still retains most of its characteristics. Appreciable decompositions occur above 400°c.
  • TRANSITION POINTS: The most important transition points of the crystalline structure of PTFE are two: the one at 19°c to which also corresponds a change in volume, and the one at 340°c to which corresponds the transition from the crystalline to the amorphous phase while maintaining a high viscosity.
  • DILATION COEFFICIENT: The coefficient of linear thermal expansion varies with temperature.
  • THERMAL CONDUCTIVITY: The thermal conductivity of PTFE, which has a very limited temperature dependence, has very low values, so PTFE can be considered a good insulator. The addition of appropriate fillers varies the thermal conductivity values according to the required applications.
  • SPECIFIC HEAT: The specific heat can be considered constant if the temperature is far from the phase transition values.

THE BEHAVIOUR OF PTFE TOWARDS EXTERNAL AGENTS

PTFE has the following behaviour towards external agents:

  • RESISTANCE TO CHEMICAL REAGENTS: PTFE is practically inert to almost all known chemical reagents. It is attacked by chlorine trifluoride and fluorine at high temperatures and pressures.
  • RESISTANCE TO SOLVENTS: PTFE is insoluble in any solvent up to a temperature of about 300°c.
  • PERMEABILITY TO GASES: In addition to obviously depending on thickness and pressure, permeability also depends on PTFE processing techniques.

PHYSICAL-MECHANICAL PROPERTIES OF PTFE

Here are some of the physical and mechanical properties of PTFE:

  • STRENGTH AND COMPRESSION RESISTANCE: PTFE can be used in continuous operation up to 260°c, while at temperatures close to absolute zero it still possesses a certain plasticity in compression. Modifying the molecular structure of the polymer, MODIFIED PTFE, improves its compressive strength.
  • FLEXURE RESISTANCE: PTFE is relatively flexible and does not break when stressed to 0.7 MPa according to ASTM D 790. Flexural modulus of elasticity is around 350-650 MPa at room temperature, around 2000 MPa at -80°c, around 200 MPa at 100°c and around 45 MPa at 260°c.
  • SHOCK RESISTANCE: PTFE possesses high resilience characteristics even at low temperatures.
  • PLASTIC MEMORY: If a piece of PTFE is stressed in tension or compression, below the yield strength, part of the deformations caused remain when the stresses cease (permanent deformations) so that stresses are induced in the piece. If the part is heated, these stresses tend to be released and the part resumes its initial shape. This property of PTFE is commonly referred to as ‘plastic memory’ and is exploited in various applications.
  • HARDNESS: The hardness measured according to ASTM D 2240 has values between 50 and 60 Shore D.
  • COEFFICIENT OF Friction: PTFE has, of all solid materials, the lowest coefficients of friction between values of 0.07 static and 0.05 dynamic.
  • WEAR: Wear depends on the characteristics of the antagonised surface and on the speed and loads. Wear is significantly reduced by charging the PTFE with appropriate fillers (LOADED PTFE).

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