Polytetrafluoroethylene (PTFE)

Polytetrafluoroethylene (PTFE):

The high thermal stability of the carbon-fluorine bond has led to considerable interest in fluorine-containing polymers as heat-resistant plastics and rubbers. 

The discovery of PTFE by Plunkett in 1938 gave an impetus to the study of fluorine containing polymers.

General Description

In addition to the presence of stable C-F bonds, the PTFE molecule possesses other features which lead to materials of outstanding heat resistance, chemical resistance and electrical insulation characteristics and with a low coefficient of friction.

However the mechanical properties are average.

Characteristic properties are -1

- almost universal chemical resistance.

- insolubility in all known solvents below 300°C,

- high thermal stability

- continuous service temp. range -270 to +260°C

- low adhesion, low coefficient of friction,

Characteristic properties are - 2

- Outstanding electrical and dielectric properties,

- Resistant to stress cracking

- High weather resistance

- Limited use in structural components because of the low modulus of elasticity.

- These properties can be modified by compounding with reinforcements or wear-reducing additives.

Preparation of monomer:

- Synthesis is based on fluorspar (CaF2), sulfuric acid and chloroform (CHCl3).

                                        CaF2 + H2SO4  ----->    CaSO4 + 2HF

- Treatment of CHCl3  with HF yields MCFM, (monochlorodifluoromethane) a gas boiling at -40.8oC.

                                        CHCl3 + 2HF   ----->    CHClF2 + 2HCl

- MCFM may be converted to TFE by pyrolysis by passing through a platinum tube at 700oC.

Structure-Property relationship

- PTFE cannot exhibit the planar zigzag formation of the crystalline regions of the polyethylene macromolecule. 

- The larger fluorine atoms hinder each other so that they can only find space along the C-C backbone in a spiral arrangement. 

Structure-Property relationship

- Below 19°C, one twist comprises 26 Carbon  atoms, above this temperature, 30. This transition causes a change in volume of 1 %. 

- The compact structure leads to exceptionally high chemical and thermal resistance. 

- The intermolecular forces of PTFE are not large.

- Hence the high melting temperature, low mechanical strength & stiffness.

Structure-Property relationship

- C-F bond is very strong (504 kJ mol-1). Because two fluorine atoms are attached to a single carbon atom there is a reduction in the C-F distance from 1.42oA to 1.35oA. 

- Since the only other bond present is the stable   C-C bond, PTFE has very high heat stability, even when heated above its crystalline melting point of 327oC.

Structure-Property relationship

- The high crystallinity & low intermolecular forces render PTFE resistant to all solvents. Only near the crystalline melting region (gel temp.) of 327 °C fluorine-containing liquids such as per fluorinated kerosene act as solvents.

- The properties of PTFE moldings are considerably influenced by the processing conditions and polymer grade. 

- Particle shape and size determine the processability and especially the number of voids in the molding.

- Molded PTFE materials exhibit high toughness; this applies at temperatures down to - 200 °C. 

- The molecular weight influences the crystallinity and hence the physical properties. Crystallinity and pore fraction are, however, also influenced by the processing conditions. The average molecular weight of commercial PTFE grades is 4x105 to 9x106. 

- The degree of crystallinity of the polymer  reaches 94%. After processing, cooling conditions determine the crystallinity of the molding. Slow cooling leads to higher crystallinity. High crystallinity affects the physical properties.