Material of the month - Polytetrafluoroethylene

Materials World magazine
,
27 Nov 2012

Polytetrafluoroethylene, commonly referred to as PTFE or the trade name Teflon, is a useful polymer with fascinating chemical and physical properties.  

Yet, PTFE was not created intentionally. It was discovered in 1938 by Roy Plunkett while he was working with tetrafluoroethylene (TFE) at DuPont. On one occasion he noticed that a gas cylinder seemed to have no gas inside, despite weighing more than an empty cylinder. He cut open the cylinder to find a white, waxy powder at the bottom. The TFE had polymerised to form PTFE. It was suspected that this was due to additional polymerisation – whereby monomer molecules attach onto each other one at a time to make the chain structure of the polymer – and this was later confirmed. The polymer was named Teflon. 

A monomer of TFE consists of two carbon atoms bonded together with a double bond and two fluorine atoms attached to each of the carbon atoms. The polymer consists of a chain of carbon atoms bonded together by single bonds and two fluorine atoms attached to each of the carbon atoms. 

PTFE has the same structure as polyethylene except that fluorine atoms replace all the hydrogen atoms and this gives it enhanced properties. Fluorine is the most electro-negative of all the atoms with the highest tendency to attract electrons, making the carbon-fluorine bonds polar. The electrons are held so tightly to the fluorine atoms that PTFE does not conduct electricity, making PTFE an excellent choice of material for wire insulation. The fluorine atoms are held close to the carbon backbone. This prevents the carbon and fluorine from reacting with other materials, so PTFE will not combine with oxygen and burn or corrode. It is therefore used in chemical storage containers, pipes and valves. 

PTFE is best known for its non-stick properties. Indeed, Ronald Reagan was known as the Teflon President because controversies did not adhere to him and tarnish his public image. The inertness of PTFE means that it has to be mechanically applied to cooking pots. This is why abrasive sponges can easily damage the non-stick qualities of Teflon pans. 

Lubricity is another important property of PTFE. The material has a coefficient of friction against polished steel of 0.05, making it suitable for use in bearings and as an externally applied lubricant, often in an aerosol spray. PTFE is also resistant to Van der Waals forces, making it one of very few surfaces that geckos are unable to stick to. 

PTFE chains are long, straight and stiff so the chains can pack together tightly, giving it the highest density of any plastic – 2g/cm3 and a high resistance to changing temperature. PTFE remains unchanged close to -286°C and does not begin to melt until 327°C. This is ideal for high-temperature applications but makes PTFE difficult to process by normal thermoplastic techniques. This means that PTFE was usually manufactured using techniques such as sintering, which are time-consuming. 

Surprisingly perhaps, PTFE’s tensile strength of 14–36MPa is comparatively low. This is because its chains are straight and unreactive, and have small pendant groups. For similar reasons, its hardness and creep-resistance are quite low. Fibre reinforcements are often used to improve its abrasion resistance, while maintaining its non-stick properties, making it suitable for use in abrasion pads. 

Gore-Tex is a porous form of PTFE invented in the 1970s by Wilbert Gore, his son, Robert Gore, and Rowena Taylor. A layer of Gore-Tex is used in rain jackets between a tough outer layer and a soft lining, replacing the traditional polyurethane layer. The pores of Gore-Tex are too small to let rain water droplets in but big enough to let sweat molecules out. Gore-Tex is used in the preservation of manuscripts, as creases can be removed using steam without the manuscript coming into contact with liquid water.