What is PTFE? Polytetrafluoroethylene

PTFE, widely known as Teflon, was accidentally discovered by Dupont Chemist Dr Roy Plunkett in 1938. PTFE is resistant to most of the chemicals and found to be one of the most slippery materials. PTFE is a white solid obtained by polymerizing TFE (Tetrafluoroethylene) monomer. PTFE is the first fluoropolymer which came into existence and has unique material properties and applications. Following PTFE, many other interesting fluoropolymers were introduced to overcome the properties and processing limitations of PTFE. Some of the most used other fluoropolymer materials are: 

• Perfluoro alkoxy (PFA)
• Ethylene tetrafluoroethylene (ETFE)
• Ethylene chlorotrifluoroethylene (ECTFE)
• Polyvinylidene fluoride (PVDF)
• Polyvinyl fluoride (PVF)
• Fluorinated ethylene propylene (FEP)
• Polychlorotrifluoroethylene (PCTFE) 

Chemical Structure of PTFE: 

Fluorine is a highly reactive element with highest electronegativity’s of all elements. The electronegativity of Carbon is significantly lower than Fluorine. As a consequence, the unshared electron pair is pulled towards F from C and resulting in high electron density / Polarity around F.  The fluorine atom being larger does not allow planar zigzag packing in crystallisation resulting in twisted zigzag morphology with F atoms packing tightly in spiral C-C skeleton. Compact interlocking of Fluorine atoms, stronger and stable C-F bonds are the reasons for high heat stability (melting point of 327°C) of PTFE. 

The electron affinity causes F atoms to be negatively charged and expected to have higher intra-molecular and intermolecular forces. However, the dipole moments of neighbouring symmetrical structures cancels the dipole moments and leaving PTFE in a neutral electronic state. And consequently this physical phenomenon leads to low coefficient of friction, low surface energy, high elongation, low strength and low abrasion resistance. Also, electronic balance and molecular neutrality leads to very high chemical resistance, low dielectric constant and high volume or surface resistance. 

In molecular level, PTFE is linear polymer with great molecular weight (Length of Polymer Chains) and Crystallinity level in the region of 90% and after processing the crystallinity is in the range 50-70% depending on the processing conditions.  Even though PTFE is a thermoplastic, because of its high viscosity, PTFE cannot be processed in conventional polymer processing techniques. Hence PTFE is processed by cold shaping operation followed by heat treatment (Sintering), during which polymer particles fuse to form a solid moulding. 

Generally, PTFE is a tough, flexible, non-resilient material of average tensile strength but with great thermal properties and excellent resistance to chemicals and passage of electric current. The coefficient of friction is unusually low and believed to be lower than any other solids.  PTFE is an outstanding insulator over a wide range of temperatures and frequency. The volume resistivity is above 1018 ohm meter with power factor very negligible over a huge range of temperatures.  The chemical resistance of PTFE is outstanding. There are no solvents which could dissolve PTFE at room temperature. The surface of PTFE at room temperature is affected only by molten alkali and fluorine in some cases. 

The properties such as chemical inertness, outstanding weathering resistance, excellent electrical insulation & heat resistance properties and low co-efficient of friction  makes PTFE to be exploited in diverse range of applications such as seals, gaskets, valves, pump parts, wire insulation, insulated transformers, PCB and coatings etc. PTFE is also used in missiles and aircraft where high temperature resistance is required.

Typical Material Properties: 

Some of the properties are listed below. These values to be considered as indicative rather than accurate. 

Material Properties	Unit	Indicative Value
Mechanical Properties
Tensile Strength	MPa	20-35
Tensile Elongation at Break	%	250-450
Tensile Modulus	MPa	700-800
Density	g/cm3	2.14-2.20
Impact Strength	J/m	No Break
Shore Hardness	D	50-56
Thermal Properties
Typical Melting Range	°C	322-342
Service Temperature	°C	-200 to +260°C
Thermal Conductivity	W/m.k	0.24-0.52
CTE (20-200°C)	K-1	~20 x 10-5
Electrical Properties
Relative Dielectric Constant at 50Hz		~2.1
Dissipation Factor Tan δ at 50Hz		~0.00005
Dielectric Strength	kV/mm	50-80
Volume Resistivity	Ohm.cm	~1018
Surface Resistance	Ohm	~1017

Typical PTFE Processing Methods: 

• Cold compression moulding and sintering
• RAM extrusion
• Paste extrusion
• Auto moulding 
• Isostatic moulding 
• Moulding and skiving 

Unique Properties of PTFE:

• Low surface energy and non-stick characteristics 
• Cryogenic and high temperature resistance 
• Very low coefficient of friction 
• Low dielectric constant 
• Electrical and thermal insulation 
• High ductility 
• High inertness 
• High corrosion and chemical resistance 
• Non-Flammable 
• Low compression modulus allowing enhanced sealing at low pressure 
• Easy blendability leading to vast range of PTFE compounds 

Limitations of PTFE:

• Non-melt processable material 
• Low tensile yield strength and modulus (when compared against PEEK, LCP and PPS)
• High wear behaviour in unfilled state 
• Not weldable 
• High creep behaviour 
• Low radiation resistance

To see how Enkidu can help your project send us an email info@enkidupolymers.com or call +44 (0) 333 4445 438