Composition, structure and physical and chemical properties of PTFE

Polytetrafluoroethylene (PTFE) resin was invented by Dr. Champlain Kate (Plankett) in 1938 and formally put into industrial production by Du Pont in 1950. PTFE has excellent, special physical and chemical characteristics, being widely used in the chemical industry and other related industries. However, due to some inherent defects of PTFE, especially its large cold flow performance and poor processing performance, its application is limited. Therefore, it has become the main research and development direction of PTFE to further study the properties of PTFE, improve its processing performance and develop new PTFE composites.

As for the high crystalline polymer of tetrafluoroethylene (TEF) monomer, PTEF is a white waxy feel thermoplastic. In PTFE, the fluorine substituted hydrogen atoms in polyethylene, because the radius (0.064nm) of the fluorine atom is larger than the hydrogen atom’s (0.028nm), which makes the carbon – carbon chain polyethylene by fully extended planar, zigzag conformation to gradually reverse helical conformation of PTFE. The helical conformation just surrounded outside the carbon chain skeleton, susceptible to chemical attack in PTFE, to form a close completely fluorinated protective layer, which makes the polymer backbone without any outside agent invasion, endowing PTFE with unique solvent resistance, chemical stability and low cohesive energy density; at the same time, the Carbon fluorine bond is very strong, the bond can reach 460.2kJ/mol, far more than the carbon hydrogen bonds (410kJ/mol) and carbon carbon bond (372kJ/mol), making PTFE has better thermal stability and chemical inertness; In addition, the electronegativity of the fluorine atom, and TFE monomers with perfect symmetry and PTFE intermolecular attraction and low surface energy, so that the PTFE has very low surface friction coefficient and good ductility at low temperature, also makes the PTFE creep resistant ability poor, prone to cold flow phenomenon; PTFE’s non branched symmetric backbone structure also makes it a high degree of crystallinity, making the processing of PTFE more difficult . PTFE’s composition and structure decide it has the following characteristics.

High degree of chemical stability. In addition to the molten, alkali metal, fluoride and high fluoride medium (e.g. three chlorine fluoride), the PTFE can withstand the effect of the other all strong acid (including aqua regia, hydrofluoric acid, hydrochloric acid, nitric acid, fuming sulfuric acid and organic acid), alkali and oxidant, reductant and all kinds of organic solvents, which are lower than the sodium hydroxide at 300 degrees Celsius.

Widely use temperature range. PTFE can be used in a wide range of -190 degrees Celsius to 260 degrees Celsius, even in the ultra -low temperature of -260 degrees Celsius ,it is still not brittle, but also can maintain a certain degree of flexibility.

Outstanding function is nonstick. PTFE is a solid material with the lowest surface tension at present, only 0.019N/m, almost all solid materials are not adhered on its surface, only the liquid with a surface tension within 0.02N/m can completely infiltrate the surface.

Abnormal lubrication. Because of the small gravitational attraction between PTFE molecules and the attraction of other molecules on the surface is very small, the friction coefficient is very small, generally only 0.04. With the increasing of load, the friction coefficient will decrease in a certain range. Usually ,PTFE’s coefficient of static friction (f) and load (W) in the presence of =0.178w F – 0.5 relationship, such as in the high speed, high pressure conditions, the coefficient of friction can be less than 0.01, which conforms it is a very excellent self- lubrication material.

Excellent electrical insulation properties. PTFE is a highly nonpolar material, having excellent dielectric properties, and the breakdown voltage is 25 ~ 40kV/mm; maximum resistance, volume resistivity at 200 degrees Celsius is still as high as 1016 psi – cm.

Excellent aging resistance and radiation resistance. PTFE not only can keep size being stable in low temperature and high temperature, retain the same performance under harsh environment, stay away from the invasion from microorganisms under the wet condition, but also possess a high protection to a variety of rays to radiation.

Excellent thermal stability. The melting point of PTFE is 327 degrees Celsius, higher than other general polymer. At 260 degrees Celsius, its fracture strength still maintains at 5MPa (about 1/5 at room temperature), and the bending strength is up to 1.4MPa. It has a very valuable non-flammable prosperty, and its limited oxygen index (L0I) is more than 95, which means it can only be molten in the flame, do not generate droplets, and only can be carbonated ultimately.

Minimal water absorption. The water absorption rate of PTFE is generally 0.005% ~ 0.001%,and its permeability is low.

PTFE has the advantages of the above, which makes it an alternative to other materials can not be anti-corrosion and friction materials. At the same time, the composition and structure of PTFE also have some disadvantages.

Poor mechanical performance. The mechanical strength of TFE is low, only 14 ~ Pa 25M (hard polyvinyl chloride is 35.2 ~ Pa 50M), and it has no back elasticity, its hardness is low either, but the elongation rate is bigger.

Larger linear expansion coefficient. Between -50 ~ 250 degrees Celsius, PTFE linear expansion coefficient of 1.13 * 10-4 ~ 2.16 * 10 -5/ degrees Celsius, which is 13 times of the steel’s, so deformation, cracking and other phenomena are easy to happen when PTFE composites with other materials.

Molding and two processing difficulties. The molding shrinkage rate of PTFE is large, with molding method, it can reach 1% ~ 5% (the rigid poly’s is 0.1% ~ 0.6%), PTFE cannot use the injection molding of plastic used, two processing molding etc.

Small creep resistance, easy to creep, and easy to cold flow. Under the long-term load, the creep of PTFE is larger, and the phenomenon of cold flow is easy to occur. The cold flow phenomenon of PTFE is one of the main reasons that limit its wide application.

Poor wear resistance. PTFE has low hardness, high abrasion, when the load (P) and sliding velocity (V) exceeds a certain condition, the friction loss will be very large, so the application value of PV have certain limitations, such as being bearing when the PV value is no more than 10kg/ (CM – s).

Poor thermal conductivity. The thermal conductivity of PTFE is only 0.24kcal/ (m·h·℃), and it is easy to cause thermal expansion, thermal fatigue and thermal deformation.

Prices are more expensive than other plastic.

These defects of PTFE limit its wide range of applications to a certain extent. At present, the research focus on PTFE in the world is to find the appropriate method to modify it, in order to improve its performance to a certain extent, so as to expand the application in various aspects.

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