ShopA Brief Discussion on the Creep Properties of PTFE
Jun
Polytetrafluoroethylene (PTFE), widely referred to as “the king of plastic”, boasts a number of noteworthy properties, including exceptional resistance to high temperatures, corrosion, and weathering. This engineering plastic boasts superior comprehensive performance and has found widespread application in a variety of industrial sectors, including aerospace, petrochemical, machinery, electronics, construction, and light textile manufacturing.
The creep property of PTFE is also known as the cold flow property. It can be utilized in the manufacture of PTFE gaskets, raw material tapes, elastic tapes and other products as sealing materials. These products are mainly applied in the field of chemical anti-corrosion. However, the creep property of PTFE imposes some limitations on its application scope.
- Structural features
It is clear that the volume of a fluorine atom is greater than that of a hydrogen atom. The C-F bond is short, which forces the molecular chain to arrange itself into an elongated helix shape. This helical conformation precisely forms a tight fluorinated protective layer outside the carbon chain skeleton in PTFE, thereby preventing external reagents from attacking the carbon atom main chain.
The strong binding and close packing of fluorine atoms and carbon atoms creates a highly rigid molecular chain. The high regularity of the molecular chain also makes PTFE highly crystalline, endowing it with high heat resistance and a high melting point.
The two fluorine atoms connected to each carbon atom are completely symmetrical, making PTFE a completely non-polar polymer and endowing the PTFE material with its excellent dielectric and electrical insulation properties.
Fluorine atoms have been shown to have a shielding effect on the carbon atoms in the framework. The high bond energy of C-F means that the material has a high degree of thermal stability.
The combination of these characteristics, along with the non-polarity and high crystallinity of PTFE, results in excellent resistance to chemical reagents and solvents.
- Creep mechanism
Under long-term loading, PTFE has a relatively large creep and is prone to cold flow.
Polytetrafluoroethylene (PTFE) molecules are characterized by their rigidity. Due to their helical structure, they have a tendency to slip between molecules. Furthermore, the crystalline region of PTFE is plastic rather than rigid, making it prone to sliding when compressed. At the same time, it is important to note that plastic deformation cannot be reversed. Throughout the force application process, plastic deformation is the predominant deformation type when compared to other motion-induced deformations. Therefore, PTFE is susceptible to creep.
The creep of PTFE is affected by factors such as temperature, time and load. Internally, it is also related to crystallinity and molecular weight. The crystallinity corresponding to the rigidity of PTFE is 75% to 80%. Should the crystallinity exceed this range, the creep resistance will decrease further.
When PTFE is rapidly cooled from a molten state, its crystallinity is known to decrease due to insufficient time for the molecular chains to be properly arranged. Within the cooling rate range of 20 to 70℃/h, there is negligible change in crystallinity. When the cooling rate is below 20℃/h, the crystallinity increases. Additionally, under identical cooling conditions, there is a decrease in crystallinity as molecular weight increases.
The majority of creep occurs within 24 hours of loading, after which the deformation rate decreases significantly. The removal of the load will result in plastic deformation. Provided the deformation does not exceed the yield point, approximately 50% of the total deformation can generally be recovered.
- Modification method
The crystallinity of PTFE can be tailored by adjusting the cooling process during sintering, thereby enhancing its creep resistance. However, this method is time-consuming, causes significant waste of energy, and has limited scope for improvement. Therefore, in actual production, the methods that are primarily adopted are filling modification, PTFE resin modification and blending modification.