Why waterproof and breathable membrane can be used in high temperature environment

Waterproof and breathable membrane PTFE is a highly crystalline polymer, and its raw material crystallinity can reach up to 92%~98%. However, after processing or sintering-cooling, the crystallinity is reduced to no more than 70% of the raw material crystallinity. When PTFE is at 19°C, it has a spatial helical conformation in which every 13 links rotate 180°C. Above 19°C, the number of chain link units required to rotate 180°C increases to 15 because of the distance between each link. The rise is 0.195nm. This change means that there is a significant difference in the hardness of powder particles below and above 19°C. Above 19°C, the bond has a certain degree of angular displacement, which increases as the temperature rises; above 30°C, this energy displacement trend continues to increase until 327°C. This means that above 19°C the particles are softer and easier to deform. This has important guiding significance for the selection of the post-treatment process of PTFE resin, the movement of materials on the assembly line, the design of equipment, the packaging method and the determination of storage temperature. It is of great significance for the use of dispersed PTFE resin after paste extrusion. The temperature used to make films using the stretching method also plays a key role.
When the temperature is below 19°C, the crystal state of PTFE is a triclinic lattice. At 19°C, the crystal form changes, the degree of rotation of the chain segments becomes slightly smaller, and the unit lattice transforms into a hexagonal crystal system. At 30°C, the PTFE crystal undergoes crystallographic relaxation, and the orderly rotation of the bonds changes to random winding. These two temperature points are usually called phase transition points.
The high crystallinity and high molecular weight of waterproof and breathable membrane PTFE resin result in PTFE having extremely high melt viscosity. The kinematic viscosity measured at 380°C is 1010~1011Pa.s. It also has no fluidity at the melting point temperature. Therefore, ordinary PTFE resin cannot be processed by the processing methods usually suitable for melt-processable plastics. Suspended PTFE resin can only be processed by methods similar to powder metallurgy, that is, suspended PTFE resin powder is placed in a mold and compressed and preformed, and the PTFE resin is dispersed. A booster (lubricant) needs to be added to form a paste before the preform can be pushed, and they must be sintered to become finished products.