The transverse expansion speed also affects the structure of the PTFE waterproof and breathable membrane. The higher the transverse expansion speed, the greater the porosity, while the average pore size decreases slightly.
During fast stretching, the length of the thicker area of the film is small, while low-speed stretching causes the edges of the film to overextend and cannot effectively stretch the central area of the base tape, so the length of the thicker area is large.
Under different stretching rates, the stretching rate affects the transfer of stress. When stretching at high speed, due to the strain hardening of the material and the rate-sensitive effect, the stress in the thinning zone of the material continues to deform, and the stress rapidly moves toward the base band. Central transmission makes the thickness and microporous structure of the PTFE waterproof and breathable membrane consistent in the transverse direction; while stretching at low speed, both sides of the base tape are first stretched, and the fibrils are pulled out and then elongated.
When the fibril is fully elongated, the stress is transferred to the middle of the base tape, resulting in large pores and thin thickness on both sides of the film.
Therefore, in order to obtain a microporous membrane with high porosity, uniform porosity and thickness, and stable dimensions, the stretching rate is one of the key factors.
Usually a higher stretching rate is required. If the stretching rate is too low, the film will appear uneven and cannot form an effective microporous structure.
At the same time, the transverse stretching rate is also affected by the stretching temperature.
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