Processing technology that affects the morphological structure of waterproof breathable membrane

1. The effect of longitudinal stretching

The resin passes through the mixing, extrusion and calendering processes and exhibits partial thinning of the film due to the formation of fibrillated features during the calendering process. During longitudinal stretching, the polycrystalline aggregates begin to extend to form fibrils, which are parallel to the stretching direction. The cross-section of the fibrils is wide and thin, and the larger width is about 100 nm, while the small fibrils are 0.5 to 1.0 nm.

2. The influence of horizontal expansion multiples

The transverse stretching stage is to control the microporous structure and thickness of the film. In the transverse stretching process, each part of the film along the transverse stretching direction is not uniformly stretched in the transverse direction, and transverse stretching is a non-uniform stretching process. Compared with the two sides of the film, the stretch ratio of the middle part is smaller, and the uneven transverse stretching results in a small pore size and low porosity in the middle region in the transverse stretching direction of the film, while the pore size and porosity of the two sides are large. The difference of the microporous structure in the transverse direction of the film will affect the waterproof and moisture permeability of the waterproof and breathable membrane and its laminated fabric.

The process conditions such as longitudinal stretching ratio and stretching temperature are fixed, and the transverse stretching ratio is changed. The film porosity and uniform pore size increase with the increase of the transverse stretching factor. At the same time, fibrils parallel to this direction are also present in the transverse stretching direction.

3. Influence of horizontal expansion speed

The lateral expansion speed also has an effect on the film structure. The higher the lateral expansion speed, the larger the porosity, while the uniform pore size decreases slightly. During fast stretching, the length of the thicker area of the film is small, while the low-speed stretching results in excessive elongation of the edges of the film, which 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. During high-speed stretching, due to strain hardening and the rate-sensitive effect, the thickness of the thinned area will increase, and the stress caused by the continuous deformation will increase, and the stress will be rapidly transferred to the center of the baseband. The thickness and microporous structure of the film in the transverse direction tend to be the same;