The Basic Film-Forming Principle Of PE Breathable White Film
PE breathable white film film forming principle: PE+CaCO3 (masterbatch) --- film forming --- stretching --- breathable film
PE breathable white film is made by adding about 50% of special calcium carbonate to the LDPE/LLDPE polyethylene resin carrier for blending, and then directional stretching at a certain ratio after being extruded into a film. Since polyethylene resin is a thermoplastic material, it can be stretched and crystallized under certain conditions. During stretching, interfacial peeling occurs between the polymer and calcium carbonate particles, and interconnected meandering pores are formed around the calcium carbonate particles. Or channels, it is these pores and channels that give the film its breathable (moisture) function, thereby ditching the environment on both sides of the film.
The Mechanism Of Action Of PE Breathable White Film
When the water vapor concentration on one side of the membrane is greater than the environment on the other side of the membrane, a humidity gradient pressure difference is formed. This provides the basic conditions for gas (steam) convection, and the humidity environment on both sides of the film tends to be relatively balanced due to the formation of convection.
PE breathable white film is used as a water-proof film, which has a barrier effect on liquids. Because there are many channels in the film, the tortuous channels formed by it have a large "length-to-diameter ratio" (L/D), which can be understood as capillaries. Therefore, under the same liquid (such as water) and the same pressure, as long as the liquid column height of the capillary is smaller than the length of the capillary, the liquid can be guaranteed not to leak out.
Therefore, the water barrier property of the film directly depends on the pore size and path length of the PE breathable white film. This performance can usually be measured by water pressure resistance (ΔP). When the water pressure exceeds the water pressure resistance value that the microporous membrane can withstand, water will leak out.
According to the above capillary principle, the water pressure resistance of microporous membrane micropores can be replaced by capillary osmotic pressure ΔP, which can be expressed by the following formula:
ΔP=2σcosα/d
in:
ΔP-osmotic pressure (Pa)
σ-surface tension (N/m)
α-contact angle (degrees)
d-capillary diameter
It can be seen that the water pressure resistance requirements (safety requirements) can be met by designing different micropore diameters and film thicknesses; obviously, the WVTR requirements can also be achieved by changing the micropore diameter and micropore density of the film.
Note: For the
PE breathable white film used in personal hygiene care products, the principle of improving the breathability of the film under the premise of ensuring safety (ΔP value) must be followed.