Breathable PE film is a new type of polymer waterproof material, which is widely used in daily life because of its breathable and impermeable performance. This paper briefly introduces the structure, technological process and characteristics of PE film backsheet.
The main component of breathable PE film is polyethylene, which is a high molecular material synthesized from polyethylene. In the production process of polyethylene, some added a small amount of 4 carbon or 8 carbon α Olefin as comonomer, but due to α Olefins are rarely used, and polyethylene retains many properties.
Polyethylene is a white waxy translucent material, soft and tough, which can be divided into low-density polyethylene (LDPE) and high-density polyethylene (HDPE). LDPE is made of ethylene under high pressure (98~250mpa), with a density of about 0.91~0.92g/ m3. HDPE is made of ethylene polymerized under the pressure of 1.0MPa, and the density is relatively high, about 0.94~0.96 g/ m3. Polyethylene is nontoxic and has excellent dielectric properties. Its glass chemical temperature is about -125 ℃. Polyethylene has excellent chemical stability. It can resist acid and alkali at room temperature, but it is susceptible to light and thermal oxidation. It can also undergo photodegradation under ultraviolet light. Polyethylene also has excellent mechanical properties. The crystalline part gives polyethylene higher strength and the non crystalline part gives it good flexibility.
From the point of view of processing technology, there are two main processing methods for breathable membrane, one is flat film casting method, the other is film blowing method. In China and even Asia, most of the European and American countries use casting method.
The main process flow of tape casting method is as follows: PE raw material + porogen → mixing and metering → extrusion granulation → extrusion tape casting → drawing and setting → cooling → trimming → coiling.
In the whole process, the influence of drawing process on the properties of the film is very important. In the process of stretching, when the binding force between the porogen CaCO3 particles and the PE polymer around it is less than the deformation force of the PE polymer caused by stretching, the polymer will separate along the edge of the CaCO3 particles, resulting in some small cavities. Liquid molecules cannot penetrate these cavities, and only gaseous molecules can pass through. Therefore, the stretched film has the performance of waterproof and breathable. The size of voids can be adjusted according to the size, form, stretching mode, stretching ratio and other process conditions of CaCO3 particles.
When the stretching temperature reaches a certain value, with the increase of stretching ratio, the pore diameter and permeability of breathable PE film increase, and the stretching performance becomes worse; As the thickness increases, the air permeability decreases.
Compared with non breathable membrane, breathable PE film has the following characteristics:
(1) It can pass through gas but not water. Therefore, it can be used as a waterproof material with moisture resistance.
(2) It can properly improve the air convection in the user's small environment and is conducive to skin breathing.
(3) Due to the characteristics of its raw materials, PE film is an environmental friendly material and will not damage the environment.
Breathable PE film has penetrated into all fields of human life. In the production process, it contains a large number of ultra-fine density inorganic fillers (CaCO3), which are treated by tape casting, micro pressing and drawing process to form micropores. The density of the micropore is relatively large, so it has a certain amount of water vapor transmission, only through gas without leakage of liquid. PE film backsheet is a new material developed rapidly in the world. It is called breathing film. Because of its breathable and impermeable characteristics, it is widely used in various fields of human beings. Such as personal care, clothing and textile, medical and health care, agriculture, food packaging and other construction, packaging, outdoor equipment, warm stickers and other fields.