The gas permeable membrane is an environmentally friendly functional material for breathable (steam) water (liquid, bacteria, dust). The appearance of microporous gas permeable membranes has been more than 20 years old and has developed rapidly in the past 10 years. China is at an early stage in terms of technology, market and applications.

A polyolefin-based gas permeable membrane obtained by adding an inert filler to a PE or PP carrier, forming a film, and stretching. For personal hygiene products, breathable membranes are an upgraded product of ordinary water-repellent membranes. The gas permeable membrane and the ordinary membrane cannot be simply replaced, and the structural design of the final product is essential for the breathable effect of the gas permeable membrane!

Breathable principle of breathable membrane

The so-called gas permeable film is obtained by uniformly adding a functional inorganic product into the polyolefin raw material to cause pores in the film forming process due to high stretching, thereby providing a gas permeable and moisture guiding function; the function of the gas permeable film In short: water-proof, breathable (moisture), the most commonly used PE as a carrier of the gas permeable membrane as an example.

2. The basic film forming principle of gas permeable membrane

Gas-permeable film forming principle: PE+CaCO3 (masterbatch) --- Film forming --- Stretching --- Breathable film

The PE gas permeable membrane is formed by adding 50% of special calcium carbonate to the LDPE/LLDPE polyethylene resin carrier, and extruding into a film to form a certain magnification. Since the polyethylene resin is a thermoplastic plastic material, it can be stretched and crystallized under certain conditions. Interfacial peeling occurs between the polymer and the calcium carbonate particles during stretching, and the intertwined and meandering pores are formed around the calcium carbonate particles. Or channels, it is these pores and channels that impart a breathable (wet) function to the film, thereby communicating the environment on both sides of the film.

3. Mechanism of action of gas permeable membrane

When the water vapor concentration on one side of the film is greater than the environment on the other side of the film, a humidity gradient pressure difference is formed. This provides the basic conditions for convection of gas (vapor), which tends to be relatively balanced due to the formation of convection.

The gas permeable membrane is used as a water-repellent membrane and has a barrier effect on liquid. Because of the many passages existing in the membrane, the tortuous passage formed by the membrane has a large "length to diameter ratio" (L/D) value, which can be understood as a capillary tube. Therefore, in the case of 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 prevented from leaking out.

Therefore, the water barrier property of the film is directly dependent on the pore size and passage length of the gas permeable membrane. This property is usually measured by the water pressure resistance (ΔP), which leaks when the water pressure exceeds the water pressure resistance of the microporous membrane.

According to the above capillary principle, the water pressure resistance of the microporous membrane micropores can be replaced by the capillary osmotic pressure ΔP, which can be expressed by the following formula:

ΔP=2σcosα/d

among them:

ΔP—soak pressure (Pa)

Σ—surface tension (N/m)

——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 breathable membranes used in personal hygiene products, the principle of improving the breathability of the film under the premise of ensuring safety (ΔP value) must be followed.

Technical principle of production of gas permeable membrane

The manufacture of the microporous gas permeable membrane can be divided into a blown film method and a casting method according to different film forming processes; and can be classified into a uniaxial stretching method, a biaxial stretching method, and a partial stretching method according to different stretching processes. The manufacture of gas permeable membranes has higher technical requirements than ordinary PE cast embossed films. In the production process, the following aspects should be given special attention:

1) Selection of raw materials

Resin selection

Stiff breathable film - high density polyethylene; flexible breathable film - low density polyethylene; high tensile breathable film - high strength polyethylene; printing (coloring, positioning cut) - high strength polyethylene

Choice of calcium carbonate

Surface treatment of calcium carbonate: fluidity, extrusion stability, die coking, pinholes, mechanical properties of the film, etc.

Choice of calcium carbonate particle size: tensile properties, moisture permeability, uniformity, processability

The choice of raw materials directly affects the processability of the gas permeable membrane (such as coking time, extrusion stability, thin film fabrication and pinhole defects, etc.) and physical properties (such as gas permeability, water pressure resistance, thermal stability and mechanical properties). Wait). Therefore, it is suitable for customers to consider the physical properties of the film and the matching with the processing equipment.

2) Dehumidification system

The high humidity of the raw materials is the main cause of pores and T film coking in the gas permeable membrane, which will seriously affect the production efficiency and product quality of the gas permeable membrane. Therefore, the production of gas permeable membranes should not only require dehumidification (<250ppm) of raw materials, transportation of raw materials and recovery of burrs, but also strict humidity control requirements;

3) Extrusion thickness control

The stretching has a magnification effect on the deviation of the extrusion thickness, so the control of the extrusion thickness is a prerequisite for ensuring the uniformity of the thickness of the final product;

4) Stretch control

There are two necessary conditions for the manufacture of gas permeable membranes, one is calcium carbonate and the other is stretching. Therefore, stretching is critical in the manufacturing process of the gas permeable membrane. The stretching temperature, the stretching ratio, the stretching rate and the stretching gap are the four elements of the stretching process, and its overall balance has a great influence on the gas permeability, mechanical properties, consistency and appearance of the gas permeable membrane;

5) Heat treatment

By heat treatment, the recovery and relaxation of the rigid polymer segment after stretching is a necessary means to improve the stability of the gas permeable membrane material in the subsequent process and during use.

Gas permeable membrane inspection

1. Water vapor transmission rate WVTR: Water Vapor Transmit Rate

Suggested conditions:

Weight gain method - 40 ° C. 75% ~ 90% humidity

Weight loss method - 40 ° C. 50% ~ 60% humidity

2. Air passing rate AP: Air Permeability ------sce/100cc

3. Shrinkage rate: %/80°C.15min

4, water pressure: mm

Principle of using a gas permeable membrane

1, the final breathable effect, the inherent characteristics of the breathable film is the basis, but the structural design of personal hygiene care products is the key;

2, as a water-repellent material, the safety of the gas-permeable membrane (water-blocking) is the premise, only on the premise of ensuring safety, can talk about gas permeability;

3, "wide - relatively large", "thin - thin, close", "package - relatively closed" type structure and relatively long-term use (such as diapers, pads, etc.) care products, the use of gas permeable membrane material is necessary;

4. Compared with ordinary PE embossed film, the gas permeable film has its inherent shortcomings. Therefore, the design of the product should be good at avoiding the shortcomings and exerting its specialties. For example, the transverse strength (CD) of the gas permeable membrane is low, the color is relatively monotonous, the thermal stability is poor, and the like;

5. The final breathability is proportional to the actual air permeability of the breathable membrane. Cherish and protect the effective ventilation area when designing the product;

6, the final product design should consider the convection of the small environment;

7. The performance of the breathable product on the user is sometimes delayed.

Gas permeable membrane application

Breathable film is used in diapers, sanitary napkins, medical mattresses, etc., and has been in stable demand in Europe and the United States, and the domestic sanitary ware market will mature with the demand for functional and comfort. The demand space is bound to grow larger and larger.