PE breathable membrane is a new type of waterproof polymer material. From the manufacturing process, the technical requirements are much higher than those of general waterproof materials. Simultaneously, from the quality point of view, the breathable membrane also has the functionality that other waterproof materials do not have features.
PE breathable film is a new material that is rapidly developing in the world. It is called a breathable film. It is widely used in various fields because of its breathable and impermeable characteristics.
1. The structure of PE breathable membrane
The main component of PE breathable membrane is polyethylene, which is a polymer material synthesized from polyethylene. In polyethylene production, a small amount of 4-carbon or 8-carbon a-olefin is added as a comonomer. However, because the amount of a-olefin used is minimal, many of the polyethylene characteristics are still maintained.
Polyethene is a white waxy translucent material, flexible and challenging, and can be divided into low-density polyethylene (LDPE) and high-density polyethylene (HDPE).
LDPE is made from ethylene under high pressure (98-250mpa), with a density of about 0.91-0.92g/m3. HDPE is produced from ethylene polymerized under 1.0mpa pressure, with a higher density, everywhere 0.94-0.96g/ m3.
Polyethene is non-toxic and has excellent dielectric properties. Its glass transition temperature is about -125℃. Polyethene has excellent chemical stability, can resist acid and alkali at room temperature, but is easy to be oxidized by light and heat, and can also undergo photodegradation under ultraviolet light. Polyethene also has excellent mechanical properties. The crystalline part gives polyethylene higher strength, and the non-crystalline part gives it good flexibility.
2. Features of PE breathable membrane
Compared with non-breathable membranes, PE breathable membranes have the following characteristics:
1. It can permeate gas but not water so that it can be used as a waterproof material with moisture resistance;
2. It can appropriately improve the air convection in the user's environment, which is beneficial to skin breathing;
3. Due to its raw materials' characteristics, PE breathable membrane is a new, environmentally friendly material and does not pollute the environment.
3. Process flow of PE breathable film
From the perspective of processing technology, there are two main processing methods for breathable membranes: the flat film casting method and the other is the blown film method, of which the casting method is more used.
The main process flow of the casting method is as follows:
PE raw material + porogen-mixing and metering-extrusion granulation-extrusion casting-stretching and setting-cooling-trimming-coiling.
In the production process of PET breathable film, the stretching process of PE Breathable Film Production Line significantly influences film performance.
During the stretching process, when the binding force between the porogen caco3 particles and the surrounding PE polymer is less than the PE polymer's deformation force caused by stretching, the polymer will separate along the edges of the caco3, resulting in some tiny particles. The cavities can not allow liquid molecules to pass through, but only gaseous molecules, so that the stretched film has the function of waterproof and breathable.
The cavity size can be adjusted depending on the process conditions such as the size, shape, stretching method, and stretching ratio of the caco3 particles.
When the stretching temperature is at a specific value, as the stretching ratio increases, the PE breathable film's void diameter increases, and the air permeability increases, while the stretching performance becomes worse; as the thickness increases, the air permeability decreases.
4. The problems and countermeasures frequently encountered in the production process of PE breathable film.
1 The compound is not strong
The composite strength of PE breathable film is very low or zero, which can not meet customers' requirements at all, resulting in product scrapping. Due to the particularity of the PE breathable film production process, low surface tension, high slip agent content, and surface contamination may lead to weak composites.
Low surface tension
Under normal circumstances, the PE breathable film's surface tension for dry compounding must be above 38 dyn/cm before it can be used.
The practice has proved that if the surface tension value of the PE breathable membrane is lower than 38dyn/cm, it may cause the phenomenon of low composite strength. Below 36dyn/cm, the combined power will be extremely low or zero.
Simultaneously, the composite strength is also related to the thickness of the PE breathable film. If it is less than 30 microns, it can be barely composited in the fashion of 36-38dyn/cm. When the thickness is 30-50 microns, and the surface tension value is 37dyn/cm, the composite structure is considered. Such as OPP/PE, the combined effect can meet the requirements. But when the thickness is greater than 50 microns, it is unusable if the thickness is less than 38dyn/cm.
Here are several sets of data for reference:
The surface tension value of the PE breathable membrane is tremendous when it is just produced, but as time goes by, the surface tension value gradually decreases. Generally, the PE breathable membrane can be stored for 3-6 months, but it can be stored for 3-6 months in summer. The storage period in the humid season is best not to exceed two months.
The practice has proved that the PE breathable film's surface tension value has a very close relationship with its thickness and the surrounding environment. The higher the environmental temperature and humidity, the faster the surface tension value decreases, and the thicker the film, the quicker the decrease.
Therefore, the PE breathable film placement should generally not exceed three months, especially in the rainy season. More attention should be paid to the ventilation and ventilation of the storage if the storage time is too long for some reason, when re-use, in addition to the roll-by-roll inspection, it is necessary to comprehensively consider whether to continue using the composite product according to the purpose of the mixed product, to avoid loss.
Influence of additives
When PE breathable film is formed into a movie, certain additives, such as antistatic agents, slip agents, and antistatic agents, are generally required to be added according to its use.
These additives are not static after film formation or compounding, especially slip agents, which generally use erucamide or oleamide.
There are two reasons for the decrease in composite strength due to slip agents:
① After the PE breathable film is placed for a short time, the slip agent migrates to the PE breathable film's surface to form a skinny dense layer. This thick layer blocks the bonding of the adhesive and the PE breathable film's character, causing the bond to fail to interact with the PE breathable film's surface. PE molecules contact, the initial composite adhesion is shallow, as the curing time increases, the combined strength does not change much, and the combined power always remains low or zero.
A high-slip PE breathable film must be used for special requirements, and it should not be left for too long under normal circumstances. If the PE breathable membrane is used for an extended period for some reason, it is best to place the PE breathable membrane at a temperature of 60-70℃ for more than 8 hours before compounding. At this time, the slip agent will partially fail, so go again at this time. Laminating does not have the problem of low composite strength, but PE breathable membranes no longer have high slip properties.
②The initial composite adhesion is acceptable, but as the curing time increases, the combined strength becomes lower and lower, and a white powdery substance appears between the two layers.
This is because high-slip PE raw materials are used in the production of PE breathable film. Due to the slip agent's high content in the movie, the movement of slip agent molecules in the film intensifies during the curing process at high temperatures (40-50℃). A large number of slip agent molecules migrate to both sides of the film. As time increases, the amount of migration to the surface gradually increases. Its migration destroys the physical bonding process between the adhesive and PE molecules and will form. The weak bond force is destroyed. The longer the time, the stronger the destructive power and the lower the composite strength.
In this case, increasing the curing temperature is generally adopted to accelerate the cross-linking speed of the adhesive so that the reaction speed exceeds the migration speed of the slip agent to compensate for the excessive slip agent in the PE breathable film. Negative impact.
PE breathable membranes are generally not polluted during the production process, but one of them is easily overlooked: the pollution of dyne fluid.
Many PE breathable membrane manufacturers use self-prepared dyne solutions to test the surface tension of the membrane. The dyne liquid is generally a mixture of ethylene glycol monoethyl ether, a colorless and transparent liquid. If it is accidentally contaminated with the guide roller due to its slow volatilization speed, the film will be degraded when passing through the guide roller. The dyne solution invades the PE breathable membrane's surface and is tightly combined with the PE molecules. It does not cross-link with the binder molecules after compounding, resulting in local composite strength of zero. This situation is complicated to find in the pre-inspection.
2 compounds white point.
This phenomenon is straightforward to occur on PE milky white film; generally, the milky white film is added with a certain amount of white masterbatch: concentrated titanium dioxide. Due to the different quality of the color masterbatch, the particle size and hardness of the contained titanium dioxide are different, resulting in various quality of the milky white film produced after compounding and different surface brightness. If the milky white film's surface feels rough and has pitting from the side, most of the products compounded with such a film will have white spots. The usual solutions to this problem are as follows: One is to increase the adhesive amount. The second is to reprint a layer of white ink. No matter which method is adopted, the cost will be increased, so the PE film must be strictly inspected before use.
3 After compounding, the finished product becomes astringent
This phenomenon has occurred in many manufacturers. The automatic packaging machine for the composite product does not run smoothly, or the bag made is difficult to open.
Dry compounding generally uses two-component reactive adhesives, which must cure at a high temperature after compounding to achieve the highest compound strength. The slip agent in the PE breathable film will change due to the change of curing temperature. At a specific temperature, the slip agent will undergo chemical changes and lose its slip effect. The higher the weather, the more significant the difference. The greater the loss, the final product will become astringent and cannot be used.
Therefore, the curing chamber's temperature must be strictly controlled at about 40℃ and cannot be increased arbitrarily. Also, according to the different uses of the product, choose other ingredients to avoid the product's astringency due to improper selection of resin grades.
4 Poor heat seal
After the compound enters the last processor enters the customer's hands, sometimes the local heat-sealing temperature is too high, the heat-sealing is not right, etc., which cannot be saved in severe cases. The reasons for this result include corona breakdown, high slip agent content, and excessive addition of recycled materials.
When the PE breathable film passes through the high-voltage discharge roller during the production process, it may cause a partial breakdown of the film due to various reasons. This phenomenon is often a longitudinal strip, and the position is generally fixed. The surface tension value is usually very high. Because the breakdown hot cover forms a substance with polar groups like the composite surface, this substance does not have heat sealability. Therefore, it will use in bag making or automatic packaging. Poor partial heat sealing occurred. The PE breathable membrane adds a compound surface tension value test item when entering the factory for inspection to avoid this phenomenon.
High slip agent content
Due to the high content of slip agent in the heat sealing layer of the PE breathable film, it will cause a large amount of it to precipitate on the PE breathable film's surface to form a dense layer, which hinders the heat sealing of the PE breathable film.
This may happen if an expired PE breathable membrane is used. Therefore, it is necessary to select the appropriate resin grades according to customers' specific requirements and try not to use dead films for high-slip products. After this phenomenon occurs, it is recommended to use a solvent to wipe the hot cover repeatedly, and the effect will improve.
Excessive addition of recycled materials
To save costs, a certain amount of recycled material is often added to PE breathable membranes' production. The recycled materials are generally regenerated at high temperatures not less than twice. There is a certain amount of impurities. After the membrane is re-made, its heat-sealing performance is much improved. Decrease, the heat-sealing temperature generally increases by 5-10℃. Experiments show that when the added amount of heat-sealing layer reclaimed material is 30%, the PE breathable film's heat-sealing temperature increases by 3-5℃. When the addition amount is 50%, the heat-sealing temperature Raise the temperature of 6-10℃ and generally requires the film manufacturer not to add recycled materials in the heat seal layer as much as possible.
5. Application range of PE breathable membrane
Because of its soft and comfortable physical and mechanical properties, PE breathable membrane has excellent tensile and elongation properties.
Typical application areas are:
- Daily necessities: raincoat, suit cover, eye mask, table cloth, shower cap, shower curtain, water bag, table cloth, etc.;
- Hygiene products: paper diapers, sanitary napkins, medical-surgical gowns, special packaging for medical treatment and food packaging, etc.;
- Packaging supplies: computers, air-conditioning appliances, dust-proof covers, car covers, cosmetic soft packaging, shopping bags, gift bags, file folders, etc.;
- Fashionable packaging: cosmetic bags, high-end stationery, saliva shoulders, wardrobes, fishing bags, handbags, and bags, etc.