The process is simple. Why can't the produced melt blown cloth reach the standard of 95 and 99?

- May 02, 2020-

The process is simple. Why can't the produced melt blown cloth reach the standard of 95 and 99?

 

The key factor of melt blown cloth not reaching 95 standard: melt blown cloth processing technology!

 

Traditional process flow

Polymer feeding one → melt extruding one → fiber forming one → fiber cooling one → net forming one → bonding (solid mesh cutting edge winding one → post finishing or special finishing).


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One

Polymer feed

The polymer is generally made into small spherical and granular slices, which are poured into the barrel or hopper and fed into the screw extruder.

 

Two

Melt extrusion

At the feeding end of the screw extruder, the polymer chips should be mixed with stabilizers, brighteners and other additives as well as color masterbatch and other necessary raw materials. After full mixing, the chips will enter the screw extruder and be heated to melt. Finally, the melt will be sent to the spinneret by the metering pump through the filter.

 

Three

Fibrogenesis

The filtered clean melt shall pass through the distribution system, and then be evenly fed into each group of spinnerets, so that the extrusion amount of each spinneret hole is the same. The spinneret of melt blown fiber is different from other spinning methods. The spinneret holes must be arranged in a straight line with high-speed air jet holes on both sides.

 

Four

Fibre cooling

A large amount of room temperature air is inhaled at the same time on both sides of the spinneret, which is mixed with the hot air stream containing microfiber to cool it down and the melted microfiber is cooled and solidified.


Five

Net forming

In the production of melt blown fiber nonwovens, spinnerets can be placed horizontally or vertically. If it is placed horizontally, the microfiber will be sprayed on the circular collecting roller to form a net; if it is placed vertically, the microfiber will fall on a horizontally moving screen to form a net.

 

Six

Bonding (fixed network)

As mentioned above, the self bonding reinforcement is enough for the melt blown cloth for some purposes, such as: the fiber mesh is required to have a relatively fluffy structure, good air retention or void ratio, etc. However, for many other applications, it is not enough to have self-adhesive reinforcement alone, but also needs hot-rolled bonding, ultrasonic bonding or other reinforcement means.

 

Technological characteristics of melt blown fiber nonwovens

(1) Short technological process and high production efficiency;

(2) the fiber is very fine, the uniformity of the fiber net is good, the handle is soft, and it has outstanding advantages in filtration, antibacterial and adsorption;

(3) the fiber orientation is poor, and the strength of the web is low;

(4) High energy consumption.

 

The process is simple, why can't it be produced?

Although the principle of melt blown nonwovens is clear and the main structure of the equipment is clear, many key technologies and production process control are not mastered by the equipment manufacturers.

Taking coating composite technology as an example, each technical service engineer usually needs years of technical accumulation to master the technical difficulties in equipment operation and provide high-quality technical service support to customers. The scientific and effective combination of multi-disciplinary and multi field technical teams and correct market direction guidance can meet the requirements of technical research and development, product design and service support in the industry, and objectively form a high barrier for professional talents.

 

After looking at the production process of the melt blown cloth, let's look at the electret treatment process of the melt blown cloth, because in order to produce a qualified composite standard mask, the two are equally important.

 

01

Filter principle of melt blown cloth fiber

N95 standard mask uses melt blown cloth to capture particles mainly through mechanical barrier and electrostatic adsorption. The mechanical barrier effect is closely related to the structure and properties of the material: when the melt blown cloth is charged by corona, it has a voltage of hundreds to thousands of volts. Due to the repulsion of static electricity, the fiber diffuses into network holes, and the size between the fibers is much larger than the size of dust, thus forming an open structure.

When the dust passes through the melt blown filter material, the electrostatic action can not only effectively attract the charged dust particles, but also capture the polarized neutral particles with the electrostatic induction effect. The higher the electrostatic potential, the higher the charge density, the more the point charge, and the stronger the electrostatic effect. Corona discharge can greatly improve the filtration performance of PP melt blown fabric. Adding tourmaline particles can effectively improve the electret efficiency, increase the filtration efficiency, reduce the filtration resistance, increase the surface charge density of the fiber, and enhance the storage capacity of the fiber mesh.

Adding 6% tourmaline to the electret has better comprehensive effect.  Too much electret material will increase the mobility neutralization of carriers. Electret masterbatch should have nanometer or micro nanometer size and uniformity. A good electret masterbatch can improve the spinning performance, not affect the nozzle, improve the filtration efficiency, reduce the air resistance, increase the density and depth of charge collection, make more charges have a greater chance to exist in the fiber aggregates, and make the captured charges in a lower energy state, not easy to get out of the charge carrier trap or be neutralized, and slow down the decline.


02

Electrostatics electret technology by melt spraying

In the process of melt blown electrostatic electret, tourmaline, silicon dioxide, zirconium phosphate and other inorganic materials are added into PP polypropylene polymer in advance, and then one or more groups of corona discharge with needle electrode voltage of 35-50kv of electrostatic generator are used to charge the melt blown material before rolling. When high voltage is applied, the air under the needle tip will generate corona ionization, partial breakdown discharge and carrier pass Under the action of electric field, some carriers will be trapped in the surface layer and become electret filter materials. The voltage of this corona process is a little smaller than that of high voltage discharge of about 200kV, and less ozone is generated. The function of charging distance and charging voltage makes the reverse effect. As the charging distance is further away, the material will capture less charge. The main requirements of electret melt blown cloth are: one set of melt blown equipment, four sets of electret masterbatch, four sets of high-voltage electrostatic discharge equipment and cutting equipment.

 

What is melt blown electret?

With the increase of charging voltage, the electrostatic potential on the surface of the melt blown electret increases. This is because with the increase of charging voltage, the polarization electric field of the host material increases, and the charge to the surface of the sample increases, so the electrostatic potential increases.

With the increase of storage time, the electrostatic potential on the surface of the electret material attenuates. Under different charging voltage, the trend of electrostatic attenuation is the same at the surface point of the material, and the retention rate is not much different, and tends to be stable in about 30 days. This shows that PP melt blown cloth has good electrostatic performance. The larger the charging voltage is, the greater the electric field strength is, the greater the electrostatic potential on the surface of the material is, but it cannot exceed a certain limit. If the charging voltage is more than 10kV, spark discharge will occur, which will cause the material to be punctured, so the charging voltage should be carefully selected.

Electret air filter material requires a large storage charge density, a long storage life of charge density and a strong storage charge stability. The stability of stored charge mainly depends on material properties, charging method, charge distribution state, storage environment conditions and so on.

According to the above requirements, in terms of the properties of electrostatic electrets, corona discharge method is the best electrostatic electret method at present; thermal polarization method is also a better electrostatic electret method when the environment is relatively stable; friction electrification method needs to be further improved in the experiment; electrostatic spinning method needs to be further developed in science and technology; low energy electron beam bombardment method needs to improve and simplify the electrostatic electret work Art.

 

03

Why is electrostatic electret needed for melt blown cloth?

Electrostatic adsorption of electret treatment is to capture the new coronavirus droplets through the Coulomb force of the charged fibers.  The principle is to make the surface of the filter material more open and the ability to capture particles more powerful. Therefore, the melt blown non-woven filter material of the filter layer must undergo electret treatment to achieve 95% filtration without changing the respiratory resistance, so as to effectively prevent the virus. The mask must achieve the barrier effect while ensuring the comfortable ventilation. The suction resistance of the medical mask is general No more than 343.2 Pascal (PA), and the suction resistance of civil masks is less than 135 Pascal (PA). Electret treatment can greatly improve the filtration efficiency without changing the respiratory resistance, and the higher the electret voltage is, the higher the material filtration efficiency is. Generally, the electret voltage is 30kv-50kv.

 

04

Where is the future development of melt blown cloth?

 

In 2020, the newly launched melt blown production line only targets one market: masks. After the epidemic, where will the new melt blown market go?

 

After the epidemic, people's understanding of masks has improved a lot. In the future, masks will be necessary for people's daily life. The market scale of masks will inevitably be expanded, but homogenization after the epidemic will inevitably bring fierce price competition to the production market of melt blown filter materials.

 

 

With the entry of large-scale state-owned enterprises such as Sinopec and PetroChina, the production capacity will be used as the national strategic material reserve after the epidemic, but it will inevitably enter the market, especially Sinopec now sells masks through its convenience stores at gas stations, which is a more direct and cash channel.

 

The newly put into production melt blown production line can not meet the standards of the corresponding masks. After the epidemic, in addition to selling off the equipment, it can also enter the field of oil absorbent cotton. Its corresponding standards are not high, but price competition will be inevitable.

 

The positive effect of the epidemic on the field of masks is that the production field of masks will be more standardized, and the quality requirements for melt blown filter materials will be greatly improved.  After that, there will be no concept of common melt blown cloth for the filter material of the melt blown mask. The bfe99 grade melt blown filter material will be the minimum requirement for the filter material of the melt blown mask field.

 

After the outbreak, the pattern of the whole melt blown industry will change even more. The enterprises deeply engaged in the field of melt spraying will invest more money in R & D, development of more environmentally friendly and lower cost products and new application fields, which will enhance the global competitiveness of the whole China's melt spraying industry.

 

During the epidemic period, most of the melt blown production enterprises showed their social responsibility, of course, there were many "chaos". The craziness of copycat is a "strange phenomenon" interwoven by many forces, such as the pursuit of profits by private small capital, the indifference of legal consciousness of small business owners and the absence of local supervision.