Why does melt blown PP melt point become low? What about brittle and hard melt blown cloth and poor filtration?

- May 11, 2020-

There are many kinds of melt blown PP materials in the market. Many melt blown PP cloth are as hard as paper after production. Not all modification plants can fully grasp the details of PP degradation process. Many big brands are outsourced due to insufficient production capacity, which also limits the stability of products.

 

GB / T 30923-2014 polypropylene (PP) melt blown special material standard issued by the state has given a specific quantitative range for melt index, ash, volatile matter, peroxide residue and molecular weight distribution, especially for PP melt blown materials with hygienic requirements.

 

If you want to produce a meltblown cloth with a grade of 95 or above, you need to understand the causes of the common problems of PP meltblown material first so as to better and faster handle and adjust it. The following is a list of common problems and cause analysis for you, mostly for the industry to share experience, for your reference.

 

It is found that the problems related to melt blown cloth are frequently reflected as follows:

·After a few days of melt blown PP, the melt finger becomes low;

·After homogenizing and drying, the melt index of the spray material becomes low;

·The melt blown cloth was just produced, but it became brittle in a few days;

·When the melt blown cloth is added to the electret masterbatch, it can achieve 95% or even higher filtration efficiency?

·How to choose the melt spray material? Powder or granule?

·How to choose the melt spray material? Powder or granule?

·How to deal with electrets? How to buy the equipment?

·What is the reason for the plugging of the spinneret hole? What should I do?

Analysis of the reason of the melt finger becoming low

 

One

After several days of melt blown PP material, why does melt finger become low?

After pelletizing on the same day, the melt finger is tested at 230 ℃, and the peroxide residue will cause PP to continue to degrade, and the melt finger will increase, so even the melt finger of 1200 may reach 1800 during the test. At the same time, there are too many residues, resulting in bubbles, which flow faster than the material, so the volume measured value is on the high side. When it comes to the customer's hands, the peroxide residue is less, and the melt finger changes little. It goes back to 1200, so it's not down to 1200, but back to 1200.

 

Two

Why does melt blown PP melt finger become lower after homogenization and drying?

As with the decomposition products of peroxides, the presence of water will also lead to the high melt index of volume method. Less water, or drying, the melt finger will become low.

Of course, there are also industry netizens to reflect that: the volume method of melt finger is not accurate, and its results should not be used. Although the current method of plugging causes poor exhaust and unstable test value, it is actually caused by the incorrect position of 30mm section tested by the current fusing finger tester. The upper and lower position of the sensor should be adjusted to align with the upper and lower scale lines of the pressing rod to test the material volume of the 30mm section.

 

Three

Why does melt blown PP become lower after crosslinking with polymer?

Melt blown PP is toughened by adding copolymerized PP or elastomer.  In fact, peroxides degrade PP in the extrusion process of homopolymer PP, but with other polymers, such as polyethylene, and rubber materials, it plays a role of cross-linking, and the result of cross-linking is that the melt index is really reduced. If the melt blown PP material becomes low due to crosslinking, it is easy to plug the die head if the melt blown PP material continues to make the melt blown cloth at this time.

 

1. Uneven spinneret and insufficient air volume of cooling fan cause uneven heat dissipation and internal stress, resulting in embrittlement. It can be improved by adjusting the melt volume of extruder, adjusting the temperature of hot air blower and increasing the air volume of cold air blower.

2. From the aspect of raw materials, it may be that low melting finger spinneret is used, and the wire is relatively thick. After 24-48 hours of complete crystallization, the melt blown cloth will become brittle and hard. The molecular weight distribution is not uniform, so it is necessary to select polypropylene melt blown material with concentrated molecular weight distribution, which can reduce the risk of brittleness of the sprayed melt blown cloth. If the melt finger 1500 melt blown material has fine, soft and even distribution, the softness of the melt blown cloth is better.

3. It is related to the high temperature of raw materials and processing.  After the fiber is produced, there is a process of secondary crystallization rearrangement in PP. in fact, it is heated seriously in the screw, which has led to serious thermal degradation. If the initiator of peroxide process is not well controlled, such problems are easy to occur. The most important thing of peroxide process is how to finish the chain break before the screw extrusion in time to prevent the secondary residue from entering into the non-woven screw to continue the reaction, which is a test of screw equipment combination and formula control.

4. Secondary decomposition makes the melt blown cloth hard and brittle.

5. Poor electret equipment and electret effect may also result in brittle melt blown cloth.

6. There is still temperature for the melt blown cloth just produced, especially for the cloth spraying machine with large drum for material collection. Maybe there is no fan, and the temperature of the melt blown cloth is high after coiling. The greater the flexibility and toughness of plastics under high temperature, the more brittle they will become after cooling.

 

According to the above content, the brittleness and hardness of the produced melt blown cloth are affected by the comprehensive factors such as raw materials, process, additives and equipment. Therefore, we must check the whole process, buy regular materials and equipment, strictly control the process parameters, and reduce the occurrence of this phenomenon.

A good high-quality melt blown cloth is related to material, melt blown process, voltage, etc. 

the biggest effect of electret masterbatch is to help us achieve N95 This standard meltblown

cloth stores electric charge and prolongs its validity! To apply electret treatment to the meltblown cloth is equivalent to charging the melt blown cloth and storing more charges. The 

electret masterbatch is the battery of our melt blown cloth!


For this reason, we also made a special call to consult the authoritative engineer of electret masterbatch. The engineer reported that the melt blown cloth reported by the customer was not up

to standard 95% The filtration efficiency and the brittleness of melt blown cloth are the

comprehensive effects of various aspects.

About the failure of melt blown cloth 95% The factors of filtration efficiency involve raw materials, auxiliaries, process, equipment, formula According to what the engineer told Xiaobian, the following

points are listed:

the filter efficiency of the melt blown cloth has a great relationship with the equipment for making the melt blown cloth. Generally speaking, the precision of the equipment mould of the large company is relatively high, the process control is good, and the plasticizing effect is good. However, the precision of equipment in many small enterprises may not meet the relevant requirements, and the filtration efficiency may not be up to the standard or even the melt blown cloth may become brittle;

Secondly, the type of electret masterbatch (organic, inorganic), the amount of electret masterbatch added, the dispersion effect of electret masterbatch and electret effect will also affect the filtration effect

of melt blown cloth;

In addition to the influence of equipment, process, electret masterbatch and other factors, the design of melt blown cloth formula and the selection of raw materials will affect the filtering effect.

■ the melt index is not stable, resulting in uneven spinning, and the filtration efficiency is not up to standard, floating phenomenon.

Therefore, many enterprises are not able to produce qualified masks, small masks, great knowledge, and very rigid technical standards. If they want to produce masks that meet the standards, they can not relax in every link.


How to choose the melt spray material? Powder or granule?

● long storage time of granular material and fast degradation of powder, especially in hot weather;

● the powder and the degradation agent are more evenly mixed, with large contact surface and more advantages;

● base material with small melting point, more degradation agent, large melting point and less adding point;

● select polypropylene melt blown material with concentrated molecular weight distribution;

● it is the key technology to find a way to eliminate the impact of base material instability.

In actual production, it should be selected MFI Big polypropylene or small?

● MFI Small: can produce high strength melt blown nonwovens.● MFI Large: high output and low energy consumption. So the current trend is to adopt a higher MFI Raw materials.

● melt blown polypropylene PP Granular material: MFI > one thousand and five hundred 。

In other words, if you find that the produced melt blown cloth is "too brittle", you have to look at the melt index of the raw material. How

to see this parameter depends on where you buy the raw materials.


◆ electret time, electret voltage and electret distance are three important process parameters that affect the effect of electret. With the increase of electret time, the equivalent surface charge density of deposit increases, and the potential of electret surface increases, so that the adsorption and polarization of electret increases. But with the increase of electret time, when the surface potential of filter material is high enough, the charge under the tip of needle will be repelled He moves where the charge density is low. At the end of the electret, the charge surface density reaches saturation state, so when the electret time increases again, the filtration efficiency of the filter material has no obvious change.

◆ the electret equipment should be positive, not negative. Because of the positive charge on the cloth, it can absorb the negative charge in the air. When the melt blown cloth contacts the skin, the negative charge will be consumed more easily. If the positive charge is carried, the

loss will be slower.

◆ dust, bacteria and viruses in the air are attached to the particles, mainly with negative electricity, and the melt blown cloth has positive electricity, which is easy to absorb the particles with negative

electricity.

◆ the ability of electret masterbatch to increase static electricity

is not great, mainly to store electric charge and slow down the

attenuation.


What's the reason for plugging the spinneret hole? What should I do?

The thread injection die is one of the most common problems in the melt blown production line. The reasons for the thread injection die are as

follows:

● the material has large ash and impurities;

● the spinneret hole is not smooth enough;

● unstable materials;

● no filter;

● the drum is not cleaned.

 

PP The production process of melt blown material is based on PP Special resin as base material, generally in powder form, melt index Thirty ~ Forty Through the twin-screw mixing extruder, combined with the mature formula and process, the polymer controlled rheological technology is adopted to process and produce PP Melt blown material.

 图片12

Schematic diagram of raw materials for upstream production of respirator


Before the outbreak of novel coronavirus pneumonia, PP melt blown products were small products, with a small market and low profit margins. The total capacity of the country was about 80 thousand tons / year, about 260 tons / day. After the outbreak of the epidemic, due to the blowout demand for masks, the demand for melt blown polypropylene special materials for masks surged in a short period of time. In order to meet the market demand during the epidemic as soon as possible, many enterprises urgently built and increased the production line of melt blown materials, and the domestic production capacity of melt blown materials rose rapidly.


At present, it is close to 5000 tons per day, an increase of nearly 20 times.

The increase in production can be roughly divided into three categories:

● the original production enterprises increase production lines and expand production capacity;

● the large and medium-sized modified plastic plants did not produce melt blown materials, so the existing modified plastic production equipment was transformed into PP melt blown material production equipment to change production;

● considering from the strategic point of view, the state has put some of Sinopec's subordinate production plants into the production and sales of PP melt blown materials, so that the production of modified granulation of PP melt blown materials is increasing every day.