The raw materials of mask are mainly composed of spunbonded nonwovens, melt blown nonwovens, hot air nonwovens, needle punched nonwovens, ear band, nose bar and other accessories.
Among them, meltblown nonwovens, as the core filter layer of masks, have the characteristics of fine fiber fineness (average fiber diameter 2 ~ 4 μ m), small pore size, large specific surface area and high porosity, which make meltblown nonwovens have certain air permeability and excellent filtering function.
Since melt blown nonwovens technology was developed by the U.S. Naval Research Institute in 1954, related enterprises at home and abroad have rapidly turned it into civilian use.
Especially in China, after SARS in 2003, H1N1 in 2009 and haze around 2014, melt blown nonwoven filter material technology has developed rapidly, and a complete industrial manufacturing system has been gradually established, including polypropylene resin raw materials, spinning and melting equipment manufacturing, spunbond / melt blown production, mask processing and so on. The overall technical strength has reached the international advanced level.
The key technologies for producing high efficiency and low resistance melt blown filter materials are as follows:
01
Micro nano technology of melt blown fiber diameter
By improving the structure design of spinneret and selecting high melt fat polymer chips, the diameter of melt blown material is reduced and the filtration efficiency is improved.
For example, some advanced spinnerets have a minimum diameter of 0.10 mm, a hole density of more than 100 holes / inch (equivalent to 3937 holes / M), a length diameter ratio (L / D) of more than 35, and a melt pressure of 10.4 MPa, which is conducive to the superfine production of melt blown fibers.
02
Electret treatment technology
Electret technology is particularly important in the industrial production of melt blown filter materials. Electret treatment can significantly improve the filtration efficiency without affecting the physical structure of the material and increasing the filtration resistance.
There are many kinds of electret technology in China, among which corona charging technology is the most common. In addition, there are water electret and hot air flow electret technology.
Under certain process temperature and humidity, the charge can move from "shallow well" to "deep well", which makes the charge storage more stable; temperature affects the charge mobility and the amount of charge stored in the material.
At present, the corona electret meltblown nonwoven filter material (kn95 mask surface density is 40-50 g / m2) is used, the filtration resistance can be less than 90 PA, and the filtration efficiency can be more than 99% (the gas flow rate used in the filtration performance test is 85 L / min).
03
Polymer modification technology
By modifying the resin with inorganic / organic materials, the electret effect can be enhanced, and the filtration performance and aging of meltblown materials can be improved.
Using polymer modification and additive technology can break through the common technical problem of short storage time of melt blown nonwoven filter materials, and greatly enhance the strategic reserve capacity of China's protective products.
The results show that the filtration efficiency is 99.4% (gas flow rate is 85 L / min) before storage. After 8 years of sealed storage, the sample can be stored in a sealed environment, The filtration efficiency measured in 2019 is 97.05%, the decrease of filtration efficiency is less than 3%, and the electret effect is stable, which can fully meet the long-term strategic reserve demand of protective masks.
The GB / T 38880 "technical specification for children's masks" issued on May 6 requires that the filtration efficiency of children's protective masks for non oily particles should be more than 95% at the test flow rate of 85 L / min, and the respiratory resistance should not be higher than 45 Pa at the test flow rate of 45 L / min; the filtration efficiency of children's health masks should be lower than 32 Pa at the test flow rate of 85 L / min Under the test flow rate of L / min, the filtration efficiency of non oily particles is more than 90%, and that of bacterial aerosol is more than 95%, and the ventilation resistance is less than 30 PA.
The above indicators put forward higher requirements for mask filter materials, which requires filter material manufacturers to have certain technical strength and production experience accumulation. During the epidemic period, if new enterprises in melt blown industry need to produce national standard filter materials for children's masks, they need to strengthen professional theory, technology, polymer performance research, or carry out relevant equipment technical transformation.
