The core material in epidemic prevention masks – polypropylene

The main material of masks is polypropylene non-woven fabric (also known as non-woven fabric), which is a thin or felt like product made from textile fibers through bonding, fusion, or other chemical and mechanical methods. Medical surgical masks are generally made of three layers of non-woven fabric, namely spunbond non-woven fabric S, meltblown non-woven fabric M, and spunbond non-woven fabric S, known as SMS structure; The inner layer is made of ordinary non-woven fabric, which has a skin friendly and moisture absorbing effect; The outer layer is made of waterproof non-woven fabric, which has the function of blocking liquids and is mainly used to block liquids sprayed by the wearer or others; The middle filter layer is usually made of polypropylene melt blown non-woven fabric that has been electrostatically polarized, which can filter bacteria and play a decisive role in blocking and filtering.

The automated mask production line greatly improves the production efficiency of masks. Large rolls of polypropylene non-woven fabric are cut into small rolls and placed on the mask production line. The machine sets a small angle and gradually narrows and gathers them from left to right. The mask surface is pressed flat with a tablet, and processes such as cutting, edge sealing, and pressing are carried out. Under the operation of automated machinery, it takes an average of only about 0.5 seconds for a factory assembly line to produce a mask. After production, the masks are disinfected with ethylene oxide and left to settle for 7 days before being sealed, packaged, boxed, and shipped for sale.

The core material of masks – polypropylene fiber

The filtering layer (M layer) in the middle of medical masks is a melt blown filter cloth, which is the most important core layer, and the main material is polypropylene melt blown special material. This material has the characteristics of ultra-high flow, low volatility, and narrow molecular weight distribution. The formed filter layer has strong filtering, shielding, insulation, and oil absorption properties, which can meet the various standards for the number of fibers per unit area and surface area of the core layer of medical masks. One ton of high melting point polypropylene fiber can produce nearly 250000 polypropylene N95 medical protective masks, or 900000 to 1 million disposable surgical masks.

The structure of polypropylene melt blown filter material is composed of many criss crossing fibers stacked in random directions, with an average fiber diameter of 1.5~3 μ m, approximately 1/30 of the diameter of a human hair. The filtration mechanism of polypropylene melt blown filter materials mainly includes two aspects: mechanical barrier and electrostatic adsorption. Due to the ultrafine fibers, large specific surface area, high porosity, and small average pore size, polypropylene melt blown filter materials have good bacterial barrier and filtration effects. Polypropylene melt blown filter material has the function of electrostatic adsorption after electrostatic treatment.

The size of novel coronavirus is very small, about 100 nm (0.1 μ m), but the virus cannot exist independently. It mainly exists in secretions and droplets when sneezing, and the size of droplets is about 5 μ m. When virus containing droplets approach the meltblown fabric, they will be electrostatically adsorbed on the surface, preventing them from penetrating the dense intermediate layer and achieving a barrier effect. Due to the fact that the virus is highly difficult to detach from cleaning after being captured by ultrafine electrostatic fibers, and washing can also damage the electrostatic suction ability, this type of mask can only be used once.

Understanding of Polypropylene Fiber

Polypropylene fiber, also known as PP fiber, is generally referred to as polypropylene in China. Polypropylene fiber is a fiber made by polymerizing propylene as raw material to synthesize polypropylene, and then undergoing a series of spinning processes. The main varieties of polypropylene include polypropylene filament, polypropylene short fiber, polypropylene split fiber, polypropylene expanded filament (BCF), polypropylene industrial yarn, polypropylene non-woven fabric, polypropylene cigarette tow, etc.

Polypropylene fiber is mainly used for carpets (carpet base and suede), decorative fabrics, furniture fabrics, various rope strips, fishing nets, oil absorbing felt, building reinforcement materials, packaging materials, and industrial fabrics such as filter cloth, bag cloth, etc. Polypropylene can be used as cigarette filters and non-woven sanitary materials, etc; Polypropylene ultrafine fibers can be used to produce high-end clothing fabrics; The quilt made of polypropylene hollow fibers is lightweight, warm, and has good elasticity.

The Development of Polypropylene Fiber

Polypropylene fiber is a fiber variety that began industrial production in the 1960s. In 1957, Italy’s Natta et al. first developed isotactic polypropylene and achieved industrial production. Shortly thereafter, Montecatini company used it for the production of polypropylene fibers. In 1958-1960, the company used polypropylene for fiber production and named it Meraklon. Subsequently, production also began in the United States and Canada. After 1964, polypropylene film split fibers for bundling were developed and made into textile fibers and carpet yarns through thin film fibrillation.
In the 1970s, the short-range spinning process and equipment improved the production process of polypropylene fibers. At the same time, expanded continuous filament began to be used in the carpet industry, and the production of polypropylene fiber developed rapidly. After 1980, the development of polypropylene and new technologies for manufacturing polypropylene fibers, especially the invention of metallocene catalysts, significantly improved the quality of polypropylene resin. Due to the improvement of its stereoregularity (isotropy up to 99.5%), the intrinsic quality of polypropylene fibers has been greatly enhanced.
In the mid-1980s, polypropylene ultra-fine fibers replaced some cotton fibers for textile fabrics and non-woven fabrics. At present, research and development of polypropylene fibers are also quite active in various countries around the world. The popularization and improvement of differentiated fiber production technology have greatly expanded the application fields of polypropylene fibers.

Structure of polypropylene fibers

Polypropylene is a large molecule with carbon atoms as the main chain. Depending on the spatial arrangement of its methyl groups, there are three types of three-dimensional structures: random, iso regular, and meta regular. The carbon atoms on the main chain of polypropylene molecules are in the same plane, and their side methyl groups can be arranged in different spatial arrangements on and below the main chain plane.
The production of polypropylene fibers uses isotactic polypropylene with an isotropy greater than 95%, which has high crystallinity. Its structure is a regular spiral chain with three-dimensional regularity. The main chain of the molecule is composed of carbon atom twisted chains on the same plane, and the side methyl groups are on the same side of the main chain plane. This crystallization is not only a regular structure of individual chains, but also has regular chain stacking in the right angle direction of the chain axis. The crystallinity of primary polypropylene fibers is 33%~40%. After stretching, the crystallinity increases to 37%~48%. After heat treatment, the crystallinity can reach 65%~75%.

Polypropylene fibers are usually made by melt spinning method. In general, the fibers are smooth and straight in the longitudinal direction, without stripes, and have a circular cross-section. They are also spun into irregular fibers and composite fibers.

Performance characteristics of polypropylene fibers

Texture

The biggest feature of polypropylene is its light texture, with a density of 0.91g/cm ³, which is lighter than water and only 60% of the weight of cotton. It is the lightest density variety among common chemical fibers, 20% lighter than nylon, 30% lighter than polyester, and 40% lighter than viscose fiber. It is suitable for making water sports clothing.

Physical properties

Polypropylene has high strength and a fracture elongation of 20% -80%. The strength decreases with the increase of temperature, and polypropylene has a high initial modulus. Its elastic recovery ability is similar to nylon 66 and polyester, and better than acrylic. Especially, its rapid elastic recovery ability is greater, so polypropylene fabric is also more wear-resistant. Polypropylene fabric is not prone to wrinkling, therefore it is durable, clothing size is relatively stable, and not easily deformed.

Moisture absorption and dyeing performance

Among synthetic fibers, polypropylene has the worst moisture absorption, with almost zero moisture regain under standard atmospheric conditions. Therefore, its dry and wet strength and fracture strength are almost equal, making it particularly suitable for making fishing nets, ropes, filter cloth, and disinfectant gauze for medicine. Polypropylene is prone to static electricity and pilling during use, with a low shrinkage rate. The fabric is easy to wash and dry quickly, and is relatively stiff. Due to its poor moisture absorption and stuffiness when worn, polypropylene is often blended with fibers with high moisture absorption when used in clothing fabrics.
Polypropylene has a regular macromolecular structure and high crystallinity, but lacks functional groups that can bind with dye molecules, making dyeing difficult. Ordinary dyes cannot color it. Using dispersed dyes to dye polypropylene can only result in very light colors and poor color fastness. Improving the dyeing performance of polypropylene can be achieved through methods such as graft copolymerization, original liquid coloring, and metal compound modification.

Chemical properties

Polypropylene has excellent resistance to chemicals, insect infestations, and mold. Its stability against acid, alkali, and other chemical agents is superior to other synthetic fibers. Polypropylene has good resistance to chemical corrosion, except for concentrated nitric acid and concentrated caustic soda. It has good resistance to acid and alkali, making it suitable for use as a filter material and packaging material. However, its stability to organic solvents is slightly poor.

Heat resistance

Polypropylene is a thermoplastic fiber with lower softening point and melting point than other fibers. The softening point temperature is 10-15 ℃ lower than the melting point, resulting in poor heat resistance. During the dyeing, finishing, and use of polypropylene, it is necessary to pay attention to temperature control to avoid plastic deformation. When heated in dry conditions (such as temperatures exceeding 130 ℃), polypropylene will undergo cracking due to oxidation. Therefore, anti-aging agent (heat stabilizer) is often added in the production of polypropylene fiber to improve the stability of polypropylene fiber. But polypropylene has better resistance to moisture and heat. Boil in boiling water for several hours without deformation.

Other Performance

Polypropylene has poor light and weather resistance, is prone to aging, is not resistant to ironing, and should be stored away from light and heat. However, the anti-aging property can be improved by adding anti-aging agent during spinning. In addition, polypropylene has good electrical insulation, but it is prone to static electricity during processing. Polypropylene is not easy to burn. When the fibers shrink and melt in a flame, the flame can extinguish on its own. When burned, it forms a transparent hard block with a slight asphalt smell.

Dongguan Liansheng Non woven Technology Co., Ltd. was established in May 2020. It is a large-scale non-woven fabric production enterprise integrating research and development, production, and sales. It can produce various colors of PP spunbond non-woven fabrics with a width of less than 3.2 meters from 9 grams to 300 grams.