1. Polypropylene (PP)
Polypropylene (PP) is a thermoplastic resin, which is one of the five general synthetic resins. Its appearance is white granular material, tasteless, non-toxic, melting point is up to 167 ° C, density is 0.90g/cm? (it is the lightest general-purpose plastic), corrosion resistance, strength, rigidity and transparency are better than polyethylene, easy to process, good electrical insulation, etc., and it is widely used in automobile industry, home appliances, electronics, packaging, building materials and furniture, etc.
Polypropylene can be divided into three types: isotactic, syndiotactic and random according to the spatial position of methyl group;
It can be divided into narrow-band, injection molding, extrusion film, fiber, extrusion, etc. according to its application; Homopolymer polypropylene and copolypropylene can be divided according to the type of monomer.
China is one of the largest polypropylene producers in the world, with a production of 20.93 million tons in 2018, accounting for 31.2% of the world's total; meanwhile, China is also the largest consumer, with a consumption of 23.96 million tons in 2018, accounting for 35.9% of the world's total consumption. In 2018, the import dependence was about 13.8%.
Upstream and downstream industrial chain:
There are many quality index items of polypropylene resin, which are important and generally can be tested and applied, mainly including isotacticity, melt index, molecular weight, ash content, chlorine content, titanium content, volatile content, density, apparent density, tensile yield strength, tensile fracture strength, fracture elongation, impact strength (notch impact strength and non notch impact strength Degree of thermal deformation temperature, softening point temperature of VEKA, embrittlement temperature, hardness, electrical properties (such as resistance coefficient, breakdown voltage intensity, dielectric loss tangent value), for spinning high strength polypropylene wire, non-woven fabric fiber grade polypropylene resin, gel particle (or "fish eye") content index and molecular weight distribution index.
(1) Isotacticity of polypropylene is the content of isotacticity polypropylene in the whole polymer, expressed by weight percentage, which is also called isotacticity index.
Isotacticity affects the crystallinity of polypropylene. The higher isotacticity is, the higher crystallinity is. The crystallinity has an effect on many properties of polypropylene. In a certain range, the crystallinity is high, the tensile yield strength is high, the hardness is large, and the impact strength, especially the low temperature impact performance is good. This is the main reason that random polypropylene can be used as impact modifier of polypropylene or other resins.
The isotacticity of polypropylene has an obvious effect on the spinning performance and fiber quality. High isotacticity and good crystallinity are beneficial to the properties and quality of silk.
The isotacticity of polypropylene not only affects the properties of products, but also affects the processing properties of resins. Low isotacticity, sticky product, poor fluidity, easy to be agglomerated and agglomerated during packaging and storage, difficult to add materials during processing, and even impossible to process.
Low isotacticity of polypropylene will also affect the normal production and operation of polypropylene. Generally, when the isotacticity is lower than 90%, the product will appear sticky phenomenon. When the isotacticity is lower than 85%, the product will stick severely, which will cause serious wall sticking in the inner wall, discharge port, flash tank and pipeline of the loop reactor, affecting the operation and normal production.
The isotacticity of polypropylene products is generally measured by boiling n-heptane extraction, infrared spectrum and nuclear magnetic resonance. Because isotacticity of polypropylene is insoluble in n-heptane, and other structure polypropylene is soluble in boiling n-heptane, it can be measured and expressed by the percentage of insoluble in boiling n-heptane in the whole sample, which is also the most simple and feasible way to measure isotacticity of polypropylene Law. In addition, the content of isotactic propylene in polypropylene can be expressed by xylene insolubles, which is slightly different from that measured by n-heptane extraction.
(2) Melt flow rate is commonly known as melt index (MI), which refers to the weight of the thermoplastic resin passing through the standard mold every 10 minutes under a certain load and temperature on the melt tester, expressed in g / 10 minutes.
Melt index is an index to evaluate the fluidity of polypropylene resin in melt state. The larger MI is, the better the melt flow performance of PP is. On the contrary, the smaller MI is, the worse the melt flow performance of PP is. As polypropylene is a thermoplastic resin, which is processed into various products in the melting state, MI is an important index affecting the processing performance of polypropylene, and also one of the most important indexes of the quality of polypropylene products.
In PP Homopolymer products, various brands are mainly distinguished by their MI. Different uses, different product brands, corresponding requirements of different specifications of MI. Polypropylene of the same brand has certain Mi range. Different Mi requires different processing temperature.
If several batches of products with large Mi difference are processed together, there will be "fisheye" in the process of processing, that is, the resin with high MI has melted, while the resin with low MI has not melted, and there will be white spots in the products, which will greatly affect the quality of products, especially in the spinning of polypropylene fiber, the presence of fisheye will have a greater impact on the silk performance and the quality of polypropylene fiber 。 Therefore, the MI of PP resin is required to be stable and controllable to ensure the good processing performance and product quality.
(3) The molecular weight of polypropylene directly affects its melt flow rate. The larger the molecular weight is, the smaller the MI is, the worse the melt fluidity is; on the contrary, the smaller the molecular weight is, the larger the MI is, the better the melt fluidity is.
(4) Ash: polypropylene resin is an organic compound composed of carbon and hydrogen elements, which will decompose at high temperature, part of it will become low molecular organic matter and volatilize, part of it will become CO2, CO and H2O and volatilize. The completely pure polypropylene should be volatilized at high temperature, not in ash, but in fact, there will always be a small amount of it at high temperature Nonvolatile substances, such as catalyst, activator and various impurities mixed in the production process. The ash content of polypropylene is generally expressed by weight percentage or weight ppm, which is the content of impurities in the whole sample that can not be volatilized after high temperature burning. Fiber grade polypropylene resin requires the ash content to be controlled below 100ppm. If the ash content is high, it means that there are many non fusible substances in the processing, which is easy to cause the blockage of equipment and affect the strength and other properties of products. But ash content has little effect on extrusion and injection molding.
(5) Tensile mechanical properties: the main three indexes are tensile yield strength, tensile breaking strength and elongation at break.
When polypropylene strip is under certain tension, it will produce deformation (i.e. strain). At first, the stress increases with the increase of strain (elongation), but when the stress and strain increase to a certain extent, the strain increases and the stress no longer increases. On the stress-strain curve in the figure above, the point (m) at which the strain increases for the first time but the stress does not increase is called the yield point, the stress FM at the yield point is defined as the tensile yield strength, the point (T) at the tensile fracture is called the fracture point, and the stress f at the fracture point is defined as the tensile fracture strength.
The fracture point may be higher or lower than the yield point. For polypropylene resin, the tensile yield strength is more important than the tensile fracture strength, because polypropylene resin is used within the yield point.
Tensile yield strength and tensile fracture strength are the most important indexes of mechanical properties of polypropylene. It represents the tensile resistance of polypropylene resin and is one of the most important bases for the application of polypropylene. In the application of polypropylene, the higher the tensile yield strength and tensile breaking strength, the better.
In the stress-strain curve, there are corresponding yield point strain (%) and fracture point strain (%) relative to yield point m and fracture point t, that is, yield elongation (%) and fracture elongation (%). Yield elongation and fracture elongation are also important mechanical property indexes of polypropylene, which are of great significance to the application of polypropylene.
Quality standard of futures delivery products:
Judging from the melt index (2.5-4.0), the main futures target is drawn grade PP. Wire drawing is mainly used for making space bags, woven bags, stationery sheets, fishing nets, etc. Injection grade (melt finger 1 or so) applicable products are: electrical shell, chemical barrel, daily products, sheet, packing belt and plastic modification series products.
The PP used as the core material of mask is high melting point PP.
2. Melt blown cloth
Melt spray (melt blow molding) is a non-woven process that is extruded through polymers. This is a filter material developed by the U.S. Navy in 1954 to collect radioactive particles from nuclear tests. Then around 1965, Exxon, 3M and other companies produced the first generation of melt spray nonwovens.
Principle of drawing nozzle hole die polymer fine flow of hot air from melt extrusion has high speed, so as to form superfine fiber or mesh screen with upper solidification and agglomeration drum. The principle of self-adhesive non-woven fabric spinning is as follows:
The filtration mechanism of medical mask is Brownian diffusion, interception, inertia collision, gravity sedimentation and electrostatic adsorption. The first four are physical factors, that is, the non-woven fabric produced by the melt blown method naturally has the characteristics of about 35% filtration; this does not meet the requirements of medical masks, we need to pole the material, charge the fiber, and capture the new coronavirus in the aerosols with static electricity.
3. Medical mask
The structure of medical mask. Disposable medical mask, except for the ear hook and aluminum plastic strip, generally adopts three-layer non-woven structure. The outermost layer and innermost layer are both spunbond non-woven fabrics which are used for medical and health care. In the middle of the two layers, a layer of dissolving and spraying non-woven fabric with filtration and bacteria prevention of over 99.999% is added, which is made by ultrasonic welding. That is, the outer layer is antifoam (spunbond non-woven fabric) + the middle layer is filtration (melt blown non-woven fabric) + the inner layer is moisture absorption (spunbond non-woven fabric). As a whole, it is called SMS non-woven fabric (English: spunbond + meltblow + spunbond) Nonwovens), a kind of composite non-woven fabric, is a composite product of spunbond and melt blown. Raw materials of medical masks. It is reported that the non-woven fabric used in the production of medical masks belongs to high melting point polypropylene, and the melting point range is 31-44. It has better fluidity and narrower molecular weight distribution, and more stable non-woven fabric products can be obtained, which is suitable for the production of masks. The melt blown layer is ultra-high melting point polypropylene, which refers to the melting point of more than 45 melting point polypropylene products.
Melt blown non-woven fabric, from the tax price of nearly 300000 / T (30-40 days later), to the invoiced price of 500000 / T, or even to the high price of 600000 / T and 700000 / T. It is understood that the price of melt blown cloth was 18000 / T in December 2019, and it has increased by more than 3000% since the end of the year, or even tens of thousands in a day. Comparing the prices of the two kinds of nonwovens, the unit price of melt blown nonwovens is 10-15 times of that of spunbond nonwovens, and because of the stricter requirements on raw materials and production technology, the production of melt blown nonwovens is low and it is difficult to obtain. At present, the state has strict control over the price of melt blown cloth, and the manufacturer does not quote at will.
The new national standard polypropylene (PP) resin GB / T 12670-2008 divides polypropylene into five categories: narrow band, injection molding, extrusion, fiber and extrusion film. Among them, the PP listed by Dashang is the narrow-band PP with the largest output and concentrated use, i.e. drawbench PP, which is mainly used for the production of woven products and BOPP film products (space bags, woven bags, stationery sheets, fishing nets, etc.); while the main raw materials of melt blown non-woven fabric and spunbond non-woven fabric as an important part of mask are melt blown polypropylene and high Melt fiber polypropylene. Fiber materials for drawing grade PP and mask are different kinds of polypropylene materials.
PP rose rapidly (compared with others