NEWS

1. Basic structure and characteristics

Polypropylene (PP) is tasteless, non-toxic Milky White granular products or powder products, the relative density of 0.90~0.91. Melting point 164~167 ℃. Has excellent mechanical properties, heat resistance, electrical insulation properties, chemical stability, and most chemicals do not occur. However, the light resistance is poor, easy to age, low temperature impact strength is poor, poor dyeing, need to be improved by adding additives, blending, copolymerization and other methods. Insoluble in water, do not absorb water, can be boiled in water, at 130 ℃ disinfection, easy processing molding.

If the methyl group is arranged in an orderly manner on both sides of the molecular main chain, it is called atactic polypropylene; when the methyl group is arranged alternately on both sides of the molecular main chain, it is called syndiotactic polypropylene. In the general production of polypropylene resin, the content of isotactic structure is 95%, and the rest is atactic or syndiotactic polypropylene. Industrial products are mainly composed of isotactic substances. Polypropylene also includes copolymers of propylene with minor amounts of ethylene. Usually translucent colorless solid, odorless non-toxic. Because the structure is regular and highly crystallized, the melting point is as high as 167 ℃, heat resistance, and the products can be sterilized by steam. With a density of 0.90g/cm3, it is the lightest general-purpose plastic. Corrosion resistance, tensile strength 30MPa, strength, rigidity and transparency are better than polyethylene. The disadvantage is that the low temperature impact resistance is poor, and it is easy to age, but it can be overcome by modifying and adding antioxidants.

2. Molding characteristics

(1). Crystalline material, moisture is small, prone to melt rupture, long-term contact with hot metal easy to decompose.

(2). Good fluidity, but the shrinkage range and shrinkage value is large, prone to shrinkage. Dents, deformations.

(3). The cooling speed is fast, the gating system and the cooling system should slowly dissipate heat, and pay attention to the control of the molding temperature. When the material temperature is low temperature and high pressure, it is easy to orient. When the mold temperature is lower than 50 degrees, the plastic parts are not smooth, and it is easy to produce poor welding, flow marks, and warping deformation is easy to occur above 90 degrees. Therefore, the temperature should be controlled at 80 degrees.

(4). Plastic wall thickness should be uniform, to avoid the lack of glue, sharp corners, to prevent stress concentration.

3. Functional characteristics

PP is a semi-crystalline material that is harder than PE and has a higher melting point. Since homopolymer PP is very brittle at temperatures above 0°C, many commercial PP materials are random copolymers with 1-4% ethylene or block copolymers with higher ethylene content. Copolymer type PP material has a lower heat distortion temperature (100 ℃), low transparency, low gloss, low rigidity, but has a stronger impact strength, the impact strength of PP increases with the increase of ethylene content. The Vicat softening temperature of PP is 150 degrees C. Due to the high crystallinity, the surface stiffness and scratch resistance of this material are very good. PP does not have environmental stress cracking problems. Usually, PP is modified by adding glass fiber, metal additives or thermoplastic rubber. PP flow rate MFR range from 1 to 40. Low MFR PP material has better impact resistance but lower tensile strength. For materials of the same MFR, the strength of the copolymer type is higher than that of the homopolymer type. Due to crystallization, the shrinkage of PP is quite high, generally 1.8 to 2.5%. And the directional uniformity of the shrinkage rate is much better than that of materials such as PE-HD. The addition of 30% glass additive can reduce the shrinkage rate to 0.7%. Both homopolymer and copolymer PP materials have excellent anti-hygroscopicity, anti-acid and alkali corrosion, and anti-solubility. However, it is not resistant to aromatic hydrocarbon (e. g., benzene) solvents, chlorinated hydrocarbon (carbon tetrachloride) solvents, etc. PP also does not have oxidation resistance at high temperatures like PE.

4. Modified polypropylene

(1) Chlorinated polypropylene

Chlorinated polypropylene (chorinated Polypropylene) has a chlorine content of up to 65%. Chlorinated polypropylene has wear resistance, acid resistance and water resistance. Commonly used as a protective coating, ink filler, can also be used as an adhesive, paper coating and film.

(2) Reinforced polypropylene

Reinforced polypropylene (reinforced polypropylene) is a mixture of polypropylene with glass or organic fibers, asbestos, or inorganic fillers (talc, calcium carbonate).

General industrial glass fiber reinforced polypropylene contains 10 to 15% of the fiber. Good heat resistance and dimensional stability due to the inclusion of glass fibers.

Reinforced polypropylene is mainly used in the manufacture of various mechanical parts, especially automotive parts, as well as corrosion-resistant pipes, pipe fittings, valves, etc.

(3) Grafted polypropylene

Polypropylene is graft copolymerized with ethylene, styrene, methyl methacrylate, etc., in order to improve the tensile strength, impact strength and adhesion strength of polypropylene to other materials.

Grafted polypropylene (graft polypropylene) can be used as olefin adhesives, coatings and waterproof coatings, as well as pipe and plate materials.

5. Production methods

5.1 slurry process

Slurry process (Slurry Process), also known as slurry process or solvent process, is the world's earliest technology for the production of polypropylene. From the first industrial plant in 1957 to the middle and late 1980 s, the slurry process has been the most important polypropylene production process for 30 years. Typical processes include Italian Montedison process, American Hercules process, Japanese Mitsui East Pressure Chemical process, American Amoco process, Japanese Mitsui Oil Chemical process and Sowell process. The development of these processes are based on the first generation of catalysts at that time, the use of vertical stirred tank reactors, the need for deashing and de-random material, due to the use of different solvents, process flow and operating conditions are different. In recent years, the proportion of traditional slurry process in production has been significantly reduced, and the retained slurry products are mainly used in some high-value fields, such as special BOPP films, high relative molecular mass blown films and high-strength pipes. In recent years, the method has been improved, and the improved slurry production process uses a second-generation catalyst with high activity, which can delete the catalyst deashing step, reduce the production of random polymers, and can be used to produce homopolymers, random copolymers and anti-flush copolymer products. At present, the production capacity of slurry PP in the world accounts for about 13% of the total production capacity of PP in the world.

5.2 ontology process

The research and development of the ontology process began in the 1960 s. In 1964, Dart Company of the United States built the world's first industrialized ontology polypropylene production device using a kettle reactor. After 1970, Japan's Sumitomo, Phillips, the United States EI Psao and other companies have realized the liquid phase bulk polypropylene process of industrial production. Compared with the slurry method using solvent, the liquid-phase propylene bulk method has the advantages of not using inert solvent, high monomer concentration in the reaction system, fast polymerization rate, high catalyst activity, high polymerization conversion rate, greater time-to-air production capacity of the reactor, low energy consumption, simple process flow, less equipment, low production cost and less "three wastes". It is easy to remove the heat of polymerization and simplify the control of heat removal, the polymerization amount per unit reactor can be increased; low molecular weight random polymers and catalyst residues which have adverse effects on product properties can be removed, and high quality products can be obtained. The disadvantage is that the reaction gas needs to be gasified and condensed before being recycled to the reactor. The high-pressure liquid hydrocarbon material in the reactor has a large capacity and is potentially dangerous. In addition, the concentration of ethylene in the reactor must not be too high, otherwise a separate gas phase is formed in the reactor, making the reactor difficult to operate, and the ethylene content in the resulting copolymer product will not be too high.

The difference between the different process routes of the bulk method is mainly the difference of the reactor. Reactors can be divided into two categories: tank reactors and loop reactors. The kettle reactor uses the latent heat of liquid evaporation to remove the heat of reaction, most of the evaporated gas is recycled and condensed back to the reactor, and the uncondensed gas is recycled back to the reactor after raising the pressure of the compressor. The loop reactor is the use of axial pump to make the slurry high-speed circulation, through the jacket cooling heat removal, due to the large heat transfer area, heat removal effect is good, so its unit reactor volume yield is high, low energy consumption.

According to the polymerization process, the bulk method production process can be divided into two types: intermittent polymerization process and continuous polymerization process. (1) Intermittent ontology process. Intermittent bulk polypropylene polymerization technology is a successful production technology developed by China. It has the advantages of reliable production process technology, low quality requirements for raw material propylene, guaranteed domestic catalyst, simple process, low investment, quick effect, simple operation, flexible product grade conversion, less three wastes, suitable for China's national conditions, etc. The disadvantage is that the production scale is small and it is difficult to produce scale benefits. The device has many manual operations, intermittent production, low automation control level and unstable product quality; the consumption quota of raw materials is relatively high; the variety and grade of the product are few, the grade is not high, and the use is narrow. At present, the production capacity of polypropylene produced by this method in China accounts for about 24.0 per cent of the country's total production capacity;(2) continuous bulk process. The process mainly includes the American Rexall process, the American Phillips process and the Japanese Sumitimo process. (a)Rexall process. The Rexall bulk polymerization process is a production process between the solvent method and the bulk method process, which was successfully developed by Rexall Company in the United States. The process uses a vertical stirred reactor and uses liquid propylene with a propane content of 10%-30% (mass fraction) for polymerization. When deashing the polymer, an azeotropic mixture of hexane and isopropanol is used as a solvent, which simplifies the rectification step. The residual catalyst and atactic polypropylene are dissolved in the solvent together and discharged from the bottom of the solvent rectification column. Later, the company and the United States El Paso company formed a joint thermoplastic company, developed a new production process known as the "liquid pool process", the use of Montedison -MPC company's HY-HS efficient catalyst, the elimination of the de-ashing step, further simplify the process. The process is characterized by high purity liquid phase propylene as raw material, the use of HY-HS and efficient catalyst, no de-ash and de-random material process. A continuous stirred reactor is used, the polymerization heat is withdrawn by the reactor jacket and the top condenser, the slurry is separated by flash evaporation, and the monomer is recycled to the reaction; (B) Phillips process. The process was successfully developed by the Phillips Oil Company in the 1960 s. The process is characterized by the use of a unique loop reactor, this simple structure of the loop reactor has a large heat transfer area per unit volume, high total heat transfer coefficient, high one-way conversion rate, fast flow rate, good mixing, will not form a plasticized block in the polymerization zone, the product switch brand time is short and so on. The process can produce polymers and random polymers in a wide range of melt flow rates;(c)Sumitimo process. The process was successfully developed by Sumitimo (Sumitomo) Chemical Company of Japan in 1974. This process is essentially similar to the Rexene bulk process, but the Sumitimo bulk process includes some measures to remove random and catalyst residues. By these measures, superpolymers can be produced for certain electrical and medical applications. The Sumitimo bulk process uses SCC complex catalyst (titanium tetrachloride is reduced by monochlorodiethyl aluminum and treated by n-butyl ether). Liquid propylene is polymerized at 50-80 ℃ and 3.0 MPa. The reaction rate is high and the isotactic index of the polymer is also high. The high-efficiency extractor is also used to remove ash. The isotactic index of the product is 96%-97%. The product is spherical particles with high rigidity, good thermal stability, oil resistance and excellent electrical properties.