The Basic Structure, Forming Function and Preparation Method of PP

1.Basic Structure and Characteristics:

Polypropylene (PP) is an odorless, non-toxic milky white granular or powdered product with a relative density of 0.90 to 0.91. Melting point 164 ~ 167℃. Has excellent mechanical properties, heat resistance, electrical insulation performance, chemical stability is good, and don’t react with most chemicals. But the light resistance is poor, easy to aging, low temperature impact strength is poor, poor dyeing, need to add additives, blending, copolymerization and other methods to improve. It is insoluble in water and does not absorb water, and can be boiled in water , disinfected at 130℃,  easy to process and mold.

If the METHYL disordered arrangement on both sides of the molecular main chain called ATACTIC  PP; when the METHYL groups are arranged alternately on either side of the main chain of the molecule, they are called SPP. In the general production of PP resin, the content of ISOTACTIC structure is 95%, the rest is ATACTIC or ISOTACTIC PP. Industrial products are mainly composed of ISOTACTIC substances. PP also includes a copolymer of propylene with a small amount of ethylene. Usually a translucent colorless solid, odorless and non-toxic. Because the structure is regular and highly transparent, so the melting point is as high as 167℃, heat resistance, products can be used steam disinfection is its outstanding advantages. Density 0.90g/cm3, is the lightest general plastic. Corrosion resistance, tensile strength 30MPa, strength, rigidity and transparency are better than PP. The disadvantage is low temperature impact resistance is poor, easy to aging, but can be modified and added antioxidant to overcome.

2. Molding Characteristics:

(1) Crystalline material, low humidity, easy to melt fracture, long-term contact with hot metal easy to decompose.

(2) Good liquidity, but the shrinkage range and shrinkage value is large, easy to occur shrinkage cavity. Indentation, deformation.

(3) The cooling speed is fast, the pouring system and the cooling system should be slow heat dissipation, and pay attention to the control of the forming temperature, the material temperature is easy to orientation at low temperature and high pressure, the mold temperature is lower than 50 ℃, the plastic parts are not smooth, easy to produce poor welding, flow marks, 90 ℃ above the buckling deformation. Therefore, the temperature should be controlled at 80 ℃.

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

3.Functional Characteristics:

    PP is a semi-crystalline material, which is harder and has a higher melting point than PE. Due to the HOMOPOLYMER type PP temperature above 0℃ is very brittle, many commercial PP materials are random copolymers with 1~4% ethylene or block copolymers with higher ratio of ethylene content. The copolymer type PP material has a lower thermal deformation 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 was 150℃.  Due to the High CRYSTALLINITY, the surface stiffness and scratch resistance  of this material are good. There is no environmental stress cracking  problem in PP. Usually, PP is modified by adding glass fiber, metal additive or thermoplastic rubber. The MFR of PP is in the range of 1 to 40. The PP material with low MFR has better impact resistance but lower tensile strength. For the same MFR, the strength of the copolymer type is higher than that of the HOMOPOLYMER type. Due to crystallization, the shrinkage rate of PP is quite high, generally 1.8~2.5%. And the direction uniformity of the shrinkage rate is much better than PE-HD and other materials. Adding 30% glass additive can reduce the shrinkage rate to 0.7%. HOMOPOLYMER type and copolymer type PP materials have excellent resistance to HYGROSCOPICITY, acid and alkali corrosion, solubility. However,it is not resistant to aromatic hydrocarbon (e.g., benzene) solvents, chlorinated hydrocarbon (carbon tetrachloride) solvents, etc. PP also does not have the same antioxidant properties at high temperatures as PE.

4.Modified PP:

(1) Chlorinated polypropylene:

CPP(Chlorinated Polypropylene) has up to 65% chlorine. Chlorinated polypropylene is resistant to abrasion, acid and water. Commonly used as a filler for protective coatings, inks, adhesives, paper coatings, and films.

 (2) Reinforced polypropylene

Reinforced polypropylene (PP) is a mixture of PP and glass fiber or organic fiber, asbestos,or inorganic fillers (talcum powder, calcium carbonate).

Glass fiber reinforced PP for general industrial use contains 10 ~ 15% fiber. Good heat resistance and dimensional stability due to the inclusion of glass fiber.

Reinforced polypropylene is used to make various machine parts, especially automotive parts, as well as corrosion-resistant pipes, fittings, valves, etc.

    (3) Graft polypropylene

The purpose of graft copolymerization of PP with ethylene, styrene and METHYL METHACRYLATE is to improve the tensile strength, impact strength and adhesion strength of PP with other materials.

GPP(Graft Polypropylene) can be used as an olefin adhesive, coating and waterproof coating,it can also be as a tube and board material.

5.Production Methods:

   5.1 Slurry  Process

Slurry Process, also known as Slurry or solvent Process, is the earliest technology used to produce PP in the world. From the first industrial plant in 1957 to the middle and late 1980s, the slurry process has been the main process for the production of  PP for 30 years. Typical processes mainly include Montedison process from Italy, Hercules process from the United States, Mitsui Toshiba chemical process from Japan, Amoco process from the United States, oil chemical process from Mitsui from Japan and Sowell process. The development of these processes is based on the first generation of catalysts at that time, using vertical stirred tank reactor, which needs to remove ash and random matter. Due to the different solvents used, the process flow and operating conditions are different. In recent years, the proportion of traditional slurry process in production has decreased significantly, and the retained slurry products are mainly used in some high-value fields, such as special BOPP film, high relative molecular weight blow molding film and high strength pipe material. In recent years, the method has been improved, the improved slurry production process using a high activity of the second generation of catalyst, can delete the catalyst demineralization step, can reduce the generation of random polymer, can be used for the production of HOMOPOLYMER, random copolymers and impact copolymers, etc. At present, the production capacity of slurry PP in the world accounts for about 13% of the total global PP production capacity.

5.2 Bulk Process

The research and development of the bulk process began in the 1960s. In 1964, DART Company of the United States built the world’s first industrial bulk PP production plant using a kettle reactor. After 1970, Japan Sumitomo, Phillips, the United States EI PSAO and other companies have realized the industrial production of liquid phase bulk polypropylene process. Compared with the solvent slurry method, using liquid propylene ontology method to aggregate has not use inert solvent, reaction system in high monomer concentration, polymerization rate, high catalytic activity, high conversion rate of polymerization, reactor space-time capacity bigger, low energy consumption, simple process, less equipment, low production cost, less “three wastes”; It is easy to remove the polymerization heat and simplify the heat removal control, which can  increase the amount of polymerization in unit reactor. It can remove the low molecular weight random polymer and catalyst residue which have bad effect on the properties of the products, and can obtain the advantages of high quality products. The disadvantage is that the reaction gas needs to be gasified and condensed before it can be recycled back to the reactor. The high pressure liquid hydrocarbon material in the reactor has large capacity and potential danger. In addition, the concentration of ethylene in the reactor should not be too high, otherwise a separate gas phase is formed in the reactor, which makes the reactor difficult to operate, so the content of ethylene in the resulting copolymerization products will not be too high.

The main difference between different process routes of bulk method is the difference of reactor. Reactors can be divided into cauldron reactors and ring – pipe reactors. The cauldron reactor uses the latent heat of liquid evaporation to remove the heat of reaction. Most of the evaporated gas is returned to the reactor after circulation condensation, and the uncondensed gas is recycled back to the reactor after pressure boost by the compressor. And the ring pipe reactor is the use of axial flow pump to make the slurry high-speed circulation, through the jacket cooling heat removal, because the heat transfer area is large, the heat removal effect is good, so its unit reactor volume yield is high, low energy consumption.

According to the polymerization process, the bulk process can be divided into batch polymerization process and continuous polymerization process.

(1) Batch bulk process:

The batch bulk polypropylene polymerization technology is a successful production technology developed by ourselves in China. It has the advantages of reliable production technology, low quality requirement of raw material propylene, domestic guaranteed catalyst, simple process, low investment, quick return, simple operation, flexible brand conversion,less waste, suitable for China’s national conditions, etc. The disadvantage is that the production scale is small, it is difficult to produce scale efficiency; Device manual operation more, intermittent production, low level of automation control, product quality is not stable; the consumption quota of raw materials is higher; the product variety brand is less, the grade is not high, the use is narrow. At present, the production capacity of polypropylene produced by this method accounts for about 24.0% of the total production capacity in China.

(2) Continuous bulk process:

The process mainly includes Rexall process, Phillips process and Sumitimo process.

(a) Rexall process:

Rexall bulk polymerization process is a production process between solvent method and bulk process. It was developed by Rexall Company in the United States. The process uses  vertical stirred reactor to polymerize liquid propylene with a propane content of 10%-30% (mass fraction).The AZEOTROPIC  mixture of hexane and ISOPROPANOL was used as the solvent for polymer  desalting, simplifying the rectification steps. The residual catalyst and ATAVTIC PP were dissolved together in the solvent and discharged from the bottom of the solvent rectification column. Later, the joint thermoplastics company formed by the company and El Paso in the United States developed a new production process known as the “pool  process”, which uses Montedison-MPC’s HY-HS high-efficiency catalyst to eliminate the desiccating step and further simplify the process. The process is characterized by the use of high purity liquid propylene as raw material, HY-HS  high efficiency catalyst, no deliming and random matter removal procedures. A continuous stirring reactor was used, and the jacket and top condenser of the reactor for polymerization heat were withdrawn. After the slurry was separated by flash evaporation, the monomer was recycled for reaction.

(b) Phillips process:

The process was developed by Phillips Petroleum in the 1960s. The process is characterized by a unique round-tube reactor, which has the advantages of large heat transfer area per unit volume, high total heat transfer coefficient, high one-way conversion rate, fast flow rate, good mixing, no plasticizing block in the polymerization area, and short switching time of product brand. The process can produce polymers and random polymers with wide range of melt flow rates.

(c) Sumitimo process:

The process was developed by Sumitomo Chemical Company in Japan in 1974. The process is basically similar to the Rexene bulk process, but the Sumitimo bulk process includes some measures to remove random materials and catalyst residues. By these measures,SUPERPOLYMERS can be made for certain electrical and medical uses. Sumitimo bulk process uses SCC complexing catalyst (reducing titanium tetrachloride with MONOETHY aluminum and n-butyl ether treatment), liquid phase propylene polymerization at 50-80℃, 3.0MPa, the reaction rate is high, the polymer ISOTACTIC index is also high, but also the use of efficient extractor descaling, product ISOTACTIC index is 96-97%. Products are spherical particles, high rigidity, good thermal stability, oil resistance and superior electrical properties.

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