MGT PetrOil Company as a major producer, supplier and exporter of many petrochemical products in the Middle East and is located in Iran. MGT PetrOil Company is professional in exporting and have many activities in case of Polypropylene supplying.
Polypropylene (PP) is a member of the polyolefin class of polymers. These polymers are produced from olefin monomers (unsaturated hydrocarbons); ethylene is the monomer for polyethylene. Polypropylene plastics began to find commercial applications in the 1950s. the second liquid popular in commercial importance only to those based on polyethylenes is belonging to Thermoplastics based on Polypropylene. World production of Polypropylene reached 30 million tonnes in 2000 and is likely to increase with new production capacity.
Unlike polyethylene, Polypropylene including three basic polymeric categories: isotactic, syndiotactic and atactic. These three polymeric forms arise because, compared to the starting substance for polyethylene CH2=CH2 (ethene), the starting substance propylene CH3•CH=CH2 (propene), has a methyl (CH3) group in place of a hydrogen. In the isotactic form of Polypropylene the methyl group has the same configuration at each tertiary carbon atom along the polymer chain. Second categories which is syndiotactic form, the methyl group alters position on alternative tertiary carbon atoms. Next for atactic form, the methyl group takes up random positions on the tertiary carbon atoms.
Schematic introduced the three different forms of polymer which are given in Figure 1. The polymer chains are actually helical and with the isotactic form the methyl groups all point outwards.
Polypropylene is produced by polymerizing propylene, a gaseous byproduct of petroleum refining, in the presence of a catalyst under carefully controlled heat and pressure Propylene is an unsaturated hydrocarbon, including just carbon and hydrogen atoms:
Fig 1. Propylene
In the polymerization reaction, many propylene molecules (monomers) are joined together to make a large molecule of polypropylene. Propylene is reacted with an organometallic, transition metal catalyst (see 1.4 Catalysts for a description of catalysts used in the reaction) to provide a site for the reaction to occur, and propylene molecules are added sequentially through a reaction between the metallic functional group on the growing polymer chain and the unsaturated bond of the propylene monomer.
One of the double-bonded carbon atoms of the incoming propylene molecule inserts itself into the bond between the metal catalyst (M in the above reaction) and the last carbon atom of the Polypropylene chain. A long, linear polymer chain of carbon atoms is formed, with methyl (CHJ groups attached to every other carbon atom of the chain. Thousands of propylene molecules would be able to added sequentially until the chain reaction is terminated.
Unlike polyethylene, Polypropylene has three basic polymeric forms: isotactic, syndiotactic and atactic. These different polymeric forms arise because, compared to the starting substance for polyethylene CH2=CH2 (ethene), the starting substance propylene CH3•CH=CH2 (propene), has a methyl (CH3) group in place of a hydrogen. In the isotactic form of Polypropylene the methyl group has the same configuration at each tertiary carbon atom along the polymer chain. In the syndiotactic form, the methyl group alters position on alternative tertiary carbon atoms. In the atactic form, the methyl group takes up random positions on the tertiary carbon atoms.
Schematic shows the three polymer forms can be seen in Fig 2. The polymer chains are actually helical and with the isotactic form the methyl groups all point outwards.
Fig 2. Schematic illustrations of the three polymeric forms of polypropylene
Commercial propylene homo-polymers are primarily isotactic with <5% of the atactic form and are high-molecular-weight semi-crystalline solids.in this process toughness is moderate, but tensile strength and stiffness are excellent. It is the semi-crystalline nature as well as the other properties that make the isotactic form the most suitable for several applications like, a commercial plastic which is used for food packaging and other applications.
There are various production processes for Polypropylene with some general similarities. But the processes are evolving continuously. consequently, the specifics can be significantly different and the following descriptions and graphic displays should be, therefore, considered exemplarily only with no direct relation to existing plant or process designs.
Generic polymerization process
In Fig 3 can be seen the similarities between the processes follow a generic olefin polymerization process scheme:
- Feedstock materials and additives should be purified and catalyst material must be prepared. And - in case of a high pressure process (not used for PP) - the gas should be compressed in many stages.
- Polymerization of propylene takes place either in the gas phase (fluidized bed or stirred reactor) or a liquid phase (slurry or solution).
- Polymer particles are then separated from still existing monomers and diluents, pelletized, dried and dispatched.
- Monomers and diluents are recovered and fed again to the process.
Fig 3. Generic Polypropylene (olefin) polymerization process, simplified
In gas-phase polymerization the propylene is contacted with solid catalyst material intimately dispersed in an agitated bed of dry polymer powder. There is Two methods to carry out this reaction:
- Firstly, In the fluidized-bed process the monomer flows through a perforated distribution plate at the reactor bottom and rapid gas circulation ensures fluidization and heat removal. Unreacted polymer is separated from the polymer particles at the top of the reactor and recycled. Fluidized-bed plants are able to produce a wide range of polypropylene. A modification uses a second reactor connected in series to perform copolymerization.
- Secondly, the stirred-bed process uses a horizontal or vertical reactor with compartments, in which the bed of polymer particles is agitated by mixing blades.
The most economical technology is the gas-phase polymerization and flexible which can accommodate a large variety of catalysts. It is by far the most common process in modern Polypropylene production plants.
Fig 4. Polypropylene gas-phase production example
In liquid-phase processes catalyst and polymer particles are suspended in an inert solvent, typically a light or heavy hydrocarbon. Super-critical slurry polymerization processes use supercritical propane as diluent.
Slurry processes run in loop reactors with the solvent circulating, stirred tank reactors with a high boiling solvent or a "liquid pool" in which polymerization takes place in a boiling light solvent. As a result of existing different catalysts using of this process is useful. Processes in solution require, as their last step, the stripping of the solvent.
Supercritical polymerization in the slurry loop has many advantages (e.g. higher productivity, improved product properties) over subcritical polymerization.
Advanced processes combine a loop reactor with one or two gas-phase reactors, placed in series, where the second stage of the reaction takes place in the gas-phase reactors. For bimodal polymers, lower molecular weights are formed in the loop reactor, while high molecular weights are formed in the gas-phase reactor.
Fig 5. Polypropylene liquid-phase production example
Nonwoven-fabrics: the most important application of Polypropylene is in the field of nonwoven fabrics, made possible by the availability of a polymer with high fluidity and a narrow MWD. nonwoven fabrics is including three different types and characterized by different properties, appearances, obtained through the following production methods: spunbonded, carded and thermobonded webs from staple fibers and melt-blown. The first are very tough, while the second are voluminous and soft. Melt-blown technology uses Polypropylene with high fluidity and produces fine fibers (2-4 microns’ diameter), suitable for fabrics with high absorbing power and selective filtering capability.