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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.


Polymerization reaction

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.


Polypropylene Production

Production Processes

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.


 Iran Polypropylene export

Fig 3. Generic Polypropylene (olefin) polymerization process, simplified


Gas-Phase Polymerization

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.


Polypropylene exporter

Fig 4. Polypropylene gas-phase production example


Liquid-Phase Polymerization

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


Polypropylene Analysis and Usages

The distinctive characteristics of Polypropylene are its low density, high chemical resistance, its toughness and it would be able to orientated (Moore, 1996). The applications to which Polypropylene has been found to be particularly suited are in the field of films and fibers and to this end different technologies have been developed which make it possible ‘to stretch’ the polymer, significantly enhancing its properties.



The Polypropylene orientation is result of heating the item to a temperature at which the crystals are partially melted, 120-160°C, stretching it to the target shape and next step is cooling it during the stretching to reform the crystals in such a way as to retain their orientation. The macromolecular chains are forced to align themselves during this processing and can very easily form crystals: Moreover, the most macroscopic effect is increase sharply in crystallinity. Alongside, the awesome phenomenon caused by orientation, there is also an increase in toughness and in the flexural modulus, which grow in proportion to the amount of stretching. The increasing of toughness and rigidity is showed by the high number of chains aligned in the stretching direction, consequently a smaller section can accept a heavier load. The orientation also has significant effects on the low temperature impact resistance, while it causes of decreasing the elongation at break. If it is reheated to a temperature close to its melt point, the item shrinks and has the tendency to assume a form similar to that of a non-orientated piece. Very important from the application point of view are Polypropylene bi-orientated films (BOPP). The properties of a BO Polypropylene can be seen in Table 1, in comparison with those of other films obtained from other polymers.







   Traction resistance (MPa)





   Flexural modulus (MPa)





   Elongation at break (%)





   Tear resistance (N/mm)





   Opacity (%)





   Transport velocity O2ASTM D1434





Table 1. Comparison of the properties of film obtained from Polypropylene and BOPolypropylene and other polymeric materials



one of the important Polypropylene application is homopolymer, consists of its use in obtaining orientated fibers which are used, for instance, in the production of the common raffia. The processing method including, obtaining a sheet of homopolymer through an extrusion process and then passing it through a series of blades to gain the ribbons.

Another method for gaining the oriented fiber is to extrude molten strands and simultaneously cool them with high-speed air. The strip of fiber thus obtained is collected on a mobile support and hot-welded into a nonwoven fabric (spunbonded). Homopolymers with medium-high fluidity and a narrow MWD are used for these applications. Another way of process, the fibers are ‘melt-blown’, orientated through the effect of the air which comes out of the nozzle along with the polymer. The melt-blown process enables very thin fibers to be obtained which contained not heat-welded.

in Table 2 the properties of orientated Polypropylene fiber are shown and compared with nylon and polyester fibers.

The fibers thus obtained are used in the production of raffia, which in turn is used to produce ropes. The development of this technology placed at the beginning of 1960s, was facilitated by the fact that it was possible to adapt the equipment typically used in producing of ropes without having to carry out any special modifications. These products would be used to supply fabric for bags, waxed fabrics, and geo-membranes for the construction of embankments and for civil engineering applications. in the carpet sector Polypropylene has several applications, both for the so-called primary or base part and for the secondary part.






   Melt temperature (°C)




   Density (g/cm3)




   Toughness (MPa)




   Elongation at break (%)




   Modulus (MPa)




   Shrinkage at 100°C (%)




   Absorption of humidity (%)




Table 2. Comparison of the properties of orientated Polypropylene fibres and fibres obtained from other polymeric materials


Specialized applications


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.

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