Polypropylene is one of the most widely used and basic polymers in the world and the largest consumer of propylene. The name of this product is polypropylene (PP) and its chemical formula is –CH2-CH(CH3)n–. What is polypropylene and what are its types and applications?
Polypropylene is produced by the polymerization of propylene under relatively mild temperature and pressure conditions in the presence of the famous Ziegler-Natta catalyst. The presence of this catalyst forms an isotactic polymer that is capable of crystallizing up to about 90%.
Polypropylene is a thermoplastic polymer that is used in a wide range of applications including film and sheet, blow molding, injection molding, food packaging, textiles, laboratory and medical equipment, pipes, industrial and construction applications, and automotive components. In addition, the polymer produced from the propylene monomer is usually resistant to chemical solvents, bases, and acids. The characteristic code of this polymer is.
The propylene molecule has an asymmetric chemical structure, so its polymerization process can lead to three types of sequences in the resulting polymer structure. Due to the effects of steric hindrance of the methyl groups, the head-to-tail sequence has a higher structural order than the other types.
Polypropylene has three different spatial configurations: isotactic (iPP), syndiotactic (sPP), and atactic (aPP). In the isotactic type, the methyl groups are on one side of the plane passing through the main chain. In the syndiotactic type, the methyl groups are alternately located on both sides of the plane passing through the chain. In the atactic type, there is no specific type of order.
A Ziegler-Natta catalyst is able to restrict the arrangement of monomers in a specific configuration, allowing only the monomers to be added to the polymer chain in the correct direction. Most conventional polypropylenes produced using titanium chloride (TiCl4) catalysts contain a high percentage of isotactic polypropylene. Because the methyl groups are on one side, some molecules tend to form a spiral, which is arranged one by one and gives the strength of conventional polypropylene.
Commercialized iPP has a variety of properties that have led to its widespread use, especially in the plastics and fiber industries. One of the most important properties of this material compared to polymers such as polyamides is its lack of moisture absorption, which makes it a suitable choice for many applications. The properties of this material can be improved by some further modifications. The most important modifications that are currently being carried out are the control of the degradation process, crosslinking, functionalization and branching. The structure of the polypropylene molecule is linear due to the nature of Ziegler-Natta catalysts, which causes its low melt strength. Low melt strength limits the application of this polymer in processes such as blow molding and thermoforming.
Compared to other polymers, polypropylene has distinctive and outstanding characteristics, including:
Relatively cheap price of propylene monomer compared to other polymer monomers
Low price of PP compared to other polymers
Light weight and specific gravity of PP
Flexibility and wide range of PP production with variable physical and chemical properties
Increase in new applications and improvement of properties of new production grades
Increase in the use of PP in medical devices and equipment and development of applications of special grade PP
Increase in the use of PP in the form of alloys with other polymers
Replacement of polymers such as PS, PE, etc. with PP
Types of polypropylene grades
Polypropylene materials are generally divided into two general categories. Polypropylene homopolymer, which is produced from the polymerization of propylene monomer alone, and polypropylene copolymer, which is produced from the polymerization of propylene with ethylene comonomer.
From the point of view of physical and mechanical properties, the difference between polypropylene homopolymer and copolymer is in impact resistance, tensile strength and hardness. Although polypropylene homopolymer has higher tensile strength and hardness than polypropylene copolymer, its main weakness is its impact resistance. In fact, polypropylene homopolymer is more brittle than polypropylene copolymer. This weakness is more evident at low temperatures, especially below zero. Therefore, the use of polypropylene homopolymer in the production of injection molded parts that are exposed to impact and low temperatures is severely limited. To overcome this weakness, polypropylene copolymer has been produced by adding ethylene monomer during propylene polymerization. Polypropylene copolymer has higher impact resistance than polypropylene homopolymer. By adjusting the amount of ethylene added to the polymer structure, adjusting the morphology of the copolymer, adjusting the type of crystallization, and also adjusting the molecular weight, the impact resistance can be adjusted and increased to a good extent. Of course, it should be noted that increasing the impact resistance of polypropylene is done at the cost of reducing the hardness and stiffness of the polymer.
Adding ethylene to the polypropylene structure during the polymerization of propylene reduces the structural order of polypropylene. Reducing the structural order, in turn, reduces the crystallization rate of polypropylene. Given the brittle structure of the crystals, the reason for the increased impact resistance of polypropylene copolymer compared to polypropylene homopolymer is the reduction in crystallinity.
Polypropylene copolymer itself is also divided into two categories: block and random copolymers.