Polypropylene

Polypropylene is a polymer whose monomer is propylene (an organic hydrocarbon with the chemical formula C₃H₆). The chemical formula of polypropylene is (C₃H₆)_n. This polymer is also known as polypropene and is often denoted by the abbreviation ‘PP’. Generally, polypropylene is produced via a chain-growth polymerization reaction involving propylene. This polymer is known to be a thermoplastic polymer, i.e. it softens upon heating and can, therefore, be remoulded. It can also be noted that polypropylene is non-polar and has a partially crystalline structure.

The physical and chemical properties of polypropylene are quite similar to those of polyethylene in some regards. However, it must be noted that polypropylene has a greater resistance towards heat than polyethylene. The hardness of polypropylene is also superior to that of polythene. Under standard conditions for temperature and pressure (STP), polypropylene is known to exist as a white-coloured rugged solid. The chemical resistance of this polymer towards certain acids and alkalis is quite high. Polypropylene is often classified as a commodity plastic since its primary application lies in the manufacture of packaging products such as bags and bottles.

Physical and Chemical Properties of Polypropylene

The density of polypropylene is known to range from 0.89 to 0.92 grams per cubic centimetre. Varying properties are exhibited by different types of polypropylene, which is usually dependent on the crystallinity, molecular weight distribution, length of the chain, and the isotacticity of the macromolecule. The key characteristics exhibited by almost all types of polypropylene are:

• • • • Toughness and flexibility (exhibited to a large extent especially when the polymerization process involves copolymerization with ethylene).
      • Large thermal expansion.
      • Resistant to organic solvents.
      • Resistant to weak oxidizing agents.
      • Soluble in some organic nonpolar solvents like xylene.

      The crystallinity exhibited by polypropylene is intermediate to those of low-density polyethene and high-density polyethene. The three primary types of polypropylene include the homopolymer, the block copolymer, and the random copolymer. The copolymerization is usually carried out with a combination of propylene and ethylene. It is important to note that polypropylene is quite susceptible to chain degradation when it is exposed to temperatures above 100 degrees Celsius. The oxidation process usually begins at the tertiary carbon centres, resulting in the breakage of the chain due to chemical reactions with oxygen.

      Applications of Polypropylene

      • • • • Due to the high resistance of polypropylene towards fatigue, it is widely employed in the construction of plastic living hinges (especially the hinges on flip-top containers and flip-top bottles).
            • This polymer is also used in the production of plastic piping systems. An important advantage of polypropylene for this application is the strength and the rigidity offered by the material. Another key advantage of polypropylene piping is that it is resistant to chemical leaching and corrosion.
            • Since this plastic has enough heat resistance to withstand the heat levels in autoclaves, polypropylene can be employed for the manufacturing of plastic laboratory items and also plastic medical items.
            • Polypropylene is also used in the manufacture of mats, rugs, and carpets for home use.
            • Another important application of this polymer is in ropes (especially because the ropes made from polypropylene are so light that they actually float in water). Also, the strength of the ropes made from polypropylene is almost similar to the strength of the ropes made up of polyester.
            • Polypropylene sheets are also employed in the production of storage boxes, stationery folders, and other packaging items.
            • This thermoplastic polymer is also used in the manufacture of loudspeaker drive units.
            • Fibres made up of polypropylene can be used to reinforce drywall joints.

            Frequently Asked Questions on Polypropylene

            What are the key characteristics of polypropylene?

            • Polypropylene is known to have a high melting point, making it an ideal material for the construction of microwave-friendly containers.
            • Polypropylene is chemically resistant towards water, alkalis, and acids.
            • This polymer is also quite resistant to stress and cracking, even when it is flexed. This is the reason why polypropylene is widely used in the construction of hinges.
            • Polypropylene exhibits high resistance to wear and tear. Therefore, this polymer is highly durable.

            What are the uses of polypropylene?

            Polypropylene is considered to be a very good material for several packaging applications because of its strong barrier properties, good surface finish, low cost, and high strength. The excellent optical clarity of polypropylene films, in combination with its low vapour-moisture transmission, makes it ideal for food packaging applications. This polymer is often moulded with a blow in order to produce bottles, pots, and crates. Thin-walled polypropylene containers are widely used for the packaging of food items. Polypropylene is also used in various consumer goods and household products, especially those containing translucent parts. Common examples include houseware, furniture appliances, toys, and luggage.

            What are the disadvantages of polypropylene?

            • Polypropylene does not have much resistance towards ultraviolet light. It also has poor scratch resistance.
            • At temperatures below 20 degrees Celsius, polypropylene is known to become quite brittle.
            • The upper service temperature of polypropylene is quite low as well, ranging from 90 to 120 degrees Celsius.
            • This polymer is known to undergo rapid swelling when it is exposed to chlorinated solvents or aromatics. Furthermore, polypropylene is known to be quite vulnerable to powerful oxidizing agents.
            • Polypropylene also offers extremely poor paint adhesion.

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