Polypropylene is widely regarded as one of the most versatile plastics. Just look around and you will surely spot it, albeit in different forms – whether it’s in your kitchen (appliances, cutlery and utensils), around your living room (furniture, carpets, rugs), your kids’ room (toys), your garage (car batteries, packaging material) or even your closet (a few selected pieces of sports apparel! Surprised?!).
Polypropylene is a thermoplastic (that which is mouldable at high temperatures, and solidifies upon cooling), made of many propylene/propene monomers. Chemists from Philips Petroleum, Robert Banks and Paul Hogan, first polymerized propylene in 1951.
Did you know? Polypropylene is the second most-widely manufactured synthetic plastic, next in line after polyethylene.
How is Polypropylene manufactured?
Propylene monomer, obtained from naphtha, is subjected to heat and pressure to obtain a translucent polypropylene. Polypropylene is typically produced by one of three methods – hydrocarbon slurry, bulk slurry, or gas phase process.
Polypropylene properties and uses
Polypropylene (PP) can be fabricated using a variety of techniques, and is well-suited for a wide range of applications, which has made it a popular choice among plastics (think Rubbermaid and Sterilite), with sustained growth in the plastic industry for many years now. Most polypropylene products are manufactured by extrusion or injection moulding.
Let’s examine a few material properties that make polypropylene so versatile:
Low density: PP has a low density, which makes it a good lightweight option for the manufacture of plates, cups, bottles, food containers, trays and toys, and industrial applications like pipes and sheets. This property also translates to significant savings in weight for manufacturers.
High chemical resistance: PP has a high resistance to organic solvents at room temperature, which makes it an ideal choice for containers of acid-based liquid cleaning agents.
High elasticity and toughness/stiffness: Polypropylene, by itself, is very tough and durable, which is why it is used in packaging material (crates, pails, boxes). PP can also be combined with other plastics like polyethylene to form composite plastics. This process, called co-polymerization, significantly changes its material properties, thus rendering it useful for robust and advanced engineering applications like medical equipment, extreme-weather gear, and automotive parts (bumpers, dashboards).
High fatigue-resistance: PP can be made into very thin pieces that can bend, without cracking or breaking, even over a wide range of motion covering almost 360 degrees. Hence, it is used in the manufacture of hinges for lids in bottles (ketchup, shampoo).
Good yield strength: Polypropylene yield strength can be in the wide range of 12-43 Mpa.
Good insulator: PP is a good insulator and is therefore, used in electronic devices. It is also used as an alternative to PVC in the insulation of electrical cables.
High melting point: PP is one of the key components in car batteries. The fact that over 95 percent of these car batteries get recycled, makes it an environment-friendly option.
High heat resistance: PP can withstand high heat, like that in an c, and is therefore, used to make consumer-grade kettles.
Pliable: The addition of rubber to PP makes it more pliable. Minerals are also added to polypropylene to form synthetic papers. These plastic papers can be easily printed on, folded, sewn and cut and are used in banners, signs, tags, menus, maps, booklets, and cards.
BOPP (Biaxially Oriented PolyPropylene): The biaxial orientation of polypropylene gives it a transparent appearance, and is used to make clear plastic bags, and as packaging for retail products.
Same material functioning as a Plastic and Fiber?! Have you felt the texture of those promotional totes given out at fairs and other events? Would you believe that they have plastic in them? Polypropylene has this unique ability to function as a plastic and a fiber, and is therefore, used in the manufacture of reusable tote bags.
Environment-friendly/Sustainability: PP is one of the most ‘green’ plastics around! PP (and PE) consumes very little energy during production, with minimal carbon dioxide emissions. Materials made from PP can be recycled several times before incineration. Even after incineration, it is possible to recover more energy from PP when compared to other plastics. Industry giants like McDonald’s and Starbucks have gradually switched to PP cups, thus reducing greenhouse emissions.
One of the main drawbacks with polypropylene is that it has a ‘slippery’ surface and therefore, cannot be ‘glued’ like other plastics. In such applications, welding is used to form a joint with other surfaces. This poor bonding property also makes it difficult to paint over it.
Polypropylene is tough and durable. The fact that it can be recycled many times over, before requiring incineration, makes it an alluring option. It is being used in almost every market that requires plastics, which, sort of, makes it a ‘superhero’ among plastics!