Polypropylene: Properties, Processing, and Applications

This versatile thermoplastic polymer got heads turning when it came onto the scene in the 1950s. Petroleum scientists Hogan and Banks, as well as European scientists Rehn and Natta, were responsible for its rapid development, and it quickly became available commercially.

Since then, polypropylene (PP) has enjoyed huge popularity and is now the second most widely used synthetic plastic in the world, second only to polyethylene (PE). You can find polypropylene in packaging, electrical work, equipment, appliances and construction work, among other applications.

Investors speculate that the global demand for polypropylene will exceed 60 million metric tons in 2020, with Asia accounting for half of the world capacity of polypropylene, followed by Europe, Middle East and Africa, North America, and Latin America in that order. According to a new research study, a compound annual growth rate (CAGR) of 3.9% for the global market for polypropylene pipes is projected, reaching up to $13.9 billion by 2024.

Here, you will learn about:

  • The physical and chemical properties of polypropylene
  • The different types of polypropylene
  • How polypropylene is produced and processed
  • The various applications of polypropylene

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Physical and Chemical Properties

Polypropylene is a linear hydrocarbon polymer. It is semi-rigid and saturated, also known as a polyolefin. Being one of the most versatile polymeric materials, PP is available either as a fibre or a plastic.

White and translucent in appearance, polypropylene is an all-around thermoplastic with high durability and lightweight. It has a low density, a slippery surface and a low coefficient of friction. It is also an excellent resistant to heat, electricity, fatigue, chemicals and organic solvents. Stress-cracking is not an issue for PP as it also displays good levels of corrosion resistance.

Here is a list of physical and chemical properties of polypropylene. Notice the high electrical resistivity and low coefficient of thermal expansion, which give PP the outstanding resistance and durability against heat and electricity.

Property

Polypropylene

Density

1.04 – 1.06 g/cm3

Elastic modulus

1.5 – 3 GPa

Impact strength, Charpy notched

2 – 6 kJ/m2 at 20 °C

Coefficient of thermal expansion

6*10-5 – 1*10-4 1/K at 20 °C

Max. service temperature, short

140°C

Melting point

160 – 168 °C

Specific heat capacity

1520 J/(kg.K) at 20 °C

Thermal conductivity

0.41 W/(m.K) at 20 °C

Flammability

UL 94 HB

Dielectric constant

2.8 at 20 °C

Electrical resistivity

1*1013 – 1*1014 Ω.m at 20 °C

 

In addition to that, and in spite of its lightweight, polypropylene is able to withstand high loads due to its good tensile strength. It is tough, resistant to biological factors, offers the possibility of colouring, and has a relatively low cost, which has led to its abundance in a variety of application areas.

Applying the polyethylene catalysts and technology to propylene gas allows polypropylene to crystallize. It can also be co-polymerized (normally with ethylene) to enhance material properties such as toughness and flexibility.

Like other thermoplastic materials, polypropylene is by definition recyclable, as new products can be manufactured by melting and reforming PP into plastic pellets.

Types of Polypropylene

Polypropylene can be produced flexibly to suit certain applications: the main forms on the market are homopolymers, block copolymers and random copolymers.

Here is an overview of materials for polypropylene describing certain aspects of each type of polymer or polymer combination.

Material

Description and benefits

PP homopolymer

It is the most common general-purpose grade of PP. It is semi-crystalline, solid, contains only PP monomers, and is suitable for a wide variety of applications, from plastic packaging to automotive and healthcare.

PP block copolymer

Ethylene-containing co-monomers (5 – 15% ethylene) are positioned in regular patterns called blocks. It is a tough and strong material with high impact resistance, suitable for industrial high-strength applications.

PP random copolymer

Ethylene-containing co-monomers (1 – 7% ethylene) are positioned in irregular patterns throughout the PP molecule. It has high flexibility and optical clarity, suitable for applications with optical transparency and good appearance requirements.

PP impact copolymer

It is a PP homopolymer with a co-mixed PP random copolymer phase, containing between 45 – 65% ethylene. With its high impact resistance, it is suitable for packaging, pipe and automotive applications.

PP terpolymer

It is a combination of propylene segments and randomly positioned monomers of ethylene and butane. It has high optical transparency and low crystalline uniformity and is a suitable material for sealing film applications.

PP high melt strength (HMS PP)

A long chain branched PP that has both high melt strength and stretchability. This polymer has a wide range of mechanical properties and high thermal and chemical properties, making it suitable to be used as low-density foams for various applications.

Expanded polypropylene (EPP)

It is a greatly versatile closed-cell bead foam with low density. It exhibits distinctive properties, such as high impact resistance, energy absorption, thermal insulation, and high strength-to-weight ratio. It is also used in many industries, such as automobiles, construction, and packaging.

Polypropylene Production and Processing

The two most common ways to make polypropylene are bulk slurry or gas phase production. In both cases, propylene, the monomer, is exposed to pressure, high temperatures and a catalyst.

Bulk slurry processing facilitates polymerisation by adding liquid propylene to the reactor. This method produces homopolymers and block copolymers successfully.

For gas-phase processing, gaseous propylene is placed with a solid catalyst inside a loop reactor, producing a fluidized bed. Random copolymers require the use of a gas phase reactor.

As a highly versatile polymer, polypropylene can adapt to various production methods. These include injection moulding, blow moulding, extrusion, and general-purpose extrusion. Some manufacturers are pushing to optimise or blend PP to be able to use it in additive manufacturing. The challenge lies in its semi-crystalline structure and severe warping.

Applications

Polypropylene’s unique qualities and ability to adapt make it suitable for an extremely wide range of applications.

Its chemical resistance makes it useful as a material for solvent containers. Living hinges are created from polypropylene plastic thanks to its shape retention and fatigue resistance properties. Electronic components also use polypropylene for electrical insulation. Other very common uses for polypropylene include flexible packaging, rigid packaging, piping, food containers, clear plastic bags, ropes, carpets and concrete reinforcement. Polypropylene fibres are used in clothing and diapers.

Polypropylene is an economical material, and polypropylene products can be seen in every industrial and commercial application area today. Those include the automotive sector, textiles, medical sector, consumer goods, and industrial applications.

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Advantages

  • Unique and flexible properties
  • Slippery surface that resists cracking
  • Low density
  • Good resistance to chemicals
  • Highly available and cost-effective