All About Polypropylene Plastic

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Polypropylene is an additional polymer made from monomer propylene. Polypropylene is regarded as the “steel” of plastics because it can be modified and customized in multiple ways to suit a wide variety of applications. It is, in fact, the second-most widely produced commodity plastic. Over 70 million tonnes of polypropylene were consumed in 2017.

History of polypropylene

Polypropylene was first made in 1951 and within 6 years, its popularity exploded. Two chemists, J. Paul Hogan and Robert Banks, from American oil company Phillips Petroleum first polymerized polypropylene in 1951. This was the first known attempt at making polypropylene.

Three years later, in two independent attempts in 1954, a German scientist, Karl Rehn, and an Italian Nobel laureate, Guilio Natta, polymerized Propylene into a crystalline isotactic polymer. These milestone events paved the way for mass production of isotactic polypropylene by an Italian chemical company called Montecatini. The use of polypropylene grew significantly in the following decades. Today, polypropylene has become the material of choice for manufacturers in a wide range of industries.

How is polypropylene made?

Polypropylene can be manufactured via three ways:

1) Hydrocarbons: The first method of producing polypropylene involves the use of inert hydrocarbon diluent that facilitates a reaction between concentrated monomer propylene and a catalyst. The reaction leads to co-polymerization of the monomer into long chains of polypropylene. Once the reaction takes place, the heat and catalyst are removed to give polypropylene. This technology is old and now losing popularity.

2) Bulk Slurry: This method is similar to the above method, but instead of using hydrocarbon, the method uses liquid propylene. The polymer formed in this process rides on the liquid propylene rather than getting dissolved with it.

3) Gaseous propylene: This last method uses gaseous propylene along with a catalyst to create polypropylene.

Who uses Polypropylene?

30% of polypropylene is used in packaging-related applications

13% of polypropylene is consumed by manufacturers of electrical components and other equipment

10% of polypropylene is used by the automotive industry

10% of polypropylene is consumed by manufacturers of domestic household appliances

5% of polypropylene is used by construction materials manufacturing 

Types of polypropylene

There are two different types of polypropylene, homopolymers and copolymers. Homopolymers are the default polypropylene material. They are considered to be the general-purpose grade of polypropylene material. Copolymers, on the other hand, have additives such as ethylene in them. Within copolymers, there are two sub-types: block copolymers and random copolymers. Block copolymers have 5% to 15% ethylene in them while random copolymers have 1% to 7% ethylene in them. This variation in the levels of ethylene and other additives improves properties like resistance and malleability in the copolymer material.

Properties of polypropylene

  • One of the major utilities of polypropylene is the fact that it can be made into a living hinge. A living hinge is a thin piece of material that can bend without breaking. Hinges are used in structural applications such as allowing doors and windows to open and close. These are heavyweight applications that use hinges made of steel and metal. But, there are other ways in which living hinges can be used in domestic applications like a lid on a bottle or a small mint box. In such applications, polypropylene can be used as a living hinge because it will not break even after repetitive bending.
  • The chemical properties of polypropylene are such that polypropylene does not react with diluted bases and acids, which are often found in liquids, first-aid products, and cleaning agents. This makes polypropylene an ideal material to manufacture containers for such liquids.
  • Polypropylene is a bad conductor of electricity and is classified as an insulator. That makes it a great material for manufacturing certain electronic components such as polypropylene capacitors and audio equipment. Electric cables also use polypropylene for insulation.
  • Polypropylene is desirable because of the way it responds to heat. Polypropylene will turn to liquid when it reaches its melting point. The liquid can then be molded into any desired shape. Injection molding uses polypropylene due to the way it responds to heat. It is also worth noting that polypropylene can be heated, liquefied, cooled, and then reheated again without any significant degradation.
  • Polypropylene has a high resistance to corrosion and chemical leaching. Hence, it is the material of choice for manufacturing piping systems. It has good resistance to freezing as well, so climatic conditions are not an issue when pipes are made using polypropylene.
  • Polypropylene can withstand heat in a pressure chamber or autoclave. Hence, it is a great material to make laboratory and other medical containers. It is even used to make food containers because it can withstand the heat in a dishwasher.
  • Polypropylene has high tensile strength and can withstand up to 4800 psi. That makes it a useful material in applications involving heavy loads and rugged use.
  • Polypropylene is highly impermeable material. It will absorb less than 0.01% of water when soaked in it for 24 hours. This makes it an ideal material for products which are to be submerged in liquids. Polypropylene is used in water-proofing of roofs as well. On the flip side, polypropylene can be treated with certain chemicals to make it absorb water. In such cases, it can be used for making things like sanitary napkins and diapers.
  • Polypropylene can be turned into a fiber form, which can allow for the making of mesh-like surfaces. Such surfaces are great for manufacturing filters used in water purification systems and air-conditioning systems.
  • Polypropylene has a low density as compared to other plastic materials. Hence, it gives manufacturers the benefit of cost savings from the low weight.

What are some of the disadvantages of polypropylene?

  • Polypropylene is highly flammable. It will melt when exposed to heat and has a flash point of 260 degrees Celsius.
  • Polypropylene tends to be affected by UV degradation. Hence, it will not be preferable in high altitude areas and geographies where UV penetration is high.
  • Polypropylene has limited high-temperature uses because it has a high thermal expansion co-efficient.
  • The surface of polypropylene has poor bonding properties and hence it is difficult to paint it. One way to address this problem is to treat the polypropylene surface with certain chemicals that can enhance the adhesive strength of paints and inks used to color the polypropylene material.
  • In temperatures above 100 degrees Celsius, polypropylene tends to suffer from chain degradation which leads to oxidation. Oxidation results in cracks and crazing. This issue can be addressed by using polymer stabilizers.

Properties and Specs

Property TypeDetail
Scientific NamePolypropylene (PP)
Resin Identification CodeCode 5
Chemical Formula(C3H6)n
Tensile Strength4800 PSI
Tensile Modulus1.0 (co-poly) 1.4 (homo-poly) Gpa
Specific Gravity0.91
Melting Temperature130oC
Flexural Strength41 MPa (6000 PSI)
Volume Resistivity19 logÙm
Shrink Rate1.5 – 2.0% (0.015 – 0.02 in/in)
Notched Izod Impact0.1 kJm-1 (co-poly) 0.07 kJm-1 (homo-poly)
Elongation at break300% (co-poly) 150% (homo-poly)
Hardness80 (co-poly) – 90R (homo-poly)
* R is for rockwell scale
Oxygen Index17%
Heat Deflection Temperature100oC
Injection Mold Temperature (Typical)32oC to 66oC

Prototype development using polypropylene

Polypropylene finds high utility in the area of prototype-making. There are 3 major processes used to make prototypes presently: 3D printing, CNC machining, and injection molding.

CNC machining

Polypropylene is used with CNC machines to create prototypes of products. This method is advised when a small number of prototypes need to be created. Polypropylene is used as sheet stock. Due to the physical properties of polypropylene, it has a low annealing temperature. Hence, it starts to deform under heat. So, very precise cutting and a high degree of precision is needed to cut the polypropylene sheet cleanly. A CNC machine with an experienced operator can achieve the desired levels of precision.

Injection molding

Polypropylene is easy to melt and has a low melt viscosity. Hence, it is a great material to use in an injection molding machine. Melted polypropylene can fill up a mold quicker than other materials with a higher viscosity. Thus, the rate of work is high and the time spent on a batch is shorter. Pellets of polypropylene are used for making prototypes with injection molding. This method is used when a high number (volume) of prototypes need to be manufactured.

3D printing

Polypropylene is not desirable to use with 3D printing machines because of two reasons. First, it is not readily available in a filament form. Filament form is critical for 3D printing. The second reason is the warping that takes place during 3D printing. Warping is difficult to prevent with polypropylene. However, there are some 3D printing companies that have recently come out with new models of printers that can print using polypropylene. Generally speaking, polypropylene is not very popular with 3D printers.