Polypropylene is a polymer plastic that is a member of the ‘polyolefin’ (polymers produced from alkenes) family. It is a highly versatile and rugged material that has many beneficial physical properties, and most importantly it is also recyclable. It resists the action of many chemical solvents.
The PP market is currently projected to reach $133 billion by 2023.
Polypropylene is an extremely versatile material and can be used for a wide range of applications. PP is tough yet flexible, being classed as semi-rigid. It is extremely resistant to heat, chemicals, and fatigue. Furthermore, it is translucent and has an integral hinge property.
PP has a wide range of uses, including:
Sandbags made from polypropylene, one of the many applications of this flexible polymer. Image Credits: fws.gov
While PP is easily among the most popular plastic packaging materials in the world, only around 1% is recycled, which means most PP is headed for the landfill. These decompose slowly over 20-30 years.
To determine how recyclable polypropylene is, companies have undertaken ‘life cycle’ studies that look at the plastic from the raw material production to the final stages of waste management to assess the sustainability of the product. The general consensus from these studies is that PP has considerable potential as a sustainable product.
To make the recycling of polypropylene economically viable, several factors must be taken into account, most importantly its difficulty and expense.
There are five steps in PP recycling, namely, collecting, sorting, cleaning, reprocessing, and producing new products.
Plastics will often have a printed ‘resin code’ (5 for PP), which is useful during recycling, as they indicate what type of plastic it is. This ensures separation and efficient recycling of different plastic types.
First, the polypropylene must be separated from other plastic polymers. This is achieved by ‘sink-float’ separation, based on the unique specific density of PP (0..93-0.95g/cm3), which allows it to float while other polymers such as PET (specific density 1.43-1.45 g/cm3) will sink.
Another separation technique is based on the melt flow index, while a third is based on dissolution and reprecipitation of PP. A simple way to identify PP is by using Near Infrared Radiation (NIR) techniques. It must be noted that this cannot work with dark-colored plastics as they absorb the radiation.
PP reprocessing includes melting at a temperature above 400 F in an extruder, followed by granulation for use in new production. Polypropylene is eventually affected by thermal degradation, which compromises the structural intensity of the plastic due to the bonds between hydrogen and carbon becoming weaker. This varies with the use of the PP, but in general, four closed loops of recycling are considered possible before the negative impact of thermal degradation is perceptible.
Recycled PP is generally mixed with virgin PP at up to 50% to produce new products such as clothes or playground equipment.
The resin identification code for polypropylene. Image Credit: dnr.wi.gov
Recycling of polypropylene is emerging as an important, and economically viable, option on a large scale.
The main benefit of recycling PP is the reduction in the consumption of raw, finite resources such as oil and propane gas. It is estimated that around 8% of the oil used worldwide (around 400 million tons) is utilized in the traditional methods of plastic production, with 4% as ‘feedstock’ and another 4% in manufacturing.
Relative to production from oil and gas, energy use can be reduced by 88% when plastic is produced from plastic.
Given its inherent flexibility, PP can be recycled back into many different products, including:
About 30% of polypropylene is recycled from major industries, but a significant proportion is still dumped into landfills. It is currently not as economically viable to recycle PP as it is to recycle other polymers, in particular, HDPE, LDPE, and PET. It is hoped that this will be changed in the near future with advancements in recycling technology.
On the other hand, it is recognized that materials recycling is not always the most cost-effective recycling method. In such cases, it would be better to use plastics for direct combustion or chemically recycle them into synthetic fuels at the expense of some embedded energy, reducing landfill significantly.
This article was updated on 14th June, 2019
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