Polypropylene is a polymer plastic that is a member of the ‘polyolefin’ (polymers produced from alkenes) family. It is a highly versatile material that has many beneficial physical properties, and most importantly it is also recyclable.
In chemical terms, it is a linear hydrocarbon polymer, with little unsaturation. The addition of a methyl group on to the hydrocarbon chain can affect physical properties such as melting temperature.
A simple way to identify PP is using Near Infrared Radiation (NIR) techniques. It must be noted that this cannot work with darkly coloured plastics as dark colours wil absorb the radiation.
Properties and Applications of Polypropylene (PP)
Polypropylene is an extremely versatile material and as such can be used for a wide range of applications. PP is tough and yet flexible and 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:
· Clear film packaging
· Carpet fibres
· Reusable containers
· Automotive components
· Laboratory equipment
· Thermal underwear
After a dip in global demand during the global recession, polypropylene is in high demand once more.
Sandbags made from polypropylene, one of the many applications of this flexible polymer. Image Credit: www.fws.gov
Recycling of Polypropylene
To make recycling of polypropylene economically viable, several factors must be taken into account. Most importantly, the difficultly and expense of the recycling process needs to be reduced. This process includes sorting, collecting, cleaning and reprocessing.
To determine how recyclable polypropylene is, companies have undertaken ‘life cycle’ studies which 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 begin with, the polypropylene must be separated from any other plastic polymers to make recycling possible. This is achieved by ‘sink-float’ separation, which works on the principal that PP has a unique specific density and therefore will float when other polymers will not. In practise, PP is often mixed with PET to produce plastic products such as drinks bottles. As PP has a specific density of .93-.95g/cm3 and PET has a specific density of 1.43-1.45 g/cm3, the PP will float on water and the PET will sink, allowing separation of the polymers. Polymers can also be separated using their melt flow index, which relates to the elasticity of the material.
If at all possible, the PP should also be sorted by colour prior to processing. This increases its value.
Once it has been ensured that the PP is homogenous, the plastic is shredded or granulated into ‘flakes’, which can be resold as recycled goods. The recycled PP may also be processed further, and compounded to produce denser plastic pellets using an extruder.
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. Though the point at which this occurs will vary depending on the use of the polypropylene, it is generally considered that PP can be recycled in a ‘closed loop’ four times before the thermal degradation has a negative impact on the polymer.
Most of the recycled PP is then mixed with ‘virgin’ plastic (i.e. plastic that has not been produced via recycling)in a ratio of around 1:3 to produce new plastic products.
Plastics will often have a ‘resin code’ printed on the bottom of a product which is using that particular plastic. These resin codes are used for the recycling of plastic, so that the different polymer types can be recycled separately and efficiently without contamination of the recyclable mix.
The resin identification code for PP is ‘5’. This does not indicate the level of difficulty involved with recycling polypropylene, rather it is simply an arbitrary number provided to keep it separate from other polymers during recycling.
The resin identification code for polypropylene. Image credit: http://dnr.wi.gov/
The Environmental Benefits of Recycling PP
Recycling of polypropylene is emerging as an important, and economically viable, option on a large scale.
A main benefit of recycling PP is the reduction in the consumption of raw, finite resources, such as oil and propene gas. It is estimated that around 8% of the oil used worldwide around 400 million tons) is implemented in the traditional methods of plastic production with 4% as ‘feedstock’ and another 4% in manufacturing.
Also, relative to production from oil and gas, there is up an 88% reduction in energy usage if plastic is produced from plastic.
Given its inherent flexibility, PP can be recycled back into many different products, including:
· Clothing fibres
· Industrial fibres
· Food containers
· Compost bins
· Speed humps
· Gardening apparatus (compost bins, garden edging and plant pots)
A reasonable proportion 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 this will be changed in the near future with advancements in recycling technology.
Special thanks are extended to Mr Scott Mudie for his input into this article.