By Gary Thomas
to Polypropylene (PP)
Applications of Polypropylene (PP)
Environmental Benefits of Recycling PP
To Polypropylene (PP)
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
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.
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
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
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/
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
A reasonable proportion of polypropylene is recycled from
major industries, but a significant proportion is still duped 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
Special thanks are extended to Mr Scott Mudie for his input
into this article.