Polypropylene (PP) is a thermoplastic polymer of the monomer propylene. It is a robust material and has high resistance to acids, bases, and other solvents making it extremely useful for a range of applications.
Molecular formula - (C3H6)n
Density - 0.946 g/cm3, crystalline
Melting point - 130°C (266°F)
MFR (Melt Flow Rate) - 5 - 60
IUPAC id - poly(propene)
Chemical and Physical Properties of PP
Polypropylene is typically an isostatic polymer (all substituent macromolecules are located on the same side of the molecular backbone – typically in a semicrystalline form). A tough, fatigue resistance and flexible plastic, polypropylene has a diverse range of applications from packaging to engineering. Easily customizable for colour, strength, etc., polypropylene can also be made translucent.
Manufacturing Process for PP
Polypropylene is manufactured by subjecting the monomer propylene to pressure and heat in the presence of a catalyst. At low temperatures, polymerisation takes place and a translucent form of the product is obtained, which can be easily colored. The properties of PP obtained can be altered by modifying the manufacturing conditions and the type of catalyst.
PP is available in three grades – homopolymers, block copolymers, and random copolymers – to suit a variety of specifications.
Current Applications of Virgin PP Material
PP is widely used and the major applications of virgin PP are in the following areas:
- Packaging and labeling
- Textiles - thermal underwear, ropes, and carpets
- Reusable containers
- Plastic parts
- Laboratory equipment
- Automotive components
- Polymer banknotes
Environmental Impact of Non-Recycled Plastics
PP and other plastics are widely used in packaging and other industries, thanks to their heat and chemical resistance. Polypropylene is extensively used in containers for hot liquids, as it has a high melting point. However, because of the short life span of packaging, most of these thermoplastics are thrown out as waste and end up in landfills. The US Environmental Protection Agency estimates that about 20% of municipal solid waste comprises plastics in some form.
PP products degrade very slowly in landfills and take about 20 to 30 years to fully decompose. This poses a serious problem because of the additives in plastic products, which include stabilizers, colorants, and plasticizers. These additives can contain a range of toxins including cadmium and lead. According to studies, plastics can account for about 28% of the total cadmium in solid municipal wastes in studied landfills, which has the potential to leach into other systems with potentially harmful consequences for a range of biosystems. Moreover, burning of thermoplastics can release toxins such as vinyl chloride and dioxins, and this makes atmospheric pollution a real issue to consider.
Hence there is a compelling need to prevent thermoplastic products finding their way into our municipal waste streams, landfills or incinerators. Recycling of PP is much needed and is a cost-effective and eco-friendly way of managing plastics throughout life. Also, since PP is a petroleum based product, recycling of these products will help us preserve our oil / petroleum resources.
A uniform code for the recycling of plastics was first introduced in 1988 by the Society of the Plastics Industry. The code was designed to make the identification of certain plastic resins easier to enable a more efficient and precise sorting stage, which would then result in more efficient recycling as specific methods apply to different types of plastics.
Plastic products are now printed with Numbers 1-7 depending on the type of plastic and the resin fundamental in its production.
Polypropylene (PP) is increasingly being recycled. With diverse applications in a range of single-use packaging products such as bottles, yogurt containers, medicine bottles, caps, straws, plastic cups and food packaging, etc, we are increasingly using more PP and thus generating extraordinarily big volumes of rubbish made from PP.
Recycling of polypropylene products involves the following steps:
- Collection - This involves accumulating used plastic products from the domestic municipal waste stream. This is usually mixed plastic waste in which all types of plastic products will be present.
- Sorting - The sorting process involves separation of PP products from products made of other types of plastics, based on their recycling codes.
- Cleaning – In this step, paper labels and coloured pails are removed, then the products are granulated and washed which gives rise to “clean flake”. This flake is converted to uniformly sized pellets for easy feeding into extruder or moulder.
- Reprocessing by melting – Here, the pellets are then fed into an extruder where it is melted at 240oC (464oF) and cut into small granules which are now ready to use in making new products.
- Creating new products using the recycled PP – In this step, key parameters of the recycled material such as the melt flow index are measured to assess their suitability for use in different applications. Depending upon these properties, the recycled PP is moulded into various products.
There are four main types of recycling processes for PP:
Primary – The primary recycling process is feasible only for industrial plastic wastes, and hence this is not a widely used process.
Secondary – Secondary recycling can be used in recycling of a combination of plastic wastes and gives rise to products such as fenceposts, which can be used instead of concrete, wood, and metal.
Tertiary – Tertiary recycling process is a widely used recycling process and produces chemical and fuels from waste plastics.
Quaternary – Quaternary recycling process involves incineration of plastic wastes at very high temperatures of up to 900 to 1000oC (1652 to 1832oF). The left over materials from this recycling process are sent to landfills. The use of incinerators is highly helpful as they prevent the release of pollutants into the atmosphere and also they reduce the amount of incoming waste by about 80% wt and 90% vol.
Applications of Recycled PP
Recycled polypropelene is currently used in the manufacture of the following products:
- Battery cables
- Ice scrapers
- Pallets and trays
- Landscape borders
- Signal lights
- Auto battery cases
- Bicycle racks
- Rakes and bins
As we refine our manufacturing and recycling processes, many more sustainable and innovative uses for recycled plastics are being developed. Companies are trying to re-use their plastic waste themselves without looping them through recyclers, which is a very encouraging step forward as it saves massive recycling cost. More extensive research needs to be conducted in the field of recycling and re-using of plastics, which will hopefully pave the way for many new opportunities in this arena.