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Recycling of Polypropylene (PP)
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. PP string is made of Polypropylene.
Properties and Applications of Polypropylene (PP)
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:
Clear film packaging
Recycling of Polypropylene
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. This raises severe environmental issues, quite apart from toxic additives in PP such as lead and cadmium. Incineration may release dioxins and vinyl chloride, both of which are poisonous.
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 PP 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 PP string such as clothes or playground equipment.
The Environmental Benefits of Recycling PP
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:
Gardening apparatus (compost bins, garden edging, and plant pots)
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.
What is Polypropylene and What It’s Used for?
Polypropylene is a tough, rigid and crystalline thermoplastic produced from propene (or propylene) monomer. It is a linear hydrocarbon resin. The chemical formula of polypropylene is (C3H6)n. PP is among the cheapest plastics available today.
Types of Polypropylene & their Benefits
Homopolymers and Copolymers are the two major types of polypropylene available in the market.
Polypropylene Homopolymer is the most widely utilized general-purpose grade. It contains only propylene monomer in a semi-crystalline solid form. Main applications include packaging, textiles, healthcare, pipes, automotive and electrical applications.
Polypropylene Copolymer family is further divided into random copolymers and block copolymers produced by polymerizing of propene and ethane:
Polypropylene Random Copolymer is produced by polymerizing together ethene and propene. It features Ethene units, usually up to 6% by mass, incorporated randomly in the polypropylene chains. These polymers are flexible and optically clear making them suitable for applications requiring transparency and for products requiring an excellent appearance.
While in Polypropylene Block Copolymer, ethene content is larger (between 5 and 15%). It has co-monomer units arranged in regular pattern (or blocks). The regular pattern hence makes thermoplastic tougher and less brittle than the random co-polymer. These polymers are suitable for applications requiring high strength, such as industrial usages.
Polypropylene, Impact Copolymer – Propylene Homopolymer containing a co-mixed Propylene Random Copolymer phase which has an ethylene content of 45-65% is referred to PP impact copolymer. It is useful in parts which require good impact resistance. Impact copolymers are mainly used in packaging, houseware, film, and pipe applications, as well as in the automotive and electrical segments.
Expanded Polypropylene – It is a closed-cell bead foam with ultra-low density. EPP is used to produce three-dimensional polymer foam products. EPP bead foam has higher strength to weight ratio, excellent impact resistance, thermal insulation, and chemical and water resistance. EPP is used in various applications ranging from automobiles to packaging, from construction products to consumer goods and more.
Polypropylene Terpolymer – It is composed by propylene segments joined by monomers ethylene and butane (co-monomer) which appear randomly throughout the polymer chain. PP terpolymer has better transparency than PP homo. Also, the incorporation of co-monomers reduces crystalline uniformity in the polymer making it suitable for sealing film applications.
Polypropylene, High Melt Strength (HMS PP)– It is a long chain branched material, which combines both high melt strength and extensibility in the melt phase. PP HMS grades have a wide mechanical property range, high heat stability, good chemical resistance. HMS PP is widely used to produce soft, low density foams for food packaging applications as well as used in automotive and construction industries.
Material Properties of Polypropylene
Keeping information about the properties of a thermoplastic beforehand is always beneficial. This helps in selecting the right thermoplastic for an application. It also assists in evaluating if the end use requirement would be fulfilled or not. Here are some key properties and benefits of polypropylene:
Melting Point of Polypropylene – The melting point of polypropylene occurs at a range.
Homopolymer: 160 – 165°C
Copolymer: 135 – 159°C
Density of Polypropylene – PP is one of the lightest polymers among all commodity plastics. This feature makes it a suitable option for lightweight\weight saving applications.
Homopolymer: 0.904 – 0.908 g/cm3
Random Copolymer: 0.904 – 0.908 g/cm3
Impact Copolymer: 0.898 – 0.900 g/cm3
Polypropylene Chemical Resistance
Excellent resistance to diluted and concentrated acids, alcohols and bases
Good resistance to aldehydes, esters, aliphatic hydrocarbons, ketones
Limited resistance to aromatic and halogenated hydrocarbons and oxidizing agents