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Polypropylene (PP)
1-2 usd/kg
A lightweight, low cost, commodity plastic useful in packaging, textiles, automotive and household goods. It is impermeable to water, highly resistant to chemicals, very durable, insulating and resistant to fatigue. The homopolymer is available in biocompatible grades suitable for healthcare applications.
It is one of the least expensive and most widely used plastics, accounting for around 17% of global annual production. Even so, only around 3% is currently recycled. This very low recycling rate is partly due to the type of applications and the variety of different grades and additive packages used. For more information, check the recycled grades.
Whereas low-grade recycled material can offer cost savings versus virgin, recycled grades of high quality (light colours, no odour and so on) can add up to 50% to the base price. Bio-based materials are up to twice the price, depending on the source and feedstock, but this is likely to come down with time and developments in the technology.
Due to it’s semi-crystalline structure, it has a naturally milky appearance. However, types (such as random copolymer) and additives (clarifiers) have been developed to produce a more transparent appearance in the finished article. Clarifiers are suitable for food application (microwave, dishwasher, freezer and hot-fill safe) and produce a clear, colour-free material. This brings PP into competition with amorphous materials, such as polycarbonate (PC) and even glass, for containers and packaging.
It is combined with various fillers – including mineral (talc), glass fibre, carbon fibre and natural materials (such as wood, hemp and bamboo) – to improve mechanical properties and, in the case of wood-fill, reduce cost and weight. Maleic anhydride grafted polypropylene (PP-g-MAH) is commonly used as the base material, because it improves coupling between the polymer and filler material.
It has low surface energy, which is useful in many cases, but also means it is difficult, but not impossible, to bond with adhesive. It will burn readily when exposed to an open flame (UL 94 HB). It is possible to add flame retardant additives to achieve UL 94 V0, but this has an impact on mechanical properties.
Sustainability concerns
Microplastics
Polypropylene (PP) is identified as #5 for recycling. Post-consumer recycled (PCR) PP comes from a range of applications, but packaging offers the most reliable sources for high quality feedstock. Films, containers, trays and other forms are readily separated and recycled. An advantage of PP is that it floats on water, which makes it somewhat easier to separate from other types of plastic.
Another large consumer of PP is the automotive industry. But in this case, it is often mixed with additives, such as glass (GF), talc (T) or calcium carbonate (K), which makes it more difficult to separate and reuse in a circular way. Municipal waste streams provide a reliable source of material, but the quality is a little more variable and this will affect the finished material more so than a relatively ‘cleaner’ source like packaging.
PP used in food contact applications can, in theory, be converted back into rPP food packaging. However, this has to happen in a controlled fashion to ensure cleanliness and quality are maintained – it is not feasible to take PP from general PCR recycling streams and convert it back into 100% rPP for food contact. Instead, food contact rPP is most likely to come from chemical recycled sources. It may also be mixed with bio-PP to further reduce the carbon footprint.
rPP is less expensive than virgin and offers incredible savings in terms of MJ/kg and kgCO2e/kg. However, there can be a significant knock-down in mechanical performance, depending on the quality of the input material. PP is vulnerable to oxidisation and degrades during the re-melting and re-processing phases. This limits the total number of cycles that might be achievable with 100% recycled – with each cycle the properties are reduced until the material becomes unusable. Also, contamination in the material, such as packaging labels, can cause weaknesses and act as crack propagators reducing the impact properties. Where this is an issue, it can be blended with virgin to reduce the negative impacts. Also, it should be considered that certain processes, like film and fibre forming, require a much higher quality of raw material than injection moulding, for example, and may not be able to tolerate any contamination.
As well as 100% post consumer recycled (PCR) rPP, it is possible to make blends with post industrial recycled (PIR) material which tends to be less expensive and ‘cleaner’ (but has lower sustainability potential). Taking recycled plastic from municipal waste streams offers the greatest savings in terms of CO2, but comes with some aesthetic and mechanical compromises (mainly due to contamination).
There are many types of rPP available off the shelf, with recycled content up to 100%. For example, rPP Moprylene by Morssinkhof – Rymoplast contains 100% recycled material from post-consumer packaging in Europe. It is sorted using near infrared (NIR) in a fully automated setup and made suitable for reuse by extensive sorting, hot-washing and compounding. It’s price and mechanical properties are impressively close to that of virgin, providing a drop in replacement in many applications. Another 100% rPP is Newplen from Sirmax made from mixed municipal waste. The source of material and range of contamination means a relatively higher knock-down in visual and mechanical properties.
Some other types include Campine, Matek, Green Iso by Sirmax, Sabic Trucircle, Borealis Borcycle, Ravago Ravapura, Kingfa, Polykemi and their subsidiary Rondo Plast, Sax, Innopol by Inno-Comp, Braskem I’m Green, MGG and MBA.
