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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.
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Sustainability concerns
Non-renewable ingredients
Raw material generates polluting by-products
Low circularity potential
Microplastics
PP bio-based composites reinforced with cellulose fibre have significantly higher stiffness and strength than unmodified PP. The cellulose fibre may be extracted and prepared using similar methods to pulp production, or from regenerated cellulose made by dissolving pulp and dry jet-wet fibre spinning (such as Lyocell, which does not use carbon disulphide, because it is toxic to workers and the environment). In comparison to other natural fibres, such as hemp and sisal, or wood flour, cellulose fibre provides higher strength and stiffness, consistent colour, better moulding, low odour, and there is a reliable supply.
These bio-based composites have the potential to be derived entirely (or almost entirely) from renewable sources if natural fibre is combined with bio-based PP (bio-PP). For example, UPM Formi EcoAce contains certified wood and cellulose fibres as well as bio-based PP sourced from UPM Biofuels plant, BioVerno naptha.
