Polypropylene (PP)
1-2 usd/kgA lightweight, low cost, widely recycled commodity plastic useful in packaging, textiles, automotive and household goods – it is one of the most widely used plastics. 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.
PP is one of the least expensive polymers and available in many different grades and formats. 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




Similar to flax, jute and ramie fibres, hemp offers an alternative to synthetic fibres in composite reinforcement. While not as predictable, strong or stiff as glass fibre, the use of hemp and other plant fibres can reduce the negative impacts of a material by reducing the consumption of non-renewable materials. The ultimate combination is organic fibre with bio-based plastic derived from renewable sources. While the conversion into an engineering material still consumes energy and CO2 emissions, the raw materials themselves have the potential to be circular.
Due to its lower density, hemp offers nearly the same strength to weight as glass fibre. This is especially advantageous in lightweight parts. In addition, natural fibres offer some advantages in terms of acoustics and vibration dampening. This combination of properties, along with their relatively low price, has resulted in widespread adoption in automotive applications. They are used as fibre reinforcement in semi-structural and non-structural interior parts such as door panels, seat parts and boot linings. They are most commonly combined with polypropylene (PP), but polyethylene terephthalate (PET), polyester, and polylactic acid (PLA) are also used as the polymer matrix.