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




The use of vegetable matter-reinforced polypropylene (PP) has been growing, mainly due to demand from the automotive and construction industries. Adding wood flour, or fibre, for example, reduces the amount of polymer used, which can reduce cost. It also adds stiffness while reducing weight (wood is lower density than plastic) – a very useful attribute for non-load bearing automotive applications, such as trays, panels and covers. In construction, PP and polyethylene (PE) filled with a high proportion of wood fibre are becoming more popular form of wood plastic composite (WPC). It is used in extruded decking, siding and fencing.
Wood flour comprises a mix of types, such as spruce, pine, oak and maple – typically whatever is available. Other popular filler materials include jute, hemp and bamboo.
While it provides a good use for waste wood, such as produced by sawmills, it results in a product with low circularity the potential. It is not biodegradable like a bio-based plastic might be, and has low recycling value due to the wood content.