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




Like flax and hemp, jute is being utilised as fibre reinforcement for composite materials. Its use helps to reduce the overall environmental impact of a material, by replacing synthetic polymer with a natural and renewable material. In some cases, jute offers technical advantages, too, such as superior vibration dampening and lower density compared to glass fibre.
The biggest drawback of natural fibres as reinforcement in plastic composites is their moisture content, which emphasises the already poor fibre-matrix adhesion. The lower the strength of the bond between the surface of the fibre and matrix, then the lower the mechanical properties of the composite. Therefore, treatments are applied to the fibre to improve surface adhesion, such as plasma or acetylation.
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