Polypropylene (PP)
1-2 usd/kg
A 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
Non-renewable ingredients
Raw material generates polluting by-products
Low circularity potential
Microplastics
Flax is strong, lightweight and sustainable. But its properties are variable and, as a result, a little unpredictable for structural applications. One major advantage it offers as fibre reinforcement in composites is vibration dampening (dissipation of mechanical energy). Therefore, it is often used in combination with glass (GF) or carbon fibre (CF) to enhance the overall performance of the composite. In this way, it has found application in a wide range of demanding applications, such as automotive seats structures and door panels, wind turbine blades and sports equipment.
On it own, it is used to reinforce both thermosetting plastic systems, as well as thermoplastics, such as polypropylene (PP), and polylactic acid (PLA) for a fully biodegradable composite. It is used in the production of furniture, automotive interiors and even musical instruments. As a natural fibre, it has more bulk than synthetics like GF and CF, which tend to lie flat. This means the best results are achieved with processes that apply positive pressure during moulding, such as compression moulding, pressing and resin transfer moulding (RTM). It is also compatible with processes like resin infusion, used in wind turbines and boat building for example, but less well-suited to dry processes that use vacuum consolidation alone.
Its high moisture absorption properties, which are very useful as a textile fibre, can cause problems in a composite. Moisture leads to micro-cracks in the matrix-fibre bond, and this weakens the structure. Therefore, in structure critical applications, the fibre must be completely sealed in with resin – for example, cut edges must be treated to avoid exposed fibre.