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




Glass fibre reinforced polypropylene (PP-GF) thermoplastic composite is fabricated as a raw material in a number of different ways, and each has its benefits. The PP has to wet-out the glass fibre by flowing around the fibres. The quality of wet-out depends on the distant the PP needs to travel, the time it has to do so, and the flow (temperature and melt flow of the PP). Therefore, increasing pressure and time will generally yield better results.
– Commingled PP-GF is made by combining yarns of PP and GF by air entanglement. The hybrid yarn is then woven or laid (unidirectional) into a fabric that can be laminated or compressed into organo plates. Commingling mixes the PP in with the glass and, in doing so, reduces the distance the melted material needs to travel during consolidation. Therefore, this material is suitable for processes that rely on vacuum alone (no positive pressure). These types of materials are used for parts that require some stiffness and will be subject to extreme impacts, such as boat hulls, racing car wheel arches and motorcycle fairing.
– PP film or fabric is placed on top of, or between, layers of glass fibre. Heat and pressure are applied to form a consolidated composite. The PP has to travel further and more pressure is required to penetrate the dense glass fibre yarns. This type of PP-GF is used to make organo sheet, which are suitable for thermoforming and over-moulding for automotive applications, for example.
– Glass fibre strands are laid onto an extruded film of PP and the materials are calendered together between hot rolls. This technique is used to produce continuous lengths of PP-GF, such as used as the skins in sandwich panels with PP honeycomb core (used in lorry trailer sides, box van and construction material, for example). The strips of unidirectional PP-GF are laid perpendicular to one another to balance the stress and so avoid warpage.
These materials are used in a range of automotive, transport and construction applications. They are recyclable, but the process reduces the length of the glass fibre, so the material cannot be used as a laminated composite. Instead, it is formed into parts by injection or compression moulding. So, for example, a compression moulded PP-GF automotive part may be recycled and converted into an injection moulded automotive part.