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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.
Sustainability concerns
Non-renewable ingredients
Raw material generates polluting by-products
Microplastics
Biaxially oriented polypropylene (BOPP), also called bi-oriented PP, is produced by stretching the film during manufacture. Compared to regular thin film, it has improved strength, stiffness and transparency. It is very smooth and shiny, and typically used for packaging (such as baked goods, snacks, fruit and vegetables, chocolate and confectionary, rice, pasta and many others), labelling and laminating.
This demonstrates a significant advantage for PP, because it can be used for all parts parts of the same product (packaging container, lid and label, for example) that can be co-recycled. A novel application for laminated PP films is in the production of self-reinforced polypropylene (SR-PP). This composite is made up of two types of PP – which have slightly different melting points to allow one to become the matrix when heated – laminated together, slit into yarn and woven. The resulting material is tough with high strength-to-weight.
It is resistant to water and most chemicals. Where other properties are required, it is metallised (oxygen barrier, conductive and metallic finish), coated, co-extruded or laminated. Heat-seal films are produced by co-extruding BOPP with a lower-melt-temperature polymer, such as polyethylene (PE).
Polymer banknotes, manufactured under the trade name Guardian and used worldwide, are produced using BOPP. Their design combines many innovative features: the printed and embedded security are encapsulated within the layers of PP, giving a glossy and wear-resistant finish; and tactility is provided by raised print on the surface.
