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Polypropylene (PP)
1-2 usd/kg
A lightweight, low cost, commodity plastic useful in packaging, textiles, automotive and household goods. 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.
It is one of the least expensive and most widely used plastics, accounting for around 17% of global annual production. Even so, only around 3% is currently recycled. This very low recycling rate is partly due to the type of applications and the variety of different grades and additive packages used. For more information, check the recycled grades.
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
Also referred to as heterophasic copolymer (HECO), polypropylene block copolymers have embedded particles of polyethylene (PE), ethylene–propylene rubbers (EPR), or ethylene–propylene–diene elastomers (EPDM). The improved mechanical properties of block copolymer types – improved toughness down to -20 degC – puts them into competition with engineering thermoplastics (ABS and PC). They are utilised in furniture and toys, as well as more demanding products, such as packaging, automotive bumpers and packaging crates. They are used as hot melt adhesive (HMA) for bonding PP and PE, for example.
Increasing the ethylene content improves impact strength and flexibility. Melt flow rate can be adjusted to suit the process and application, from low MFR/MFI (high molecular weight) to high flow.