Polypropylene (PP) — types, properties, applications and transport

Definition

Polypropylene (PP) is a thermoplastic from the polyolefin group, obtained by polymerisation of propylene, supplied to processors in the form of granulate with a density of about 0.90–0.91 g/cm³, the lightest of the commodity plastics, used in the automotive industry, packaging and the textile industry.

Polypropylene and polyethylene are the two pillars of the polyolefin market and the two most frequent cargoes in granulate logistics. From the terminal’s perspective they are transported very similarly — both are light, both sensitive to contamination — but their properties and end uses differ enough that the processor never confuses them, and neither can we.

Types of polypropylene

Polypropylene occurs in three basic variants, differing in chain structure and the properties that result from it:

As with PE, the properties are also determined by the melt flow rate (MFR) and the structure (isotacticity) affecting crystallinity and rigidity. For the processor these are critical parameters; for logistics — another reason why batch traceability and the absence of grade mixing are absolutely required. Mixing a homopolymer with a copolymer renders the material useless to the recipient.

It is worth understanding the logic of this division. The homopolymer is “pure” polypropylene with a regular, strongly crystalline structure — hence its rigidity and high service temperature, but also its brittleness in the cold. Introducing ethylene units into the chain (copolymerisation) disturbs this regularity: the random copolymer, with ethylene dispersed randomly, gains transparency and a lower melting temperature; the block copolymer, with ethylene in separate segments, gains impact resistance at low temperatures. One family of materials, yet three completely different application profiles — and three streams that must not be confused in logistics.

Properties and granulate form

The most characteristic feature of polypropylene is its low density — about 0.90–0.91 g/cm³, which makes it the lightest commodity plastic; it is the only common plastic that floats on water. PP has a higher melting temperature than PE (about 160–170 °C), is rigid, chemically resistant and resistant to repeated bending — hence the famous living-hinge effect, used in “click” closures. Its weakness is brittleness at low temperatures (in the homopolymer variant) and sensitivity to UV without stabilisation.

It is precisely this combination of lightness, rigidity and thermal resistance that makes polypropylene so versatile. Chemical resistance allows it to be used for packaging aggressive substances, the high softening temperature — for sterilised and hot-water-resistant products, and the exceptional fatigue resistance under bending — for hinges moulded directly into the product, working hundreds of thousands of cycles without cracking. From the transport perspective these properties do not change the way it is carried — granulate is granulate — but they explain why demand for PP is so stable and why it makes up such a significant part of the stream handled by bulk materials terminals.

It reaches processors in the form of granulate 2–5 mm, natural or coloured, with a bulk density of around 0.50–0.55 kg/l. Like PE, it is a volume material — a silo tanker fills up by volume before it reaches the permissible combination weight, which is why high-capacity trailers with a low tare weight are selected for its transport.

Polypropylene is also, like other polyolefins, prone to electrostatic charging during pouring and pneumatic conveying — which promotes caking and the sticking of grains to walls. Combined with sensitivity to abrasion (the formation of dust and filaments during intensive pneumatic conveying) this is an argument for gentle transloading methods, especially for grades with high quality requirements. It is also worth remembering the sensitivity of PP homopolymer to low temperatures: in frosty conditions the material becomes more brittle, which matters not so much for the transport of the granulate itself as for selecting the grade for the end product.

Applications

The versatility of polypropylene means it is present almost everywhere:

• Automotive — bumpers, dashboards, interior elements, battery housings. The lightness of PP translates directly into vehicle mass and fuel consumption.
• Packaging — cups, buckets, closures, BOPP and CPP films, temperature-resistant food packaging (e.g. for microwaves).
• Fibres and technical fabrics — carpets, geotextiles, sacks, and also the fabric from which big bags are made. This is an interesting logistics paradox: PP is at once a cargo and the packaging material in which it is carried.
• Technical products and household appliances — garden furniture, crates, pipes, washing-machine and dishwasher components.
• Medicine and hygiene — disposable equipment, pharmaceutical packaging, hygiene nonwovens; here the granulate cleanliness requirements are the highest.

Each of these groups has different requirements for the material: the automotive industry expects repeatable impact resistance and thermal resistance, food packaging — food-contact approvals, fibres — stable spinnability, and medical products — absolute cleanliness. For the logistics operator this means that “polypropylene” is never one material: it is dozens of grades, each travelling to a different recipient and tolerating no mistakes.

This diversity means that — as with polyethylene — under the single name “PP” hide dozens of grades with strictly defined parameters, each with its own recipient. What is more, polypropylene increasingly replaces heavier plastics and metals where mass reduction matters — in the automotive industry and packaging its low density translates directly into material savings and lower transport costs for the finished product. This drives demand and means that PP, alongside PE, remains a pillar of the polyolefin market, and its logistics — a permanent element of the work of every bulk materials terminal.

Transport and logistics of PP granulate

Polypropylene is transported exactly like polyethylene: in big bags at the production, import and storage stage, and in bulk by silo tanker on the last leg to the processor’s silo. Because of its near-twin characteristics with PE, we handle both these plastics in a single terminal stream — the 2000-big-bag buffer and the 200-tonnes-per-day throughput work the same way, and the only hard rule is keeping PP and PE batches separate so they do not mix.

Some PP granulate comes from European production, some arrives by import from Asia and the Middle East — by the same chain as PE: plant → port → inland terminal → recipient’s silo. On this last, shortest leg to the processor’s plant in Central and Western Europe, efficient, clean bulk delivery is what matters — and it is this that determines the total cost of delivering material ready for processing.

The big bag and the silo tanker are therefore not competitors but successive stages of the same journey: the sack works for import and storage, the silo tanker — for fast, maintenance-free bulk delivery straight to the recipient’s installation. The transloading terminal is the link that joins these two stages, and the transloading method determines the state in which the granulate arrives at the end of the chain.

Quality requirements

The cleanliness requirements for PP are the same as for PE and just as absolute. The three main risks are:

• Contamination — foreign granules, dust, black specks; for a film or technical-product processor it means rejection of a batch.
• Angel hair and fines — filaments and particles that form during high-pressure pneumatic conveying, clogging the recipient’s installations.
• Moisture — impairing processing despite the material’s low hygroscopicity.

That is why for PP granulate, as with PE, we use pneumatics-free transloading. With polypropylene the additional argument is its sensitivity to abrasion — intensive pneumatic conveying easily chips dust and filaments off the soft grain, while a gravity flow through a sieve does not.

Polypropylene recyclates (R-PP)

An ever-larger part of the PP stream is recycled material — R-PP. It is driven by the same regulation as with polyethylene: the EU Packaging Regulation (PPWR) introduces mandatory recycled-content levels, raising demand for high-quality regranulate. Polypropylene is one of the most frequently recycled plastics — it is recovered from, among others, crates, bumpers, packaging and nonwovens.

From a logistics standpoint, R-PP is transported the same way as virgin material, but it sets even higher cleanliness and traceability requirements. Recyclate has greater batch variability, is sometimes more expensive to source and is especially sensitive to contamination — contamination wastes all the effort put into its recovery. This is another reason why gentle, well-documented transloading gains in importance with the development of the circular economy.

Polypropylene versus polyethylene — which when

Although twins in transport, both plastics have clearly different application profiles. PP is chosen where rigidity, temperature resistance and lightness matter — in the automotive industry, rigid packaging, fibres. PE dominates where flexibility, low-temperature impact resistance and a barrier are needed — in films, pipes, bottles. To simplify: PP is stiffer and more heat-resistant, PE — more flexible and frost-resistant. Visually the two granulates are sometimes indistinguishable, which makes separating the streams all the harder and all the more important.

For the processor these are completely different materials; for the logistics operator — two streams that must be kept strictly apart, because mixing them is the most frequent and most costly error in granulate trading. Hence in work with polyolefins so much emphasis on batch traceability, cleanliness of the installation between loads and documentation — this is not bureaucracy but real protection against a mistake whose consequences are borne by the processor at the end of the chain.

Related topics

Polypropylene is best understood in comparison with polyethylene, with which it shares logistics but differs in properties and end use, and with the method of pneumatics-free transloading, which determines granulate quality at the interface of container and silo. PP granulate is transported in big bags and in bulk by silo tanker. The full offering of bulk material transport can be found on the PHS Magnum portal.

Sources

• ISO 1873 / ISO 1183 standards — classification of polypropylene and determination of the density of plastics.
• PP producers’ technical materials (granulate data sheets).
• Industry guidelines on granulate cleanliness (Operation Clean Sweep).
• Operational practice of the SMIALA terminal, Chorula — Aleksy Pasternak.