How Polypropylene Recycling Sets the Grade of the Pellet

Polypropylene recycling is well understood as a collection problem. PP arrives in more shapes than any other commodity plastic, much of it too small or too dark to sort, and that story has been told many times.

What gets far less attention is what happens after you already have a polypropylene stream. Two plants running visually identical washed PP flake can produce pellets that sell at very different prices, and the difference is not wash quality. It is polymer chemistry that was decided before the material ever reached the plant. This article is about that half — what recycled PP is as a resin, and what that makes it worth.

Washing decides how clean the flake is. Polymer chemistry decides what the pellet is worth.

What Polypropylene Recycling Produces

Polypropylene recycling converts washed PP flake into a pellet through granulating, melt filtration, degassing and cutting. The output is graded not by purity alone but by melt flow, colour, odour and filler content. Those four properties decide which buyer can use the material, and three of them are fixed before the line runs.

Think of a recycled PP pellet as a resin with a specification rather than as clean plastic. A moulder asks the same questions they would ask of a virgin grade — melt flow index, low-temperature impact behaviour, filler content, colour, odour. A washing line answers only two of those, and only partly. The rest was determined by what the material was originally made of and how much heat it has seen.

That is why polypropylene deserves separate treatment from polyethylene even though the two wash on almost identical equipment. Their behaviour on the line converges. Their behaviour in the melt does the opposite.

Polypropylene Degrades by Chain Scission and That Sets the Clock

Polypropylene has a tertiary carbon on every second backbone atom, and that site is where thermal and oxidative attack begins. The result is chain scission — the molecule breaks rather than crosslinks. Average molecular weight falls, melt flow index rises, and impact strength falls with it on every heat cycle.

The two polyolefins degrade in opposite directions, and the melt flow index shows which.

The contrast with polyethylene is the part worth remembering, because it is counter-intuitive and it changes how you read a test result. Polyethylene under the same conditions tends to branch and crosslink, so its melt flow rate usually falls with reprocessing. Polypropylene scissions, so its melt flow rate rises. A rising melt flow index on a PP batch is not a measurement error; it is the degradation history showing itself.

Three practical consequences follow.

Melt flow index has to be measured per batch, not assumed from the source. The measurement itself is standardised as ASTM D1238, which fixes the temperature and load so that two labs report comparable numbers, and quoting a melt flow figure without those conditions tells a buyer very little. Two lots of washed PP flake from the same supplier can differ if one came from thicker mouldings that saw more heat, or from material that has already been through a recycling loop once. The rigid plastic granulating pelletizing line specification names this directly, describing quick parameter adjustment for different material types and melt flow indices — because the line is expected to see a moving target.

Chain scission moves recycled PP towards moulding and away from extrusion. Injection moulders generally want a high melt flow so thin sections fill. Extruders generally want a low melt flow so the melt holds together leaving the die. Degradation pushes PP up the melt flow scale, which is a large part of why so much recovered polypropylene goes to crates, pails and mouldings rather than to extruded product.

Gentler processing is worth real money on PP. Every avoidable heat cycle, every over-sheared minute in a barrel, and every stagnation point costs melt flow you cannot get back. That is a stronger argument on PP than on polyethylene, and it is the reason the pelletizing stage matters more here than the wash stage does.

Why Recycled Polypropylene Usually Needs Its Stabiliser Rebuilt

Virgin polypropylene is never sold unstabilised. It carries an antioxidant package that protects it during processing and in service, and that package is consumed by the work it does. By the time a part reaches a recycling line, much of the protection it started with has already been used up.

The protection a virgin grade carried is largely spent by the time the flake reaches the extruder.

Three things deplete it in sequence, and they add up. The original moulding or extrusion consumes some. Service life consumes more, particularly outdoors where ultraviolet exposure attacks the same sites that heat does. Then hot alkaline washing is by design an aggressive chemical environment for anything on or near the polymer surface.

So recycled PP arrives at the extruder with less protection than virgin resin, at exactly the moment it takes another heat cycle. Restoring an antioxidant package at the compounding step is standard practice for this reason, and it is why recycled polypropylene is usually described as compounded rather than simply pelletized. The equipment implication is a line that can meter an additive accurately into the melt.

One Resin Code Covers Three Different Polymers

Everything stamped 5 is polypropylene, and polypropylene is three commercially distinct materials plus a family of filled compounds. They sort identically under near-infrared detection, float identically in water, look identical as flake, and behave differently in both the melt and the finished part.

One resin code, three polymers and a filler question. None of it is visible in the flake.

The last row is the one that catches people out, and it does so twice.

First, filler changes the density. Unfilled polypropylene sits around 0.90–0.92 g/cm³ and floats comfortably. A mineral-filled grade at the loadings common in automotive and appliance parts can pass 1.0 g/cm³ and sink, which means a float-sink tank will discard it with the grit rather than recovering it. A stream containing filled PP therefore loses yield in a place nobody was watching.

Second, mineral filler is abrasive. Talc and glass wear screw flights, barrel liners and the sealing faces of screen changers, and the wear is proportional to loading and residence time. A line expected to see filled compounds needs wear-resistant screw and barrel elements specified at the outset, not retrofitted after the first campaign.

None of the three families can be separated from the others by any mechanical means once they are in the same bale. What can be done is to characterise the mix, price the output accordingly, and choose a destination that tolerates it — which is what the grade ladder below is for.

The Density Overlap Is Beaten Upstream, Not in the Tank

Polypropylene at 0.90–0.92 g/cm³ and polyethylene at 0.92–0.97 g/cm³ both float, so a float-sink tank cannot tell them apart. This constraint is well known from the collection side. What matters on a production line is narrower — what the tank can still do, and where the separation has to happen instead.

The tank is still worth having, because everything heavier than water leaves through the bottom discharge — sand, grit, metal fines, PET and PVC fragments, which between them account for a large share of what would otherwise reach the melt filter. Dropping out heavies and splitting two polyolefins are different jobs, and only the second one fails.

There is one adjustment available inside the tank, and the PP PE soft film recycling washing line specification names it — saline solution can be dosed to fine-tune the separation cut point. Raising the density of the medium moves the boundary, which is useful, but the PP and PE bands sit close enough together that this rarely yields a fraction clean enough to sell as single resin. In practice the split is made before the material reaches the plant, by detector sorting or by source separation, or it is not made at all and the output is priced as a mixed polyolefin grade with real markets of its own.

Odour Is a Melt Problem Before It Is a Wash Problem

Odour is the property that most often stops a technically clean recycled PP pellet from being bought. Polypropylene absorbs organic compounds into the polymer rather than holding them on the surface, so what a container held during its service life is partly inside the material by the time it arrives.

Washing reaches the surface fraction. Only the melt reaches what the polymer absorbed.

That distinction decides which equipment addresses it. Hot alkaline washing removes the surface fraction, and it removes a lot — this is the largest single reduction available. What washing cannot reach is the absorbed fraction, because it is not on a surface.

The absorbed fraction leaves in the melt or not at all. Vacuum degassing in the extruder pulls volatiles out of the polymer while it is molten and the diffusion path is short, which is why the number of degassing zones matters more on odour-sensitive PP work than on almost anything else. Where one stage is not enough, a second extruder adds another pass of degassing and filtration. Neither eliminates odour from a stream of long-stored food packaging — the process reduces the burden rather than removing it, which is why buyers into odour-sensitive applications specify tested input rather than relying on tested output.

Where Recycled Polypropylene Goes and What Each Destination Demands

Recovered PP has real, established markets, and each one has an entry requirement expressed in the same four properties. Reading the ladder from the destination backwards is more useful than reading it from the material forwards, because it tells you which property to fix first for the market you actually want.

Each destination has an entry requirement. Colour is the one that rules out the most buyers.

Two patterns in that table are worth stating explicitly.

Most of the volume is moulded, and that is consistent with the chemistry. Chain scission raises melt flow, high melt flow suits injection moulding, and the biggest destinations are moulded. The extruded destinations sit lower down precisely because they need the low melt flow degraded PP no longer has.

The woven sack route is the cleanest closed loop polypropylene has. Washed woven PP recovers into pellet that goes back into raffia tape and then into new sacks — the same polymer family returning to the same application, which very little post-consumer plastic manages. It works because the input is homopolymer, single-source and unfilled. Note that tape extrusion at the far end is a different machine from anything in a recycling plant — the pellet is the deliverable and the buyer runs the tape.

Which Line Configuration a Polypropylene Stream Needs

Physical form decides the front end and melt behaviour decides the back end, and those two decisions are made independently of each other. Size reduction and washing follow the shape the material arrives in, while filler content, odour risk and target melt flow together set what the pelletizing section has to be able to do.

Form decides the front end. Melt behaviour decides the back end. They are separate purchases.

On the front end, the published configurations follow the form. Rigid containers run on rigid-container washing architecture at 300–2,500 kg/h, cutting to 10–15 mm flake with hot alkaline washing at 60–80°C and finishing below 1% moisture. Sacks and bulk bags need the PP jumbo bag recycling washing line at 300–2,000 kg/h, because woven fabric wraps a conventional rotor. Film and raffia take the soft film route at 250–2,000 kg/h, cutting to 10–20 mm and finishing below 5%.

The back end is where the polymer questions get answered, and three choices carry most of the outcome.

Melt filtration stage count follows the contamination, not the capacity. Because a polypropylene bale is heterogeneous by nature, it delivers more small foreign fragments to the melt than a bottle stream does. That is why the rigid pelletizing line specifies multi-stage filtration with continuous or discontinuous screen changers, and why an undersized filter announces itself first as rising melt pressure and shrinking intervals between screen changes.

Degassing zone count follows the odour risk. Food packaging streams need far more devolatilisation than industrial scrap does, and the lever for that is vacuum capacity in the barrel. The PP PE film compacting pelletizing line publishes one to two vacuum exhaust zones plus an optional second extruder.

Cut method follows melt behaviour, and PP shifts over time. Rigid PP with a moderate melt flow strands well. High-melt-flow degraded PP does not hold a strand as reliably, and a water ring or underwater cutter is the more forgiving choice — which is why the published lines offer both rather than one. Our comparison of plastic recycling pelletizing configurations covers the related single-stage against double-stage question.

Frequently Asked Questions

What does polypropylene recycling produce?

A pellet graded by melt flow, colour, odour and filler content rather than by purity alone. Those four properties decide which buyer can use it, and three of them are set by the material’s history before it reaches the line rather than by how well it is washed.

Why does the melt flow index of recycled PP go up?

Because polypropylene degrades by chain scission. A tertiary carbon on every second backbone atom gives thermal and oxidative attack a starting point, the molecule breaks rather than crosslinks, average molecular weight falls and melt flow rises. Polyethylene does the opposite and its melt flow usually falls.

Does recycled polypropylene need additives?

Usually yes. The antioxidant package a virgin grade carried is consumed by the original processing, by service life and by hot alkaline washing. Restoring it at the compounding step is standard, which is why recycled PP is normally described as compounded rather than simply pelletized.

Are all number 5 plastics the same polymer?

No. Code 5 covers homopolymer, random copolymer and impact copolymer polypropylene, plus filled versions of all three. They sort identically under near-infrared detection and look identical as flake, but they differ in stiffness, low-temperature toughness, melting point and density.

Does filled polypropylene float in a washing line?

Not always. Unfilled PP sits around 0.90 to 0.92 grams per cubic centimetre and floats, but a mineral-filled grade at automotive or appliance loadings can pass 1.0 and sink, leaving with the grit. Filler is also abrasive, so wear-resistant screw and barrel elements should be specified upfront.

Can washing remove odour from recycled polypropylene?

Only the surface fraction. Polypropylene absorbs organic compounds into the polymer, and hot alkaline washing reaches what sits on the surface. The absorbed fraction leaves in the melt through vacuum degassing or not at all, which is why degassing zone count matters on odour-sensitive work.

What is recycled polypropylene used for?

Crates, pallets, pails, automotive non-appearance parts, technical profile and cable duct, garden furniture, and woven raffia tape back into sacks. Most of the volume is injection moulded rather than extruded, which follows from chain scission raising melt flow into the moulding window.

What a Polypropylene Line Borrows From Neighbouring Streams

• Plastic Recycling Machine Overview — the six process decisions every stream passes through, PP included
• HDPE Rigid Milk Bottle Recycling Washing Line — the rigid washing architecture PP tubs and crates share, separated at intake
• Jumbo Bag Shredder — the anti-winding rotor woven PP needs before any wash stage runs
• Friction Washer — the scrubbing stage that lifts residue but cannot fix polypropylene odour