Material Recycling Guides
PET vs. HDPE vs. PP: How the Material Decides the Process Route
Three of the most common recycling polymers, three different process routes — and the differences come from density and form, not from preference.
PET vs. HDPE vs. PP: How the Material Decides the Process Route
Key takeaways
- PET is denser than water, so it sinks; HDPE and PP float. That single fact drives separation.
- PET is usually processed as bottles into clean flake; HDPE and PP are processed as bottles, containers or rigid regrind.
- Labels and adhesives are the core problem for PET, which is why hot washing is standard.
- HDPE and PP from the same stream cannot be separated by density — they need other criteria.
- The target output (flake or pellet) determines how far down the process route the line goes.
The one physical fact that organises everything
PET has a density greater than water. HDPE and PP have densities lower than water.
That means a float-sink tank separates PET from HDPE and PP cleanly and cheaply — PET sinks, the others float. Almost every mixed-bottle recycling route is built around this fact.
It also means the harder problem is separating HDPE from PP, because they both float. That separation cannot be done by density and requires other criteria: near-infrared sorting, or accepting a mixed polyolefin output.
PET: bottles into clean flake
Input form: post-consumer bottles, baled or loose, from municipal collection or deposit systems.
Contamination profile: labels, adhesives, caps and rings (PP and PE), sand, residual liquid, and sometimes coloured or opaque material mixed in.
Why the route looks the way it does:
- Hot washing with detergent is standard, because adhesive and label residue do not release in
cold water.
- Density separation removes the PP and PE cap and ring fraction, which floats.
- Colour and grade sorting matters, because clear, light blue and coloured streams go to
different end products.
- Moisture control before extrusion is critical, because PET is sensitive to moisture at melt
temperature.
Typical route: bale opening → sorting and metal removal → label removal → crushing → hot washing → friction washing → density separation → drying.
Typical output: clean PET flake, or recycled PET pellets if the route extends to extrusion.
HDPE and PP: containers and rigid parts
Input form: bottles, drums, buckets, crates, pipes and rigid regrind, from post-consumer or post-industrial sources.
Contamination profile: soil, residual product, oil and grease (particularly from containers that held oil or dairy), labels and closures.
Why the route looks the way it does:
- Pre-washing removes the bulk of soil and grit, protecting downstream equipment.
- Hot washing is often needed for containers with oily or food residue; less often for clean
industrial scrap.
- Density separation is used here to remove any PET that has contaminated the stream, not to
separate HDPE from PP.
- Size reduction to a controlled flake is what makes the material saleable or extrudable.
Typical route: feeding → size reduction → pre-washing → hot or friction washing → rinsing → density separation → dewatering → drying.
Typical output: clean HDPE or PP flake, or pellets.
Comparing the three
| PET | HDPE | PP | |
|---|---|---|---|
| Density vs. water | Sinks | Floats | Floats |
| Separable by density from the others | Yes | Yes, from PET | Yes, from PET |
| Separable from each other by density | — | No | No |
| Typical form | Bottles | Bottles, containers, rigid | Woven bags, containers, rigid |
| Core contamination problem | Labels and adhesives | Soil, residual product, oil | Soil, printing, residual product |
| Hot washing | Standard | Often | Often, or not, depending on source |
| Melt processing concern | Moisture at melt temperature | Moisture and volatiles | Printing ink and volatiles |
Flake or pellet?
The material does not decide this — the destination does. Washing lines produce flake, which is saleable and can be shipped. Pelletizing adds extrusion, filtration and pelletizing stages, and produces a denser, more consistent product that is easier to feed into a moulding or film process.
Integrated washing-to-pelletizing systems exist precisely because some projects need to go from contaminated input to pellets in one line, without the interface risk of running two separate processes.
Related: Material Recycling Guides · Solutions by Material · PET Bottle Solutions · HDPE/PP Bottles & Containers
ABOUT THE AUTHOR
Plastic Recycling Process & System Engineering
The Retech Engineering Team works across material analysis, process design, recycling line configuration, factory testing and project commissioning.
Questions
PET vs. HDPE vs. PP: How the Material Decides the Process Route — FAQ
Can I separate HDPE and PP from each other?
Not by density — both float. Separating them requires other criteria such as near-infrared sorting, or accepting a mixed polyolefin output. Many recycling routes process them together as a mixed PE/PP stream.
Why is hot washing standard for PET but not always for film?
PET bottle recycling is dominated by label and adhesive removal, which requires heat and detergent. Many film streams carry soil and organic residue rather than adhesive, and that is removed by pre-washing and friction washing without heat.
Should I produce flake or pellets?
It depends on where the material is going, not on the material itself. Flake is saleable and shippable; pellets are denser and more consistent for moulding or film production. Integrated washing-to-pelletizing lines exist for projects that need to do both in one pass.
More answers in the <a class="retech-x-link-arrow" href="/faq/">full FAQ</a>.
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