HDPE vs LDPE and HDPE vs PP get asked as material selection questions and answered with property tables. On a recycling line they are a different question entirely — not which one to specify, but which one is currently in your hand, in your bale or in your feed hopper.
That question is harder than it looks. All three are hydrocarbons of carbon and hydrogen only. All three float in water. All three melt and extrude. There is no colour, no smell and no obvious handling difference that separates them cleanly, and the two most widely repeated field tests do not distinguish them at all.
Three polymers, one element pair, and no visual difference worth relying on.
What Actually Separates the Three Polyolefins
All three are hydrocarbon chains of carbon and hydrogen, and the differences come from chain architecture rather than chemistry. Branching and the propylene methyl group change how tightly chains pack, which sets crystallinity, and crystallinity sets density, stiffness and melting point. Three properties, one underlying cause.
LDPE is made under high pressure by a process that grafts branches onto the growing chain. Those branches act as spacers, crystalline regions stay small, and the material ends up soft and clear. HDPE is grown over catalysts that keep the chain straight, so it packs efficiently and ends up stiff and opaque. Polypropylene is a different monomer, carrying a methyl group on every second carbon, and packs into its own crystal form with a much higher melting point.
Crystallinity itself cannot be seen, but both of its consequences can be measured with a bucket of water and a hot plate. The molecular background matters here only as far as it explains why the numbers land where they do, and the numbers are what the sorting decision is made on.
The Three Materials Side by Side
Density and melting point are the two numbers that do the real work, and everything else in the table below is a field proxy for one of them. Read the density column first, because it is the one that decides whether a separation stage can act on the difference at all.
Two measurable numbers at the top, five hand checks underneath that are proxies for them.
| LDPE and LLDPE | HDPE | PP | |
|---|---|---|---|
| Density (g/cm³) | 0.910–0.925 | 0.941–0.965 | 0.895–0.910 |
| Melting point | 105–115°C, LLDPE nearer 120–125°C | 125–135°C | 160–165°C for homopolymer |
| Crystallinity | Low, roughly 45–55 percent | High, roughly 70–80 percent | Moderate, and a different crystal form from PE |
| Feel and surface | Soft and waxy, stretches under a fingernail | Waxy, takes a fingernail mark easily | Harder and slicker, resists a fingernail mark |
| Sharp crease behaviour | Stretches and necks rather than creasing | Creases with mild whitening, hinge fatigues quickly | Whitens strongly at the fold and survives repeated flexing |
| Sound as film | Quiet and soft, the bread bag and dry cleaner bag sound | Loud crinkle, the carrier bag sound | Sharp rustle, often woven rather than cast |
| Boiling water behaviour | Distorts readily | Softens and can deform | Holds shape, which is why it is the microwave and dishwasher polymer |
| Optical appearance | Clear as thin film | Always opaque or hazy, never clear | Translucent, and can be made genuinely clear with a clarifier |
| Typical products in a waste stream | Packaging film, liners, agricultural sheet, squeeze bottles, lids | Milk and detergent bottles, crates, drums, pressure pipe | Caps and closures, woven sacks and bulk bags, crates, thin-wall tubs, automotive trim |
The boiling water row is the single most useful field check in the table and the least often mentioned. Drop three unidentified rigid containers in hot water and the one that holds its shape is PP. It takes two minutes, needs no equipment, and resolves the PP-against-PE question that density cannot.
Where the Density Bands Overlap and Water Stops Working
Plain water separates polyolefins from PET and PVC and does nothing to separate them from each other, because all three float. Splitting PP from HDPE needs a medium near 0.92, and LDPE and LLDPE sit directly on that cut line, so they report to both fractions.
The gap between HDPE and PP is real. The band LDPE occupies sits right on top of the cut you would need to make.
Lay the numbers on a single scale and the problem is visible immediately. PP runs from about 0.895 to 0.910. LDPE runs from 0.910 to 0.925. They meet. HDPE starts at 0.941, comfortably clear of both, and MDPE fills the 0.926 to 0.940 gap between them. Water at 1.00 sits well above everything in the polyolefin family, which is why a plain float-sink tank sends all of it to the float fraction together.
| Separation medium | What floats | What sinks | What it cannot resolve |
|---|---|---|---|
| Plain water, 1.00 | PP, LDPE, LLDPE, HDPE, all as one float fraction | PET at about 1.38, PVC at 1.3–1.45, grit, metal fines, mineral-filled fragments | Any distinction inside the polyolefin family |
| Controlled medium near 0.92 | PP, and part of the LDPE and LLDPE band | HDPE, MDPE, and the rest of the LDPE and LLDPE band | LDPE and LLDPE, which straddle the cut and split across both outputs |
| Controlled medium near 0.93 | PP, LDPE and LLDPE together as one light polyolefin fraction | HDPE cleanly, with MDPE mostly following it | PP against LDPE, which is often the pairing that actually matters |
The third row is the practical compromise most plants land on. Pull HDPE out cleanly with a cut above the LDPE band and accept that PP and LDPE leave together, then deal with that pair somewhere else — usually by keeping them apart at intake rather than trying to split them downstream. There is a formal method behind these figures; ASTM D1505 sets out the density gradient column technique that produces the published bands in the first place.
Two operating realities widen the problem further. Fillers move the number, so a talc-filled PP tub can sit well above unfilled HDPE and sink where its unfilled twin would float. And trapped air floats anything, which is why density separation is always placed after size reduction — a sealed container reports to the float fraction regardless of what it is made of.
Field Tests That Work and What Each One Resolves
No single hand test separates all three, and the most commonly recommended one is the weakest. A burn test reliably tells a polyolefin from PVC, PET or polystyrene, and tells you almost nothing about whether the polyolefin in your hand is polyethylene or polypropylene.
Each test answers one narrow question. Chaining two of them resolves most of what arrives on a tipping floor.
| Test | What you do | What it resolves | What it does not resolve |
|---|---|---|---|
| Water float | Cut a chip, sink it in water and agitate to shed bubbles | Polyolefin against PET, PVC, PS and anything filled or reinforced | Nothing inside the polyolefin family, since all three float |
| Hot water | Hold a sample in near-boiling water for two minutes | PP against both polyethylenes, because only PP holds its shape | HDPE against LDPE, which both distort |
| Sharp crease | Fold a section hard and flex it back and forth | PP by its strong stress whitening and durable living hinge | Little on thin film, where geometry dominates the response |
| Fingernail scratch | Draw a nail firmly across a clean flat area | HDPE and LDPE, both of which mark, from harder PP | HDPE against LDPE, which feel similar under a nail |
| Film sound and stretch | Crumple a piece of film next to your ear, then pull it | HDPE film by its loud crinkle against quiet stretchy LDPE | Anything rigid, and any film that has been coated or laminated |
| Burn test | Ignite a small sliver in a ventilated space and observe flame, drip and smell | Polyolefin against PVC, PET and PS, which all behave distinctly | PE against PP, which give near-identical flame, drip and smell |
Two rows there run against common advice. The burn test is presented in most online guidance as the field method for polymer identification, and for this particular question it is close to useless — both PE and PP burn with a blue flame and a yellow tip, both drip, and both smell of burning wax. It also destroys the sample and carries an obvious safety cost.
Hot water is the reverse case. It appears in almost no identification guides and resolves precisely the pairing density cannot. Run water float first to confirm you have a polyolefin, then hot water to pull PP out. That turns a three-way question into a two-way one, and the remaining HDPE against LDPE call is usually obvious from form alone.
How Near-Infrared Sorting Tells Polyethylene From Polypropylene
Near-infrared sorters do not measure density. They read how a material absorbs light across a band of infrared wavelengths, and polypropylene carries a methyl side group that polyethylene does not. That one structural difference shifts the absorption pattern enough for a detector to separate the two reliably.
The sorter reads a surface reflection. Everything it gets wrong follows from that one fact.
The mechanism is worth understanding because its limits follow directly from it. A polyethylene chain is a run of methylene units and nothing else. A polypropylene chain carries a methyl group on every second carbon. Carbon-hydrogen bonds in those two environments absorb infrared light at slightly different wavelengths, and the resulting reflectance patterns are different enough that a detector reading a few thousand times a second can classify each fragment as it passes. This is the one technique that resolves the PP-against-LDPE pairing that density cannot.
The technique has five failure modes, and all of them come from the same source — it reads a surface, not a bulk.
| Blind spot | Why it happens | What it does to the output |
|---|---|---|
| Black and very dark items | Carbon black absorbs across the near-infrared band, so almost nothing returns to the detector | The item is not misclassified, it is invisible, and it leaves with the residue |
| Full-sleeve labels and heavy print | The reading comes from the outermost layer, which is the label rather than the container | A PP tub in a PE sleeve is classified as PE, and vice versa |
| Wet or dirty surfaces | Water has strong absorption bands of its own that sit over the polymer signal | Classification confidence drops, and marginal fragments get rejected or passed at random |
| Multilayer and coated material | The surface layer is genuinely a different polymer from the bulk | Correct reading, wrong answer, since no reading describes a laminate as a whole |
| Small fragments and overlapped items | Below a certain size a fragment does not fill the detection area, and stacked items shade each other | Both are read as one mixed or indeterminate signal and ejected together |
Read alongside the density section, the conclusion is the same from both directions. Density separation handles bulk and is blind to what is printed on a surface. Optical sorting reads the surface and is blind to what is underneath it. Neither is a complete answer, and a plant with a genuinely mixed polyolefin intake usually needs both, plus the intake discipline that makes each one’s job smaller.
What Mixing Them Actually Costs You
Polyethylene and polypropylene both melt and both extrude, which is why mixed polyolefin looks harmless on a pellet analysis. They are thermodynamically immiscible, so the blend is two separate phases with weak bonding between them, and impact strength falls faster than the contamination percentage suggests it should.
A blended pellet is not an averaged material. It is two materials sharing a granule.
| What goes wrong | The mechanism | Where the customer sees it |
|---|---|---|
| Impact strength drops out of proportion to the contamination level | The two polymers form separate phases and the boundary between them carries almost no load, so cracks run along it | Parts that pass a tensile check and fail a drop test, especially in cold conditions |
| Melt flow index describes neither component | The measured figure is an average of two very different rheologies rather than a property of one material | A specification sheet that looks acceptable and a process that will not hold settings |
| Gels and unmelted inclusions in film | PP melts around 160–165°C, well above LDPE, so a film-line temperature profile may not fully melt and disperse it | Pinholes, gel specks and web breaks on a blown or cast film line |
| Warp and dimensional drift in moulded parts | The two phases crystallise at different temperatures and shrink by different amounts as the part cools | Out-of-tolerance parts and inconsistent shrinkage between production runs |
| Stress cracking resistance falls | Phase boundaries act as initiation sites for the slow crack growth that HDPE is normally good at resisting | Containers that split in service weeks after passing every incoming check |
| The pellet drops a grade commercially | Buyers who need a defined single polymer will not take it, so it sells into applications where properties matter least | A price per tonne set by the mixed polyolefin market rather than by the resin it mostly is |
None of that means mixed polyolefin has no market. It has a real one, and compatibilisers exist that improve adhesion between the phases well enough to make usable compound. What it means is that the mixed product is a deliberate, lower-value output rather than a slightly imperfect version of the clean one, and it should be priced and planned as such rather than discovered after the first shipment is rejected.
There is also a direction to the damage. A small PP fraction in an HDPE stream costs more than the same fraction of HDPE in a PP stream, because HDPE’s headline properties — stress crack resistance and low-temperature toughness — are the two most sensitive to phase boundaries. If a plant has to tolerate one contamination direction, that is the one to argue about.
Which Line Each Material Runs On
Material identity decides wash chemistry and pelletizing route, but physical form decides the machine. Rigid HDPE, PP woven sacks and PE film share almost no hardware, so a plant handling more than one of them is buying more than one line rather than one line with options.
Read the form column, not the polymer column. That is the one that decides the machine.
| Material and form | Washing route | Published configuration | Pelletizing route |
|---|---|---|---|
| Rigid HDPE containers and bottles | HDPE rigid milk bottle recycling washing line | 300–2,500 kg/h, 10–15 mm flake, hot alkaline wash at 60–80°C, final moisture below 1 percent | Rigid plastic granulating pelletizing line, 3–8 mm in and 2–5 mm out |
| PP and PE soft film, liners, agricultural sheet | PP PE soft film recycling washing line | SHW300 to SHW2000, 250–2,000 kg/h, 10–20 mm flake, final moisture below 5 percent | PP PE film compacting pelletizing line, or a squeezer ahead of an extruder |
| PP woven sacks and bulk bags | PP jumbo bag recycling washing line | Configured against bag size, fill residue and print load rather than against the polymer | Compacting pelletizing, since woven tape behaves as film rather than as rigid flake |
| Rigid PP crates, caps and thin-wall tubs | Rigid washing route, sized as for HDPE but separated at intake | Same family of hardware as the rigid HDPE line | Rigid granulating pelletizing, kept as a separate campaign from HDPE |
One published figure is worth pulling out of that table. The film line’s steam and chemical consumption both start at zero, running up to 600 kg/h and 12 kg/h — clean industrial film may need no hot wash at all while agricultural film needs both, so the hot stage is a real configuration choice rather than a fixed feature.
The point underneath all of it is that identification pays off at intake and nowhere else. Once three polyolefins have gone through the same rotor together, no washing stage, no density tank and no melt filter separates them again. Configuration options across materials are set out in the plastic recycling machine range, with the film-side decisions in our guide to choosing a plastic film washing line and the rigid side in the HDPE recycling machine guide.
Frequently Asked Questions
How do you tell if plastic is PP or PE?
Hot water is the quickest reliable check. Hold a sample in near-boiling water for two minutes — PP holds its shape while both polyethylenes soften and distort. A burn test does not help here, because PE and PP give almost identical flame, drip and smell. Near-infrared sorting resolves it industrially.
What is the difference between HDPE and LDPE?
Chain branching, and everything that follows from it. Branches keep LDPE chains from packing closely, which holds its density to 0.910 to 0.925 and leaves it soft and clear. HDPE grows essentially unbranched, packs tightly, and reaches 0.941 to 0.965, which is where its stiffness and opacity come from.
How can you tell HDPE film from LDPE film?
By sound. Crumple a piece next to your ear — HDPE film gives a loud crinkle and LDPE film is soft and quiet. The everyday reference points are a supermarket carrier bag for HDPE and a bread bag or dry cleaner bag for LDPE. LDPE also stretches under a pull where HDPE tears.
Is PP stronger than HDPE?
It depends which property is meant. PP is stiffer, harder and holds shape at higher temperatures, which is why it goes into microwave-safe tubs and living hinges. HDPE has better impact toughness at low temperature and better resistance to slow crack growth, which is why pressure pipe and cold-chain containers use it.
Do HDPE, LDPE and PP all float in water?
Yes, all three sit below the density of water, so a plain float-sink tank sends them to the float fraction together. Water separates polyolefins from PET, PVC and mineral-filled fragments, and does nothing to separate the polyolefins from each other. That takes a controlled-density medium or optical sorting.
What happens if PP gets mixed into an HDPE recycling stream?
The two are immiscible, so the melt forms separate phases with weak bonding at the boundaries. Impact strength and stress crack resistance drop faster than the contamination percentage would suggest, the measured melt flow index describes neither component, and the pellet sells into the mixed polyolefin market instead of as recycled HDPE.
Why can a float-sink tank not separate PP from LDPE?
Because their density bands touch. PP runs to about 0.910 and LDPE starts at about 0.910, so any medium set to cut between them splits the LDPE band across both outputs. HDPE at 0.941 and above is far enough clear to be separated cleanly, which is why that is the cut most plants actually make.
Separating the Three Is Only Step One
- Plastic Recycling Washing Line — four washing lanes side by side, for plants that must buy more than one
- Plastic Film Squeezer Granulating Machine — the one-machine route for film flake that will not gravity-feed an extruder
- Choose a Plastic Film Washing Line — seven ordered decisions once you know which film you are actually buying for
- What Plastics Can Be Recycled — where the other four resin codes sit once you step outside the polyolefins
- Plastic Recycling Machine Lines — the full machine range once you know which of the three you are sorting for
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