The quickest way to tell polyethylene (PE) from polypropylene (PP) at home is to flip the item over and look for the stamped recycling code: PE carries a #2 or #4, while PP carries a #5. When no code is present, you can lean on differences in flexibility, density, melting point, and even the way each plastic burns. For labs and recycling plants, spectroscopic instruments settle the question in seconds, but plenty of low-tech methods work surprisingly well in a garage or workshop.
Start With the Recycling Code
Most rigid plastic containers sold in the United States and many other countries carry a resin identification number inside a triangular chasing-arrows symbol molded into the base. High-density polyethylene (HDPE) is marked #2, low-density polyethylene (LDPE) is marked #4, and polypropylene is marked #5.1ScienceDirect. Evaluation of mixed #3–7 plastic waste from material recovery facilities (MRFs) in the United States – Section: Introduction A milk jug, for example, is almost always #2 HDPE; a yogurt cup is almost always #5 PP. The code is your fastest, most reliable field indicator when it is present.
The catch is that many everyday PE and PP items never receive a code. Plastic bags, sheeting, automotive trim, industrial tubing, and raw pellets typically carry no stamp at all. Colored or heavily textured surfaces can obscure a faint molded number. And even when a code exists, items made from PE/PP blends or multi-layer laminates may be stamped #7 (“other”), which tells you nothing about the dominant resin. In any of these situations, you need a physical or chemical test.
The Float Test
Both PE and PP are polyolefins, meaning they are lighter than water, so dropping a sample into a glass of tap water will not separate them from each other. The density gap between them is real but small. HDPE sits at roughly 0.94 to 0.97 grams per cubic centimeter, LDPE at about 0.91 to 0.94, and PP at about 0.90 to 0.91. All three float in pure water (density 1.00), but PP floats a touch higher because it is the least dense of the group.
To exploit this difference you need a liquid with a density between the two plastics. A mixture of water and isopropyl alcohol (rubbing alcohol) tuned to roughly 0.92 g/cm³ will let a piece of PP float while a piece of HDPE sinks. You can fine-tune the ratio by adding alcohol drop by drop until a known HDPE sample just barely sinks. LDPE, however, overlaps with PP in density so closely that the float test cannot reliably distinguish LDPE from PP. This method works best when you already know the item is made from a rigid, high-density polyethylene and you want to confirm it is not polypropylene, or vice versa.
How They Feel and Flex
Experienced plastics workers often identify PE and PP by touch alone, and there are real physical reasons their instincts hold up. Polyethylene, especially HDPE, has a waxy, slightly slippery surface. If you drag a fingernail across a PE bottle or cutting board, it tends to leave a visible scratch or a white mark fairly easily. Polypropylene feels smoother and a bit harder, almost dry to the touch compared with PE’s greasier slip.
Flexibility is another clue. A thin PE sheet or bag tears without much resistance and can be stretched before breaking. PP film, on the other hand, tends to crackle and crease sharply when you bend it, and thin PP hinges (like those on a flip-top cap) can be flexed back and forth hundreds of times without snapping. That “living hinge” behavior is nearly unique to polypropylene. If the item has an integral hinge that still works after repeated use, it is almost certainly PP.
When polyethylene is bent or stretched aggressively, the deformed area often turns white, a phenomenon called stress-whitening. Polypropylene shows stress-whitening too, but its whitened zones tend to be more pronounced and more sharply defined.2Polymer Engineering & Science. Healing of stress‐whitening in polyethylene and polypropylene at or below room temperature If you bend a mystery plastic sharply and a crisp white line appears at the crease, that leans toward PP. A more diffuse, waxy whitening suggests PE.
The Burn Test
Burning a small sliver of plastic is a classic workshop identification technique. Use a well-ventilated area and a flame you can hold steadily, like a lighter or a match. Both PE and PP will ignite, burn with a blue-based flame tipped in yellow, and drip molten plastic as they melt. The key difference is smell. Polyethylene gives off a distinct paraffin or candle-wax odor, essentially the same smell you get from a burning birthday candle. Polypropylene’s smell is similar but tends to be slightly sharper and faintly sweet, sometimes compared to burning oil or hot rubber.
The flame behavior itself is nearly identical between the two, which limits how far a burn test can take you. Both plastics continue to burn after you remove the flame, and both drip. This sets them apart from many other common plastics: PVC, for instance, self-extinguishes and smells strongly of chlorine, and polystyrene burns with a sooty, black-smoke flame. So a burn test is excellent for confirming that you are dealing with a polyolefin rather than some other plastic family, but telling PE from PP by flame alone requires a practiced nose.
Melting Point as a Practical Divider
If you have a heat gun, an oven, or even a pot of oil with a thermometer, melting point is one of the most reliable low-tech ways to tell PE from PP. HDPE melts at roughly 130 to 135 °C (about 265 to 275 °F). LDPE melts lower, around 105 to 115 °C. Polypropylene does not melt until roughly 160 to 170 °C (320 to 340 °F). That gap of 25 to 35 degrees between HDPE and PP is wide enough to exploit easily.
A simple approach: set your oven to 140 °C (285 °F) and place a small sample on a piece of aluminum foil. After about ten minutes, HDPE will soften and deform visibly, while PP will still hold its shape. Increase the temperature to 165 °C and the PP sample will slump. LDPE will already be a puddle at that point. This works because the crystalline structures of these polymers melt at distinctly different temperatures, a principle that underlies the lab-grade technique called differential scanning calorimetry (DSC), which measures the precise heat flow as a plastic melts. DSC can even quantify blends, pinpointing how much PE and how much PP are mixed together with errors as low as about one percent by weight.3ScienceDirect. Accurate determination of polyethylene (PE) and polypropylene (PP) content in polyolefin blends using machine learning-assisted differential scanning calorimetry (DSC) analysis
If you do not have precise temperature control, the oven test at least narrows the field: anything that softens well below boiling-water temperature is likely LDPE, anything that resists up to about 130 °C is probably HDPE, and anything that stays rigid past 150 °C points strongly to PP.
Spectroscopy Settles the Question Definitively
All of the methods above rely on physical properties that can overlap, especially when fillers, colorants, or blending muddy the picture. When certainty matters, spectroscopy is the standard answer. The three main techniques used for PE-versus-PP identification are Fourier-transform infrared spectroscopy (FTIR), near-infrared spectroscopy (NIR), and Raman spectroscopy. Each works by shining a beam of light at the sample and reading back which wavelengths get absorbed or scattered, producing a molecular fingerprint unique to each polymer type.
FTIR is the workhorse in analytical labs. A database of FTIR spectra for the six most common industrial plastics, including both PE types and PP, has been published with thousands of reference scans, allowing automated matching of unknown samples against known fingerprints.4PubMed Central. FTIR-Plastics: A Fourier Transform Infrared Spectroscopy dataset for the six most prevalent industrial plastic polymers In practice, a lab tech presses the sample against the instrument’s crystal, collects a spectrum in under a minute, and runs it through a library search. PE and PP produce recognizably different spectral peaks, making misidentification rare.
Raman spectroscopy uses a laser instead of broadband infrared light and reads the scattered light to build its fingerprint. It works well on colored, dark, or wet plastics that sometimes give FTIR trouble. Remote Raman setups can even identify polymer types at a distance, which has obvious appeal for waste-sorting applications.5Scientific Reports. Remote Raman detection of seven groups of plastics
NIR spectroscopy is the backbone of industrial plastic sorting. Recycling facilities use high-speed NIR sensors to scan items on a conveyor belt and trigger air jets that blow different polymers into separate bins. One study demonstrated successful classification of PP, polystyrene, and other plastics from waste electronics using NIR.6ScienceDirect. Auto-sorting commonly recovered plastics from waste household appliances and electronics using near-infrared spectroscopy The technique is fast enough to handle thousands of items per minute in a sorting plant.
Handheld Devices for On-the-Spot Checks
You do not need a full laboratory to access spectroscopy anymore. Miniaturized handheld NIR spectrometers, small enough to fit in a coat pocket, have been validated for fingerprinting and classifying PE, PP, PVC, PET, and PS directly on unwashed waste samples at a recycling plant.7PubMed Central. Miniaturized Near-Infrared (MicroNIR) Spectrometer in Plastic Waste Sorting These devices cost a fraction of a benchtop instrument, typically a few thousand dollars, and give a result in seconds. Portable Raman spectrometers occupy a similar niche. For anyone regularly sorting plastic scrap, buying or dismantling mixed-resin bales, or verifying incoming material at a fabrication shop, a handheld unit pays for itself quickly by preventing costly mix-ups.
Consumer-grade “plastic identifier” gadgets and smartphone-based apps have started to appear as well, though their accuracy varies. The underlying principle is sound, but cheaper sensors and smaller optics mean narrower spectral ranges and more room for error. If you are buying one for occasional home or small-business use, look for a device that provides actual spectral data and a library match score, not just a color-coded yes-or-no readout.
Why Mixing Them Up Causes Real Problems
At first glance PE and PP seem so similar that confusing them might not matter. They are both polyolefins, both lightweight, both resistant to water and many chemicals, and both recyclable in theory. In practice, though, even a small amount of the wrong one contaminating a recycling stream degrades the quality of the output. When polypropylene sneaks into a batch of recycled HDPE, the two resins do not blend well at the molecular level. The recycled pellets come out with weak spots, haze, and unpredictable mechanical properties.
Detecting that contamination requires sensitive analytical methods. Temperature-based fractionation techniques can identify PP impurities in recycled HDPE at concentrations as low as two percent by weight, catching contamination that DSC alone tends to underestimate.8Polymer Testing. Quantification of PP contamination in recycled PE by TREF analysis for improved the quality and circularity of plastics More advanced fractionation methods can even distinguish between different types of polypropylene in the blend, such as homopolymer PP versus random copolymer PP, and quantify each separately.9Polymer Testing. Quantifying the content of various types of polypropylene in high density polyethylene blends This level of detail matters because different PP grades disrupt recycled PE in different ways.
Currently, PE (#1 as PET and #2 as HDPE) accounts for the vast majority of plastics that actually get reprocessed. Polypropylene (#5), despite being one of the most produced plastics on Earth, is recycled at much lower rates partly because it so often gets mixed with PE and other resins in bales destined for landfill.1ScienceDirect. Evaluation of mixed #3–7 plastic waste from material recovery facilities (MRFs) in the United States – Section: Introduction Better identification at the consumer and sorting-plant level is one of the simplest levers for improving PP recycling rates.
Thermal Degradation and Pyrolysis Fingerprints
When plastic waste is thermally recycled, meaning broken down by heat into oils or gases, PE and PP behave differently under the same conditions. Pyrolysis studies show that the energy required to start breaking down HDPE is higher than for PP. Activation energies for thermal degradation run in the range of 238 to 247 kJ/mol for HDPE and 179 to 188 kJ/mol for PP, with LDPE falling in between.10ScienceDirect. Thermal degradation behaviors of polyethylene and polypropylene. Part I: Pyrolysis kinetics and mechanisms In plain terms, polypropylene starts to decompose more readily than polyethylene under the same heating conditions, and the two follow different decomposition pathways at the molecular level.
This distinction is useful for identification in the form of a technique called pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). A tiny sample is rapidly heated until it breaks into fragments, which are then separated and identified. Each polymer produces a characteristic pattern of breakdown products. Researchers have shown that PE and PP can be reliably told apart this way, even after significant UV weathering that changes the surface chemistry of both plastics.11ScienceDirect. Aging effects on low- and high-density polyethylene, polypropylene and polystyrene under UV irradiation Py-GC/MS is especially valuable for identifying tiny microplastic particles found in environmental samples, where no recycling code or physical test is feasible.
When the Usual Methods Struggle
Black plastics are a notorious headache. Carbon black pigment absorbs infrared light so thoroughly that both FTIR and NIR struggle to get a usable spectrum. This is a significant real-world problem because a large fraction of automotive parts, electronics housings, and food trays are black. Raman spectroscopy fares somewhat better on dark samples, and advanced NMR-based approaches have been developed specifically for situations where vibrational spectroscopies fail. Automated carbon-13 NMR analysis can determine the molecular microstructure of polyolefins, distinguishing PE from PP and even characterizing copolymers, in a matter of minutes using modern high-temperature probes.12PubMed Central. Automated Ultra-Fast 13C NMR Analysis of Polyolefin Materials NMR requires dissolving the sample at high temperature, so it is not a quick field method, but it provides unambiguous identification when other techniques hit a wall.
Blends and multilayer packaging pose another challenge. A chip bag, for instance, might have a PP outer layer, a PE inner layer, and an aluminum barrier in between. No single surface measurement will capture the full story. Solvent-based dissolution techniques can help here: PE and PP dissolve at different temperatures in the same solvents, so researchers can selectively dissolve one layer, recover it, then dissolve the other at a higher temperature.13IOP Conference Series: Earth and Environmental Science. Solvent and non-solvent selection for the chemical recycling of waste Polyethylene (PE) and Polypropylene (PP) metallized film packaging materials This selective dissolution is both a separation technique for recycling and an identification tool: if the plastic goes into solution at the lower temperature, it is PE; if it waits for the higher temperature, it is PP.
Surface Energy and Printability
One practical difference between PE and PP that matters outside the recycling world is how they accept ink, adhesive, and coatings. Both plastics have naturally low surface energy, which means liquids tend to bead up rather than spread. If you have ever tried to write on a plastic bag with a permanent marker and watched the ink pull away into droplets, you have seen low surface energy in action.
Untreated PE and PP have similar surface energies, typically in the range of 30 to 33 millijoules per square meter. Industrial processes like corona treatment or atmospheric-pressure plasma treatment raise the surface energy dramatically, enabling inks and adhesives to stick. After plasma treatment under optimized conditions, PE’s surface energy climbed to about 54 mJ/m² with a water contact angle of 40°, while PP reached about 53 mJ/m² with a contact angle of 42°, essentially converging the two materials to comparable wettability.14Journal of Physics D: Applied Physics. Surface modification of polyethylene and polypropylene in atmospheric pressure glow discharge Before treatment, though, subtle differences in how inks and labels behave on PE versus PP can serve as yet another identification clue. Labels designed for PE containers sometimes peel or wrinkle when applied to PP, and vice versa, because adhesive formulations are tuned to each resin’s specific surface characteristics.15Progress in Rubber and Plastics Technology. Application of Contact Angle Measurement to Wettability, Surface Energy and Adhesion Properties of Polymers
For anyone doing heat-sealing, welding, or gluing at home or in a shop, correctly identifying which polyolefin you are working with determines the right temperature setting, the right adhesive chemistry, and whether the bond will hold. Welding PE to PP, for example, produces a joint that looks fine but tends to be weak because the two resins do not co-crystallize well at the interface. Knowing which plastic you have before you start saves wasted material and frustrating failures.