How to Tell Polypropylene From Polyethylene With a Simple Float Test

Why Knowing the Difference Between PP and PE Matters for Recycling

Two plastics look very similar to the naked eye. Polypropylene and polyethylene appear in countless everyday items. A milk jug looks like a yogurt container. A shampoo bottle resembles a ketchup bottle. The untrained eye sees white or translucent plastic and stops there.

The recycling world treats these two materials differently. A recycling facility sorts plastics by type. Polyethylene goes into one stream. Polypropylene goes into another. Mixing them together lowers the value of both. A bale of polyethylene with polypropylene contamination sells for less money. A manufacturer buying recycled polypropylene expects a pure material.

The float test offers a simple way to separate them. No expensive equipment is needed. No chemical knowledge is required. A container of water does the job. One plastic floats. The other sinks. That one difference allows anyone to tell them apart in seconds.

Plastic Type	Common Short Name	Behavior in Plain Water
Polypropylene	PP	Floats on the surface
Polyethylene (low density)	LDPE	Floats on the surface
Polyethylene (high density)	HDPE	Floats but sits lower in the water
Polyethylene (some grades)	HDPE with additives	May sink slowly

Water has a fixed density. Anything lighter than water floats. Anything heavier sinks. Polypropylene and most grades of polyethylene are lighter than water. A simple float test does not separate polypropylene from low density polyethylene because both float. The test separates both of them from heavier plastics like PET or PVC.

What Happens When You Drop a Piece of Plastic Into Water

A piece of plastic dropped into a container of water experiences two forces. Gravity pulls the plastic down. Buoyancy pushes the plastic up. The balance between these two forces decides whether the plastic sinks or floats.

Gravity pulls based on weight. A heavier piece of plastic feels more downward force. Buoyancy pushes based on the volume of water displaced. A larger piece of plastic pushes aside more water and feels more upward force. The comparison between weight and displaced water volume gives the density.

If the plastic weighs less than the water it pushes aside, it floats. If the plastic weighs more than the water it pushes aside, it sinks. The shape of the plastic does not matter for this basic comparison. A flat sheet and a dense chunk of the same material behave the same way. Both float or both sink.

Water temperature changes density slightly. Cold water sits a little denser than warm water. A piece of plastic that barely floats in warm water may float higher in cold water. The difference is small. For telling polypropylene from polyethylene, room temperature water works fine.

Salt water has a higher density than fresh water. Adding salt to water makes more things float. A piece of polyethylene that sinks in fresh water may float in salt water. The float test for separating polypropylene from polyethylene uses plain fresh water. Adding salt would change the result.

A container deep enough to fully submerge the plastic piece gives a clear result. A shallow bowl lets the plastic touch the bottom even if it would float. The plastic needs room to rise to the surface if it is lighter than water. A clear glass or plastic container helps with seeing what happens.

How the Density of Polypropylene Compares to Water

Polypropylene has a density that sits below that of water. A piece of polypropylene floating in water shows most of its volume above the waterline. Only a small portion sits below the surface. The material feels light in the hand. A polypropylene container holds its shape well but weighs very little compared to its size.

The float test works for polypropylene in all its common forms. A polypropylene bottle cap floats. A polypropylene yogurt cup floats. A polypropylene hinge on a box lid floats. The manufacturing process does not change the density enough to affect the test result.

Some polypropylene products have fillers or additives. A mineral filler like talc increases density. A polypropylene part with too much filler may sink. Most household polypropylene items do not contain enough filler to change the result. A pure polypropylene piece floats every time.

The float test gives a clear yes or no answer for polypropylene. A piece that sinks is not pure polypropylene. A piece that floats could be polypropylene or low density polyethylene. The test does not separate these two, but it separates both from heavier plastics. That separation alone helps in many recycling situations.

A person handling many plastic parts quickly learns to recognize how polypropylene feels in water. The material resists sinking. Pushing a polypropylene piece under water requires force. Letting go sends it shooting back to the surface. That behavior differs from plastics that barely float or sink slowly.

Where Polyethylene Sits on the Density Scale

Polyethylene comes in several grades. Low density polyethylene feels soft and stretchy. A piece of LDPE floated in water sits high on the surface, similar to polypropylene. The two materials behave almost the same in a simple float test. Telling them apart requires another method or a closer look at how high they float.

High density polyethylene feels stiffer and more rigid. A milk jug made of HDPE floats but sits lower in the water than a polypropylene cup. The difference is visible when comparing side by side. The HDPE piece may sit with more of its thickness below the waterline while the polypropylene piece sits with less.

Some polyethylene products have a density very close to that of water. A thick walled HDPE container may barely float. The top surface of the water sits just above the bottom of the piece. A small push sends it under. It slowly rises back to the surface.

The float test does not separate polyethylene from polypropylene because both float. The test does separate both from polyethylene terephthalate (PET) and polyvinyl chloride (PVC), which sink. A person sorting a mixed pile of plastic can pull out all the floaters as either PP or PE and all the sinkers as other materials.

Standard polyethylene production stays below the density of water. A piece of polyethylene that sinks has something added or has absorbed something heavy. That situation happens rarely with everyday household items.

How to Set Up a Float Test With Things Found in a Workshop

A float test needs only a few items. A container holds the water. A clear glass or plastic jar works well. A bucket or a kitchen bowl also works. The container needs to be deep enough to fully cover the plastic piece.

Water from a tap fills the container. Room temperature water gives consistent results. Very hot or very cold water changes density slightly, but the change rarely affects the test outcome. Using water straight from the tap saves time.

The plastic piece for testing should be clean. Dirt, grease, or food residue adds weight. A dirty piece may sink when the clean plastic would float. Washing the piece with soap and water removes surface contamination. Drying the piece before testing removes water weight from the surface.

A small piece of plastic works better than a large one. A piece around the size of a coin gives a clear result. A very large piece may touch the bottom of a shallow container even if it would float. A piece that is too small may cling to the side of the container by surface tension.

Dropping the plastic into the water from a low height prevents splashing. Splashing air bubbles cling to the plastic surface. Those bubbles add buoyancy. A piece that would sink may float if bubbles stick to it. Gently placing the plastic on the water surface gives the cleanest result.

Watching for a few seconds gives the answer. A floating piece rises to the surface and stays there. A sinking piece drops to the bottom and stays there. A piece that hangs in the middle of the water column has a density very close to water. That situation happens rarely with polypropylene or polyethylene.

What the Sinking or Floating Result Tells You About the Plastic Type

A plastic piece that rises quickly to the surface and sits high in the water belongs to the family of plastics that are noticeably lighter than water. Polypropylene and low density polyethylene both behave this way. The test does not tell them apart, but it confirms that the material is not a heavy plastic like PET or PVC.

A plastic piece that floats but sits low in the water, with only a small portion above the surface, suggests high density polyethylene. The density of HDPE comes closer to that of water than polypropylene does. A piece of HDPE may have most of its thickness below the waterline while a polypropylene piece of the same size rides higher.

A plastic piece that sinks to the bottom is not polypropylene or polyethylene. The material has a density greater than water. Common plastics in this category include PET found in soda bottles, PVC found in pipes, and certain engineered plastics. A sinker could also be polypropylene or polyethylene that contains heavy fillers, though that happens less often with everyday items.

A plastic piece that hangs suspended in the middle of the water column has a density very close to that of water. This situation occurs rarely with polypropylene or polyethylene. A piece that neither floats to the top nor sinks to the bottom may need a second test or a closer look at its surface for trapped air bubbles.

The float test gives a quick answer but not a complete answer. It separates plastics into two groups: those lighter than water and those heavier than water. For many recycling and sorting needs, that separation is enough. A person knows that all the floaters can go together into one bin and all the sinkers into another.

Why Some Polyethylene Sinks While Most Floats

Pure polyethylene always floats on fresh water. The polymer itself has a density lower than water. No manufacturing process changes that basic fact. When a piece of polyethylene sinks, something else is present inside the material.

Additives cause most cases of sinking polyethylene. Colorants, especially white pigments, add weight. A bright white polyethylene bottle may contain titanium dioxide. That pigment increases the density of the final part. If enough pigment is added, the overall density can cross the threshold and the bottle sinks.

Fillers serve a similar purpose. Some polyethylene products contain calcium carbonate or talc to reduce cost or change the surface feel. Those mineral fillers are much heavier than the plastic itself. A small amount of filler may not change the floating behavior. A large amount pushes the density over the line.

Absorbed liquids also cause sinking. A polyethylene bottle that held oil or detergent may absorb a small amount into the material. The absorbed liquid adds weight. A bottle that would float when clean may sink after years of use. Washing the bottle removes surface liquid but cannot remove liquid absorbed into the plastic itself.

Labels and adhesives left on a piece of plastic add weight as well. A bottle with a heavy paper label and thick glue may sink when the bare plastic would float. Removing the label before testing gives an accurate result. A person testing a piece with the label still attached may draw the wrong conclusion.

The thickness of the plastic piece does not cause sinking. A thin piece and a thick piece of the same material have the same density. Both float or both sink. A thick piece sits lower in the water because it displaces more water relative to its weight, but it still floats if the material density is below that of water.

How to Test a Plastic Container That Has a Label or Paint

A label changes the test result. Paper labels absorb water and become heavy. A floating container with a paper label may sink because the wet label pulls it down. Removing the label gives a clean test. Peeling off as much paper as possible solves the problem.

Adhesive left behind after label removal still adds weight. A sticky residue may hold small air bubbles. Those bubbles add buoyancy. Wiping off the adhesive with a little cooking oil or rubbing alcohol removes the residue. Washing with soap after cleaning removes the oil.

Paint on a plastic surface adds weight, but usually not enough to change the result. A thin layer of paint does not contain enough material to push the density over the line. A thick coating of heavy paint could cause sinking. Sanding off a small area of paint gives access to the bare plastic underneath.

Ink printed directly on the plastic causes no trouble. The ink layer is too thin to add measurable weight. A printed plastic cup floats just as easily as an unprinted one. The ink does not absorb water or change the density of the part.

A multi layer container presents a different challenge. Some containers combine polyethylene with a barrier layer of a heavier plastic. A ketchup bottle may have an inner layer of EVOH that is denser than water. The whole container may sink even though the outer layer is polyethylene. Testing a single layer of material works better than testing the whole container.

Cutting a small sample from the container solves the label and multilayer problems. A piece cut from a side wall with no label gives a clean test. A piece from the top edge where the container seals also works well. The sample does not need to be large. A piece the size of a coin is enough.

What Other Simple Tests Confirm the Float Test Result

The float test alone does not separate polypropylene from polyethylene. A few other simple tests done at home or in a workshop help tell them apart. These tests use no special equipment and give quick answers.

The burn test offers a clear difference. A small piece of plastic held near a flame behaves differently depending on the type. Polypropylene drips and smells like hot candle wax. Polyethylene drips and smells like paraffin or candle wax as well, but the flame color differs slightly. Caution is needed with any burn test. Working near a flame requires a safe space away from flammable materials.

The feel test separates the two materials by touch. Polypropylene feels waxy and slightly flexible. A polypropylene container bends easily without cracking. Polyethylene, especially the high density grade, feels stiffer and smoother. A milk jug made of HDPE crinkles when squeezed. A polypropylene yogurt cup snaps back without crinkling.

The scratch test uses a fingernail or a dull knife. Polypropylene scratches more easily than high density polyethylene. A light scratch on a polypropylene surface leaves a visible mark. The same pressure on HDPE leaves little or no mark.

The float test can be made more sensitive by adding salt to the water. As salt dissolves, water density increases. A piece of polyethylene that barely floats in fresh water may float higher in salt water. A piece of polypropylene floats high in both. This difference becomes visible when the water density is adjusted carefully.

The sink float separation in a salt solution works as follows. Start with plain water. Add salt a little at a time, stirring until dissolved. Drop the plastic piece in after each addition. Polypropylene floats at all salt levels. Low density polyethylene also floats. High density polyethylene may sink once the salt concentration reaches a certain point. That sinking point tells the user the material type.

When the Float Test Does Not Give a Clear Answer

Some plastic pieces do not cooperate with a simple float test. A piece covered in grease or oil picks up extra weight. The oil layer traps air bubbles. The piece may float when it should sink or sink when it should float. Cleaning the piece thoroughly solves the problem.

A very small piece of plastic may cling to the side of the container. Surface tension holds the piece against the glass or plastic wall. The piece never gets a chance to float or sink freely. Using a larger piece or dropping the piece into the center of the container prevents this problem.

A very thin piece of plastic may trap air underneath. The air bubble lifts the piece even if the plastic itself would sink. Flexing the piece under water releases the trapped air. A thin film of polyethylene may float on the surface forever because of surface tension, not because of its density.

A piece with a complex shape, such as a bottle cap with threads, may trap air inside the threads. The trapped air acts as a floatation device. The piece floats even if the plastic density exceeds that of water. Tapping the piece to release air bubbles or cutting it into smaller pieces gives a true result.

Some plastics absorb water over time. Nylon absorbs enough water to change its density. Polypropylene and polyethylene do not absorb significant amounts of water. An old piece of polyethylene that has been underwater for years may show no change. An old piece of polypropylene also stays dry inside. Water absorption is not a concern for these two materials.

The human eye can misjudge floating and sinking. A piece that sits on the bottom but lifts slightly when touched has not sunk. A gentle stir of the water shows whether the piece wants to rise. A true sinker stays on the bottom no matter how much the water moves.

When in doubt, repeating the test with a clean, dry piece from a different part of the same object gives confidence. Two pieces from the same container behave the same way. If one piece floats and another sinks, something changed between the two tests. Contamination or trapped air causes the difference, not the material itself.