Polyethylene (PE) and Polypropylene (PP) are the two most common members of the polyolefin family. Both are widely used in consumer products, from packaging film to rigid containers, and understanding how to tell them apart is important for specialized recycling or material selection. Distinguishing between these polymers relies on simple observation and a few physical tests, moving from non-destructive visual clues to a definitive destructive test.
Visual and Tactile Clues
Non-destructive examination provides the first strong hints about a plastic’s identity. Polyethylene (PE), especially the low-density variety (LDPE), is highly flexible and often appears translucent or milky white. High-density PE (HDPE) is stiffer but still pliable, commonly used in milk jugs, and often feels slightly waxy or slippery.
Polypropylene (PP) is a stiffer and more rigid material overall, exhibiting higher resistance to bending than PE. It frequently has a higher gloss finish and a natural, bright white tone when uncolored. PP’s resistance to fatigue makes it the plastic of choice for “living hinges” found on bottle caps or storage containers that must be opened repeatedly.
The Simple Density Test
A practical, non-destructive method for differentiating PE and PP involves using their specific gravity (density relative to water). Both PE and PP have a specific gravity less than 1.0, meaning both will float in standard water. This observation immediately rules out many other common plastics, such as PVC or PET, which sink.
To differentiate the two floating polyolefins, the density difference must be observed closely. PE generally has a lower density than PP. In a cup of water, PE will float higher and appear more buoyant on the surface. PP, being the denser of the two, will often float lower in the water or may even remain partially suspended just below the surface.
Safety and the Definitive Burn Test
The most definitive identification method is the burn test, which reveals chemical composition through combustion characteristics. This test must be performed with extreme caution on a small sample in a well-ventilated area, away from flammable materials. Since both plastics are highly flammable, they will continue to burn after the flame source is removed.
When ignited, polyethylene melts easily and drips like hot candle wax. It burns with a clean flame that is blue at the base and yellow at the tip. The most telling characteristic is the odor, which is distinctly similar to burning paraffin wax.
Polypropylene also burns easily with a blue-based, yellow-tipped flame, but it has a higher melting point than PE. This higher melting point means PP melts and drips more slowly and less readily than PE. The definitive identifier for polypropylene is the smell it produces, which is often described as sweet, slightly acrid, or similar to burning motor oil.