Polypropylene (PP) and Polyethylene (PE) are two of the most common and structurally similar plastics produced globally, both belonging to the family of polyolefins. They share a similar chemical backbone, making them lightweight, durable, and resistant to chemicals. Distinguishing between these polymers is crucial for material selection, optimizing performance, and ensuring proper recycling. A systematic examination of their physical, density, and thermal properties reveals clear distinctions.
Quick Visual and Tactile Examination
A preliminary, non-destructive assessment can often narrow the possibilities based on the material’s feel and appearance. Polypropylene tends to be noticeably stiffer and more rigid than Polyethylene, holding its molded shape more effectively. Conversely, PE is often described as feeling waxy or slightly slippery to the touch, and it exhibits greater flexibility and pliability in thin sections.
The visual clarity of the plastic provides another clue, though it is highly dependent on the type of PE. High-density Polyethylene (HDPE) is typically opaque, while low-density Polyethylene (LDPE) is generally translucent or milky. Polypropylene is often translucent, offering better clarity than HDPE but rarely achieving the optical clearness found in some PE grades.
A simple bending test can highlight the materials’ difference in fatigue resistance. Polypropylene is famously used for “living hinges” on containers, meaning it can be repeatedly bent a thousand times or more without breaking. When you crease a thin section of PP, it will retain a sharp, clean fold, whereas PE tends to resist a permanent crease and will eventually tear rather than maintain a sharp hinge.
Differentiation by Specific Gravity
Density, or specific gravity, offers a reliable non-destructive method for distinguishing between these two polyolefins. Specific gravity is the ratio of a material’s density to that of water (1.0). Both PP and PE have specific gravities less than 1.0, meaning both will float when placed in water.
Polypropylene is the lightest of the two, with a specific gravity ranging from 0.90 to 0.91. Polyethylene, while still floating, is slightly denser, with Low-Density Polyethylene (LDPE) around 0.92 and High-Density Polyethylene (HDPE) ranging from 0.94 to 0.96. The small gap between the two plastics’ densities allows for separation using a liquid solution denser than PP but less dense than PE.
To separate them, a specialized liquid test is necessary, often involving an alcohol-water mixture to achieve the required intermediate density. By slowly adding water to isopropyl alcohol, the solution’s density increases gradually from the alcohol’s lower specific gravity. When the solution reaches a specific gravity around 0.94, the lighter Polypropylene will remain floating on the surface. At this point, the slightly denser Polyethylene will begin to sink toward the bottom, providing a definitive visual distinction.
Observable Behavior When Exposed to Heat
A destructive flame test provides the most definitive indicators, especially through the characteristics of the melt and the resulting odor. Safety Warning: This test produces fumes that should not be inhaled and presents a fire hazard. When exposed to a small flame, both polymers ignite readily and continue to burn even after the flame source is removed.
Polyethylene has a lower melting point, melting between 105°C and 137°C depending on its density, causing it to burn quickly and drip like hot candle wax. The odor produced by burning PE is distinctive, often described as smelling exactly like paraffin candle wax. The flame is usually yellow with a blue base, and the dripping material is a reliable characteristic.
Polypropylene has a higher melting point, generally in the range of 160°C to 175°C. This means it burns more slowly than Polyethylene, and the dripping is less profuse. The most differentiating characteristic of burning PP is the smell, which is often described as a sweet, almost gassy, or petroleum-like scent. The flame typically presents as a yellow tip over a blue base, and the plastic will continue to burn steadily until consumed.