Polyethylene and polypropylene are both lightweight, versatile plastics, but they differ in heat tolerance, flexibility, density, and best-use applications. Both belong to the polyolefin family, meaning they’re made from simple hydrocarbon building blocks. The practical differences come down to their molecular structure: polyethylene is built from ethylene (two carbon atoms), while polypropylene is built from propylene (three carbon atoms with a methyl side group). That extra branch in polypropylene’s chain changes nearly everything about how the two plastics behave.
How Their Structures Create Different Properties
Polyethylene’s simpler chain structure means it can pack together in different ways, which is why it comes in several major varieties. Low-density polyethylene (LDPE) has lots of branching in its chains, making it soft and flexible. High-density polyethylene (HDPE) has far less branching, so the chains pack tightly together, creating a stiffer, stronger material. Both are still the same basic polymer, just organized differently.
Polypropylene’s methyl side group forces its chains into a more regular, crystalline arrangement. This gives polypropylene a higher melting point and better resistance to bending fatigue, but it also makes the material more brittle at low temperatures. If you’ve ever noticed a plastic container crack in the freezer, it was likely polypropylene.
Density and Weight
Polypropylene is the lighter of the two. Its density ranges from 0.895 to 0.92 g/cm³, which means it floats easily in water. LDPE sits at 0.91 to 0.94 g/cm³, while HDPE ranges from 0.93 to 0.97 g/cm³. In practical terms, the weight difference is small for a single product, but it adds up in packaging and shipping. Polypropylene’s lower density is one reason it’s favored for lightweight automotive parts and thin-walled food containers where every gram of material matters.
Heat Resistance
This is one of the sharpest differences between the two. Polypropylene withstands temperatures up to about 160°C (320°F) before melting. LDPE softens and melts at just 105 to 115°C (220 to 240°F), and HDPE holds up a bit better at 130 to 137°C (266 to 279°F). That gap matters in kitchens and industrial settings alike. Polypropylene is the plastic you’ll find in microwave-safe containers and dishwasher-safe lids. Polyethylene would warp or melt under the same conditions.
On the cold end of the spectrum, polyethylene has the advantage. It stays flexible well below freezing, which is why it’s used for ice bags, frozen food packaging, and cold-weather piping. Polypropylene becomes stiff and prone to cracking in sub-zero temperatures.
Flexibility and Fatigue Resistance
If you’ve ever opened a flip-top bottle cap and bent the little hinge back and forth, you’ve used what’s called a living hinge. Polypropylene is the go-to material for these because of its excellent fatigue resistance. You can flex a thin polypropylene hinge thousands of times without it breaking. Polyethylene can work for simpler hinge applications (snap-on lids, for instance), but it doesn’t match polypropylene’s durability under repeated bending.
LDPE, on the other hand, is the more flexible material overall. It’s soft, stretchy, and easy to squeeze, which is why it’s used for squeeze bottles and plastic film. HDPE is noticeably stiffer. Polypropylene falls somewhere in between for general flexibility but excels specifically at surviving repeated stress in a single spot.
Chemical Resistance
Both plastics resist a wide range of chemicals, which is a big part of why they’re so common in laboratories, cleaning product bottles, and industrial containers. Polyethylene (both HDPE and LDPE) shows little or no damage after 30 days of exposure to strong acids like hydrochloric acid, sulfuric acid, and phosphoric acid, as well as strong bases like sodium hydroxide and potassium hydroxide. It also handles hydrofluoric acid, which destroys glass.
Polypropylene is nearly as resistant as polyethylene across the board, and in a few specific cases it performs better. Neither plastic holds up well against strong oxidizing agents or certain organic solvents like toluene or xylene. For most household and lab purposes, either material will work, but polyethylene has a slight edge in broad chemical compatibility.
Where You’ll Find Each Plastic
The easiest way to tell them apart in daily life is by their recycling codes. HDPE is marked with a “2” inside the recycling triangle, LDPE with a “4,” and polypropylene with a “5.”
- HDPE (code 2): Milk jugs, detergent bottles, playground equipment, cutting boards, water pipes, and sturdy storage bins. It’s rigid, impact-resistant, and one of the most commonly recycled plastics.
- LDPE (code 4): Grocery bags, cling wrap, squeeze bottles, and flexible tubing. It’s soft and transparent in thin sheets.
- Polypropylene (code 5): Yogurt containers, bottle caps, straws, microwave-safe food containers, automotive bumpers, and reusable food storage sets. Its heat tolerance and stiffness-to-weight ratio make it ideal for anything that needs to survive warm temperatures without adding bulk.
Recyclability
HDPE is one of the easiest plastics to recycle and is accepted by most municipal programs. LDPE is trickier. Its thin, flexible form (think grocery bags) can jam recycling machinery, so many curbside programs won’t take it. Some grocery stores collect LDPE film separately. Polypropylene is recyclable in theory, but acceptance varies by location. The general rule with plastics is that lower recycling code numbers are easier to process, and that pattern holds here: HDPE (2) is recycled far more consistently than LDPE (4) or polypropylene (5).
Cost
Raw polyethylene and polypropylene resins trade at very similar prices globally. As of early 2025, both hover around 7,400 to 7,500 CNY per metric ton on commodity markets, making them nearly interchangeable on cost alone. The choice between them almost always comes down to performance requirements rather than price. When a product needs heat resistance or a living hinge, polypropylene wins. When it needs chemical resistance, cold tolerance, or low-cost flexible film, polyethylene is the better fit.