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How does polyethylene differ from polypropylene in typical products and properties?

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plastic soft-drink bottles — plastic Plastic soft-drink bottles are commonly made of polyethylene terephthalate (PET).

Polyethylene (PE) and polypropylene (PP) are both versatile polyolefin resins, but they differ in their chemical structure and physical performance.12

Chemical and Physical Differences

Structure: Polyethylene is composed of ethylene units ( $C H_{2} = C H_{2}$ ), while polypropylene is formed from propylene ( $C H_{2} = C H C H_{3}$ ), which adds a pendant methyl group to every other carbon atom in the chain.12
Thermal Properties: Polypropylene has a higher melting point (approximately 176 °C) compared to polyethylene (typically 110–137 °C), making it more suitable for high-temperature applications.342
Mechanical Properties: Polypropylene is generally stiffer, harder, and stronger than polyethylene. While both are highly crystalline and tough, polypropylene exhibits exceptional fatigue resistance, allowing it to be used for "self-hinged" containers.256

Typical Products

Polyethylene: Commonly used for packaging film, grocery bags, milk bottles, and wire insulation.71
Polypropylene: Frequently used for dishwasher-safe food containers, appliance housings, automobile battery casings, outdoor furniture, and textiles like upholstery and carpets.25

Polyethylene is identified by recycling codes #2 (HDPE) or #4 (LDPE/LLDPE), while polypropylene is marked with code #5.712