In terms of heat resistance, polypropylene (PP) outperforms polyethylene (PE). Typically, the melting point of polypropylene is about 40–50% higher than that of polyethylene-ranging from 160°C to 170°C-allowing products to withstand sterilization at temperatures above 100°C and remain undeformed at 150°C in the absence of external force. In daily life, "No. 5" polypropylene containers are commonly used to heat food in microwave ovens (where temperatures generally reach 100–140°C), whereas polyethylene is unsuitable for microwave use-whether for containers or cling film-due to its poor heat resistance. Similarly, regarding standard packaging films, polyethylene bags are best suited for temperatures below 90°C, while polypropylene bags can withstand relatively higher temperatures.
Regarding rigidity and tensile strength, polypropylene is characterized by low density and superior mechanical properties compared to polyethylene, particularly its exceptional rigidity. Polypropylene is increasingly competing with engineering plastics (such as PA and PC) and is widely used in the electronics, electrical appliance, and automotive sectors. Furthermore, its high tensile strength and excellent flexural resistance have earned it the nickname "fold-resistant plastic"; it can withstand a million bending cycles without stress whitening at the fold. This characteristic serves as a clue for identifying polypropylene products and acts as an implicit marker for sorting them during recycling.
In terms of low-temperature resistance, polypropylene is inferior to polyethylene. Its impact strength at 0°C is only half of that at 20°C, whereas polyethylene remains resilient at much lower temperatures; its brittleness temperature is generally below -50°C and can drop as low as -140°C depending on relative molecular mass. Therefore, if a product is intended for use in a low-temperature environment, polyethylene is the preferred raw material. Trays used for refrigerated food, for instance, are typically made from polyethylene.
Regarding aging resistance, polypropylene is inferior to polyethylene. Although their molecular structures are similar, the presence of a methyl side chain in polypropylene makes it more susceptible to oxidative degradation caused by ultraviolet light and heat. A common example of polypropylene products prone to aging is the woven bag; prolonged exposure to sunlight causes these bags to crack easily. In fact, while polyethylene has better aging resistance than polypropylene, its performance in this regard is not outstanding compared to other materials; the presence of trace double bonds and ether linkages in polyethylene molecules results in poor weatherability, making it susceptible to aging from sun and rain exposure.
In terms of flexibility, polypropylene possesses high strength but poor flexibility-or, technically speaking, poor impact resistance. Consequently, its application differs from that of polyethylene, particularly in film products; polypropylene film is more commonly used for surface packaging and printing. Regarding piping, standard polypropylene is rarely used on its own; instead, cross-linked polypropylene-commonly known as PPR-is required. Because ordinary polypropylene has poor impact resistance and is prone to cracking, impact modifiers must be added for practical applications; for instance, additives are used to enhance impact resistance in products such as automotive bumpers.