From heat resistance, the heat resistance of
polypropylene is higher than that of polyethylene. Usually, the melting temperature of
polypropylene is about 40%~50% higher than that of polyethylene, which is about 160-170 ℃. Therefore, the product can be sterilized at temperatures above 100 ℃, and it does not deform at 150 ℃ without external force. In our daily life, we will find that “5”
polypropylene lunch box is often used to heat food in microwave oven (the general temperature of microwave oven heating is 100-140 ℃), and polyethylene can not be used as plastic for microwave oven, including lunch box and fresh-keeping film because of its poor heat resistance. Similarly, in the field of general packaging film, polyethylene packaging bags are more suitable for use below 90 ℃, while
polypropylene packaging bags can be used at relatively high temperatures.
From rigidity and tensile strength,
polypropylene is characterized by low density, superior mechanical properties to polyethylene, and outstanding rigidity. For example,
polypropylene has gradually developed competition with engineering plastics (PA/PC), which is widely used in the fields of electronics, electrical appliances, and automobiles. At the same time, due to its high tensile strength and good flexural resistance,
polypropylene is called “100 fold gum”. It does not change the whiteness of the bent parts for 1 million bending times. This also provides clues for us to identify
polypropylene products, and at the same time becomes a hidden mark for product recycling and classification.
From the point of view of low-temperature resistance, the low-temperature resistance of PP is weaker than that of PE, and the impact strength of PP is only half of that of PE at 0 ℃, while the brittleness temperature of PE is generally below – 50 ℃ and the lowest is – 140 ℃ with the increase of relative molecular weight. Therefore, if products need to be used in a low-temperature environment, PE should be chosen as raw material as far as possible. Generally, the trays used for refrigerated food are made of PE raw materials.
From aging resistance,
polypropylene has a weaker aging resistance than polyethylene. The structure of
polypropylene is similar to polyethylene, but it is easier to oxidize and degrade under the action of ultraviolet light and heat energy because of its side branched-chain composed of methyl.
The most common aging
polypropylene product in daily life is woven bags, which are easy to crack when exposed to the sun for a long time. Although the aging resistance of polyethylene is higher than that of
polypropylene, it is not very outstanding compared with other raw materials, because polyethylene molecules contain a small number of double bonds and ether bonds. Its poor weather resistance, sunshine, the rain will also cause aging.
From flexibility,
polypropylene has high strength, but poor flexibility, which means poor impact resistance from the technical point of view. Therefore, when it is used as film products, its application field is different from that of polyethylene.
Polypropylene film is more used for surface packaging printing. In the aspect of pipe material, it is seldom produced with simple
polypropylene, and cross-linked
polypropylene, also known as PPR pipe, is needed. Because ordinary
polypropylene has poor impact resistance and is easy to crack, it is necessary to add impact modifier in practical application and use additives in bumper to improve impact resistance.
