Which is better, polypropylene or polyethylene?

Performance difference between PE and PE

1 In terms of heat resistance, the heat resistance of polypropylene is higher than that of polyethylene. In general, the melting temperature of polypropylene is about 40%-50% higher than that of polyethylene, about 160- 170℃, so the product can be sterilized at a temperature above 100℃, while the melting temperature of polypropylene is about170 without external force. In life, we will find that "No.5" polypropylene lunch box is often used to heat food in microwave ovens (the general temperature of microwave ovens and polyethylene can not be used as plastic for microwave ovens, including lunch boxes and plastic wrap. Similarly, in the field of ordinary packaging films, polyethylene packaging bags are more suitable for use below 90℃, while polypropylene packaging bags can also be used at relatively high temperatures.

In terms of rigidity and tensile strength, polypropylene is characterized by low density, superior mechanical properties and outstanding rigidity. For example, at present, polypropylene has gradually competed with engineering plastics (PA/PC) and is widely used in electronic appliances, automobiles and other fields. At the same time, polypropylene is called "100-fold adhesive" because of its high tensile strength and good bending performance. It won't turn white after being folded in half for 6,543.8+0,000 times, which also provides clues for us to distinguish polypropylene products and becomes a hidden symbol for product recycling and classification.

3 From the point of view of low temperature resistance, the low temperature resistance of polypropylene is weaker than that of polyethylene, and the impact strength at 0℃ is only half that at 20℃, while the brittleness temperature of polyethylene can generally reach below -50℃; With the increase of relative molecular weight, the lowest molecular weight can reach-140℃. Therefore, if the product needs to be used in a low temperature environment, polyethylene should be chosen as the raw material as much as possible. Usually, trays used to refrigerate food are made of polyethylene.

From the point of view of aging resistance, the aging resistance of polypropylene is weaker than that of polyethylene. The structure of polypropylene is similar to that of polyethylene, but because its side chain is composed of methyl, it is more prone to oxidative degradation under the action of ultraviolet light and heat energy. The most common aging polypropylene products in daily life are woven bags, which are easy to break when exposed to the sun for a long time. In fact, although the aging resistance of polyethylene is higher than that of polypropylene, compared with other raw materials, its performance is not very outstanding, because polyethylene molecules contain a small number of double bonds and ether bonds, and its weather resistance is not good, and sun and rain can also cause aging.

5 From the perspective of flexibility, polypropylene has high strength, but poor flexibility, that is, poor impact resistance from a technical point of view. Therefore, when it is used as a film product, its application field is still different from that of polyethylene, and polypropylene film is more used for surface packaging printing. In pipeline, simple polypropylene is rarely used in production, and cross-linked polypropylene is needed, which is a common PPR pipe. 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 and other applications to improve impact resistance.

Extended data:

When methyl groups are arranged on the same side of the molecular backbone, it is called isotactic polypropylene; If methyl groups are arranged on both sides of the molecular main chain in disorder, it is called random polypropylene; If methyl groups are alternately arranged on both sides of the molecular main chain, it is called syndiotactic polypropylene. In general industrial polypropylene resin, the isotactic structure content is about 95%, and the rest is random or syndiotactic polypropylene.

Industrial products are mainly composed of isotactic substances. Polypropylene also includes a copolymer of propylene and a small amount of ethylene. Usually a translucent colorless solid, tasteless and nontoxic. Because of its regular structure and high crystallinity, its melting point can reach 167℃. Its outstanding advantages are heat resistance and corrosion resistance, and the product can be sterilized by steam. Low density, it is the lightest general plastic. The disadvantages are poor impact resistance at low temperature and easy aging, but they can be overcome by modification respectively.

* * * Polymer-based PP material has low thermal deformation temperature (100℃), low transparency, low gloss and low rigidity, but it has strong impact strength, and the impact strength of PP increases with the increase of ethylene content. The Vicat softening temperature of polypropylene is 65438 050℃. Because of its high crystallinity, this material has good surface hardness and scratch resistance. There is no environmental stress cracking problem in PP.

The melt mass flow rate (MFR) of PP is usually 1~ 100. PP with low MFR has better impact resistance, but lower tensile strength. For materials with the same MFR, the impact strength of poly type is higher than that of homopolymer type. Due to crystallization, the shrinkage of PP is quite high, generally 1.6~2.0%.

Polyethylene (PE) is a polymer polymerized from ethylene. Its products have been developed for about 60 years, and the global polyethylene output ranks first among the five commonly used resins.

Polyethylene can be divided into high density polyethylene, low density polyethylene and linear low density polyethylene according to polymerization method, molecular weight and chain structure.

Low density polyethylene (LDPE), commonly known as high pressure polyethylene, is the softest material because of its low density, and is mainly used in plastic bags and agricultural films. ?

Compared with LDPE and LLDPE, high density polyethylene (HDPE) has higher temperature resistance, oil resistance, steam permeability resistance and environmental stress cracking resistance. In addition, it has good electrical insulation, impact resistance and cold resistance, and is mainly used in blow molding, injection molding and other fields. ?

Linear low density polyethylene (LLDPE) is a mixture of ethylene and a small amount of high-grade polyethylene. Olefin polymers polymerized in the presence of catalysts. LLDPE is similar to LDPE in appearance and low in transparency, but it has good surface gloss, low-temperature toughness, high modulus, bending resistance and stress cracking resistance, and good low-temperature impact strength.

The application of LLDPE has penetrated into almost all low density polyethylene markets. At present, LLDPE and HDPE are in the growth period of life cycle; At the end of 1980, LDPE gradually entered the mature stage, and few LDPE equipments were put into production in the world. Polyethylene can be formed by extrusion, injection, molding, blow molding and melt spinning. Widely used in industry, agriculture, packaging and daily use industries. It is widely used in China. Film is its biggest user, consuming about 77% of low density polyethylene and 65,438+08% of high density polyethylene. In addition, injection molded products, wires and cables, hollow products, etc. They all account for a large proportion in their consumption structure, all in the plastic industry.

References:

Baidu encyclopedia-polyethylene? Baidu encyclopedia-polypropylene