PE, polyethylene, is a polymer with a variety of structures and properties. It is mainly divided into low density polyethylene (LDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE), linear low density polyethylene (LLDPE), and special properties of ultra high molecular weight polyethylene, low relative molecules. Mass polyethylene, high molecular weight high density polyethylene, very low density polyethylene, etc. Generally, a polyethylene having a relative density of less than 0.920 is generally referred to as a low-density polyethylene; a polyethylene having a relative density of 0.940 or more is referred to as a high-density polyethylene; and a polyethylene having a relative density of 0.926 to 0.940 is referred to as a medium-weight polyethylene. Density polyethylene. As can be seen from the classification of PE, the density is the main indicator related to the difference in performance of PE plastics, followed by the relative molecular mass, and the density is caused by the difference in crystallinity and molecular linear structure of the resin. The linear structure of PE has high crystallinity, high density, and high melting temperature, hardness, yield strength and modulus of elasticity.
Although the force between the PE molecules is not large, the molecules are densely packed and the strength is increased mainly due to the high crystallinity. On the contrary, PE with a large degree of branching has a small crystallinity, and the density is low, and the extensibility and toughness are large, that is, a flexible material.
The relative molecular mass and its distribution will directly affect the crystallinity, which in turn affects a range of properties, such as: strength, hardness, toughness, wear resistance, chemical resistance and aging, and low temperature brittleness, and elongation at break. reduce. The relative molecular mass distribution is narrow, and the toughness and low temperature brittleness are improved. While resisting long-term load deformation, the environmental stress crack resistance is reduced. Therefore, the width and width of the relative molecular mass distribution are also closely related to the type and performance of the PE product. In addition, the melt index is a quantitative indicator of the melt flowability of polyethylene and a sign reflecting the molecular weight of polyethylene. In general, the higher the melt index of PE, the lower the molecular weight; on the contrary, the lower the melt index of PE, the higher the molecular weight. The melt index of PE has a great influence on its processing. If the melt index is large, the fluidity is good, which is advantageous for injection molding.
However, for direct extrusion blow molding, it is not desirable that the melt index is too high, especially HDPE, the melt index is large, and the parison is liable to fall, affecting the parison. Normal molding. In order to blow large products, high molecular weight high density polyethylene (codenamed HMWHDPE) should be used. Its weight average molecular weight is in the range of 300,000 to 500,000, and its molecular weight is not only significantly higher than that of general HDPE (weight average molecular weight is 15~). 200,000), and the molecular weight distribution is wide, and the melt tension is large. When using direct extrusion blow molding, the parison of large parts is not easy to fall. Plastic articles made with HMWHDPE also have good impact resistance, creep resistance and stress crack resistance.
Introduction of properties of commonly used polyethylene
(1) Properties of low density polyethylene: LDPE is milky white waxy granule, which is non-toxic, tasteless and odorless. It is the lightest variety in PE with low crystallinity of 55%~65%. The melt flow rate is wide, about 0.2~50g/10min, and has good flexibility, elongation, transparency and cold resistance. It has excellent processability, chemical stability and gas permeability, and excellent electrical insulation performance. However, its mechanical strength, moisture permeability, aging resistance and heat resistance are lower than that of high density polyethylene.
(2) Properties of high-density polyethylene: HDPE is white powder or granular, non-toxic, odorless, odorless. Compared with LDPE, it has fewer branches, higher crystallinity, higher density, and relatively more than a dozen molecular masses. From 10,000 to several hundred thousand, the melt flow rate range is narrow; it has high rigidity and toughness, excellent mechanical properties and heat resistance, and also has good solvent resistance and vapor permeability resistance.