polyethylene

日期:2022/7/4 8:50:07 / 阅读: / 来源:本站

polyethylene
The raw materials of foamed plastics are composed of resins and various additives. Resin is the main component of it. Its performance determines the basic characteristics of foamed plastics, and it is also the main basis for formulating the molding process. The main purpose of adding various additives to the raw material formula is to improve the formability of the resin and the service performance of the products. In actual production, different additives should be selected according to the needs and requirements of different products. This chapter will briefly introduce the properties and applications of raw materials and common additives of foamed plastics. Hollow board production line www.handern com
Polyethylene (PE) is one of the largest and most widely used plastic products at present, accounting for about 30% of the world's total output. Due to its excellent performance, easy molding, abundant raw materials and low price, polyethylene has developed rapidly.
Polyethylene structure polyethylene is a polymer formed by polymerization with ethylene as monomer. The synthetic methods of polyethylene can be divided into high pressure method, medium pressure method and low pressure method. The properties of polyethylene are different with different production methods. Usually, low-density polyethylene is produced by high-pressure method, and high-density polyethylene is produced by medium pressure method and low-pressure method. Low density polyethylene density 910~925kg/cm ³, Crystallinity 55%~65%, crystallization melting point 105 ℃; The density of HDPE is 920~965kg/cm3, the crystallinity is 85%~95%, and the crystallization melting point is 135 ℃. Hollow board production line www.handern com
Polyethylene is a kind of plastic with the simplest structure and orderly molecular arrangement. It is a typical crystalline polymer. In the solid state, the crystalline part and amorphous part coexist, so it has a crystallinity. Generally, the higher the density, the higher the crystallinity. The crystallinity is proportional to the density. The structure of polyethylene molecular chain is shown in Figure 2-1. As shown in Figure 2-1, polyethylene molecules have a certain degree of branching. High pressure polyethylene has a high degree of branching. Because the branching chain reduces the regularity of macromolecules, its crystallinity, density, rigidity and hardness are low. The branching degree of low-pressure polyethylene is very low, so its yield, density, rigidity and hardness are high.
The relative molecular weight of polyethylene is between 10000 and 1million, and those with a relative molecular weight of more than 1million are called ultra-high relative molecular weight polyethylene. The higher the relative molecular weight, the better the physical and mechanical properties, but with the increase of the relative molecular weight, the processability decreases. Therefore, polyethylene with appropriate relative molecular weight should be selected for processing according to the use requirements. Polyethylene with ultra-high molecular weight is suitable for processing structural materials and load materials, while polyethylene with low molecular weight is suitable for coatings, polishes, lubricants and softeners. Hollow board production line www.handern com
The relative molecular weight of polyethylene is expressed by its melt flow rate (MI). With the increase of the relative molecular weight of polyethylene, the melt viscosity increases. Under the same test conditions, the higher the melt flow rate is, the smaller the melt viscosity of the resin is, the smaller the average relative molecular weight is, the lower the molding temperature is, and the mechanical properties of the product are relatively poor; On the contrary, the higher the melt flow rate, the higher the molding temperature, and the better the mechanical properties of the products. Therefore, when selecting the melt flow rate of polyethylene, we should consider not only the molding performance, but also the performance requirements of products. Hollow board production line www.handern com
Polyethylene performance polyethylene is milky white powder or waxy particles, odorless, tasteless, non-toxic, flammable, with a density of 910~965kg/m3. Melting point 110~130 ℃, maximum service temperature ≥ 100 ℃, minimum service temperature < -70 ℃, ignition temperature 340 ℃. Processing temperature 140~180 ℃ (LDPE), 160~220 ℃ (HDPE). Polyethylene has good low temperature resistance and good mechanical strength at -60 ℃. But the use temperature is not high, low-density polyethylene is below 80 ℃, high-density polyethylene is about 110 ℃. Polyethylene is a non-polar crystalline polymer with good chemical stability and resistance to general acid, alkali and salt corrosion. There is no solvent at room temperature, but it can be soluble in xylene (60 ℃) and tetralin + hydrogenated naphthalene (100 ℃) under enhanced conditions (such as heating up), and the higher the crystallinity. The greater the relative molecular weight, the less soluble it is. The low water absorption of polyethylene indicates that it has good water resistance. Polyethylene has good air permeability, so it is suitable for making food packaging or other packaging films and agricultural films. Generally, the permeability of LDPE is about 5 times higher than that of HDPE. Hollow board production line www.handern com
Because polyethylene is a non-polar material with excellent electrical insulation performance, its dielectric constant and dielectric loss are almost independent of temperature and frequency, and its high-frequency performance is very good, which is suitable for manufacturing the insulation layer of high-frequency cables and submarine cables.
Polyethylene has low softness, medium tensile strength and high impact strength. Its mechanical properties depend on the relative molecular weight, branching degree and crystallinity of the polymer. The tensile strength of HDPE is 20~25mpa, while that of LDPE is only 10~12mpa.
Polyethylene melt has lower viscosity than PVC, good fluidity and easy molding, so it has good molding performance. Various molding methods can be used to produce foamed plastic products. Hollow board production line www.handern com
. Polyethylene foamed plastic polyethylene is a crystalline polymer. When the temperature exceeds the melting point, the melt viscosity decreases sharply. This characteristic is not conducive to the production of foamed plastics by foaming molding. Because in the foaming process, the melt must have a certain viscoelasticity to cover the bubbles generated in the residence, and the temperature range of crystalline polymers with this property is narrow. Therefore, it is necessary to expand the temperature range suitable for foaming by adding additives. In actual foaming molding, polyethylene macromolecules are usually crosslinked by adding crosslinking agent to improve the viscoelasticity of the melt. Polyethylene has a large specific heat capacity, and its melt will undergo crystallization process during cooling and curing, giving off crystallization heat. In foaming molding, in order to keep bubbles, the cooling rate of melt should not be too slow. Therefore, rapid heat dissipation should be carried out during cooling to meet the cooling requirements of polyethylene foam Polyethylene foam can be foamed by chemical and physical foaming methods, and the molding methods are extrusion, injection molding and molding. It is widely used in shock-absorbing packaging materials, thermal insulation materials, floating materials and daily necessities.

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