Sell polyethylene every day, can you name the characteristics of various types of polyethylene?
" There are many kinds of polyethylene, usually according to the age of industrialization, there are the first generation of industrialized polyethylene in 1939, that is: high pressure polyethylene (low density polyethylene); The second generation of industrialized polyethylene in 1953, that is: low pressure polyethylene (high density polyethylene); The third generation of industrial polyethylene in 1977, namely: linear low density polyethylene (LLDPE); The fourth generation polyethylene, ultra-low density polyethylene (VLDPE), industrialized in 1984; Ultra-high molecular weight polyethylene (UHMWPE), industrialized in 1958, and metallocene polyethylene (MPE), which appeared in the 1990s. In the production process of the above polyethylene, some added a small amount of 4 carbon or 8 carbon α Olefin as a copolymer, but because of the α Olefin is used very little, so it still retains many of the characteristics of polyethylene. low density polyethylene (LDPE) (high pressure polyethylene) LDPE features are: (1) LDPE is a white waxy granular solid with a density of 0.91~0.925g/cm3, tasteless, odorless and non-toxic; (2) LDPE is a typical crystalline polymer with a crystallinity of 55%~65% and a melting point of 105~126¡æ; (3) LDPE is non-polar material, easy to carry static electricity, low surface energy, so in printing, composite should be corona treatment, in order to improve the surface energy, processing process, should pay attention to anti-static, avoid electrostatic accumulation affect product quality or spark discharge, cause fire; (4) LDPE excellent transparency, excellent heat sealing, can be widely used in the production of transparent low temperature frozen packaging products; (5) LDPE has excellent moisture resistance, and is a high-quality raw material for making dry food or good moisture-proof articles packaging. But LDPE gas resistance is large, easy to pass through all kinds of gases; (6) Although LDPE has a certain oil resistance, its oil resistance and organic solvent resistance is not as good as polypropylene, therefore, when the thickness is small, it is not suitable for long-term placement of gasoline, alcohol, grease and so on. When using LDPE, the best thickness should be more than 50mm; (7) LDPE has inflammability, when burning, the flame is smokeless and colorless, and there is burning drop phenomenon and candle gas, is a characteristic of identification. LDPE extrusion blown film should choose the melt index (MI) of 2 ~ 6g/10min blown film particles, not only has good opening, but also good heat sealing. The temperature of the extruder homogenization section is 150~180¡æ, and the blowing ratio is 2~3. The traction ratio should be balanced with the blowing ratio. Squeeze blowing or injection blowing hollow containers, choose MI less than 2g/10min squeeze blowing grade or injection blowing grade LDPE particles, particles greater than 2g/10min is easy to produce uneven thickness of the bottle or no good container at all. When extruding salivary LDPE film, generally choose 8 ~ 15g/10min MI, too high MI film strength is too low, extrusion temperature depends on the use of salivary film. If it is for heat sealing, the temperature should not exceed 200¡æ, if it is for composite use, in order to improve the extrusion fastness of PE with other substrates, it can be increased to 300¡æ or even higher temperature, but when it exceeds 315¡æ, the time can not be too long, to avoid increasing the interface and reducing the performance. medium density polyethylene (MDPE) medium density polyethylene is a density of 0.926 ~ 0.94 g/cm3, With LDPE has the same performance of a kind of polyethylene, due to the increase in density, the crystallinity of MDPE is as high as 70% ~ 80%, and the increase in density and crystallinity, it improves the melting temperature of MDPE, the hardness and strength of products. MDPE is between LDPE and HDPE. It should be pointed out that PE can also be formed into sheets and films by calendering method, but because LDPE melt fluidity is too good, so the calendering process is used in PE filling modified materials, such as: sheet for vacuum blister packaging products. high density polyethylene (HDPE) high density polyethylene (0.94 ~ 0.965 g/cm3) rigidity, strong toughness, mechanical The strength, solvent resistance and stress cracking resistance are better than LDPE. Due to MI< 1g/10mm HDPE has a high strength, therefore, for garbage bags is one of the important uses of film grade HDPE, melting point 126 ~ 136¡æ, crystallinity of more than 90%. Rotary molding with HDPE can choose MI3 ~ 20g/10min of particles or powder, the production of large high-strength packaging containers. The injection turnover box can be HDPE with MI of 30 ~ 50g/10min. HDPE molding temperature is 180 ~ 250¡æ. HDPE is characterized by high strength and poor transparency. The linear low density polyethylene (LLDPE) LLDPE besides some properties of LDPE, it also has the following features: (1) LLDPE has excellent heat sealing property, although the melting temperature of LLDPE is about 5¡æ higher than LDPE, but the heat sealing property of LLDPE has good ion-like performance, that is: Like ionic polymer surlyn, even if there is serious pollution on the thermal surface, it still has a high heat sealing strength. High heat sealing strength can be achieved at a temperature higher than the initial heat sealing temperature. The general plastic, such as: LDPE heat sealing temperature, the higher the heat sealing strength, therefore, such as LLDPE as the heat sealing material, you can only use a lower heat sealing temperature can achieve a highly reliable heat sealing strength, suitable for high-speed hot sealing machine use. (2) The melting viscosity of LLDPE is quite large, 10 times that of LDPE, and the melting viscosity of LLDPE is not sensitive to temperature, but the sensitivity to processing stress is very strong. That is to say, LLDPE can not use the method of increasing the temperature to reduce the viscosity, but can only use the method of increasing the processing speed, that is, improving the shear stress to improve the melting fluidity. Therefore, the screw design for processing LLDPE is special, and the power of the main motor is often more than 2 times that of LDPE. We often use a mixture of 50% LDPE+50%LLDPE to improve the performance of both, and make it possible to process LLDPE on the equipment processing LDPE. The metallocene polyethylene (mPE) mPE with all kinds of PE is not the same as above, It is not polymerized with the usual ziegler-Natta catalyst, but with a new catalyst composed of dicenylchlorozirconium and methyl aluminum oxides. It is characterized by the same properties as LLDPE, but it can also be used to increase the temperature, or increase the shear force to improve the flow. It has been widely used in plastic packaging as soon as it comes out. ultra-high molecular weight polyethylene (UHMWPE) UHMWPE is one of the molecular weight of more than 1 million polyethylene, Due to the particularly high molecular weight, its melting fluidity is almost zero, not suitable for general processing equipment to process, but only suitable for the use of hot pressing or cold pressing sintering process, now the use of other PE blending method with extrusion method can also be extruded. At present, in addition to being used in packaging containers, it is rarely used for other purposes. on polyethylene formula design should pay attention to the following: (1) all kinds of polyethylene and ethylene copolymer. Such as: ethylene with vinyl acetate (EVA), ethylene with vinyl alcohol copolymer, etc., have good compatibility, so can be blended modified to improve the performance of the blend. (2) The same product type, for example: film, can be manufactured by a variety of production processes. The same kind of raw material, due to the different production process, the performance of the product will be very different. Even if the same product is the same raw material and production process, the process parameters used are different, the performance of the product will be different, therefore, in paying attention to the production formula, the process should also pay attention to the production of temperature, shear stress, tensile ratio and other parameters. (3) When the use of rubber and plastic blending modification, it is best to use powdered rubber, or should be used double roll rubber mixing machine fully mixed with plastic, after cutting and reproduction, otherwise rubber is not easy to disperse into the plastic, and in the use of functional materials such as: When modifying fillers, antioxidants, antistatic agents, etc., it is best to use masterbatch to achieve a more uniform adding effect. (4) The important principle of the formula is to use the cheapest and simplest process in the case of fully meeting the requirements of the use of the product.
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Polyethylene is the plastic packaging products the largest use of a class of packaging raw materials, because it is produced by the petroleum processing process of cracking gas ethylene as a raw material polymerization, ethylene monomer non-toxic, so all kinds of polyethylene raw materials, Even if it contains 200 ~ 300ppm of ethylene monomer, it is still a non-toxic polymer, which can be used in packaging occasions in direct contact with a variety of food and drugs.
There is a large difference between the melting temperature and the thermal separation temperature (above 315¡æ) of various types of polyethylene; The melting fluidity is better, so the melting of all kinds of polyethylene can be processed and formability is better, and the extrusion, injection, compression, blow molding and other methods of plastic molding can be used to produce various packaging products.
All kinds of polyethylene are non-polar polymers, there is good compatibility between them, can be combined into blends with each other in any proportion to improve performance. Because the melting temperature of all kinds of polyethylene is relatively low, and there is a high degree of thermal adhesion, therefore, in flexible plastic packaging, polyethylene is often used as a thermal sealing material for packaging.
The modification of plastics has chemical modification and physical modification, blendingFormulation is a physical modification, and Lamination is also a modification. Even two perfectly compatible materials should be thoroughly mixed to have a good effect. The properties of a blend are the sum of the properties of two perfectly compatible substances multiplied by the percentage of their weight in the blend.
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