High polymer
DATE：2022/3/28 9:09:17 / READ： / SOURCE：This station
High polymer
Polymer is the main component of foamed plastics. The processing and use performance of foamed plastics depend on the chemical and physical properties of polymer. Therefore, it is necessary to understand the basic properties of the polymer before formulating the raw material formula. Here are some of the polymers commonly used in foam plastics. Hollow board production line www.handern.comPolystyrene (PS) is a linear macromolecule with random arrangement in space. The K molecular weight used in industry is generally in 45,000 ~ 70,000. PS properties are hard, rigid, high transparency, its transparency can reach 88% ~ 92%, second only to plexiglass, water resistance and chemical corrosion resistance. Can dissolve in benzene, toluene, carbon tetrachloride, chloroform, o-dichlorobenzene, ketones (except acetone), esters and some oils. However, PS is very stable to water, acids, alcohols and bases. The electrical properties of PS are good, and the dielectric constant and dielectric loss are almost not affected by frequency (50 ~ 10°Hz), and are also little affected by temperature. Hollow board production line www.handern.com
On the other hand, PS properties are more brittle, low impact strength, poor heat resistance, PS operating temperature is generally required to be less than 75℃. The heat resistance of PS is related to its molecular weight and the content of low molecules. The softening point of PS containing 5% monomer decreases by 30℃. PS in sunlight or long-term storage are easy to aging, yellow, cloudy. If 0.04% sulfur is present, the speed and degree of yellowing will be greatly accelerated. Ps has good compatibility with styrene butadiene rubber, isoprene rubber, nitrile rubber, butadiene rubber and cellulose acetate, but poor compatibility with polyethylene, polyvinyl chloride. PS foam thermal conductivity is low, widely used as insulation material, it has buffering performance and bubble body has a certain stiffness, so it is widely used in packaging for a variety of pad blocks, it can also be made of metal, paper, textiles and plastic composite sheet, used in packaging, sound insulation, insulation and other aspects. PS foam plastics mostly use physical foaming agents for high foaming, commonly used physical foaming agents such as pentane, butane, fluorine chlorothane, N2, carbon dioxide, etc., there are also chemical foaming agents for foaming PS low foaming products. Commonly used chemical foaming agent such as: azodicarbonamide (AC foaming agent), 4, 4-oxydiphenyl sulfonyl fat, 2,2 '-azodiiso-butyronitrile, toluene sulfonyl, sodium bicarbonate and dinoleic acid. PS can be made into very light foaming materials. PS foams with high foaming ratio are mostly made of physical foaming agent. The density P is 0.02 ~ 0.15g/cm³, and now the foaming ratio of high foaming PS has reached 80 times. PS and butadiene-styrene elastomer can also be 1:1 dosage ratio made of elastic structure bubble. PS powder particles can be made into elastic pre-foaming PS particles (EPS) after developable and pre-foaming treatment, which is used for PS molding and high foaming molding (see Chapter 8 for details). PS foam molding method is the main method of molding, extrusion and injection molding. Hollow board production line www.handern.com
Two, polyethylene (PE) for crystalline polymer. According to the different synthesis methods, PE can be divided into high pressure PE, commonly known as low density polyethylene (LDPE); Low pressure PE, commonly known as high density polyethylene (HDPE). Their molecular structure and properties are different. LDPE molecule has branched chain structure, crystallinity about 35% ~ 75%. HDPE molecules have a straight chain structure with a crystallinity of about 60% ~ 90%. PE density is proportional to crystallinity, so density can be used as a marker of crystallinity. The molecular weight of LDPE is 19000~48000, and that of HDPE is 70000 ~ 350000. The melt index of PE indicates the flow performance of PE melt. This parameter has obvious influence on the physical and mechanical properties of the bubble. PE is a crystalline polymer. When the temperature exceeds its melting point, the melt viscosity decreases sharply. This characteristic of PE is very unfavorable to foaming molding. Because in the foaming process, in order to maintain air bubbles, must have certain viscoelastic melt, and crystal plastic has the performance of the temperature range is very narrow, are difficult to control, therefore need to add fertilizer, to expand to appropriate bubble between the temperature of the foaming area, general often adopt the method of adding cross-linking agent to make PE melt crosslinking, to improve the performance of the melt viscoelastic bend 7 Expand the temperature range suitable for foaming of melt. Crosslinking agent of type a lot, but suitable for PE foam molding, there are many different kinds of crosslinking agent, for it not only has certain intermiscibility with PE melt, and its decomposition temperature must be higher than the melting point of PE, lower than the decomposition temperature of the foaming agent, generally is the difference between each other around 10 ℃, such making that PE melt before the decomposition of foaming agent, crosslinking agent has decomposition, The PE melt has viscoelasticity suitable for foaming, and then the foaming agent decomposes and releases gas to form the bubble body. In the process of foaming forming and setting, the cooling speed of melt should not be too slow in order to make the fully expanded bubble solidify and set in time. PE specific heat capacity is relatively large, PE melt in the cooling solidification process to undergo crystallization process, which is exothermic process. Therefore, in order to improve the cooling speed of PE vesicle, the strength of cooling must be improved to ensure the necessary cooling efficiency and cooling speed. Get rid of a lot of heat in time. One of the important factors affecting the foam stability in the foaming process of PE plastic melt is the permeability coefficient S of the gas released by the foaming agent in the melt. The gas with too large permeability coefficient S is not suitable for foaming agent. The permeability coefficient S is the product of diffusion coefficient D and solubility C. However, THE rigidity of PE foam is worse than that of PS foam, and the creep performance is greater than that of PS foam. In order to improve the stiffness of PE foam and reduce the creep, it is more effective to use filler. There are many kinds of fillers that can be added, such as glass fiber, calcium carbonate, talcum powder, mica, etc., and the amount added is generally 10% ~ 40%. If the surface of the filler is activated, the effect is better. Someone did the test, with the length of 3 ~ 6mm glass fiber 20%(mass) to add PE foaming, so that the elastic modulus of THE PE foam plastic obtained increased by 2 times, tensile strength increased by 2 times, bending strength increased by 3 times, flame retardant performance improved by 3 times, thermal stability from 40℃ to 70℃, but impact strength decreased by 2 times. PE foam molding can be used more chemical foaming agent, such as azodicarbonamide, diisopropyl azodicarboxylic acid, N, N' -nitropentylene methyl tetramine, p-toluene sulfonyl amino, 4,4 '-oxydiphenyl sulfonyl volume. PE foaming molding can also use physical foaming agent, such as fluorochlorothane, butane, N, CO, etc., physical foaming agent for high foaming occasions. PE foam molding is more plastic injection method, extrusion method, molding method. Hollow board production line www.handern.com
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