What properties does polypropylene have
Because industrial polypropylene has a high degree of stereoscopic regularization, it has good crystallinity, and its crystallinity is generally 50-70%, sometimes up to 80%.The higher the isometric degree, the better the crystallinity and the higher the density.The density of polypropylene is generally 0.90 ~ 0.91g/cm3.Polypropylene has many crystalline forms.The conditions of crystallization (i.e. the cooling process) are different, forming different forms of crystals first, thus affecting the hardness, rigidity, impact strength and transparency.Industry is about 164 ~ 170 ℃, melting point of polypropylene and polyethylene, has good heat resistance, and its products can temperature above 100 ℃, low load can be used in 110 ℃ continuous and intermittent use temperature can reach 120 ℃, no load ability under 150 ℃, but as a result of pp molecular chain methyl exist alternately, the polyethylene molecular chain rigidity, high glass transition temperature (- 10 ℃ or so) cold resistance is poorer, high temperature embrittlement.All together low temperature using the temperature limit of polypropylene is commonly - 20 ℃ to 15 ℃.Actually near 0 ℃, polypropylene is a bit of a crisp.Polypropylene tensile strength, yield strength, rigidity, hardness are higher than polyethylene.As the molecular weight increases (the melt index decreases), the tensile strength and elongation increase, but as the low molecular weight increases, the crystallization slows down, the yield strength, rigidity and hardness decrease slightly.The higher the isotropy of polypropylene, the higher the yield strength, rigidity and hardness will be.The impact strength of polypropylene is low, especially when the molecular weight is low and the isoparametric degree is high.In order to improve the impact resistance and reduce the embrittlement temperature, copolymerized polypropylene (usually produced by copolymerization of propylene with a small amount of ethylene) can be used, or the method of blending modification can be used to toughen polypropylene.The mechanical properties of polypropylene are characterized by excellent flexural fatigue resistance, especially the oriented polypropylene, which can withstand more than one million bends.By using this feature, we can produce a single hinge box with bottom cover.Electrical properties of polypropylene and polyethylene are similar, with excellent electrical insulation, volume resistivity Ω. > 1016 cm, dielectric constant (2.2 ~ 2.6) and dielectric loss (0.0004 ~ 0.001) is very low, and affected by the frequency of small.The basic chemical properties of polypropylene are similar to polyethylene and stable to most media.Inorganic acid, alkali or salt solutions, in addition to the strong oxidation of rendering, under 100 ℃ for PP almost no damage.No liquid dissolves polypropylene at room temperature.When the temperature rises, some liquids, such as aromatics and chlorinated hydrocarbons, increase the swelling of polypropylene.Above 80 ℃, polypropylene can melt in aromatics and chlorinated hydrocarbons, such as xylene, decalin, carbon tetrachloride, etc.Because of the alternation of tertiary carbon atoms on the main chain of polypropylene, polypropylene is sensitive to oxygen and is prone to heat oxygen aging and photooxygen aging. The aging speed is much faster than polyethylene.Copper ions have a strong catalytic effect on the aging of polypropylene.Therefore, for polypropylene products that have been exposed to high temperature for a long time or are used outdoors, attention should be paid to strengthening the anti-aging system in the distribution. In the case of electric insulation of copper contact, copper inhibitor should be added or copper-damaging polypropylene resin with copper inhibitor should be selected.The polarities of polypropylene are non-high polymer, and its water absorption rate is very low. Generally, it does not need to be dried before forming.When necessary, can be in 80 ~ 100 ℃ drying 3 ~ 4 hours.Polypropylene processing temperature is 210 ~ 250 ℃.Too high temperature or too long heating time, the molecular weight will decrease obviously due to thermal degradation, resulting in poor performance.The specific capacity of polypropylene changed greatly in the forming process due to thermal shrinkage and crystallization.For injection moulding products, bubbles and indentations are easy to be produced in the box holes, reinforcing bars, nearby areas and areas with large wall thickness.For some parts with high dimensional precision, it is better to choose modified low shrinkage polypropylene.Polypropylene melt cooling crystallization under normal processing conditions will reduce the number of crystal nuclei due to slow nucleation of the bulk, so it is easy to form large spherulites, resulting in strong impact and poor transparency.The addition of effective crystalline nucleating agent can increase the number of crystal nucleus, refine the crystal, increase the impact strength and improve the transparency.The additive, which makes polypropylene transparent, has been commercialized in recent years.After the addition of nucleating agent, the size of the spherulites is less affected by temperature, which can reduce the structural unevenness caused by the uneven cooling of thick-walled products.In addition, after adding nucleating agent, the maximum rate of crystallization temperature to 140 ℃ (usually polypropylene maximum crystallization rate of temperature is 120 ~ 130 ℃), the speed is greatly accelerated, crystallinity increase, reduce the crystallization and shrinkage deformation of molding products.