The crystallization of polypropylene
random PP for amorphous polymers, is the production of isotactic PP by-products, as plastic of little value, developed in the 1980 s as a carrier of the filler masterbatch, resin received good results. The crystallization of PP has a strong ability, crystallization rate and crystallinity and relative molecular mass. PP isotactic degree is higher, the crystallization rate, the greater the degree of crystallinity is higher; The relative molecular mass, the greater the macromolecular chain spread more difficult, crystallization rate decreases, and crystallinity is low. Usually, the crystallinity of PP is approximately 70%.
the relative molecular mass distribution effect on the properties of PP is more complex. From the different relative molecular mass of PP blend the results that the relative molecular mass distribution is wider, the smaller the tensile strength and elongation at break; Roughly the same relative molecular mass distribution and relative molecular mass increases, the PP melt flow properties, melt strength increase, The relative molecular mass closer to the wider distribution, the melt flow rate change is not big, but the material mechanical strength reduced.
the crystallinity of PP increased, tensile strength, stiffness, hardness, melting point increases. In the processing, the crystallinity of PP and depends on the cooling rate of melt crystallization form. From the crystal, which is formed by the molten state cooling are generally with spherulite structure. Slow cooling rate, high crystallinity, crystallization rate, high crystallinity, easy to form a larger size of the spherulites. Opposite to low crystallinity, formation of the spherulite size is smaller. Large spherulite increase the rigidity and heat resistance of products, but the impact strength is reduced, the spherulite products has good transparency and softness.
heterogeneous nucleation will change the crystallization in the process of polymerization, presently used in nuclear technology of transparent PP varieties because of the addition of nucleating agent improved the crystallization temperature, decrease the spherulite size and increased the number of spherulite. At the same time, due to the crystal size decrease and tend to be more uniform and the internal stress of materials and products, improve toughness. PP commonly used nucleating agent with sodium benzoate and alkaline diformate, aluminum, etc.
if methyl arranged alternately in composed of main chain flat on both sides, with between a set of PP is called syndiotactic PP, when methyl in the main chain of plane irregular arranged on both sides as a random set, called a random PP. Due to the highly isotactic PP molecular chain regularity of the vertical structure, easy to crystallization, and thus has high mechanical strength, is the industrial production of the main varieties, about 95% of the total PP production; But with the application of metallocene catalyst in PP synthetic, syndiotactic PP production technology have made some breakthrough, the product has obtained the development and application of different level.
as a result of the PP Tg under room temperature, products often can continue to crystallization at room temperature, causing contraction after products, this phenomenon is known as the late crystallization, it is in the processing of plastic injection molding can be most within 24 h after finish. Industrial production of PP molecular weight - in 200000 700000, with the increase of relative molecular mass, the melt viscosity of PP, the tensile strength, elongation at break and impact strength are improved. But due to increasing the relative molecular mass crystallization is difficult, can lead to yield strength, hardness, rigidity, heat resistance.
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