Cao Xinxin1, He Xiaofang1, Wang Haijuan1, Zhang Haibo1, Chen Xiaolang2 (1. School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo, 4.54000; 2. College of Polymer Science and Engineering, Sichuan University, Chengdu, 610065)
Abstract: The crystallization process of polypropylene (IV), the influence of the crystal structure of PP molecule on its properties and the classification of nucleating agents, such as dibenzyl sorbitol derivatives, organic phosphates, alkyl carboxylates, and rosin, are introduced. The recent research results of the effects of nucleating agents on the isothermal crystallization behavior, melting properties, mechanical properties, aging resistance, optical properties and processing properties of PP were reviewed.
Keywords: Polypropylene; Crystal structure; Nucleating agent; Dibenzyl sorbitol derivative; Organic phosphate; Alkyl carboxylate; Rosin
Polypropylene can be processed into injection-molded products, hollow molded products, films, sheets, and fibers with various uses due to its simple synthesis method, rich source of raw materials, low cost, good chemical resistance, electrical properties, and mechanical properties. As one of the fastest growing varieties of plastics, its output ranks third in the five general plastics, so it is widely used in everyday products, packaging materials, office supplies, electrical appliances and automotive components. However, since polypropylene is a crystalline polymer, a large amount of spherulites exist inside, resulting in a very low impact strength of polypropylene and a serious post-shrinkage phenomenon of the product. In use, it does not have sufficient rigidity, dimensional stability, or transparency. Such as sex, which seriously affected the use of polypropylene resin performance. Therefore, many researchers have modified polypropylene from the aspects of polymerization technology, molding technology, and composite materials technology to improve its performance. Among them, a nucleating agent is added to improve the crystallization speed in the molding process, and the crystal grains are refined to improve the impact resistance, transparency, and gloss of the product. This is a common method for realizing the high performance of polypropylene.
1 Crystallization of Polypropylene
1.1 Crystallization process [1]
The polymer melt can be crystallized under the following conditions: (1) The molecular structure of the polymer can cause the crystals to be arranged in an orderly manner, such as incomplete movement of the main chain, irregular distribution of side groups in a certain position, branching, and large Side chains can interfere with crystallization. (2) The crystal nucleus must be capable of initiating crystallization, and thus form microcrystals, and the crystallites self-arrange into a superstructure, ie, spherulites. (3) The crystallization temperature is between the melting point (f) of the polymer and the glass transition temperature (tg) so that the molecular chain has the necessary mobility. Above tm, a stable crystal nucleus cannot be formed; below tg, the segmental motion freezes and the growth rate of crystal nuclei is zero. (4) The crystallization process includes nucleation and nucleus growth. The crystallization rate can be calculated from the nucleation density and the growth rate of spherulites. The crystallization process of a polymer is actually a process in which the chain segments of the molecular chains are arranged in an orderly manner.
1.2 Effect of Molecular Crystalline Polypropylene on Performance
1.2.1 Polymorphs of Polypropylene [2.3]
The molecular chains of isotactic polypropylene (iPP) are all 3s helix configuration, and can form five crystal structures of α, β, γ, δ and quasi-hexagonal under different crystallization conditions, of which α and crystal forms are more common. . (1) The alpha crystal is monoclinic and is the most common and stable form of polypropylene. It has a melting point of 167°C and a density of 0.936 g/cm3. The main crystal form in commercial polypropylene is α. Crystal form.
(2) The β crystal form belongs to the hexagonal system with a melting point of 150°C and a density of 0.922 g/cm3. Under normal processing conditions, the PP is mainly monoclinic with a nearly stable a. The β crystal form is much more evasive than the a crystal form, and has a good absorption effect on the impact energy. Therefore, the impact strength of the β crystal PP is 1-2 times higher than that of the a crystal PP. However, the crystalline form is a crystalline field that is thermodynamically quasi-stable and kinetically unfavorable to formation. It can be obtained only by special means, such as selecting an appropriate melting, crystallization temperature, and a certain temperature gradient, shear orientation, and use. The nucleating agent and the like transform the crystalline form of a into a crystalline form, wherein the addition of a specific nucleating agent capable of inducing the formation of a crystalline form is currently recognized as one of the most effective ways to obtain a high content of crystalline form PP.
(3) γ-crystalline PP, melting point 150°C, density 0.946g/cm3, less chance of formation than crystals. For polypropylene with relatively high molecular weight, a crystal to γ ​​crystal can occur at high pressure. The phase transition has the highest degree of order.
(4) The pseudo-hexagonal state is also known as twin, with a density of about 0.88 g/cm3, which is a thermodynamically unstable crystal structure, also called a secondary crystal structure or a discotic liquid crystal, belonging to an amorphous to crystalline state. The interim transitional structure has poor regularity of the molecular chain arrangement, loose structure, easy to be stretched, and high transparency.
1.2.2 Spherulitic structure of polypropylene [3]
Polypropylene slowly cools from the molten state and generally has a spherulite structure. The higher the crystallization temperature, the larger the spherulites formed, the more brittle the properties; the lower the crystallization temperature is, the smaller the spherulites are, so the spherulite size directly affects the impact strength of the material. In Table 1, Type I, II, and hybrid types are prone to rubber-like deformations, so the yield elongation is large, while Form III and Type IV are easily cracked perpendicular to the tensile direction with less distortion.
1.3 Nucleation of nucleating agents in polypropylene crystallization [4]
When polymer materials are melted and crystallized, it is most likely to form polygonal crystal grains and dendritic crystals.
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