Cao Xinxin1, He Xiaofang1, Wang Haijuan1, Zhang Haibo1, Chen Xiaolang2 (1. School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo, 4.54000; 2. School of Polymer Science and Engineering, Sichuan University, Chengdu, 610065)
Abstract: The crystallization process of polypropylene (IV), the influence of the crystal structure of PP molecules on its properties and the classification of nucleating agents, such as dibenzyl and sorbitol derivatives, organic phosphates, alkyl carboxylates and rosins, are introduced. Recent research results on the effects of nucleating agents on isothermal crystallization behavior, melting properties, mechanical properties, aging resistance, optical properties and processing properties of PP were reviewed.
Key words: polypropylene; crystal structure; nucleating agent; dibenzyl sorbitol derivative; organic phosphate; alkyl carboxylate; rosin
Polypropylene has simple synthesis method, abundant raw material source, low price, good chemical resistance, electrical properties and mechanical properties, and can be processed into injection molded articles, hollow molded articles, films, sheets and fibers for various purposes. It has become one of the fastest growing varieties of plastics, and its output ranks third among the five general-purpose plastics, so it is widely used in daily necessities, packaging materials, office supplies, electrical appliances and automotive parts. However, since polypropylene is a crystalline polymer, there are large spherulites in the interior, which causes low impact strength of polypropylene, serious post-contraction of the product, and does not have sufficient rigidity, dimensional stability or transparency in use. Sex, etc., which seriously affects the performance of polypropylene resin. Therefore, many researchers have modified polypropylene from polymerization technology, molding technology, and composite technology to improve its performance. Among them, by adding a nucleating agent, the crystallization rate of the molding process is improved, and the crystal grains are refined to improve the impact resistance, transparency and glossiness of the product, and is a commonly used method for realizing 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 ordered, such as incomplete movement of the main chain, irregular distribution of side groups at a certain position, and branching and large Side chains will prevent crystallization. (2) The crystal nucleus must be able to initiate crystallization, and thus form crystallites, and the crystallites themselves are arranged in a superstructure, that is, spherulites. (3) The crystallization temperature is between the melting point (f) of the polymer and the glass transition temperature (tg) in order to impart the necessary mobility to the molecular chain. Above tm, a stable crystal nucleus cannot be formed; below tg, the segment motion freezes and the crystal nucleus growth rate is zero. (4) The crystallization process includes nucleation and nucleation growth, and the crystallization rate can be calculated from the nucleation density and the growth rate of the spherulites. The crystallization process of a polymer is actually a process in which the segments of the molecular chain are ordered.
1.2 Effect of polypropylene crystal form on properties
1.2.1 Crystal form of polypropylene [2.3]
The molecular chains of isotactic polypropylene (iPP) are all in 3s helical configuration, and can form five crystal structures of α, β, γ, δ and quasi-hexagonal states under different crystallization conditions, among which α and crystal forms are more common. . (1) α crystal form is monoclinic, is the most common and stable form of polypropylene, melting point is 167 ° C, density 0.936 g / cm 3 , the main crystal form in commercial polypropylene is α Crystal form.
(2) The β crystal form belongs to the hexagonal crystal system, the melting point is 150 ° C, and the density is 0.922 g/cm 3 . Under normal processing conditions, PP is mainly close to the most stable a monoclinic crystal. Since the β crystal form is more dispersed than the a crystal form and has a good absorption effect on the impact energy, the impact strength of the β crystal form PP is 1-2 times higher than that of the a crystal form PP. However, the crystal form is a thermodynamically quasi-stable, kinetically unfavorable formation of a crystal field, which can only be obtained by special methods, such as selecting appropriate melting, crystallization temperature and certain temperature gradient, shear orientation, and use. A nucleating agent or the like converts a crystal form into a crystal form, wherein the addition of a special nucleating agent capable of inducing a crystal form is currently recognized as one of the most effective ways to obtain a high content of crystalline form PP.
(3) γ crystal form PP, melting point of about 150 ° C, density of 0.946g / cm3, the formation of less than the crystal, for the relatively high molecular weight of polypropylene, under high pressure can occur from a crystal to γ ​​crystal solid Phase transitions have the highest degree of order.
(4) The quasi-hexagonal state, also known as twins, has a density of about 0.88 g/cm3. This is a thermodynamically unstable crystal structure, also known as a subcrystalline structure or a discotic liquid crystal, which belongs to the amorphous to crystalline state. The transition state structure has a poor molecular structure, a loose structure, is easily stretched, and has a high transparency.
1.2.2 Spherical structure of polypropylene [3]
The polypropylene is slowly cooled from the molten state and generally has a spherulite structure. The higher the crystallization temperature, the larger the spherulites formed and the more brittle the properties; the lower the crystallization temperature, the smaller the spherulites, and thus the spherulite size directly affects the impact strength of the material. In Table 1, I, II and the mixed type are prone to rubber-like deformation, so the yield elongation is large, and the type III and type IV are easily cracked perpendicularly to the stretching direction, and the deformation is small.
1.3 Nucleation of nucleating agents in the crystallization of polypropylene [4]
When the polymer material is melt crystallized, it is most likely to form polygonal crystal grains and dendrites.
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