Structural characteristics and crystallization pro

Structural characteristics and crystallization process of polypropylene

since its advent, polypropylene has been widely used in many fields, such as injection molding, film and fiber production, with excellent thermal and mechanical properties. This versatility and economy make polypropylene surpass polyvinyl chloride and polystyrene ethylene, and become the second largest synthetic resin after polyethylene. Especially with the commercialization of various crystalline polypropylene, polypropylene has a very broad prospect in engineering plastics and functional materials

from the structural characteristics of polypropylene, it can be seen that polypropylene has different primary structures due to the asymmetric carbon atoms in the main chain of polypropylene. When the polymer crystallizes, it can only partially crystallize, and it is difficult to obtain inorganic high-purity crystals. However, with the change of crystallization conditions, it can cause the change of molecular chain conformation or stacking mode, and form several different crystal forms, which is the so-called homogeneous polycrystalline phenomenon in crystals

like other crystalline polymers, the crystallization process of polypropylene includes two stages: nucleation and nucleation growth

in the nucleation stage, the polymer chain segments are regularly arranged to form a sufficiently large and thermodynamically stable crystal nucleus, and then the crystal nucleus grows to form spherulites, and the crystallization process enters the crystal nucleus growth stage. The nucleation methods can be divided into homogeneous nucleation and heterogeneous nucleation according to whether there is heterogeneous nucleation in the crystallization process. Homogeneous nucleation refers to the process of spontaneous nucleation of polypropylene melt in amorphous state due to temperature changes. This nucleation method often obtains a small number of crystal nuclei, slow crystallization speed, large spherulite size, low crystallization rate, and poor processing and application properties of products; On the contrary, heterogeneous nucleation refers to the process in which solid-phase "impurities" (such as nucleating agents) or undamaged polypropylene nuclei exist in the polypropylene melt and form nuclei by adsorbing polypropylene molecules on its surface. Obviously, heterogeneous nucleation can provide more crystal nuclei, accelerate the crystallization speed, reduce the size of spherulites, and improve the crystallinity and crystallization temperature of products under the condition of constant spherulite growth rate. These changes in crystallization parameters will give polypropylene many new properties. Therefore, heterogeneous nucleation is actually the theoretical basis of polypropylene crystallization modification

isotactic polypropylene has many crystal forms, namely α、β、γ、δ And pseudo hexagonal crystal. Among them α Crystal forms are the most common, and other crystal forms can only be obtained under special circumstances, such as shear, pressure, nucleating agent, etc. But since β Since the advent of crystalline nucleating agents, they have shown attractive value in commercial applications. With traditional α Compared with isotactic polypropylene, β Crystalline isotactic polypropylene has good impact strength at room temperature and low temperature, high thermal deformation temperature, high toughness and ductility under high-speed tension, and is not easy to embrittle. These characteristics greatly expand the application field of polypropylene. β The formation of crystalline isotactic polypropylene mainly includes three methods: temperature method, shear orientation method and adding nucleating agent, among which adding β Crystal nucleating agent, which has become a high β Crystal content of polypropylene is a practical and effective method

β The addition of nucleating agent induces a considerable part of it, so it is better to choose sample 1 in Figure 1 α Crystallographic orientation β Crystal transformation shows a completely different cluster morphology of bundled wafers, and there is no clear boundary between spherulites, which is explained from the microscopic point of view β The mechanical properties of crystal form are relatively good α The reason for good crystal form

β There are many kinds of nucleating agents, and the nucleation system is relatively dispersed, which can be roughly divided into organic and inorganic. According to the molecular structure, they can also be divided into three categories: fused ring compounds with quasi planar structure, some salts of group IIA metal elements, complexes of dicarboxylic acids and some metal oxides

in the late 1960s, foreign countries β After the first report of the research on crystal nucleating agents, China also accelerated the research in this area, and in 1980, through the use of organic carboxylic acid and metal salt complex β Crystal nucleating agent was obtained β Crystalline polypropylene. In the 21st century, China has used rare earth complexes as polypropylene for the first time β Crystal nucleating agent, 95% β It was found that in CaCO3 toughened polypropylene system, nucleating agent and CaCO3 β There is a synergistic effect in the formation of crystals, which makes our country β The research and development of crystalline polypropylene has reached a new level

in terms of commercial products, Shanxi Institute of chemical industry has developed efficient β Crystal nucleating agents tmb-4 (substituted aromatic amides), and tmb-5 (substituted benzamides), β The crystal form can reach 90%; Starnu-100 nucleating agent newly launched by Japan physicochemical company is an aromatic amine compound, which can make β When the content of crystal reaches more than 90%, the thermal deformation temperature is increased by 15 ℃, and the impact property is increased by many times; Rare earth complexes produced by Guangdong weilinna functional materials company β Crystal nucleating agent can make polypropylene β When the crystal content reaches 90%, the thermal deformation temperature increases by 15 ℃, and the impact strength increases by many times

β Polypropylene modified by crystal nucleating agent has a high thermal deformation temperature (generally higher than 110 ℃, and some can be as high as 135 ℃), while pet, PVC, PS and other thermal deformation temperatures are lower than ℃, so β Crystalline isotactic polypropylene has a wider range of applications than pet, PVC, PS, etc., especially for appliances that require high transparency and need to be used or disinfected at high temperatures, such as transparent hot cups, microwave cookers, baby bottles, disposable fast food boxes, etc. In addition, the price and service life of transparent polypropylene are lower than those of pet, PVC, PS, etc

with efficient β The appearance of crystal nucleating agent makes high β The synthesis of polypropylene with crystalline content becomes possible, so in terms of application, more people begin to use traditional molding technology to β The mechanical properties of crystalline polypropylene were studied

β The addition of nucleating agent makes polypropylene β The crystal content is greatly increased, β The unique bunched aggregation structure of the crystal produces crack bands when stressed, and the toughness is greatly increased; stay β When the mass fraction of nucleating agent is 0.6%, the notch impact strength and elongation at break of simply supported beam reach the maximum, which are more than twice that of polypropylene. In this way, the application field of polypropylene has been greatly expanded. because β The rigidity of crystalline isotactic polypropylene is much higher than that of polyethylene, so when producing plastic pipes with the same diameter and ring stiffness, the β The special material of crystalline isotactic polypropylene can save 15% compared with bimodal polyethylene. Therefore, through the coordination of crystallization refinement, stiffness and toughness, polypropylene can be used to replace bimodal polyethylene to manufacture large-diameter structural wall tubes

through comparative research, β Crystal ratio α Crystal has lower elastic modulus and yield strength, and high contracting parties can apply for a specific exemption for a period of 5 years. In particular, the tensile strength can be nearly 2 times higher than that of traditional polypropylene. According to these characteristics, β Crystalline polypropylene is expected to be more widely used in stretched films. An American company has β Crystal nucleating agent is used for the preparation of biaxially stretched microporous polypropylene membrane. The membrane has both waterproof performance and good air permeability. It is used in functional clothing materials and has achieved commercial production. In addition, use β The mechanical properties of crystalline polypropylene can also replace some engineering plastics as structural materials

polypropylene β Great progress has been made in the study of nucleating agents, but there are also some problems. Low cost, processing without changing the traditional molding and processing equipment, and better universal nucleating agent will be the main BASF development in the future: expanding the high nickel production capacity and licensing patent cooperation direction

on the whole, β The application potential of crystalline polypropylene promotes β The development of crystal nucleating agents seeks more effective, universal, low-cost and good processability β Nucleating agents have become a research hotspot. With the application of new experimental technology, in-situ study of kinetics and molecular simulation, the nucleation theory is enriched, and polypropylene is studied at a deeper level β The study of nucleating agents is of great significance