There are thousands of types of polymer materials, some polymer materials are amorphous, some are crystalline, main influencing factor is structure of polymer chain, because a polymer needs its own polymer to crystallize. Chains can be straightened and arranged in parallel to form a "close-packed" structure.
1. Factors affecting polymer crystallization
1.1 Chain symmetry and regularity
Like two parallel lines, more symmetrical structure of polymer chain, more irregular its microscopic appearance, more easily molecular chain crystallizes. For example, main chain of polyethylene consists of only carbon atoms, no other impurity atoms, and all carbon atoms are hydrogen atoms with high regularity, and it is polymer compound that is easiest to crystallize.
Polyethylene and its molecular formula
1.2 Copolymerization, branching and crosslinking
Copolymerization: Homopolymers with two copolymerization units have same crystal structure, so copolymer can also crystallize; random copolymerization destroys symmetry and regularity of polymer chain, making polymer chain disorganized, thereby weakening ability to crystallize. .
Branching: Branching breaks symmetry and regularity of polymer chain, which reduces ability of polymer material to crystallize. For example, crystallizability of LDPE is less than that of HDPE.
Crosslinking: The formation of crosslinking structures between polymers limits mobility of each chain, which in turn weakens ability of polymers to crystallize.
2. Classification of polypropylene nucleating agents
All molecular chains of isotactic polypropylene (iPP) are in 3S helical configuration, with α and β crystal forms being more common. Commercial polypropylene is mainly in α-crystalline form, which is a monoclinic crystal system, which is most common and stable form of polypropylene, with a melting point of 167°C and a density of 0.936 g/cm3.
Polypropylene cools slowly from molten state and usually has a spherulitic structure. The nucleating agent is an impurity that can promote crystallization of polypropylene and acts as a crystal seed during crystallization process. Adding a nucleating agent to polypropylene can accelerate rate of crystallization.
According to composition of nucleating agent for commercial polypropylene, there are mainly two types of nucleating agents: inorganic and organic.
Table 2-1 Polypropylene nucleating agent
Among them, due to poor compatibility of inorganic nucleating agent itself with resin matrix, transparency will be affected. During this epidemic, transparent nucleating agents chosen for PP meltblown are mainly organic nucleating agents. This article provides some technical guidance for manufacturers.
Third, influence of polypropylene nucleating agent on mechanical properties of polypropylene
Table 3-1 Influence of polypropylene nucleating agent on mechanical properties of polypropylene
Conclusion: With addition of nucleating agents, mechanical properties of polypropylene changed significantly. The general impact trend is that tensile strength and flexural strength increase with addition of nucleating agents, and notched impact strength decreases.
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