A brief discussion on polypropylene α and β crystalline forms and their performance
Polypropylene is a semi-crystalline polymer with a general crystallinity between 40% and 70%.
Under different conditions, it has been found to have 5 crystal structures, namely α, β, γ, δ, and pseudo-hexagonal crystalline states.
The physical properties and even optical properties of polypropylene products are affected by the degree of crystallization and crystalline form.
Among them, the control techniques and related additives of α-crystalline polypropylene are quite mature and widely used in the polymerization end and back-end processing industry of synthesis plants.
This article will compare and describe β-crystalline polypropylene and α-crystalline polypropylene.
(α)Alpha crystalline polypropylene Overview
As early as the 1960s, when polypropylene was first invented, the crystal characteristics of the alpha form of polypropylene had been described in detail: "Monoclinic system, each unit cell has 4 1/3 helices, a relatively dense and complete structure, a thermodynamically stable crystalline form."
In the general heating and cooling process of pure polypropylene, the crystalline form of polypropylene is mostly α crystal form.
Therefore, subsequent polypropylene related processes and development have a large area surrounding the control of α crystal form. Techniques, such as controlling the crystal size and crystal density of α crystals, are used to improve product characteristics such as transparency and rigidity, etc.
(β)Beta crystalline polypropylene Overview
In 1959, the beta crystalline form of polypropylene was discovered. However, compared to the research on the α crystal form, for a long time, pure β crystal polypropylene could not be made due to the instability of the β crystal form during the processing, so its structure could not be clearly described. Naturally, there is no further explanation of its application. Nowadays, β-crystalline polypropylene can be easily induced through some β-crystal nucleating agents. The β crystal state belongs to the hexagonal crystal system, and the macromolecular chains in polypropylene are arranged in a spiral shape in the same direction into the crystal lattice. The lamellae in the beta crystalline state will be parallel or gathered into bundles from the center of the spherulite, and then branch, or first grow in an outward spiral and then branch.
(Figure (b)).
