Crystallization characteristics of thermoplastics
The crystallization characteristics of thermoplastics are one of their most important physical properties, directly affecting their mechanical, thermal, and processing capabilities. During the molding process, the molecular chains of crystalline thermoplastics gradually align from a disordered molten state into an ordered crystalline structure. This process is called crystallization. The degree of crystallinity and the integrity of the crystal structure determine the performance of the plastic. For example, high-density polyethylene ( HDPE) has a high degree of crystallinity, which gives it excellent rigidity, hardness, and chemical resistance, while low-density polyethylene (LDPE) has a lower degree of crystallinity, which exhibits better flexibility and transparency.
The crystallization process is influenced by many factors, temperature being one of the most critical. Crystallization must occur within a certain temperature range, typically between the glass transition temperature and the melting point. When the melt temperature is above the melting point, the molecular chains move violently, making it difficult to form an orderly arrangement. When the temperature is below the glass transition temperature, molecular chain movement is restricted, and crystallization cannot proceed. For example, the optimal crystallization temperature for polypropylene (PP) is approximately 100-120°C. Within this temperature range, the molecular chains can align in an orderly manner, forming a complete crystal structure. If the cooling rate is too rapid, and the temperature quickly drops below the glass transition temperature, the crystallinity of PP decreases, resulting in a decrease in its rigidity and heat resistance.
In addition to temperature, cooling rate also significantly affects the crystallization properties of thermoplastics. Faster cooling prevents the molecular chains from aligning, resulting in lower crystallinity and a less perfect crystal structure. Conversely, slower cooling allows the molecular chains ample time to align, leading to higher crystallinity and a more complete crystal structure. For example, in the production of polyamide (PA6) products, slow cooling can increase crystallinity, enhancing the product’s strength and wear resistance. Rapid cooling, on the other hand, reduces crystallinity, imparting greater toughness and transparency. An automotive parts manufacturer, while producing PA6 gears, increased crystallinity from 50% to 70% by controlling the cooling rate, extending the gear’s service life by 30%.
Molecular structure is also a significant factor influencing the crystallization properties of thermoplastics. Plastics with regular, symmetrical molecular chains are more likely to crystallize and have a higher degree of crystallinity. For example, polyethylene (PE) has a linear, symmetrical molecular chain structure, resulting in a high degree of crystallization. Random polypropylene, on the other hand, has an irregular molecular chain structure and poor crystallization ability, often exhibiting an amorphous state. Furthermore, the length and degree of branching of the molecular chain also affect crystallization. Longer molecular chains result in slower crystallization but potentially higher crystallinity. Higher levels of branching result in less regular molecular chains and lower crystallinity. For example, high-density polyethylene (HDPE) has a low degree of branching and a crystallinity of 80%-90%, while low-density polyethylene (LDPE) has a high degree of branching and a crystallinity of only 50%-60%.
Additives also regulate the crystallization properties of thermoplastics. Nucleating agents are commonly used additives that form a large number of crystal nuclei in the plastic melt, promoting the orderly arrangement of molecular chains around the nuclei, thereby increasing the crystallization rate and degree of crystallinity, and refining the crystal particles. For example, adding a nucleating agent to polypropylene (PP) can increase the crystallization temperature by 5-10°C and shorten the crystallization time by 30%-50%, while also improving the rigidity and heat resistance of the product. Conversely, certain plasticizers reduce the forces between molecular chains, hindering their orderly arrangement and thus reducing crystallinity. A plastics processing plant added 0.2% of a nucleating agent to PP film, which increased the film’s crystallinity by 15% and its tensile strength by 20%, significantly improving product quality.
