Why are PP products prone to brittleness in winter and a decrease in temperature in winter? Many cities experience temperatures below zero, which can lead to a series of problems such as PP polypropylene plastic products freezing and cracking. This is mainly determined by the inherent molecular structure characteristics of PP polypropylene, with a key parameter being the glass transition temperature Tg. Tg is a characterization temperature used to measure the flexibility of polymer materials. The smaller the Tg, the better the flexibility of the chain
What is the glass transition temperature Tg
The vast majority of polymer materials can typically be in the following three physical states (or mechanical states): glass state, high elasticity state (rubber state), and viscous flow state. The glass transition refers to the transition between the high elastic state and the glass state, and the glass transition temperature refers to the temperature corresponding to the transition point of polymer chains from freezing to thawing, and from activity to freezing. When the temperature is below the glass transition temperature, the polymer is in a glassy state, and the molecular chains and segments cannot move. Under external forces, only very small deformations occur. At this point, polymer materials exhibit obvious brittleness. When subjected to external impact, the material itself cannot absorb and consume energy, resulting in damage. However, when the ambient temperature is above the glass transition temperature, the molecular chain segments have the ability to move and exhibit high elasticity. External energy shocks can be absorbed through the movement and deformation of molecular chain segments, thus avoiding damage.
According to the instrument test, the glass transition temperature of polypropylene is around 0 ℃. In winter, the environmental temperature in southern cities will approach the glass transition temperature of polypropylene, and the molecular chain activity and toughness of PP products will decrease. The temperature in the north is generally lower than the glass transition temperature of PP, so in winter, PP products in the north appear very brittle, and more toughening agents need to be added to improve low-temperature brittleness. In order to improve the low-temperature toughness of polypropylene, it is necessary to choose materials with lower glass transition temperature than PP, including PE materials, POE elastomers, etc.
High transparency polypropylene material can copolymerize a small amount of ethylene onto the main chain of polypropylene, which can damage the crystallization of PP and slightly reduce the glass transition temperature of polypropylene molecular chains by a few degrees. Therefore, it can improve the toughness of PP products to a certain extent.
