Since the start of its industrial production in 1976, PET bottles made through injection stretch blow molding have developed rapidly, becoming the third largest application of PET after fibers and biaxially oriented films. Today, PET is the second most widely used polymer for blow molding, with molded bottle volumes ranging from 10 ml to 30 L.
PET bottles are primarily used for packaging carbonated beverages, but also serve in the packaging of alcoholic drinks (such as beer and wine), fruit juice, mineral water, edible oil, condiments (soy sauce, jam, vinegar), pharmaceuticals (like eye drops and syrup), cosmetics, pesticides, and detergents.
Blow-molding grade PET features a high intrinsic viscosity (IV) of 0.70–0.85 dl/g, providing excellent mechanical strength and optical clarity. Although PET is a crystallizable polymer, its crystallization rate is relatively low. By precisely controlling the crystallization temperature and cooling rate, PET products can be either amorphous or crystalline, and their crystal type and quantity can be adjusted.
The crystallinity of PET generally ranges from 0% to 30%. As illustrated in the figure, the crystallization rate is very slow at temperatures slightly above the glass transition temperature or just below the melting point. In contrast, the crystallization time shortens significantly at around 175 °C. PET crystallizes only when heated, and the process tends to form large spherulites slowly. These large crystals scatter light, making the product appear white and brittle.
Additionally, as the intrinsic viscosity increases, the crystallization rate decreases. Molecular orientation aligns PET chains in an orderly manner, which promotes the formation of strain-induced crystals. These crystals are very fine and do not scatter light, resulting in a transparent product. If oriented and partially crystallized PET is further thermally processed to induce more crystallization, the product remains transparent.
In summary, crystallization and orientation are critical factors influencing both the molding process and the final properties of PET bottles.
