Shrinkage Behavior and Mechanism of Isotactic Polypropylene Based on Anti-Nucleating Agent Modification

The high shrinkage of isotactic polypropylene (iPP) during molding limits its application in high-precision products. Although nucleating agents are commonly used to regulate crystallization, they frequently increase shrinkage. In this study, l-isoleucine (l-Ile), an anti-nucleating agent that inhibits crystallization, was incorporated into iPP to regulate shrinkage behavior, with the nucleating agent HPN-68L used for comparison. Shrinkage tests show that l-Ile reduces iPP shrinkage by 11.8% in the flow direction and 14.2% in the transverse direction, whereas HPN-68L increases shrinkage by 15.4% and 27%, respectively. Crystallization behavior was analyzed by using differential scanning calorimetry (DSC), flash differential scanning calorimetry (Flash DSC), polarized optical microscopy (POM), and X-ray diffraction (XRD). Results indicate that l-Ile suppresses nucleation and decreases the crystallinity, while HPN-68L exhibits the opposite effect. The shrinkage-reducing mechanism of l-Ile is attributed to its ability to reduce the crystallinity of iPP, thereby increasing the proportion of the amorphous polymer characterized by a high free volume. Moreover, the relationship between crystallinity and ultrafast cooling rates was established using Flash DSC, providing process optimization guidance for the production of low-shrinkage iPP products. This study presents not only a novel strategy for low-shrinkage modification of iPP but also new insights into enhancing the dimensional stability of semicrystalline polymers.