Study on Bending‐Induced Whitening of Isotactic Polypropylene and Whitening Resistance of aPP / iPP Blends

ABSTRACT Stress whitening is a commonly observed phenomenon in plastic products such as polypropylene. This study examined the mechanism underlying bending‐induced whitening in iPP products and proposed a practical industrial solution. The α‐relaxation temperature ( T α ) was identified as a key transition point for the different whitening mechanisms of iPP. Below T α , with increasing deformation degree, the amorphous phase cavitation appeared in the early stage, followed by the development of fibrillar structure, which together led to a significant whitening phenomenon. Above T α , although cavitation still occurred in the initial deformation stage, its contribution diminished due to increased plastic deformation capacity. Moreover, crystal slipping became the dominant deformation factor in the later deformation stage, resulting in reduced whitening. Based on these insights, a simple blending modification method was proposed to reduce the whitening of iPP by utilizing the excellent plastic deformation capacity of aPP. The experimental results confirmed that the deformation mechanism of the aPP/iPP blended system was dominated by crystal slipping, which effectively inhibited cavitation. The blends exhibited up to approximately 50% enhancement in whitening resistance and 10% improvement in toughness by optimizing the aPP content. This concurrent improvement offered a viable industrial strategy for mitigating stress whitening in iPP applications.