Effect of nucleating agent on the crystallization behavior, crystal form and solar reflectance of polypropylene

Nucleating agents of polypropylene (PP) have a function in accelerating the crystallization rate, decreasing the spherulite size and transforming the crystal form. Materials with high solar reflectance are widely used in the field for cooling purpose. In this work, three types of nucleating agents were applied to improve the optical properties: malonic acid, sorbitol, and carboxylate nucleating agent (NTC). Differential scanning calorimetry (DSC) measurement was conducted to investigate the crystallization behavior. Wide angle X-ray diffraction (WAXD) was used to study the crystal structure and the crystal form. Polarized optical morphology (POM) was chosen to observe the crystal morphology. UV–vis-NIR spectrometer was performed to record the solar reflectance from 280 to 2500 nm. It was found that nucleating agents could increase the crystallinity, decrease the spherulie size and transform the crystal form of PP, which in turn improved the solar reflectance considerably. Especially, β crystal form with high birefringence exhibited more excellent solar reflectance in comparison with neat PP, PP/malonic acid and PP/sorbitol. The order of solar reflectance is as follow: spherulites of β crystal form>small spherulites of α crystal form>large spherulites of α crystal form>disordered polymeric chain. All these types of PP have high thermal emittance more than 0.85, 1 wt% NTC even improves this value of PP to 0.90.