Preparation of a Novel Oriented Structure in Carbon Black Filled Isotactic Polypropylene Composites by Microwave Selective Heating

Abstract

Microwave heating technology has numerous advantages compared with the traditional heating methods and has been widely used to process materials. However, most thermoplastics do not possess a sufficiently high dielectric property to be heated by microwaves. In this study, carbon black (CB) was utilized as the microwave absorber to improve the microwave heatability of isotactic polypropylene (iPP). Effects of CB contents on the microwave heatability of iPP/CB composites were studied. The temperature of iPP/CB composites with relatively low CB content (5% and 10%) increased slowly and tended to remain unchanged after 120 s of microwave exposure. In contrast, iPP/CB composites with relatively high CB content (15% and 20%) presented a much faster heating rate and the temperature of the sample kept increasing with the prolongation of exposure time. On the basis of the fact that iPP/CB composites with different CB contents have different microwave heatability, a novel oriented structure, in which the core layer has relatively high orientation and the surface layer has relatively low orientation, was prepared by selective microwave heating. Two-dimensional wide angle X-ray diffraction (2D-WAXD) analysis indicates that the orientation parameter calculated by the (040) plane of the surface layer (0.45) was lower than that of the core layer (0.83). The novel oriented structure is different from the common skin-core structure formed in the samples of semicrystalline polymers by traditional polymer processing methods, in which orientation of the skin layer is higher than that of the core layer. The novel oriented structure has not been reported before to our knowledge and its formation mechanism is also discussed in this paper.

Keywords:

• carbon black
• isotactic polypropylene
• microwave
• orientation
• selective heating

Acknowledgments

We would like to express our sincere thanks to the National Natural Science Foundation of China (50873072, 51010004) and the Special Funds for State Key Laboratory for financial support. We are indebted to the National Synchrotron Radiation Laboratory (NSRL) in University of Science and Technology of China and Prof. Guoqiang Pan (NSRL) for his help in synchrotron WAXD experiment.