The Modification Of Polypropylene By Melt Grafting And Blending And Its Fatigue Resistance

In recent years, there exists a rapid development in china's paper industry. At present, many foreign-funded enterprises mainly import the paper mandrel plastic pipes, while some domestic companies are also stepping up the research and development of high-rigidity and high toughness plastic pipes. Based on our early research, two PP pipes made with different catalysts were analysed, and the results showed that there wasα-βtransition in the Ziegler-Natta isotactic polypropylene (ZNPP) pipe after cyclical fatigue process, while there was no newβcrystal formation in the metallocene isotactic polypropylene (MPP) pipe after the same fatigue process. The metallocene catalyst is expensive, so most polypropylene in domestic industry is prepared by Ziegler-Natta catalyst. If the crystalline transition in Ziegler-Natta polypropylene can be inhibited in the course of use, the service life of materials can be extended.In this paper, the ZNPPs were grafted by polyfunctional monomer with twin extruder, and blended with a small amount of nylon 6 andα-nucleating agent to control the crystalline structure of polypropylene. These methods changed the chemical structure and the crystallization behavior in order to control the aggregate structure and the crystalline transition. The dynamic fatigue test was used for the pretreatment of specimens, and the microstructure and properties of the samples were characterized by differential scanning calorimetry (DSC), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), polarized optical microscopy (POM) and mechanical property test. The main conclusions were drawn as follows:(1) The grafting modification and fatigue resistance:(a) Pentaerythritol triacrylate (PETA) was successfully grafted onto the polypropylene chains via melt reactive extrusion, which improved the mechanical properties of the isotactic polypropylene(PP)/glass fiber(GF) composites, enhanced the interfacial adhesion between PP and GF, and increased the melt strength, melting and crystallization temperatures, crystallization rate. In addition, the grafted acrylic polymers restrained the formation ofβcrystal in fatigue experiment.(b) The polypropylenes grafted by pentaerythritol tetra-acrylate(PETeA) can not only improve the crystallization and melting temperatures and lower the spherulite dimension, but also increase the melt strength. The grafting polymers restrained the formation ofβcrystal. Similarly, the grafted polypropylenes generated no newβcrystal after the dynamic mechanical fatigue, which significantly improved the fatigue resistance and would extend the service life of the materials.(2)The blending modification and fatigue resistance:a) Isotactic polypropylene (PP) was modified by PA6 which acted as nucleating agent. PA6 improved the processability of PP, and PA6 significantly suppressed the formation ofβcrystal during processing and restrained the formation ofβcrystal in fatigue experiment. b) The addition ofα-nucleating agent also lowered the spherulite dimension, andpromoted the formation ofαcrystal. The modified polypropylene had no newβcrystal after the fatigue process which indicated thatα-nucleating agent had the effect of restraining the formation ofβcrystal.The characteristics and innovation of the paper are the following:(1) The Ziegler-Natta isotactic polypropylene was grafted by polyfunctional monomer to improve its melt strength, and the dynamic fatigue test showed that the grafting modification can improve its fatigue resistance.(2) The dynamic fatigue test also showed that PA6 andα-nucleating agent can significantly restrain the formation ofβcrystal in both processing and fatigue processes.(3) Two different kinds of dynamic fatigue test were used to simulate the fatigue process in the actual service process of polypropylene pipes.