| Polypropylene (PP), an important commodity plastic, is widely used in many applications, such as wire, cables, housing, electronic and electric industries, automobiles, etc. However, the notched impact performance of PP is very poor, especially at low temperature. In order to improve the strength and toughness of PP materials, many researchers dedicated to study the modification of PP. The commonly used method is polymer blending. The blends of PP with other polymers, e.g., ethylene-co-vinyl acetate (EVA), poly (ethylene-co-octene, POE), ethylene-propylene-diene monomer (EPDM), thermoplastic polyurethanes (TPU), etc., will remarkably improve the mechanical properties of the material. TPU has good elasticity and abrasion resistance. The blending of TPU with PP can not only improve the notch sensitivity of PP, but also improve the rigidity of TPU, and the blends show excellent mechanical properties. However, the inherent flammability of PP limits its wide applications in many fields where good fire retardancy is required. In addition, the blends of TPU with PP will remarkably increase the flammability of the materials, which directly results in the flame resistance for PP blends being much less as compared to pure PP. Therefore, more and more researchers have recently been interested in the flame retardant PP and its blends. In order to improve the flame retardancy of PP and its blends, various flame retardants are required to be added. Among the flame retardants, intumescent flame retardants (IFR) have aroused great attention because they are not only more environmentally friendly than the halogen-containing flame retardants but also have higher flame retardant efficiency than inorganic metal hydroxides, e.g., aluminum hydroxide (ATH) and magnesium hydroxide (MH). Generally an IFR system is generally made up of three elements:an acid source, a carbonic agent and a blowing agent. The intumescent char layer, produced from the synergistic interaction among these elements, protects underlying polymeric materials from oxygen and heat. For example, the combination of ammonium polyphosphate (APP), melamine (MEL) and pentaerythritol (PER) is widely used as an IFR system for fire retardant polymeric materials.Expandable graphite (EG) is another typical of IFR; it is a flake graphite intercalation compound. It is a layered crystal consisting of sheets of carbon atoms tightly bound by covalent bonds to each other in the same plane. When exposed to heat source, EG will expand and generate a voluminous insulative layer on the surface of the polymer matrix, thus providing flame retardant properties. In addition, EG has the properties of non-toxic, smoke less, excellent thermal insulation, anticorrosion and EG added into the polymer does not affect the physical properties of the polymer.In this work, EG was added to the PP/TPU blends by melt blending. Maleic anhydride grafted PP (PP-g-MAH) was added as a compatibilizer of imissicble PP/TPU blends. The effect of two different particle sizes of EG on thermal degradation behavior of PP/TPU/EG composites was studied. The surface modification and synergistic flame retardant effects and mechanism on PP/TPU/EG composites were also studied. The main results are as follows:(1) The thermal stability, fire behavior and char morphologies of PP/TPU containing two different particle sizes of EG were investigated by TGA, cone calorimeter tests, UL-94, carbonation tests and morphology characterization. The results showed that the thermal stability and combustion behavior were improved with the EG addition to PP/TPU. In addition, with decreasing the size of EG, the thermal stability and combustion behavior enhanced. At the same time, the mechanical properties of PP/TPU/EG composites decreased with increasing the size of EG.(2) The thermal stability, flammability and mechanical properties of PP/TPU composites filled with untreated and treated EG were studied. Increases in the thermal stability were achieved with EG and the HRR, PHRR, THR, TSR values and the emission of CO of the PP/TPU/EG composites decreased significantly because of the presence of flamme retardant. Compared with unmodified EG, suface modification of EG could improve the dispresibility and compatibility of EG in PP/TPU composites, therefore PP/TPU/treated EG composites showed better mechanical properties and flame retardancy.(3) In this part, we focused on studying the flammability behaviors, thermal stability. and the mechanical properties of EG in halogen-free flame retardant PP/TPU composites. The flame retardant performances were enhanced with the increase of the flame retardant content, indicating the increase of LOI values. The mechanical properties of PP/TPU/EG composites declined with increasing the content of EG. (4) Red phosphorus masterbatch (RPM) wae used as synergistic flame retardants for PP/TPU/EG composites. The synergistic effects of PP/TPU/EG composite were studied by LOI, CCT, TGA and mechanical properties tests. The results showed that RPM was an excellent synergistic flame retardant for PP/TPU/EG composites, the combustion behavior and thermal stability of PP/TPU/EG/RPM composites was significantly improved.(5) In this part, melamine coated ammonium polyphosphate (MAPP) was used as synergistic flame retardants for PP/TPU flame retardant composites. The synergistic effects were studied by LOI, CCT and mechanical properties tests. The results showed that the fire behaviors of PP/TPU composites, especially the smoke behaviors were improved and the mechanical properties were enhanced when MAPP was used to partly replace the RPM. |
PDF Full Text Request