Structures And Properties Of Expandable Graphite Filled Polypropylene

Polypropylene (PP) is always the most important products of polymer materials, but the low limiting oxygen index, high burning rate, high quantity of heat release, and easy causing of fire, which limit the application range. Although many researches have been done on flame retardants filled PP, but introducing of retardants greatly affects the mechanical property of PP. In this paper, a new kind of polymer composites is obtained by adding a new kind of retardant-Expandable Graphite (EG). Combustion performance, mechanical property, melting and crystallization has been studied. The main conclusions were listed as followings.EG which intercalated by sulfur acid and nitrate acid has a similar crystal structure with graphite. The main force between the carbon atoms is covalent bond in each layer while between the layers is van der waals force. There are many voids between layers in EG after expansion. After treated in high speed mixer, mechanical force makes the average diameter and average area of EG become small and layers decrease.EG/PP composites made by compression has a better combustion performance than by injection molding. The reason summaries as follows: As a flame retardant, EG after combustion acts as a physical obstruct on the surface of the matrix and separated oxygen and PP. It is found that the composites are strongly affected by the integrity of the structure of EG which can be easily damaged by shear stress. Compare to the injection molding, there is less shear force in compression processing. So combustion property of compression molded EG/PP composites is better.With increase of EG, Izod impact strength of PP/EG composites first increase, then decrease and reaches a maxim when the content of filler is 5 percent. Tensile strength decreases while Yang's modulus increases. Annealed sample presents a great increase of impact strength.Lamellar thickness and uniformity improves when adding EG into PP. As a nucleate agent, EG has many positive effect; it makes PP from homogenous nucleation to heterogeneous nucleation, it shorten the crystal time and size of a crystal. After annealing, PP and PP/EG composites have a better crystal structure.βcrystal is induced by EG and found in core section of impact bars.βcrystal disappeared when annealing in Differential Scanning Calorimeter(DSC) due to high degree of supercooling. The great increase of impact strength after annealing is not caused by increase of quantity ofβcrystal, may be due to elimination of innerstress and improve of crystal structure.