The Charring Process And Flame Retardant Mechanism Of HIPS/OMMT Composites

In this thesis, HIPS/OMMT nanocomposites with different contents and layerorientations of OMMT were prepared by melt intercalation. The flammability of thecomposites was evaluated by cone calorimeter (CONE). The structures of the charresidues of HIPS/OMMT nanocomposites at different burning stages were studied bythe digital camera, scanning electron microscope (SEM), X-ray diffraction(XRD),transmission electron microscope (TEM), DBP oil absorbed method and gasadsorption. Finally based on the results of experimentation, the effects of the charresidues structure (coarse-structure and fine-structure) on the flame retardancy ofHIPS/OMMT nanocomposites, charring process and flame retardant mechanisms werediscussed.The CONE results for HIPS/OMMT, HIPS/OMMT/NiO and HIPS/MMTcomposites demonstrated that the HRR for HIPS/OMMT nanocomposites wassignificantly reduced in comparison with the pure HIPS. The flammability ofHIPS/OMMT/NiO composites was further reduced compared with HIPS/OMMTcomposites, while HIPS/MMT composites did not give any significant reduction inHRR. The XRD and TEM analyses showed that galleries of OMMT were intercalatedand dispersed in the HIPS matrix, while HIPS was incompatible with MMT.Observation of structures of residues for HIPS/OMMT composites with SEM revealedthat the layer structures were divided into coarse-structure and fine-structure.Combined with interstice rate results, the research showed that residues were morecompact when the mass fraction of the OMMT increased. The structure of charresidues had a great effect on the flame retardancy of the composites.In this work, the changes of the residues structure for HIPS/OMMT composites atdifferent burning stages were discussed, and the charring process and flame retardantmechanisms of HIPS/OMMT composites were further studied. The SEM resultssuggested that the residues for the composites during burning divided into three parts:deep burning layer at the upper layer, incomplete burning layer in the middle part andunburnt layer at the bottom. Some soot aggregations were formed in the pellicle layer,also played an important role in reducing combustion efficiency and reducing flammability as a thermal shielding effect. It was observed that the net-like structure ofOMMT in the residue char was filled with nano-scale round NiO particles, and it is theparticular char structure that promoted the charring and improved flame retardantperformance of polymer layered silicate nanocomposites. Vertical orientation layerswere all found in the coarse-structure and fine-structure of the residues for theHIPS/OMMT composites with vertical layer orientation. The flame retardant ofHIPS/OMMT composites with vertical orientation was not as good as that for thecomposites with horizontal orientation. The residue structures of composites withvertical orientation showed that the MMT flocks in the PLS composites aftercombustion aggregated horizontally. This is an important observation which hardly canbe explained by currently available models.Finally combined with experiment results and theoretical analyses, a model forthe thermal deformation of MMT flocks which aggregated into a net-like structure toform char layer was put forward. The model includes the decomposition of organiccompounds connected between MMT layers, when subjected to heating; the distortionof MMT by high temperature, burning flame, upward rising bubbling in the melt bulk;aggregation under high temperature to form a net-like structure.