Flame Retardancy Of PA6, Polycarbonate/Thermally Stable Montmorillonite Nanocomposites

Research on polymer/clay nanocomposites have been carried out for more than twenty years and this type of materials showed enhanced mechanical properties over conventional composites. However, since the commercial modifiers are quaternary ammoniums, which possess limited thermal stability, it restricts heavily on process and mechanical properties of most engineering plastics. Furthermore, the flame retardancy property of these materials is also negatively affected by the limited thermal stability of quaternary ammonium salts. Generally, flame retardancy of polymer nanocomposites is fulfilled by the incorporation of conventional flame retardants. Among those, halogen-containing flame retardants tend to be replaced due to their polluting products during combustion. Phosphorus-containing and metal hydroxides have their problems in application, as well. Nitrogen-containing flame retardants are more environmentally benign, thus are a type of promising option in research and industry. Therefore, there are two main purposes in this study. One is replace conventional quaternary ammonium salts with more thermally stable modifiers; the other is try to employ nitrogen-containing flame retardants to blend with polymer matrix and study their fire response.A thermally stable benzimidazolium surfactant containing a benzene ring fused to imidazole was used in the current investigation instead of the conventional quaternary ammonium salts to modify silicate layers (montmorillonite) and address the issue of thermal stability.In the current investigation, multi-wall carbon nanotube (CNT) was used as an additional nanofiller to layered silicates to improve the packing density and homogeneity of the carbonaceous-silicate barrier. Nitrogenous compounds like melamine are environmentally friendly and do not add any new elements to those already present in PA 6. Thus melamine was used as a flame retardant in this study.The higher steric hindrance in the presence of benzimidazolium modified silicate layers resulted in poor dispersion in a PA6 matrix. Furthermore, this had a significant negative effect on the dispersion of CNT in the ternary nanocomposite. The additional presence of melamine also had a negative effect on the thermal stability of CNT reinforced nanocomposites. In the presence of CNT, heat release rate (HRR) and mass loss rate (MLR) were greatly reduced compared to neat PA 6; however, in the absence of CNT, the poor dispersion of B-clay resulted in only minor reductions of HRR/MLR relative to neat PA 6. The poor dispersion of B-clay in PA 6 matrix even had a negative effect on the dynamic viscosity of the melt at a test temperature of 250℃. CNT reinforced materials exhibited high viscosities due to the entangled network formation of CNT, in turn preventing vigorous bubbling and melt dripping during fire exposure tests. PA 6/B-clay/melamine nanocomposite achieved an HB rating in the horizontal burning test. This was attributed to the dripping of the low viscosity melt.In order to improve the compatibility between organoclay and polymer matrix, two one-chain benzimidazolium modifiers were synthesized. Synthesis time of these two modifiers is shorter than two-chain ones; yield rates are higher as well. Better compatibility between PA6 and organoclay with which modified was achieved and exfoliated structures were obtained due to lower steric hindrance. These two PA6/organoclay nanocomposites showed high thermal stability, PA6/B18 showed higher thermal stability than neat PA6 during the whole TGA testing temperature range. In cone calorimetric test, reduced HRR of these two nanocomposites were observed than neat PA6, but not strikingly. This indicated that conventional flame retardants still need to be added in this system.Polycarbonate (PC) blended with benzimidazolium modified pristine clay was also studied. Guanidine hydrochloride modified clay/PC was studied as a comparison. Guanidine barbiturate (GB) was chosen as a flame retardant in this system. Unfornately, the presence of GB promotes the degradation of PC due to the weak basic capacity of GB. It was found that these two organoclay resulted in agglomerated structure due to the increased steric hindrance. PC/organoclay/GB failed in twin-extruder procession due to the weak alkalinous property by our postulation. Two PC based composites showed minor HRR reduction in cone calorimetry test. Guanidine modified clay promoted char formation in this system. PC/B-clay obtained a V1 rating in UL 94 test.Generally, polymer nanocomposites show enhanced mechanical properties than neat polymers, thus mechanical and thermal behavior of PA6 based nanocomposites in this study were investigated. In DMA test, CNT and organoclay show synergism in storage modulus enhancement. One-chain benzimidazolium modified clay disperses homogeneously in polymer matrix, enhances storage modulus of nanocomposites significantly and increased glass transition temperature as well. DSC test shows that the exfoliated structure of one-chain benzimidazolium modified clay in polymer matrix promotes the formation of y crystal phase. Nanoindentation test indicates that both CNT and organoclay improve elastic modulus and hardness of the nanocomposites. On the same loading level, CNT plays a more effective role in improvement of above properties than orgaoclay.