Graphitic Carbon Nitride Based Hybrids/polystyrene Composites: Preparation, Combustion Properties And Flame-retardant Mechanism Investigation

As a general thermoplastic manufactured on a large scale, polystyrene (PS) is widely applied in the area of automobile, building, and electric appliance, etc., due to its outstanding properties, such as thermal resistance, low density, excellent mechanical durability and convenience of processing and molding. However, PS is highly flammable. The flammability of PS has a closed relationship with its thermal degradation behavior, and the species and chemical composition of released gas during the degradation. Furthermore, a large amount of toxic gas and smoke, together with severe melt-dropping will be evolved during combustion of PS, leading to heavy casualties and property losses. It is of great importance to perform the flame retardancy of PS. It is demonstrated that graphitic carbon nitride (g-C3N4) as a kind of two dimensional nanomatcrial can be potentially applied in the flame-retardant field of PS. Nevertheless, there are two important problems left for further investigation:(1) g-C3N4 is poorly dispersed in PS host, (2) g-C3N4 does not provide enough flame-retardant efficiency to meet requirement for practical applications when incorporated into PS matrix. Therefore, this dissertation primarily focuses on improving dispersion of g-C3N4 and enhancing thermal stability and flame retardancy of PS/g-C3N4 composites.In this dissertation, a series of g-C3N4-based flame retardants (FRs) were successfully prepared and the structures of the FRs were well characterized. These FRs were added into PS matrix through the layer by layer self-assembly (LBL), solvent mixing-precipitation and melt blending processes. Alternate deposition of g-C3N4 and multiwalled carbon nanotube (MWCNT) onto surface of PS led to significant improvements in the thermal stability, flame retardancy and total pyrolysis gaseous products suppression. Then, the synergistic effect was employed to improve the distribution of additives and thermal, flame-retardant and smoke suppressed properties of the polymer matrix. In order to further improve flame retardancy of PS/g-C3N4 system, the microencapsulation technology was adopted. Finally, combining the condensed phase mechanism with the gas phase mechanism could provide flame-retardant PS composites to meet the practical requirement.1. Significant improvements in thermal and flame retardant properties of polymeric materials at low loadings hold tremendous promise for fire safety materials. A highly effective g-C3N4/MWCNT bilayer was deposited on PS sphere for reducing its fire hazards. The PS sphere allowed the intimate assembly of the g-C3N4/MWCNT bilayer on its surface through electrostatic interactions. Morphology characterization indicated that the ultrathin g-C3N4 nanosheets were obtained by a simple isopropanol-assisted exfoliation approach. Structure and morphology analysis demonstrated the successfully alternative decoration of g-C3N4 and MWCNT onto the surface of PS host. Thermogravimetric analysis-Fourier infrared spectra results showed that the thermal stability of PS was dramatically improved, including increase of 15.5,20.0℃ and 6.5 wt% in the initial decomposition temperature (T-10), the temperature at maximum decomposition rate (T-max) and char residue, respectively after assembling of the g-C3N4/MWCNT instead of g-C3N4 or MWCNT solely. Moreover, the generation of total gaseous products was distinctly inhibited by the g-C3N4/MWCNT bilayers. The results obtained from microscale combustion calorimeter indicated that the ternary assembled systems showed the significant improvements in flame retardancy, i.e., peak of heat release rate (pHRR) and total heat release (THR) decreased by around 45% and 47%, respectively. These enhancements were attributed to the building of the tight barriers of the g-C3N4/MWCNT bilayers and the chemical interactions between the two components.2. The g-C3N4 nanosheets are endowed with extraordinary chemical and thermal stability, and good optical and photoelectrochemical properties and expected to use in a wide range of fields. The direct dispersion of hydrophobic g-C3N4 nanosheets in water or organic solvents without the assistance of dispersing agents was considered to be a great challenge. Here we reported novel g-C3N4/organic functionalized montmorillonite (OMMT) nanohybrids, which were synthesized through electrostatic interaction and then introduced into PS matrix to fabricate nanocomposites by a simple solvent blending-precipitation method. Hybridizing the g-C3N4 with OMMT could easily form stable aqueous colloids through electrostatic stabilization. These nanohybrids were evenly dispersed in PS and showed strong interfacial interactions with the polymer matrix. It was noted that the concentration of total gaseous products was dramatically reduced by combination of g-C3N4 and OMMT. Moreover, flame retardancy was improved upon incorporation of the nanohybrids into PS host. These improvements were due to the strong interactions at interface of ternary systems, synergism between g-C3N4 and OMMT, and physical barrier effect of the two components.3. Three kinds of g-C3N4/organic aluminum hypophosphites (g-C3N4/OAHPi) hybrids, i.e., CPDCPAHPi, CBPODAHPi and CDAHPi, were synthesized by esterification and salification reactions, and then incorporated into PS matrix to prepare composites through melt blending method. Structure and morphology characterization indicated the successful synthesis of PDCPAHPi, BPODAHPi, DAHPi and their hybrids. The g-C3N4 could protect OAHPi from external heat and thus improve thermal stability of OAHPi. These additives showed enhanced interfacial interaction with PS. Moreover, introduction of the hybrids led to reduction in pHRR, THR and smoke production. These properties improvements could be attributed to the gas phase mechanism and physical barrier effect: phosphorus-containing compounds generated from OAHPi effectively captured free-radicals evolved from PS; on the other hand, as well as its barrier effect, g-C3N4 retarded the disappearance of these free-radical scavengers. Therefore, g-C3N4/OAHPi hybrids will provide a potential strategy to reduce the fire hazards of PS.4. A series of g-CsN4 wrapped ammonium polyphosphate (APP) (CNAPP) were firstly prepared, and then incorporated into PS. The results indicated the successful wrapping of APP by g-C3N4. The CNAPP exhibited the higher thermal stability than pure APP. Enhanced interfacial interactions between CNAPP and PS were obtained upon introduction of CNAPP. In the case of PS/CNAPP composites, the thermal stability was significantly improved, compared with that of PS containing an equal amount of APP. Moreover, cone calorimeter results showed that the values of pHRR and THR were reduced greatly for PS/CNAPP20. It was confirmed that the formation of P-O-C and P-N-C structures could remarkably improve the stability of char layer and thus result in the better flame retardancy of CNAPP, besides the enhanced thermal stability.5. PS/CNAPP20/OAHPi composites with different loading level and proportions of CNAPP20 to OAHPi were fabricated by the melt compounding method. The results indicated that the additives had good interfacial interaction with PS matrix. Under nitrogen condition, both T-10 and T-max of PS composites were decreased while their residual yields were increased, compared with those of pure PS. Furthermore, the T-10 was reduced, and char residues increased with increasing the weight ratio of DAHPi to CNAPP20. It was found that the thermal oxidation led to an increase in char residues of composites. In comparison with DAHPi, PDCPAHPi showed the increased T-10 in PS composites at same loading level in inert atmosphere. It was noted that only the PS-1 achieved the UL 94 V-0 rating, and value of limited oxygen index was also the highest among all the samples. Moreover, the pHRR and THR of all the composites were reduced as compared to those of virgin PS. For instance, the 77.5% reduction in pHRR was obtained for PS-1. Combining CNAPP20 with DAHPi induced the formation of a cohesive and uniform carbonaceous residue. However, some holes or cracks occurred in the external char residues as the content of fillers decreased. It was noted that the sphere-like particles mainly formed in external char. In addition, the amount of carbon dioxide and smoke was reduced while the ultimate concentration of carbon monoxide was increased upon addition of CNAPP20 and DAHPi. It was concluded that the condensed phase and gas phase mechanisms contributed to enhancement in fire retardance of PS. The formed continuous and dense char residues acted as protecting layers to prevent the heat and mass permeation. On the other hand, the evolved gas could dilute the oxygen and retard the further combustion of flammable compounds. In addition, the formed free-radical scavengers resulted in terminal scission of polymeric chains.