Abstract
The cage and ladder structured phosphorus-containing polyhedral oligomeric silsesquioxanes (DOPO-POSS) have been synthesized through the hydrolytic condensation of 9,10-dihydro-9-oxa-10-phosphenanthrene-10-oxide (DOPO)-vinyl triethoxysilane (VTES). The unique ladder and cage–ladder structured components in DOPO-POSS endowed it with good solubility in vinyl epoxy resin (VE), and it was used with tetrabutyl titanate (TBT) to construct a phosphorus-silicon-titanium synergy system for the flame retardation of VE. Thermal stabilities, mechanical properties, and flame retardancy of the resultant VE composites were investigated by thermal gravimetric analysis (TGA), dynamic mechanical analysis (DMA), three-point bending tests, limiting oxygen index (LOI) measurement, and cone calorimetry. The experimental results showed that with the addition of only 4 wt% DOPO-POSS and 0.5 wt% TBT, the limiting oxygen index value (LOI) increased from 19.5 of pure VE to 24.2. With the addition of DOPO-POSS and TBT, the peak heat release rate (PHRR), total heat release (THR), smoke production rate (SPR), and total smoke production (TSP) were decreased significantly compared to VE-0. In addition, the VE composites showed improved thermal stabilities and mechanical properties comparable to that of the VE-0. The investigations on pyrolysis volatiles of cured VE further revealed that DOPO-POSS and TBT exerted flame retardant effects in gas phase. The results of char residue of the VE composites by SEM and XPS showed that TBT and DOPO-POSS can accelerate the char formation during the combustion, forming an interior char layer with the honeycomb cavity structure and dense exterior char layer, making the char strong with the formation of Si-O-Ti and Ti-O-P structures.
Highlights
As shown in Scheme 1, Firstly, DOPO-vinyl triethoxysilane (VTES) was synthesized through the addition reaction between P-H groups of DOPO and C=C groups of VTES [32], DOPO-Polyhedral oligomeric silsesquioxanes (POSS) was synthesized from the hydrolytic condensation reaction of DOPO-VTES
When 4 wt% DOPO-POSS and 0.5 wt% tetrabutyl titanate (TBT) were incorporated into vinyl epoxy resin (VE), the limiting oxygen index (LOI) value reached 24.2, which was improved by 24.1% compared with that of the pure VE
The results revealed that TBT played a key role in increasing the LOI value of the vinyl epoxy resin [38,39,40], and phosphorus, silicon, and titanium showed a good synergistic effect in the flame retardancy of VE composites
Summary
DOPO and its derivatives are widely used to improve the flame retardancy of epoxy resins due to their high thermal stability and flame-retardant efficiency [11,12,13,14,15,16,17,18]. It has been reported that by adding both POSS and DOPO into epoxy resins, the flame retardant efficiency was increased significantly due to the synergistic effect between phosphorus and silicon [24,25,26]. We have synthesized ladder and cage structured phosphorus-containing polyhedral oligomeric silsesquioxanes and named them DOPO-POSS, DOPO-POSS were used as co-additive with TBT to construct multi-component synergistic flame retardant system containing phosphorus, silicon and titanium. The effects of DOPO-POSS and TBT on the thermal stabilities, mechanical properties, and flame retardancy of the VE composites were investigated. The flame-retardant mechanism of the prepared VE composites was revealed
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