Blowing-out effect in epoxy composites flame retarded by DOPO-POSS and its correlation with amide curing agents

A novel polyhedral oligomeric silsesquioxane containing 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-POSS) has been used to flame retard DGEBA (diglycidyl ether of bisphenol A) epoxy resins cured by two amides, namely the aliphatic oligomeric polyamide 650 (PA650) and the aromatic 4,4′-diaminodiphenylsulphone (DDS). The epoxy composites with DOPO-POSS showed different flame retardant properties depending on the amide used. The results of UL-94 tests show that the DEGBA/DDS with DOPO-POSS exhibits a blowing-out effect through vigorous emission of pyrolytic gases, but the DEGBA/PA650 does not. Moreover, only 2.5 wt.% DOPO-POSS imparts to the epoxy resin DEGBA/DDS a LOI value of 27.1% and UL-94 V-1 rating. In contrast, 10 wt.% DOPO-POSS in the DEGBA/PA650 results in a LOI value of 25.9% and a UL-94 V-1 rating. The details of fire behaviour, such as the values of TTI, HRR, p-HRR, COPR, and CO ₂PR have been tested using a cone calorimeter. DOPO-POSS in the DEGBA/DDS causes a lower value of p-HRR and longer TTI than in the DEGBA/PA650. The DEGBA/DDS with even as little as 2.5 wt.% DOPO-POSS easily forms a compact char. However, the DEGBA/PA650 with DOPO-POSS does not char until 10 wt.% DOPO-POSS has been added. The thermal stability and pyrolytic gases of the two kinds of epoxy resins were assessed by TGA-FTIR under a nitrogen atmosphere. DOPO-POSS performs better in accelerating charring in the DDS curing system compared with the PA650 curing system. It is postulated that for the DEGBA/DDS/DOPO-POSS, fast and dense charring and accumulating of pyrolytic gases in the char contribute to the blowing-out effect. By contrast, the aliphatic chain of the PA650 is easy to break down and produce combustible gases, so does not easily form a crosslinked structure in the condensed phase until enough DOPO-POSS has been added. These results may be very helpful for investigation of the conditions under which the blowing-out effect in epoxy resins can be caused by synergy of phosphorous (DOPO) and silicon (POSS).