Investigations of epoxy resins flame-retarded by phenyl silsesquioxanes of cage and ladder structures

Cage-type octaphenyl silsesquioxane (OPS) and ladder-type polyphenyl silsesquioxane (PPSQ) have been used as flame-retardants in epoxy resins (EPs) in the presence and absence of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). The flame retardancy of these EPs have been tested by the LOI and UL-94 standard tests, and details of fire behaviors, such as TTI, HRR, p-HRR, THR, COPR, and CO2PR, have been tested using a cone calorimeter. The results have shown that OPS has distinctly different effects on the flame retardancy of EPs compared to those of PPSQ. In the UL-94 test, the flame-retarded EP with OPS showed a weak blowing-out effect, but the flame-retarded EP with PPSQ did not; further, the flame-retarded EP with DOPO/OPS showed a significant blowing-out effect, but the flame-retarded EP with DOPO/PPSQ did not. According to the cone tests, addition of PPSQ to EPs, with or without DOPO, causes higher p-HRR; on the contrary, the addition of OPS to EPs leads to lower p-HRR compared to that of the neat EP. The thermal stability of these EPs has been investigated by TGA. The morphology of the chars after the cone tests has been investigated by visual observation, SEM, and XPS. Observation of the chars suggested that OPS can assist the EP, especially the EP with DOPO, to form stronger and denser chars than PPSQ, although PPSQ with a ladder structure has higher thermal stability than that of cage-type OPS. It was also observed that the Si concentration in the interior chars from the EPs with PPSQ was higher than that in those from the EPs with OPS. It is supposed that in the composites of EP or EP/DOPO, slow charring of PPSQ cannot match the intumescent and charring process of the EPs during combustion, but OPS can. This may explain why OPS exhibits significantly different flame retardancy on the EPs compared to PPSQ.