Limiting oxygen indices of silicone block polymer

The limiting oxygen index (LOI) has been determined for each of a large number of silicone‐containing resins, principally dimethylsiloxane (DMS) block polymers varying widely in DMS content and hard block type. A synergistic enhancement in LOI of varying strength is seen in several families of resins. For the major families explored, the synergism varies with hard block type in roughly the following order: bisphenol‐A carbonate and bisphenol fluorenone carbonate > styrene and 2,6‐diphenyl‐1,4‐phenylene oxide > bisphenol chloral polycarbonate > methyl methacrylate and phenolphthalein carbonate, the enhancement being nil in the last case. Strength of the synergism is discussed in terms of DMS degree of dispersion, char‐forming tendencies, melt viscosity, and other factors.