Abstract
Silicon compounds containing R3SiO1/2 (M-unit), R2SiO (D-unit), RSiO3/2 (T-unit), and SiO2 (Q-unit) in various combinations are commonly used as flame-retardant auxiliaries for polycarbonate (PC) plastics, and the use of branched DT-type silicones as the main flame retardant has also gained interest. We have found, based on cone calorimetry, that the flame retardant effect of such silicones is determined primarily by silicone mobility during combustion and the silicon content in the plastic, and conversely that the aromatic units and branched structure of the silicone are of little importance. The flame retardant effect of linear D-type silicone, in particular, was found to be superior to that of DT-type as well as other silicones, apparently because of its relatively simple molecular structure and a resulting high mobility in the plastic under combustion. We found that D-type silicones increase PC resin recyclability, whereas the branched structure of DT-type silicones tends to lower recyclability. Our investigation indicates that the linear D-type silicones cause little or no PC decomposition during molding, in contrast to the PC decomposition encountered with DT-type silicones due to interaction with their numerous chain ends. The results indicate that the excellent flame retardance, high impact resistance, and superior moldability and recyclability of PC resin containing linear D-type silicone will make it suitable for applications in office machines, computers, and other equipment. Furthermore, the most important and essential fact is that silicon plays an important role of flame retardance, not silicone.
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