2. Product Information and Characteristics

2.4.2. Inhibition Chemistry

When styrene monomer (M) is exposed to light and/or heat, it forms radicals (see Figure 5). These styrene radicals (R*) react either with oxygen to form peroxide radicals or with styrene to form polymer chains. In the presence of sufficient oxygen (at least 10-15 ppm), the peroxide formation is significant because this reaction (r1) is much faster (see Reference 3) compared to the polymer formation with styrene (r2). In the subsequent step if the inhibitor (TBC) content is >10 ppm, the peroxide radicals are scavenged via quinone formation (r3), whereas if no or too little inhibitor is present, the peroxide radicals react with styrene to form peroxide chains (r4). Oxygen works as the polymerisation inhibitor while TBC controls its efficient use. In the absence of TBC the peroxy-radicals continue to react with the monomer molecules until the oxygen is completely depleted. The poly(peroxides) are soluble in styrene and will not collect in pockets. Hazardous situations may occur during distillation of styrene contaminated with unstable poly(peroxides), which may trigger a runaway reaction since with temperature increase a significant number of radicals may be formed.

Note: Considering the minimum oxygen concentration to support combustion (see Figure 4), the IMO related Safety Of Life At Sea (SOLAS) Convention has defined an atmosphere with < 8%-vol. oxygen as inert gas for styrene. As a dissolved oxygen level of 3 ppm is in equilibrium with 1%-vol. oxygen in the vapour phase, the minimum requirement for the proper functioning of TBC is therefore being met with sufficient certainty.

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