"Non-combustible" or "self-extinguishing" might be true if you try to burn an individual piece of that plastic, but an entire mass of it can be a different story.
Agree. The "self-extinguishing" material rating is misunderstood by many and would appear so in this case in the decision to store probably what was UL-94 rated plastic material under the bridge.
The problematic part of that rating is that the self extinguishing test is in part a heat transfer issue.. A single specified piece of material suspended per the test may well receive the rating, but that same material in a confined area, with reflected heat, other ongoing ignition source, or otherwise different heat transfer/escape characteristics means it may burn rather violently.
Learned this in an evaluation of an issue in a fairly large digital system using an extensive amount of ABS UL94 rated plastics. The plastics first melt into a black tar like residue (plastics being a petroleum derivative) and then ignites and burns with the ferocity of a petroleum fire. The materials engineer in this investigation mentioned that the bromine and other compounds added to plastics for the flame retardant rating yields considerable chlorine gas when burned.
The byproduct of burning halogenated plastics or those containg halogen fire retardants is hydrogen chloride (or hydrogen bromide, which will combine with moisture in the air or lungs to form hydrochloric acid). At high temperature, plastics will burn like gasoline and fire retardants will have a negligible effect, except for making the smoke more toxic.
