I should also point out that there are plastics that are even harder than polyurethane to burn safely, namely, anything containing chlorine or fluorine. The common ones are polytetrafluoroethylene (Teflon) and poly(vinyl chloride) ("vinyl"). These, regardless of how you burn them, are going to produce compounds of fluorine and chlorine, typically gaseous hydrofluoric and hydrochloric acids, which will EAT YOUR FUCKING LUNGS. HF will also poison the fuck out of you. On the plus side, these gases are light and will disperse rapidly if they're not confined.