A curable composition comprises: a) 91 to 98.2 weight percent of: (i) at least one polymerizable compound containing at least one carbamylene group; or (ii) at least one polyurethane precursor system; and b) 0.2 to 9 weight percent of alpha alumina particles having a particle size distribution with a Dv50 of from 0.1 to 1 micron, wherein the weight percentages of a) and b) are based upon the total amount of a) and b). Cured compositions and their use in thermoforming are also disclosed.