A polyol is an alcohol that contains a plurality of hydroxyl groups, and includes diols such as the glycols (dihydric alcohols) ethylene glycol, propylene glycol and trimethylene glycol; triols such as glycerin (1,2,3-propanetriol); esters of hydroxyl containing densification agents may also be used, with mono- and di-esters of glycerin, such as monoglycerides and diglycerides, being especially de