Hardeners are basically liquid resins with amino nature that can react with the oxiranic ring of the epoxy resin giving a solid product with high molecular weight. The hardeners are characterized by their hydrogen equivalent (HEW), that is the weight in grams needed to have one reactive hydrogen atom. From this parameter we can calculate the so called PHR (per hundred ratio), that is the weight of hardener that we need for a stechiometric reaction with 100 grams of epoxy resin.