Furan resin refers to polymers produced from various furan compounds, of which the most common starting materials are furfuryl alcohol and furfural. In the resin and in the cured polyfurfurol, the furan rings are not connected by conjugation. The resins are generally used as binders for sand castings. The furan monomer is typically converted to a free-flowing resin with mild acid catalysis. Curing is achieved using strong acid.