Synthetic resins are resins produced by polymerization, polyaddition or polycondensation, with possible modification through natural substances (fats, oils, natural resins and similar). Synthetic resin is also understood to be natural resin altered by chemical reaction (esterification, saponification, etc.). Unlike natural resins, many synthetic resins can be converted by cross-linking into thermoset materials. Phenolic resins are generally used as impregnation for porous face materials.