Epoxy resin is a class of thermosetting polymers made from monomers that contain at least two epoxide groups. Epoxy resins are usually made up of four ingredients: the monomeric resin, a hardener, an accelerator, and a plasticizer. The most common epoxy resins are produced from a reaction between epichlorohydrin (ECH) and bisphenol-A (BPA), though the latter may be replaced by other raw materials such as aliphatic glycols, phenol, and o-cresol novolacs to produce specialty resins. The epoxy resins can be obtained in either liquid or solid states. The two processes are similar. Firstly ECH and BPA are charged into a reactor. A solution of 20-40% caustic soda is added to the reaction vessel as the solution is brought to the boiling point. After the evaporation of unreacted ECH, the two phases are separated by adding an inert solvent such as methylisobutylketone (MIBK). The resin is then washed with water, and the solvent is removed by vacuum distillation. The resin itself is made of bisphenol and epichlorohydrin, and the most common type of bisphenol is a combination of acetone and phenol. Epoxy resins are polymeric or semi-polymeric materials or an oligomer, and as such rarely exist as pure substances, since variable chain length results from the polymerization reaction used to produce them. High purity grades can be produced for certain applications, e.g. using a distillation purification process.