The epoxy resins are thermosetting polymers containing a three-atom epoxy ring in the liquid precursor. The epoxy resins are vitreous at room temperature and are mixed with thinners in order to lower the viscosity at appropriate levels to impregnate fibres. Thinners are styrene monomer and styrene liquid. The viscosity of an epoxy resin without thinner may vary a lot from liquid to solid. The epoxy resins are provided of superior physical characteristics and quicker reaction times compared to the polyester and vinylester ones but its price is higher.