Also, there may be used as the acrylate monomer, monomers having the formula H2 C???C(CH3)COOR' where R' is an alkyl group of 8 to 18 carbon atoms such as n-octyl methacrylate, n-dodecyl methacrylate, hexadecyl methacrylate and n-octadecyl methacrylate and the like.