a stripline board material of hollow glass microspheres chemically bonded to a coating material and physically held in a polymeric matrix , preferably homo - or co - polymers of ethylene , propylene and / or styrene . the coating material is a difunctional silane with one functional group bound to the glass and the other bound either to the matrix or to a copolymer of a structure similar to the structure of the matrix . in the latter case , the sphere - silane - copolymer forms an intimate mixture with the matrix .