Typical examples of classes of materials which are useful as the void-generating agent in the process of this invention include ketones such as acetone or methyl ethyl ketone, hydrocarbons such as benzene or toluene, ethers such as tetrahydrofuran, alcohols such as methanol or isopropanol, halogenated alkanes such as ethylene dichloride or propylene dichloride, esters such as ethyl acetate or buty