The method of claim 1, in which said additive contains a plurality of groups selected from the group consisting of the following general formulae:(1) B1 --, (2) B2 --O--, (3) R1 --, (4) R2 --Si.tbd., (5) (R3)(R4)(R5)Si--, (6) (R6)(R7)(R8)Si--O--, (7) [--Si(R9)(R10)--O--]a, and (8) [--Si(R11)(B3)--O--]b ;in which each of R1 and R2 independently is a monovalent group selected from the group consisti