In your question, you stated," between hydroxylated Hollow glass microspheres and natural fibres."  Does this mean that you are interested in forming bonds between two solids?  If so, the chances are low because they do not form good contacts unless the filler content is close to close packing, which causes processing nightmare.  If you want, however, form bonds between a polymer matrix, such as epoxy and hollow glass microspheres as well as natural fibers, a glycidoxypropyltrimethoxysilane (epoxy-functional silane) would be good.  The hydrolyzed methoxy will form bonds with glass spheres as well as the fibers and epoxy functional group will form covalent bond with the matrix.  

In your question, you stated," between hydroxylated Hollow glass microspheres and natural fibres."  Does this mean that you are interested in forming bonds between two solids?  If so, the chances are low because they do not form good contacts unless the filler content is close to close packing, which causes processing nightmare.  If you want, however, form bonds between a polymer matrix, such as epoxy and hollow glass microspheres as well as natural fibers, a glycidoxypropyltrimethoxysilane (epoxy-functional silane) would be good.  The hydrolyzed methoxy will form bonds with glass spheres as well as the fibers and epoxy functional group will form covalent bond with the matrix.