Phosphate vs silane vs titanate

Silanes hydrolyse to Si–OH and condense with surface hydroxyls to form Si–O–Si or Si–O–M. They need moisture, are sensitive to humidity and self-condense in storage. Best on glass and silicate ceramics.

Titanate and zirconate coupling agents suit inorganic filler treatment and dramatically reduce viscosity, but hydrolyse easily and are unusable in waterborne systems.

Phosphate esters need no hydrolysis and react directly. Their double bond copolymerizes into the network, so they do not migrate. On metals, zirconia, alumina and calcium-containing ceramics they usually anchor more strongly than silanes.

A practical order: is the substrate siliceous (glass, quartz, whiteware)? Then silane or a silane/phosphate blend. If not (steel, aluminum, zirconia, concrete), choose phosphate. Is the system free-radical? Phosphate is the natural fit.

The boundary is non-polar plastics: PP and PE defeat all three chemistries and need a chlorinated polyolefin primer or corona treatment.