It is quite possible that, in my recent emulsions, the formalin just sat there doing nothing. However,last night I did a quick search of crosslinkers for PVA. I not only found refferences to formalin and glyoxal, but to borax and even to bromine salts. Rather than argue over it, I plan to do some experements. I will compare the water solubility of films made from Kuraray silane / PVA with and without formalin and some other components of the emulsion.
Yes, silane coupling agents are toxic. That is one reason to use Kuraray product instead of pure coupling agents. But I think that, for me, it is way too late to worry about it now. I started working with organo-silanes in 1974. I am careful, but time is time and quantity is quantity.