Export a Textures Alpha Channel as a Multipass?

Hi there

I’ve modelled a book in c4d. I want the text to disappear as the scene goes on. To do this I’ve used two materials on the page: the first one is the material for the actual paper. The second is the material for the text with an animated alpha channel created in AE.

To be able to add some effects to the disappearing text in AE I would like to export this materials alpha channel as a separate pass but I haven’t found the way to do it yet.

Any ideas?

Cheers

Steve