On patches professionally made the back is sealed with a glue?
This is going to depend on the size of the patch as to if glue is used. Now glue could still be used, but again depending on size, may want to also stitch those patches on as well.
This is very thick and heavy. Is this just heat seal off a roll or something poured or brushed on?
What I have done is used a roll that would have this adhesive on that was double sided. Treat it before we would cut the patches to size and do the merrow stitch and ship. Most of the spray on adhesives are for temp use (and typically in the production of patches not the final application, treating it as if it was applique)), could there be a product out there for otherwise that I'm unaware of... sure, but I would be leary with any spray on product as I would be worried about the consistency of the application (and this would apply to pour on or brushed on(but I know of no such product for any of that type of applying with regard to patch production (personally, I firmly believe in stitching the patches on, but that's me))).