Hi again, the glossy photo paper and printable disks are coated with different materials - polycarbonate being much more difficult to coat than paper as it has higher surface energy and a smooth surface.
A general rule is that you can’t coat a surface with a material that has a lower surface energy. Polycarbonate disks have high surface energy, so the printable surface coating has to have even higher surface energy, otherwise it won’t stick to the disk. That means the ink must be higher still to form a good film. Technically that’s difficult.
As far as I know, there have been 2 options for most printable disk media (until this recent development). The first is absorbent polymer coatings that can coat the polycarbonate disks, and swell when inkjet ink is applied, but can also have poor water resistance and lower gloss, plus long drying time.
The second type is a metal oxide coating, which is more difficult to coat onto the disks due to surface energy, is more glossy but gives poorer rub resistance.
The new disks also seem to have been developed with metal oxides - but they have made a big leap in the coating formulation to allow proper coating with the printable layer. I haven’t looked at the patents or anything, but suspect that they have used different metal oxides with “nano-particles” which has a lower surface energy, more absorbent surface.
They sound like a really good development, and I’d like to try some out myself.