Announced today, starting with Kaladesh cards printed for Japanese language cards will use a new, 100% recyclable cardstock.

From the release:

Additionally, people may notice an additional stiffness in the cardstock, and the very observant might note that the cards will fluoresce differently under UV light than previous Japanese printings.

These changes were made in concert with our Japanese printing partners to continually upgrade the quality of our printing process, including our paper. We believe these differences—particularly the recyclable nature of the cards and the sustainability of the paper—are significant steps forward in our printing process.

While we believe most Magic players may not notice these differences, if players are found to be intentionally exploiting these differences for advantage during a tournament, such players may be subject to tournament penalties based on the discretion of the tournament judge.

Magic continues to make strides towards sustainability. The cardboard booster packs from Modern Masters 2015 Edition were not well received. It remains to be seen what reactions players will have to the new feel of their cards.