Complex sentence: After this time, the DVDs become unreadable to DVD players because they contain a chemical that, after the set period of time, will display a "No Disc" message when inserted into a DVD player.
Split into simpler sentences: After this time, the DVDs become unreadable to DVD players. The discs contain a chemical, that after the set period of time, will display a "No Disc" message when inserted into a DVD player.