Unfortunately ACID FREE has become a somewhat misunderstood term and, actually, is misleading. The proper term is "Acid Neutral". An ingredient called calcium carbonate (the main ingredient in Maalox) is added to the mat materials. This is a good thing and makes better mats just like Maalox makes my stomach better. It neutralizes the natural acidity in standard core mats. When measured with a PH meter, these mats measure "Acid neutral". A good thing.
Some big bad marketing genius decided that "Acid Free" sounded better than "acid neutral". But, this is where the misleading part comes, it is not permanent. It helps to retard the return of acidity it does not cure it.
So, a good thing (better mats) got turned into a misleading thing (They are cured!, They are cured! And it's free!).