Thermal paper rolls are fine and special paper coated with materials that come formulated to change color when exposed to heat. Therefore, the thermal printer head heats the POS paper roll, making it legible.
Manufacturers of POS paper rolls apply specially made and heat-sensitive materials.
However, it helps to know that the mixture and formulation of these chemicals may vary depending on the manufacturer's decision and formula.
In that regard, you may experience different qualities of thermal paper rolls available in the market today.