This product is custom manufactured to the ISO specification. (Many items currently available are inferior/non-conforming off-the-shelf foams of the polyester-urethane type, which have different hardness and chemical properties.) To ensure improved shelf life and reduce yellowing through exposure to UV radiation in daylight, it is heat-sealed in a black UV-stable polythene pack. Available in packs of 25 sheets and 1,000 38 mm cut discs.