A passive cool box is basically an insulated container designed to keep the temperature inside stable. This works according to the following principles:
Insulation: The walls of a passive cool box consist of one or more layers of insulating material, such as polyurethane or polystyrene. These materials are poor heat conductors and effectively prevent heat from entering the cool box from the outside or cold from escaping from the inside.
Cold packs or ice: Before filling the cool box, place cold packs or ice inside. These serve as a source of cold and lower the temperature inside the box.
Heat exchange: The cold from the ice packs is slowly released into the food and drinks. At the same time, the heat from outside tries to penetrate the cool box. The insulation slows down this process considerably so that the contents remain cool for a longer period of time.
Phase transition: When ice melts, it absorbs heat from its surroundings. This phase transition from ice to water traps heat and helps to keep the temperature inside the cool box low.
Put simply, a passive cool box works like a thermos flask, but for larger quantities. The insulation keeps the cold in and the heat out, so your food and drinks stay fresh for longer!