in stress in the region of densification. Resistance to dynamic fatigue Relating to the longevity of the material, the resistance to dynamic fatigue is tested by recursively compressing a foam and allowing it to relax. The resistance of the foam to dynamic fatigue can then be measured either by visually observing the structure of the cells to note what proportion of cell walls has broken or ruptured, or by measuring the change in physical properties like the thickness of the material. Thermal Conductivity The low thermal conductivity of latex foams is affected by four factors: heat conduction of the polymer, heat conduction of the gas within the air bubbles, convection of gas inside the cells (less important for small to medium size cells), and radiation through the foam. There are several ways conductivity can be affected through these factors: lower temperature to lower heat radiation; decrease cell size to decrease convection and radiation (due to more reflections within the cell walls); decrease foam density to decrease conduction through the solid polymer; replace air for a less conductive gas inside the cells. Energy Absorption Energy absorption is a particularly important quality of latex foam. Most energy absorption occurs in the first and second regions of the strain-stress curve. In less elastomeric polymers, the cell walls are more brittle and therefore can get crushed more easily. In this case, most of the absorption occurs in the second region of the curve caused by the deformation and crushing of cell walls. This means that each cell can only contribute once to such absorption (that is, cells are getting crushed and therefore used up). For a more elastomeric polymer, the cell walls are more flexible and can take more impact. The cell wall in this case may bend and the cell becomes squeezed, but the cell will eventually return to its original shape. Most energy absorption therefore occurs in the first region of the stress-strain graph. The foam can also handle more instances of impact as the cells do not become depleted as easily. This is a significant environmental improvement. Classification and Additives Choice of Polymer Traditional polymer choices Historically, natural rubber latex was used, and foams were produced using the Dunlop processes. Styrene-butadiene rubber latex rose to prominence once high-solids concentrates, which were designed specifically for foaming, began to be sold on the market. Properties of this polymer were fairly similar to natural