For a material to be visco-elastic, it has to be temperature sensitive and have an ability to rebound fairly quickly and return to its normal shape. Memory foam is graded by its visco-elastic nature and its durability by breaking it down into the following grading system:
1. Weight (Density in pounds per square foot):
The weight of a foam is determined by the amount of chemicals used in the composition of the polyurethane foam. The more chemicals, the higher the density and the more visco-elastic it will become. This density will not determine the hardness of the material; this is done through utilizing the ILD rating structure. Many memory foam mattresses use a 4 and 5-lb. density, but some use as low as 1, 2 and 3-lb. densities.
2. ILD Rating (Indentation Load Deflection):
The ILD rating is going to tell you how hard or soft a material is. The 25% ILD rating is the number of pounds required to achieve a 25% compression of a 4″ thick foam using a 50 square inch indentation. An example of this is as follows: 20-ILD foam indicates that this material took 20 lbs. of pressure to indent this foam 25%. Keep in mind that the higher the ILD, the firmer the foam. This rating is synonymous with the abbreviation IFD (Indentation Force Deflection). IFD is still in use, even though ILD was created just for the purposes of grading polyurethane foam. You can assume that a lower ILD for the top layer of your memory foam mattress or bed topper can be a real advantage in pressure point reduction. However, too soft of an ILD or too thick of a very soft surface layer, especially without enough support underneath, can sometimes be problematic for low back sufferers. That is why the best memory foam mattresses use a layered effect that properly offers a supple enough surface to disburse pressure points, but gradually firms up in the layers below, with a very resilient support base used in the final layer.
This measures the foam’s springiness by determining the percent rebound of a steel ball dropped from a height of 36″. The term “H.R” (highly resilient) foam refers to a highly resilient foam that will give a very high “ball rebound” reading. In general, the higher the resiliency, the better and more durable the foam will be with compression forces. However, with viscoelastic memory foam, less resilience indicates a better force dampening. This is due to less rebound pressure fighting the force of your body as it sinks into the mattress. Very high quality memory foam mattresses combine lower high resilient layer(s) with memory foam on the surface, so you get the pressure point reduction, while retaining the extra support below.
4. Tensile Strength:
This indicates the extent to which foam can be stretched, measured in pounds per square inch, and how much elongation in terms of percent of stretch before rupture. This value has little relevance to memory foam mattresses because they are not usually stretched, and will become compressed only when slept on.
* It is important to remember that these ratings can vary from each foam batch that is poured and that no mattress company can predict an exact rating on their mattress each time they produce the final product. As an example, a 10-ILD rated foam could vary from 8 to 12-ILD each time the foam is poured.