For any type of polyurethane foam to be considered visco-elastic, it has to have a few characteristics. It must be sensitive to temperature and pressure and it must be able to return to its original form once pressure and temperature has been removed. Once it is determined that a polyurethane foam as visco-elastic it is considered memory foam and it's graded based on its nature and durability using four different grading methods.

The ILD Rating

ILD stands for Indentation Load Deflection and is a direct measurement of how hard or how soft the memory foam is. A 20% ILD rating is the number of pounds that are required for a 3 inch thick foam mattress with an indentation of 40 square inches to achieve a 20% compression. For instance, if a memory foam mattress has an ILD 20 pound rating it would indicate that the mattress would need 10 pounds of pressure to indent the mattress 10%. The bottom line is that the higher the Indentation Load Deflection, the more firm the memory foam mattress is.

Resilience

This is an interesting measurement. Resilience measures the foams springiness. It's actually measured by dropping a small, steel ball from 36 inches above the mattress and then figuring out the percent of rebound the steel ball has when it bounces off the foam. If the memory foam mattress give a high percent rebound and that mattress will be determined to have a high resilience (so it will be classified as HR foam). The basic rule of foam is the higher the resiliency of the mattress the stronger and more durable the foam will be when exposed to pressure.

The weight of the foam is measured by its density in pounds per square foot. The actual weight of the foam can be determined by the number and amount of chemicals that are used in the polyurethane foam's composition. The higher number of chemicals that are used, the higher the density and visco-elasticity of the foam will be. Density is not determined by the thickness or the hardness of the foam. The thickness and the hardness of the foam is measured with the ILD rating. Most mattresses on the market that are memory foam use a density of 5 or 4 pounds but you can find some that have densities of 2 and 3 pounds.

Tensile Strength

Tensile strength is used to indicate the exact extent to which memory foam can be pulled and stretched until it tears or ruptures. The measurement is determined in pounds per square inch and a percentage of elongation before it tears. This is the least relevant measurement of all for measurements because memory foam mattresses are not often stretched or elongated.

An important fact to remember is that the ratings used for memory foam mattresses can vary greatly between different batches of foam. Also, because of this variation, manufacturers are often unable to determine an exact rating on their final products. For example, a memory foam mattress with a 20 ILD rating could vary from an 18 to a 22 ILD rating for each batch of foam that is created.