What Makes Memory Foam Have Memory?
In order for foam to have memory, it all begins with synthetic polyurethane foam material and certain types of chemicals, which add an increase in weight or density to the foam. The combination of these chemicals along with the synthetic polyurethane foam material creates a non-toxic, visco-elastic foam material. visVisco-elastic and memory foam are interchangeable, or you may even see it referred to as "viscoelastic memory foam". Although this is how memory foam is made, there are many other factors that go into what you receive as the final product. In order to understand the differences in the final product, you have to understand how to grade the foam.
1. Weight (Density in pounds per square foot):
The weight of the foam is determined by the amount of chemicals that was used in combination with the synthetic polyurethane foam material to create the visco-elastic foam. It is directly proportional to the amount of chemicals, so the more chemicals the higher the density and vice versa. However, density does not determine the hardness of the memory foam. Most memory foam mattresses are between 4 - 5 pounds density, however some use as low as 2 - 3 pounds densities.
2. ILD Rating (Indentation Load Deflection):
The ILD Rating is what you will need to look at to see the hardness of the memory foam. For instance, a 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. The ILD directly correlates with the firmness of the foam since the higher the ILD, the firmer the foam. The ILD may also be referred to as; IFD (Indentation Force Deflection) since they are synonymous. The ILD or IFD refers to how hard or soft the top layer of the memory foam mattress or bed topper is. You want to avoid getting an ILD that is too hard or too soft since, if there is no support underneath, this can flair up lower back problems especially if you are already susceptible to them. In order to get the best memory foam mattresses, you will want to confirm there is a layered effect that will properly disburse pressure points. The layers should gradually get a little firmer below the top layer so that the final layer on the bottom is the support base and is the firmest of the layers.
This is the measurement of the foam's "springiness" and is determined by dropping a dropping a steel ball from a height of 36" and measuring the percent rebound. The higher the resiliency, the more durable the foam will be with compression forces. This is where it may get a little tricky since with viscoelastic memory foam, less resilience indicates a better force dampening. To compensate for this, and to make sure you are getting a high quality memory foam mattress, you will want to see that you get one that has the higher resilient layer(s) toward the bottom combined with memory foam on the top. This way you will get the pressure point reduction while still retaining the extra support below.
This means the extent to which the memory foam was stretched. This value actually has very little bearing on memory foam mattresses since they are not usually stretched.