Memory Foam Mattresses
NASA’s Ames Research Center funded a development project in the early 70′s designed to create a substances that would relieve astronauts of the g-forces they experience during lift-off. They found that reducing g-forces on the body was aided by using a material that conforms to a person’s body. Mold could accomplish this but any movement would create incorrect pressure points. To remedy this, they created a foam material that was viscoelastic – able to conform to a shape, but come back to an original shape once pressure was removed. Fagerdala World Foams of Sweden, which at the time was one of Europe ‘s largest foam producers, perfected this type of foam for consumer use in the 80’s.
The Swedish Mattress company, Tempur-Pedic®, introduced this material in a mattress form to the people of Sweden in 1991. In 1992, Tempur-Pedic® launched the products in North America. With the success of Tempur-Pedic®, other foam manufacturers developed their own viscoelastic memory foam materials flooding the market with a variety of options and price levels. With this we also saw the use of viscoelastic foam in products other than mattresses.
Why does viscoelastic foam have memory?
Take synthetic polyurethane foam, add certain chemicals to add an increase density. As you can imagine, there are many combinations of foam and chemicals that will determine what type of memory foam you end up with. To understand the differences let’s start with understanding how foam is graded.
How Foam is Graded?
For a material to be considered viscoelastic, it has to be temperature sensitive and have an ability to return to its normal shape relatively quickly. Memory foam is graded by its viscoelastic properties and its durability in the following grading system:
Weight (density in pounds per square foot):
The weight of a foam is determined by the quality of the foam and chemicals used in the composition of the foam. The density measurement is not a reflection of the hardness or stiffness, which is determined by the ILD rating below. You will see memory foam mattress range from 4-5lb densities to as low as 2-3lb densities.
ILD Rating (Indentation Load Deflection):
The ILD rating tells you how hard or soft the material is. The 25% ILD rating is the number of pounds required to compress a 4” sample of foam by 25% using a 50 square inch indentation. The higher the ILD, the firmer the foam. It is important to note that the ILD rating can vary from lot to lot due to the chemical reactions that take place. So an 8 ILD rated foam can vary from 6 – 10 ILD by lot.
This measures the foam’s springiness by calculating the percent rebound of a steel ball dropped from a height of 36″. The term “H.R” foam refers to highly resilient foam with a high “ball rebound.” In general, a foam with a higher resilience will be more durable to the forces of compression.
This indicates the how much the foam can be stretched, measured in pounds per square inch, and how much elongation in percent of stretch before the foam breaks. We believe this value has little relevance to mattresses because they are rarely stretched when slept on.
Memory or Latex?
In general, latex foam has a higher elasticity or resilience than memory foam. The result is that latex will feel springy opposed to memory foam, which will feel solid. Memory foam does not feel like it is pushing up against your body. Many mattress companies are blending both and seeing good results. Sometimes allergies to latex can be triggered, but this is usually due to skin contact, and does not normally occur with the latex blends that are being used today. It is thought that proteins that trigger allergies are washed away in many of the latexes used today. There are no chemicals used in latex composition, so look at the ILD rating to determine the feel of the product.