A memory foam mattress is constructed of visco-elastic foam that is sensitive to the slightest pressure placed upon it, and when that pressure is removed, its elasticity enables it to return to its original state. Some people choose a memory foam mattress which has the added advantage of a layer of latex to give a bouncier surface while still retaining the properties expected from a memory foam mattress. The original memory foam was developed during the 1970’s to assist astronauts against the enormous g-forces they experienced at lift-off.

The concept was developed at the Ames Research Centre at NASA, with the idea of some kind of force-protection that fitted into the contours of the astronaut’s body. Foam was considered but with movement, excess pressure would be put on the joints of an astronaut’s body, and it was this they were attempting to reduce. They were looking for something that would conform to the shape of the person and then return to its original shape once movement altered the original position. From this the scientists were able to construct a polyurethane foam material which had visco-elastic properties, based on the addition of chemicals to give the foam more body and density.

Later, during the 1980s, in Sweden, Fagerdala World Foams started to look at the possibilities for developing its use for the consumer market. The result was the Tempur-Pedic, which came on the market in 1991 and became popular in Sweden. A year later, Tempur-Pedic launched the product in the USA and Canada, with similar acclaim.

Many mattresses are being sold as memory foam but are not necessarily the quality product you would expect if you are using the Tempur-Pedic as a guideline. A Tempur-Pedic is constructed of two layers and is a full eight inches thick. The top layer has an ILD rating of 14, with a visco-elasticity of 5.3lb weight. The layer below is a non-viscous layer, not containing any visco-elastic, but constructed of high density polyurethane which is five inches thick and weighs 3.5lb. The whole mattress is covered by a good quality plastic cover and a cotton material covers that.

Grading

Foam is graded in different ways according to various factors:

• Weight in terms of density – lbs/sqft
• ILD Rating – indentation load deflection
• Resilience
• Tensile Strength – based on yield strength, ultimate strength, maximum stress and breaking strength

Weight

When polyurethane foam is manufactured, different quantities of chemical are used. When more chemicals are used the density increases and this also increases the degree of visco-elasticity. The density of the foam, however, does affect how hard or soft the foam is.

ILD Rating

Basically, the firmest kind of foam is achieved by manufacturing to the higher ILD rating. An ILD rating is the measurement used to determine whether a material is hard or soft. This is measured in lbs/per % compression. A 4 inch foam thickness which has a 25% rating can achieve 25% compression of an area of indentation of 50 square inches. In ILD terms, 20lbs of pressure was required to create an indentation of 25% in this foam.

Resilience

Foam is made to be springy, and to determine how springy the foam is, a steel ball is dropped from 36 inches and the amount it rebounds is measured in percentages. Foam which is designated with ‘HR’ is highly resilient. This is important in terms of mattresses as a higher resilience will also provide much more durability.

Tensile Strength

This is measured in lbs/square inch and shows how much foam can be stretched before it breaks.

All the above factors in the right proportions go into making the perfect mattress and the one thing that sets apart memory foam are the much appreciated visco-elastic properties.