Density is the most commonly talked about attribute of a memory foam mattress. In general, higher quality beds are made from higher density foam and shoppers look to density as a way of assessing quality in a mattress. High density memory foam beds are more expensive because they have more foam per bed, but they offer better support and more durability.
According to the definition from Wikipedia, Memory foam is made from polyurethane with additional chemicals that add to its viscosity level, thereby increasing its density. It has been gaining popularity in recent years seeing the declining manufacturing costs.
Although some manufacturers claim that the memory foam is good for everyone, it is not entirely true. First of all, some people just never have their body fit to the memory foam mattress. Secondly, in some cases, the foam used may not be genuine in order to sell it at a lower price.
The best part is that you will never feel your partner move in the middle of the night. You can literally set a glass of water on one side of your memory foam bed and jump up and down on the other side and it will barely even move.
Foam is made up of thousands of tiny cells. The cells are formed by a polyurethane wall and filled with space (air). Higher density foams have thicker cell walls and less space inside. Lower density foam has thinner walls, greater space. If foam were made too dense, its cell walls would be too thick / stiff. It would be hard to the touch and would not conform to the body properly. It costs more to make dense foam, and at some point it can be made too dense. So there is a balance that needs to be struck between performance, cost, and durability. The marketplace today has gathered around a 5 lb memory foam mattress as the optimal choice for bedding. While more expensive than lower density foams, it provides better, long term contouring support.
Source: http://www.greenthumbcorner.com/articles/garden-tools/memory-foam-beds-7060/
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