Memory foam

Memory foam consists mainly of polyurethane with additional chemicals that increase its viscosity and density. It is often referred to as "viscoelastic" polyurethane foam, or low-resilience polyurethane foam (LRPu). The foam bubbles or 'cells' are open, effectively creating a matrix through which air can move. Higher-density memory foam softens in reaction to body heat, allowing it to mold to a warm body in a few minutes. Newer foams may recover their original shape more quickly.[1]

Mechanics

Memory foam derives its viscoelastic properties from several effects, due to the material's internal structure. The network effect is the force working to restore the foam's structure when it is deformed. This effect is generated by the deformed porous material pushing outwards to restore its structure against an applied pressure. Three effects work against the network effect, slowing the regeneration of the foam's original structure:

The effects are temperature-dependent, so the temperature range at which memory foam retains its properties is limited. If it is too cold, it hardens. If it is too hot, it acts like conventional foams, quickly springing back to its original shape. The underlying physics of this process can be described by polymeric creep.[2][3]

The pneumatic and adhesive effects are strongly correlated with the size of the pores within memory foam. Smaller pores lead to higher internal surface area and reduced air flow, increasing the adhesion and pneumatic effects. Thus the foam's properties can be controlled by changing its cell structure and porosity. Its glass transition temperature can also be modulated by using additives in the foam's material.[2]

History

Memory foam was developed in 1966 under a contract by NASA's Ames Research Center to improve the safety of aircraft cushions. The temperature-sensitive memory foam was initially referred to as "slow spring back foam"; most called it "temper foam".[5] Created by feeding gas into a polymer matrix, it had an open-cell solid structure that matched pressure against it, yet slowly returned to its original shape.[6]

Later commercialisation of the foam included use in medical equipment such as X-ray table pads, and sports equipment such as American / Canadian football helmet liners.

When NASA released memory foam to the public domain in the early 1980s, Fagerdala World Foams was one of the few companies willing to work with it, as the manufacturing process remained difficult and unreliable. Their 1991 product, the Tempur-Pedic Swedish Mattress eventually led to the mattress and cushion company Tempur World.

Memory foam was subsequently used in medical settings. For example, when patients were required to lie immobile in bed, on a firm mattress, for an unhealthy period of time, the pressure on some of their body regions impaired blood flow, causing pressure sores or gangrene. Memory foam mattresses significantly decreased such events, as well as alternating pressure air mattresses.

Mattresses

A memory foam mattress is usually denser than other foam mattresses, making it both more supportive and heavier. Memory foam mattresses are often sold for higher prices than traditional mattresses. Memory foam used in mattresses is commonly manufactured in densities ranging from less than 24kg/m3 (1.5 lb/ft3) to 128kg/m3 (8 lb/ft3) density. Most standard memory foam has a density of 16–80 kg/m3 (1 to 5 lb/ft3). Most bedding, such as topper pads and comfort layers in mattresses, has a density of 48–72 kg/m3 (3 to 4.5 lb/ft3). High densities such as 85 kg/m3 (5.3 lb/ft3) are used infrequently.

The firmness property (hard to soft) of memory foam is used in determining comfort. It is measured by a foam's indentation force deflection (IFD) rating. However, it is not a complete measurement of a "soft" or "firm" feel. A foam of higher IFD but lower density can feel soft when compressed.

IFD measures the force in newtons (or pounds-force) required to make a dent 1 inch into a foam sample 500 x by a 323 cm3 (50 sq in, 8-inch-diameter) disc—known as IFD @ 25% compression.[10] IFD ratings for memory foams range between super soft (IFD 10) and semi-rigid (IFD 12). Most memory foam mattresses are firm (IFD 12 to IFD 16).

Second and third generation memory foams have an open-cell structure that reacts to body heat and weight by molding to the sleeper's body, helping relieve pressure points, preventing pressure sores, etc.[11] Manufacturers claim that this may help relieve pressure points to relieve pain and promote more restful sleep, although there are no objective studies supporting the mattresses' claimed benefits.

See also

• Low-resilience polyurethane
• Sorbothane
• Neoprene
• List of polyurethane applications