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Memory foam is polyurethane with additional chemicals increasing its viscosity and density. It is often referred to as "viscoelastic" polyurethane foam, or low-resilience polyurethane foam (LRPu). 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 more quickly to their original shape.
Memory foam was developed in the 1970s under a contract by NASA's Ames Research Center to improve the safety of aircraft cushions. Ames scientist Chiharu Kubokawa and Charles A. Yost of the Stencel Aero Engineering Corporation were major contributors to this project. The temperature-sensitive memory foam was initially referred to as “slow spring back foam”; Yost called it "temper foam". Created by feeding gas into a polymer matrix, the foam has an open-cell solid structure that matches pressure against it, yet slowly springs back to its original shape.
Yost later founded Dynamic Systems Inc. in collaboration with NASA to commercialize the foam, including it in both medical equipment such as X-ray table pads and sports equipment such as American / Canadian football helmet liners. After Dynamic Systems sold memory foam products to Becton, the range of products was expanded.
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 the foam, 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, it was commonly used in cases where the patient was required to lie immobile in their bed on a firm mattress for an unhealthy period of time. The pressure over some of their body regions decreased or stopped the blood flow to the region causing pressure sores or gangrene. Memory foam mattresses significantly decreased such events. Claims have also been made that memory foam reduces the severity of pain associated with fibromyalgia.
Memory foam was initially too expensive for widespread use, but became cheaper. Its most common domestic uses are mattresses, pillows,shoes and blankets. It has medical uses, such as wheelchair seat cushions, hospital bed pillows and padding for people suffering long-term pain or postural problems; for example, a memory foam cervical pillow may alleviate chronic neck pain. Its heat-retaining properties may help some pain sufferers who find the added warmth helps to decrease the pain.
Unfortunately, the heat-retaining properties can also be a downside when used in mattresses and pillows so in the second generation memory foam, companies began using open cell structure to better help with breathability. In 2006, the third generation of memory foam was introduced. Gel visco or gel memory foam consists of gel particles fused with visco foam to reduce trapped body heat, speed up spring back time and help the mattress feel softer. This technology was originally developed and patented by Peterson Chemical Technology, and gel mattresses became popular with the release of Serta’s iComfort line and Simmon's Beautyrest line in 2011. Gel-infused memory foam was next developed with what were described as "beads" containing the gel which, as a phase-change material, would achieve the desired temperature stabilization or cooling effect by changing from a solid to a liquid "state" within the capsule. Changing physical states can significantly alter the heat absorption properties of an element, which is why the technology was applied to memory foam.
Since the development of gel memory foam, other materials have been added. Aloe vera, green tea extract and activated charcoal have been combined with the foam to reduce odors and even provide aromatherapy while sleeping. Rayon has been used in woven mattress covers over memory foam beds to wick moisture away from the body to increase comfort. Phase-change materials (PCMs) have also been used in the covers that are used on memory foam pillows, beds, and mattress pads.
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 1.5 lb/ft3 to 8 lb/ft3 density.
The property of firmness (hard to soft) of memory foam is used in determining comfort. Firmness 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 pounds-force) required to make a dent 1 inch into a foam sample 15" x 15" x 4" by an 8-inch-diameter (50 sq in) disc—known as IFD @ 25% compression. 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).
Some [according to whom?] report that IFD is a poor way to measure softness of memory foam, and that foam density as a measure of quality is more important, but not always true. Foam density of 5 pounds per cubic foot (80 kg/m3) or greater is considered high quality, although most standard memory foam has a density of 1 to 5 lb/ft3 (16–80 kg/m3). Most bedding, such as topper pads and comfort layers in mattresses is 3 to 4.5 lb/ft3. Very high densities such as 5.3 lb/ft3 (85 kg/m3) are used infrequently in mattresses.
The new 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. Most memory foam has the same basic chemical composition, however the density and layer thickness of the foam makes different mattresses feel very different. A high-density mattress will have better compression ratings over the life of the bedding. A lower-density one will have slightly shorter life due to the compression that takes place after repeated use.
Emissions from memory foam mattresses may directly cause more respiratory irritation than other mattresses. A 2002 American-Norwegian study claims children's "foam" mattresses are 3.3 to 8.1 times more likely to host dust mite feces than spring mattresses, yet the study did not mention if the foam tested was of the standard or shape memory type and simply referred to covered or non-covered foam mattresses, which could lead to believe the foam in question could be the standard less expensive non-memory type. Thirteen mattresses were excluded of the study without further notice and the paper was written self-admittingly to debunk a 1981 Australian paper that claimed the opposite, namely that kapok and spring mattresses had 2-3 times more mites than foam mattresses, yet the American-Norwegian team explain "We speculate that children tend to get their parents’ old foam mattresses". Neither study addressed memory foam. 
Memory foam, like other polyurethane products, can be combustible. Laws in several jurisdictions have been enacted to require that all bedding, including memory foam items, be resistant to ignition from an open flame such as a candle or cigarette lighter. US bedding laws that went into effect in 2010 change the Cal-117 Bulletin for FR testing. There is concern that high levels of the fire retardant PBDE, commonly used in memory foam, could cause health problems for users. PBDEs are no longer used in most bedding foams, especially in the European Union.
Manufacturers caution about leaving babies and small children unattended on memory foam mattresses, as they may find it difficult to turn over, and may suffocate.
The United States Environmental Protection Agency published two documents proposing National Emissions Standards for Hazardous Air Pollutants (HAP) concerning hazardous emissions produced during the making of flexible polyurethane foam products. The HAP emissions associated with polyurethane foam production include methylene chloride, toluene diisocyanate, methyl chloroform, methylene diphenyl diisocyanate, propylene oxide, diethanolamine, methyl ethyl ketone, methanol, and toluene however not all chemical emissions associated with the production of these material have been classified. Methylene chloride makes up over 98 percent of the total HAP emissions from this industry. Short-term exposure to high concentrations of methylene chloride also irritates the nose and throat. The effects of chronic (long-term) exposure to methylene chloride in humans involve the central nervous system, and include headaches, dizziness, nausea, and memory loss. Animal studies indicate that inhalation of methylene chloride affects the liver, kidney, and cardiovascular system. Developmental or reproductive effects of methylene chloride have not been reported in humans, but limited animal studies have reported lowered fetal body weights in rats exposed.
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