The Default Material for Foam Mattresses
The most common material found in foam mattresses is flexible polyurethane foam (FPF).
What Is Flexible Polyurethane Foam?
Flexible polyurethane foam is foam that is flexible (bendable and compressible) made of polyurethane. Polyurethane (PUR) is a polymer, linking carbamate molecules in chains which are cross-linked into three-dimensional webs. Until recent developments, polyurethane has always been made by reaction of isocyanates (mono-, di- and tri-) and polyols. Both of these are derived from petroleum, but some polyols can be made from plant oils.
Polyurethane was first made by Otto Bayer of the German firm IG Farben in 1937. According to Wikipedia, the initial attraction of this development was that PUR was not covered by existing patents at that time, as were polyester and its precursors. While IG Farben intended to produce foams and fibers, the first use was protective coatings for aircraft.
Polyurethane is a versatile plastic. It can be modified in many ways to adjust its properties to desired specifications. For instance, applied as a coating to metal and wood, it becomes a hard film protecting the underlying substrate from rust or weathering. It can be flexible enough to not crack with moderate flexing, as in aircraft skins. Like polyester and rayon, it can be extruded into sheets or fibers. And by injecting or generating gases, it can be expanded into foam.
Two kinds of polyurethane foam can be made: rigid and flexible. Rigid foams are mostly used in construction to fill and support thin shells or as insulation.
Flexible foams were developed after World War II, but were not mass produced until after the general availability of isocyanates in 1952. In 1954, it began to be widely used for cushioning furniture upholstery. The firmness and resiliency of the foam can be tailored to meet particular specifications.
Into Bed with Polyurethane Foam
From seat cushions, the use of flexible polyurethane foam spread to beds. Sheets of FPF were used in cushioning the tops of innerspring mattresses. The foam was less expensive than natural materials such as cotton and wool battings. It could also be cushier than batting, and did not quickly become clumpy as fiber fillings tend to do.
Foam mattresses soon followed. Polyurethane foam mattresses cut into the market share of latex (foam rubber) mattresses. Eventually, most latex mattresses had support cores of very firm FPF with a layer of latex foam on top, since this lowered the overall cost of the mattress and made it more competitive. While many customers bought foam mattresses, innerspring mattresses were still the predominant favorite. That did not change until the arrival of memory foam on the mattress market.
Polyurethane Foam with Memory
The United States began manned space flight in 1961. The National Aeronautics and Space Administration (NASA) needed better cushioning for astronauts to withstand the g-forces at lift-off. The ideal would have been to mold each launch seat to fit the person using it, but that would have been very expensive, and the seat could not be used for an astronaut with a very different build. NASA commissioned Stencel Aero Engineering Corporation to develop a cushion foam that would mold itself to the body of the person using it. It would “remember” the shape of the astronaut while in use, then return to its original shape afterwards, ready for the next occupant.
Charles Yost, an engineer at Stencel who worked on developing the new material from polyurethane foam, called it “temper foam.” Why? It was a firm PUR foam that became soft when it became warm (“temper” referred to the foam being temperature-sensitive). The foam had to be firm to cushion the extra weight created by launch acceleration, and it had to soften to change its shape. The source of the heat for softening was the body of the astronaut. As the foam warmed and softened, the astronaut sank in until the firmness and the contouring equalized. Since the astronaut lay on his back at lift-off, the hollows of the contour lessened pressure on the hips and shoulders, but in between still supported the lumbar.
In 1981, NASA released the process for their temper foam for commercial development. Only one firm completed this development, Fagerdala Foam of Sweden. In 1991 they introduced their Tempur Material to the market in Sweden in cushions and mattresses. It became popular. They next year they founded Tempur-Pedic to sell Tempur mattresses in North America, where it also became popular.
Popularity of Memory Foam
By then, waterbeds had become popular alternatives to innerspring mattresses (1/5 of mattress sales). Waterbeds provided pressure relief. Memory foam mattresses also provided pressure relief, but without the extra weight of water and the risk of a leak. Waterbed sales declined, and memory foam took its own bite out of the innerspring market.
Ironically, the popularity of memory foam mattresses resulted in more sales for latex mattresses. By popularizing foam mattresses in general, it made latex look better compared to innersprings. Latex was also the alternative for those who liked memory foam, but wanted a bouncier mattress.
Safety, Health, and Environmental Concerns
Polyurethane foam is extremely flammable. Some labeled it “solid gasoline.” When ignited, it can quickly heat to 1400ºF. It also produces thick, black, toxic smoke containing cyanide. Federal consumer safety regulations led to putting flame retardant chemicals into the foam. However, these are toxic and were escaping the foam through outgassing. These chemicals are now not allowed in the foam.
The next flammability solution was to wrap the foam in a fire sock. Flammability regulations that went into effect in 2007 (16 CFR Part 1632) required a level of protection for the foam that could be met by treating the cover or fire sock with toxic chemicals. Not as bad as in the foam, but still not healthful. So non-chemical means of fire protection have been developed, such as compressed wool and silica-infused rayon.
Another health concern of polyurethane foam is emissions of volatile organic compounds (VOCs). Most of those that are volatile are also those that are toxic. Research on polyurethane foam then focused on reducing toxic VOCs. CertiPUR and CertiPUR-US were established to certify foams that were free of toxic compounds and had only a minimum of VOCS (and those non-toxic).
Environmental and sustainability have become major concerns, including for mattresses. Since regular polyurethane foam is made completely from petroleum, it is a petroleum product. Diisocyanate, and the other isocyanates are toxic, so toxic that workers using them have to take extreme precautions. That’s half the ingredients in polyurethane. The other half is polyols, a certain form of oil, not as toxic as isocyanate.
Some writers are particularly concerned because isocyanate is an ingredient of polyurethane foam, since it is toxic. However, it reacts with the polyols to form the long chains, which are themselves not toxic. It is like reacting sodium with chlorine. Chlorine is a toxic gas. It has been used as a battlefield weapon. The effect was so horrendous that international agreements outlawed its use as a weapon. On the other hand, sodium is a very alkaline metal. In its pure form, it is very reactive. It will burn when put into contact with water. But sodium chloride is a salt, a necessary ingredient in our diet. Almost no one gets upset by having salt on hand, fearing that the chlorine will poison the entire household. Nor will hardly anyone think of salt as a fire hazard because it has sodium in it.
Another concern is the environmental risk of extracting, shipping and refining petroleum. Also, it is a non-renewable resource. Both concerns are addressed by using polyols derived from plant oils. This would reduce the use of petroleum in making flexible polyurethane foam. Most of the source oils comes from soybeans and castor beans.
Initially, only between 5% and 10% of the petroleum polyols could be replaced by plant-derived polyols and still maintain quality polyurethane foam, especially so for memory foam. However, since before September 2004, research has focused on ways to replace most or all of the petroleum in FPF (Diakoumakos; Kotzev). By 2011, progress was being made on replacing isocyanates with plant-sourced materials (Helou; Carpentier; Guillaume). In 2016 a research paper concluded that “these compositions represent the first examples of film-forming, linear isocyanate-free polyurethanes with mechanical integrity and processability” (Zhang; Nelson; Talley; Chen; Margaretta; Hudson; Moore; Long).
FXI (formerly Fomex International) claims that with its Variable Pressure Foaming process it can make safer, greener foam. And Cargill, the agribusiness giant, claims that polyurethane foams made with its BiOH® polyols out-perform petroleum-only foams, though they do acknowledge that (so far, at least) they replace only a portion of the petroleum. However, Cargill’s claims are disputed by some, including the manufacturer of modified latex foam mattresses, Essentia.
The Future of Flexible Polyurethane Foam
For the foreseeable future, flexible polyurethane foam will be used in mattresses, whether as support cores, FPF cushioning, or in the form of memory foam. Results of materials research for polyurethane indicate that at some point all or mostly plant-based FPF will replace a good percentage of petroleum-based FPF. How soon and how much depends on a combination of materials development, relative cost, consumer demand, and regulation.