Structure of Polyurethane:
Polyurethane is made from reacting different monomer materials; isocyanates and polyols, both derived from refined crude oil. Polyurethane foam is the most common upholstery material used today.
- 67% of Polyurethane is composed of the chemical ingredient, Isocyanates, which are used in the manufacturing of highly flexible foams, fibers, or coatings. The two main isocyanates used in the chemical reactions are Toluene Diisocyanate and Methyl Diphenyl Diisocyanate.
- 33% of the finished weight of this memory foam is created from Polyols. This is a form of alcohol, which is almost always made entirely from petrochemicals
- An important part of transforming all polyurethane into its foam-like appearance is the use of a blowing agent. This is a chemical in the form of a gas which is blown into the mixture to turn it into foam. Different types of polyurethane and memory foams simply use different blowing agents, some foams are water expanded, which is the most natural way.
Structure of an Isocyanate, consisting of Hydrogen, carbon, and nitrogen. This is a figure of a specific isocyanate, which is mentioned above, Methyl Isocyanate (google images)
Polyol Structure consisting of the same elements found in Isocyanates (google images)
The more in-depth step by step process of forming a Polyurethane structure from isocyanates and polyols (http://www.essentialchemicalindustry.org/polymers/polyurethane.html)
Properties of Polyurethanes:
Typically, polyurethanes have high-density, but in the case of memory foam in our cushioning found home, these polyurethanes have low density, and are viscoelastic and are more durable than standard pillows and mattresses.
- Durability of Polyurethanes: To simplify, the more dense polyurethane or memory foam is, the more durable it will be. The challenge with regular polyurethane foam is that as we increase the density, it becomes more brittle. As the density increases so too does the firmness, which creates the unfortunate side effect of the foam no longer being comfortable. This is the primary reason why most high density polyurethane foams are of the memory foam variety. Memory foam is more apt to offer additional flexibility and forgiveness as the density increases. The figure below compares the durability of polyurethane to rubber when subject to intense loading. (http://www.tmasc.ca/memory-foam.html)
- Viscoelasticity: The "memory" in memory foam is actually due to the viscoelastic nature of memory foam versus traditional polyurethane foam. The term "viscoelasticity" is simply the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Both varieties of foam will try to return to their original shape. Memory foam and polyurethane foam do break down over time. The only difference is that memory foam typically takes longer wear out. A high quality memory foam mattress usually provides 6-8 years of optimal support and comfort compared to the average life span of 3-5 years for traditional pillow top-variety mattresses. (http://www.tmasc.ca/memory-foam.html)
Figure of Durability of Polyurethane when subject to high stress (http://lep.net.nz/materials-plastics-selection-guide/lurethane-or-polyurethane/)
- Correlation to Properties and its Composition:the low density of the polyurethane found in mattresses and pillows is what makes them more flexible and resistant to fatigue, which is why in memory foam pillows, it active molds your body when subject to heat and pressureallowing the surface to evenly distribute body weight when occupied and return to its original shape once pressure is removed. Furthermore, The high durability which allows all of this pressure relief and support is dependent on the density of the polyurethane. While the "memory" of the memory foam is given that composition from the viscoelastic property. (http://www.tmasc.ca/memory-foam.html)
The properties of Polyurethane in memory foam clearly correlate to its composition, as its density changes, the material's durability and ability to return to its original shape changes as well. And the viscoelasticity of the polyurethane is where the "memory" in memory foam comes from.