Viscoelastic (Memory) Foam Systems

Viscoelastic (Memory) Foam Systems

Definition of Viscoelastic and Viscoelastic Polyurethane Foam

Viscoelastic is defined as the physical property of a material exhibiting both viscous and elastic characteristics when subjected to deformation. When a load is applied to a viscous material, the material deforms and when the load is removed, it "forgets" its original shape and remains deformed. On the other hand, when a load is applied to an elastic material, the material deforms and when the load is removed, it "remembers" its original shape and quickly returns to it. A viscoelastic material possesses both of these properties; when a load is applied and removed, the material returns to its original shape through time-dependent behavior. Additionally, a viscoelastic material has temperature sensitivity. The ambient temperature affects the material's behavior depending on its glass transition temperature. Today, viscoelastic properties are quite common in polyurethane foams. Viscoelastic polyurethane foam has the property of distributing the pressure applied to it. When pressure is removed, the foam slowly returns to its original shape. Due to this slow recovery or rebound, these foams are called memory foam, slow-recovery foam, or low-resilience foam. This type of polyurethane foam easily conforms to the shape of the human body, softens with body temperature, distributes the weight of the human body, and provides an extra feeling of comfort and relaxation.

Invention and Development of Viscoelastic Polyurethane Foam

Viscoelastic polyurethane foam was invented by NASA Research Center in 1966 for use in aircraft seats and space suits. Due to the material properties of viscoelastic foam, it is able to distribute the G-force on the bodies of pilots and astronauts. Following its invention, NASA opened viscoelastic polyurethane foam to the public in the 1980s. Shortly after, this material began to be used in medical applications. People confined to wheelchairs and beds began to benefit from this material's pressure-distributing property in preventing pressure sores and improving blood circulation. Later, in 1991, Swedish mattress manufacturer Tempur became the first company to commercialize viscoelastic polyurethane foam by using it in mattresses. Today, apart from medical applications, viscoelastic polyurethane foam is used in pillows, mattresses, insoles, helmet liners, office chairs, and vehicle seats. [caption id="" align="aligncenter"] Figure 1. Slow recovery of viscoelastic polyurethane foam[/caption]

Physical Properties of Viscoelastic Polyurethane Foam

Four separate effects work together inside viscoelastic polyurethane foam to create a combined effect. These are the network effect, relaxation effect, pneumatic effect, and adhesion effect. While the network effect causes the foam to return quickly to its original shape, the remaining three effects balance this elastic recovery force, causing viscoelastic foams to return slowly to their original shape. Network Effect: This effect is the main force that causes the foam to return to its original shape. Thanks to this effect, all flexible polyurethane foams show a tendency to rebound or spring back when the load on them is removed. Relaxation Effect: This effect is the chemical aspect that enables the foam's slow recovery. The glass transition temperature (Tg) of viscoelastic foams is quite high compared to standard foams. This high glass transition temperature causes some segments in the foam's network structure to remain stiff or frozen at room temperature and their movement to be restricted. This reduces the foam's elasticity and causes the foam to behave with delay. The polymeric chain lengths and cross-link density in the structure, and the plasticizer concentration (if any), determine the foam's glass transition temperature. As the glass transition temperature approaches room temperature, the foam's recovery becomes slower. Pneumatic Effect: This effect is a part of the physical aspect that enables the foam's slow recovery. This relates to the cell opening of the foam, and the smaller and more closed the cells are, the longer it takes for air to enter and exit the foam, causing the foam's slow recovery. Adhesion Effect: This effect is another part of the physical aspect that enables the foam's slow recovery. This relates to the adhesiveness of the foam's surfaces, and the more adhesive the surfaces are, the longer it takes for the foam to return to its original shape, causing the foam's slow recovery. And the foam's adhesiveness is greater when the foam's temperature is close to its own glass transition temperature. As a result of the combined effect of the above, viscoelastic polyurethane foams have low resilience and memory properties and apply less recovery force to an applied stress compared to standard polyurethane foams. As shown below, the force curves coincide in a standard polyurethane foam, while the force curves of a viscoelastic polyurethane foam differ from each other. [caption id="attachment_131362" align="aligncenter"] Figure 2. Stress-Strain force curves of viscoelastic polyurethane foam[/caption]

Structure of Viscoelastic Polyurethane Foam

Viscoelastic polyurethane foam contains different types of polyols with different characteristics. By using low OH, high molecular weight polyols, softer and more elastic foams are obtained, while by using high OH, low molecular weight polyols, firmer, more highly cross-linked, and slower-recovering foams are obtained. High OH, low molecular weight polyols leave more open chains in the foam structure, shifting the foam's glass transition temperature to higher values. Producing polyurethane foams at high isocyanate index values increases the aromaticity in their structure, shifting the foam's glass transition temperature to higher values. If the glass transition temperature is high and close to room temperature, the viscoelastic foam's recovery will be slower. If the ambient temperature increases, the viscoelastic foam's hardness decreases and its recovery also accelerates. [caption id="attachment_131363" align="aligncenter"] Figure 3. Structures of standard flexible foam and viscoelastic polyurethane foam[/caption] In addition to the polyols mentioned above, depending on customer needs, plasticizers and flame retardants may sometimes be added to viscoelastic foam formulations. Generally, viscoelastic polyurethane foams are produced at higher densities than standard polyurethane foams for greater comfort and longer service life.

Applications and Benefits of Viscoelastic Polyurethane Foam

Today, viscoelastic polyurethane foams are used for comfort purposes and in medical applications. The pressure-distributing property of these materials and their ability to conform to the shape of the human body by softening with body temperature provide unique comfort and make them an indispensable part of our lives. [caption id="attachment_131364" align="aligncenter"] Figure 4. Pressure distribution map of the human body on a viscoelastic foam mattress compared to that on a conventional polyurethane foam mattress[/caption] As seen in Figure 4, the pressure distribution of the human body on a viscoelastic foam mattress is much better than that on a standard foam mattress. This pressure-reducing property of viscoelastic foam is particularly useful for preventing pressure sores and preventing blood circulation obstruction. Besides comfort and medical applications, viscoelastic polyurethane foams are also used as shock-absorbing, vibration-dampening, or sound-absorbing materials. Poleks Kimya viscoelastic systems are produced at different densities, hardnesses, and recovery times for different sectors. While low-density, soft, and slow-recovery products are available for pillows and medical pillows, high-density, firm, and faster-recovery products are available for insoles. As for production technology, viscoelastic pillows and insoles are produced using the mold casting method, while viscoelastic mattresses are produced using the block or box casting method. The technical properties of Poleks Kimya viscoelastic systems are given below. [caption id="attachment_131365" align="aligncenter"] Table 1. Technical data of Poleks Kimya viscoelastic systems[/caption] The advantages of Poleks Kimya viscoelastic foam systems are: • Smooth skin and homogeneous cell structure, • Effective flow and filling in large molds, • Wide iso/poly ratios, hardness and recovery time range, • Fast curing and short mold opening time, • Long aging life. References Hager et al., Viscoelastic Polyurethane Foams, Bayer Material Science LLC, US 2013/0289150A1, 2013. M. Krebs and R. Hubel, The Adjustment of Physical Properties of Viscoelastic Foam - the Role of Different Raw Materials, Evonik Nutrition & Care GmbH, American Chemistry Council, 2016. In-Touch Publication, Viscoelastic (Memory) Foam, Polyurethane Foam Association, Vol.11, No:1, 2016.

Yücel Aktay Chemical Engineer Research and Development and Technical Support Manager Poleks Kimya San. ve Tic. A.Ş

Dr. Murat Baranak Chemical Engineer Research and Development and Quality Control Manager Poleks Kimya San. ve Tic. A.Ş.