Liquids which flow freely such as petrol, paraffin, alcohol and water are said to be mobile whilst those that do not easily flow such as treacle, honey, glycerine and lubricating oils are called viscous. The property of resistance to flow in fluids is called viscosity. Elasticity is the ability of a material to return to its normal shape after being subjected to stretching or compression by an external force.
“Viscoelasticity” is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscoelastic foam also known as memory foam, confor foam, slow recovery or low resilience foam is characterised by having significantly lower rates of recovery and resiliency when subjected to an applied stress compared to conventional polyurethane foam.
An important characteristic of viscoelastic foam is its sensitivity to temperature. Conventional polyurethane foams are generally manufactured using polyols having high molecular weights and the “ Tg” ( glass transition temperature) for these foams is nominally between -40oC to ‑50oC. Tg is defined as the temperature at which a material upon cooling transforms to a glassy/brittle state. The low resiliency/slow recovery properties of viscoelastic foams is achieved by elevating the glass transition temperature of conventional PU foams above 0oC and closer to ambient temperatures. This can be achieved by substituting some of the high molecular weight polyol with lower molecular weight polyol/polyols. Other factors which influence the properties of visco-elastic foams are the type of isocyanate used, foaming index and functionality of the polyols. The pneumatic effect due to a high closed cell content ie, low air flow can also contribute to slow recovery behaviour.
Viscoelastic foams can be manufactured using conventional slabstock foam machinery and can also be moulded. Commercial foam conversion equipment can be used for fabrication but due to its slow recovery properties the foam needs to be cut at lower speeds than for conventional polyurethane foam.
The first commercial use of viscoelastic foam was when it was developed during the mid 1960’s as a result of NASA’s AMES research technology transfer programme, where it was used as a cushioning material to reduce the G forces experienced by astronauts during take- off and landing. Since the late 1980’s it has been used in different applications such as:
- Soundproofing: Due to the foams ability to dampen vibrations as well as absorb shock it has been used as a noise and vibration harshness (NVH) foam in automotive applications.
- Speciality packaging foam for protecting sensitive electronics equipment.
- Military and commercial pilot seating.