Viscoelastic foams are widely used in comfort applications, ranging from mattresses and cushions to footwear innersoles. They respond slowly to dynamic or static stress, and are selected for many applications where it is desirable to spread pressure evenly over a wide area, giving consumers a feeling of luxury and comfort.
While conventional flexible polyurethane foam formulations are designed to give a rapid elastic response to dynamic or static stress, viscoelastic foams are designed to have a more damped, higher viscous response.
Formulations for viscoelastic foams rely on changing the morphology of conventional foams so that the amount of elasticity of the foam is reduced close to room temperature, or the temperature at which they will be used. Their glass transition temperature is close to room temperature.
There are a number of ways in which the morphology of conventional foams can be altered. Some formulations use low molecular weight crosslinkers. Other approaches include using blends of high and low molecular weight polyols, or mixtures of polyols. Plasticisers can also be used. This paper will concentrate on the effects of adding a new polyester polyol into the formulation alongside traditional polyether polyols.
The combination of polyether and polyester polyols in viscoelastic foam formulations is not new, but a number of problems with traditional formulations limit the use of polyesters.
For example, traditional polyester polyols have much higher viscosities than the corresponding polyether polyols with the same hydroxyl number. This can make it harder to successfully disperse the polyols within each other during mixing and dispensing.
It is now becoming increasingly important for polyurethane foam to be produced with a greater proportion of recycled content, or which can help to reduce the environmental impact of the finished product. This is being driven by legislation, and also by consumer demand.
Polios NT170 LV, a new polyester polyol from Purinova, overcomes the problem of viscosity that often affects polyester polyols, and helps to increase the recycled content of flexible viscoelastic foams.
The polyol is an aliphatic, saturated, linear polyester polyol. Despite its linear structure, it ensures that appropriate specific stiffness will be introduced to the viscoelastic foam.
Traditionally, many polyether polyols based on propoxylated glycerol have been used in viscoelastic foam formulations. The new Polios NT170 LV has a high recycled content, being made from by-products formed during the oxidation of cyclohexane to cyclohexanol and cyclohexanone. This process is described in US Patent 9982089, and Polish Patents 229712 and 211684.
The industrially important processes that produce caprolactam and adipic acid, the monomers used to make nylon 6 and nylon 6,6, also create cyclohexane oxidation products. If Purinova did not use these by-products in this application they could be considered waste, and it is classified as pre-consumer recycled content under EN ISO 14021. The new class NT170 LV materials contain a minimum of 65% recycled and renewable content.
Purinova has modified the structure of the polyester polyols to reduce their viscosity. The new grade has viscosity between 500–1000 mPa.s at 25°C with KOH values of 160–180mgKOH/g and acid number of 1.5mg KOH/g. The yellow-to-brown liquids can be supplied in 20, 200 and 1000 litre IBC containers, or in bulk.
The polyol has been tested in a number of formulations designed to resemble those in use commercially. Table 1 shows the formulations for the polyol used in TDI crosslinked systems, and Table 2 shows formulations using Polios NT170 LV in pMDI crosslinked systems.