Flexible Polyurethene Foam

Polyurethane foam is characterised by compression under load and quick recovery after compression.

Polyurethene foams are produced by a chemical reaction between a set ratio of Polyols (-OH) with Isocyanates (-NCO) in the presence of a catalyst, and combined with other additives, such as…

Flame Retardancy Chemicals – Reduces the flammability of the final product, but can also increase the harness of the foam.

Blowing Agent – e.g. water, reacts with the Isocyanates to create carbon dioxide. It is the carbon dioxide bubbles stirred into the mix which creates the characteristic cell structure. Some high density foams do not use a blowing agent, but instead froth the polyol prior to use. Along with the ratio of the mould volume to raw chemicals, the blowing agent helps to determine the density of the foam.

Surfactant – e.g. silicon oil. Used to modify foam characteristics. Helps to regulate the cell size and stabilises the foam’s cell structure.

Pigment – Determines the colour of the foam.

Foam Production Process

Component chemicals are injected with, into a mould at a predetermined temperature that aids the chemical reaction. The chemicals will then foam until the mould is filled. The foam then sets to produce stable foam.

Once removed from the mould, the foam is then crushed to release any trapped carbon dioxide.

Foam Products Production

Polyurethene foam products are either cut from large blocks of foam, or are manufactured in a product specific mould. Moulded foam products have the advantage of being manufactured to an exact and replicable three dimensional shape.

Flexible Polyurethane Foam Structure

Polyurethane foam has an open structure of interlocking cells described as windows and struts.

It is the elasticity of the supporting struts that allows the foam to compress and recover under loading and unloading. In effect, the struts act like springs.

Foam Density

Foam density is the mass per unit volume. Foam hardness and density are largely independent, and so it is possible to have high density soft foam (e.g. packaging foam), or low density hard foam.

As density relates to ratio of material to air in foam, low density foams have less struts supporting the load compared to high density foams. High density foams are usually expected to have a longer lifespan than low density foams as they have more struts over which the load can be spread. Density related to comfort, support and durability.

Different density foam products are used for different applications, as shown in the chart below.

Generally anything above around 30 kg/m3 is considered to be a high density product.

Foam Hardness

The hardness of foam is the feel or ability of a foam to push back against a given load. It is tested using an indentor which measures the force required (in Newtons, or KPa) to squash a 50mm thick piece of foam to 40% of its original height. The ideal cushion hardness allows the user to immerse into the cushion, allowing pressure to be redistributed over a greater surface area, without ‘bottoming out’ (squashing the foam flat). In very soft cushions, excessive cushion deflection during repeated loading may create stress within the foam cell structure reducing the durability of the cushion. Foam hardness relates to comfort and conformity. Research by Stockton and Rithalia (2009) into comfort perception found that if a cushion as discontinued by a user, it was invariable a softer ‘medium’.

Foam Fatigue Class

Fatigue class is determined by test stipulated in British Standard BS3379 where the foam is pounded until there is a predetermined loss of hardness and thickness.

Fatigue class is an indicator of product longevity.

Foam Yellowing

Polyurethane foam made from isocyanates can discolour when exposed to visible light.

Visco Elastic (Memory Foam)

Visco elastic foam is a type of flexible polyurethane foam created in the same way as Polyurethane foams by modifying the catalysts and polyols used.

Visco elastic foams are used for a wide variety of applications, including…

  • Archery targets
  • Bullet proof vests
  • Unsinkable rafts
  • Pressure redistribution cushions

Visco Elastic Foam Density

The density of visco foam is measured in exactly the same way as the density of polyurethane foams. However, due to the tendency of visco elastic foams to progressively compress, a higher density visco foam is generally required to maintain support and durability.

Visco Elastic Foam and Hardness

The hardness of visco foam is measured in exactly the same way as the density of polyurethane foams. As visco elastic foams progressively compress, they are more prone than polyurethane foams to bottoming out. Therefore it is important that foam hardness is appropriate for the user weight. A base layer of flexible polyurethane can also greatly reduce the risk of bottoming out over time.

ViscoElastic Foam and Fatigue Class

Due to the slow recovery characteristics of visco elastic foam, it is not possible to test for fatigue class.

References

Aissaoui et al. 2001. Effect of seating cushion on dynamic stability in sitting during a reaching task in wheelchair users with paraplegia. Arch Phys Med Rehabil Vol 82, Feb 2001.

Polyurethene Foam Association. In Touch. Volume 11 Number 1, June 2003.

Polyurethene Foam Association. In Touch. Volume 3, Number 4, July 1994.

Polyurethene Foam Association. In Touch. Volume 1, Number 2, May 1991.

Stockton, L. and Rithalia, S. (2009) Pressure-reducing cushions: Perceptions of comfort from the wheelchair users’ perspective using interface pressure, temperature and humidity measurements. Journal of Tissue Viability (2009) 18, 28-35