Properties of polyurethane foam
by Duncan Geddes
Polyurethane (PU) foams are renowned for their superior resilience and versatility, the likes of which make them the ideal choice for use across the construction and automotive industries. This article aims to uncover even more about the physical properties of polyurethane foam and establish how these characteristics have helped to make this type of foam such a popular choice.
Polyester vs. polyether
There are two primary foam types derived from the polyurethane family; polyester and polyether. To understand the properties of polyurethane foam, a clear distinction must first be made between these two.
The first type of flexible polyurethane foam to be developed was polyester foam in the early 1950s. With the development of polyether based polyols in the late 1950s, polyether foams offering a wider range of properties started to be manufactured. While both are established types of polyurethane foam, there are differences in their physical characteristics that determine their suitability for different applications.
Physical properties of polyurethane foam
Perhaps the most obvious difference between polyester and polyether foams is the lower resilience of polyester foams. This difference in resiliency has led to a preference for polyether foams in cushioning applications, particularly upholstery and bedding.
In general, compared to polyether foams, polyester foams have higher tensile strength, elongation at break and hardness. As a result, polyester foams have good abrasion resistance – hence their use in polishing foam applications. Their lower resiliency and higher energy absorbency also make them more suitable for use in packaging applications.
Polyurethane foams are known for being resistant to a wide range of solvents. Polyester foams are generally superior to polyether foams here, particularly in their resistance to dry cleaning solvents. For this reason, application areas for flexible polyester foam include the manufacture of foams for textile laminates, where their superior resistance to dry cleaning solvents, flame bonding performance and elongation properties make them the preferred product.
Polyurethane foams are also subject to degradation by aqueous acids, alkalis and steam. Ester, amide and urethane groups represent sites for hydrolytic attack. Since the ether group is not readily attacked, polyether foams are generally more resistant to hydrolysis than polyester foams.
Polyurethane foams have been found to perform quite well in fungal and microbial growth studies when compared to other polymers. Bacteria struggle to adhere to the polymer surface, making growth more difficult. For this reason, they’re used in medical and hygiene applications.
Polyester polyurethanes are more readily degraded by microbial action than polyether polyurethanes because of the susceptibility of the ester group to hydrolysis, which a large number of microbial enzymes catalyse. Several antimicrobial additives are available which can be introduced into the PU foam at the manufacturing stage to enhance its antimicrobial properties.
Yellowing and discolouration
Exposure to UV light can cause discolouration in both polyester and polyether foams – the degree of yellowing being dependent on the intensity of the radiation. Polyether foams tend to yellow at a faster rate than polyester foams, although the yellowing has no significant effect on the physical properties of either type of foam.
Both types of foam are manufactured using aromatic type diisocyanates. Non-yellowing foam can be produced with the use of aliphatic isocyanates, although these are considerably more expensive than the type normally used.
Low fire retardancy
Like many other organic type materials, flexible polyurethane foams are easily ignited by small ignition sources because of their low density and high surface area.
The ignition resistance of polyurethane foams can be improved by the addition of flame retardants at the manufacturing stage or by post-treatment with polymer latices containing flame retardant additives. Post-treatment techniques are generally employed where the foam has to meet stringent flammability requirements, such as building regulations. Overall, however, it is recommended to choose a more fire retardant foam if fire safety is high on your list of necessary properties.
How are the properties of polyurethane foam determined?
Flexible polyurethane foam characteristics are predominantly determined by the starting materials and the formulations used in their manufacture. Commercial polyurethane foams are nominally manufactured within a density range of 15 to 80kg/m3. The choice of density that a foam converter uses for a particular application will be determined by the end use of the foam.
All in all, polyurethane foam is a flexible, durable material solution across dozens of industries. If you require precision engineering of polyurethane components, get in touch to discover how our expert foam engineers can help.