Some Properties of Polyurethane Foams
by Duncan Geddes
The characteristics of flexible polyurethane (PU) foams 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.
As was mentioned in the previous blog (Polyurethane Foams 28/09/2015) 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 ( PU ) foams offering a wider range of properties than polyester foams started to be manufactured. 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. 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. Polyester foams have good abrasion resistance hence their use in polishing foam applications. Their lower resiliency and higher hysteresis values (energy absorbing characteristics) compared to general polyether foams also make them suitable for use in packaging applications.
Polyurethane foams are resistant to a wide range of solvents. In this respect, polyester foams are generally superior to polyether foams, particularly in resistance to dry cleaning solvents. This is a further reason why polyester foams are preferred in textile laminate applications. Polyurethane foams are 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 and are used in medical and hygiene applications. Bacteria struggle to adhere to the polymer surface making growth more difficult. 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 ( such as Candida cylindracea lipase ) catalyse. Several antimicrobial additives are available which can be introduced into the PU foam at the manufacturing stage to enhance its antimicrobial properties.
Exposure to UV light can cause discolouration (yellowing) 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, the yellowing has no real 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. These isocyanates are considerably more expensive than the aromatic diisocyanates normally used.
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 eg building regulations.
All in all, polyurethane foam is a flexible, durable material solution across dozens of industries. If you require precision engineering of polyurethane components, find out more about our polyurethane foam manufacturing services here.