Polyurethane foam UV resistance
by Duncan Geddes
Polyurethane foam has many uses and applications, some of the notable properties of PU foam include providing acoustic and thermal insulation. Where a flexible foam is required, it can also be manufactured for fitting and sealing purposes.
The UV resistance and stability of polyurethane foam very much depends on what it has been manufactured for (and therefore what it has been made with).
Is polyurethane foam UV stable?
Where flexibility is a key property of the polyurethane foam, aromatic isocyanates will be included in the manufacturing process. Polyurethane foams which are based on aromatic isocyanates will all exhibit various shades of yellowing when exposed to light (including ultraviolet radiation).
The most commonly used aromatic isocyanate for flexible polyurethane foam is Toluene di-isocyanate (TDI), whereas for high resilient, semi-flexible and microcellular foams various types of Diphenylmethane di-isocyanate (MDI) are used.
The yellowing that can occur in UV exposed polyurethane foam is a surface effect and has no significant impact on the physical properties of the foam.The degree of yellowing depends on the intensity of the radiation that the foam has been exposed to.
Ultraviolet radiation breaks down bonds in the polymer via a free radical reaction (free radicals are atoms that contain an unpaired electron). These reactions can be complex, resulting in various coloured species being formed.
Enhancing the UV resistance of polyurethane foam and preventing discolouration
To prevent the discolouration of polyurethane foam, and enhance its UV resistance, light protecting agents and stabilisers can be added to the foam formulation at the manufacturing stage. These additives can be split into the following two groups:
Ultraviolet Absorbers (UVA)
UVAs function by absorbing harmful ultraviolet radiation and dissipating it as thermal energy.
Hindered Amine Light Absorbers (HALs)
HALs do not absorb ultraviolet radiation but act as radical scavengers to inhibit degradation of the polymer. Due to the fact that hindered amine light absorbers are regenerated rather than consumed during the stabilisation process, they tend to be more capable of providing better, longer-term light stability than UVAs.
UV stable polyurethane foam
For applications that require excellent polyurethane UV resistance, non-yellowing foam can be produced with the use of aliphatic isocyanates. Aliphatic isocyanates are isocyanates where the NCO (isocyanate group) is not directly attached to an aromatic/benzene ring and produce ultraviolet stable, colourless polyurethane foam.
Aliphatic isocyanates are generally considered as speciality materials since they are used in significantly lower quantities than aromatic isocyanates, which account for the bulk of isocyanates used for the production of polyurethane foam and other polyurethane products.
Aliphatic isocyanates are considerably more expensive than the aromatic isocyanates normally used, and therefore the manufacturing costs to produce light stable foams are higher than for standard polyurethane foam manufacture.