Ensuring the UV Stability of Polyurethane Foam
by Duncan Geddes
- Polyurethane foams which are based on aromatic isocyanates will all exhibit various shades of yellowing when exposed to light, including to ultra violet radiation. Yellowing 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. Ultra violet 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.
In flexible polyurethane foam manufacture the isocyanate most commonly used is Toluene di-isocyanate (TDI), whereas for high resilient, semi-flexible and microcellular foams various types of Diphenylmethane di-isocyanate (MDI) are used. Both TDI and MDI are classified as aromatic isocyanates, as they are both based on benzene ring-shaped molecules. Polyether polyurethane foams tend to yellow faster than polyester foams.
Preventing polyurethane foam discolouration
Light protecting agents and stabilisers can be added to the foam formulation at the manufacturing stage. This enhances polyurethane UV stability and can aid in the delay of the yellowing process. These additives can be split into the following two groups.
- Ultraviolet Absorbers (UVA) – these function by absorbing harmful ultra violet radiation and dissipating it as thermal energy.
- Hindered Amine Light Absorbers (HALS) – these do not absorb ultra violet radiation but act as radical scavengers to inhibit degradation of the polymer. Due to the fact that HALS are regenerated rather than consumed during the stabilisation process, they tend to be more capable of providing better longer term light stability than UVA’s.
For applications that require excellent polyurethane UV stability, 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 ultra violet 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. These isocyanates are considerably more expensive than the aromatic isocyanates normally used, and therefore the manufacturing costs to produce light stable foam are higher than for standard polyurethane foam manufacture.
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