UV Stability of Polyurethane Foam
Polyurethane foams which are based on aromatic isocyanates will all exhibit various shades of yellowing when exposed to light i.e. to ultra violet radiation. Yellowing is a surface effect and has no significant effect 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.
Light protecting agents/stabilisers can be added to the foam formulation at the manufacturing stage to aid in the delay of the yellowing process. These additives can be split into 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 polyurethane foam applications that require excellent ultra violet 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.