Discolouration

Some polyurethane foam was spilt on unprotected window frames during application. Despite removing the spillage with the cleaning fluid, the u-PVC frame started to turn brown several months later – why was this?

You did not manage to remove all the spillage from the u-PVC surface; a thin, almost invisible film has been left. Over the course of time this film has turned brown under the action of UV light; to which polyurethane foam is not resistant.