I have a white duvet cover, pillow cases and white cotton fitted sheets which are still as white as the day I bought them 17 years ago. I don't have any special secret, although obviously I always wash them separately. Nowadays I wash them with an Eco Egg, usually at 40 or sometimes at degrees.
I agree, rosenoir. Cotton and linen will stay properly white if washed at high temperatures, but any fabric with synthetic yarns in it won't stay white. It's the nature of the beast, I'm afraid.
I don't know if this will help Joelsnan but I always put a Colourcatcher paper in with each wash. This will need to be either pinned onto something big or placed inside a laundernet bag - or it may block the pipework. I suppose the colours are bound to deteriorate in time though.
I have a number of ‘first world’ questions however the one that is bugging me at the moment is:
Why, when I always wash whites separately on a hot wash, do they eventually turn a dull grey? I do occasionally add that whitening powder but still they do not maintain their new bought whiteness.