IBM 5251 - different keycap plastics?

[EnthusiastDude]

So I bought an IBM 5251 recently and noticed that some keycap legends are yellowed, while others are not at all yellowed. First I thought that the reason was unequal exposure to sunlight, but after I removed the keycaps I noticed a pattern emerging. All the yellowed keycaps have an all-white bottom, while the unyellowed ones are white and grey at the bottom.
There are even more differences when closely inspecting the keycaps, but I'll not going into too much detail since the color is enough to distinguish between them.

I have no clue if this has been reported yet, but I thought that this couldn't be coincidence as there was not a single exception on the whole keyboard.

Facts aside, here's what I think about this: Maybe IBM at some point changed their manufacturing process and with it the plastics used. However, I'm not sure if bromine was added, since the bottoms are a bit yellowed as well and shouldn't have seen too much sunlight.

I'm also looking for a way to fix the yellowing, if anyone here has a clue on how I could approach this, just let me know.

IMG_20200409_103429__01.jpg (2.71 MiB) Viewed 2182 times

IMG_20200409_103359__01.jpg (2.67 MiB) Viewed 2182 times

[inmbolmie]

Yes, that's something usual specially in the "internationalized" keys like your "Ü". There seems to be different batches of those keycaps or different factories or something that produced different qualities and those with an inferior quality are more prone to yellowing.

I recommend you not to try to fix the yellowing, as with using retrobrite processes with double shots you usually end up ruining the dark plastic and that is far worse and more noticeable than the yellowed legends. Anything you do you will have to do it for both plastics (dark and white) at the same time, and to get a perfect white you will certainly bleach the dark part. Also you won't have anything to test the process first, so you will have only "one try" to get it perfect.

Example: viewtopic.php?t=10065

[Invisius]

You're right about the manufacturing process changing, especially in EU factories. While these were caps originally made in the late 70s, replacement caps were available well into the 80s.

It's possible the ones yellowing are replacements that were manufactured in the Greenock factory, who were notorious for using ABS spacebars on late 80s Ms. One of the only parts that's yellowed over time on any IBM, even without sun exposure. You can see how similar of a shade it is in the picture attached.

Since the grey portion of the caps looks unaffected, I think it would be difficult to get a uniform retrobright without streaks or turning the greys lighter. Without knowing the composition of each "shot", it's hard to say for sure.

[EnthusiastDude]

... that's something usual specially in the "internationalized" keys like your "Ü".

I'm not so sure on that, it appears all over the keyboard and some international keycaps like "Ä" are absolutely pristine.

I recommend you not to try to fix the yellowing, as with using retrobrite processes with double shots you usually end up ruining the dark plastic and that is far worse and more noticeable than the yellowed legends. Anything you do you will have to do it for both plastics (dark and white) at the same time, and to get a perfect white you will certainly bleach the dark part. Also you won't have anything to test the process first, so you will have only "one try" to get it perfect.

I thought so too, but am still hopeful that there is a way to fix this. I'll probably try to get some replacement keycaps from a selectric typewriter though before I do anything irreversible.

[Erderm_]

Yes, that's something usual specially in the "internationalized" keys like your "Ü". There seems to be different batches of those keycaps or different factories or something that produced different qualities and those with an inferior quality are more prone to yellowing.

+1 for this. My 3278 has only the Ü and the number 1 key are the only yellowed keys. All other ones are white as bone.
(not really sure why the 1 key is yellowed though as I dont think its one of the ones that would be changed for a different layout.