Different kinds of plastic

[stratokaster]

Recently I visited our local flea market and acquired an old IBM keyboard for 10 UAH (~40 cents at the current exchange rate). It's an IBM KB-6323. Its color was what grabbed my attention. Every other vintage keyboard I have ever seen showed some kind of discoloration, usually yellowing. But this thing, despite being 20 years old and incredibly filthy, was still white.

I opened it up for cleaning and there was a label inside saying it was made of HIPS, which apparently stands for "high impact polystyrene." The case is very sturdy and solid, and like I said before, it shows no signs of discoloration.

The keycaps are also their original color without any yellowing. They are definitely not PBT, they feel more like ABS, but somewhat harder and more slippery. The material is very similar to that of the case, so I assume they are also made of HIPS. The legends are lasered and then infilled with paint.

The fact that this keyboard still retains its original color after all those years kind of makes me wonder why keyboard cases and keycaps made of HIPS are so rare... Googling it led me to understand that it was used only by Chicony. Are there some downsides to using HIPS or are ABS plastics just cheaper to produce?

[Findecanor]

HiPS and ABS are two related alloys (I.e. mixes), composed of polystyrene and other chemicials.

HiPS = Styrene + Butadiene, where Butadiene is a type of rubber that makes it more flexible. Pure polystyrene can be quite brittle.
ABS = Acrylene + Styrene + Butadiene, where Acrylene adds strength. You could say that ABS = HiPS + Acrylene.
There are many different types of HiPS and ABS out there with different proportions of each component. Some ABS used for keycaps is very hard while ABS and HiPS moulded in sheets for vacuuforming is very soft in comparison.
Then there is PC-ABS, which is ABS alloyed with Polycarbonate. Polycarbonate itself is the toughest and impact-resilient of the components.
They are also different to work with. There are different melting temperatures, and I think that ABS and PC-ABS need to be moulded at narrowed temperature intervals.

Then there are brominated flame-retardants in many mixes used for electronics. These turn the plastic yellow with exposure to sunlight (UV light). If there are less/no flame-retardants and/or the item has been stored in a room with no windows then it should not have yellowed.
Retr0brite (for de-yellowing plastic) affects predominately the bromine at the surface. When a retro0brited item has yellowed again it is believed to be because bromine deeper in the plastic has migrated to the surface.
Use of bromine in plastics should be lower in new products because of reports of flame-retardant compounds found accumulating in fish and other marine life.

I have seen keyboard cases of all three of these: HiPS, ABS and PC-ABS. There is sometimes text on the inside for different alloys and an arrow that points to the one that has been used for just this one. Often the plastic manufacturer's designation for the alloy has been used instead of just "ABS", which you can look up on the Internet to find what plastics it is composed of and if it contains any flame-retardant chemicals.

[andrewjoy]

I suspected not all ABS is created equal, look at beamspring caps vs some crappy lenovo keyboard

now we need caps made of carbon fibre