Indeed, ABS gets more brittle with age, especially when exposed to UV. However, ABS is just a general term for a type of thermoplastic. There are many ways to adjust its behavior when it is formed along with many additives and techniques used to augment various properties. There is definitely something fundamentally different between the Model F and Model M cases. I have had many Ms of different ages and even the oldest M is nowhere near as brittle as any F I have ever touched. In fact, a brand new Unicomp case is much more brittle than an almost 30 year old M, even though both are ABS. I wish Unicomp would spring for the nicer materials. Their products would have a much higher quality feel to them.
Well, as I said, the primary difference between the M and F is the
engineering and not the material. Remember that on the Model M, all of the plastic that makes up the chassis is
structural whereas on the Model F the chassis plastic is largely
cosmetic. The F is also guaranteed to be older than the M, meaning more brittle. In terms of economics, where production overlaps (1986), they wouldn't have used two different plastics. Just wouldn't make sense. As far as thorough chemical analysis, well, I'll defer to a chemist on that because I really can't say 100% if they did something silly like that. It doesn't strike me as IBM-like and I will say that I've had the same problem with 'cosmetic' IBM plastic cracking for many many years across many many different products.
I'd have to get out the calipers, but to the best of my recollection, the actual shell on the F is substantially thinner than the M at various points and also lacks various reinforcing elements that are present in the M. Basically the entire F shell is more like the hasty cutout area on the M13. (Unlike the well planned cutouts on M5-1 and M5-2 which avoided structural elements.)
As for current Unicomp ABS being brittle, yeah, there's major reformulations been required over the years due to EPA regulations and such. But seriously, ABS has been a done deal there for a very long time. And it's not like it's a high cost item either - the QFR is ABS. I can't explain it either. There just seems to be a continuing downward spiral there - a friend's brand new Unicomp, case never opened, lost enough melt-rivets to start failing in less than a year. I offered to fix it, but the chassis had serious cracks and warping.