Hugh, You put one in my wheelhouse. The Delrins are actually all polyacetal homopolymer, not copolymers. Delrin (and other brands of polyacetal homopolymer) are a very good plastic. Originally, the only brand was DuPont, but now there are a few Asian sources, which I assume are just as good. The different grades are primarily based on additive packages, but melt flow (molecular weights) and coloring are differentiated by grade.
We routinely (every week) verify the identity of incoming Delrin raw material lots for an Italian medical device company that makes artificial heart valves and stents as well as for their outgoing products.
Differentiating DuPont Delrin from other brands of polyacetal homopolymer is not easily done as they all look alike for any of the basic tests. However a simple FT-IR test will tell me if a derailleur body is a polyacetal homopolymer (such as Delrin) or another type of plastic.
I recently purchased from our friends at Reperages Velo in France a NOS white Delrin long cage SLJ rear derailleur and I will post photos of it for you tomorrow. I tend to think the white body version is pretty rare (and pretty good looking too).
Jon M. Crate FAI Materials Testing Laboratory, Inc. 825 Chance Road Marietta, Georgia 30066 http://www.FAI.US
-----Original Message----- From: firstname.lastname@example.org [mailto:email@example.com] On Behalf Of Hugh Thornton Sent: Monday, January 31, 2011 9:03 AM To: Classic Rendezvous Subject: [CR] Simplex "Plastic" Derailleurs - Questions For MaterialScientists
The plastic in Simplex derailleurs is often referred to as Delrin (A Dupont acetal copolymer). As far as I can remember back in the early 1960s, when they first appeared, the publicity material referred to Delrin by name and the name has been associated ever since. Does anybody know whether Simplex continued to use Delrin or whether they changed to another acetal or something else altogether?
Another thing I am interested in is the deterioration of the finish and whether it is reversible. Occasionally early models in white resin come on the market but very yellowed. I have wondered whether that is through absorption of oil. grease and dirt into the material such that it is effectively permanent discoloration, or whether it is on the surface and can be removed.
The common finish deterioration on the black plastic moldings of later derailleurs is a blooming or whitening of the finish. I assume this is caused by atmospheric pollution or exposure to light. This does appear only to be "skin deep" and it can be removed rather laboriously with a mildly abrasive liquid such as car paint renovator. Does anybody know if there is a better way, such as using some not too noxious chemicals, that would make it easier to renovate, especially in the nooks and crannies which are hard to reach any other way?
You might well ask why anybody would want to renovate a Simplex derailleur when it is so much easier to throw it away, but the better models such as the LJ 4000, referenced recently by a listmember, are not so plentiful. The others seem to be considered so plentiful and so disposable that a world shortage of them could develop too.