Why acetate instead of polyester?

Peter;

I'm not sure of all of the reasons, but Kodak's Estar or Dupont's Cronar support is very stable dimensionally, and much tougher than the triacetate.

I have seen Estar jams in high speed motion picture cameras rip the teeth off the sprocket guide mechanism. It is hard to cut and hard to tear, so the slitting and chopping blades would probably wear more rapidly. A jam in the coating process might be quite a big deal.

So, I would say that it is used where dimensional stability and strength are required. This is found in some photomechanical applications and photo mapping for example.

Ron Mowrey

Machophot 820 IR also uses a polyester base, I believe.

Kodak's BW motion pciture film on ESTAR base is predicted to last up to 500 yeas in temperature and humidity controled vaults.

Floppies are also polyester based too. With changes in temp; even polyester wanders too. It even has a grain; ie a directional history of how it was made. A perfect circular track on a floppy; polyester negative; becomes an oval; at the micro percentages; with a change in temperature. One place I worked we had an experimental super floppy; of a super tight track pitch; and I did alot of testing of this ovaling problem. Polyester costs more than acetate; when buying film for engineering drawings; and printing purposes. In mapping with our old process camera; polyester films are the norm for stability of dimensions.

I found that polyester based films curl terrible and are hard to flatten (or did I miss something ?)

I'm no expert on this by any means, but I have heard from conservators that apparently polyester is hard to spool in long lengths--has some static problems and is tough to splice etc. Mostly had to do with motion picture films. In the old, pre-digital compositing days, acetate was used because you could strip negs & chromes together by dissolving the base (actually a technique used now to save deteriorated acetate base negs). I know for a fact that the standards for long term records retention and reformatting in gov't archives are for polyester based b&w microfilm masters that are brown toned or toned in silverlock. this is the standard that all other types of media are measured against.

Rowland: Yeah, polyester base can be tougher than camera components--Old Zorkis have a hook on the take-up spool that goes through a sprocket hole to hold the film on. When you rewind it just tears out that sprocket hole.

Unless it's Tech Pan that is--then the little hook gets bent way out of shape until you realize that maybe you're applying way too much force, even for a tractor-factory Zorki. Lucky I have spare take-up spools...

it's the most stable film support there is though...typical acetate based neg can eventually shrink up to about 10% of it's size during it's life. what happens is not that the film "turns to vinegar" as stated above, but that it gasses off acetic acid as it ages. colder temps & lower rh will slow this down, but the emulsion however becomes wrinkled and buckled on top of the shrinking acetate. The oines I've seen always remind me of a potato chip in an odd way. You can print them, but they're all wrinkled and screwed up. At this point, your options are very limited as to what you can do--you can either attempt to float the emulsion off onto a new polyester base--or most likely you will just have to live with it. It costs about $100 or more per neg to treat them this way, and it's not a guaranteed process either. The best thing to do is a preventive measure actually of dropping your storage temps & rh as low as you can realistically hold them year-round (forever)--OR--to shoot polyester based films or just live with the fact that most acetate based films have about 50 or so years.

With polyester, alot of the current storage standards have the room temp & rh at an almost "normal" range, compared to acetate. There are certain printing papers made on polyester as well--Ciba Classic for one. Some of the Fuji c-print materials are made on a poly base too. The very best plastic storage sleeving materials for film & prints are made out of polyester too. The grade is Mylar D--now called Dupont Archival Polyester. They use this stuff in rolls in archives for preservation encapsulation as well as for protecting objects in exhibit mounts. The only drawback really is in the static issue and the sharp edges and smoothness of the surface--can cause ferrotyping with certain types of materials.

On the whole though--it's the material used most in preservation work. If you compared it to film sleeves for example--they used Cellulose Acetate for many years, but this fell out of favor for the same reasons as the film base. OTOH--mylar D enclosures often sell for about triple the price of other products, but you get what you pay for really.