So, on the question of rubber decay. What exactly are the specific circumstances that differentiate between “dry rot” and “wet rot”? Well, it’s tough to say from what I’ve learned from my previous sources, but I’ll try to put together the best information I can, with the help of one additional source.
20191216/DuckDuckGo rubber dry rot polymersolutions
20191216/https://www.polymersolutions.com/blog/why-does-rubber-dry-rot/
First of all, temperature cycling decreases the wear and tear on rubber. Especially extreme temperature cycling does its toll. If you have really high seasonal hots and really low seasonal colds, then you shouldn’t expect rubber to last too long in such an environment. These temperature extremes take their toll due to the thermoset plastic nature of rubber. In particular, they snip the long rubber polymers shorter, which either causes rubber to become gooey and sticky (“wet rot”) or dry and brittle (“dry rot”). Oxygen exposure, UV light exposure from the sun, and high temperatures also take their toll.
But how do you know what the ultimate fate of rubber is, dry rot or wet rot when exposed to the extremes? Well, suffice it to say, I believe the end effect depends on the particulars of the rubber you use.
For example, I had two rubber objects in an second floor bedroom in a house with a south side window (hence more exposure to solar heating). Under these conditions, they experienced a number of temperature cycling stresses that they wouldn’t have experienced had they been in the basement, for example. However, both such pieces were placed in semi-closed containers that restricted, but didn’t eliminate, exposure to air flow, and indeed there is more air flow in an upstairs room of a house to keep the temperature regulated than, say, in the basement. One piece failed in a “dry rot” mode, the other piece was degraded in a “wet rot” mode, but only on the surface.
Additionally, I also had a brass-like piece of metal in the same area as the “wet rot” rubber piece. The metal was noticeably tarnished, but only on the surface, and this was easily cleaned and rubbed off.
Natural rubber latex is more sensitive to higher temperatures than synthetic rubber, so chances are if you have some rubber that is failing in a “wet rot” mode, it is more likely to be natural. But “dry rot” rubber failures, that is more characteristic of synthetic rubbers. Not to mention, that for some synthetic rubbers in particular, they are made flexible by plasticizers that can leak out into teh atmosphere. Without the plasticizers, they would otherwise be a hard and brittle plastic, i.e. what they end up turning into.
Some other interesting notes, some rubbers that are more like natural rubber latex may have additives mixed in that, when the rubber is worked and stretched, they make their way to the surface to help protect the rubber. But, when the rubber is not used, they stay within the rubber, and the rubber fails to get its protection that it would otherwise get under regular use.
But, a good closing note. Beyond computerized temperature monitoring, one cheap way to assess the climate stability of an environment for preserving rubber is to place a metal object in it that would tarnish when exposed to a sub-optimal archival atmosphere. If after a few years it shows tarnish, chances are that rubber objects stored in the same climate would become either gooey or brittle after that same period of time. But, if the metal preserves its shine, so would the rubber likely better preserve its stretch.
Particularly, in the case of certain kinds of computer electronics like vintage Quantum hard disk drives commonly found in vintage Macintosh computers, any “wet rot” of rubber that can happen at all is an issue because it can cause the read/write hard drive head to stick to the rubber bumpers in the disk mechanism. Luckily, if you are unsure of the archive-worthiness of a particular storage location, you can use a piece of metal as a simple and cheap indicator.