I'm trying to understand the shelf life of platinum cure silicon rubbers, for mold making in particular. Given the price point of these materials, I find myself really wanting to understand in deep technical detail, questions about the how and why why of whatever it is that makes the two-part components, A and B "go bad". I'm hoping to make more informed decisions when purchasing, using, storing, and maybe even salvaging the stuff.
First off, I'd like to know what are the primary cause(s) are that contribute to a two part platinum cure silicone rubber to "go bad", and in what happens when they do go bad? Does the mixture just tend to fail to cure? Does it take longer and longer to cure? Is it more sensitive to cure inhibition as it ages? Are their any indicators that would tell you that a batch has gone bad (besides doing a small trial cure), and if so what are they?
Also, there are two components, A (liquid silicone base usually) and B (platinum based catalyst usually). As the product ages, is it just the catalyst that goes bad, or the liquid silicone base?, or do both go bad? If both, then which component goes bad faster? I would think at least one component would tend to go bad quicker than the other.
Let's say I had been given a large container of part "A" that was well past it's prime, say 1.5~ 2 years since its batch manufacture date. How likely would it be to cure properly if I replaced the missing part "B" with a brand new tub (just of "B", mixed with my aging "A")? Of course I ask this because I do happen to have this exact situation. I was gifted with a large pail of Smooth-On Mold Star Series platinum cure silicone, but just the part "A" mind you, and even that was from a batch manufactured in February of 2018 (I called Smooth-On about purchasing just part "B" and they do sell it that way upon special request, but did not seem confident that a batch that old would still work. But the guy I talked to at least, did not know the specifics about how it goes bad etc., he just knew my part "A" was well beyond it's shelf life.
Smooth-On Mold Star Series, Part A (only) of a gallon kit
the catch (besides missing Part B): it's pretty old. Not good for a product with a 12 month shelf life
I looked it up though, and the bucket I had It was half of a gallon order. To give you a value context, I looked it up, and a brand new 1 gallon order of Mold Star costs $185.00. The bucket of "A" that I have seems mostly full too. So if it did still work, I could get it basically a gallon of the stuff for about half off (having to replace the part "B" with new product). Almost $100 worth of rubber is on the line here, but then again I could end up wasting the purchase of part "B", or at least have to go back and fork out another $100 to get the matching new part "A" in order not to have wasted the first $100 I spent on the new, replacement part "B". For more information about the product, here is a link to the product page on Smooth-On's site:
https://www.smooth-on.com/product-line/mold-star/
Sure, I have this dilemma before me now to solve, and I'm fully expecting since it's so far out of date, there is probably a snowball's chance in hell that I'll be able to salvage anything useful out of this bucket of old Mold Star part "A", but it did get me thinking and wondering about the shelf life of silicone rubber components in general and I'd like to understand a lot more. Especially at this price point, I want to know everything I can that could help me avoid making expensive mistakes.
I fell like if I understood more about how it goes bad, as in what are the primary mechanisms that degrade the stuff, I would be better equipped as craftsman in the stewardship of the materials of my craft.
For example, let's do a little thought experiment that supposes that I did have that missing understanding about how the product components degrade on the shelf. Lets say that I found out that it was actually exposure of both of the components, A and B to the oxygen in the air that was the primary mechanism that degrades these material components over time... If that were true, I would certainly think twice before opening a newly purchased set of rubber "just to try the stuff out" when I got back from the store. In fact I'd probably wait until I had the actual project I need it for was right in front of me before I actually opened the containers and exposed both components to air for the first time since manufacture. I would also be very mindful about keeping the lids tightly sealed at all times, except when I was dispensing it for a project. I may even consider storing the components under vacuum, to minimize the amount of oxygen the components are exposed to in storage.
Of course, in reality, I DON'T actually know that its oxygen that degrades the A and B component(s) over time, though, that was just a thought experiment to illustrate how useful such knowledge could be. Can someone that understands this decay process of platinum cure silicone rubber components over time, explain it to me?