In the July Q&A on Patreon, Belinda asked: I have two questions...1) do you know if polyquaternium 10 has an expiration date" I've checked the website I bought it from (Personal Formulator) plus a couple more that sell it, and I've not seen anywhere if it does, indeed, expire. The reason I ask is because I'm having problems getting it to dissolve in my water phase, and I was wondering if the reason was that it's not "fresh". 2) Is acetic acid and aspen bark extract a good preservative system for hair conditioner" I've seen this listed on a product being sold on Etsy, and I have not found any evidence to back up the fact that this is a good preservation for a conditioner. I told my friend she should throw it out, but am now wondering if I was over-reacting. 
The quick answer to the first question about polyquaternium 10 is that everything has an expiry date at some point, so we just have to find it. This version has a shelf life of a year, as does this one. So I'd say a year" Which sucks because I have some that I've had a lot longer than I thought!

Acetic acid (aka ethanoic acid) is the acid found in vinegar. In this post (scroll down to Edina), Perry Romanowski says he doesn't think dehydrated acetic acid will work as a preservative, and he says it again in this post. I've seen variations on acetic acid in preservatives, like dehydroacetic acid like we find in Optiphen ND, but never just acetic acid. Could this person be u...