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Posted on April 3rd, 2013

Why Do Some E-liquids Crack Plastic Tanks?

Cracked-ecig-TankHey everyone! Pat, Jet’s residence science guy, reporting for duty! And for my first proper science post, I’m going to be talking about a problem that a lot of advanced ecig users complain about: plastic tank cracking. If you don’t know what this means, you don’t have to worry about it – this is an issue exclusive to people who buy bottled e-liquid and fill their own tanks with it, which is a fairly small proportion of our customers. Nonetheless, I think this post could be interesting to even those who have never experienced the frustration of a cracked tank caused by banana e-liquid, so please read on – I promise I’ll do my best to explain everything you might not be familiar with.


First off, what are these “tanks” I’m talking about? Basically, they’re an alternative to the cartomizers with which all of you should be familiar. They’re usually cylindrical, clear, plastic tanks with a post down the middle, although there are plenty of variations on this basic design. And while there are some technical differences in the ways that cartomizers and tanks work, if you’re not familiar with tanks, you can basically just imagine them as big cartomizers made of plastic. (or, in some cases, glass, but today we’re talking about plastic ones)


There are plenty of advantages and disadvantages to using tanks, rather than cartomizers, and I’m not going to go over them here. You can find lots of heated debates on the subject on any ecig forum. But a known issue with tanks is that some e-liquids, including both our Apple Medley and Mojito Mint e-liquids, can cause certain tanks to crack after a day or two of use. A cracked tank can be an annoying expense – there are some pretty fancy ones out there – and the idea of vaping a liquid that can cause a plastic tank to crack makes some vapers nervous. So I’m here to tell you a little about what I know about tank cracking, and why it’s not really a cause for worry – just a fact of life that you need to be aware of if you’re a fan of tanks.


The first step in understanding tank cracking is finding out what plastic the tanks are made of. There are lots of different plastics out there, ranging from the polyvinyl chloride (aka PVC) used in your plumbing to the numerous varieties of polyethylene (aka PE), which are used in soda bottles, plastic bags, and all kinds of other things. Perhaps it goes without saying, but these plastics have very different physical and, importantly, chemical properties.


So what are ecig tanks made out of? I’ve seen a handful of tanks which use polypropylene (aka PP), but it seems that a majority use polycarbonate (aka PC). It’s easy to tell these two types of tanks apart – PP is a somewhat opaque plastic that looks kind of off-white in color, while PC tanks are as translucent as window glass, which may be why PC tanks seem to be more popular. I’ve never heard of a PP tank cracking, so the problem is, to the best of my knowledge, seems exclusive to PC tanks. Therefore, those are the ones we’re going to focus on.


Polycarbonate is in many ways an ideal material for a tank. Physically, it’s a very durable – scratch resistance is one of the main properties for which it’s known – and it’s fully transparent. It’s almost custom made for plastic eyewear, one of its major uses, and since a lot of vapers like to see their colorful juices and check their levels, it’s easy to see why companies have designed tanks using polycarbonate.


Unfortunately, polycarbonate has a dark side. I suspect many of you are familiar with the Bisphenol-A (aka BPA) controversy. For those of you that aren’t, I’ll keep it short: the main ingredient used to make polycarbonate plastic is probably bad for you, and using PC containers to store foods, drinks, and other things you ingest probably isn’t a great idea. I personally don’t think there’s much risk of using polycarbonate tanks to store e-liquid, but a) I’m not intimately familiar with the details of the hazard that BPA may or may not pose to your health and b) I’m not a doctor. If you want to be proactively cautious about your health, you might want to avoid plastic tanks.


But this is all sort of a digression from my main point – lots of people happily use polycarbonate tanks but want to know why they crack with certain e-liquids. So the other, more topical drawback of polycarbonate is that it’s rather chemically reactive compared to other plastics. This means that there are a lot of things that you shouldn’t store in PC containers because they will, at best, rapidly degrade its structural integrity, causing it to crack fairly quickly. It’s pretty easy to find a list of these compounds – just do a search for “polycarbonate compatibility” and look for chemicals labeled “not recommend”, “forbidden”, “severe effect”, or anything else to that effect


If you do this search, you’ll find that there are many, many things you shouldn’t (and arguably can’t) store in polycarbonate. Among them are plenty of nasty things that are unlikely to be present in even the sketchiest foreign-made e-liquid, like chloroform and concentrated hydrofluoric acid. But, hidden in the lengthy compatibility charts for PC are several Generally Recognized As Safe (GRAS) flavorings, a group of chemicals which includes all flavors that are appropriate for use in ecigs. Prominent examples of these polycarbonate-incompatible flavors include benzyl acetate, a compound commonly used in apple and banana flavors, so that explains why those particular fruity flavors are somewhat infamous for cracking tanks.


At this point, you might ask “So we know why those two flavors crack tanks, but what about the other tank cracking flavors? Why does Mojito Mint crack tanks, for example?” Well, I can’t tell you exactly what compound is responsible, though I can assert that chemical compatibility lists are not exhaustive, and that many GRAS flavorings are fairly obscure chemicals and unlikely to appear on any compatibility chart. And if that’s not good enough, I can point you to Cole Parmer’s polycarbonate compatibility page, which classifies “ketones” as having a “severe effect” on polycarbonate, explicitly stating that polycarbonate products are “not recommended for any use” where they might come into contact with ketones…


If you don’t remember ketones from chemistry class, well, let’s just say that lots and lots and lots of flavors are ketones. There are ketones used to impart fruity flavors, there are ketones used to impart minty flavors, and there are ketones used to impart earthy flavors. Name any flavor/smell you like, and a seasoned flavor chemist can probably name you a GRAS ketone which would be useful in recreating it. And, according to Cole Parmer’s compatibility chart, an e-liquid that contains that ketone would crack any PC tank in a couple of days.


So there you have it. Just about any flavor out there can crack a polycarbonate tank, because polycarbonate is a chemically-weak plastic. Some e-liquids won’t crack tanks, and if you like to use plastic tanks, you should stick to those e-liquids.


But if you don’t use tanks, you certainly don’t need to avoid tank-cracking e-liquids on the spurious logic that “anything that cracks plastic can’t be good for a person”. If polycarbonate were a decent proxy for your lungs, you could inhale phosgene all day without any ill effects. And, while I can’t claim there aren’t questions about the long term safety of the myriad flavorings out there which are used in e-liquid, I can tell you that amyl acetate, a well-known tank cracker, is one we’re more confident about the safety of.


That’s about all I have to say on the subject. In my next post, I plan to talk a bit about glycerin, as I hinted in my intro post, though that plan is subject to change without notice. So until next time, happy vaping! And remember to send any ecig questions you might have to – no question is too simple or complex for me to at least take a stab at answering!