Propy-what now? That's a question I hear quite a lot throughout a week. While the base ingredients of e-liquid are published in media on a nearly daily basis nowadays, there still is no shortage of people who remain unaware. And even those that know are not necessary aware of the details. Quick recap: Eliquid base is Propylene Glycol and Vegetable Glycerin in various ratios. Added to that is more often than not food grade flavourings, but not necessarily: flavourless e-liquid is most certainly a thing. Nicotine can usually be added in various amounts ranging from none to 18mg per ml, with the increasingly rare occurence of places that will go up to 24mg per ml. Food flavourings and nicotine are deep subjects of their own, and will likely be the subject of their own blog entry. Today, it's down to the basics: Propylene Glycol and Vegetable Glycerin.
Propylene Glycol (PG)
Before going any further, here is the NFPA Safety Index for PG:
Flammability: 1 (Materials that require considerable preheating, under all ambient temperature conditions, before ignition and combustion can occur. Includes some finely divided suspended solids that do not require heating before ignition can occur. Flash point at or above 93 °C (200 °F).
Health: 0 (Poses no health hazard, no precautions necessary and would offer no hazard beyond that of ordinary combustible materials)
Reactivity: 0 (Normally stable, even under fire exposure conditions, and is not reactive with water)
So aside from being about as chemically safe as it gets, what is PG? It's a solvent that is used mostly due to it's extremely low oral toxicity and various anti-viral properties. It's been used as the solvent of choice for asthma pumps by the pharmaceutical industry, it is periodically pumped through hospital air systems to help keep them free of harmful bacteria. It is also a key ingredient in many cosmetics, foods, personal lubricants, smoke machine liquids, hand sanitizers and food grade antifreeze. Wait, did you say antifreeze? We really are vaping antifreeze after all? Well, yes, we are. Antifreeze is simply something that freezes (and thus, most often vaporizes) at lower temperatures than water. Salt water qualifies. This is also why these work in electronic cigarettes: specifically because they vaporise with greater ease than water. And for the record, propylene glycol has by and large replaced its predecessor ethylene glycol as an antifreeze because it is food safe.
Just a little note: PG allergy is known to exist. It isn't very common, but it does happen. PG is so common that if you were to be one of the few who is allergic to it, you would most certainly already know.
Vegetable Glycerin (VG)
Health: 1 (Exposure would cause irritation with only minor residual injury).
Contrarily to popular belief, VG is actually the more hazardous of the two base liquids. So why that 1 on the health scale, and why is no one flipping out about it? If VG is heated to 270 C it begins to degrade into harmful acrolein. Your average e-cig will not reach those temperatures, and even if it does, it will taste of burnt oil, and whoever is using it will immediately stop. This isn't the kind of occurence that one does not notice. With that out of the way, VG is a sugar-alcohol compound and fills many of the same roles as PG. It covers most of the same bases as a solvent, antifreeze and food additive. It's main difference is it can be used as an alternative sweetener, which explains why e-liquids are always at least at little sweet.
Liquids will tend to vary between 70%PG/30%VG, all the way up to around 10%PG/90%VG and everything in between. Practically speaking, what does that mean?
First off, PG and VG have different viscosity. VG is by far thicker than PG. Smaller atomisers with low wicking properties will struggle with liquids above 60%VG, mainly because they are too thick to reach the coils as fast as it is being vaporised. Whereas, the bigger atomisers which can handle high VG liquids easily might tend to be a little prone to leakeage with liquids below 50% VG, because the much thinner liquid will tend to overwick.
PG carries flavour a lot better than VG does. It takes much less flavouring in PG to reach the same intensity of taste than in VG. Plus, the flavours tend to come out clearer in PG if only because the sweet taste of VG warps perception somewhat. As such, higher VG liquids will either have a higher ratio of flavouring in them, be more muted, or restrict themselves to flavours that are already considerably powerful. VG however is mellower and is softer on the troat.
Lastly, there is one thing that VG does better than PG and that thing is clouds. VG produces a denser more voluminous vapour than PG. For that reason, high VG liquids are of great popularity on dripping atomisers and in sub-ohm tanks.
Hopefully, that clears up any question remaining on liquid bases.
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