How (in)flammable is "isopropyl alcohol" really?

Question

I have an electric device which frequently gets very oily (it's a vaporizer for herbs). It came with a few individually packaged small "tissues" drenched in "isopropyl alcohol". I've used up them all now to clean the insides.

After each cleaning session, I wait a few minutes before I put in new herbs and heat it up again (to 365-420 degrees F). Every time, it feels scary, because "isopropyl alcohol" is said to be "extremely" flammable. I feel like it's going to explode every time, but it luckily never does. Since they shipped those with the product, and didn't mention a word about this in the "manual", I assume it's safe.

But how can it be safe? Is it not hot enough? Does it evaporate so quickly as to never actually be left in there when I heat it up?

I also bought a separate, very expensive bottle of what I assume is "high-quality" isopropyl alcohol and poured it into a glass container with a firm plastic lid, which I keep in a wardrobe. Luckily, it doesn't seem to evaporate into nothing, like used to happen with other alcohol which I previously attempted to store in the same container. After a couple of weeks, half of it had disappeared! That doesn't happen with "isopropyl" alcohol, apparently, but why doesn't it?

As you can hear, I wonder both how (in)flammable it really is, and what makes it not evaporate inside the box, but does evaporate very quickly (apparently) when not restricted by a closed container.

Answer 1

The Flash Point of a liquid is the minimum temperature at which a liquid gives off vapor within a test vessel in sufficient concentration to form an ignitable mixture with air near the surface of the liquid. The flash point is therefore dependent on the boiling point and consequent vapor pressure of the liquid (Source of Vapor Pressure):

$$ \begin{array}{l|c c c} \text{Flammable Liquid} & \text{Boiling Point at $\pu{1 atm}$} & \text{Flash Point} & \text{Vapor pressure at $\pu{25 ^\circ C}$} \\ \hline \text{Diethyl ether} & \pu{35 ^\circ C} & \pu{-45 ^\circ C} & \pu{532.7 mmHg} \\ n\text{-Pentane} & \pu{36 ^\circ C} & \pu{-40 ^\circ C} & \pu{434.3 mmHg} \\ \text{Hexane} & \pu{69 ^\circ C} & \pu{-26 ^\circ C} & \pu{151.2 mmHg} \\ \text{Ethyl acetate} & \pu{77.1 ^\circ C} & \pu{-4 ^\circ C} & \pu{94.7 mmHg} \\ \text{Methanol} & \pu{65 ^\circ C} & \pu{11 ^\circ C} & \pu{126.89mmHg} \\ \text{Ethanol} & \pu{78 ^\circ C} & \pu{12 ^\circ C} & \pu{58.8 mmHg} \\\text{Isopropanol} & \pu{82.2 ^\circ C} & \pu{12 ^\circ C} & \pu{43.6 mmHg} \\ \text{Water} & \pu{100 ^\circ C} & \text{N/A} & \pu{23.7 mmHg} \\ \end{array} $$

By definition, any liquid with a flash point less than $\pu{37.8 ^\circ C}$ ($\pu{100 ^\circ F}$) is considered to be a flammable liquid. Any liquid with a flash point between $\pu{100-200 ^\circ F}$ is considered combustible. In general, the relative hazard of a flammable liquid increases as the flash point decreases.

However, OP's isopropyl wipes are soaked with about 70% isopropanol and DI water (the best combination for better sanitation). For example, see these wipes on market. As a consequence, the flash point ($\approx \pu{18 ^\circ C}$) is increase on these wipes. Also, since vaporizer has good ventilation system (I assumed), these vapors never would get to saturation point in addition to combination of water and isopropanol vapors. That would reduce the flammability of vapors even with a spark (here are heating coils, I assume). That's probably why the user manual did not warn fire hazard.

OP's question about evaporation from an open bottle: As you can see in the given table, vapor pressure of ethanol and isopropanol is quite high in room temperature ($\approx \pu{25 ^\circ C}$) compared to that of water. If bottle is open, the vapor escapes without ever getting saturated like wipes in closed box. Thus, alcohol in the bottle keep evaporate and, as a result, volume in the bottle reduces with time. On the other hand, the closed space in the box of wipes eventually get saturated with alcohol/water vapors, and evaporation stops at that point.

Answer 2

Let's read the information from a source that has to be right, a supplier's (Fisher Scientific) Safety Data Sheet.

SECTION 9 : Physical and chemical properties

Appearance (physical state,color): Clear colorless liquid

Explosion limit lower: 2%

Explosion limit upper: 12.7%

...

Flash point (closed cup): 12.0°C

Auto/Self-ignition temperature: 425.0°C

...

OSHA defines "(in)flammable" as having a flash point below 93°C, which this material certainly does. But to generate the flash you need a spark. Without the spark you have to go to the much higher auto ignition temperature to make fire, which is the 425°C/797°F figure. With the spark -- well, you're above that 12°C/54°F figure. Fortunately in your case there were no sparks.

If you are heating or even working with isopropanol, keep smoking and other flame or spark sources away, because one spark ... see above.