Editing Propellant (section)

===Liquified gas propellants===
{{See also|Aerosol_spray#Aerosol_propellants}}

====Principle====
Liquefied gas propellants are gases at atmospheric pressure, but become liquid at a modest pressure. This pressure is high enough to provide useful propulsion of the payload (e.g. aerosol paint, deodorant, lubricant), but is low enough to be stored in an inexpensive metal can, and to not pose a safety hazard in case the can is ruptured.

The mixture of liquid and gaseous propellant inside the can maintains a constant pressure, called the liquid's [[vapor pressure]]. As the payload is depleted, the propellant vaporizes to fill the internal volume of the can. Liquids are typically 500-1000x denser than their corresponding gases at atmospheric pressure; even at the higher pressure inside the can, only a small fraction of its volume needs to be propellant in order to eject the payload and replace it with vapor.

Vaporizing the liquid propellant to gas requires some energy, the [[enthalpy of vaporization]], which cools the system. This is usually insignificant, although it can sometimes be an unwanted effect of heavy usage (as the system cools, the vapor pressure of the propellant drops). However, in the case of a [[freeze spray]], this cooling contributes to the desired effect (although freeze sprays may also contain other components, such as [[chloroethane]], with a lower vapor pressure but higher enthalpy of vaporization than the propellant).

====Propellant compounds====
[[Chlorofluorocarbon]]s (CFCs) were once often used as propellants,<ref>{{cite magazine
| author     =<!--Staff writer(s); no by-line.-->
| title      = Fires Halted Quickly by "Lazy" Freon Gas
| url        = https://books.google.com/books?id=zt4DAAAAMBAJ&q=%22Freon%22&pg=PA115
| magazine   = [[Popular Mechanics]]
| quote = Freon chemical compounds in household refrigerators, air-cooling systems and as '''a [[DDT]] carrier in aerosol [[Fogger#Fogger composition|insect bombs]]''' have been found to be more effective in extinguishing fires than carbon dioxide.
| volume = 87
| page =  115
| publisher  = [[Hearst Communications|Hearst Magazines]]
| date       = April 1947
| access-date= June 7, 2019
}}</ref> but since the [[Montreal Protocol]] came into force in 1989, they have been replaced in nearly every country due to the negative effects CFCs have on Earth's [[ozone layer]]. The most common replacements  of CFCs are mixtures of volatile [[hydrocarbon]]s, typically [[propane]], n-[[butane]] and [[isobutane]].<ref>{{Cite journal|last1=Yeoman|first1=Amber M.|last2=Lewis|first2=Alastair C. |date= 2021-04-22 |title=Global emissions of VOCs from compressed aerosol products|url=https://eprints.whiterose.ac.uk/172292/18/elementa.2020.20.00177.pdf |journal= Elementa: Science of the Anthropocene|language=en|volume=9|issue=1|pages=00177|doi=10.1525/elementa.2020.20.00177|issn=2325-1026|doi-access=free}}</ref> [[Dimethyl ether]] (DME) and [[methyl ethyl ether]] are also used. All these have the disadvantage of being [[flammable]]. [[Nitrous oxide]] and [[carbon dioxide]] are also used as propellants to deliver foodstuffs (for example, [[whipped cream]] and [[cooking spray]]). Medicinal aerosols such as [[asthma inhaler]]s use [[hydrofluoroalkanes]] (HFA): either [[HFA 134a]] (1,1,1,2,-tetrafluoroethane) or [[HFA 227]] (1,1,1,2,3,3,3-heptafluoropropane) or combinations of the two. More recently, liquid [[hydrofluoroolefin]] (HFO) propellants have become more widely adopted in aerosol systems due to their relatively low vapor pressure, low [[global warming potential]] (GWP), and nonflammability.<ref>{{Cite web|url=https://prod-edam.honeywell.com/content/dam/honeywell-edam/pmt/oneam/en-us/medical-propellant1/documents/pmt-am-solstice-propellant-DataSheet.pdf|title=Solstice® Propellant Technical Bulletin|website=Honeywell|date = 2017}}</ref>

====Payloads====
The practicality of liquified gas propellants allows for a broad variety of payloads. [[Aerosol sprays]], in which a liquid is ejected as a spray, include paints, lubricants, degreasers, and protective coatings; deodorants and other personal care products; cooking oils. Some liquid payloads are not sprayed due to lower propellant pressure and/or viscous payload, as with [[whipped cream]] and [[shaving cream]] or shaving gel. Low-power guns, such as [[BB guns]], [[paintball]] guns, and [[airsoft]] guns, have solid projectile payloads. Uniquely, in the case of a [[gas duster]] ("canned air"), the only payload is the velocity of the propellant vapor itself.