Hydrocarbons

The hydrocarbon propellants are propane (freezing point —42.1), n-butane (freezing point — 0.5) and isobutane (freezing point — 11.7). Commercial butane is always a mixture of n-butane and isobutane. Mixtures with propane in different ratios are usually used to adjust the pressure. Hydrocarbons are cheap, stable and of low toxicity, but are highly flammable.

Aerosol propellants

A few years ago only one group of aerosol propellants, the fluorocarbons, was used. Due to the well-founded suspicion that these chemicals damage the ozone layer, they are now virtually obsolete. The gap has been filled by the increased use of pump systems, and by hydrocarbons and dimethyl ether.

W/O stabilizers

A relatively simple way of obtaining highly stable W/O emulsions is to use block copolymers such as PEG-45 dodecyl glycol copolymer. These products, with molecular weights of 2000 to 4000, can improve the stability of W/O emulsions by protection of the droplets from coalescence. The hydrophilic middle part is responsible for a good anchorage in the water phase, while the highly branched ends give a steric hindering effect. The high molecular weight imparts excellent skin compatibility. Contrary to other stabilizers, for example metal soaps, which increase the viscosity of the outer emulsion phase, these copolymers typically reduce the viscosity. It is possible to obtain light, highly stable W/O lotions that are good bases for sun preparations and which give a skin-feel very close to emulsions.

Single W/O emulsifiers

A limited number of W/0 emulsifiers are available. This is because ionic emulsifiers will not work in the case of W/0 emulsions and, since very low HLB is required. variations using ethylene oxide are not possible. W/0 emulsifiers have HLB values between 3 and 6. However, the type of the alkyl chain is also important. In practire, oleyl derivatives have shown good effects. The most widely used W/O emulsifiers are sorbitan monooleate, sorbitan sequioleate and glycerol monooleate, and with increasing importance, polyglycerol esters.
In addition to oleate, ricinoleates and isostearates have aIso found use.

The methoxy PEG-22 dodecyl glycol copolymer is a high molecular weight W/O emulsifier, which is especially effective in formulations containing mineral oils. It is a saturated ether and is therefore stable against oxidation and hydrolysis.

Water-in—oil (W/O) emulsifiers

From the dermatological point of view, W/O emulsions are preferable to O/W emulsions. The natural lipid fiIm on the skin is also a W/O emulsion. W/0 emulsions can improve the hydration of the skin and are an ideal base for lipid-soluble active ingredients. A problematic point is that they usually leave a ‘fatty-feeling’ on the skin, and this is not particularly appreciated by customers.
Another problem can be ‘oiling-off’ or separation of the oil phase on storage. Viscosity a ad stability are extremely process sensitive.