Eric Rotheim, undoubtedly the inventor of the aerosol can, discovered dimethyl ether (DME) in 1926. Today, almost 100 years later, aerosol cans still exist and DME is used as a propellant.
Broadly speaking, if we had to give a profile to the ideal propellant, what characteristics should it have?
- Good pressure
- Chemically inert
- Miscible with water
- Not toxic
- Not flammable
- Not explosive
- No smell or taste
Today, there is no propellant that meets all these parameters at the same time.
The most that we can find are certain gases that present optimal conditions with respect to each of the points mentioned and depending on the formulation needs of our final product, choose between several possibilities.
Under these premises, the DME is presented as the best of all the propellants on the market since it is the one that covers most of the previous points in the best conditions.
The main advantages that distinguish it from other propellants known to date are:
DME is produced by a process of catalytic dehydration of methanol that gives water as a by-product.
Thanks to this process, a minimum purity of 99.9%, a maximum methanol content of 1 ppm and 100 ppm of pure water are guaranteed, as well as a stable pressure. This results in the security that we will always know with what type of gas purity we are going to work, unlike other propellants, mixtures of different gases and origins that do not guarantee the previous parameter.
- Miscibility with water
DME is miscible in water. By using from 6% to 34% weight of DME it is possible to form a single liquid phase with water: stable, homogeneous, and transparent. For other amounts of DME, phase separation is formed, although just adding 6% by weight of ethyl alcohol prevents such formation.
The water formulation is positive for several reasons. Water is cheap, non-flammable, it is not a VOC (volatile organic component), and it is a great solvent. On the other hand, it is colourless, odourless, and ecologically accepted. The addition of certain amounts of water reduces the flammability of certain products (and even in some cases cancels it), increases the solubility of certain active principles and reduces the total cost of the formulation and in formulations where the use of perfumes is required, facilitates its development.
- Solvent power
Its great solvent power is remarkable and very practical in certain formulations. A theoretical value of this parameter is its solubility value. These values are calculated considering, among various factors, the heat of vaporization and the molar volume. Propane and butane have solubility values between 5.8 and 6.6. The DME presents a value of 7.3.
A more recognized value to illustrate solvent power is the Kauri Butanol number. This value is obtained by using a 20% standard solution of a Kauri resin and titrating it with a solvent or a propellant. The number of millilitres required to achieve the cloud point at 20°C is defined as the Kauri Butanol number. Propane and butane have values between 15 and 20. The DME is markedly higher, around 91.
A practical way to demonstrate this solvent power of DME is its influence on the “cloud point”. DME lowers the “cloud point” of aerosol formulations, which is particularly important in hair spray formulations where most of the most common polymers are compatible with DME.
DME increases the solvent power of formulations; This makes it an excellent component for hairspray, spray paints and some technical products such as spray polyurethane foams, adhesives …
DME can be used in multiple ways:
- as the sole propellant.
- mixed with the most common hydrocarbons, minimizing their major disadvantages (solvent power, miscibility with water).
- mixed with CO2, being the best solvent for this product.
- mixed with the new generation of HFCs such as 134A, 152A, 365 fa, 227 …
- Fundamental in reducing VOCs in formulations
To reduce the emission of VOCs (volatile organic components) of current formulations, it is necessary to replace some of its components with others that are not considered VOCs.
The ideal substitute in most cases is WATER, as it is colourless, odourless, non-flammable, non-toxic, non-VOC, compatible with the skin, inexpensive, and a good solvent.
This means that we must choose a propellant that is miscible in water. Today, the only one that offers this possibility is the DME.