After many years of scientific research, it has been found that HFCs, which used to be used as aerosol propellants, have a certain destructive effect on the ozone layer, so the production of CFCs in Europe and the United States is restricted by law.
The main substitutes for CFCs are liquefied petroleum gas (LPG), dimethyl ether (DME), difluoroethane (DFE), carbon dioxide (CO2), compressed air, etc. LPG and DME are widely used.
In recent years, people have become increasingly aware of the greenhouse effect of volatile organic compounds (VOCs) on the global environment as they enter the atmosphere. California and New York State have mandated that VOCs in aerosol products must be controlled to less than 80% of the total volume, and will continue to do so to reduce this percentage. This requirement is gradually being accepted by many countries in Europe.
1) DME is highly soluble in both polar and non-polar substances, so it is miscible with water (is cheap and can reduce VOC ratio) to keep the product homogeneous; while LPG is non-polar, so it cannot be compatible with water.
2) Water and alcohols are generally used as solvents to cooperate with DME, which formulation is low-flammability compared to LPG.
3) The price of DME is only 1~2 times of LPG, so the difference is not big.
Therefore, it seems that the aerosol product with DME as the propellant has a good market prospect.
One of the major disadvantages of aerosol products with DME water-based formulations is that they are more corrosive to packaging materials, especially tinplate cans.
As a result, products such as hairsprays or air fresheners are sometimes sprayed with an unpleasant smelling brown liquid.
Corrosion occurs because the feed liquid in the acidic conditions and the inner wall of the can produced an electrochemical reaction.
This is typically manifested in the form of tinolysis of the tin-coated outer layer, where the hydrogen ions in the liquid are reduced to hydrogen gas.
Sn → Sn+2 + 2e E=0.1364V
2H– + 2e → H2 ↑ E=0 V
When developing aerosol formulations, the PH is maintained mainly by paying attention to the choice of ingredients.
Let’s take hair spray gels as an example. Choosing the right hair fixative (hear fixative polymer) is very important. Different types of hair fixatives have different PH levels due to their different structures.
In pure alcohol formulations, there is no corrosion because no effective electrolyte is formed. However, in the water and DME formulations, there may be free hydrogen ions, so that an electrochemical reaction occurs.
The widely used fixatives on the market are now formulated for comparison, as recommended by the manufacturer for neutralization.
The six representative hair fixers were made into water and DME formulations for storage experiments and stored at a constant temperature of 50°C for 4 months. Their advantages and disadvantages were found from the following indicators.
|5||-17||-23||-23||room temperature||room temperature|
10 scores: no corrosion
8 scores: spotted tin removal
6 scores: red or black corrosion in the feed liquid
4 scores: dotted or pitted etching
2 scores: severe corrosion occurs
0 scores: can erosion
10 scores: no corrosion
8 scores: slight mottled discoloration of the weld and the welded area.
6 scores: moderate discolouration corrosion
4 scores: severe discolouration corrosion
2 scores: complete discoloration corrosion
0 scores: perforation occurs
The above experimental data show that hair fixers with a lower pH value are more likely to cause corrosion. Therefore, when determining the formulation, great attention should be paid to the pH of the components.
Although alkaline conditions have an inhibitory effect on corrosion, but too strong alkaline often changes the properties of the product.
Let’s still take hair spray gel as an example. Too strong alkaline not only damage the hair, but also makes the fixative not work as well as the original styling.
So it has been reported in the literature that hair spray gel liquid pH is generally controlled at about 8. 5.
When the metal is immersed in a liquid with a high dielectric coefficient, the solvent molecules with a high polarity are attracted to the ions on the metal to produce solventization. As a result, the bonding force between some metal ions and other ions on the lattice is weakened, so that they can leave the metal surface and enter the nearby water layer. The remaining electrons are left on the metal, making the metal negatively charged.
Due to the electrostatic attraction, the positive ions in the solvent are active near the surface of the metal, if there is a positive ion stronger than the metal ion itself, it will continue to absorb the electrons left on the metal, and the metal will continue to lose electrons, so that the metal ions continue to diffuse into the solvent.
The purpose of adding corrosion inhibitors is to isolate these electron-prone ions in the solvent from the double layer structure of the metal surface.
There are two types of corrosion inhibition mechanisms: passive and film-forming. Passive inhibitors, also known as oxidizers, are mostly composed of a “passive” positive field consisting of phosphates and silicates.Film-forming inhibitors form a barrier between the metal surface and the solvent environment and can be organic or inorganic.
Organic film-forming inhibitors are long-chain molecules with a polar terminal that is adsorbed to the metal surface. The reaction of the polar end of the organic aqueous environment shows repulsion, and the inorganic film-forming inhibitor participates in the formation of an insoluble layer on the surface of the metal molten metal.
One problem that must be noted is that for pressure packaged containers, corrosion is present not only in the liquid phase part, but also in the vapor phase part. The tiny liquid droplets in the solvent vapor can also react with the can wall electrochemically. Therefore, the choice of corrosion inhibitors should be balanced. Also note that corrosion inhibitors should not be used in personal hygiene products, such as hairsprays, mousse, etc. if they are toxic.
Liquid phase preservative
Vapor phase preservative
When formulating, corrosion inhibitors can be selected according to the requirements of different formulations and the properties of other raw materials. For example, the Japanese literature reported the use of a fixed proportion of benzoic acid, EDTA-2Na, diethanolamine as corrosion inhibitors in hair spray gels. Neither the liquid nor the vapor phase sites will corrode.
1) Although the existence of hydrogen ion directly affects the effect of corrosion resistance, but the existence of halogen ion is not to be ignored. Once the metal is exposed, the typical hydrolysis reaction in water will occur.
M–+CI–+H2O → MOOH+H–+CI–
The MOH here can also form metal oxides, i.e. rust, and the hydrogen ions generated from it can aggravate the corrosion process. Therefore, it is required that the chlorine or bromine ions in the feed solution should be controlled below 3PPM.
2）The presence of oxygen in the system is also very dangerous. The oxidation of metals can interact with oxygen and water to form hydroxides.
4M+2H2O-O2 → 4MOH
Therefore, the oxygen in the vessel should be removed by vacuum sealing or by introducing nitrogen into the feed under over-pressure using a tube to remove additional oxygen.
3）The strong polarity of DME aqueous solutions is soluble for many polymers. So in order to prevent corrosion and leakage, the valve seals should be made of butyl or chlorobutyl rubber, and the surface of the valve should preferably be coated with Polyethylene or polypropylene coating.
4）For general anhydrous formulations, the thickness of the inner wall of the tinplate, i.e. the thickness of the tin plating, is 0.56 g/m2 for white tinplate cans, while for water-based formulations that require a certain corrosion margin, the minimum thickness of the tin should be 1.12g/m2.
The entire aerosol industry at present, due to price, capacity and other factors, there is still a larger part of the tin cans that are being used and will continue to be used. So under the premise of ensuring product performance, in-depth research on corrosion prevention of tinplate cans is very meaningful.
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