Paint strippers are essential for removing paint layers and for recovering artifacts to be repainted. According to the type of paint and the material to be stripped, we find different products with specific characteristics.
Alkaline paint stripping
Mainly suitable for iron products, it can also be partially used on aluminum, avoiding alloys that are too reactive with alkaline agents. Solvent-based alkaline paint stripping is usually more effective than water-based stripping, as it has a faster action even at room temperature. The paint, in this way, denatures and dissolves in the product. Water-based products require higher temperatures and execution times, but sometimes are able to remove even very thick and complex paints, detaching them.
A historic alkaline water-based paint stripper, shown in the examples, contained a huge percentage of water and soda and a minimal part of solvent and additives. The strength of this paint stripper lay in the fact that it is possible to heat it, even in autoclaves, up to 90 ° C, thus being able to remove tough and thick coatings, for example from bodywork and military artifacts.
A disadvantage of solvent-based paint strippers is the saturation which quickly reaches the bath; the paint, being pulverized, creates cloudiness and compresses on the bottom, requiring continuous filtration and handling.
Acidic paint stripping
This type of paint stripping removes the film by acting between the metal and paint interface, raising it. Generally, it is used only for aluminum, since the acid pH can oxidize the iron artifacts if in contact for too long. Acid paint strippers were once widespread, but they used raw materials that were harmful to health that are now disappearing from the market. Among the most used acids we find formic (often in combination with dichloromethane), but now classified as toxic.
However, very effective aqueous acid-based products can be formulated very well, using organic acids and solvents that are miscible or rendered soluble in water.
Gel or paste paint remover
Very common in the bodywork and aeronautics sector, where large metal sections are covered with paint stripper which must remain vertical on the surface. Due to the contact with the operator who has to apply the product, these paint strippers should be as less dangerous as possible. These products can remain on the surface even for several hours or overnight, a plastic cover could also be added to prevent evaporation and allow greater penetration of the product.
At the end of the paint stripping, the paint can be removed by scraping or high pressure washing.
Dichloromethane is renowned for being the main stripping solvent. The molecule of this solvent is able to penetrate the paint in a very short time and lift it from the substrate. The dichloromethane can easily remove paints such as polyesters and epoxy polyesters in a few seconds, but it must be combined with other ingredients to make it functional on more resistant paints such as epoxies. The historical formulations were based on dichloromethane, formic acid, phenols and dodecylbenzenesulphonic acid, the latter able to keep the acid solute in the solvent. Additives completed the formulation, for example corrosion inhibitors or oily products which, by stratifying on the surface, limited the evaporation and the harmfulness of the paint stripper. Dichloromethane was slowly replaced by pyrrolidones, especially NMP and then NEP. Both portentous, they were also declared harmful and reprotoxic, which is why they are now less and less used, together with other well-known solvents such as methyldiglycol. The search for new raw materials at the moment seems to have stopped on the renowned benzyl alcohol, which seems to have no competitors on a par with low toxicity. Benzyl alcohol is practically active on all types of paint and is compatible with a large variety of other solvents, bases and acids. Its functionality even when cold makes it preferable to other noteworthy solvents such as DMSO, which seems to be effective in a reasonable time only in hot conditions and in a strongly alkaline environment, where it manages to complex into a super-base. Furthermore, DMSO is not always well seen due to its odor and its ability to penetrate tissues and skin. There are also other solvents such as DBE (dibasic esters) which have the ability to denature and slowly penetrate even in multi-layer paints. This solvent is increasingly used, especially for those paint strippers declared "green", in fact it does not present any toxicity to humans or the environment.
As far as gel paint strippers, more or less the same indications as traditional paint strippers are apply, however there are greater difficulties due to the physical state of the product which, being gel or pasty, finds it more difficult to penetrate into the paint. For this, reaction catalysts such as hydrogen peroxide, amines, ammonia or strong acids are often used.
For the stripping of wood, neutral products based on volatile solvents (such as acetone, dioxolane, MEK) are preferable which, thanks to their evaporation, cannot penetrate into the wood fibers. Unfortunately, toxic solvents, such as methanol and xylene, are still used today.
As gelling additives, modified carboxymethylcelluloses are usually used for solvents, which make the final product quite viscous, or kaolin or bentonites which make a pasty and less sticky effect.
Economical traditional paint stripper - For high temperatures and long time
- Mains water: 70-75%
- Sodium gluconate: 2-4%
- NaOH 30%: 15-20%
- MEA: 2-4% (optional)
- Butyl Diglycol: 2-3%
Quick paint stripper at room temperature - Flammable, high toxicity
- Benzyl alcohol: 45-55%
- Xylene: 25-35%
- Formic acid 85%: 10-15%
Gel paint stripper for wood - Flammable, non-toxic
- Dioxolane: 50-70%
- Butyl acetate: 15-20%
- Methyl ethyl ketone: 5-7%
- Paraffin in hydrocarbon solution: 2-4%
- Fatty alcohol C12-C15: 1-2%