Paint strippers are essential for removing layers of paint and for recovering items to be repainted. Based on the type of paint and the material to be stripped, we find different products with specific characteristics.
Alkaline paint stripping
Suitable mainly for iron products, it can also be used on aluminum, avoiding too reactive alloys with alkaline agents. Alkaline solvent-based paint stripping is usually more effective than water-based stripping, as it has a faster action even at room temperature. The paint thus detaches and dissolves in the product. Water-based products, on the other hand, require high temperatures and longer execution times, but sometimes managing to remove even very complex paints, detaching them. It is absolutely forbidden to treat aluminum with alkaline water-based paint removers, while it is allowed to treat them with solvent-based alkaline (after testing)
Acidic paint stripping
This type of paint stripping removes the film by acting between the paint and metal interface, raising it. Generally used only for aluminum, as the acidic pH can oxidize iron products if in contact for too long. At one time, acid strippers were widespread, but they used raw materials harmful to health that are now disappearing from the market. Among the most used acids we find the formic, now however classified as toxic. However, very effective acid-based aqueous products can be formulated using organic acids and solvents that are miscible or made soluble in water.
Gel paint remover
Very common in the bodywork and aeronautics sector, where large metal sections must be covered with a paint stripper that must remain vertical on the surface. Because of the contact with the operator who has to apply the product, these paint strippers must be the less dangerous as possible. These products can remain on the surface even for several hours or overnight, often a plastic cover is also added to avoid evaporation and allow greater product penetration. At the end of the paint removal, the paint can be removed by scraping or high pressure washing.
Examples of formulation
Everyone knows the "king" of the solvents used for paint stripping, dichloromethane. The molecule of this solvent is in fact able to penetrate the paint in a very short time and raise it from the substrate. Dichloromethane is able to easily remove paints such as polyesters and epoxypolyesters in a few seconds, but it must be combined with other ingredients to make it functional even on more resistant paints such as epoxies. The historical formulations were based on dichloromethane, formic acid, phenols and dodecylbenzenesulphonic acid, which managed to keep the acid solute in the solvent. Additives completed the formulation, such as for example corrosion inhibitors or oily products which stratified on the surface limited the evaporation and the harmfulness of the paint stripper. These products were number one, but were then replaced by pyrrolidones, especially by the NMP and then by the NEP. Both portentous, were declared harmful and reprotoxic, for this reason they are now less and less used, together with other 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 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 also to other noteworthy solvents such as DMSO, which seems to have efficacy in reasonable times only when hot and in a strongly alkaline environment, where it manages to complex in a super-base. Furthermore, DMSO is not always well regarded because of its smell and its ability to penetrate tissues and skin. Then there are 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 for gel paint strippers, the same indications apply as traditional paint strippers, however, there are greater difficulties due to the physical state of the product which, being gel, finds it more difficult to penetrate the paint. For this they often use reaction catalysts such as hydrogen peroxide, amines, ammonia or strong acids. Certainly a higher speed of paint stripping is guaranteed thanks to the use of volatile solvents which, however, are not always desirable, except for the stripping of the wood in which solvents such as acetones and dioxolanes are still widely used. The gelling agents used are for example the classic modified carboxymethylcellulose for solvents such as Metocell, or solutions such as kaolin or some bentonites that make a "paste" effect, less sticky.