By nanotechnology we mean those applications in which technologies, products or raw materials that are a few nanometers in size are used. In the metal pretreatment sector, nanotechnology helps to make the surface more regular and constant, as well as increasing adhesion with the paint.
How does it work
Nanotechnology is used as final metal treatment process, typically after degreasing or after amorphous phosphating. The best results are obtained if the surface on which this technology is applied is well cleaned and degreased. Nanotechnological products are usually represented by molecules called silanes, sometimes used in synergy with specific inorganic acids capable of reacting with the substrate thus forming a film called "ceramic"; the silane provides adhesion strength between the substrate and the paint while the acid (usually zirconic) guarantees considerable resistance to corrosion.
Nanotechnologies are the last frontier of pretreatment and over time tend to replace the much more complex tricationic phosphating. An advantage over the latter are certainly the economic and toxicological advantages (simplier industrial plant, reduced controls by the operator or laboratory, less chemistry used). Furthermore, with an optimized nanotechnological process management, it is possible in some cases to achieve the same performance as a tricationic one, in fact we are talking about more than 1000 hours of resistance in salt spray. The negative aspects are the price, shelf life and reactivity of the silanes which, if used incorrectly, can hydrolyze or combine and coagulate with other chemical elements, even with the ions of non-demineralized water.
Formulating silane-based preparations is never easy, precisely because of their marked reactivity. There are various types of silanes although we can say that those selected for the pretreatment of metals are for the most part amino-silanes, amino-alkyl-silanes or epoxy-silanes. In the surface finishing aesthetic sector are instead used modified silanes (for example with silica) to give considerable resistance to scratches and wear. However, these are particular final treatments which we will not focus on in this context.
In synergy with silanes, as already mentioned, small percentages of acids (such as hexafluozirconic acid or hexafluotitanic acid) can be added to the bath, the dosages must be done careful not to damage the silane (often alkaline) and the surface on which is applied. The acidity must be fixed around pH 5, while ensuring a non-excessive reactivity with the metal.
Beyond this base formulation, it is possible to add wetting agents to the formula, making sure that they do not remain on the surface before painting; the final rinse is important to remove these wetting additives and to stop the acid reaction on the metal. If there are no wetting agents and other additives that could compromise the painting, the final rinse can be omitted. In general, it is advisable to simplify the formula as much as possible, helping the silane to naturally attach itself to the substrate, without interference from other molecules.
In the industrial plant, the nanotechnology solution is preferably nebulized by a soft fog and directed to waste water, to avoide contamination of the silane bath. Instead, when used by immersion, they require daily pH and conductivity checks, visually no flocculations and opalescences should form. Any dilution of silane or posthumous washing must be performed with good quality demineralized water.
Parallel to the silanes, products based on the chemistry of phosphorus are spreading, they are called "phosphines". The operability of these raw materials is roughly similar to that described for silanes, with the advantage of being able to be used at much lower dilutions (on the other hand, they have a much higher price). The corrosion resistance performance would seem to be even higher respect silanes, exceeding 1200 hours of salt spray. The phosphines also work in synergy with hexafluozirconic acid; this element should be declared by suppliers, given that phosphine-based products are becoming increasingly popular as alternative and ecological products, in reality they could contain small amounts of toxic acids not declared in the safety data sheet.