Thin-layer technology

Thin-layer technology is a single-stage pre-treatment system for preparing metal surfaces to painting.

The products that exploit this technology are composed of mixtures of solvents and suitable synthetic resins that, in combination with each other, are able to degrease and clean the surfaces, coating them at the same time with a micrometric insulating coating. This coating has the function of protecting the surface from oxidation, incorporating contaminating oils during drying, promoting the adhesion of the paint.

The process operates with a single immersion or spray bath, at room temperature and without creating any type of wastewater.

The management of this pre-treatment is simple, the pieces to be cleaned are hung, immersed or nebulized by the product, left to drip for about 10 minutes and finally placed in the oven or left to dry in the air (in the versions based on flammable solvents). Most of the oil present on the dirty pieces is removed and dissolved in the solvent, while the remaining part remains on the piece and is incorporated by the resins during polymerization (up to a maximum threshold of about 4% oil).

Thanks to this system, the bath will hardly become saturated with oil and will never need to be replaced, but only topped up with fresh product, as it runs out.

Coarse pollutants (workshop dust, shavings and dirt) will be removed with bag or magnetic filtration, through gentle movement of the bath.

Formulation examples

The technology has now been widely distributed for more than 30 years (sometimes called Plaforization), initially low-cost flammable solvents such as acetates, alcohols and xylene were used, preferring the air drying process. Over the years, solvents have been almost completely replaced with non-flammable and medium molecular weight glycols, increasing safety in the workplace and indirectly allowing better drying of the resins, thanks to the use of the oven in the final step (up to 150° C). Lately there has been ongoing research into new glycols with a lower environmental impact or eco-friendly surrogates; the essential thing is that they maintain a good dissolving power of the oils (as well as the resins contained), a modest evaporation capacity in the oven and a viscosity such as not to compromise dripping. Usually the amount of solvent in the formula exceeds 98%. 

The resins involved in this technology must have certain characteristics: thermosetting, easily cross-linkable, non-yellowing, compatible with oils and solvents and not too viscosifying so as not to create excess thicknesses unsuitable for painting. Resins are often used in combination with each other, to increase the cross-linking, aesthetic and adhesion power with the paint. The total percentage of resin varies from 1 to 2%.

Efficiency

Thin film technology products are ideal for treating metals (from iron to aluminum) with a standard level of dirt and free from oxides and encrustations. Galvanized metals and alloys that may have extrusion greases and oils on the surface that are not easily removable are not suitable.

They certainly have benefits compared to traditional phosphating and detergent products, but they also have limitations, now exposed:

Considerations

Although they are often advertised for their great resistance to salt spray corrosion, they offer mediocre resistance, comparable to a good traditional degreasing treatment (about 300 hours). Some products still on the market (acidicic and single resin based) even provide lower resistance, comparable to phosphodegreasing. These values ​​are referred to standard ferrous surfaces; steels and aluminums resist up to 1000 hours of salt spray, but this is not due to the thin film technology, but rather to the intrinsic resistance of the materials.

A myth and a mistake related to these products was the fact of acidifying to make the surface reactive and create a chemical link with the resin, but this does not actually happen because there is no water and there is not enough oxidative reaction, indeed, the excess acid can create, during drying in the oven, blackish overthicknesses. This refined ability to oxidize the surface is instead verified with the so-called Wash Primer products, used for the manual treatment of artefacts for hobby, automotive and other surfaces where a tenacious primer is required.

A particular controversy is due to the statement that glycol-based products are free of VOCs (Volatile Organic Compounds) and therefore suitable for a low environmental impact perspective. The question is only partially true, high boiling glycols certainly represent the ideal solution for treating manufacts without problems of fumes or acute toxicity, however a passage in the oven is required to allows the total evaporation of the solvent phase of the product (approx. 98%). This translates into both energy consumption and harmful fumes to be reduced to avoid their emission into the atmosphere.

From this point of view, phosphodegreasing would become competitive again, especially if the process could be optimised to be able to operate at room temperature, thus eliminating the energy costs of the burners. Further ideas for updating phosphodegreasers are described in the dedicated post: https://www.surface-treatment.it/l/amorphous-phosphating/