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 which, in combination with each other, are able to degrease and clean the surfaces, while coating them with a micrometric insulating coating. This coating has the function of protecting the surface from oxidation, incorporating contaminating oils during drying, promoting paint adhesion.
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 sprayed by the product, left to drain for about 10 minutes, then placed in the oven or left to dry in the air. Most of the oil present on the dirty pieces is removed and dissolved by the solvent, while the remainder, present on the piece, is incorporated into the resins during their polymerization (up to a maximum threshold of about 4% of oil).
Thanks to this system, the bath will hardly become saturated with oil and will never be replaced, but only topped up with fresh product, as it runs out. The part of workshop dust, filings and dirt, will be removed with a filter or magnetic filtration, through a gentle movement of the bath.
The technology has now been widely distributed for more than 30 years, initially low-cost flammable solvents such as acetates, alcohols and xylene were used, preferring the air-drying method. Over the years the solvents have been almost completely replaced with non-flammable and medium molecular weight glycols, increasing safety in the workplace and also improving the drying of the resins on the piece, although this involves the use of an oven. Lately there is a search for new glycols with an increasingly low 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 capacity of evaporation in the oven and a viscosity such as to do not compromise the dripping. Usually the amount of solvent exceeds 98%.
The resins involved in this technology must have certain
characteristics, they must be thermosetting, easily crosslinkable,
non-yellowing, compatible with oils and solvents and not too thick to avoid
creating excess thicknesses not suitable for painting. Resins are often used in
combination with each other, to increase the cross-linking, aesthetic and
adhesion strength with the paint. The total percentage of resin varies from 1
Products with thin-layer technology are optimal for treating metals (from iron to aluminum) with a standard degree of dirt and free from oxides and encrustations. Unsuitable galvanized metals or alloys that could have extrusion greases and oils on the surface.
They certainly have benefits over traditional phosphating and detergent
products, but they have limitations that we will now expose:
Phosphating or Degreasing
- Tanks heating
- Waste and sludge to be disposed of
- Periodic change of the bath
- Daily analysis
- Inconsistent quality
- Very acidic or basic products
- Multimetal and flexible
- Rust and greases remover
- Economical products
- Room temperature working
- Occasional analysis
- No wastewater
- Single stage
- Protective coating
- neutral pH
- Initially expensive
- No rust and greases remover
- Necessary to break down fumes during drying
While they are often advertised for great resistance to salt spray corrosion, they offer medium-high resistance, comparable to a good traditional degreasing treatment (about 300 hours). Some products still on the market (acid and single resin) have even lower resistance, comparable to a phospho-degreasing. This applies to standard ferrous surfaces; steels and aluminum withstand up to 1000 hours of salt spray, but this is not due to the resin technology.
A myth and an error linked 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 exist because there is no water and there is not enough oxidative
reaction, indeed, the excess of acid can create blackish over-thicknesses during
drying in the oven. This specific ability to oxidize the surface is instead
verified with the so-called Wash Primer products, used for the manual treatment
of artifacts such as hobby, automotive and other surfaces where a strong primer