Dan said:
It is also simple physics.
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It's bunk, and I'm glad to call them out on it.
It is
not bunk and it is simple
chemistry.
Check my posts here and elsewhere and you will see that I
am biased because I am a formulator/manufacturer. However I also have the insight provided by a scientific background and a specific expertise in cleaning chemistry. There is one over-riding principle which underlies aqueous cleaning system. That principle is
wetting. In order for a soil to be removed by an aqueous system, it needs to be wetted so that it may be incorporated into the wash solution. If you fail to do this, you need additional assistance, most commonly (in this scenario) mechanical assistance by way of a sponge/mitt/other wash media.
So to focus on wetting. How does one wet an un-specified soil? Well there are different types of soils. For example, you have dry particulate soils which are relatively easily wet or can be wet with small amounts of the appropriate surfactant. Then there are oily soils which repel water and are extremely hard to wet, even
with the correct surfactants. Most loose soiling generally could be described with the first generalisation and this can be easily removed with water or moderate pressure washing. Unfortunately this does not provide a touchless wash because we have traffic films which are inherently oily and are relatively immune to water in isolation and are all but impossible to 'wet' fully. There are then several things one can attempt to do to improve this. The most gentle is appropriate surfactants and solvents to bridge between hydrophobic oily soils and the bulk aqueous medium. This is a time dependent reaction, it takes time for the surfactants to surround the oily molecules and to liberate them from the surface to which they are bonded. Heavier duty cleaners will use high alkalinity as an addition. This will chemically react with the oily film and actually convert it to a soap (remember your high school chemistry lessons about 'saponification'). So not only is the film dissolved but you form a surfactant which further enhances the wetting of remaining soils. Again, this is a time dependent process.
So there you have the fundamental of what is going on. Both of the highlighted mechanisms are time dependent. The longer the process carries on, assuming adequate chemical for the 'reactions' to proceed, the more effectively the soils will be wetted and the more completely they will be able to be removed with a pressurised water rinse. So now it should be obvious that there is a very simple method to enhance the cleaning process
without the need for increasing the strength of the basic wash solution - dwell time. There are several ways to achieve this, most of which I would not discuss (a man's gotta keep some of his secrets...) but the one we all know well is a foam. With a foam, it is possible to cover the surface and keep it covered much longer than with a thin liquid film. Moreover, the kinetics of a foam mean that it (should) break down in time resulting in the initial, interface (i.e. foam/paint) being continually refreshed by new foam from the bulk foam region. So we achieve both a long dwell time
and a constant refreshing of the essential surfactants/bases/solvents. So chemically, foaming works. In practice, it also works, just not as well as some would hope. The reasons for this I outlined previously - briefly, the dilution levels and the additional effective dilution caused by volumetric parameters mean that the effective chemical concentration is in fact much lower than with traditional prewash (or TFR) methods.
This is not something which is really for debate, it is accepted theory. The debate is really reserved for the latter part, the effective concentration at the surface. If one goes as far as to use the foams at a level that would achieve the 1:20 levels often used with prewashes, the level of cleaning tends to be notably better.
