RESENE Architect’s memo no. 136: Eolian transport PART TWO
Author: Colin Gooch, Resene technical Director.
Australia is continuously generous to New Zealand, never more so than in 1928 when, over four days 206 gms m2 were deposited “throughout most of New Zealand” following a massive dust storm.* Taking the area affected to be 56% of our land mass (because it makes the maths easier), this equates to a total of 300 million tonnes. Thanks cobber! Pollens are another huge source of dust but are orders of magnitude larger than the mineral dust we have been talking about – typically between 10 microns and 1 millimetre (1000 microns). SCION tells us that one mature radiata pine produces 500-750 grams of pollen per year. At the upper end, it would it would take as little as 3.3 hectares of pine forest to produce a tonne! Resene
Other natural sources of dust are mould spores (10-30µ), algae spores (0.004-0.1µ) and wood smoke (0.2-3µ). Not so natural is road dust which comes along mixed with unburnt carbon and fuel, fuel additive residues, abraded material from brake pads and rubber particles from tyre wear.
All these dusts will land on exterior paintwork and interact in various ways depending on their size, chemistry and electrical charge that they carry.
Of the components in paint, it is relatively soft, polymeric binders to which dust particles are likely to permanently adhere and cause disfigurement. Binders bring durability to the film paint but the higher the binder level, the greater the risk of dirt pick-up. Harder binders are a help, but such binders need VOC additions in order to form good films.
My good friend and Resene colleague Greg Percival researched these topics and explored the exterior performance of more highly pigmented paints. As expected, he found that paints with higher levels of hard extender pigments had lower dirt pick-up levels but, unexpectedly, he found that the size of extender pigments used, and thus the size of the surface pores created had a profound effect on the dirt pick-up performance. The finer the pores, the more difficult it is for dust particles to lodge. More unexpectedly, the very fine-pored surface would not allow entry of mould spores and mould growth changed to algal as only these very fine spores could enter. (These studies were done for tropical exposure).
The two major tactics for reducing dirt pick-up (apart from making harder films) is to make the surface either super-hydrophilic or super-hydrophobic.
In the first case, water so loves the substrate that it heads there displacing all other surface contaminants and sloughing them off. We continue to monitor this technology in our labs and while, at its best, it can be very, very good it has to be acknowledged that it is not the most user-friendly technology.
Super-hydrophobic coatings are predicated on the assumption that any contaminant on a superhydrophobic surface must be more hydrophylic than the surface and, therefore would be swept off that surface by rain. The technology flattered to deceive until the Professor Wilhelm Barthlott elucidated, and then taught us, that the reason that the lotus flower remained beautifully clean was not simply due to the constant supply of hydrophobic waxes that the lotus flower secreted but also due to the very fine hairs on the surface, which dramatically reduced the contact area of any water droplet on the surface which, in turn lead to a stunning increase in beading effect.
Armed with this knowledge, paint technologists started blending silicone resins with a designed micro-scale topography to produce, when correctly formulated, extremely durable, ‘lotus effect’ surfaces.
We launched our version of this technology, Resene AquaShield, about 20 years ago, and, while it may not have taken the whole market by storm (it does need to carry a small price premium), it is one of the products of which I am most proud and one which is regularly specified by the cognoscenti. In these staunchly non-commercial memos, perhaps I could give it the commendation of ‘Not so Dusty’.
* A re-examination of the Trans-Tasman Dust Transport Event. Marx & McGowan.