Architects memo no.
74: November 2003
out damned spot!
Wipeable; washable; scrubbable; fully scrubbable - what does
it all mean? Not an awful lot if one is seeking clear guidance to the
ease with which soiling can be removed from a coated surface!
One thing is clear; no-one wants their painted surfaces to be marred
with unwanted marks, and the ability to readily remove such marks is
a very desirable feature in any coating. So what features in a coating
lend themselves to soiling resistance? Intuitively everyone realises
that hard, smooth, chemically resistant surfaces such as glass, vitreous
enamel, stainless steel etc are the easiest surfaces to remove marks
from. The dense atomic structure of their surfaces means that stains
can not penetrate, and their surface smoothness means that there are
few pits or pores in which soiling can lodge.
Does this mean that these surfaces are easy to clean? This would have
to be given a very qualified yes. The qualification comes from the fact
that smooth polished surfaces also show marks very easily and no-one
who has tried to clean and produce a totally smear-free set of ranchsliders
would say that glass is easily cleaned!
Brushed stainless steel does not highlight marks as much as its highly
polished cousin but the minute scratches in this surface can, in fact,
harbour more dirt.
Some organic surfaces approach the surface properties of the above,
Teflon and Formica bring to mind two famous brand name examples. Paints
can also approach these materials especially if they can deliver a highly
crosslinked matrix via two component and/or high temperature, UV or
E.B. curing systems.
Typical waterborne wall paints do not come anywhere near such a degree
of hardness. The polymers that they are made from are invariably thermoplastic
materials and current VOC trends tends to mean that these polymers
have to be quite soft to be able to form a film without the use of coalescing
solvents. Soft films are more prone to penetration by soil.
There is quite a range of suitable polymers ranging from PVA co-polymers
that are quite hydrophilic to styrene/acrylics that are quite hydrophobic.
This means that the PVA will be quite sensitive to waterborne stains
such as fruit juices but relatively resistant to oily stains such as
lipstick or crayon whilst the reverse will be true for the styrene/acrylic.
Flat finishes are much more aesthetically pleasing on broadwall areas
but, as noted earlier, the micro-roughness induced to achieve such effects
will harbour dirt more than a smooth surface. The trick then becomes
to reduce the gloss as much as possible whilst keeping the surface as
smooth as possible - quite a balancing act.
Even if the soil can be removed from the coating, the coating itself
must be able to withstand the rigours of the cleaning process. Rubbing
can induce the softening of thermoplastic polymers and breakage and/or
realignment of flattening agents. This can result in the treated area
being clean but permanently glossier than its surrounds - out of the
frying pan and into the fire.
There are additives that can be used to improve cleanability but these
are often impermanent, coming off themselves after the first wash and
once more leaving the surface vulnerable.
No memo on cleanability would be complete without reference to 'surfactant
leaching'. Under today's technology no waterborne paint can be made
without a range of surfactants to hold the system stable, and the greater
level of tinter added, the greater the surfactant burden. Under good
drying conditions these surfactants simply get absorbed by the film
and no harm results. Under adverse drying conditions (cold, humid, poor
ventilation) a portion of the surfactants can become deposited on the
top of the film.
This fine layer can subtly change the gloss but invariably it remains
totally unnoticed until the wall is marked. When the mark is cleaned
off the layer of exuded surfactant in the same area is removed showing
the true gloss of the film. What was unnoticed in its uniformity now
stands out in contrast and often the whole wall needs to be washed with
a 'dry bright' cleaner.
Maintaining pristine surfaces is never going to be easy and, at a
personal level, the author has responded by increasing his levels of
tolerance to a few blemishes. We could also stop breeding, keeping pets
and having parties to keep walls pristine, however I for one would rather
enjoy these and put up with a few blemishes here and there.
For some useful advice on maintaining your paint finish, read the
Resene Caring For Your Paint Finish brochure available from Resene ColorShops or see it online.