Architects memo no.
70: September 2002
It was Benjamin Franklin
who said that "In this world nothing can be said to be certain,
except death and taxes." The modern world has shewn us another
certainty and that is 'that things will change'.
Although the more obvious examples are in the high tech
area, the New Zealand building industry is no sluggard. The New Zealand
Building Industry Code allows for a lot of innovation and the industry
has not been slow to exploit new opportunities. Plastered monolithic
structures remain extremely popular and the change from the standard
three coat solid plaster systems (that could be up to 22mms thick) to
current thin layer, polymer modified plasters is a dramatic example
Polymer modified plasters are hybrid materials that are
a cross between a highly filled acrylic texture coating and a hydraulic
plaster. In fact the materials are inter-penetrating networks of initially
a polymer bound matrix followed by the development of the hydraulically
setting cement matrix.
The dilemma that exists is that these two networks develop
by entirely different mechanisms: loss of water to develop the water
resistance of the acrylic portion and presence or addition of water
to cure the cementitious portion. The most elegant method is for the
water to be lost to the acrylic by uptake by the cement. In reality
however, significant amounts of water can be lost to the atmosphere
and, where the layer of plaster may be less than 2mm thick, insufficient
is left to cure the cement portion.
This leaves the surface as a pretty 'hot' proposition
for subsequent overcoating.
The problem is exacerbated if lime is used as a trowelling
aid in the plaster. For years lime has been valued as giving a nice
'buttery' consistency to plasters. Lime will slowly react, in the presence
of water, with silica sand but in its unreacted form it is slightly
soluble in water; aggressive to many organic media and can transport
through carboxylated polymers to form efflorescence or 'lime staining'
on the surface plasters.
There are more modern cellulosic additives, which can
be used in plasters to give similar application characteristics to lime
without the detrimental side effects.
Not all paints are equally susceptible to lime migration;
research has shewn that certain binders (and more often certain thickening
systems) allow the passage of dissolved lime more freely than others.
This is not necessarily a quality issue, more a formulating style.
It is also interesting to note that the acrylic binders
are unaffected by the lime, they merely allow its passage. More affected
than the binders are some organic pigments, which can suffer premature
loss of colour by UV light in the presence of lime.
Whilst there is no solution as good as allowing the plaster
to fully cure by keeping it damp after the initial set of the polymer
matrix, there is an excellent alternative. Going back to its roots in
concrete and cement technology, Resene have come up with a product,
which, if applied immediately after plastering, will hold the water
in the plaster aiding the set of the hydraulic components. The polymer
chosen is also engineered to present a tight barrier to the transport
of dissolved lime.
Limelock this cool new preparatory product offers an insurance against
damage from 'hot' substrates.