hot topic

Architect's memo 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 of change.

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.

Called Resene Limelock this cool new preparatory product offers an insurance against damage from 'hot' substrates.

Tech talk and Architect's memos
Technical information on a variety of topics relating to paint, finishes and coatings.

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