Architects memo no.
62: March 2000
cool coalescings
Some rules are so fundamental that they are writ, actually or
figuratively, in tablets of stone. Changing such basic tenets requires
some pretty sound backing before the changes can be accepted.
For waterborne paints, the edict 'Do not apply at temperatures below
10°C, or when it is liable to drop below 10°C during the drying
period' has held sway for the 50 years that they have been commercially
available. The appearance of 2°C instead of 10°C on some labels
and data sheets therefore demands explanation.
Waterborne paints (acrylics, PVAs etc) are typically based on tiny,
thermoplastic particles that deform and stick to one another during
the stresses of drying and film formation. Particles that were deliberately
engineered to be soft, formed films easily, even at quite low temperatures.
The softness, however, extended to the finished film, which became prone
to damage from dirt retention. Hard particles could easily be made,
but they required heat, or large amounts of plasticising solvents, in
order to form films.
A compromise had to be reached and the industry accepted particles
that needed some plasticising solvent, and some heat (10°C) to form
useful films.
An extended application window would be valuable and, as polymer engineering
improves exponentially, novel technologies have arisen that can overcome
the hardness/film-formation dilemma.
One of the methods is to build these sub-micron plastic particles
in two separate phases - a soft phase that will coalesce at very low
temperatures; and a hard, tough phase that will contribute good film
properties. Sounds tricky, but picture peanut toffee and you won't be
far from the mark.
Resene has been evaluating this style of technology for about five
years and the results are excellent. Because coalescing solvents are
not necessary, an added bonus of the technology is that it is very green
with low VOC.
Waterworld II (Architects Memo No. 52) discussed
the influence of weather conditions on the drying of waterborne paints,
including low temperature and high humidity. These rules cannot be broken,
but it is possible to formulate paints without humectants that are typically
added to slow the dry in hot weather.
Last year, Resene introduced a Wintergrade version of Resene
Lumbersider and this year adds a Resene
Wintergrade Hi-Glo. Because tinters contain a substantial amount
of humectant-like materials, the colour range in these two products
will be limited to the colours off white, and a few factory colours.
Two phase polymers are only one of the strategies that Resene is researching
to open the painting window further - drying below freezing point continues
to elude us!
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