cool coalescings

Architect's memo 62: March 2000

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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