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humidity, temperature, water - waterworld ll

Architect's memo 52: June 1998

Water - this chemical oddity dominates our lives. It supports life and destroys it. It shapes and reshapes our environment. It fills our oceans, lakes, and rivers. It is in the very air that we breathe, and, in the variability of the amount present in the air, causes headaches for the paint chemist.

Everyone who has hung washing out to dry will be familiar with the fact that some days are good drying days and some are poor. At any specific temperature, air has the ability to hold a specific amount of water in it. For example, at 15°C one cubic metre of air can hold exactly 12.8 gms of water as vapour - and no more. Washing, or paint, put out to dry under such conditions simply will not dry as the air can hold no further water. Air saturated with water like this is referred to as having 100% relative humidity.

At 5°C, air can only hold 6.8 gms of water and so if the air, referred to above, cools down to 5°C, 6 gms of water vapour will condense from it as dew. Conditions such as this regularly occur during spring and autumn as warm day temperatures fall sharply in the late afternoon leading to a dramatic increase in relative humidity, up to and past the dew-point. This imposes disciplines on painters regarding how long they can keep painting on a beautiful late autumn afternoon. Many a litre of roof paint has ended up literally down the drain due (no pun intended) to the painter trying to squeeze an extra hour of productivity from the day.

High temperatures and low humidities have the opposite effect and lead to very rapid evaporation of water.

Under still conditions the evaporated water can lie like a blanket over the surface, raising the relative humidity locally, and slowing the evaporation of the remaining water.

A breeze however, will disperse this layer and allow the rapid evaporation of the water to continue unhindered.

Waterborne paints, in the main, are particulate materials carried in a medium of water. They require some time to achieve their optimum packing during film formation and too rapid drying can lead to less than optimum film properties. Too rapid drying can also lead to wet edge or open time problems where the leading edge of the applied paint sets up before the painter can get back onto it with his next brush load. This prevents smooth melding and leads in turn to aesthetically unacceptable finishes.

Too rapid drying of waterborne paints can be mitigated by avoiding painting in the full sun; pre-dampening porous surfaces; and by the use of temporary humectants like hot weather additives. Wet edge or open time problems can be further alleviated by technique. Most waterborne paints set after they lose a specific percentage of their water. A leading edge of paint that is feathered out to a thickness of 10 microns will set ten times faster than one that is left at 100 microns. Therefore when applying waterborne paints, the thicker one keeps the leading edge, the better will be the final appearance.

Formulating paint to cope with our temperate climate can be very taxing but, as ever, for the paint chemist water has a role - either to wash down the aspirin or to dilute (a little) the Scotch.

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