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Q & A: drying an adhesive

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Most recent answer: 10/22/2007
Q:
DEAR SIR WE ARE IN THE BUSINESS OF COATING A WATER-BASED ADHESIVE ON BOPP (BI ORIENTAL POLY PROPANE ) FILM WE HAVE TO EVOPARATE THE WATER RROM THE ADHESIVE IN THE OVEN THE RATE OF PRODUCTION DEPENDS ON RATE OF EVOPARATION OF WATER FROM THE WATER BASED ADHESIVE. I WILL LIKE TO KNOW THE RELATIONSHIP BETWEEN TEMPERATURE OF AIR, VOLUME OF AIR . IN SHORT IF I HAVE TO EVAPORATE 50 GRAMS OF WATER FROM 1.6 SQ. MT OF THE FILM IN ONE MINUTE AT AVERAGE TEMPERATURE OF 70 DEG. CENTIGARDE WHAT SHOULD BE THE HOT AIR FLOW (AIR TEMP. WILL BE 70 DEG. CENTIGRADE)IN THE DRYING OVEN.
- dilip desai (age 55)
DUBAI, UAE
A:
We can only provide a minimum estimate of the total volume of air needed. At a given temperature, water has a well-defined vapor pressure, for which the vapor is in equilibrium with liquid water. You wonít get the water content of the air to be higher than whatís expected from the 70 deg C vapor pressure, so that sets a lower limit on the volume of air needed. In fact, since your source is water bound up in some adhesive, its vapor pressure is almost certainly lower than that of liquid water, which means that you will need an even bigger volume of air to carry away the 50 grams. Also, if you want the process to go reasonably quickly you will want to use more air. Even if you use much more air than needed, the rate at which the water leaves the film is limited by diffusion processes in the film, so I donít know whether one minute will suffice.

The vapor pressure is, according to a standard table, 234 mm Hg. Thatís about 30% of atmospheric pressure. Youíve got to get rid of almost 3 moles of water. 3 moles of gas would occupy around 60 liters of volume at standard temperature and pressure. So I think youíll need at least 200 liters of air.

Mike W.

There are a couple of other variables to look into here. One is how humid the air is before it even gets to your drying oven -- more humid input air will require more air to dry your film. The good news here is that the capacity of air to hold water vapor goes up with temperature, and so the relative humidity goes down as you heat air up -- even if the cold outdoor air is humid, it may be less of an issue when heated up. But your air requirements and drying times will certainly depend on the weather. You may want to recirculate air for a while to save on energy costs, but youíll have to constantly add new air and remove moist, hot air if youíve got a continuous manufacturing process going.

Another thing to worry about is your adhesive. Mike already mentioned that the vapor pressure over a mixture will be less than that of pure water. I might add that you sometimes can get a "skin" of dry adhesive floating on top of wet adhesive. White school glue is a good example of something that gets a tough, dry skin on top of it, preventing the drying of the wet glue underneath. This is less of a problem if your film is thin (and it seems to be), but for some adhesives, it is nearly impossible to get all of the water out quickly.

It sounds like a little experimentation with the air flow rate in order to see what the results are like is in order here!

Tom

(published on 10/22/2007)

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