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Humidity

Humidity

An article written by Dan Oaks, reprinted from tech-nique'





Humidity or lack of humidity affects paper in strange ways. Paper is made up of cellulose fibers which act as sponges to absorb or release moisture very readily. As the fibers absorb moisture, they swell; as they lose moisture, they shrink. In fact, as the amount of moisture changes, the expansion or contraction of the fibers is very dramatic. If I may play Mr. Wizard for a moment; measure a piece of forms bond at room humidity, then soak it and measure it again. I think you'll be surprised at the results.

I tried this with a 9 1/2 x 11" piece of forms bond and the results were startling. At room humidity the form measured 10.985" long by 9.469" wide. (Obviously we are not getting our money's worth on our 9 1/2 x 11" forms.) After soaking the sheet the measurements were 11.008" long by 9.844" wide! That's an increase in length of .023", which is considerable . . . But the width grew a full 3/8"! This is a crude experiment, but from these results, it looks like paper fibers expand about 20 times more in width than they do in length, with the absorption of water. Using that same ratio, it a ten foot length grows 1/16" from moisture absorption, the same paper, if it were 10 feet wide, would have grown 1 1/4" in width. That should make you happy that all paper that runs through a forms collator is grain long!

Understanding the nature of the paper fiber will give you a little insight into the problems that humidity variations cause. Printing on steel would be a lot easier in some respects.

Humidity an temperature control is one of the best things a forms manufacturer can do for the quality of his product; and for maintaining top production of multi-part forms. Control of both humidity and temperature is important. Relative humidity is not a measurement of how much water is in the air. It is a statement relating to the capacity of the air for holding water. Warm air can hold a lot more water than cold air. Her's a very unscientific explanation of that statement: if one cubic foot of air at 70 degrees could hold a maximum of nine ounces of water at it's saturation point, it would be stated as 100% relative humidity. Another way of stating it would be: nine ounces of water can be evaporated into one cubic foot of air at 70 degrees . . . no more. It now contains 100% of the moisture it is capable of holding evaporated.

However, if the air were to be warmed ten degrees, it could hold one more ounce of water. That same cubic foot of air now is at 90% relative humidity.

If the original cubic foot of at at 70 degrees and 100% relative humidity were cooled ten degrees, it would lose some of its capacity to hold water. As air cools, anything over its 100% capacity must go somewhere else, so it condenses on the side of the container, or turns to rain. Wherever it goes, it must turn back into water in its liquid state since the cooling air cannot support it in its evaporated state.

So, "relative humidity" is a statement of the percentage of water holding capacity of air utilized at a given temperature. Relative humidity, without a reference to the temperature, is meaningless.

Absolute humidity is the actual amount of water that is in the air. In the above cited example, one cubic foot of air at 70 degrees has a relative humidity of 100% and holds 9 ounces of water (absolute humidity). The same cubic foot of air at 80 degrees and 90 % relative humidity still holds 9 ounces of water (absolute humidity).

Now let's assume that your plant is temperature and humidity controlled at 70 degrees and 50% relative humidity and that at that temperature and relative humidity each cubic foot of air is holding 4 ounces of water. Let's also assume that your paper warehouse is at 80 degrees and 70% relative humidity. In the original statement we said that the air at 80 degrees could hold about 10% more water that when at 70 degrees. So even though the air in the warehouse is at 70% relative humidity, the actual amount of water (absolute humidity) is higher than the air in the production area. Each cubic foot of 80 degree air holds about 4.4 ounces of water.

Now let's make one more assumption: the paper stored in your warehouse has the same relative humidity as the air. That is, 70% relative humidity at 80 degrees, holding 4.4 ounces of water (absolute humidity) per cubic foot of paper. When that paper comes into the production area, it is going to cool to 70 degrees. As it cools it loses it's ability to hold moisture. The result is that as it cools to 70 degrees, it's relative humidity tries to go up because there is more than 4 ounces of water per cubic foot. Since there is nothing to hold that moisture in, the excess water evaporates into the air in an attempt to find a balance at 70% relative humidity. This loss of moisture continues until the temperature, relative humidity and absolute humidity in the paper matches the surrounding air. And what happens to paper as it loses moisture? It shrinks!

The process of losing or gaining moisture from a tight-wound, wrapped roll is very slow. It can take about two to four weeks for a roll of paper to stabilize. Unwrapping the roll is even worse. An unwrapped roll loses moisture very rapidly from the outer edges; the outer edges shrink and you end up with a web that is baggy in the middle. This causes wrinkling problems.

The complete answer to humidity problems in paper is unattainable in practical life. The complete answer would be to buy your paper far enough in advance to store it, still wrapped, in a warehouse, with exact temperature and relative humidity control for four to six weeks. Then move it into an equally controlled production environment. I don't know of any forms manufacturer that can afford that kind of turnover in inventory.

There are some things that can help you work around humidity problems. For one thing, understanding how relative humidity works, you may be able to improve your control situation somewhat. Running your press work no more than a day or two ahead of the collator will work wonders. Rolls that must set over a weekend before they are collated should be wrapped in plastic to keep them from losing or gaining moisture. The plastic wrap will also help if you are having problems with varying humidity in the pressroom. But when you're wrapping, remember that humidity is like moisture under pressure. It is extremely difficult to contain.

Please remember that the examples that I have used were purely hypothetical. The exact amount of water air can contain (absolute humidity) is not as stated above. However, I have been out fishing these Florida lakes when it was obvious that there was at least a bucketful of water in every cubic foot of our 90 degree air.



- Dan Oaks, 2-87, Orlando





*We can't control the humidity in your warehouse but we can help you move your rolls a lot easier! CLICK HERE