Paper Moisture Content and the Characteristics of Equilibrium Moisture Changes

Paper moisture content

Paper moisture content has a great influence on printing. If the moisture is too low, the paper becomes brittle and static builds up easily during printing; if too high, the ink dries with difficulty. Moisture changes also strongly affect paper properties: grammage, tensile strength, flexibility, and folding endurance all change with moisture, and the paper dimensions expand and contract, causing curling, warped edges, wrinkles, and wavy edges. Every paper has an optimal moisture content at which its properties are at their best: for cast-coated paper and cast-coated white board the optimum is7%±2%; for white cardboard, 4%–7%; for single-side coated white board, 8%±2%. These are the moisture contents at the time of delivery.

Equilibrium moisture

During storage in a printing plant, paper, being a hygroscopic material, absorbs moisture from the air—the rate depending on relative humidity and temperature—and also releases moisture to the air, at a rate depending on the paper's moisture content and the ambient temperature and humidity. When the absorption and desorption rates are equal, the paper and the air are in equilibrium and the paper's moisture content stays constant; the moisture held at this point is called the equilibrium moisture.

Relative humidity is the ratio of the water vapor in the air at a given temperature to the maximum amount the air can hold at that temperature (saturation). The same paper has different equilibrium moisture at different relative humidity.

In addition, equilibrium moisture changes have the following characteristics:

 ① Influence of paper properties

At the same relative humidity, the more hydrophilic the paper, the higher its equilibrium moisture, and vice versa. Unfilled paper has higher equilibrium moisture, while filled, sized, and coated papers have lower equilibrium moisture. For the same grade, thicker products have higher equilibrium moisture than thinner ones because they contain a higher proportion of absorbent base paper.

② Influence of temperature

At the same relative humidity, a temperature change of about15 °C changes the equilibrium moisture of paper by roughly 0.5%. However, register printing requires that the paper moisture content be controlled within ±0.1%, otherwise register accuracy suffers; color printing workshops must therefore control temperature within ±3 °C while controlling relative humidity.

③ Influence of the path to equilibrium—moisture absorption or desorption

At a given relative humidity, the moisture content at equilibrium reached by absorption from a lower moisture level is lower than that reached by desorption from a higher moisture level—this is the paper's moisture hysteresis effect. To bring the paper's moisture content back to its original value,，you must adoptan "over-correction" approach. That is, paper equilibrated at a certain relative humidity that has absorbed moisture in a high-humidity environment must be placed in an environment with lower relative humidity than before to desorb back to the original equilibrium moisture, and vice versa. In addition, desorption is much slower than absorption. Either way, the process is relatively fast at first and slows as equilibrium approaches; it also depends on the paper type and air circulation. Under standard humidity and temperature, a single sheet of cigarette paper reaches 5.8% equilibrium moisture in 35 minutes, printing papers need 2–4 hours to reach 5%–8%, and packaging board takes even longer. This lag between relative humidity changes and paper moisture changes produces a hysteresis effect between paper deformation and humidity changes.

④ Influence of paper directionality

Directionality means that the equilibrium moisture differs between the machine direction and the cross direction; the cross-direction expansion rate is far greater than the machine-direction rate, so cross-direction dimensional changes are larger. Tests show that a single fiber expands about 20 times more across its width than along its length.However, since fibers in paper are arranged in many directions—merely with more aligned in the machine direction—the difference between the two directions is not as extreme as for a single fiber. Taking a relative humidity change from 50% to 60%, the ratio of machine-direction to cross-direction expansion is about 3:7, i.e., roughly 2.3 times. This ratio varies with the degree of fiber orientation: the stronger the orientation, the larger the ratio. Making paper with randomly oriented fibers is one of the keys to reducing cross-direction expansion and avoiding misregister.

⑤ Influence of the two-sidedness of paper

When the environment of a perfectly flat sheet of paper changes, the paper absorbs or desorbs moisture from the air. If it absorbs moisture, the reverse side expands more than the front and the sheet curls toward the front; if it desorbs, the reverse side shrinks more and the sheet curls toward the reverse. Whichever way it curls, the axis is always the machine direction, because the degree of oriented fiber alignment differs between the two sides. This is especially pronounced in paperboard.

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