Moisture Content of Paper: What does this mean and why do I care?

Summer vs. Winter:

Why do we have wavy paper in the summer?

Why do we have static in the winter?

Why do we fight curl year-round?

The answer: EQUILIBRIUM—the balance of moisture

First, let me provide you with some facts and definitions:

Moisture Content of Paper:

Typical fine paper is produced to an absolute moisture content between 4.0 and 6.0%, meaning 4 to 6% of the paper’s total weight is made up of water. So, the sheet is designed to be stable in a 45-55% relative humidity (RH) at 72°F environment.

Different Moisture Contents of Different Papers:

• Laser grades (4.5%) – Less moisture, less reaction to fuser heat
• Preprint grades (5.0%) – Less moisture, usually errors to laser grades
• Offset grades (5.5%) – Higher moisture, less reaction to fountain solution

The key: make sure you purchase the right paper for the right use.

Moisture Content of a Print Shop:

The moisture content of a print shop is measured as relative humidity, not to be confused with absolutehumidity, which is the actual amount of water content in the air.

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• Absolute humidity (AH) is the amount of water in avolume of air, typically given in grams of water vapor per cubic meter of air. It is also the closest term related to the moisture content in paper.

• Relative humidity (RH) is the ratio of the amount of water in the air (AH) at a given temperature, expressed as a percent. Air at a given temperature can support only a certain amount of water and no more. This is referred as the saturation humidity point or dew point.

Ideal Pressroom Conditions:

Wausau Paper recommends the RH be between 40 and 55% at 72°F (50% is ideal). The further away from these standardized conditions, the more “paper problems” will be created.

So, to be clear, to achieve equilibrium, paper is recommended to be used in a 72°F room at 45% RH. For example, if the room temperature is 65°F then the RH should be adjusted to 60% to obtain the same amount of moisture in the air. If the room temperature is 80°F, the humidity should be adjusted to 40% RH. All of these adjustments will keep the same amount of water in the air and be compatible with the amount of water in the sheet.  Also, if a typical pallet of paper were to equilibrate to an atmosphere of 10% RH (72°F), the moisture loss would be about 80% or 9.6 gallons of water.

Other Factors:

• Acclimation is a process where the temperature of paper completely matches the temperature of the room so that the transfer of moisture is limited. DO NOT open the wrappings of paper until the paper is the same temperature as the room. For a full pallet, 10 hours for every 10 degrees is a good rule of thumb—otherwise, for a carton, about 24 hours should be adequate. Opening the carton and taking the individual reams out to warm up is okay.

• Curl is caused by several factors; however, in this blog, I’m going to discuss wavy edges and tight edges, as they relate to relative humidity levels. As paper fibers lose moisture, they shrink in diameter—when they pick up moisture, they swell. The problem with open reams in higher humidity areas lies in the fact that only the edges that are exposed to the moist air will swell (or shrink in lower humidity areas) or only about the outer .5” will be affected, while the inner part of the paper stays the same size. As the outer edges swell (or shrink), the paper starts to distort in shape, creating wavy or tight edges—most consumers just call it curl.When excessively wavy paper is fed through a copier, jamming can occur—not because the paper was made wrong, but, instead, because it was in an unsuitable environment for paper.

• Static Electricity is an electrical charge that builds up on moving paper (plastic, hair, etc.). Typically, in moderate humidity conditions, the charge is easily released (discharged) and does not have an opportunity to build up enough to cause issues; however, in low humidity conditions, the dry air acts as an insulator and the charges build-up causing the paper to stick together. This is when operators will try to use tinsel or anti-static sprays as an outlet for the built-up charge.  These methods typically meet with limited success.  The air must have enough moisture in it to conduct electricity or the static will not dissipate, causing the sheets to stick together.

6 Simple Tips for Maintaining Proper Moisture Equilibrium:

1. Storage of product - Keeping the paper in the same room that it was manufactured in is the best way to prevent moisture swings because the paper will become better acclimated to the room’s temperature. Keep the paper wrapped and away from outside walls—I have seen pallets frosted tightly to an outside wall. Long-term storage of paper in a non-controlled environment is acceptable, provided adequate time is available for proper temperature acclimation prior to opening.

2. Acclimate the paper prior to opening - Before you remove the wrap, make sure that the paper is the same temperature as the surrounding air. If you bring in a cold skid (say <48°) into a 72°F room at 50% RH , moisture from the air will start to condense and, just like a cold glass of ice tea in the summertime, water may form on the wrap. If the same paper is unwrapped, the moisture will collect and make the sheets stick together.  Trust me, I know this stuff, I have been in the back of a truck with paper coming from northern Canada when it was raining in the springtime.

3. Keep your pressroom between 45 and 55% RH - I once worked at a place where the pressroom temperatures were around 70°F in the winter—the RH was about 45% and this worked well, with the exception of the times when we would run plastics, in which case we needed to raise the RH to 50%+.  This was done to prevent static issues!

4. Keep temperatures constant - Turning up or down the temperature in a room at night or over the weekend is a bad idea. It is often done to save money, but when paper issues arise, the related downtime adds up fast and the savings are lost. Watch the outside doors for moisture loss/gain, especially from loading docks; consider installing a second barrier door between the docks and production areas.

5. Acclimate the paper after the press - Heat-set webs, IR dryers, and laser printers all play a part in moisture-loss. If a sheet has been excessively dried out, it will become brittle.  As a result, the fibers will be crisp and tend to crack and split thereby separating. This moisture will comeback eventually, even if you do not want it to, causing the paper will grow. So, let the paper acclimate for a couple of days (at least hours) prior to binding, folding, shrink wrapping, etc. If the paper must be stored between processes in less than ideal conditions, cover it with plastic bags to help control the moisture pick-up/loss.

Storage of printed product - Shrink wrap is the best way to lock in the optimized moisture from your print shop.  But, remember: if the paper is dry or damp when it is wrapped, it will stay that way. Please help educate your customers on the importance of acclimation.