As a cellulose product, paper is naturally hygroscopic, making it dimensionally unstable and vulnerable to temperature and humidity changes. In attempt to reach an equilibrium state with its surroundings, paper collects and releases moisture, which may affect its flatness, dimensions, strength, performance, conductivity and fold. The material only reaches an equilibrium state when it neither absorbs nor loses moisture in an environment. Fortunately, it is possible to manipulate and control an environment using humidity control for paper production. By understanding how humidity affects paper and pulp production, you can prevent costly problems and better ensure customer satisfaction.
Humidity and Paper
Relative humidity is the amount of water that air holds at a certain temperature. Absolute humidity is the quantity of water vapor in a sample of air. As the temperature in the air increases, so does its ability to retain moisture. Similarly, cooled air loses its ability to retain as much moisture. Despite temperature changes, cooled air can retain the same absolute moisture as heated air, but will experience an increase in relative humidity levels. Paper relative humidity, or paper RH, is the relative humidity level in the air in which the paper does not give off or absorb moisture.
The environment within a paper production facility is the primary factor that affects paper’s moisture content. The ideal relative humidity level for a production facility is a debated subject among industry professionals because the ideal level depends on a facility’s geographic location and the season. Since papermakers can’t produce paper with relative humidity levels that function well in all environments, they must produce paper that accommodates most environments. In general, the best environment for a paper mill to promote equilibrium is one that has relative humidity levels of 40 to 50 percent and a temperature of 72°F. However, factors such as fiber, fillers, adhesives and manufacturing techniques affect how paper respond to atmospheric conditions.
The Effects of Dry Conditions on Paper Production
Low relative humidity levels in paper mills can cause paper fibers to lose moisture and curl, shrink and lose dimensional stability quickly. Sometimes the damage to the paper is not apparent until the second passing through a press or the sheeting stage, when the paper’s tension loosens, allowing the fibers to change shape and develop tight edges. At low relative humidity levels, paper is also susceptible to fold cracking and static electricity, which can cause packing issues, such as feeding and jamming problems.
The Effects of Too Much Humidity on Paper Production
When relative humidity levels exceed 65 percent, the cellulose fibers in paper draw in moisture in as little as 30 seconds. As a result, the fibers increase in size and may make sheets of paper limp or its edges appear wavy. Because of the manner in which paper fibers are in alignment with machinery, the greatest changes in dimension generally occur along the across-grain dimensions, making the environment that paper is exposed to between processing steps critical. High relative humidity levels can also lead to warping, wrinkling and mold growth.
Understanding how paper interacts with its environment is critical to ensuring its appearance and performance. Because of atmospheric changes that occur throughout the seasons, HVAC units often do not control conditions appropriately or uniformly. The best way to promote equilibrium in paper fibers is to control the relative humidity levels and temperatures in the processing and storage environments using temporary climate control solutions from Polygon. These solutions improve production efficiencies and the quality of paper while reducing waste. Because engineers customize the equipment to your facility’s exact needs, the systems complement existing monitoring and control devices, improve humidity control, save energy and reduce operating costs. Get in touch with Polygon today to learn more about simplifying humidity control for your paper production operations.
[Austin Granger via CC License 2.0]