What is the Range of Paper Moisture Content?
Paper moisture content (also referred to as water content) is defined as the ratio of the mass lost after drying according to a specified method to the original mass of the sample, typically expressed as a percentage (%). The moisture content of commonly used printing papers is generally controlled within the range of 4% to 7%. Depending on the paper type, raw materials, and intended application-such as coated paper, offset paper, or newsprint-the optimal moisture content may vary slightly.
How to Determining Paper Moisture Content?
The oven-drying method is commonly used to determine paper moisture content. The specific testing procedure is as follows:
Weigh the sample before drying; Dry the sample at a temperature of 103°C to 107°C until constant weight is achieved; Weigh the sample again after drying; Calculate the moisture content using the following formula: Paper Moisture Content=(Mass Before DryingMass Before Drying−Mass After Drying)\mass before drying×100%
Factors Affecting Paper Moisture Content
1.Hygroscopic Nature of Paper Components
Paper is primarily composed of plant fibers and other hygroscopic materials used as fillers, such as clay, kaolin, and gypsum powder. The physicochemical properties of these components give paper its capacity to absorb moisture. Paper not only absorbs water from dampening solutions during printing and releases moisture (desorbs) in a drying oven, but it also absorbs moisture from humid air or releases moisture to dry air.
2.Relative Humidity
When paper comes into contact with surrounding air, it reaches equilibrium with the ambient humidity. Therefore, the same paper will exhibit different moisture content levels under different humidity conditions. Conversely, different types of paper will show varying moisture content levels even under the same humidity conditions.
Effect of Paper Moisture Content on Printing Quality
1.Dimensional Changes and Register Accuracy
Paper has a distinct directional structure formed during the papermaking process, known as the grain direction. When different fibers absorb or release moisture, they undergo varying degrees of deformation: typically, the increase in diameter can reach up to 30%, while the increase in length is only 1% to 2%. Consequently, the expansion or contraction parallel to the grain direction (longitudinal) is relatively small-approximately one-half to one-third of that in the cross-grain direction-and has a limited impact on register accuracy. In contrast, the expansion or contraction in the cross-grain direction (transverse) can be as large as 0.3 mm or more, making it a primary factor affecting multi-color register accuracy.
In addition, uneven moisture content within a sheet can cause localized deformation, further compromising register accuracy. When large quantities of paper are stored, only the edges are fully exposed to the surrounding air, while the central portion is less affected. When ambient temperature and humidity change, the edges of the paper respond more quickly, resulting in either "wavy edges" (edge expansion due to moisture absorption) or "tight edges" (edge shrinkage due to moisture desorption). This pre-existing deformation, when subjected to printing pressure, prevents the printed image from aligning precisely with the plate. During multi-color printing, if a partially printed sheet develops wavy or tight edges due to changes in environmental humidity, the geometry of the already-printed image will shift, leading to register errors in subsequent color units.
2.Changes in Physical Properties and Printability
Moisture content variations not only affect register accuracy through dimensional changes but also alter the physical properties of paper, thereby influencing overall printability. The specific effects are as follows:
When moisture content is too high: The tensile strength and surface strength of the paper decrease, while plasticity increases. The drying rate of the printed ink slows down, making the paper more prone to set-off (backside smudging).
When moisture content is too low: The paper becomes brittle and stiff, with reduced elasticity. During printing, friction can easily generate static electricity, leading to poor sheet delivery and stacking issues, which in turn increase paper waste.
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