Definition

Moisture content is the amount of water present in linerboard expressed as a percent moisture content and refers to both the overall average moisture content of a roll of linerboard as well as machine-direction (MD) or cross-direction (CD) variation.

Influence

Linerboard moisture content should be uniform throughout a roll, both in the cross-machine and machine directions, at an average level that is neither too now nor too high and which varies relatively little from roll to roll.

Variation can lead to warping of the combined board, which will cause feed problems on kicker-type converting equipment and will cause printing and slotting register variation. Warp can also affect setup, packing, and the sealing of boxes on automatic and semiautomatic packaging lines. Moisture content also can affect bond formation between the flute tips and the facing, the dimensional integrity of lightweight facings, and the folding properties of combined boards.

Potential Problems

Very low average moisture content can retard the receptivity of the linerboard to adhesive, causing a particular problem on the single-facer, where a "green bond" must be formed quickly. Production speed may have to be reduced to avoid weak ("zipper") bond or loose paper. Scoreline cracking is a function of many paper properties, including fiber length and fiber structure and fibrillation due to refining. Also, combining and converting factors can contribute to this defect. However, excessively low moisture levels in linerboard may compound the problem. Obviously, there is no single answer to the scoreline cracking problem.

Very high average moisture content can lead to excessive CD shrinkage on the corrugator preheaters, particularly on lightweight liners, causing edge misalignment and loss of trim.

Roll-to-roll moisture content variation can cause combined board warp beyond acceptable tolerances. Many industry corrugators are able to control warp if the initial moisture content difference between opposing facings is excessive.

CD moisture streaks within a roll of linerboard can cause "S" warp and bond variation across the machine. MD moisture variations can lead to warp and lineal bonding differences.

Extremely dry edges can cause edge curl, loose bonding at the edges, and increased web breaks as a result of tension differentials at the edges.

Source
McGrattan, W. "Key characteristics of linerboard, corrugating medium, and roll stock mechanical condition and their influence on the manufacture of corrugated products, part 1." Tappi Journal (1990): 99-108.

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