Two-Sidedness (Color, Composition, Smoothness)
Problems of two-sided paper can be defined as undesired differences in the appearance or properties of the top and bottom surfaces. Such problems are most often associated with production on Fourdrinier paper machines, where the water is removed mainly through one side of the paper as it is formed. That kind of formation process has the potential of producing a sheet that differs in composition on its two surfaces. In particular, it is likely that fine particles can washed out of the wire-side of Fourdrinier paper such to the hydrodynamic shearing of hydrofoils and other dewatering elements. Two-sidedness in the degree of fiber orientation is likely to result when there is a significant difference in velocity between the jet and the forming fabric at the point of impact.
Other possible sources of two-sidedness, once the paper leaves the forming section, include the possibility that the wet-press treatments have a distinctly asymmetric effect on the two sides of the paper. For instance, it is known that a single-felted press nip tends to produce a dense zone near to the surface of the paper on the side facing the felt. Also, the weave patterns of some felts can be transferred to paper, especially if the nip pressure is aggressive. Unlike some of the other factors we will consider here, these wet-press issues are unlikely to be helped by any chemical strategies. Rather, it may be necessary to review the wet-press fabric selections, their present condition, and conditions of loading. It is worth keeping in mind, however, that wet-press felt performance can be degraded if the felt is filled with pitch or other debris. Continuous or intermittent spray treatments with alkaline, acidic, or solvent-containing solutions, usually with detergents added, can be used to minimize felt filling.
Aside from rebuilding Fourdrinier machines with twin-wire formers or hybrid formers, the second most promising way to minimize two-sidedness of sheet composition is to employ an effective retention aid system. The idea is that a high proportion of the fiber fines, filler, and other small particles remain attached to the longer fibers throughout the forming process. It has been shown that there is a high correlation between the value of first-pass retention and the z-directional uniformity of paper composition.
Two-sided color of paper can be readily detected by folding a sample of the paper so that both sides can be viewed simultaneously. In addition to the aforementioned use of a retention aid program, papermakers have used two main ways to achieve even-sided coloration of paper made on Fourdrinier machines. The most traditional approach involved adjusting the ratios of dyes having the same hue but different affinities for the fibers vs. fine materials in the sheet. With the advent of good online color monitoring and control systems it has become increasingly common to make color corrections by differential addition of dye to one or both sides of the paper at the size press.
PLEASE NOTE: The information in this Guide is provided as a public service by Dr. Martin A. Hubbe of the Department of Wood and Paper Science at North Carolina State University (firstname.lastname@example.org). Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification. Go to top of this page.