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Imperfections Formed During Paper Production

In a perfect world, paper would have no imperfections at all whenever it is made. Reality, however, paints a different picture. One of the paper machine design goals is to reduce these nonuniformities. These imperfections have numerous causes, including the fact that pulp is usually constituted from several sources, filler agents are not uniformly distributed, and each individual fiber is of variable size. Some of paper's desired properties, however, rely on the existence of these nonuniformities. There are four types of nonuniformities associated with the physical properties of paper(See diagram below):

Image: Diagram of Paper Formation

  • Machine and Cross Direction Anisotropy--Anisotropy means that there exists a difference between properties measured in the machine direction(i.e. the direction parallel to which the paper travels in a paper machine) and those same properties measured in the cross direction(i.e. direction perpendicular across the width of the forming paper web). One can easily discern a paper's machine and cross direction in the following manner: cut about a 5 cm diameter circle from a piece paper and float this piece on top of some water. The paper will curl up and the long axis of this curl is the machine direction. The cross direction lies perpendicular to this axis. The difference in properties along the two respective directions is a consequence of the uneven distribution of fibers oriented in the machine direction and those oriented in the cross direction. The orientation distribution ratio of the paper fibers changes somewhat as the forming paper web is pulled down the line in a paper machine. The direction that most of the fibers lie in is mostly in the machine direction("Wire Side" fibers in Fig 1), since these fibers lie slightly askew of the machine direction. If plotted by angles of orientation against number of fibers lying at those angles on a polar diagram, the shape that appears is dumbbell or hourglass shaped(enlarged ends with a somewhat constricted middle) lying on the central diagonal running through the origin of the graph(See Fig. 2). The differential angle of the general fiber orientation that is swept out from the machine direction(MD in Fig.2) is called the fiber orientation angle(Angle Θ[Theta] in Fig. 2). This angle, in conjunction with another angle known as the TSO angle, is partly responsible for paper properties as well as how well paper is run through a paper machine. The factors that have a major influence on fiber orientation lie at the "wet-end" of the paper machine, where headbox consistency, water drainage rates, fiber length, angle at which the water jets spray, and wire tension, among others. There are several ways to measure fiber orientation:

    • Ultrasound -- Sensitive wet end and downstream fiber orientations.

    • Polarized Laser Light -- Measurement of angles accurate within 0.5. Lasers are pulsed on both the top and bottom of the forming sheet. Resultant scattering pattern is indicative only of the orientation of the paper fibers.

    • Transmitted Light -- Sensitive only to the orientation of wet-end fibers.

    The way the fibers are oriented affects many of paper's physical properties including:

    • Paper Machine Runnability(Lack of resistance of paper when run through a a paper machine)

    • Hygroexpansion(Expansion due to the absorption of water)

    • Stiffness

    Paper that has fairly equal numbers of fibers oriented in the machine and square direction results in what is known as square paper. The squareness of paper is expressed as its squareness value, and this value is different for each paper grade.
    It is possible for fibers to be oriented through the Z-dimension, or thickness, of the paper, but it is overwhelmingly more probable that the fibers lie in the two-dimensional plane of the paper itself.

  • Machine and Cross Direction Variation-- As paper stock flows out of the headbox onto the forming table, its grammage, or weight in grams per square meter, is not uniformly distributed along the machine and cross sections. These variations are known as grammage variations and are responsible for moisture variations in the finished paper product. Failure to control these properties results in the paper being dried or coated unevenly and also may cause paper to cockle(wrinkle). Uniform grammage in the machine direction is partly due to factors such as stock flow and consistency, headbox, and performance of the retention aid system, among others.

  • Uniform Paper Web Formation--During paper web formation, the paper fibers aggregate into patches, or flocs(See Fig. 1). In rare cases, these flocs are so extremely localized that they actually leave holes in the paper(called pinholes in the paper industry). Observation of paper web formation(and the quality thereof), can be achieved by holding paper up to a light source(this is called observing the look-through). Paper with flocs that are readily observable is called wild or cloudy paper. Every paper grade has flocs, but they are kept to a minimum and fiber distribution is kept at an optimal uniform level. Uniform fiber distribution(and keeping flocs at a minimal level) is very important for all paper grades, as it is linked to several desirable paper qualities:

    • Smooth ink absorption in printing papers.

    • Uniform surface coating for magazines, catalog, and similar paper grades.

    • Tear resistance for all paper grades. As a chain is only as strong as its weakest link, so paper is at its thinnest point. This property also facilitates the manufacture of paper grades produced in fast paper machines.

    There are many ways that formation is quantified in the industry:

    • As stated above, varying degrees of translucence can be found by shining light through the paper, or variances in grammage can be found by firing beta() particles or X-rays through the paper.

    • Full sheet imaging, a new technology which provides 100% of a sheet to be seen at once, instead of the usual 1% that is normally seen with conventional equipment.

    • Profilometry, where a sensitive pencil-like tip is run over the paper's surface.

    • Testing of tensile strength as compared to a handsheet(i.e. handmade sheet) made from the same material.

  • Two-Sidedness--This is a property difference between the two sides of paper. Sometimes this can be caused by a difference in textures between the wire side and the top side of the paper. This effect can be reduced by using twin-wire paper machines, as they allow water drainage off both sides of the paper web.

Paper imperfections within a paper grade are sometimes what makes that a paper grade unique, as pointed out above. Imperfections and inconsistencies are sometimes part of the uniqueness of a piece of paper, but it is these 4 types of defects described above that must be kept to a minimum for high quality paper to be achieved.