Variability (Color, Sizing, Drainage, Retention, Strength)
VARIABILITY IN GENERAL probably includes more than half of all of the serious issues faced by a typical papermaker. To try to identify the key sources or variability and correct the situation it makes sense to look at the time history of the following quantities: first-pass retention or white water solids, pH, electrical conductivity of the process water (often correlated with carry-over from pulping or bleaching), cationic demand, stock freeness, stock consistency, and water surface tension. Many of these variables affect the efficiency of different papermaking additives. In addition to identifying quantities that vary with time, it is critical to find out whether any of them are highly correlated with the observed problem.
VARIABLE COLOR is likely to be a much more serious issue than a steady error in color relative to a standard. Some likely causes of variability in color include variations in first-pass retention, variations in broke content, or variations in pulp quality or bleaching. It is recommended to look for any periodicity in the color variations. Sometimes color variations can follow the batch-wise preparation cycles of a starch (possibly subject to biological decay), retention aid (possibly related to swings in first-pass retention), or biocide (also affecting retention efficiency in certain cases). Also, if the problem is new, one needs to ask about recent changes in the process or the types and qualities of the materials. Efforts to control retention to a steady value, appropriate biocide use, and attention to dye additives usually will resolve problems with color variations.
Factors that are likely to cause variability of SIZING are some of the same factors that can cause shifts in retention efficiency and other changes in the wet end. One needs to find out whether changes in water resistance of the paper or in the demand for sizing agent can be correlated to other cycles in the system. For instance, there may be episodes of increased levels of surface-active materials that act as anti-sizing agents. Such materials may include black liquor carry-over, components of coated broke formulations, nonionic surfactants from deinking operations, components of dye formulations or slimacides formulations, and various anti-foam surfactants. The total surface area of filler particles in the furnish may be cycling up and down, or maybe there are instabilities in the retention aid system. It is also possible that variations in the cationic demand of the furnish are causing cycles in retention efficiency, which are also affecting sizing efficiency. It may help to review some of the key factors mentioned in the section dealing with problems involving different classes of sizing agent.
Additional measures may be appropriate if the DRAINAGE OR RETENTION efficiency varies over time. The best approach is to try to identify the root cause of the variation. For instance, does retention efficiency always get worse when the proportion of coated broke entering the system increases? Does it get worse only when the paper grade is changed to a lower basis weight? Do the cycles have anything to do with the preparation cycle of batches of retention aid polymer? Is there a problem of unsteady water pressure, that may affect the delivery of retention aid polymer to the addition point? Many more similar questions can be formulated by reviewing the process diagram for a particular papermaking operation. You also can review some of the factors mentioned in the previous paragraphs when considering what might be responsible for observed changes in retention. Once the root cause is identified, it is often possible to make changes to smooth out or reduce the variations.
VARIATIONS IN STRENGTH can have a wide range of causes. Some of the most important factors that can cause strength variations are the freeness of the furnish (an indication of the degree of refining), the fiber length distribution, the filler content, the dosage and quality of dry-strength agents such as cationic starch at the wet end, and the quality and application rate of size-press starch and other polymeric additives.
One thing to consider is whether there is a variable amount of surface-active materials in the wet end system. Regardless of the possible source, this can be determined by whether there are sizing variations that correlate with the strength variations, whether there are foaming issues that correlate with the strength changes, and whether the process water surface tension varies significantly.
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.