Dirt in paper can be defined as visible specks of off-color material. Papermakers have various ways of quantifying dirt levels. These include TAPPI method T437 that is based on visual observation and various tests based on scanning devices (e.g. TAPPI Method T563). See the list of references at the end of this section.
Dirt can come from many different sources. The first step in solving a dirt problem is to try to identify the material. Light microscopy is a good place to begin. Often the chemical nature of a dirt speck can be narrowed down by finding out whether the specks are soluble in acid, base, or organic solvents. Melting point tests or scratch tests for hardness might be considered.
Many mill sites and most chemical vendors have access to analytical tools that are especially well suited to the identification of chemical components in dirt specks. In particular, some infra-red (IR) spectrometers are able to microscopically focus on a small spot of interest. IR spectra provide the user with information about the kinds of chemical groups that either (a) may be present or (b) are definitely absent from the discolored material in the paper. The reason for the word "may" is that many IR absorbance peaks overlap. An experienced spectroscopist may be needed to interpret a complicated spectra resulting from a mixture of various materials.
Some dirt problems can be traced back to their source, based on some preliminary knowledge of the composition of the material. For instance, bits of plastic, rust, or scale deposits may be becoming worn away from an exposed surface somewhere in the paper machine system. Alternatively, the dirt may consist of plastic, wood resin, bark, or even sand coming in with wood chips and passing through faulty screens in the pulping system. If a papermaker runs out of other folks to try to blame, also it is worth considering whether wet-end additives are involved.
Some practical steps to reduce dirt in a paper machine system include (a) inspecting screening equipment and possibly using a finer screen, (b) inspecting the hydrocylone cleaner system and possibly increasing the rejects flows, (c) inspecting and repairing any pump or agitator packings that may be coming apart, (d) shutting the system down for a clean-up ("boilout"), and (e) reducing the dirt content of incoming pulp, especially in the case of recycled pulp or pulp that contains bark. If the dirt is on the surface of the sheet it makes sense to inspect the surfaces of felts and rolls, cleaning them as necessary. Dirt also may fall from dryer hoods or dirty ventilation air.
Even though this website is mainly involved with chemical additives, additives seldom can solve dirt problems. Exceptions include (a) cases where the "dirt" consists of aggregates or scale formed from the additives themselves, (b) cases where additives tend to decompose after addition to the wet end, e.g. sizing agents, and (c) cases where bacterial slime problems can be managed by avoiding excess, unretained starch and treating with appropriate biocides. Canister-type filters near to the containers for chemical additives are invaluable for preventing sediment, grit, paint coatings and other potential dirt from reaching the paper. It is a good idea to visually inspect such filters if the chemical additive system in question is considered as a possible source of the observed dirt material.
When using recycled pulps, dirt problems should be mainly addressed during various deinking, screening, floatation, or washing operations. Such operations lie outside of the scope of this website. To a moderate extent, dirt problems related to sticky materials from recycled pulp sometimes can be reduced in severity by adding (a) finely divided talc mineral or certain organic polymers designed to detackify the wetted surfaces, (b) spray application of barrier chemicals (very dilute solutions of highly cationic polymers) to the forming fabric and sometimes to wet-press fabrics, (c) detergent or even solvent treatment of wet-press felts, and (d) use of doctor blades to remove tacky or sticky materials from dryer can surfaces.
Anon., "Identification of Specks and Spots in Paper," TAPPI Useful Method UM 589, 1984.
Rosenberger, R. R., "Putting the New Dirt Count Method into Perspective: A Discussion of TAPPI Method T-563," Prog. Paper Recycling 6 (1): 9 (1996).
Soderhjelm, L., "Dirt and Shives in Pulp, International Standardization," Paper Technol. Ind. 37 (10): 51 (1996).
Zeyer, C., Heitmann, J. A., Venditti, R., and Joyce, T. W., "Image Analysis with an Optical Scanner," Prog. Paper Recycling 3 (3): 29 (1994).
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.