Friction (High, Low, Paper to Paper, Paper to Equipment)
The end-uses of many kinds of paper are critically dependent on frictional properties. Who among us hasn't at some point experienced the aggravation of multiple feeds of paper in xerographic copiers or ink-jet copiers? That's an example of a problem that tends to occur when paper-to-paper friction is too high or too variable. Problems with slippery paper can be just as problematic, especially if one is trying to stack up the paper or die-cut it in precise locations. In many end-use applications it turns out that the frictional properties of the paper against a metal or rubber surface is the most critical, rather than problems associated with paper-to-paper coefficients of friction. For more comments related to xerographic copying, click here.
Let's start with a definition. The coefficient of friction can be defined as the ratio of two forces. The numerator is the force needed to slide two objects in a direction that is perpendicular to a force that is pushing them together (often gravity, in simple laboratory tests). The denominator is the value of this second force. Two more useful definitions are static friction and dynamic friction. The static frictional coefficient is the ratio of tangential to perpendicular force at the point where motion (slippage) is initiated. The dynamic coefficient is the same ratio at a defined rate of sliding, and usually the average value is reported.
PAPER-TO-PAPER FRICTION PROBLEMS
Factors that can cause paper to be unexpectedly BELOW specification limits for paper-to-paper friction coefficient include (a) the presence of waxy materials such as wax, high levels of alkylketene dimer (AKD) size, or certain silicone materials, (b) high smoothness, especially in combination with the first factor, though friction coefficients sometimes increase with increasing smoothness due to a higher effective area of contact, and (c) surfaces covered with solid polymeric materials well above their glass transition temperatures.
Often papermakers can overcome problems related to low friction by making moderate changes in chemical additives. For example, if AKD sizing is causing the paper to become to slick to be handled by an automatic stacker, papermakers have the option of reducing the dosage of the sizing agent. To compensate, it may be necessary to add a hydrophobic polymer, such as styrenemaleic anhydride, to the size press starch solution. Papermakers also can replace the sizing agent with alkenylketene dimer size, a version of AKD having unsaturated hydrocarbon tails. This small change renders the product more liquid-like, rather than wax-like, and it has much less effect on frictional properties compared to the ordinary type of AKD. Another approach is to add fillers such as scalenohedral precipitated calcium carbonate, or (for the biggest effect) high-surface-area precipitated aluminosilicate or amorphous silica particles in the size range of about 1-5 micrometers.
Factors that can cause paper to be unexpectedly ABOVE specification limits for paper-to-paper friction coefficient include (a) the presence of high-surface-area mineral products capable of adsorbing waxy materials from the paper, (b) the presence of tacky polymeric materials or rosin-like materials, and (c) high roughness, especially in combination with the first two items.
Sometimes papermakers need to decrease paper-to-paper friction, as in the case of the multi-feed problem mentioned earlier. Increased calendering sometimes lowers the paper-to-paper friction, but the relationship is not expected to be simple, and usually there are firm specification limits for smoothness. Papermakers have the option of adding wax emulsions, AKD size, or delaminated clay. Addition of such substances at a size press are usually more effective, based on the amount used, compared to adding them at the wet end.
In high-speed operations the coefficient of friction between paper and various transfer surfaces is often dominated by contaminants from the paper. For example, transfer of AKD size to feed rolls sometimes interferes with the operation of high-speed xerographic copiers. Such problems are most likely if the level of AKD size is high and if the paper is strongly heated during the process of interest.
In cases where the paper-to-equipment friction coefficient is too low, the most promising strategies to solve the problem usually involve either (a) ways to reduce the amount of whatever material is acting as a lubricant, or (b) addition of materials that can adsorb the lubricating substances onto their surfaces and reduce their effect on friction, even though the materials remain in the paper. When problems are serious and protracted, it may be worth carrying out a trace analysis of the transfer surfaces to determine the chemical nature of slippery materials.
When papermakers face the opposite problem of excessively high paper-to-equipment coefficients of friction, one likely solution is to add waxy materials to the paper surface. Where appropriate, this can be done during manufacture of the paper, or later at the time when the paper is being converted. A good example of this is the use of wax lubricants during corrugation of "medium" for the production of corrugated containers. In the absence of lubricants, the force needed to draw the paperboard through the corrugator may become too high.
XEROGRAPHIC COPYING AND FRICTION
Especially in the case of xerographic copying, it is worth noting that the relationship between paper friction and product performance is not likely to be simple. Charles Green suggested that problems are likely to occur in xerographic copying under the following circumstances:
Those wishing further information can go to Charles Green's website (note articles no. 103 and 131).
Brungardt, C. L., and Gast, J. C., "Improving the Converting and End-Use Performance of Alkaline Fine Paper," Proc. TAPPI 1994 Papermakers Conf., 155 (1994).
Gunderson, D. E., "Concerning Coefficient of Friction," TAPPI J. 83 (6): 39 (2000).
Hoyland, R. W., and Neill, M. P., "Factors Affecting the Frictional Properties of Paper - the Effect of AKD Neutral Size," Paper Technol. 42 (3): 45 (2001).
Johansson, A., Fellers, C., Gunderson, D., and Haugen, U., "Paper Friction - Influence of Measurement Conditions," TAPPI J. 81 (5): 175 (1998).
Withiam, M. C., "The Effect of Fillers on Paper Friction Properties," Tappi J. 74 (4): 49 (1991).
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.