Tribological Characteristics of Paper

Friction in paper

Friction coefficient is an important parameter in paper production. Friction coefficient of paper is modified according to various applications.

High friction coefficient between papers is required for applications such as:

• Storage and transportation of stacks of paper – If there is low friction coefficient between papers then the stacks of paper will not stay on one another and slip
• Rolling of paper – If there is low friction coefficient between papers then it would be difficult to roll long lengths of paper

Low friction coefficient between papers is required for applications such as:

• Automatic feeding of paper in photocopiers and printing machines – If there is high friction coefficient between papers then the machine would not be able suck the paper.
• Cash counting machines – If there is high friction coefficient between papers then fast repetitive motion used to pull the notes can jam and get a wrong figure to the operator.

Measuring friction coefficient of paper

• Inclined-plane method – This test method is very popular for the measurement of paper-to-paper friction in the paper industry. This method determines the coefficient of static friction by measuring the angle at which one test surface begins to slide against another inclined surface as the inclination angle is slowly increased.
• Horizontal-plane method: This method determines the coefficient of friction by measuring the force required to initiate or maintain horizontal motion when one test surface rests on another surface in a horizontal plane.

Wear in paper

Wear can be defined as progressive damage involving material loss. Wear by paper is generally witnessed in printers, scanner, copiers, paper press, paper shredders, paper cutting and paper punching machines. Abrasive wear which refers to ploughing a grove into a softer counterpart by asperities or embedded hard particles of harder surface, generally prevails as one of the common wear mechanism.

Hard particles can be present in paper as:

• Impurities: Improper cleaning of wood pulp can lead to presence of dirt, dust, forest fire ash, mineral ore particles in the finished products. These impurities act as abrasives.
• Modifiers: Fillers are used to improve certain properties of paper. Fillers talk, hydrous kaolin, china clay, titanium oxide are generally used. China clay has Aluminum and Silicon Oxide particles which act as abrasives.

The rate of abrasive wear by paper depends on:

• Weight of hard particles
• Shape of hard particles
• Quantity of hard particles
• Hardness of counterpart
• Presence of paper debris

The surface of the counterpart worn by paper can have two main type of appearance:

• Polished – Highly smooth surface with shiny finish
• Grooved – Blisters and craters on the surface with high roughness

Measuring paper wear

Abrasivity of paper can be determined by wear tests that can simulate a particular application. With paper wear testing is not an easy task as it has to be made sure that during wear testing little or no damage should occur to paper.  There are very few Tribometers that are that much sensitive and precise. One such Tribometer is the Advancer Paper Tribometer by Bruker which promises precise measurements of friction force, paper wear and paper fatigue. It also allows the user to perform paper-on-paper sliding, paper-on-roller sliding, pen on paper sliding, eraser on paper sliding tests.

Reducing paper friction and wear

• Addition of Fatty acids in paper production reduces surface free energy of paper thereby reducing their coefficient of friction.
• Wood extractives and other chemical compounds mixed during paper production should be more hydrophobic in nature as they provide certain amount of lubricity effect.
• A coated paper has a lower friction coefficient than base paper or uncoated paper
• Use of wax emulsion as lubricants for paper production improve the internal lubricity of coatings thereby further reducing coated paper friction
• Fillers (talk, hydrous kaolin – low COF) are also believed to reduce coefficient of friction but only to a minimal amount.