COF Testing: Why Does it Matter?

Coefficient of friction (COF) is a property critical to the optimum processing and handling of packaging from filling operations through to the consumer. COF is defined as the force required to move one surface over another, divided by the total force applied normal to those surfaces. The values are between zero and one and are dimensionless. There are two common basic COF testers — sliding plane and incline plane.

Sliding Plane

Testing is performed by sliding a metal plane at constant speed beneath a block. The print specimen is attached to the base of the block. The block is connected to a stationary gauge that measures the frictional force between the plane and the block. To simulate a particular use condition, a piece of print or other packaging material may be attached to the block.

Static COF is the number generated the instant motion starts between the surfaces. Kinetic COF is the value indicated after motion is established between the surfaces.

Incline Plane

This COF tester is a simple and robust device. It is used for routine measurement and control of static COF. A sample is clamped to the inclined plane; another is clamped to the sliding block. The tester gradually inclines the samples until the block starts to slide and automatically locks and captures the angle of slide. The tangent of this angle is the static coefficient of friction.

Why does it matter? If the static COF is too high, problems with stacking and unstacking of cartons, boxes, and sacks will occur. Flexible films may bind when sliding onto filling collars.

If the static COF is too low, problems in maintaining stability in stacks could result, as well as difficulty pulling materials through automatic processing machinery.

If kinetic COF is too high, this can cause hangups along packaging lines, undue folds, erratic web tension, and a buildup of static electricity. If the kinetic COF is too low, guide rolls may not rotate, tracking along the production line becomes irregular, web tensions again may be erratic, and the material will run eccentrically.

Getting the COF right is mostly a process of qualifying a packaging product and then supplying this product at a consistent COF specification based on a given instrument and testing procedure. Be aware that different instruments and procedures have poor correlation.

In packaging films, the traditional technology to lower COF is to incorporate a fatty amide, which over time migrates to the film's surface. Newer technology uses more stable proprietary non-migratory slip packages.

Inks and coatings employ waxes, fatty amides, and silicones to lower COF and silica particles to increase COF. Many converting variables cause COF to vary. This property is sensitive to application viscosity, coating weight, drying conditions, and environmental humidity. In energy-cure systems, COF is affected by the degree of cure. So, once again, running to proven standards ensures low waste and fewer field problems and returns.

Process improvement expert David Argent has 30+ years of experience in process analysis with particular emphasis on ink and coating design and performance. Contact him at 314-409-4304; djvargent@sbcglobal.net.


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