COEFFICIENT OF FRICCION (CoF)
The acknowledgment of this very important property for flexible packaging films is defined as the relative slip resistance between two surfaces. Both of them could be formed by plastic films or one plastic surface and other one metallic.
This property called Coefficient of Friccion or CoF is the index between the frictional force and the perpendicular force already installed on among the both surfaces regularly the gravity force. Fricction evaluation could be as follows:
• Static Opposite direction resistance in regards the starting of the relative movement between both surfaces;
• Dynamic Opposite direction resistance in regards the continuity of the relative movement between both surfaces.
CoF number is a great toll to evaluate and to specify flexible packaging and label constructions. It is really important due to keep the correlation between the material real performance and the packaging equipment machine and/or the stacking. So, Static CoF is used for stacking and Dynamic CoF is to evaluate output on the packaging equipment.
The CoF evaluation reproducibility depends on the respect to the applied methodology and also the “environmental conditions” to make the analysis. Please, look at below some factors that could be disturb CoF reproducibility:
• Surface smoothness or surface roughness; • Compatibility between the surfaces;
• Plastic material composition (slip agentes, anti-blocking, pigments);
• Electrostatic energy; • Surface Treatment (corona, flame, plasma, chemical);
• Warehouse conditions (temperature and relative humidity);
• Film manufacturer process.
Slip agent additives used to decrease CoF values and it means low friccion. These slip agentes could be:
• Migration In general, the additive is part of film core and then its migration goes to the surface layers over time.
• Non migration Applied directly on the desired film layer.
ASTM D-1894 standard describe the right methodology to evaluate CoF and it is much more precise than inclined plane analysis. The ASTM standard define a metallic block of 200 ± 5g (0.44 ± 0.01 lb) as weight and the environmental conditioning of, at least, 40 hours at 23 ± 2ºC (73 ± 4°F) as temperature and 50 ± 5% as relative humidity.
The CoF results has the range between 0.00 (highest slip) and 1.00 (lowest slip). No units are used for CoF, because it is an index. Click here to download in PDF