Slip Manufacturing Defined and Discussed

16 Aug 2010

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Most of our blog posts have information about our products, how they work, or just the general specifications about polyethylene. We talked briefly about the history of how poly bags came to be and how they are widely used. In this blog we're going to get a bit technical to inform you what “slip” is and how it is manufactured in plastics. If you have never heard of slip before, we believe you’ll find this blog to be quite enlightening.  

Slip is an organic chemical that is added to the blend during film extrusion process to modify the coefficient of friction or COF. COF is a measurement of the amount of friction between two surfaces as they begin to slide and as they continue to drag against one another. The amount of energy it takes to put an object in motion is always greater than the amount of energy that it takes to keep it moving while it is in motion. Although there are a number of chemicals that are used to modify the COF, the two main ones used in Polyethylene and Polypropylene are Erucamide (which is considered a slow bloom) and Oleamide (which is a fast bloom.) The materials are sometimes called primary amides.

Slip is a very efficient molecule it has been added to a plastic film in very low concentrations typically 500 to 1500 parts per million. The slip additive can be let down at the levels of 1 to 3% by weight in blending while extruding the plastic film. Slip molecules are very low in molecular weight as compared to the polymer in which they are used. These molecules are somewhat incompatible because of their nature. This means the material is bi-polar and has one end that has a positive charge and the other has a neutral charge. This incompatibility causes the molecule to migrate referred to as bloom through the polymer to the surface of the plastic film. The more slip that migrates on the surface the lower the COF and the more slippery the plastic film becomes.

The COF relates to packaging because the substrates used to package objects always come in contact with other surfaces or themselves. As most packaging operations are very high speed, COF plays a very large role. The packaging material must have just the right COF in order to track properly on the packaging machines. If a material is too slippery the film will not track properly and may cause issues such as bad sealing or cause a powdery substance that builds up on the tracking rolls which will cause packaging failures. If a material becomes too sticky it can have the opposite effect listed above. It may not allow the film to un-wind properly and cause web brakes or other tracking problems. Either scenario can shut a packaging line down.

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