Adhesion in Extrusion Coating: Time in the Air Gap Revisited, 2016 PLACE Conference
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In extrusion coating, the polymer melt experiences a number of physical and chemical changes that affect adhesion performance as it travels through the air gap from the die exit to the chill roll. These include oxidation, stress and orientation, and cooling. Antonov and Soutar [TAPPI, 1991] proposed a parameter to characterize this region which they called Time in the Air Gap (TIAG). TIAG is simply the air gap length divided by the line speed. Adhesion generally increases with increasing TIAG, and because of its ease of calculation, it has caught on as an important parameter for benchmarking performance.
This paper reviews some of the key findings from the literature regarding TIAG and adhesion performance. A close look at the formula for TIAG shows that it does not capture the physics of the flow in the air gap. It assumes that the flow instantaneously accelerates to the final line speed at the die exit. Other formulas are presented for calculating the “true” time in the air gap, focusing on those that are still easy to program in spreadsheet software. A discussion of the pros and cons of each formula is provided. In the end, TIAG is one of the best tools for assessing adhesion performance and is suitable for most situations.