Converting Magazine

Eldridge M. Mount

Measuring treatment, Part 1
January 10, 2008

In one of my last posts, I talked about treatment being a poor indicator of surface quality. However, it is widely used so we need to talk about how to use it. And to do this, we need to know how best to measure it. 

Measurement of the surface energy or treatment level can be conducted many different ways to obtain simple wetability measurements, to obtain more detailed information about the nature of the surface forces, the level of surface oxidation and the composition of the surface chemistry. Each method is more complex, time-consuming, informative and expensive than the preceding method. Each will be briefly described and its use outlined. Those wishing to study this topic further are directed to the recent book of Chan[i], which reviews the many methods of polymer surface modification and characterization. We will start with the simplest methods and move to the more complex methods in the days to come.

For the simple determination of the existence of treatment, pre-made treatment pens may be purchased to wipe on a film surface to check for minimum surface treatment levels. However, due to contamination of the pen tip by actual testing of the film by surface wiping, the pens should not be routinely used to determine the treatment level but as a "go, no-go" test for the presence of treatment. To save money or time if no treatment pens are available, it should also be possible to determine which commercial markers will show treated and non-treated surfaces. As some markers are solvent-based and others are water-based, it should be quite possible to test some to determine which will show proper film treatment levels for your process by the formation of a uniform mark or a mark which breaks up and does not wet your substrate.

Surface treatment level: This is a straightforward and fast measurement to determine the surface tension or wetability of the surface. The test is detailed in the ASTM procedure D2578 for the measurement of Surface Tension of polyolefin films from 30 to 56 dynes/cm. While simple to do, reading the results are somewhat subjective, and is therefore prone to operator variability based upon training and experience. The test consists of wetting a cotton-tipped swab into a series of solutions of Formamide and ethylene glycol monoethylether (ethyl cellosolve^TM) blends, Table 7.1, and spreading the solution on the surface by wiping the wet swab lightly over the surface. Alternatively, several drops of solution may be spread or drawn down by a metering rod. Approximately 1 sq in. of surface is covered and then the breakup of the liquid film is watched for. The surface tension is defined as the solution composition where the liquid film breaks up at two seconds. This is determined by testing the surface with a series of solutions. Great care is needed at each step, which is why the operator's experience is important. A fresh swab needs to be used for each attempt, too much fluid on the cotton will impact the reading by varying the liquid film thickness, too much pressure on the swab tip can disturb the film surface changing the result, the cotton itself must be free of sizing, and the solutions need to be fresh to insure proper composition. In general, the test yields a value, which can vary 1 to 2 dynes for a single tester and between testers

Posted by Eldridge M. Mount on January 10, 2008 | Comments (0)