Q: What, why and how do we measure substrate properties?

By Dr. Eldridge M. Mount

A: I think I will tackle “adhesion” testing. There are many adhesion tests, such as the T-peel test (ASTM D1876) and the AIMCAL TP-105-92 for metallized films. Many tests can be found in the testing standards for many products and end uses, perhaps specifically for your substrate and end use. In large part, this is – in my estimation – because the “adhesion” test in most cases is subjective or, perhaps better, sample-specific. The tests have value, of course, but not in the way many think they do. Seldom are standardized “adhesion” tests numerical values and thus predictive for new product performance in specific applications. There is a longer discussion of this in the AIMCAL Metallizing Technical Reference [1]. The tests are, however, very valuable for in-house quality control, product development and process studies, and developing correlations between substrate and lamination results.

Standardized adhesion tests are poor predictors of a particular film’s performance in a commercial lamination product. This is simply because the standardized test defines the construction of a specific sample structure, which is physically different than any particular commercial lamination. This becomes a limiting factor in predicting a product’s particular numerical performance in any customer’s laminations because standardized “adhesion” tests rely on measuring the peel force required to separate a test sample or lamination, generally with a T-peel test. You are not, in fact, measuring the “adhesion” but the total force required to separate the sample at an interface, within a layer of the sample or the tearing of the sample, depending on the mode of failure. In this case, the measured force contains the total force required to distort the entire structure, as well as the separating force. Consequently, the composition of the final lamination and the test sample construction all combine to give a specific numeric “adhesion” value. I would suggest that the interested reader review other discussions of peel testing, such as Peel Test - an overview | ScienceDirect Topics (www.sciencedirect.com/topics/engineering/peel-test) and, in particular, the discussion in Adhesives Technology Handbook (Third Edition), Chapter 12.4 Peel [2].

Testing the actual lamination

To determine the lamination bond strength or perhaps metal adhesion of a particular film in a particular lamination, it is my view that you have to make the actual lamination. This is because the construction, lamination-process conditions and lamination materials used in the lamination combine to determine the measured peel force. A standard laboratory bond or metal-adhesion strength test will not predict the actual lamination or metal-bond strength of a component film in all applications. The tests should be used for process control, product development and to ensure uniformity of your product. For instance, I have found the standardized metal-adhesion tests very useful in process studies. Designed experiments of metallizer conditions, lamination conditions, base-film product design or film-manufacturing process experiments help determine what process conditions control and optimize metal adhesion and peel strength.

In the case of the AIMCAL “Qualitative” metal-adhesion test, while it may not predict the numerical performance of a metallized film at a customer’s lamination plant, it can be used to determine the production uniformity of your metallization process or base film. If you decide to control-chart your results, you can develop product and process control. This allows you to be confident in your product’s uniformity and meeting your specifications for it. Comparisons between in-house test results and customer product results require performance correlations to be developed. Then, when a problem arises with a customer’s lamination, you can look for the cause more effectively. This is what a standard “adhesion” test can and should be used for – to test a product for production uniformity.

Testing for component performance

For testing the performance of a component film in a final lamination, you must have a standard sample construction for testing. This is really what a standard peel test, such as the ASTM or AIMCAL test, provides. This is important especially if you are comparing values generated at different testing locations or produced somewhere else. For me, this understanding came about from a large misunderstanding of test results with a lamination customer. We would obtain different testing results, even using the same samples, the same model of testing machines and the same conditions. After a lot of finger-pointing, (and maybe a bit of shouting back and forth), it finally was decided to sit down together and go over the fine details of the testing procedures. Turns out each lab did everything exactly the same, making the same test-sample design, except we each were using a different sample-backing tape. The change in backing tape changed both the peel force and the mode of failure. Once everyone used the same tape, the results were comparable.

Another good example was in the development of a metal-fracture-resistant film for extrusion lamination. It had a homopolymer-PP metallization surface in the place of a copolymer metallization surface. The homopolymer metallized film always gave 100% metal transfer in the lamination, sometimes at 200 gm/in., which everyone liked, and sometimes less than 40 gm/in., which nobody liked. At the same time, the standard adhesion test always gave high metal-transfer bond strengths. A subsequent lamination process study clearly showed that the extrusion-lamination conditions were responsible for the wide variation on the laminations’ peel force, irrespective of the 100% metal lift.

So standard adhesion tests are a valuable tool and, if used and understood properly, can improve your understanding of product and process performance and product uniformity. However, they should not be considered a predictive tool for specific adhesion values in any specific application.

References

1.   1. “Purpose and Use of TP-105-92 for Adhesion Testing,”  AIMCAL Metallizing Technical Reference, 4th Edition, Ed: Eldridge M. Mount & Charles A. Bishop, AIMCAL, 2007, pp 12-13. 

2.  2. “Testing of Adhesive Bonds, 12.4 Peel,” Sina Ebnesajjad, Ph.D., Arthur H. Landrock, Adhesives Technology Handbook (Third Edition), 2015.