Seeing is Believing – Qualitative vs. Quantitative Measurements

Posted on 6/10/2020 7:33:04 AM By David Speth

One of the things I have noticed recently is an increased reliance on quantitative measurements.  Advances in testing equipment have certainly made our results more accurate, reliable and reproducible. Automated screening tools allow us to evaluate hundreds of formulations to find unexpected islands of performance (ASC Formulations Short Course 2019: "High Throughput Design" by Dr. Kshitish A. Patankar Research Scientist Dow Chemical).

I wonder, however, if we are losing our grasp of the importance of the qualitative part of the data. This may be especially true now that many are working remotely and relying on others to report the data to them for analysis. For me, it is important to have eyes on the samples as they are prepared and tested and then again after failure to see what story they tell beyond the numbers.

The most important qualitative factor is the locus of failure. Whether a formulation gives adhesive failure at the substrate surface or cohesive failure within the adhesive layer is a critical factor in developing a successful product. While it is common to note the failure locus when testing pressure sensitive adhesives, it is not always true for structural adhesives where there can be multiple difficult to discern failure modes (adhesive, cohesive, near surface, primer layer or substrate). A colleague once said that getting the right strength when bonding plastic or steel was easy, but getting the failure mode the customer wanted (usually cohesive) was difficult. If the failure mode isn’t noted, it’s impossible. As an example, I recall watching a technician test a series of bonded plastic specimens. Failure started when the plastic yielded outside the bond line and ended when the yielded volume crept under the adhesive bond so all the numbers were the same. In this case the numbers did not tell a clear story.

Another area where the failure mode is important is accelerated aging tests. Managers and salesmen often ask; “Why does a 6 week test take 6 weeks? Can’t we go faster?” Since everyone knows that increasing the test temperature by 10°C increases the rate of a chemical or physical process by about a factor of 2, why not just increase the test temperature? To be meaningful the failure mode observed in the accelerated test must mimic that seen under normal aging conditions. If the failure mode changes, the test is evaluating something other than the desired performance. How the sample fails is as important as the strength at failure.

Failure mode is not the only qualitative factor that should be noted. For pressure sensitive adhesives, the shape of the peel curve is also informative. Is the peel trace smooth or rough? Does it show a periodic saw tooth pattern (slip-stick)? For structural adhesives it is important to know the shape of the formulation’s stress-strain curve. Does the curve rise in a straight line until failure? Does it rise steeply from the origin and then flatten (yielding)? Is the area under the curve large (tough) or small (brittle)? There is no single right answer to these questions. The mechanical performance needed depends on the application.

Years ago I was consulting with a colleague when a student brought him a table of stress-strain data. I was surprised when all the various samples had exactly the same elongation at break-512%. Having just done similar series of tests, I knew that elongation at break has a high level of variation due to flaw sensitivity. I also knew that the samples being tested showed a wide range of elongation. It took my colleague three days to figure out that the 512% came from the Excel default plotting choice. Because he had never watched the test being run or related it to what he could feel with hand pulls, he did not know that the data was faulty.

There are many other qualitative aspects of a product that can be informative and important. Is the melt clear or cloudy? Does filler settle out over time? Does the sample have an objectionable odor? Direct observation using all your senses provides useful information. So, go into the lab (or use a remote camera) and watch the testing or train your technician to report his or her observations-whatever they may be. Stay involved.

I hope that as we all navigate this unusual time that we keep our wits about us and focus on using all our senses to create successful products.

Stay safe. Stay healthy.

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