Volume 8, Issue 11 - December 2007
What’s in a Number?
by Walt Polifka
In the world of pressure-sensitive adhesives (PSA) you will find no shortage of physical property data. Peel strength, shear strength, loop tack and temperature information are all readily available. The question is: How useful is all of this data? The short answer is: not very.
As a provider of PSA solutions, we coach our salespeople to understand the application and the conditions to which the PSA is subjected before we can select a product from our pressure-sensitive adhesive tape products.
How Do You Compare?
The problem is people love numbers. They would sometimes rather compare numbers in the comfort and privacy of their offices than interact with a vendor and divulge “secret” information. Customers may think that if they confide in suppliers they lose control over their projects when, in fact, the opposite is true. By providing comprehensive accurate information they can tap into our expertise in the chemistry and application of PSAs.
Before looking at the different properties and what can affect the numbers, it is important to start with the source of the information and the test procedures themselves. There are several sources of test procedures but the most common are tests sanctioned by the Pressure-Sensitive Tape Council and tests developed by the American Society of Test Methods. In addition, some vendors report results from tests they have developed in-house. Large customers, particularly in the fenestration, automotive and electronics markets, require a myriad of test procedures to qualify. If you do compare numbers, it is important to know who conducted the tests and the test methodology. Comparing numbers from two different test methods is like comparing apples to oranges.
How the numbers are generated is equally important. Did the vendors do the testing themselves, or did they utilize an independent outside testing facility? The American Association of Laboratory Accreditation (A2LA) has established standards for laboratories that include equipment calibration, technician training, laboratory environment and record-keeping. For example, A2LA has certified our company to ISO 17025, the standard for independent laboratories signifying data published by our laboratory is accurate and free from subjective commercial influences.
Factors at Play
Loop tack is a common measure of “quick stick”—how fast the PSA will adhere to the substrate. A tensile-type machine lowers a loop of the test tape on to a substrate, typically stainless steel, and contacts for a specified period of time, typically one second or less, and then it is pulled away with force, usually measured in pounds per inch. When looking at loop tack numbers, it is important to know the machine speed as well as the backing used as these factors will influence the final number.
When speaking about tack it also is important to consider “thumb appeal” or the time-honored practice of using finger pressure to predict the quick stick properties of a PSA. This test does not produce any numbers that can be compared, and it is not much more reliable than depending on an arthritic knee to predict the onset of a hurricane. In addition to being subjective, the thumb appeal test is susceptible to many unpredictable distortions, such as, what variety of breakfast sandwich the tester handled that morning. We have engineered many systems with mediocre thumb appeal but with an excellent affinity for the substrate for which they were designed.
Peel numbers, usually expressed in pounds per linear inch, are impacted by the speed of the test apparatus, the dwell time or the elapsed time the adhesive has been in contact with the test surface following application, the backing material (PET film is common but higher numbers can be generated by the use of dead-soft aluminum foil as a backing material) and the chemistry. The chart at right shows the impact of both peel angle and backing material for a typical acrylic PSA.
If one does not know the test conditions, comparing the numbers is useless. Rubber-based systems in general will exhibit higher peel numbers than acrylic-based systems. PSA manufacturers typically will report immediate peel numbers since this test procedure is performed routinely as part of the company’s quality assurance process. Unfortunately these will be the lowest numbers seen on a system and seldom relate to actual use. The chart at right shows the impact of dwell time on peel strength, with different backing materials and substrate types. As is apparent, some conditions show little impact with dwell time while with others, the impact is quite dramatic.
There are many ways to generate numbers so it is important that the conditions under which they were produced be well understood. Static shear testing is done by hanging weights onto a sample of PSA and measuring the time to cohesive failure (the adhesive film splits). If the adhesive fails (i.e., cleanly delaminates), that would not be considered a measurement of shear strength of the adhesive system. Shear testing may be done at room temperature or at an elevated but constant temperature. Variables that need to be considered before comparing shear testing results include the amount of the weight and the area of the test sample.
Lighter weights are sometimes used at higher temperatures. Often the tests are discontinued if the sample is still “hanging” after 7 days and reported as “7+ days+.” One way to discern differences is to test to failure of the first candidate.
Dynamic shear testing defines the strength of the adhesive as measured by the force required to generate a shear failure. In this case, the units are force or pounds per square inch rather than time. Since the rate of application of the force is the determining factor, it is critical that the rate be known or reported as part of the test results.
Service temperature is one of the more misunderstood and misused properties of a PSA tape product. The service temperature is primarily a factor of the adhesive itself. Service temperatures are reported typically as constant and intermittent. Intermittent results always need further definition. Is it five minutes at the elevated temperature and five minutes at the reduced temperature, or is it five days at each temperature? Vastly different results can be expected. Other factors that influence service temperature include the type of substrate that is bonded, the other ambient conditions such as how much humidity is present, how was the PSA applied and how much dwell time preceded the test.
Another number that customers commonly use to compare adhesives is Shear Adhesion Failure Temperature (SAFT). The test sample preparation is identical to the shear testing, but in this case the test temperature is raised in increments until the sample fails. The key word is failure.
The temperature reported bears little relationship to the conditions of an actual use situation. All of the variables affecting service temperature come into play for the SAFT test as well. When comparing these numbers, be sure the test conditions were identical.
Side-by-Side Tests are Crucial
Comparing products in actual use conditions and/or environment involves time and effort but it is infinitely better than comparing by numbers alone. So, when comparing numbers on PSA data sheets, what’s in a number? The answer is: Not much, really.
Walt Polifka has been part of the business development team of Adchem Corp. in Riverhead, N.Y., for more than seven years.