The use of tackiness in the adhesives industry, to measure the peak bonding strength of a product, demonstrates an integral application of the property. Two ASTM methods (D2979 and D3121) have been adopted to measure tackiness2 for these types of applications, but it is worth noting that method D2979 has since been withdrawn with no replacement due to limited industrial use3.
ASTM D2979 was a method used to measure the tackiness of pressure-sensitive additives, which are additives that will form very strong bonds even with little pressure applied. The testing method was modeled to be a more quantitative version of the subjective finger test using a probe with a diameter of 5 mm, which would press down lightly into the sample and then pull up at a constant rate of 600 mm/min. This measures the amount of force necessary combined with the maximum pulling force totaling the overall tackiness of the sample2.
This testing method has fallen out of favor due to its lower levels of reproducibility because sample preparation can cause varying results4. ASTM method D2979 has since been replaced by other testing methods such as loop tack, rolling ball tack, and quick-stick. These have not yet gained ASTM adoption except for the rolling ball tack which has become ASTM method D31212. ASTM D3121 is a standard that measures the tackiness of pressure-sensitive additives by rolling a ball with a diameter of 11.1 mm down an incline of 21° 30’ which is coated in the sample and measures the distance that the ball travels before ceasing. This method has been widely embraced due to its low cost and strong reproducibility. However, it is not a reliable approximation to practical applications and is thus used more commonly in quality-assurance testing where it can detect variations between batches.
It is essential in the ink industry to discern tackiness in order for it to be able to adhere to the paper properly and to design printers around the ink’s constraints. ASTM method D4361 which, despite its withdrawal in July 2019 due to lack of updating, is the primary testing method used to quantify the tackiness of inks. ASTM D4361 employs a device known as a three-roller tachometer. The bottom roller in this machine is an oscillating rubber roller, the middle roller is a brass roller which can maintain a certain temperature, and the top roller is a static rubber roller that does not oscillate. This trio of rollers is designed to simulate the rolling conditions that occur in printers where the freshly applied ink must be able to stay on the page without smearing as it is fed through. This method has proven extremely effective for the ink industry due to its high reproducibility, correlation to real-world performance, and quantitative results.
Tackiness is pivotal to the petroleum market since determining the adhesive properties of a grease or oil sample can yield vital information regarding the compatibility of a certain lubricant in regard to a specific task. The selection of the appropriate lubricant is essential to guarantee that an adequate amount of lubrication remains adhered to industrial components; thereby, maintaining operational efficiency and integrity. One of the most common and uncomplicated methods of testing petroleum products is using the finger test which is performed by simply pressing the grease or oil between two fingers and then pulling the two fingers apart. Unfortunately, this method does not produce quantitative results but instead produces relative qualitative results. Furthermore, reproducibility with this method is low as one individual’s definition of tacky may differ from another’s. Another problem with this method is that it is not effective as an indicator of real-world performance since the human finger is not an adequate approximation of the plastics and metals used in automotive and industrial components. Another test method that has also been developed involves applying grease to two disks, pressing them together, then pulling them apart and measuring the force needed. While this method produces a quantitative result and predicts real-world performance, it has lower reproducibility as there is no universal testing standard.
A new development in testing greases and oils is using a tackiness tester. The Koehler K95200 (Figure 1) is an innovative quantitative instrument capable of testing a multitude of petroleum-based products. The machine consists of a stationary metal plate on the bottom and a second metal plate on the top. To run the test, 5 mL of the sample is placed on the bottom plate, then after pressing start, the top plate will come down and press against the sample allowing it to bond to both plates.
Figure 1. Koehler K95200 Tackiness Tester
After this step, the top plate retracts at a constant speed and measures the force needed to pull the grease apart (Figure 2). This testing method has excellent repeatability with a gauge equipment variance of only .097.
Figure 2. Grease being pulled apart by Tackiness Tester
While basic qualitative test methods have been in existence for years to test tackiness in a variety of industries, recent advances in technology and laboratory instrumentation has allowed for more advanced test methods yielding quantitative results. The physical material property of tackiness has proved to be an important property to study specifically in the food, ink, adhesive and petroleum industries. With the new methods for obtaining a quantitative result of tackiness, food, ink, adhesive, grease and lubricant manufacturers can gain a better understanding of the property and thus improve their product formulations.
Dr. Raj Shah is currently a Director at Koehler Instrument Company and an active ASTM member for the last 25 years. He has been elected a fellow by his peers at NLGI, STLE, IChemE, INSTMC, AIC, EI, and RSC and is a Chartered Petroleum Engineer and a Chartered Chemical Engineer.
Kareem Mehdi is a chemical engineering student at Stony brook university where Dr. Shah is currently a member of the industrial advisory board. Mr. Mehdi is also part of a thriving internship program at Koehler instrument company, Holtsville, NY.
Cindy Galdamez has a Masters Degree in Mechanical Engineering from Stony Brook University. She has been working with Koehler Instrument Company, Inc. for over 5 years as the Technical Application Specialist. She is heavily involved with ASTM, specifically committee D02 for Petroleum Products and is also an active member of STLE.
References
1. Measure Stickiness. Stablemicrosystems.com. https://www.stablemicrosystems.com/MeasureStickiness.html.
2. Roberts R. Review Of Methods For The Measurement Of Tack. Pira International; 1997. http://www.adhesivestoolkit.com/Docu-Data/NPLDocuments/P%20A%20J/PAJ%20Reports/PAJ1%20Reports/PAJ1%20Report%205.pdf.
3. ASTM D2979 - 16 Standard Test Method for Pressure-Sensitive Tack of Adhesives Using an Inverted Probe Machine (Withdrawn 2019). Astm.org. https://www.astm.org/Standards/D2979.htm.
4. Tack of Pressure Sensitive Adhesives | Glue Machinery Corp. Glue Machinery Corp. https://www.gluemachinery.com/tack-pressure-sensitive-adhesives/.
5. Electronic Inkometer Model 106. Web.tech.uh.edu. https://web.tech.uh.edu/digitalmedia/materials/3252/Inkometer.pdf.
6. ASTM D4361 - 10 Standard Test Method for Apparent Tack of Printing Inks and Vehicles by a Three-Roller Tackmeter (Withdrawn 2019). Astm.org. https://www.astm.org/Standards/D4361.htm.