Tackiness is one of the most ambiguous terms in the lubricant industry vocabulary. Formulators and engineers agree on the practical importance of tackiness, but they are still far from a consensus on how to define, measure and control it. A novel additive that stabilizes tackifiers against degradation at high temperatures and a new instrument for evaluating lubricant tackiness are putting formulators on firmer footing.
While tackiness is more commonly associated with grease, it is also a desirable property in some oils, and can be increased with the use of tackifiers. According to Erik Willett and Daniel Vargo of Functional Products Inc. in Macedonia, Ohio, these additives are very long polymers that add elasticity when used in oils. The most widely used tackifiers are high molecular weight polyisobutylene polymers and olefin copolymers. They are used in products such as bar and chain lubricants to prevent fling-off from rotating devices, prevent leaking, avoid mist formation and improve flow properties.
To evaluate tackifiers, Willett and Vargo use a simple test with a bizarre name: the ductless siphon. Typically between 0.01 and 1 percent polymer by weight is dissolved in oil and poured into a glass cylinder. A glass capillary tube is positioned vertically, and one end is immersed in the sample. The other end of the tube is connected to a flask and a vacuum line.
A vacuum is applied to suck sample through the tube and collect it in the flask. When the sample level in the cylinder drops below the open end of the tube, a tacky fluid forms a string (technically a free jet). Fluid flow continues until the string breaks. The longer the string is at its breaking point, the more tacky the mixture.
Preserving Tack in Oils
Willett and Vargo reported results from their investigations into the effects of heat and base stocks on tackifiers at the National Lubricating Grease Institutes 2017 annual meeting. Their work led them to recommend a particular tackifier preservative for lubricants.
To investigate thermal stability in PIBs, which are more efficient tackifiers than OCPs, they dissolved a high molecular weight PIB in an API Group III base oil. Samples were preconditioned at temperatures between 70 and 200 degrees Celsius for various periods of time before measuring string lengths with a ductless siphon. Results showed that the particular PIB that was tested could remain stable for months below 80 C, weeks between 80 and 100 C, days from 100 to 150 C and just hours above 150 C.
At high temperatures, PIBs (and OCPs) are more stable in API Group III and IV base stocks than in Group I and II oils. Willett and Vargo hypothesized that specific bad-actor molecules in Groups I and II accelerated thermal degradation of PIBs. To investigate, they tested PIBs in Group III oil spiked with chemicals that modeled common types of bad actors present in Group I and II oils-molecules containing aromatic groups, sulfur and nitrogen.
Surprisingly, thiosulfates (sulfur) improved PIB thermal stability, while a primary alkyl amine (nitrogen) accelerated degradation. Effects of alkylated benzene and naphthalene (aromatics) seemed to depend on a PIBs molecular weight.
Willett and Vargo screened many more chemicals for their effects on PIB and OCP tackifiers in Group III oil. Samples were preconditioned for 24 hours at 150 C or 2 hours at 200 C. One chemical significantly reduced loss of string length for PIBs at both temperatures and for OCPs at the lower temperature. The researchers found it was much easier to formulate lubricants with blends of PIB and this new tackifier preservative than with other polymers that have inherently high temperature tolerance, including polyethylene terephthalate, polysulfone, polyphenylsulfone and polyetherimide.
Tackiness in Grease
Ask any hands-on formulator about grease tackiness, and theyll scoop up a blob and watch it form peaks and string-like filaments as they separate their thumb and forefinger, like a youngster playing with a moist wad of bubble gum. But its not childs play to use the finger tack test to evaluate greases.
Willett explained to LubesnGreases, Therere two ways to look at grease tack: cohesive or adhesive. Cohesive tack refers to the internal strength of grease. It depends on the attractive forces between molecules in a substance. Cohesive tack helps grease resist being pulled apart by applied shear. Greases with good cohesive tack are stringy, show little oil bleed, and perform well in mechanical roll stability, worked cone penetration and water spray-off tests.
He continued, Adhesive tack means the external attraction between grease and other surfaces. It depends upon attractive forces between dissimilar molecules. Adhesive tack helps grease stick to surfaces and resist being rubbed away or flung off.
Grease needs both adhesive and cohesive tack to fulfill its purpose. Grease with no adhesion would slide off surfaces and never spread inside bearings, and grease without cohesion would be flung off surfaces due to centrifugal forces in rotating machinery.
Tackifiers tend to increase both adhesive and cohesive tack. For lubricating greases, tackifier performance depends on thickener and base stock. Theres still no definitive way to quantify tack in grease, although tackiness is one of the first things people notice when handling grease. So, grease formulators take a trial-and-error approach.
Vargo proposed a simple pull-off test for grease tackiness at the 2014 NLGI annual meeting. Grease was loaded in a shallow trough made from steel sheet. A flat steel sheet was placed on top of the grease, and an inexpensive spring scale was attached. He calculated the pull-off force required to separate the flat steel sheet from the grease. Results depended on the adherence of the grease to the steel as well as the cohesiveness and string-forming ability of the grease.
Vargo compared OCP, styrene-ethylene-butylene and OCP-anhydride tackifiers formulated in NLGI grade 2 lithium complex greases. Pull-off test results corresponded to cone penetration tests, which measure grease consistency, primarily cohesion; water washout tests, which primarily measures adherence to a substrate; and water spray-off tests. He concluded that grease tack is a composite property and recommended a combination of pull-off, water spray-off and other tests for qualitative evaluation and comparison of grease additives.
The Latest Tack Tester
Tackiness is not to be confused with stickiness, which is adhesive force, Emmanuel Georgiou of Falex Tribology Belgium explained to LubesnGreases.Tackiness is the ability of a grease to stick well and form sufficiently long threads when two surfaces separate so that grease redistributes itself between the surfaces.Grease must be able to travel from one surface to another and stick well to work well in applications.
A grease with a good combination of adhesiveness, cohesiveness and thread formation is said to have high tackiness.But, Georgiou noted, this is a subjective definition.
It is important to tailor grease tackiness to match the velocity and geometry of an application, he pointed out. Slow-moving, open gears may need a lot of tackiness, but too much tackiness may form long threads that cause resistance and energy loss in faster-moving systems. In applications such as food processing, its important to avoid strings that might break easily and contaminate products.
Georgiou, his colleague Dirk Drees and Michael Anderson of Falex Corp. USA presented a new pull-off test methodology and insights about grease adhesion and tackiness at the Society of Tribologists and Lubrication Engineers annual meeting in May.
They used a high-precision instrument to measure the forces needed to gradually move a probe into a film of grease on a solid substrate, retract the probe in the opposite direction through the grease, and then form threads until the threads break. The instrument operates on the same principles as an atomic force microscope but on a much larger engineering scale.
Georgiou analyzed graphs of force versus probe position, known as approach-retraction curves. Unlike the finger tack test, approach-retraction curves can be used to objectively and quantitatively compare greases. He used proprietary software to analyze the curves and calculate maximum pull-off force, an indication of adhesive strength; string length, which indicates tackiness; and separation energy, or the work needed to form strings from the grease.
I believe that greases perceived as tacky are those that can produce long threads with minimum effort, i.e., low separation energy during retraction, he noted at the meeting in Minneapolis. This means the ratio of string length to separation energy can be important.
To compare two NLGI grade 2 lithium greases, Georgiou prepared grease films 200 microns (0.008 inches) thick on stainless steel and used a copper ball with 3.175 millimeter diameter as a probe.
At faster retraction speeds, the pull-off force and separation energy were much greater, and there was a smaller effect on string length. Grease A generally had stronger adhesion and better stickiness, as well as longer strings and better tackiness than grease B.
Increasing the temperature from 20 to 100 C resulted in loss of adhesion for both greases. String length and separation energy for grease A decreased but remained relatively stable for grease B with increasing temperature.
Georgiou noted that its important for end users to understand how grease properties depend on application conditions. He recommended using three-dimensional maps of pull-off force and string length vs. speed and temperature to compare formulations.
Georgiou told LubesnGreases about some unexpected results from further studies. We compared three greases with low, medium and high finger tack rankings from an industrial contact. We were surprised when maximum pull-off force from approach-retraction curves did not correlate with high finger tackiness. We found that the finger tack test is very sensitive to the rate at which the operator separates their fingers. This shows the value of measuring ARCs over a range of separation speeds.
We also learned that, in order to form the longest possible strings, grease needs to separate and flow more easily, he continued. This means that the grease needs to have a high pull-off force for adhesion, while at the same time, a low separation energy for easy thread formation. The ARC is the only means to measure this combination of properties in a single test.
The original research instrument used in Georgious project has been redesigned into a more compact, automated Tackiness Adhesion Analyzer, made by Sugar Grove, Illinois-based Falex Corp. According to Michael Anderson of Falex, the instrument can vary retraction speed, load and temperature in order to fully characterize a greases tackiness and adhesion under many conditions, much like a Stribeck curve represents tribology testing.
The next steps will be for the grease industry to identify the most ideal retraction speed, load and temperature conditions and develop new standard test methods and specifications for grease adhesion and tackiness, Anderson said.
Mary Moon, Ph.D., is a professional chemist, consultant and technical writer and is technical editor of The NLGI Spokesman. Contact her at email@example.com or (+1) 267-567-7234.