However, microcapsules made using traditional techniques cannot withstand the high temperature, high pressure and high shear conditions in most lubrication environments. Knowing the technology could benefit lubricants, Stephen Hsu, professor in the Department of Mechanical and Aerospace Engineering at George Washington University’s School of Engineering and Applied Science, has developed a way to create more resilient microcapsules.

Hsus hardy microcapsules can be used to replenish depleted additives in lubricating oil and reduce interactions with other chemistries. Placing additives in a capsule also avoids additive agglomeration and helps maintain optimal concentration of additives in the lubricant, he told attendees at the 2018 Society of Tribologists and Lubrication Engineers annual meeting in Minneapolis.

The technology could also be applied in process fluids and coolants that are subjected to similarly harsh operating conditions as lubricants.

Encapsulated Protection

Hsu has worked for a few years on this approach to lengthen the effectiveness of oil additives, which become depleted as an engine oil degrades through its service life. Some additives, such as friction modifiers, may lose effectiveness quickly, reducing the energy efficiency or fuel economy performance of the lubricant, Hsu noted in an extended abstract submitted to the 2017 World Tribology Congress in Beijing, China.

Additive chemistries may compete with one another, especially if an additive that has become depleted is added to an in-service formulation. Hsu gave the example of a friction modifier interacting with zinc dialkyldithiophosphate, an antiwear additive, which may negate its functionality because the friction modifiers molecular structure does not interact well with that of ZDDP.

That’s where microencapsulation could be beneficial. Capsules allow the additive to be isolated from other additives until it is released into the lubricant when triggered to do so, Hsu later told LubesnGreases. This enables a timed release of a selected additive to reinforce the depleted additive to maintain the friction reduction property or to enhance the fuel economy performance. Because all additives deplete quickly, many of the other additives have been used up and the encapsulated additive can function without much interference, he explained.

Solid lubricant particles such as molybdenum disulfide and graphite can also be encapsulated to be used as friction and wear control agents in lubricant and grease formulations. Microencapsulation would prevent them from getting tangled with viscosity index improvers, dispersants and detergents, said Hsu.

Preparation of the microcapsules includes an oil phase, containing the additive to be encapsulated, and a water phase. An emulsifier is used to create an emulsion, and then a monomer and catalyst are added to begin polymerization. Parameters such as pH, temperature, reaction sequences, cooling and polymer chemistry must be carefully controlled during the process in order to produce the desired size, shape and uniformity.

We chose this method because it can provide a very controllable shell thickness, Hsu explained during his presentation, adding that researchers can control the shells thickness to be between 0.1 micron and several microns.

Various polymers can be used for the shell, including polyurea, polyurethane, polyamide, polyester and polymethacrylate.

The process works with additives that have a symmetrical structure, such as ZDDP, as well as amines, inhibitors (rust and corrosion inhibitors, antioxidants), viscosity index improvers, dispersants, detergents and both molybdenum based and organic friction modifiers. Multiple additives can be packaged together. You can take different things and put them in a single capsule, like an aircraft carrier, Hsu said.

Ensuring Performance

To test the mettle of the microcapsules, Hsu recommended in the 2017 abstract that a batch of capsules filled with additives be weighed and solubilized in oil or a solvent and their shells broken either by shear or thermal stress. The additives and the polymer shell material should be separated through filters and sieves, then weighed again. The recovered additive can then be tested against the performance of the virgin additive to establish equivalency.

Since the microcapsules are present in various sizes and the release rate during performance testing could vary depending on the test geometry and testing conditions, the actual performance could vary somewhat, Hsu wrote.

Testing of the encapsulated additives must take into consideration their intended use, said Hsu. For slow release through a porous shell, the release rate needs to be taken into account. For timed release, the lubricant needs to be subjected to oxidation and periodic samples taken for property measurements.

He also emphasized that the amount of polymer from the microcapsule shells in the lubricant needs to be kept at a low level to avoid oil thickening tendencies. The ideal polymer chemistry for lubricant application would be a basic polymer chemistry that is soluble in oil at elevated temperatures, he expanded.

Making Capsules Last

Engine conditions can challenge the performance of microcapsules. High shear stress from the oil pump and sliding interfaces can break the polymer shells, stray fuel and water can act as solvents and damage the capsules, and engine temperatures ranging from -30 to 125 degrees Celsius demand excellent thermal stability, Hsu noted.

In addition, additives already present in the engine oil such as dispersants and detergents, which are made to capture oil-insoluble oxidation products, can become entangled with the capsules and prevent them from acting properly. Hsu added that acidic contaminants resulting from blow-by gases in the engine, exhaust recirculation and oil degradation can affect the capsule shells. The engine is a hostile environment, so the number one question for the first three years was, How do we make the capsule stable and survivable?

The first generation of capsules had about a 50-50 additive-to-polymer ratio and would sit in the engine oil without releasing their additives because of the thickness of the shell. So Hsu and his team had to adjust the polymer chemistry to make the shell much thinner and more easily breakable.

The researchers had focused on making the capsules as small as possible. However, this resulted in too much polymer in relation to the amount of additive. We want to maximize the additive content and minimize the polymer content, Hsu said. The ideal ratio is about 70 percent additive filler to 30 percent polymer shell, he said.

For automotive applications, the microcapsules should be between 5 and 30 microns to avoid getting caught in oil filters, which normally have pore sizes of 28 microns in diameter. It all depends on how you design the capsule. If the capsule is easily breakable, when it hits the [oil] filter it breaks up and releases the additive, Hsu also noted.

Measuring Up

Hsu gave an overview of a test measuring the timed release of friction modifiers in SAE 0W-20 and SAE 0W-16 engine oils at the conference in late May 2018. The test sought to simulate extended drain interval conditions by subjecting samples to oil thickening caused by oxidation.

The oxidation test was conducted at 170 C with an air bubbling rate of 10 cubic centimeters per second. One percent iron and 1 percent copper naphthenate were used as catalysts for oxidation, and the five oil samples contained 0, 0.5, 1.0, 1.5 and 2.0 percent friction modifier capsules.

Samples were drawn to test for friction performance at 0, 12, 24, 36 and 48 hours. Each of the oil samples showed reduced friction coefficients compared to the baseline fluid.

After 36 hours, the viscosity of the oil increased and the oil degraded, indicating that the friction modifier had been depleted. However, the samples with microcapsules maintained better performance compared to the sample without friction modifier capsules. Capsules allow for slow release [of the friction modifier] so it doesn’t get consumed very fast, Hsu highlighted.

Hsu said his team has learned how to achieve the optimal amount of additives in the microcapsules and ramped up manufacturing processes. Microencapsulation of lubricant additives is still not close to being commercially available, but Hsu has applied to patent the process with the United States Patent and Trademark Office.

Since the technology for encapsulating additives is still new, Hsu said that more application testing needs to be conducted to determine its effectiveness. He noted in his presentation that some engine tests have already been conducted, and now the researchers aim for real-world condition engine tests. A sequence test, to some extent, is not really reasonable to evaluate the technology, so now well want to move to a vehicle test because its more realistic.