KEY CONCEPTS
• Industry is shifting toward multifunctional and environmentally friendly additives.
• Ionic liquids are on the rise as a new class of next generation lubricants additives.
• There are challenges, limitations, opportunities and innovation for biodegradable additives.

Lubricants have advanced human civilization with the changing priorities from “transportability” in prehistoric times, to “longevity” in the modern times, and most recently to “environment friendly.”¹

Modern lubricant additive technologies are critical for reducing wear and friction in machinery and are now being pushed to do more: to perform better, last longer and align with a cleaner, greener future. However, some traditional petroleum-based lubricants have serious environmental drawbacks. The growing emphasis on ecofriendly solutions has led to increased interest in renewable biolubricants derived from alternative sources.¹ Some of these biolubricants are biodegradable and renewable, but they face challenges such as extreme temperature performance and oxidative instability—not to mention availability and cost. 

The present review investigates the role of innovative lubricant additives in overcoming these challenges through practical insights into emerging additive technologies like biobased solutions and ionic liquids, addressing both technical performance and sustainability which are key concerns across the industry. The topics discussed balance technical depth with forward-looking trends.

Evolution of multifunctional additives 
Lubricant additives start by tracing their journey from beginnings, when early civilizations used materials like graphite and sulfur, to today’s advanced chemical formulations that are tailored for everything from reducing friction to improving oxidation stability. Along the way, the science behind lubrication has grown more sophisticated, and so have the demands. Additives now play a crucial role in how machines perform, how long they last and how cleanly they run. 

STLE member Dr. James Wei, senior chemist, lubrication scientist, LANXESS, explains that multifunctional additives evolved in grease formulations over the past few years, and they have had a positive impact on performance and formulation efficiency: “The grease additives, which mainly include antioxidants, corrosion inhibitors and antiwear (AW) and extreme pressure (EP) additives, are essential to enhance performance, efficiency and lifetime. Additive packages for greases have changed significantly over the years and depend on the type of grease that is to be additized. Many additives are now changing their chemistries such that they are “label free,” i.e., meeting the regulation requirements with less toxic/hazardous components than their previous counterparts. Commonly used multifunctional additives include antimony, molybdenum and zincdialkyldithiocarbamates and dialkyldithiophosphates. Recently, there were requests for low metal content or metal-free additives, which led to development and applications of metal-free dialkyldithiophosphates and dialkyldithiocarbamates. Both products offer excellent AW, antioxidation and EP properties. They have been successfully used in grease formulations. There are products that have been recognized as environmentally friendly additives and included in the European Ecolabel Lubricant Substance Classification (LuSC) list. More recently, multifunctional additives, such as hybrid salicylates and metal-free, low ash additives, have been developed. They offer excellent rust inhibition, detergency, thermal oxidation stability, AW and friction-reducing functions. Initial studies show that they have excellent corrosion protection performance when added into lithium greases and calcium sulfonate complex greases. Along with making greases less hazardous, a large push in the grease industry is to reduce the overall number of additives in a product by making the base product have inherent properties that can replace some additives currently being used in the grease. Calcium sulfonate complex grease is a great example, where a lot of EP and wear performance is inherent to the soap, and thus less additives are required. Polyurea greases have excellent inherent antioxidation behavior and thus reduce the concern when formulating with antioxidants.”

Ionic liquids
One particularly exciting area this article explores is the rise of ionic liquids (ILs). These are a new class of liquid salts that bring some incredible properties to the table, things like high thermal stability, low volatility and the ability to be tailored to specific performance goals. Since researchers discovered around 2012 that certain ILs could mix well with traditional non-polar base oils, interest has surged.² What’s especially impressive is that even tiny amounts, sometimes less than 1%, can significantly improve wear protection and reduce friction. Some of these ILs are already outperforming traditional additives like ZDDP, all while being engineered to avoid toxic elements like fluorine or heavy metals.

STLE member Dr. Jun Qu, Corporate Fellow, group leader, Surface Engineering and Tribology Materials Science and Technology Division, Oak Ridge National Laboratory, explains what makes ILs uniquely suited for use as lubricant additives compared to traditional additives like ZDDP: “ILs are composed of organic cations and anions, which possess distinct physicochemical properties compared with the neutral molecules of traditional additives. First, the anions are adsorbed onto a metal surface, often positively charged, by static electricity attraction. Then another layer of cations is attracted by the first layer of anions on the metal surface.³ Such a physically adsorbed organic ions in a layered structure could function like friction modifiers. Upon contact and sliding, ILs would chemically react with the metal surface and wear debris to form a solid tribofilm for wear protection,^4,5 which shares some similarities with traditional ashless AW additives. However, the anions of ILs are readily available for bonding with the metal cations unlike the molecules of traditional additives that need energy input to break down first before reactions. As a summary, ILs are more responsive in chemically forming the protective tribofilm on the surface area where wear occurs and also reduce friction by their stronger physical adsorption onto the bearing surface.”

Qu explains the compatibility issues that formulators should be aware of regarding ILs interaction with different base oils: “There are many different groups of ILs. The compatibilities between ILs and base oils depend on their chemistries. Solubility and possible chemical reactions between an IL and a base oil often are among the first items to check.^6,7 More complex are the compatibilities between ILs and other additives in the package, such as detergent, friction modifier, antioxidant and other AW additives.^8-10 Their interactions could be rather complex and require careful formulation.”^11-13

Looking ahead, Qu explains the greatest opportunities for ILs in the lubrication industry: “ILs have demonstrated superior wear protection and friction reduction compared with traditional additives in both fundamental investigation and full-scale engine and gear operations^11-13 in our previous work jointly with industry companies. More recently, we have developed a new class of ecofriendly ILs that not only outperform traditional additives in terms of tribological performance but possess significantly lower toxicity and interesting ability of enhancing the oil biodegradability.^14-16 Therefore, I see great potential for ILs in many applications, especially when energy efficiency and/or environmental impact are critical, such as transportation^11-13 and waterpower.”^14-16

Wei shares his experience of ILs integration in grease formulations: “ILs possess a set of great properties, such as excellent thermo-oxidative stability, inherent polarity, non-flammability and low volatility. Since the mid-20th century, amine partially neutralized phosphate esters have been developed and used as surfactants, corrosion inhibitors and dispersing agents in various industrial applications including grease formulations. In greases, they function as AW and EP additives and also as corrosion inhibitors. Recently, different types of ILs including oil soluble products have been developed in the research labs. However, they are not widely commercialized yet. Some studies show that they have good friction-reducing and AW functions in lithium greases. The major concerns for ILs include their biodegradability and compatibility with certain materials such as seal materials. Moreover, development of low sulfur, phosphorus and halogen oil soluble ILs are needed to meet the strict environmental protection requirements.” 

Biodegradable lubricants 
In recent years, the industry has faced a new kind of challenge: sustainability. As environmental regulations tighten and the need to reduce our industrial footprint grows, the spotlight has shifted toward ecofriendlier options like biolubricants made from renewable sources such as vegetable oils. These biobased lubricants offer clear benefits—they’re biodegradable and lower in toxicity—but they also come with performance trade-offs: oxidative instability, high viscosity at low temperatures and limited thermal endurance, etc. That’s where innovative additive technologies come in. New developments like epoxidized vegetable oils and ester-based antioxidants are already making strides in boosting the stability and usability of these green alternatives. 

Vittoria Lopopolo, professional engineer, director technology, finished lubricants, global, HF Sinclair, explains performance gaps and challenges which are commonly faced with additives used to design more ecofriendly finished lubricants: “Regulatory bodies, increased environmental awareness and sustainability pressures are driving change at a fast pace and reshaping lubricant formulations. Legacy additives are under increased scrutiny and face increased risk of hazard classification changes and restrictions for use or allowable limits.”

Key performance gaps and challenges commonly faced with additives used to design more ecofriendly finished lubricants include: 
• Availability of additives that are permissible yet effective—for example, when designing an environmentally acceptable lubricant (EAL) hydraulic fluid, finding effective additives on the European Union (EU) Ecolabel LuSC list, such as corrosion inhibitors, is challenging.
• Effectiveness of additives that are designed for use in petroleum or synthetic lubricants, in biobased lubricant formulations. For example, biobased fluids are known to struggle with low-temperature performance due to their compositions and molecular structures, which may include components such as saturated fatty acids or long-chain paraffinic hydrocarbons. These components crystallize and solidify at low temperatures, increasing their pour point and hindering the lubricant’s ability to flow and lubricate machinery in cold conditions. Conventional pour point depressants (PPDs), designed for use in petroleum or synthetic lubricants, are not equally effective in depressing the pour point of biobased fluids due to the different types of crystal structure formed at low temperatures.
• Ability to achieve the same level of performance, such as oxidative stability, thermal stability, hydrolytic stability or lubricant life versus traditional lubricant componentry.
• Compatibility with traditional lubricant compositions and chemistries.
• Futureproofing. Managing change for ecofriendly finished lubricants is becoming increasingly challenging for designers due to the constant reclassification of additives. Validating and seeking industry and OEM approvals for new lubricant product designs can be time consuming and expensive, which is challenging when dealing with constant changes.

In addition to the biodegradable additives, it is paramount to use biodegradable base stocks. Wei says that there are some unique challenges when incorporating biodegradable additives into grease systems compared to fluid lubricants: “Biodegradable base stocks mainly include triglycerides, polyglycerols, esters and PAOs. Before these biolubricants are blended into greases, their advantages and disadvantages should be well understood, and several factors need to be carefully considered, such as compatibility with other components, low and high temperature performance and volatility. Most biodegradable lubricants on the market today tend to perform to the same capability as their current industrial counterparts, or even better showing that the properly designed products can deliver both performance and biodegradability.

In order to balance biodegradability with performance demands, especially in high-stress applications like hydraulics or gear oils, Lopopolo suggests the following: “Designing a high-performance lubricant that is also readily biodegradable, such as an EAL hydraulic fluid, requires a unique selection of base fluids and additives that meet the criteria of having a final formulation where greater than 90% of the fluid’s components must be readily biodegradable, minimally toxic to aquatic and terrestrial organisms and non-bioaccumulative in the environment. While difficult, it is possible to accomplish an effective balance with minimal compromise between biodegradability and product performance.”

Per Wei, the most promising direction for developing high-performance, environmentally friendly greases over the next five to 10 years is the demand for more environmentally friendly products is always a major topic of discussion, but this rarely leads to real commercialization. Wei says, “However, the real change will happen in the next five to 10 years with the implementation of new government regulations. For example, European REACH legislations were launched in 2005, pan-European Ecolabel was issued in 2004 and the Code of Federal Regulations Title 21 Section 178.3570: odorless, colorless, tasteless and suitable for lubricants with incidental food contact was updated by U.S. FDA in 2011. The biggest driver we are seeing is the moving from mineral based products to more refined food grade approval products. These will use white oils (Group II+), or synthetics (Group III, IV and V). These products tend to not only have better performance than their mineral counterparts (Group I) but tend to come from sources that are more refined and controlled. By using products with greater performance, the changeover of the greases can be extended, saving money and waste. There is a greater push to using oils from recycled sources (e.g., recycled oil). While these may not have the performance of their synthetic counterparts, it gives these oils more useful life than just one and done. As greases tend to be an end-of-life operation, reusing oils that were going to be discarded anyway is a great way to use them such that they can still get some uses. High-performance sulfonate complex greases have been developed and used for nearly 50 years due to their unique properties such as high thermal oxidation stability, upper temperature limit and excellent AW and EP performance. However, boric acid as a key component was used in the process and formula to achieve excellent performance. Due to significant regulatory restrictions in the EU, boric acid has been classified as a substance of very high concern under REACH regulations. Therefore, the conventional greases containing boric acid including sulfonate complex greases will be out of market, and novel boric acid-free products will emerge. Development of biodegradable greases will continue to be a hot subject. Depending on applications, specific biobased stocks need to be selected and formulated into the grease formulations to achieve desired performance and lifetime. In addition, biobased additives such as corrosion inhibitors and AW/EP additives will be developed to meet the new requirements, which can further enhance the biodegradability and applications of novel biobased greases.”

Despite these advancements, hurdles remain, such as performance, scalability issues and a lack of governmental support, which hinder widespread adoption.1 The industry needs research and development support and joint projects with governmental environmental groups, in addition to the establishment of standards to support the transition to green lubrication technologies.

Conclusions
The next generation of additives are emerging, and they are closing the performance gap and push toward a more sustainable future. Researchers are tirelessly working on their integration into commercial formulations overcoming challenges such as compatibility with base oils, toxicity concerns and cost. As discussed previously, one of the most pressing challenges in lubrication is balancing biodegradability and renewability with performance limitations. These complex topics are widely discussed in addition to other challenges to overcome on their own.

Environmental regulations, demand for longer equipment life and circular economy principles will be the major driving forces shaping additive innovation in the future. Collaborations between R&D, government and end-users will be necessary to continue scaling these technologies to make them commercially acceptable. Lubricant additive development will continue to move toward multifunctional, hybrid systems that combine performance with sustainability, and biodegradability enabling the transformation of the lubricants in the next years.

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