KEY CONCEPTS
• Grease compatibility chart recommendations can be contradictory and misleading, especially when based on thickener type without accounting for influences of base oils and additives.
• Grease incompatibility can occur due to incompatibilities between the thickeners, the base oils or the additives. 
• When due care is taken, switching from one grease to another is quite possible without detrimental impacts to equipment.

Grease compatibility is thought to be an important consideration for both grease manufacturers and end-users. When switching from one grease to another, the supplier is typically asked if the two greases are compatible. Two incompatible greases could be detrimental to the equipment and even cause bearing failure in extreme cases. However, provided due care is taken, switching from one grease to another is quite possible. 

From the manufacturing side, different types of grease can be made in the same kettle and/or pass through common dispensing lines and mills before packaging. If two greases are not compatible with each other, this interaction may have detrimental impacts on quality. From the user side, grease compatibility is of most concern when one type of grease is switched over to another type. In general, grease compatibility is determined using the ASTM D6185 test method, which has some limitations. 

Different types of grease compatibility charts are available throughout the industry. However, the recommendations from these charts can differ significantly and be misleading. These charts typically do not provide enough data to properly determine compatibility between two greases. Often the issue can be related to proprietary data, making it difficult to compare greases made by different manufacturers. Also, these charts are based on thickener type and may not take the influences of base oil and additives into account. However, with proper awareness and understanding of the risk and reward of a change, maintenance personnel need not be overly concerned about grease incompatibility. 

What greases are made of 
Greases have three main components: base oils, thickeners and additives. The base oil used in a grease influences high and low temperature properties, viscosity and thickening efficiencies. The base oil can be 55%-95% of the grease.¹ Mineral oils are the most common base oil used today and operate best in moderate conditions. Synthetic base oils are used to meet the demands of more severe operating conditions. 

The thickener is what sets a grease apart from a fluid lubricant and makes up 5%-30% of the grease.¹ The thickener is a molecule, polymer or solid that is typically partially soluble in the lubricating fluid and imparts a semi-solid consistency to the grease. The more thickener used, the stiffer the grease. Soaps can be grease thickeners, while non-soap thickeners include polyurea, organophilic clays (e.g., bentonite) and other materials. Thickeners may be fibrous, spherical or platelets, and have an impact on the final grease properties. The type of thickener influences grease properties such as temperature resistance, which relates to viscosity, and water washout, which is the ability of the grease to still perform and remain intact when submerged in water.² 

Additives supplement desired properties not provided by the thickener and base oil. A goal of additives is improving grease performance in stressful operating conditions.²

Is grease compatibility overstated? 
Andy Waynick, currently a consulting research fellow in Bulverde, Texas, sees grease compatibility as “a potential issue whenever a user switches from one grease to another.” If the transition between greases is not handled properly, issues can include “costly equipment downtime but can be as serious as complete failure and permanent damage of the equipment being lubricated.” He says, “A poorly handled transition from one grease to another is usually the result of a lack of understanding of the potential of grease incompatibility, or an unwillingness to spend the time and resources to ensure that the transition is properly handled.” 

Waynick notes, “Most people who have heard about grease incompatibility tend to think of it in terms of only the potential incompatibility of different thickener systems.” He observes: “The various grease incompatibility charts that have been put forth over the past many decades have contributed to this mentality.” However, Waynick says, “Grease incompatibility also can result from an incompatibility of the base oils and/or the additives.” 

G.R.P. Singh, head of QAG, One Shared Services Technology Group TATA Steel, in Jamshedpur, India, notes that grease compatibility is an important criterion in steel plant operations. Lubricants are selected “based on their performance in the given environment.” When a business decides to change from one grease to another, they “need to evaluate compatibility of the current lubricant with the proposed lubricant.” 

Singh gives the example of a process that has been using a clay-based grease in continuous caster bearing lubrication that wants to change over to polyurea grease for enhancing reliability. In this situation, “the compatibility of the bentonite thickener must be checked with that of polyurea, or else the incompatibility may result in costly failure of the machine.” Singh provides another example of a process using polyurea grease and wanting to switch over to a calcium sulfonate grease. In this situation, the changeover process should include intermediate runs with calcium sulfonate to get smoother machine operation. 

Singh has witnessed bearing failure due to grease incompatibility in a caster operation. In that case, the steel plant wanted to change from a lithium complex grease, that caused choking of small diameter pipelines from the distributor because of heat, to a polyurea thickener. Compatibility of the greases was not checked prior to the changeover. Singh notes that this type of checking “is especially important for bearings that are lubricated through a centralized lubrication system.” 

Understanding and interpreting compatibility 
Waynick says, “In fact, incompatibility can even result due to a combination of the different base oils and additives.” He offers the following example: 

“Consider the case where Grease A uses a certain base oil with additives dissolved in that base oil, and Grease B uses a different base oil with different additives dissolved in it. The base oils of the two greases are miscible in each other, but the additives in the Grease B are not soluble in the base oil of Grease A. What will happen if Grease A and B are mixed? You will have additives coming out of solution and perhaps no longer providing their needed function.” 

In this type of situation, even when the additives are soluble in both base oils, “they may interact in an antagonistic way resulting in significantly reduced performance.” A further complication is that base oils may not be mutually soluble. An example is silicone oils, which are not soluble in most other base oils. He offers: “Some polyalkylene glycols (PAGs) are insoluble with mineral oil base oils and polyalphaolefins (PAOs).” Mixing greases with mutually insoluble base oils will result in a mess. 

STLE member Anoop Kumar, senior staff scientist at Chevron Lubricants in Richmond, Calif., notes that approximately 70% of the worldwide grease market is based on lithium and lithium complex greases. When changing from one thickener type to another, he says, “There is a strong possibility one can end up changing lithium grease to another thickener type of grease like calcium sulfonate, aluminum complex, polyurea, clay base, etc.” He concludes that due care must be taken during the changeover period. 

Manufacturing 
Kumar notes that grease compatibility “is very important for both manufacturers as well as for end-users.” Mixing of incompatible greases, whether mixing occurs intentionally or unintentionally, has the potential to adversely impact the end-use properties of greases and can even result in catastrophic failures. 

Kumar notes, “From the manufacturer’s viewpoint, it is not uncommon to share the same kettle for manufacturing different types of greases due to efficiency and other operational considerations.” Lubricating greases are semi-solids and are designed to adhere to metal surfaces, which makes it difficult to completely clean the kettle, connecting lines, pumps, milling/homogenizer and filling lines used in manufacturing. 

Kumar states, “In general, lubricating greases are prepared by chemical reaction at elevated temperatures up to 400 F or sometimes even beyond.” Residual grease from a previous production batch may interfere with thickener structure formation, which “may adversely affect the physico-chemical and performance characteristics of the resultant grease.” Different greases may use different types of base oils, and he notes, “Certain synthetic base oils are not compatible with most widely used mineral base oils.” Also, some additives are not compatible with certain thickeners, which may thin down the grease. Kumar gives the example of “thickeners like polyurea and aluminum complex that are not compatible with certain class additives’ chemistries.” 

Testing procedures and protocols 
Kumar reports that the compatibility of two grease mixtures is typically tested using ASTM D6185. Following this test procedure, 90:10, 50:50 and 10:90 mixtures of the two greases are prepared by the prescribed methodology and tested for primary tests (i.e., drop point, shear stability and storage stability at elevated temperatures). Kumar notes, “If the blends pass the primary tests, secondary (i.e., non-mandatory) testing also is suggested as agreed between supplier and consumer.” 

Singh says testing compatibility is normally performed by “mixing the two greases in 50% ratio and running a 60-stroke penetration test and seeing the change in consistency and whether the mixture is hardening or softening after the mixing.” Also, Fourier transform infrared spectroscopy (FTIR) can be used to check whether the wavelengths match or not. 

Kumar notes that the various compatibility charts available for reference can be confusing, misleading and even contradictory. He observes that these compatibility charts are based on thickeners, while additives as well as base oils may influence compatibility. 

Waynick agrees with Kumar that grease compatibility charts have a number of flaws and some “are not even consistent among themselves.” He provides the example of charts that indicate lithium and calcium soap greases are incompatible; however, the class of mixed lithium-calcium soap greases are commercially successful lubricating greases. In addition, “some grease thickener categories cover a lot of ground,” says Waynick. An example is polyurea greases, which can vary significantly in both chemical structure and rheological properties. Waynick concludes that it is irresponsible to make a blanket statement about whether polyurea grease is compatible with another thickener type. 

To really determine whether two greases are compatible, Waynick notes: “You need to run more than just aged penetration tests on mixtures of the two greases.” True testing must include mixing the two greases at different temperatures to ensure that elevated temperatures do not impact functional performance. Also, testing needs to ensure that the extreme pressure, antiwear or corrosion protection performance properties of the grease remain acceptable. 

In addition, Waynick says, “It is always best to speed up the transition from one grease to another in the user’s equipment by pushing a much larger amount of the new grease through so as to more quickly purge out the old grease.” In situations where that approach is not possible, “then sampling and testing the grease in the equipment during the transition period is a good idea.” 

Singh notes, “There are few practices to address the switching over to incompatible greases,” and lists three. If the lubricated parts are accessible manually, the first and easiest method is to clean them completely. The second method “is to empty the existing grease from the centralized lubrication system and run a grease campaign with a thickener compatible with both the existing grease and proposed grease.” A third method is to exhaust the existing grease, put in the new grease and run the centralized system in manual mode continuously until the new grease comes out of the bearing. At that point, the centralized lube system can be set to auto mode with the desired on-off timing. 

Conclusions 
Kumar observes, “The phenomenon of grease compatibility is sometimes overblown unproportionally and scary to the end-users.” While it is true that incompatibility may cause serious problems, he says, “If due care is taken, it is possible to change over grease from one incompatible grease to another.” 

Waynick “always assumes that the new and old grease are incompatible unless comprehensive testing has proven otherwise.” Taking a very conservative approach when switching one grease to another can prevent problems. 

REFERENCES
1. Holdmeyer, D. (2022), “Grease fundamentals: What color grease is best?,” TLT, 77 (5), pp. 34-36. Available here. 
2. Aikin, A. (2020), “The art of manufacturing grease,” TLT, 75 (3), pp. 44-46. Available here. 

Andrea R. Aikin is a freelance science writer and editor based in the Denver area. You can contact her at pivoaiki@sprynet.com.