Grease Colors Tell Us About Grease Quality
The color of grease can indicate its overall quality. Grease can darken as a result of operating and environmental factors. Darkening may be a sign that the grease reached its usability limit.
A change in color can also indicate that one form of grease has combined with another. It is imperative that immediate action be made to determine how and why this occurred. You should be cautious about applying the wrong grease to your machine. For different types of grease, use different fittings, such as standard fittings for stand-alone bearings and button-head fittings for electric motor bearings to avoid contamination. This will take care of your grease discoloration problems. Another option is to leave a mound of grease on the grease fitting to demonstrate the correct color.
Although grease color can provide useful information, it mainly serves as a label to identify the type of grease. However, no manufacturer guarantees that a specific grease color corresponds to a certain thickening type, even within a single selection.
Grease Colors
Greases are available in white, black, blue, red, green, purple, transparent, yellow/golden, and other colors. Grease color can influence impressions and perceptions. Manufacturers add colorants to help identify greases and make them more visually appealing. If a grease gun dispenses the wrong color, users can quickly spot the error.
White grease typically indicates suitability for food machinery, while black greases usually contain molybdenum or graphite for extreme operating conditions. Although no strict rules exist, red often signals high-temperature applications, blue indicates cold-temperature use, and green suggests environmentally friendly formulations.
Grease Whiteners
The most common whiteners in grease are titanium dioxide (TiO₂) and zinc oxide (ZnO), both solid powders. Depending on the base grease formulation, these compounds can make up 0.5–10% or more of the grease.
A recent study tested lithium, lithium complex, aluminum complex, and calcium sulfonate greases with varying percentages of TiO₂ and ZnO. Researchers evaluated how these whiteners affect dropping point, penetration, mechanical stability, anti-wear performance, storage hardening, and oil separation.
Polymers and Tackifiers
Manufacturers add polymers to improve grease consistency, shear stability, water resistance, adhesion, and tack. Polymers can negatively affect cold-temperature performance and flow properties.
In calcium sulfonate greases, polymers enhanced penetration and provided good mechanical and roll stability in the presence of water. However, higher polymer concentrations reduced water spray-off performance.
Researchers also tested grease pumpability using the United States mobility test. At 77 °F, polymers had little effect on pumpability.
Environmental Factors: Sunlight, Heat, and Water
Technicians must assess the environment where grease will be applied, considering sunlight, heat, and water, whether due to operating conditions or poor container storage.
- Sunlight: Exposure can fade grease color, reducing its visual appeal.
- Heat: High temperatures can darken grease or lighten its appearance over time.
- Water: Water can change grease color; for example, red grease emulsified and turned orange after floating in water for 24 hours.
As a result, grease color has little bearing on overall performance once applied.
Liquid Dyes
Manufacturers add dyes in very small amounts, so they rarely pose a concern. However, certain dye classes can present environmental or health risks. Depending on the required color intensity, dye concentrations typically range from 0.001 to 0.5% or more.
Polymers or Tackifiers
Manufacturers add polymers to improve grease consistency, shear stability, water resistance, adhesion, and tack. However, polymers can negatively affect cold-temperature performance and flow properties.
In the presence of water, both polymers enhanced penetration and showed good mechanical and roll stability in calcium sulfonate grease. Higher polymer concentrations, on the other hand, resulted in poor water spray-off qualities
Researchers also tested the pumpability of several grease samples using the United States mobility test. At 77 °F, polymers did not significantly affect pumpability.
Environmental Factors: Sunlight, Heat, and Water
Technicians must assess the environment in which they apply grease, taking into account exposure to sunlight, heat, and water, whether due to application demands or poor container storage. Light exposure can fade the grease’s color, reducing its visual appeal compared with its original appearance.
When grease is exposed to heat or high temperatures, it turns dark or turns a dull/light color. Furthermore, the color of grease tends to darken over time when applied. The color of grease is also affected by the presence of water. Red grease, for example, emulsified and changed color from red to orange after floating in water for 24 hours. As a result, the color of grease has little bearing on its overall performance when applied.
Analyzing Used Grease Samples
Changes in Grease
Contamination, oil loss, and mechanical shearing can cause grease to harden or soften in service. If you observe changes, talk to CRE consultants and analysts to help you determine what the possible causes of these issues and determine what is the best course of action.
Grease Analysis to Determine the Condition of Grease and Equipment
FTIR Spectroscopy
Fourier transform infrared (FTIR) spectroscopy identifies the type of base oil and thickener in used grease. By comparing a fresh grease reference to a used grease sample, technicians can detect additive depletion or contamination by another grease type.
Compared to FTIR spectroscopy of oil, measuring and interpreting a grease spectrum is more complex. Thickener compounds can dominate key areas of the spectrum that are normally used to calculate water content or oxidation. FTIR spectroscopy works on the principle that molecules in a lubricant absorb infrared light at wavelengths corresponding to their typical structures. Technicians may detect additive degradation if a grease contains high-pressure additives. Comparing with fresh grease can also reveal excessive water content.
Dropping Point
Grease stability can decline as it ages. For application, the OEM requires that grease meet a minimum dropping point. When technicians increase the temperature, grease does not melt like butter or coconut oil; the thickener keeps the base oil stable, so the grease resists significant changes. The soap structure dissolves only when the thickener reaches its critical temperature.
During the Dropping Point test, technicians heat samples until the grease liquefies and drops into a test tube. If a sample melts at temperatures above 300 °C, technicians consider it as not having a dropping point.
Oil Bleeding Test
To properly lubricate a bearing, grease must release part of its oil during operation. Grease oil bleed rates for bearing lubrication typically range from 1 to 5%. The bleed rate depends on the viscosity of the base oil and the operating temperature, which must be high enough to ensure appropriate bearing lubrication.
Analytical Ferrography
Technicians begin analytical ferrography by using a ferrogram slide maker to separate magnetic wear debris from lubricating oil. They dilute the oil sample to improve particle precipitation and adhesion. The diluted sample flows down a specially prepared glass slide called a ferrogram. A magnetic cylinder pulls ferrous particles from the oil onto the slide. Technicians then identify the collected particles under a microscope.
Penetration Test
Technicians often use the ASTM cone plunger method to measure grease stiffness. They can also assess grease mobility using other techniques, such as rheological testing, which applies varied loads to grease samples.
Grease color alone cannot determine performance, but proper analysis can. From FTIR spectroscopy and dropping point testing to oil bleed and ferrography, the right testing strategy ensures your lubricants and equipment perform as expected.
Contact us today to schedule a grease analysis or speak with a reliability specialist about improving your lubrication program and preventing costly failures.