Can you judge a grease by its colour ?

[Dhanush G]

The DIN standard for grease is a widely used system for classifying and labelling lubricating greases. The DIN 51502 classification code is a combination of letters and numbers that describe grease’s application, base oil, consistency and temperature range. The classification typically follows the below format,
Lubricating grease type| Lubricating material type | Consistency index | Upper & Lower service temperature
Types of Grease Lubricant

Ex. K P 2 K- 30

K	Bearing grease
G	Enclosed gear box grease
OG	Open gear grease
M	Paste Lubricant

Lubricating Material

Ex. K P 2 K- 30

D	Hydraulic oil
E	Water miscible lubricant
F	Lubricant with solid lubricant additive
L	Lubricants with active ingredients for corrosion & ageing resistance
M	Coolant with mineral oil component
S	Coolant with synthetic oil component
P	Lubricants with EP & AW additives
V	Lubricants diluted with solvents

Synthetic & Semi synthetic oil

Ex. K P HC 2 K- 30

E	Ester
FK	Per fluoro Liquid
HC	Synthetic Hydrocarbon
PG	Polyglycols
SI	Silicone
X	Others

The previous grease supply was lighter in shade, pails from the latest batch supplied look a bit dark, should we worry ? Has it been over cooked ? Would it provide the same lubrication performance ?

Well these are some questions that customers ask when a perceived change in colour is observed from batch to batch.
Greases can show slight variation in colour between batches and is primarily dependent on the colour of the base oils. Base oil colour change could occur in different batches due to different source of crude oil and manufacturing process followed by different refineries. However the raw materials are inwarded and put in use only after the required quality parameters meet the specification requirements as per the defined control plan in place. While appearance is also a control parameter, it would be approved if if falls within the accepted colour gradiation/shade.
The colour of grease is also determined by the thickeners, additives and dyes used in its manufacturing. The purpose of the dye would solely be to differentiate from other grease products. A red, green or purple grease is only a colour differentiator and has no impact on lubrication performance.
Colour Change During Long Term Storage A change in colour of the grease stored in the original sealed container is also not a regular occurrence. However a gradual change also does not necessarily mean the product has degraded. As long as the grease is stored correctly, a simple colour change should not be mistaken for a change in its integrity or performance.
A rare case : Colour change observed in one of the comparator brand product of different batches and age stored in ambient conditions with corresponding IR readings of the samples
Colour change observed from regular white to pink in multiple batches with varying age
Colour Change when in Use
When the grease is put in use (on the friction partners, example bearing) environmental and operational factors can influence the colour of grease over time. Exposure to sunlight, heat, water process contaminants can cause the colour to lighten or darken without compromising its lubricating performance. For instance, a water resistant grease, red in colour might develop an orange hue when exposed to water, yet its functional properties remain intact. Similarly, heat can darken a grease, but this cosmetic change is not necessarily an indication of loss in lubrication performance for the intended period or until the next defined re-lubrication interval.
A better method would be to collect sample of the used grease and test the same in a certified or well equipped lab for its functional parameters.
Identical peaks and troughs for the white as well as pink sample indiccate no change in chemical and performance characteristics of the grease
A typical test could be to compare IR graphs of the used grease with that of the master sample. The spectracopic graph would depict the identical peaks and troughs of additives, base oil and thickener content of both used grease versus the fresh master sample. Any significant deviation would infer depletion in the health of the used grease.
A case of colour variation observed with two batches of Berutemp P207. However the IR spectroscopy graph are identical of both indicating no change in chemical characteristic of the grease
Note on IR :
Infrared (IR) specrtascopy is absorption measurement analysis of different IR frequencies by a sample positioned in the path of an IR beam. The main goal of IR spectroscopic analysis is to determine the chemical functional groups in the sample, compound identification, its structural elucidation and purity.

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