There are a vast number of different particle types which can be found in lubricating greases. As is well known, there are numerous types of desirable particles in grease, such as molybdenum disulfide, Teflon, graphite, etc. and many important purposes for their intentional incorporation into greases. In this article we shall examine the other side of the coin: particles in greases which are viewed as harmful, or at best, not serving a useful purpose.
Types of particles
First, let us divide all particles broadly into two classes: abrasive and non-abrasive. In the abrasive category, we find such undesirable materials as metallic fines from machining (Figure 1) – iron, steel, copper, brass, bronze. Other metallic particles may include wear debris – iron, steel, copper, brass, bronze, lead, and oxides of the same (Figure 2). Corrosion of iron and its alloys creates iron oxides, which vary in hardness and therefore abrasiveness. And lastly, there is the ubiquitous dirt, usually composed mostly of sand, or silicon (Figure 3).
In the non-abrasive class, some of the more commonly found undesirable particles consist of agglomerates of either soap or non-soap thickeners. Some cases of unintentional agglomerates of, or insoluble additives have been observed. Finally, we have the desirable additive solids – MoS2, graphite, etc.
Figure 1: Metallic machining debris Figure 2: Metallic particle under magnification
Figure 3: Metallic and other debris.
Sources of deleterious particles
There are easily as many sources of unwanted particles as there are types. Starting at the beginning, particles may be introduced into grease during manufacture via the raw materials. Contamination entering the grease this way may be due to contamination by the supplier’s process or handling. An example of this might be incomplete reaction during preparation of a multifunctional additive, resulting in a material which is either insoluble or agglomerated, which could result in insolubles or agglomerates in the grease. Another example might be contamination from the supplier’s packaging, such as rust, dirt, paper fiber or plastic debris in additive drums or bags.
In addition, contamination from raw materials may occur as a result of improper storage or handling by the grease manufacturer. Poor raw material inventory management can lead to materials degrading due to aging or incorrect storage conditions. One example is over aged / poorly stored lithium hydroxide bags, allowing the formation of lithium carbonate particles. Another example is the undesirable formation of dimers in liquid diisocyanates used in the manufacture of some types of polyurea greases, caused by improper storage temperatures, or excessive age.
Next, after potential raw material sources of contamination, we have the manufacturing process, itself. During manufacturing, there are many possibilities for contamination to occur – let’s face it, most grease plants tend not to be the cleanest of environments! First, one must be aware of the potential for improper incorporation of raw materials – wrong temperature, insufficient mixing or milling, inadequate filtration, etc. Any of these errors can result in agglomerates or undissolved ingredients (Figure 4: (see right) Unreacted lime particles in soap thickener matrix). During the process of adding raw materials to a batch of grease, it is also quite easy for dust, dirt and other environmental contamination to enter the grease. Another less obvious source of contaminating particles is from poor equipment cleanliness, such as residual product or hardened thickener from prior batches, or environmental debris resulting from poor housekeeping practices. And finally, during manufacturing, if there are worn pumps, poor seals, etc. this may cause metallic or elastomeric contamination to enter the grease as it is recirculated.
Now, assuming the raw materials and manufacturing process are well controlled, there is still the potential to introduce contamination to the grease during the packaging process. Sources of dirt and other contaminants include dirty (improperly flushed) packaging equipment, inadequate filtration and contaminated containers.
Even if a grease is contaminant free post-manufacture and packaging, once it arrives at a customer’s location, there are numerous opportunities for it to become contaminated. At the end user facility, storage and handling conditions and inventory control are critical. Poor housekeeping after containers are opened can lead to ingress of environmental contaminants such as dirt, dust or other airborne particles. And if a grease is stored beyond its shelf life, there is the potential for additives to recrystalize. Some rust inhibitors and anti oxidants are particularly prone to this problem.
And finally, if the grease is still free of contamination up to the point where it is introduced to the application, poor relubrication practices (not cleaning grease fittings prior to re-greasing, etc.) or poor seals (allowing ingress of environmental contaminants such as water, dirt, dust, etc.) are common causes of contamination by deleterious particles.
The next installment of this article will address problems caused by these particles and testing for deleterious particles. The final installment will look at specifications and standards for cleanliness and measures to control particles.
Chuck Coe is the President of Grease Technology Solutions, LLC, President of NLGI, and a member of STLE. You can find his contact information in the STLE member database. He holds both the NLGI CLGS certification and the STLE CLS certification. He is chairman of the STLE/NLGI Grease 101 course, which has been offered at past Annual Meetings, and will be offered again at the 2014 STLE Annual Meeting in Orlando, FL.
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