While many manufacturers are tight lipped about the directions these developments are taking, there are public-domain examples from other industries and academia that may offer hints at where they are heading.

The number and variety of grease thickeners in everyday use is large. Metal soaps are usually formed in situ in highly exothermic chemical reactions (they produce heat) with natural fats, oils or derivatives. They are based on lithium, calcium, sodium, aluminum and barium salts, in the rough order of prevalence. Each has its strengths and drawbacks depending on many factors, including production cost.

The performance of these materials in greases can be highly dependent on the manufacturing process. The transfer of heat within the reaction vessel is critical to the rate of reaction, which affects all aspects of performance of the finished greases.

In integrated lubricant and grease companies, the lubes people are often reminded that grease is different because manufacturing is usually not a simple mixing process. Additive manufacturers, most notably Lubrizol Corp. and Lanxess, have offered readymade thickeners, however.

These soaps can all be complexed by replacing some of the fatty acid with, to a greater or lesser extent, dicarboxylic acids, such as azelaic or adipic acid, phosphoric and boric acid or related esters.

Non-soap thickeners such as bentonite and hectorite clays are used in high-temperature applications, as is fumed or methylated derivatized silica, yet in more niche areas. Calcium sulfonate greases have a layered structure, like clays, that can shear and provide lubricity. Polymers, such as polyisobutylene are added as supplementary thickeners.

Waste Not, Want Not

There are more sources of thickeners found in obscure places. Hard-to-recycle plastics can be mixed with bitumen for use as road surfacing. Shredded fine-grade polyethylene, polypropylene and polystyrene coat the stone chips and are then mixed with the familiar sticky black substance, replacing around 10 percent of the bitumens volume as an extender. Chemist Rajagopalan Vasudevan from Thiagarajar College of Engineering in Tamil Nadu, India, first brought this concept to prominence in the early 2000s.

This is an effective way of removing low-value mixed plastic waste from landfill sites but it may be difficult to see what has this to do with greases. In the case of plastic roads, the recycled waste can replace higher-value virgin plastics that would be too expensive for such applications. These costly virgin plastics are already used as supplementary grease thickeners and appeared on the market in the last decade as primary thickeners, too.

So can recycled plastics be used as grease thickeners instead? The answer seems to be yes, albeit partially.

Thicken the Plot

Polyurea and polytetrafluoroethylene greases have been on the market for several decades but polyethylene and polypropylene, two of the highest volume polymers available that have been in commercial production for more than 60 years, have only been used as primary thickeners in the 21^st Century.

In the 1990s, bearing company SKF and grease manufacturer Axel Christiernsson began joint activities in the area of polymer-thickened grease, culminating in Axel Christiernsson bringing a dedicated polymer grease manufacturing unit on line in the mid-2000s. According to Axel at the time, the polymer component was a mixture of different chain lengths of polypropylene.

In 2017, Axel Christiernssons group technical manager Johan Leckner and senior development engineer Rene Westbroek presented their findings on the now-mature polypropylene-thickened greases at the European Lubricating Grease Institutes annual general meeting.

Leckner and Westbroek reported an in-depth study of the mechanistic differences in lubrication performance that led to a polypropylene-thickened grease giving up to seven times longer grease life in rolling element bearings compared with a lithium complex grease.

The first stage of polypropylene grease manufacture – melting the polymer in the solvent – is identical to that described for the bitumen extender above. In the old days, bitumen was extended with high-grade virgin polymers, like those used in the Axel Christiernsson greases being produced today. So, why not follow Vasudevans approach and melt waste plastic into oil? Would a mixed plastic make a stable grease?

Another type of polymer that has been receiving attention in many chemistry labs for the past 20 years is the biodegradable polymer. Cellulose and its derivatives, for example, are already common in personal care as a rheology modifier, so it was logical for someone to ask the question of whether it could modify the flow characteristics of mineral oil in a grease.

Putting It All together

One research group where these issues have been studied extensively is at the Centre for Research in Product Technology and Chemical Processes at the University of Huelva in Spain. Professor Jos Mara Franco Gmez has led the activities of the complex fluids research group in projects including the improvement and innovation of lubricant and biolubricant grease formulations.

The Huelva team began studying recycled plastics in greases in 2004. Early work was on recycled low density polyethylene and polypropylene as supplementary thickeners in metal soap and clay-based greases. Some later results were encouraging. For example, suggesting that a blend of high-density polyethylene (as used in pipes) and polypropylene could be considered a suitable potential viscosity modifier for lithium lubricating greases over a wide range of in-service temperatures.

In 2000-2002, we worked on two projects related to traditional lubricating greases, on the one hand, and liquid bio-lubricants, on the other. We realized that all the efforts to develop biolubricants were focused on replacing mineral oils by vegetable oils or some derivatives, Franco told LubesnGreases.

Most biodegradable greases were based on non-biodegradable thickeners, so the team set out to develop new renewable thickening agents that also provide adequate technical performance. Most studies have been based on cellulose and other wood products.

The challenge is to find the compatibility with the base oil in order to stabilize the colloidal system, Franco explained.

A significant and novel chemical step forward is to use highly reactive small molecules to cross-link the base fluid and thickener. Some of these formulations are technically competitive with conventional lubricating greases, said Franco.

The Huelva team creates soft gel-like biobased polyurethanes dispersed in oil. The complex chemical modifications that make the process of obtaining formulations (are) not completely environmentally friendly, although the final product is.

Most of the work is in the public domain. We have filed a couple of patents on the use of ethylcellulose as a viscosity modifier for vegetable oils – by gelling action – and another on the use of residual highly viscous oleins, instead of vegetable oils, said Franco.

With two significant industrial collaborators, the anticipated markets are food lubes and total loss applications. The team has also studied estolides, (as described in LubesnGreases EMEA Vol. 12, Issue 3, March 2018) as viscosity modifiers for vegetable oils.

Environmental Impact

Do these greases work in the field? One industry insider, who preferred to speak to LubesnGreases on condition of anonymity, recalls one major trans-Alpine tunnel project in recent years where the tunneling company asked lubrication suppliers for cellulose-thickened greases for one of the massive boring machines.

This may have been driven by a desire to minimize environmental impact by using chemicals are found in the woodland above the tunnel. But it also implies there is a body of knowledge that indicates such greases would be suitable for the application.

Public concerns over the presence of microplastics in the environment may mitigate against a migration from soap-based to plastic grease thickeners, even those based on waste plastic. Hundreds of thousands of tonnes of grease are lost to the environment per year, so the sustainability target of grease product developers has to be biodegradability, rather the reuse of waste. Whereas bitumen can accommodate chemical contaminants and mineral particles of millimeter diameters, these would render a grease useless, therefore waste plastic for grease would have to be cleaned.

For this reason, the desirable environmental route to replace metal soaps with materials that are less harmful to the environment and/or less carbon-intensive could lead towards bio-derived products currently classed as waste or that have low value. Therefore, cellulose and lignin may have a future as grease thickeners, whereas the environmental advantages of greases based on waste plastic may be limited.