Grease Thickeners and Additives - Smooth Operating with AEROSIL® Fumed Silica

 

What is lubricating grease?

Grease is a specific type of lubricant in solid or semi-solid form that is used when oil cannot be used. This generally applies to applications operated under high temperatures, pressures or shock loads as oil is often too thin to lubricate these properly. The thixotropic nature of grease, however, allows it to stay in place more easily. This also means that grease doesn’t require as many re-applications as oil as it behaves like a sponge which releases lubrication slowly. The thicker alternative also has the added benefit of keeping out dust, dirt and water thus protecting the system from contaminants and corrosion.

 

How are grease viscosities categorised?

Greases are typically characterised by their viscosity according to the following methods:

NLGI (National Lubricating Grease Institute)

The NLGI number is a standardised method of categorising the viscosity of a grease where the higher the viscosity, the higher the number given to it. This is measured by the cone drop method where a cone is placed on a grease at 25 °C and the depth of cone penetration is then measured. NLGI 1 grease is typically considered soft, a number 2 considered standard and a number 6 considered hard. Semi-fluid greases are often referred to as a ‘00’. These are referred to as ‘semi-fluid’ due to their liquid-like consistency. This low viscosity is important in applications where grease is to be pumped through tubes via a grease reservoir so lubrication is automated.

4 ball weld test

Perhaps one of the most common and well-established tests in the lubrication industry, the 4-ball weld test is used to test the wear and load-carrying properties of a grease. Three steel balls are held in a container and a fourth ball is rotated against these at speed. Between the 3 steel balls and the top one, grease is applied. A load is then applied and gradually increased until the rotating ball welds to the 3 stationary balls, removing all the grease between them. This is called the weld point.

 

Figure 1: The 4-ball weld test is used to study the wear and load-carrying properties of greases.

 

At this point, the applied load is recorded and due to the standardised nature of the test, this can be compared between lubricants. Values of between 200 kg and 250 kg are typical and values above 250 kg are achieved from high load-carrying lubricants for demanding applications.

 

What are the components of grease?

The broad definition of grease allows for many different types so it is important to choose the correct type based on the environment and demands of the application. Generally speaking, greases are made up of a base oil, additives and a thickener (2 – 15 %). Greases are typically classified as soap-thickened or non-soap thickened depending on the material used as a thickener.

Soap thickened Greases

Lithium as a grease thickener

Widely used as a multi-purpose grease, lithium greases provide good water resistance, as well as protection against corrosion and oxidation. Lithium greases are often used in the industrial and automotive sectors.

Aluminium as a grease thickener

Aluminium greases remain stable in high-temperature applications, offer water resistance and can withstand high shear in fast-moving machinery making them ideal for food industry applications.

Calcium as a grease thickener

Due to their ability to perform well at low temperatures, calcium greases can be used in marine applications as well as in the industrial and agricultural sectors.

Non-soap Thickened Greases

Greases that have been thickened with alternatives to soap, such as polyurea, bentonite or fumed silica, have the advantage of significantly increasing the dropping point of the grease. This can allow the grease to reach the temperature at which it transitions from a semi-solid paste to a liquid (300 ˚C) hence these greases can be used in higher temperature applications such as roller bearings. Different thickeners allow different benefits including mechanical properties and water resistance, an overview of which can be found in Table 1.

 

Name of Grease	Lithium Grease	Calcium Grease	Aluminium Grease	Non-soap Based Grease
Thickener	Lithium Soap	Calcium Soap	Aluminium Soap	Polyurea, Bentonite, AEROSIL® Fumed Silica, Carbon Black
Base Oil	Mineral Oil	Mineral Oil	Mineral Oil	Mineral Oil
Dropping Point	170 - 190 ˚C	80 - 90 ˚C	70 - 90 ˚C	> 300 ˚C
Temperature Range	-30 - +130 ˚C	-20 - +70 ˚C	-10 - +80 ˚C	-10 - +130 ˚C
Mechanical Properties	Excellent	Depends	Good	Good
Water Resistance	Good	Good	Good	Good
Applications	Widest range (bearing)	Low speed and Heavy load users	Vibrating Bearings (adhesion)	Low to high temperature applications, General roller bearings

Table 1: Non-soap thickened greases have the advantage of a higher dropping point and often a larger temperature range while still giving good performance.

 

Aerosil® as a grease thickener

Aerosil® Fumed Silica from Evonik is widely used in greases as a thickener a high-temperature stability is required. Stable up to 1000 ˚C, Aerosil® thickened greases are only limited by the oil's temperature stability. The established stable matrix also prevents oil separation and anti-settling of solid particles, so with no ecological or health issues, Aerosil® offers an excellent alternative to soap-thickened greases.

When compared to other non-soap thickeners, Aerosil® outperforms bentonite and polyurea with improved performance in almost all key areas such as chemical resistance, water resistance, thickening and thixotropic properties. Where the colour of the grease needs to be white or transparent, typically for use in industries such as the food or pharmaceutical industry, Aeorsil® Fumed Silica is the most appropriate thickener. A full comparison can be seen in Table 2.

 

Thickener	Bentonite	AEROSIL® Fumed Silica	Polyurea
Continuous Operation	150 ˚C	Only Limited by Oil	160 - 180 ˚C
Dropping Point	> 300 ˚C	> 300 ˚C	> 300 ˚C
Chemical Resistance	o	+	-
Water Resistance	o	+	+
Shear Strength	Not as strong as Fumed Silica	Strong	Stronger than Fumed Silica
Thickening Performance	o	+ Better than Polyurea	+
Thixotropic Performance	o	+	o
Environmentally Friendly	+	+	-
Colour	Dark	White/Transparent	Yellow
Oil Separation	Inferior than Soap Thickeners
Performance	Low	High	Medium

Table 2: A comparison between some common non-soap thickeners for greases showing the advantages of using AEROSIL® Fumed Silica 

 

Hydrophobic vs hydrophilic

In general, the hydrophilic grades of fumed silica require lower addition rates to increase a greases’ rheology compared to their hydrophobic counterparts. However, for anti-rusting applications, a hydrophobic grade such as Aerosil® R974 or R816 can be used to increase moisture or chemical resistance.

Below is a table of recommended Aerosil® grades in combination with base oils commonly used in the lubrication industry.

 

	Recommended Aerosil® grades
	Hydrophilic	Hydrophobic
Mineral Oil	Aerosil® 200 and 200 HV	Aerosil® R974 and R816
PAO	Aerosil® 300 and 150	Aerosil® R816 and R805
Naphthenic Oil	Aerosil® 200 and 200HV	Aerosil® R816 and R202
Silicone Oil	Aerosil® 200 and 200HV	Aerosil® R805 and R816
Paraffin Oil	Aerosil® 150 and 200HV	Aerosil® R816

Table 3: A summary of which Aerosil® grade should be used for which grease base for both a hydrophilic and hydrophobic option.

 

Incorporation methods

To attain a homogenous and effective lubrication system, care must be taken to ensure Aerosil® Fumed Silica is properly dispersed. Poorly dispersed Aerosil® can cause oil separation, particle settling and a drop in viscosity over long storage periods.

The correct dispersing equipment is dependent on the viscosity of the system. For Aerosil® silica thickened greases Planetary Dissolvers and Planetary Mixers are the most suitable mixing equipment. Other equipment used in this market also includes Three Roll mills, Ball Mills and High-Pressure Homogenisers.

Food Industry considerations

Certain grades in the range such as Aerosil® 200, 200F, R202, R208, R972, R972 and 380 are registered NSF H1 for use in the food industry for incidental food contact. Please contact Lawrence Industries for further information on these grades.

 

Why are additives used in greases?

After choosing the correct grease type for the application and ensuring the NLGI number is suitable for the application other desirable end properties can be considered and adjusted. For example, a large temperature range, improved rheology profile or even rust-resistant properties may benefit the end product and can be added via additives.

Solid lubricants

PTFE lubricants

Polytetrafluorethylene, PTFE, or Teflon as a brand name is a fluoropolymer that is widely used within the lubrication industry as it has wide operating temperatures, excellent friction reduction properties, and omniphobicity. . Despite generally being considered the safer “cousin” of fluorchemicals, PTFE is part of the PFAS family of “forever chemicals” which are highly persistent in the environment and living organisms. In February 2023, The ECHA published a proposal to restrict the use of PFAS with an aim to reduce emissions into the environment and improve product safety however this is still an ongoing decision process. Many consumers are already looking to avoid products containing PFAS and by extension PTFE due to these health and environmental concerns.  For manufacturers wishing to provide a PTFE-free lubricant, some alternative additives can provide similar high-temperature stability.

 

Figure 2: Lubrication is a vital part of many industries and selecting the correct grease and thickener for the system is the best way to get a smooth-running, low-maintenance process.

 

Copper flake or pigment

Copper flake is often used in anti-seize greases for increased lubricity. Greases are also often coloured for identification purposes when several greases are used for instance in a factory environment for numerous functions.

Graphite and Molybdenum Disulphide

These can be used for increased lubrication as they leave a thin protective layer on metal surfaces even when there is no longer any grease remaining. This means that the surface is still offered some protection between lubrication intervals.

Boron Nitride

Boron Nitride platelets, often referred to as ‘white graphite’ and manufactured by 3M, are a good option for the replacement of PTFE due to the extremely high-temperature stability of the material. Besides being lubricious, boron nitride also has the added benefit of offering a cooling effect by transferring heat away from components due to its thermally conductive properties. For certain applications and systems, this can potentially extend the life of the equipment to offer overall cost savings.

Boron nitride is also NSF HX 1 approved for food contact lubricants and so is suitable for greases used on food production lines.

Graphene Nanoplatelets

Graphene Nanoplatelets are an emerging raw material for use as a grease additive and can act as a solid lubricant through the localised sliding of graphene multilayers. As a secondary benefit, the inclusion of graphene nanoplatelets has the potential to render the grease thermally and electrically conductive, useful for example, in car battery packs.

 

Figure 3: Lubricating capacity of graphene nanoplatelets by addition level (arbitrary units).

 

How to select the correct grease system?

Selecting the most suitable grease for your application is critical and is based upon the industry, the aspects of the application and the environment it would be subjected to. It is also important to note that the quantity of grease used is as crucial as selecting the correct grease. As a general rule of thumb, under-greasing is better than over-greasing as excessive lubrication can cause increased hydraulic resistance and as a consequence, more energy and wear on the part. It is also very difficult to remove excessive grease so it is good practice to grease ‘little and often’. An added advantage of this is that it provides engineers the opportunity to also carry out condition monitoring of surrounding equipment.

Grease compatibility

When replacing a grease in an existing system, it is important to ensure that the grease to be applied is compatible with any existing grease which may already be present. If there is incompatibility, there is a high risk of equipment failure where grease solidifies and releases all its oil. It is important to flush any old grease out of the system before replacing it with new.

 

Our knowledgeable technical team are always available to help if you would like any further details on the above article or any of the products mentioned in it. Do not hesitate to get in contact with us to discuss your needs and requirements for grease thickeners and additives.