8 Analytical Instrumentation


SUSTAINABLE GREASE FORMULATIONS: EVALUATING KEY PERFORMANCE PARAMETERS AND TESTING METHODOLOGIES


1 Introduction


Grease is an essential lubricant used across various industries to ensure the effi ciency and longevity of machinery by reducing friction, wear, and corrosion. Grease forms a protective barrier that seals out contaminants such as dirt, dust, and water, preventing corrosion and damage, especially in harsh operating environments [1]. Moreover, grease reduces maintenance costs by prolonging the intervals between necessary servicing and repairs, as its ability to stay in place and continue lubricating over extended periods reduces the need for frequent reapplication (2].


However, traditional grease formulations derived from petroleum-based mineral oils have a signifi cant environmental footprint, contributing to greenhouse gas emissions and environmental degradation throughout their lifecycle [3]. With global energy consumption predicted to increase substantially in the coming decades by as much as 50% by 2050, the grease industry’s reliance on mineral oil exacerbates environmental pressures associated with traditional grease production and use. Therefore, there is an urgent need for more sustainable alternatives [4].


One promising solution is the development and adoption of bio-based greases derived from renewable resources such as vegetable oils, animal fats, or microbial sources [5-7]. Compared to petroleum-based greases, bio-based greases offer several advantages: reduced dependency on fi nite fossil fuels, lower carbon footprint, and often improved biodegradability, minimizing environmental impact in case of leaks or spills [8, 9].


Despite the clear environmental benefi ts, the adoption of bio-based greases faces challenges. Their performance characteristics, such as viscosity, thermal stability, and wear protection, must meet or exceed those of traditional greases to gain widespread market acceptance [6, 10]. Furthermore, the economic feasibility and scalability of producing bio-based greases for a global market need to be addressed.


This paper provides a comprehensive overview of grease formulations, lubrication, and the key performance parameters of lubricating grease, with a particular focus on the ASTM standards and test apparatus used to evaluate these parameters. The paper also examines the challenges and research directions necessary to advance the fi eld of sustainable grease formulations, considering the broader implications for the environment and energy consumption within the grease industry.


2 Overview of Grease 2.1 Grease formulations


Unlike oil, which can be thin and runny, grease contains a thickener system that provides consistency and structure allowing grease to excel in applications where fl uidity makes oils unsuitable. Thicker consistency allows grease to adhere to surfaces, making it ideal for high-pressure environments where oil can be squeezed out, high-speed applications where oil gets fl ung away, and environments with water or dust where oil can be washed away or contaminated. Grease acts as both a lubricant, reducing friction and wear, and a sealant, keeping out contaminants and protecting machinery in harsh environments with extreme temperatures, moisture and abrasive particles [1].


Grease formulations typically consist of three main components: base oils, thickeners, and additives. Each component plays a role in determining the performance and suitability of the grease for various applications.


Table 1. Various vegetable oils and their applications [15-19]. Vegetable Oil


Major Applications


Canola oil Castor oil


Coconut oil Jojoba oil Linseed oil Olive oil Palm oil


Rapeseed oil Saffl ower oil Soybean oil Sunfl ower oil Tallow oil


2.1.1 Base oils


The base oil is the primary lubricating component, typically constituting 65-95% of the grease formulation. It provides essential properties, including viscosity and fi lm strength, which are crucial for reducing friction and wear between moving parts [10]. Base oils can be classifi ed into three main categories: mineral, synthetic, and vegetable oils.


Mineral oils derived from crude petroleum are cost-effective and perform well across a wide range of applications. They offer good lubrication properties and thermal stability, making them suitable for many industrial and automotive applications. Synthetic oils, such as polyalkylene glycol (PAG) and polyalphaolefi ns (PAO), offer improved oxidative stability, low- temperature fl uidity, and high-temperature viscosity retention, making them ideal for applications involving high loads, high speeds, or extreme temperatures [11]. Vegetable oils, derived from plant sources such as soybean, rapeseed, or palm oil, are biodegradable and renewable, making them attractive for environmentally friendly grease formulation. However, they may require stabilizing additives to enhance their oxidative stability and performance characteristics [12]. Oxidative stability is crucial for any base oil due to oxidizing reactions within the base oil composition. If the oil is too unsaturated, it breaks down faster, leading to ineffective grease. This is particularly important for vegetable oils, as many are unsaturated and require stabilizers for additional support. Table 1 shows various vegetable oils and their applications in grease formulations.


The choice of base oil depends on factors such as the intended application, operating temperatures and speeds, load conditions, and environmental considerations. In some cases, a blend of different base oils may be used to achieve the desired performance characteristics. For example, blending rapeseed oil with PAO signifi cantly improves its


PIN OCTOBER / NOVEMBER 2024


Hydraulic oils, tractor transmission and metal working fl uids Gear lubricants, greases Gas engine oils


Cosmetic industry, lubricant applications Stains, varnishes, lacquers Automotive lubricants Grease, rolling lubricant


Air compressor-farm equipment, chain saw bar lubricant Enamels, light-colored paints, diesel fuel, resins


Plasticizers, hydraulic oil, printing inks, pesticides, disinfectants Grease, diesel fuel substitutes Lubricants, plastics


World Production (Mt/ year)


49.7 1.9 3.4


0.018 0.66 18.7 73


27.4 0.08 59.2 19.4 9.8


low-temperature properties, enhancing fl uidity and reducing pour point temperature from -22°C to -31°C [13]. Additionally, a mixture of saffl ower oil with polyol esters has shown promising results in enhancing oxidation stability [14]. These blends are not only effective in improving specifi c performance attributes but also maintain compatibility with anti-wear additives, ensuring that the lubricity of the fi nal product is not compromised. Such blends can be used as base stocks in the production of environmentally friendly greases, meeting multiple performance criteria while also being eco-friendly.


2.1.2 Thickeners


Thickeners, comprising 5-35% of the grease formulation, are responsible for giving grease its characteristic semi- solid consistency [20]. Their primary function is to stabilize grease positioning and maintain continuous contact with the lubricated surfaces, preventing leakage or being squeezed out under pressure. Thickeners play a crucial role in ensuring effective lubrication, even in applications involving high loads or complex geometries.


In addition to providing the desired consistency, thickeners also infl uence other important properties of the grease, such as mechanical stability and water resistance. The choice of thickener can signifi cantly impact the grease’s ability to withstand mechanical shear forces, as well as its resistance to water washout or degradation in the presence of moisture.


The most commonly used thickeners in grease formulations are metallic soaps, particularly lithium, calcium, sodium, and aluminum soaps [21]. These soaps are classifi ed as surfactant amphiphiles, which are compounds with a dual affi nity for both water (hydrophilic) and oil (lipophilic) [20]. The unique molecular structure of amphiphiles, with a nonpolar hydrocarbon tail and a polar ionic head group, allows them


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