Green Lubrication: Transforming Waste into High-Performance Grease
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The global lubricants market, valued at over $120 billion annually, relies predominantly on mineral oils derived from non-renewable crude oil. These conventional greases pose significant environmental risks, including soil and water contamination from improper disposal, as well as greenhouse gas emissions during production and use. A study revealed that industrial lubricants contribute 2.3% of global CO₂ emissions, highlighting the urgent need for sustainable alternatives.
The concept of green lubrication emerged as a response to these challenges, emphasizing biodegradability, renewable feedstocks, and waste valorization. A landmark study demonstrated that industrial by-products—specifically waste oils and red gypsum—could be repurposed into high-performance greases. This innovation addresses three critical issues:
- Waste Emulsion Pollution: Machining processes generate oil-water emulsions containing heavy metals and chemicals, which require costly treatment.
- Red Gypsum Disposal: The titanium dioxide industry produces 5–7 million tons of red gypsum annually, often landfilled due to its impurity content.
- Economic Viability: Commercial greases cost 1,500–3,000 per ton, whereas waste-derived formulations reduce raw material expenses by 60–70%.
By converting these wastes into functional lubricants, industries can achieve circular economy goals while meeting stringent environmental regulations.
Fig 1. Grease stability graph by four types of base oil. (Razali M. N.,
et al., 2017)
Material Science of Waste-Based Grease Formulation
- Base Oil Selection
Grease performance hinges on the base oil's viscosity, thermal stability, and compatibility with thickeners. The 2017 study tested four base oils:
- Silicone Oil: Exhibits exceptional thermal stability (up to 300°C) and oxidation resistance, making it ideal for high-temperature applications.
- Waste Emulsion Oil: Recovered from machining processes, this oil contains residual emulsifiers that enhance grease adhesion to metal surfaces.
- Waste Cooking Oil: Rich in triglycerides, it provides biodegradability but requires careful purification to remove food residues.
- Used Engine Oil: Contains oxidized hydrocarbons and metal particles, rendering it unsuitable for grease formulation due to instability.
Silicone oil and waste emulsion oil emerged as the top performers, with silicone-based grease achieving a dropping point of 240°C—surpassing commercial lithium-based greases (180–200°C).
- Thickener Optimization
Thickeners determine grease consistency and shear stability. The study evaluated fumed silica and red gypsum (calcium sulfate dihydrate) as thickeners:
- Fumed Silica: A synthetic amorphous silica with a high surface area (200–300 m²/g), it forms a three-dimensional network that traps base oil. At 50% concentration, it increased grease consistency from semi-fluid (NLGI Grade 0) to firm (NLGI Grade 1).
- Red Gypsum: Acts as a secondary thickener, providing cost savings (red gypsum costs 20–30 per ton vs. 500–800 for fumed silica). However, excessive red gypsum (>60%) caused oil separation due to insufficient thickening power.
The optimal ratio—50% fumed silica, 50% red gypsum—balanced cost and performance, achieving 0.5% oil separation after 30 hours at 177°C (ASTM D-1742).
- Additive Engineering
Additives enhance grease properties under extreme conditions:
- Molybdenum Disulfide (MoS₂): Reduces friction by 30% in boundary lubrication regimes, as shown in Tribology International (2019).
- Iron Octoate: Acts as an antioxidant, extending grease shelf life by preventing base oil oxidation.
These additives, comprising 2–3% of the formulation, are critical for meeting ASTM D-4950 standards for automotive and industrial applications.
Performance Evaluation: Benchmarking Against Commercial Greases
Thermal Stability
The dropping point test (ASTM D-2265) measures the temperature at which grease transitions from semi-solid to liquid. Waste-derived greases exhibited:
- Silicone-Based Grease: No melting up to 240°C, outperforming lithium complex greases (200°C).
- Waste Emulsion-Based Grease: Stable up to 220°C, suitable for steel mill bearings.
This thermal resilience reduces downtime in high-temperature industries like cement manufacturing and glass production.
Mechanical Durability
The cone penetration test (ASTM D-217) classifies grease consistency (NLGI Grades 000–6). The 50% fumed silica formulation achieved:
- Penetration Value: 340 (10-1 mm), corresponding to NLGI Grade 1—ideal for centralized lubrication systems.
- Worked Penetration: After 60 strokes, the value increased by only 15%, indicating excellent shear stability.
In contrast, commercial calcium-based greases softened by 25% under the same conditions, leading to leakage and uneven lubrication.
Environmental Impact
Life cycle assessment (LCA) data reveals that waste-based greases reduce:
- Carbon Footprint: By 45% compared to mineral oil-based greases, due to avoided crude oil extraction and refining.
- Water Consumption: By 70%, as red gypsum and waste oils require no additional water for processing.
Overcoming Technical Challenges
Particle Size Control
Fumed silica particles must be dispersed uniformly to prevent clumping. The study employed:
- High-Shear Mixing: At 1,500 rpm for 2 hours, achieving a particle size distribution of 5–10 µm.
- Sonication: Ultrasonic treatment reduced agglomerates by 80%, improving grease texture.
Oil Separation Mitigation
Oil separation occurs when thickeners fail to retain base oil. Solutions include:
- Hydrophobic Modification: Treating fumed silica with hexamethyldisilazane (HMDS) increased oil retention by 25%.
- Polymer Additives: Polyisobutylene (PIB) at 1% concentration reduced separation to 0.2% after 72 hours.
Scalability for Industrial Use
Transitioning from lab-scale (1 kg batches) to industrial production (500 kg batches) requires:
- Continuous Mixing Systems: Twin-screw extruders ensure homogeneous blending.
- Quality Control: Real-time rheometry monitors viscosity during production.
Future Directions in Green Lubrication
- Bio-Based Thickeners
Researchers are exploring cellulose nanocrystals (CNCs) derived from agricultural waste as renewable thickeners. CNCs exhibit:
- Thermal Stability: Up to 250°C in silicone oil matrices.
- Biodegradability: 90% degradation within 180 days under composting conditions.
- Ionic Liquid Lubricants
Ionic liquids (ILs) like [BMIM][BF₄] offer:
- Non-Volatility: Negligible vapor pressure at high temperatures.
- Corrosion Inhibition: Protect steel surfaces in marine environments.
- AI-Driven Formulation Optimization
Machine learning models can predict grease performance based on:
- Base Oil Viscosity: Neural networks correlate viscosity indices with dropping points.
- Thickener Morphology: X-ray diffraction data trains models to optimize particle packing.
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Reference
- Razali, Mohd Najib, et al. "Synthesis of grease from waste oils and red gypsum." Australian Journal of Basic and Applied Sciences 11.113 (2017): 154-159.
Used Oil and Grease Products
This article is for research use only and cannot be used for any clinical
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