The dominant tacky-adhesive grease chemistry and the workhorse NSF H1 food-grade thickener — aluminium complex (AlX) grease combines aluminium isopropoxide with stearic acid and benzoic acid to form a translucent, stringy, naturally tacky tri-soap with drop point above 240°C. The chemistry serves three distinct markets: open-gear and wire-rope grease for mining and infrastructure, NSF H1 food-grade for the food and pharmaceutical industries, and specialty industrial applications where extreme adhesion is the primary performance need. This guide covers the in-situ saponification SOP, raw-material specifications, NLGI grade range, ASTM performance targets, and reactor equipment.
Aluminium complex grease is a soap-thickened grease in which the soap is an aluminium tri-soap — specifically, an aluminium complex of stearic acid and benzoic acid — dispersed in mineral or white oil base. The chemistry exploits aluminium's tri-valent oxidation state: each aluminium atom can bond with three carboxylate groups, allowing a mixed soap structure where two stearate and one benzoate are bonded to each Al atom (typical formulation), giving a thermally stable complex.
The in-situ reaction is straightforward: 2 R-COOH + C₆H₅COOH + Al(OiPr)₃ → Al(OOC-R)₂(OOC-C₆H₅) + 3 iPrOH. The aluminium isopropoxide (the most controllable aluminium source) reacts with the mixed acid charge to form the aluminium tri-soap, releasing isopropanol which is stripped off and recovered. The reaction occurs in the base oil; the soap forms as fine fibres that re-crystallise on cool-down into the characteristic translucent, stringy, tacky gel.
The tackiness is intrinsic to the chemistry, not a property of an added tackifier. The aluminium tri-soap fibres are short and ribbon-like, with strong inter-fibre cohesion driven by the benzoate aromatic stacking. This gives the grease its unique combination of high yield strength (it does not flow under gravity) and very high tack (it sticks aggressively to metal surfaces). The translucent appearance — another distinctive feature — comes from the small fibre size and high refractive-index matching with the base oil.
Three application properties make AlX commercially important: (1) natural tackiness for open gears, wire ropes, slide ways and any application where grease throw-off is a problem; (2) NSF H1 compliance — all raw materials (aluminium isopropoxide, stearic acid, benzoic acid, USP white oil) are HX-1 listed, making AlX the workhorse food-grade thickener; (3) good water resistance with drop point >240°C, enabling marine and outdoor industrial applications.
| Component | Grade / Specification | Treat Rate (NLGI 2) | Function & Sourcing |
|---|---|---|---|
| Base oil — primary | USP white oil (NSF H1) or Group II SN500, KV40 90–110 cSt | 55–65% | USP white oil for NSF H1 variant; Group II for industrial open-gear AlX. The base oil drives application suitability. |
| Base oil — secondary | USP technical white oil 70/80 or Group II SN150 | 15–25% | Diluent for acid dissolution. Match the primary base oil for NSF H1 compliance. |
| Stearic acid | Tech grade or USP (for NSF H1), AV 195–205, SV 200–210 | 8–10% | Primary fatty acid. NSF HX-1 listed grade required for food. Specify saturated (IV <5). |
| Benzoic acid | USP / FCC grade, >99.5% pure, AV 458–463 | 2–4% | Complexing aromatic acid. NSF HX-1 listed grades available. Drives the complex drop-point gain. |
| Aluminium isopropoxide | Al(OiPr)₃, technical grade, ≥95% pure, white solid or solution in iPrOH | 1.5–2.5% | The aluminium source. Stoichiometric ratio: 1 mol Al per 3 mol total acid (2 stearate + 1 benzoate). Sourced from Sasol, domestic chemical suppliers. |
| Antioxidant | Hindered phenol (BHT) + aryl-amine (HX-1 for NSF) | 0.5–1.0% | Standard antioxidant package. HX-1 listed grade for NSF H1. |
| Rust inhibitor | Sorbitan monooleate + amine carboxylate (HX-1) | 0.3–0.6% | Standard rust package. Modest dose since AlX already has good rust resistance. |
| Tackifier (open gear / wire rope) | PIB 2400 cSt or polyisobutylene | 1–5% | For open-gear and wire-rope variants. AlX is naturally tacky but PIB enhances it dramatically for harsh service. |
| Bitumen extract (mining only) | Asphaltic residue or extract | 0–10% | Used in cost-driven mining wire-rope grease for ultimate adhesion and load capacity. Not for NSF H1. |
| Solid lubricant (optional) | PTFE micropowder (HX-1 for NSF) or MoS₂ (industrial only) | 1–5% | PTFE for NSF H1 high-load variants; MoS₂ for non-H1 open-gear loaded service. |
The aluminium complex SOP follows the soap-grease pattern with one critical difference: isopropanol is generated as a by-product and must be stripped off through a condenser. The reactor must have a condenser with isopropanol recovery on the vent line — both for environmental compliance and to recover the alcohol for re-use. Total cycle time for a 200–400 kg batch is approximately 8–10 hours.
| Property | ASTM Test Method | Typical Value | Industrial / NSF H1 Spec |
|---|---|---|---|
| Worked penetration, 60 strokes | ASTM D217 | 265–295 (0.1 mm) | 265–295 |
| Penetration change, 100,000 strokes | ASTM D217 | +15 to +35 dmm | +50 max |
| Dropping point | ASTM D2265 | 245–260°C | 240°C min |
| Water washout @ 79°C | ASTM D1264 | 2–5% | 10% max |
| Four-ball wear scar | ASTM D4172, 40 kg, 75°C, 1h | 0.45–0.55 mm | 0.60 max |
| Four-ball EP weld point | ASTM D2783 | 160–250 kgf (with EP additive 250–315) | 160 kgf min (general); 250 min (industrial) |
| Oxidation stability, 100h @ 99°C | ASTM D942 | 3–8 psi pressure drop | 10 psi max |
| Oil separation, 24h @ 100°C | ASTM D1742 / D6184 | 3–6% | 10% max |
| Copper corrosion, 24h @ 100°C | ASTM D4048 | 1a | 1b max |
| Apparent stringiness (visual) | Visual assessment | Highly stringy, translucent | Characteristic property — not a numerical spec |
| Failure Mode | Root Cause | Diagnostic Test | Fix |
|---|---|---|---|
| Drop point below 240°C | Top temperature below 200°C; or benzoic acid ratio too low | D2265 drop point; recheck stoichiometry | Raise top hold to 215°C, 15 min; verify benzoic acid:stearic molar ratio 1:2 minimum; verify Al stoichiometry covers both |
| Grease opaque / not translucent | Cool-down too fast; or top temperature too low for fibre development | Visual; D2265 drop point | Slow cool to 90 minutes from 215°C; ensure top hold at 210°C minimum; verify benzoic acid contributing to complex |
| Grease not tacky / stringy | Over-milled (broke long fibres); or insufficient soap content | Sensory; D217 penetration | Reduce mill passes to 1–2; widen final gap to 50 µm; consider raising soap content to 12–14% |
| Severe foaming during Al isopropoxide addition | Rapid isopropanol generation overwhelming condenser; or acid mass too viscous | Visual; reactor vent flow rate | Slow Al isopropoxide addition to 45 min; oversize condenser capacity; verify acids fully melted before Al addition |
| Acid value remains high after step 3 | Al isopropoxide degraded (moisture-aged); or aluminium dose stoichiometrically low | Acid value; check Al supplier CoA | Verify Al(OiPr)₃ stored in dry sealed condition; recalculate Al dose at 1 mol per 3 mol total acid + 5% excess |
| Discolouration (yellow / brown) | Top temperature exceeded 220°C; or oxidation during high-temp hold | Visual / colour scale | Control top hold strictly at 210°C; add antioxidant earlier; use nitrogen blanket if persistent |
| Isopropanol not fully stripped | Dehydration hold insufficient; or vent restricted | Water/iPrOH content of bulk <0.5%; flash point check | Extend hold at 150°C to 90 min; verify condenser not flooded; sample bulk and confirm iPrOH below detection |
An aluminium complex grease plant is similar to a lithium soap plant with two specific additions: a condenser system for isopropanol capture (both environmental compliance and isopropanol recovery), and (for NSF H1 variants) dedicated stainless reactor and packaging segregation to maintain H1 cleanliness. See our Plant Setup service for complete specification and ISO 21469 facility planning.
The aluminium tri-stearate + benzoate complex soap forms a translucent, stringy, gel-like structure that is naturally tacky — this is an intrinsic property of the aluminium soap chemistry, not from added tackifier. The tackiness gives the grease excellent adhesion to vertical and high-vibration surfaces, preventing throw-off in open gear, wire rope, and slide-way applications.
PIB tackifier (1–5%) and bitumen (mining only) can be added for extreme adhesion in wire rope cores and open gears. The natural tackiness comes from the short ribbon-like fibres held together by aromatic stacking of the benzoate groups.
Tell us your target application — NSF H1, open-gear, wire rope or general industrial — NLGI grade and reactor scale. We respond within one business day with an honest assessment and indicative timeline.