Aluminum stearate is an inorganic chemical compound, the aluminum salt of stearic acid, commonly used in various cosmetic and industrial applications. It is valued for its multifunctional properties, including its role as an anticaking agent, colorant, emulsion stabilizer, and viscosity controller.

Chemical Composition and Structure

Aluminum stearate is an organic compound that consists of aluminum ions combined with stearic acid, a long-chain fatty acid. This combination gives the compound its unique properties and wide range of applications.

Physical Properties

Aluminum stearate typically appears as a white, fine powder. It is insoluble in water but soluble in oils and organic solvents. The compound is known for its stability, resistance to heat, and ability to form gels with oils, making it highly useful in various formulations.

Chemical Industrial Synthesis Process

• Preparation of reagents. The main raw materials include stearic acid and an aluminum compound such as aluminum chloride (AlCl₃) or aluminum sulfate (Al₂(SO₄)₃).
• Saponification. The stearic acid is heated and treated with a sodium hydroxide (NaOH) solution to form sodium stearate (sodium stearate soap).
• Precipitation of aluminum stearate. The aluminum chloride or aluminum sulfate is slowly added to the sodium stearate solution with constant stirring. This reaction forms a precipitate of aluminum stearate.
• Filtration. The resulting suspension is filtered to separate the solid aluminum stearate precipitate from the aqueous solution.
• Washing. The aluminum stearate precipitate is washed with deionized water to remove any soluble impurities and residual reagents.
• Drying. The washed aluminum stearate is dried at controlled temperatures to remove residual moisture and obtain a dry powder.
• Grinding. The dried aluminum stearate is ground to obtain a fine and uniform powder. This step may involve the use of ball mills or other grinding machinery.
• Classification. The dried powder is classified to ensure a uniform particle size. This step may involve sieving or the use of air classifiers.
• Stabilization. The aluminum stearate powder is stabilized to ensure its stability during transportation and storage, preventing aggregation and degradation.
• Quality control. The aluminum stearate undergoes rigorous quality testing to ensure it meets standards for purity, safety, and functionality. These tests include chemical analysis, spectroscopy, and physical tests to determine particle size and rheological properties.

What it is used for and where

It is used as an agent to gel or thicken aliphatic and aromatic hydrocarbons.

Cosmetics

Restricted cosmetic ingredient as  IV/150 a Relevant Item in the Annexes of the European Cosmetics Regulation 1223/2009. Substance or ingredient reported:

• Aluminium, zinc, magnesium and calcium stearates. 

Cosmetics - INCI Functions

• Absorbent: It absorbs substances dispersed or dissolved in aqueous solutions, water/oil, oil/water.
• Anticaking Agent: Aluminum stearate is used as an anticaking agent in powders and other dry products to prevent clumping and improve flowability. It helps maintain the free-flowing nature of products, enhancing their usability and shelf life.
• Colorant: While aluminum stearate itself is not a colorant, it is often used in conjunction with pigments to enhance their dispersion and stability in cosmetic formulations. It helps achieve uniform color distribution in products like foundations and eyeshadows.
• Emulsion Stabilizer: In emulsions, aluminum stearate acts as a stabilizing agent, preventing the separation of oil and water phases. This property is particularly valuable in creams, lotions, and other emulsion-based products, ensuring a consistent and stable formulation.
• Viscosity Controller: Aluminum stearate is used to control the viscosity of various cosmetic and industrial formulations. It helps thicken products, providing the desired consistency and texture in items such as creams, ointments, and paints.

Molecular Formula    C₁₈H₃₉AlO₄

Molecular Weight    346.5 g/mol

CAS    7047-84-9

UNII   P9BC99461E

EC number 230-325-5

Synonyms:

Aluminum monostearate

Safety

Careful consideration should be given to the risk of cumulative aluminum intake, which cannot be ruled out because this ingredient can be found in both cosmetic products and widely consumed food products such as bread, various baked goods (1).

References_____________________________________________________________________

(1) Tietz, T., Lenzner, A., Kolbaum, A.E. et al. Aggregated aluminium exposure: risk assessment for the general population. Arch Toxicol 93, 3503–3521 (2019). https://doi.org/10.1007/s00204-019-02599-z

 Abstract. Aluminium is one of the most abundant elements in earth’s crust and its manifold uses result in an exposure of the population from many sources. Developmental toxicity, effects on the urinary tract and neurotoxicity are known effects of aluminium and its compounds. Here, we assessed the health risks resulting from total consumer exposure towards aluminium and various aluminium compounds, including contributions from foodstuffs, food additives, food contact materials (FCM), and cosmetic products. For the estimation of aluminium contents in foodstuff, data from the German “Pilot-Total-Diet-Study” were used, which was conducted as part of the European TDS-Exposure project. These were combined with consumption data from the German National Consumption Survey II to yield aluminium exposure via food for adults. It was found that the average weekly aluminium exposure resulting from food intake amounts to approx. 50% of the tolerable weekly intake (TWI) of 1 mg/kg body weight (bw)/week, derived by the European Food Safety Authority (EFSA). For children, data from the French “Infant Total Diet Study” and the “Second French Total Diet Study” were used to estimate aluminium exposure via food. As a result, the TWI can be exhausted or slightly exceeded—particularly for infants who are not exclusively breastfed and young children relying on specially adapted diets (e.g. soy-based, lactose free, hypoallergenic). When taking into account the overall aluminium exposure from foods, cosmetic products (cosmetics), pharmaceuticals and FCM from uncoated aluminium, a significant exceedance of the EFSA-derived TWI and even the PTWI of 2 mg/kg bw/week, derived by the Joint FAO/WHO Expert Committee on Food Additives, may occur. Specifically, high exposure levels were found for adolescents aged 11–14 years. Although exposure data were collected with special regard to the German population, it is also representative for European and comparable to international consumers. From a toxicological point of view, regular exceedance of the lifetime tolerable aluminium intake (TWI/PTWI) is undesirable, since this results in an increased risk for health impairments. Consequently, recommendations on how to reduce overall aluminium exposure are given.