With the development of society, the demand for lubricating greases is increasing. Therefore, the production of lubricating greases must not only consider their performance but also their economic, environmental, and sustainable aspects. With growing awareness of environmental protection, the development of high-performance, “environmentally friendly” greases is the future direction of development.
Calcium carbonate, with the chemical formula CaCO₃, is a white, odorless solid and an important inorganic compound in nature. In nature, calcium carbonate is primarily found in mineral resources such as limestone, calcite, marble, and dolomite, with limestone being the most abundant. Depending on the production method, calcium carbonate can be divided into heavy calcium carbonate and light calcium carbonate.
1. Heavy Calcium Carbonate
Heavy calcium carbonate, abbreviated as heavy calcium carbonate (GCC), is produced directly from high-quality limestone, calcite, marble, chalk, and other raw materials through crushing and grinding (fine or ultrafine grinding) to produce a powder. Heavy calcium carbonate powders are characterized by irregular particle shape, a wide particle size distribution, and large particle size. The whiteness and crystal shape of heavy calcium carbonate vary depending on the raw material. Its particle size is dependent on the powder processing equipment, with the finest reaching 0.1μm.
2. Light Calcium Carbonate
Light calcium carbonate, also known as precipitated calcium carbonate (PCC), is produced from limestone through calcination, digestion, and carbonization. It features fine particle size (average primary particle size of 0.07μm), high whiteness, and adjustable crystal shape. Light calcium carbonate is divided into three types: ordinary light calcium carbonate, activated light calcium carbonate, and nano-calcium carbonate. Light calcium carbonate has dozens of crystal morphologies, with seven common ones being spindle, cubic, needle-shaped, chain-shaped, spherical, flake-shaped, and amorphous.
3. Applications of Calcium Carbonate in Greases
Grease is a semi-fluid to solid lubricant prepared by thickening a base oil with a thickener. Thickeners, base oils, and additives are the three main components of greases. Together with the grease preparation process, these three components are crucial factors in determining and influencing grease performance. Modern grease production technology and grease formulation are a science, and grease is a high-tech product.
3.1 As a Raw Material for Thickening Calcium Greases
Calcium soap is an important component of calcium greases, and calcium carbonate is one of their basic raw materials. Calcium carbonate decomposes at high temperatures to produce calcium oxide, which then reacts with water to produce calcium hydroxide (which acts as a saponifying agent). Finally, calcium hydroxide undergoes a saponification reaction with fatty acids to produce calcium soap.
3.2 As Grease Additives
Grease additives are substances added to grease to improve its performance. They can enhance the grease’s inherent properties or enhance properties it lacks. With the rise of nanomaterials science and technology, research on lubricant additives has gradually shifted from traditional conventional materials to nanomaterials, and new nano-lubricant additives have also entered the spotlight.
3.3 Fillers as Grease Volume and Weight Increasers
Fillers are also used in grease production. These are non-oil-soluble solid additives, mostly in the form of non-oil-soluble solid powders that lubricate, enhance sealing properties, and improve barrier properties. Most grease fillers function as solid lubricants themselves, improving the lubrication properties of greases.
Based on research on its use as a grease additive, the following points should be considered for calcium carbonate to be used as a filler for increasing grease volume and weight:
1) Surface modification. Calcium carbonate is a polar inorganic substance with a hydrophilic and oleophobic surface, poor oil compatibility, and a tendency to agglomerate. Base oil is the largest component by mass in grease. To enhance its compatibility with oil, calcium carbonate should undergo surface modification before use as a grease filler.
2) Ultrafine particle size reduction. Nanomaterials possess unique and excellent properties, and numerous researchers are currently investigating their application in greases. Nanoparticles can be used as grease additives primarily due to their three advantages: First, their strong diffusion and self-diffusion abilities allow them to form a good penetration or diffusion layer on metal friction surfaces, achieving an anti-wear effect; second, their small size, nearly spherical in shape, acts like a “rolling bearing” on the friction surface, altering the lubrication method of the friction pair and reducing the coefficient of friction; third, the particles can polish and strengthen the friction surface, supporting external loads and improving the load-bearing capacity of the grease.
3) Calcium carbonate crystal morphology. Crystal form is a crucial technical specification for calcium carbonate, determining its intended use. When used as a filler in grease, the appropriate crystal form must be selected. Currently, the main morphologies of calcium carbonate used in lubrication applications include cubic, spherical, and amorphous.
Calcium carbonate, as a grease filler, offers the advantages of being environmentally friendly and inexpensive. Using calcium carbonate to increase grease volume and weight without compromising grease performance can reduce the use of raw materials, benefiting the environment and lowering production costs.