However, polar groups within the acid molecule may exert a positive effect because of intermolecular hydrogen bonding. Thus, hydroxystearic acid is a popular soap substrate. Unsaturated fatty-acid molecules are not used as grease thickeners because their relative oil solubility reduces both consistency and dropping point. The consistency, as well as other physical properties, of soap-based greases may be modified by incorporating certain chemical agents called structure modifiers. The polar nature of these materials assist in the dispersion of thickener molecules.
Particle size is also an important parameter affecting the consistency of soap-based greases.
Optimal particle sizes may be obtained by carefully regulating temperatures during processing. As a result of the small, closely packed fibers which constitute their structural framework, products thus produced have a smooth texture. At low temperatures, they suffer no phase transformation and remain pumpable. Calcium-soap greases are primarily used to lubricate water pumps, wire ropes and machinery components operating under mild conditions. Their use in plain and rolling-element bearings is severely limited by their low thermal stability and susceptibility to shear.