IntroductionA lubricant, an additive to reduce friction, is an essential component of a drug formula since lubrication is often required to ensure the success of pharmaceutical manufacturing. Historically, use of animal fats as lubricants to reduce friction in transportation can be traced back to Egyptian time. For pharmaceutical operations such as blending, roller compaction, tablet manufacturing, and capsule-filling, lubrication is essential in order to reduce the friction between the surfaces of manufacturing equipment and that of organic solids as well as to ensure the continuation of an operation . In this review, we first present some fundamental principles on lubrication in terms of action mechanisms: friction reduction, friction and adhesion, and lubrication in pharmaceutical processes . Then, a general overview on the use of lubricants in the pharmaceutical industry is provided. Specifically, magnesium stearate as a common lubricant will be examined in detail. Contrary to the common belief, friction was actually first studied by Leonardo Da Vinci, but it was wrongly credited to Amontons, which is often referred to as Amontons’s law .
Since Amontons’s law was derived from observing sliding wooden blocks, there is no consideration of adhesion. By definition, the energy of adhesion is the energy required to break two dissimilar surfaces . As expected, friction always increases with the adhesion between surfaces. JKR adhesion force and the surface roughness parameter, respectively . In addition, under compression, pharmaceutical powders may undergo a plastic deformation due to particle fracture or the deformation of excipients, or both.
In the next section, we will focus on the mechanisms of reducing these forces using lubricants. As their names implies, the former three mechanisms are related to the usage of liquid lubricants to some extent. In the pharmaceutical industry, boundary lubrication is the most common mechanism functioning in unit operations . In these processes, friction occurs at either powder-tool interfaces or particle-particle interfaces. The larger the wall friction angle or the coefficient of wall friction, the greater is the wall friction. Although the wall friction angle is an important parameter to consider, in this paper, our discussion will focus on the coefficient of wall friction. In this case, lubricants are used to reduce the shear stress required to move a tablet out of a die for a given normal stress.
Similarly, lubricants can be used to decrease the internal friction among powder particles. Chemically, stearic acid is a straight-chain saturated monobasic acid found in animal fats and in varying degrees in cotton seed, corn, and coco . The commercial material of stearic acid has other minor fatty acid constituents such as myistic acid and palmitic acid. Physical properties of pure solid fatty acids.