The preparation of pharmaceutical formulations often incorporates lubricant ingredients that are used to increase bulk powder density, improve the flow of powder blends, reduce any friction that may exist between the surfaces of manufacturing equipment and die walls, as well as ensure adequate ejection forces.
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Magnesium stearate (MgSt), which is the most frequently used lubricant for these purposes, exhibits specific characteristics that can adversely impact the physical and chemical properties of tablets. It is therefore critical to investigate how the manufacturing processes and utilization of lubricant powder particles can play a role in determining the efficiency of pharmaceutical formulations.
Factors Affecting Lubricants
Within the pharmaceutical industry, boundary lubrication, which is one of the four types of lubrication methods, is typically achieved through the use of long-chain molecules with active end-groups, such as stearic acid and its metallic salts.
These lubricant compounds function by forming several layers or films to reduce any unintended contact between the surfaces and interfaces and lubricant powder particles. Lubricant efficiency is often determined by measuring the extent to which these layers can conceal the field of force of the underlying surface.
The type of lubricant and its concentration, as well as the lubricant preparation techniques, such as the amount of mixing time, operating method and type of mixer used, can all play a role in determining powder compactibility and the intermolecular forces of formulations. These factors therefore ultimately contribute to the final mechanical properties of tablets and other pharmaceutical formulations. Regarding mixing parameters, research has found that adjusting the strain and shear rate used during formulation can better predict tablet quality.
The Effects of Powder Particle Size
Different vendors will often distribute lubricant solutions that exhibit varied powder particle sizes, surface area, and shapes. As a result of this variance, researchers have become increasingly interested in determining how to assess these particle characteristics and how they contribute to the mechanical properties of applied pharmaceutical formulations.
Regarding particle size distribution (PSD), researchers have found that reducing the size of the lubricant particles can increase tablet porosity by creating greater cohesive forces between the particles. Increased porosity causes the powder particles to rearrange themselves in a way that also increases surface area, thereby leading to intensified interparticle interactions and ultimately a significantly greater tablet strength.
Powder Particle Size and Hardness
While a reduction in powder particle size has demonstrated certain advantages, it can also lead to sticking that occurs following the unwanted entrance of extra fine particles. The contamination of these particles can form an unwanted additional thin film that forms on the inner surface of the die wall. Sticking can cause compressed tablets to remain trapped within the die and ultimately unable to eject.
Additionally, the increase in surface area attributed to extremely small particle size can produce a higher concentration of lubricant needed to cover the die walls completely. It is therefore imperative for researchers to carefully prepare their particle size fractions to obtain the desired hardness of pharmaceutical tablets.
In a recent study that investigated how different particle sizes and lubricant concentrations affect the hardness of pharmaceutical tablets, it was determined that the lubricant concentration of MgSt needed to achieve optimal tablet hardness of approximately 18 kiloponds (kp) was a significant factor. Furthermore, the researchers concluded that maintaining optimal particle size prevented the segregation of particles and reduced weight variation during compression, as well as improved hardness and aesthetic appearance of the final product.
Conclusion
Overall, it can be determined that in addition to tablet hardness, lubricant particle size can also affect various physical and chemical properties of pharmaceutical formulations. The optimum particle size varies for every type of lubricant being used and what the desired properties are for each pharmaceutical preparation.
References
- Li, J., & Wu, Y. (2014). Lubricants in Pharmaceutical Solid Dosage Forms. Lubricants 2; 21-43. DOI: 10.3390/lubricants2010021.
- Rajani, C., Kumar. ,D. D., Jaya, D., & Kumar, J. A. (2017). Effects of granule particle size and lubricant concentration on tablet hardness containing large concentration of polymers. Brazilian Journal of Pharmaceutical Sciences 53(3). DOI: 10.1590/s2175-97902017000300149.
- Razavi, S. M., Gonzalez, M., & Cuitino, A. M. (2018). Quantification of lubrication and particle size distribution effects on tensile strength and stiffness of tablets. Powder Technology 336; 360-374. DOI: 10.1016/j.powtec.2018.06.001.
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