Excipients: Formulation Effects on Tablet Surface Properties and Film Coating Adhesion

Aqueous film coatings can enhance the appearance, stability, swallowability, and brand identity of pharmaceutical tablets. Coatings can also be used to mask the taste of a bitter or otherwise unpleasant active pharmaceutical ingredient or control its release. The properties of the tablet being coated, including tensile strength, surface roughness, and hydrophobicity can greatly influence coating quality and performance. 

This article describes a study conducted to investigate the influence of different commonly used lubricants and disintegrants on tablet surface roughness and film coating adhesion. In addition, a commercially available coprocessed excipient for direct compression (Prosolv Easytab) containing silicified microcrystalline cellulose (SMCC), lubricant, and disintegrant was part of the study. The study investigated the influence of two lubricants, magnesium stearate (MgSt) and sodium stearyl fumarate (SSF), and three disintegrants, croscarmellose sodium (CCS), crospovidone (PVPP) and sodium starch glycolate (SSG). The researchers prepared tablets of pure microcrystalline cellulose (MCC), five physical mixtures containing MCC and either disintegrant or lubricant, and the coprocessed excipient. The tensile strength and surface roughness of the tablets as well as the film coating adhesion of aqueous HPMC coatings on these tablets were studied. 

Materials 

The study used microcrystalline cellulose Vivapur 12 (MCC), sodium stearyl fumarate PRUV (SSF), croscarmellose sodium Vivasol (CCS), crospovidone Vivapharm PVPP XL (PVPP), sodium starch glycolate Explotab (SSG), Prosolv Easytab SP, and magnesium stearate Ligamed MF-2-V (MgSt) as tablet-core excipients. The tablets were coated with the ready-to-use HPMC coating Vivacoat A. All ingredients came from JRS Pharma, Germany, except for the MgSt, which came from Peter Greven, Germany. Methods Compaction and coating. Seven different tablet core formulations (Table 1) were compacted at a pressure of 125 megapascals using a compaction simulator (Styl’One Evolution, Medel’Pharm) equipped with bi-planar round punches with a diameter of 11.28 millimeters. The tablet cores were coated using a perforated drum coater (Solidlab 2, Bosch Hüttlin) with a tablet bed temperature of 38 ± 2°C. Tablet characterization. The tensile strength of the tablets was measured with a hardness tester (MultiTest 50, Sotax). The surface roughness was measured by profilometry (DektakXT stylus profiler, Bruker). Scanning electron micrograph (SEM) images of the tablet surfaces were taken using a tabletop microscope (TM-1000, Hitachi) and evaluated in terms of texture, morphology, and quality. Adhesion between the film coating and the tablet surface was measured using a material testing machine (Retroline BZ2, Zwick Roell) by applying double-sided adhesive tape to fix the tablet to even punches. Prior to adhesion measurement, the coating was carefully detached at the edge of the tablets using a scalpel. 

Results 

Tensile strength

A high tablet tensile strength indicates good compaction behavior and is important for withstanding the tablet coating process¹. However, it is well known that lubricants decrease a tablet’s tensile strength². Of the tablet core formulations tested, pure MCC showed the highest tensile strength, while the formulation containing MgSt displayed the largest reduction in tensile strength, as shown in Table 2. The coprocessed excipient, which contains SSF, showed a higher tensile strength (hence, better compactibility) than the physical mixture of MCC with SSF. 

The disintegrants only slightly decreased the tensile strength of the tablet cores, and that decrease was nearly the same for CCS, PVPP, and SSG. This can be attributed to the disintegrants’ high capacity for forming hydrogen bonds, which are crucial for their disintegration properties and enable strong interactions with the MCC particles.

Surface roughness

Tablets containing MgSt exhibited higher surface roughness than pure MCC tablets (Figure 1). Tablets containing SSF possessed a smoother tablet surface than pure MCC tablets, and the coprocessed excipient provided the smoothest tablet surface of all formulations tested, as shown in the SEM images in Figure 2. Disintegrants increased the tablet cores’ surface roughness compared to pure MCC. 

Film coating adhesion

The investigation of film coating adhesion (measured as stress of failure) showed that the lubricants strongly decreased the adhesion of the aqueous HPMC coating (Figure 3). MgSt showed a stronger effect than SSF. The disintegrants also decreased film coating adhesion, but to a lesser extent than the lubricants (Figure 4). 

Discussion 

Magnesium stearate has a layering effect³ that leads to lower overall bondability between particles in a formulation and, therefore, lower tablet tensile strength, higher friability (data not shown), and higher surface roughness for the tablets produced. Surface roughness, in combination with the hydrophobicity of MgSt, leads to reduced film coating adhesion. This layering effect occurs to a lesser extent with SSF⁴ and may even be reversed in some cases⁵. As a result, SSF yielded tablets with higher tensile strength than MgSt.

Furthermore, the lubricating effect of SSF decreases the tablet core’s adhesion to the punch face, which, together with its higher tensile strength, results in smoother tablet surfaces compared to pure MCC cores and cores containing MgSt. Since SSF is less hydrophobic than MgSt, higher film coating adhesion values were observed for tablets using SSF. Within the group of tablets containing lubricants, the coprocessed excipient yielded the highest film coating adhesion values due to the tablets’ high tensile strength and smooth surface. 

Conclusion 

Any addition of further compounds reduced MCC tablet performance. Lubricants showed a strong negative effect on film coating adhesion, while disintegrants exhibited a less distinct and less diverse adverse effect. Surface roughness and hydrophobicity play a vital role in this context. The coprocessed excipient achieved higher tensile strength, a smoother tablet surface, and higher film coating adhesion compared to the physical mixtures of excipients.

References 

1. B. A. C. Carlin, “Direct compression and the role of filler-binders,” In L. L. Augsburger and S. W. Hoag, (Eds.) Pharmaceutical Dosage Forms—Tablets: Rational Design and Formulation, 3rd ed., Informa Healthcare USA, New York, pages 173-216 (2008). 

2. W. A. Strickland, E. Nelson, L. W. Busse, and T. Higuchi, “The physics of tablet compression IX: Fundamental aspects of tablet lubrication,” Journal of the American Pharmaceutical Association, Vol. 45, No. 1, pages 51-55 (1956). 

3. M. S. H. Hussain, P. York, and P. Timmins, “A study of the formation of magnesium stearate film on sodium chloride using energy-dispersive X-ray analysis,” International Journal of Pharmaceutics, Vol. 42, No. 1-3, pages 89-95 (1988). 

4. A. W. Hölzer, and J. Sjögren, “Evaluation of sodium stearyl fumarate as a tablet lubricant,” International Journal of Pharmaceutics, Vol. 2, Nos. 3-4, pages 145-153 (1979). 

5. R. Louw, “Evaluation and comparison of magnesium stearate and sodium stearyl fumarate (Pruv) as lubricants in directly compressible tablet formulations: Their effect on tablet properties and drug dissolution,” Master thesis, North-West University (2003) available at repository. nwu.ac.za/handle/10394/426.