Tablet Coating Failure Using Micro Scratch Testing

Tablets are covered by various coatings for many reasons, such as controlling how the pill dissolves once ingested. These requirements inside the body sometimes conflict with the mechanical properties needed to keep the appearance and durability before use. During the manufacturing and packaging of pharmaceutical tablets, many situations can harm the coverage and appearance of the coating. The two main sources of damage occur when the pill travels on the metallic conveyor and when it is inside the container in contact with other tablets. The coating is intended to withstand conveying and packaging with minimal failure while still meeting its requirements when ingested. The reliability of the tablet coating process is critical to existing tablet production and will be even more so with new or improved pharmaceutical coatings.

A major concern for manufacturers of pharmaceutical tablets is ensuring strong coating adhesion and/or the ability to withstand marring and cracking. A tablet coating that loses its adhesion and/or begins to crack is a serious product failure when thousands of tablets are coated with the same process. As tablet medication continues to grow, quality assurance plays a crucial role in establishing a reliable coating process. Although adhesion or cracking failure may be inevitable, or even intended, over time, it is crucial that these failures are investigated, known and controlled. Using the micro scratch test, precisely controlled loads can be used to investigate a tablet’s coating for these very failures.

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Generic: the critical load at which failure (delamination) occurred was 2.07 N, with the substrate exposed by delamination.

Brand: the critical load at which failure (cracking) occurred was 0.64 N, with the substrate exposed by cracking on the sides of the scratch.

From the hardness test we can see that the generic tablet is covered by a softer and more elastic coating. During scratch testing, the failure seen on the generic tablet shows how the more elastic coating fully delaminates by ripping open at 2 N. The coating of the brand tablet is much harder and more brittle: during scratch testing it fails with cracks and chipping at 0.6 N. Here the harder coating is pushed into the soft substrate, as indicated by the exposure of substrate on each side of the scratch. Visually, the failure will be more apparent above 2 N on the generic sample than on the brand samples. This is a typical scenario when comparing soft and brittle coatings on a soft substrate.

As shown, the value of micro scratch testing for tablet coatings is the ability to quantify, with superior repeatability, the adhesion (adhesive failure) and cracking (cohesive failure) of tablet coatings. Many indenter tips can be used to simulate various levels of scratches. Additionally, the NANOVEA Mechanical Tester could also have been used to measure wear, friction coefficient, compression, fracture toughness, roughness and many others. It is a complete and powerful tool for tablet coating research and control.