Tablets

Test Principle

Quantify tablet coating adhesion force.

Background

Tablet coating has numerous functionalities including tablet strengthening, controlled release, ease of handling and packaging, protect the tablet from moisture, improve taste, facilitate swallowing, and provide tablet identity. The film coating often contains polymers, plasticisers, water-soluble dyes, pacifiers (such as titanium dioxide), various inorganic materials (such as iron oxides), and talc. Plasticisers improve flexibility and reduce brittleness otherwise caused by polymers. The titanium dioxide serves to increase adhesion and the talc minimises tackiness between coated tablets. These compounds service to improve tablet appearance (e.g., colour and opacity), film coating and barrier properties.

The adhesion of a coating to a tablet is influenced by the strength of interfacial bonds between film and tablet surface and the internal stresses within the coating. Poor adhesion results in peeling, markedly reducing film functionality. The mechanical protection provided by the coating can also be compromised by loss of adhesion, leading to the accumulation of moisture at the film-tablet interface. This can have profound effects on the stability of moisture-labile drugs.

Over the years, researchers have investigated variables that can affect the pharmaceutical quality of the final product. Among these variables, the composition of the tablet, compression force of the tablet, tablet surface roughness, coating formulation, and coating conditions all influence adhesion.

The tablet coating adhesive accessory provides a quantitative measure of the adhesion strength of the coating to tablet surface. The test is useful in monitoring effect of changes in tablet coating formulations or for process control and as a QC check.

Method

Equipment

• CT3 with 4.5kg load cell
• Tablet Coating Adhesion Accessory
• Standard Base Table (TA-BT-KIT)

Settings

• Test Type: Adhesion Test
• Pre-Test Speed: 1.0 mm/s
• Test Speed: 1.0 mm/s
• Post-Test Speed: 10.0 mm/s
• Target Load: 1500 g
• Hold Time: 5 Seconds
• Trigger Force: 450 g

Materials

• Scalpel
• Double-sided adhesive foam tape
• Scissors

Note: For the test to function correctly, the post-test speed indicated is necessary. A slower withdrawal speed tends to cause the whole tablet to peel from the tape in contrast to the quick “pull-off” at 10.0 mm/s.

Sample Preparation

Remove the tablet coating from around the circumference of the tablet using a scalpel.

Procedure

1. Cover both fixture cavities (upper and lower) with the adhesive foam tape.
   
   Note: The choice of adhesive tape in this test will have an effect on the results. For the purpose of comparison, once a suitable tape has been selected, this tape should be used for all subsequent tests.
   
   
2. Push the tape firmly into each cavity before removing the exposed surface backing.
   
   Note: The upper and lower attachment of the adhesion fixture has cavity dimensions complementary to, and consequently defined by, the customer’s sample tablet.
   
   
3. Remove the surface backing, then neatly mould the excess tape down the sides of the fixtures
4. Position the tablet coating centrally in the lower cavity.
5. Commence test.

Results

A typical plot of a tension test on two types of hard gelatine capsules.

Click to enlarge

The Figure1 graph shows the coating adhesive force of four tablets (9 mm in diameter) taken from the same batch. The action taking place in the initial 6 second interval is the upper fixture applying a force of 1500g on the tablet. The test measurement for the adhesion force commences between 7 and 8 seconds after the beginning of the text.

Click to enlarge

The Figure 2 graph shows the load versus deformation of tablet coating adhesive force of four tablets.

Results obtained from 4 tablet samples produce a typical mean maximum adhesion force value shown below:

Adhesive force
(± S.D.)
(g)
300.1 ± 49.6

Observations

When the trigger force of 450 g is attained, the upper fixture proceeds to apply a load force on the tablet of 1500 g. At this stage, the tablet is compressed into both cavities. This is followed by a quick withdrawal (10mm/s) of the upper fixture, either removing the upper coating or leaving the lower coating on the base table. The “tablet coating adhesion force” then becomes the peak negative force required to separate the tablet from its coating.

Note: It is possible for the tablet itself to be pulled apart, rather than the removal of its coating, indicating that adhesive forces between coating and tablet are stronger than the tablet’s particle cohesive forces.