In-filling of Logos/Break Lines

Infilling of logos can be caused by many factors. One potential cause is the application of a foamy coating solution with high air entrapment. Any foamy coating solution that deposits on the exposed surfaces of the tablet will tend to collapse because of the shear stresses that develop as the tablets slide over one another. However, coating that deposits inside the logo (intagliation) will not be subjected to such stresses, and thus will remain in a foamy state, causing coating material to build up within the logo as shown in the figures below, leading to filling in of the logo.

Setting up the spray guns correctly is extremely important, if issues like poor coating uniformity, localized overwetting and spray drying are to be minimized. The following figure gives some guidelines as to how spray guns should be set up in a typical film-coating process to achieve optimal spray coverage.

Correct operation of spray guns is critical to the success of a film-coating operation. Proper maintenance of spray gun parts that can easily be damaged during gun breakdown, cleaning, and reassembly is very important. The photograph shown below provides details of parts that can be easily damaged.

Lubricants ease the ejection of tablets from the die after compaction, and minimize adhesion of the tablet surfaces to punch faces. Each of these benefits requires a lubricant to be effective at the surface of the tablet. Because many lubricants are hydrophobic (such as magnesium stearate) this surface functionality can compromise film-coating adhesion, as shown in the table below. Alternative lubricants, such as stearic acid, can help to minimize this problem.

On the other hand, careful selection of tabletting excipients can influence film-coating adhesion to tablet surfaces, as shown in the following table.

The bonding of a film coating to a tablet surface is primarily related to the positive interaction of the applied coating with the tablet surface. Among many factors influencing this interaction is the potential to increase the surface area of interaction. Tablet porosity, which allows the applied coating to penetrate into the surface of the tablets, is one factor that can increase this contact area, and its positive impact on subsequent adhesion of the film coat is highlighted in the table shown below.

The correct formulation of the film coating can have a significant effect on film adhesion and thus the ability to minimize problems, such as logo bridging and other adhesion-related problems. The positive impact of careful film coating selection on maximizing coating adhesion and minimizing problems such as logo bridging is illustrated below.

In dealing with logo-bridging problems, adjusting coating process conditions often has limited impact. Nonetheless, the potential exists for adjusting process conditions to minimize logo-bridging problems. For example, an increase in product temperature can have a positive impact. Another process adjustment is reducing spray rate, as shown in the graph below.

Problems such as edge chipping and tablet-surface erosion problems can often be minimized by optimizing the tablet core shape. For example, minimizing the sharpness of tablet edges can also minimize the risk of edge chipping. Such a positive impact of tablet design can often be achieved by employing compound radius punches rather than the standard curvature punches, as shown in the following figure.

Formulating robust tablets is critical to minimizing defects that arise during film-coating application. Tablet breaking force (or tensile strength) must be maximized, while minimizing tablet friability. Using binders that deform plastically, such as hydroxpropylcellulose (HPC), can be very beneficial, as shown below.

Not only does HPC increase tablet breaking force, but also it reduces tablet friability. In both of these examples, all binders except HPC were incapable of allowing the metformin tablets to be compacted above 10 kN because of capping issues.

A film coating can offer increased protection for the tablet core. Potential examples include reduced surface erosion and edge-chipping problems. The challenge, however, is to get the coating on fast enough, especially during the early stages of the coating process. Process thermodynamics is often a limiting factor that dictates how fast the coating can be applied. Certainly, coating that can be applied at high-solids loading can facilitate faster coating deposition.

In terms of effective process conditions, preheating tablets to a higher temperature can lead to faster spray rates, at least initially, as indicated in the diagram below.

Tablet shape can be a factor in tablet-surface erosion during film-coating application. Curvature of the punch faces can be particularly problematic.

The data shown below highlights how tablet surface hardness (that is, resistance to abrasion) is impacted by punch curvature. Tablets with a high degree of curvature tend to be softer in the center of the tablet face, while those that have a flatter profile, although less prone to tablet-face erosion, are more prone to edge erosion and chipping because the edges tend to be softer.

Incorrect logo placement (intagliation) can increase surface erosion. For example, a centrally placed logo can further weaken the tablet surface in the region of the logo, increasing the risk of surface erosion. Under these circumstances, placement of the logo around the periphery of the tablet surface can alleviate the problem. An example of such logo placement is shown below.

Capping problems are an inherent feature of poorly formulated tablets. Less-severe capping problems often become evident when such tablets are film coated, primarily because of the stressful conditions employed in a typical film-coating process.

Oftentimes, capping problems occur when a tablet formulation with incipient capping issues is overcompressed (see right side of figure below). Under these circumstances, reducing compaction forces may be all that is required to alleviate the problem.

Tablet-color variability after film coating, while primarily related to the amount of coating applied, is also impacted by the hiding power (often defined in terms of contrast ratio) of a colored film coating. The type of pigment used, as well as the level employed in the coating formulation, can be important factors for consideration. The impact of pigment concentration and its effect on the amount of coating required to mask the core (defined as achieving a contrast ratio of > 90%) is highlighted in the table shown below.

Avoiding twinning (that is, the building up of multiples in a coating process where tablets have become stuck together) is often as simple as avoiding flat surfaces. Although it is generally appreciated that one should not try coating flat-faced tablets, it is not always recognized that other flat surfaces may be presented for film coating. For example, capsule-shaped tablets with deep side walls can be very problematic. A simple resolution is to employ capsule shaped tablets with some degree of curvature on the side walls (see photographs below).

Film-coated tablets often exhibit some degree of roughness simply because atomized droplets of coating suspension have to be dried rapidly to avoid overwetting. That said, even allowing for some degree of roughness, elegant film-coated tablets can still be produced.

Although tablet-surface roughness is often linked to process conditions (for example, excessive spray drying) the film-coating formulation can also be a factor. The photographs shown below indicate how an Aquarius™ Preferred HSP film coating system provides smoother results than can be achieved with a PVA-based coating, when both coatings were applied under equivalent conditions.

Many aqueous film-coated tablets contain moisture-sensitive ingredients that can lead to discoloration of the final coated product. One notable active pharmaceutical ingredient in this regard is ranitidine. Multivitamin tablets are problematic because they contain many ingredients with the potential for interaction in the presence of moisture, and the discoloration exhibited can be non-uniform (often called leopard spotting). A common interaction in the presence of moisture is that between iron salts and ascorbic acid. The coating formulation can provide a solution to such problems, especially if it minimizes the amount of moisture penetrating into the core during coating. The photographs shown below indicate how an Aquarius™ Preferred HSP film coating system can give better protection in this regard than a standard HPMC-based coating or even one based on PVA, which is reputed to have good moisture-barrier properties.

In each case, the storage condition was 37°C, 75% RH.

One potential cause of edge wear and edge chipping is badly worn punches. The result is that flashing builds up around the tablet edges, and this can easily break off as tablets rotate in the coating pan.

Examples of tablets produced with worn punches are shown below.

Large tablets that have deep side walls, and have elongated shapes (such as capsule-shaped tablets), can “lock” together (like bricks in a brick wall), causing them to slide en masse, thus increasing the chances of scuffing effects.

The data shown in the graph illustrates how the dissolution behavior of certain APIs (in this case, ibuprofen, which has a relatively low melting point) can be influenced by heat. Such potentially negative behavior needs to be taken into account during tablet core formulation development.

The applied film coating tends to have little influence, per se, on drug dissolution behavior. Sometimes, the nature of the film coating (for example, the type of polymers and plasticizers used, as well as the presence of other additives) can influence drug dissolution behavior. In which case, a change of coating formulation may be warranted, as shown in the graph.

The most common cause of poor dissolution performance after tablet film coating is exposure to common process conditions, in particular the high processing temperatures that are associated with aqueous film-coating processes.

Under these circumstances, employing film-coating systems that allow an aqueous process to be carried out at lower temperatures can be beneficial, as shown in the graph (data are for ibuprofen tablets, and in each case the temperature listed is product temperature).