problems in tablet manufacturing

1 Pharmaceutics

Tablet Mfg Sugar Coating Film CoatingBinding Chipping of coating PickingsSticking Cracking Orange peel( Roughness)Picking Twinning Edge wear- ChippingFilming Mottling-Uneven color Film CrackingCapping Blooming IntagliationsLamination Sweating Film peelingChipping Marbling In filling of logosCracking BlisteringWeigh variation Hazing dull/ dull film

BlushingCrateringPitting

Problems in Tablet Manufacturing and Tablet Coating

Parag Mehta, Final yr B.Pharm, MET’s Institute of Pharmacy, Bhujbal Knowledge City.

2 Pharmaceutics

Sugar Coating

1] CHIPPING OF COATINGS: Sugar coating are inherently brittle and thus prone to chipping if mishandled. Addition of small quantities of polymers(such as cellulosic, polyvinyl pyrrolidone, acacia,

or gelatin) to one or more of the various coating formulations often help to improve structural integrity, and thus reduces chipping problems.

Excessive use of insoluble fillers and pigments tends to increase the brittleness of sugar coatings, and thus should be avoided where possible.

2] CRACKING OF THE COATING: Tablet cores that expand, either during or after coating, are likely to cause the coating to

crack. Such expansion may result from moisture absorption by the tablet core, or may be

caused by stress relaxation of the core after compaction (a phenomenon which is known to occur, for example, with ibuprofen).

Moisture absorption can be minimized by appropriate use of a seal coat, whereas expansion due to post compaction stress relaxation can be resolved by extending the time between the compaction event and commencement of sugar coating.

3] NONDRYING COATINGS: Inability to dry sugar coating properly special those based on sucrose is often an

indicator that excessive levels greater than 5% of invert of invert sugar is present. Inversion of sucrose is exacerbated by keeping sucrose syrups at elevated temperatures

under acidic conditions for extended period of time. Such conditions occur when sugar coating solutions containing aluminum lakes are kept

hot for too long or such sugar coating formulations are constantly being reheated to redissolve sugar that is beginning to crystallize out

4] TWINNING (OR BUILD UP OF MUTIPLES) . Sugar coating formulations are very sticky, particularly as they begin to dry and allow

adjacent tablets to stick together. Buildup of multiples really becomes a problem when the tablets being coated have falt

surfaces which can easily come into contact with one another. This can be particularly troublesome with high dose, capsule shaped tablets that have

high edge walls. Appropriate choice in tablet punch design can be effectively used to minimize the

problem.


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5] UNEVEN COLOUR.:Because it has a major impact on final tablet appearance, the color coating stage of the sugar coating process is critical to ultimate tablet quality. Uneven distribution of color, particularly with darker colors, is often visually apparent, and thus a major cause of batch rejection. Many factors may contribute to this type of problem, including:

Poor distribution of coating liquids during application. This may be caused by poor mixing of tablets in the coating process, failure to add sufficient liquid to coat completely the surface of every tablet in the batch.

Color migration of water soluble dyes while the coating is drying. Unevenness of the surface of the sub coat when using dye colored coatings. This

unevenness causes a variation in thickness of the transparent color layer that is perceived as different color intensities.

“Washing back” of pigment colored color coatings. While pigments do not migrate on drying, if excessive quantities of coating liquid are applied during the coloring process, there is a tendency for the previously applied(and dried) color layers to be redissolved and distributed non uniformly; thus giving rise to no uniform appearance. This problem is particularly noticeable for formulations predominantly colored with aluminum lakes where the level of pacifying pigments (such as titanium dioxide) is low.

Excessive drying between color applications. This can cause erosion of color layer and contributes to unevenness in color coat.

Sr. No.

CAUSE REMEDY

1.Improper mixing, uneven spray pattern, insufficient coating, migration of soluble dyes-plasticizers and other additives during drying.

Go for geometric mixing, reformulation with different plasticizers and additives or use mild drying conditions.

6] “BLOOMING AND SWEATING”: Residual moisture (in finished sugar coated tablets) can often be a problem. Over a

period of time, this moisture can diffuse out and affect the quality of product. Moderate levels of moisture egress cause the polish of product to take on a fogged appearance, a phenomenon often termed as blooming.

At higher levels the moisture may appear like beads of perspiration on the tablet surface. This second phenomenon, often called sweating, can be much more serious, since tablets stored in closed containers will ultimately stick together.

7] “MARBLING”: One of the secrets of achieving a high quality, sugar coated product is to assure that

color is uniformly distributed in the color layer, and at the same time at the end of the application of the color coating that a smooth coating surface is obtained.


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Failure to achieve the requisite smoothness often results in marbled appearance on polishing. This problem occurs as a result of collection of wax in the small surface depressions of a rough coating and is particularly evident with darker colors.

FILM COATED TABLETS1] PICKING:Picking results when, for example the coating on two adjacent tablets is not sufficiently dry before contact between them occurs. Because the partially dried coating can be extremely tacky, once the two tablet make contact they adhere to one another only to break apart later (under the influence of attrition ) once the coating has dried. In extreme cases, tablets with flat faces or flat edges (as with capsule shaped tablets that have thick side walls) may become permanently glued together so that twinning (or the build up of multiples) occurs.

Certain types of coating formulations (e.g. those based on hydroxypropylcellulose, many of the enteric-coating formulations, and several of the acrylic aqueous latex-coating system) are inherentely tackier during application, and are thus more likely to create the conditions under which picking occurs.

Sr.No.

CAUSE REMEDY

1. Inefficient drying. Use optimum and efficient drying conditions or increase the inlet air temperature.

2. Higher rate of application of coating solution

Decrease the rater of application of coating solution by increasing viscosity of coating solution.

2] “Orange peel” (Roughness): For a successful film coating process, it is critical that the droplets of coating liquid dry

(at least to the point of being tack-free) very soon after they make contact with the surface of the product being coated.

Consequently, almost all film-coated tablets exhibit a characteristic known as orange peel (so-called because of a resemblance to skin of an orange). Optimized coating processes will allow this characteristic to keep to a minimum such that it is not readily visible to the naked eye.

Certain process conditions, however, are likely to cause this inherent roughness to become visible. Such process conditions include low spray rates coupled with excessive drying conditions (high processing temperatures and air flows) and use of excessive atomizing air pressures (for air sprays system) which accentuate premature drying of the droplets of coating liquid.

The problem may also be compounded by attempting to spray coating liquids with excessively high viscosities such that optimal atomization is difficult to achieve.


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Sr. No. CAUSES REMEDIES

1. Rapid Drying Use mild drying conditions

2. High solution viscosity Use additional solvents to decrease viscosity of solution.

3] Edge Wear (chipping)Since Tablet edges are often exposed most to the attritional effects, fracture at this point is common and results in parts of the tablet surface being exposed. Cause of this problem includes:

Tablets core having high friability values. Worn tablet punches (that produce “flashing” on the tablet edges). Minor lamination problems (with the tablet cores) that exacerbate edge erosion

problems. Brittle film coatings that offer insufficient protection to tablet edges.

4] Film Cracking\Splitting: Cracking of film coatings occur when the internal stress (that develops within the

coating on drying) exceeds the tensile strength of that coating. These problems are totally unacceptable for products where the applied coating is a

major factor in modifying drug-release characteristic. While cohesive failure is often associated with brittleness of the coating, the problem is

certainly exacerbated by thermal expansion effect particularly when significant differences exist between the thermal expansion coefficients for core and coating respectively.

5] Bridging of Logos (Intagliations): Internal stress is also a major causative factor in logo bridging. This phenomenon occurs when a competent of the internal stress becomes sufficiently

high so as to cause partial or complete detachment of the coating (from the substrate) in the region of the logo.

As a result of detachment, the film is able to “shorten” and thus partially relieve the stress within the film. In doing so, legibility of the logo can be significantly reduced. Typically this type of problem becomes progressively worse as more coating is gradually applied during the process.

Solutions to bridging involve improving film adhesion and/or reducing stress within the film, and usually require some reformation of either the tablet core or the coating. In addition, appropriate design of tablet punches (especially with respect to the logo) may help to alleviate the problem, whereas adjustment of process conditions may prove beneficial.

6] Film peeling:On occasion (particularly during application of aqueous coating formulations), if cohesive failure of the coating occurs, that coating may subsequently peel back from the surface of the substrate. While both cohesive and adhesive failures are implicated here, appropriate solutions


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typically involve addressing the initial cracking problem by increasing the mechanical strength of the coating.

7] In filling of logos: While visually similar to bridging, in filling of logos typically occurs during the spray

application of aerated aqueous film –coating solutions. When a foaming coating solution impinges on a regular part of the tablet surface it will,

under the shear forces generated, form a film with “normal “characteristics. However those droplets of coating liquid that reside in the logo , being protected from

the shear forces at the surface , gradually dry to form a solid foam that eventually obliterates the legend .

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CAUSE REMEDY

1. Bubble or foam formation because of air spraying of a polymer solution

Add alcohol or use spray nozzle capable of finer atomization.

8] Cratering:Definition: It is defect of film coating whereby volcanic-like craters appears exposing the tablet surface.Reason: The coating solution penetrates the surface of the tablet, often at the crown where the surface is more porous, causing localized disintegration of the core and disruption of the coating.

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CAUSES REMEDIES

1. Inefficient drying.

Use efficient and optimum drying conditions.

2. Higher rate of application of coating solution.

Increase viscosity of coating solution to decrease spray application rate.

9] Blistering:When coated tablets require further drying in ovens, too rapid evaporation of the solvent from the core and the effect of high temperature on the strength. elasticity and adhesion of the film may result in blistering.

Sr. No. CAUSE REMEDY


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1. Effect of temperature on the strength, elasticity and adhesion of the film.

Use mild drying condition.

10] Blushing:Definition: It is defect best described as whitish specks or haziness in the film.Reason: It is thought to be due to precipitated polymer exacerbated by the use of high coating temperature at or above the thermal gelation temperature of the polymers.

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CAUSES REMEDIES

1. High coating temperature Decrease the drying air temperature

2.

Use of sorbitol in formulation which causes largest fall in the thermal gelation temperature of Hydroxy Propyl Cellulose, Hydroxy Propyl Methyl Cellulose, Methyl Cellulose and Cellulose ethers.

Avoid use of sorbitol with Hydroxy Propyl Cellulose, Hydroxy Propyl Methyl Cellulose, Methyl Cellulose and Cellulose ethers.

11] Hazing/Dull film: This is sometimes called bloom. It can occur when too high a processing temperature is used. Dulling is particularly evident when cellulosic polymers are applied out of aq media at

high temperatures. It can also occur if the tablets are exposed to high humidity conditions and partial

solvation of film results.

12] Pitting:Definition: It is defect whereby pits occur in the surface of a tablet core without any visible disruption of the film coating.Reason: Temperature of the tablet core is greater than the melting point of the materials used in the tablet formulation.

Sr. No.

CAUSE REMEDY

1.Inappropriate drying (inlet air ) temperature

Dispensing with preheating procedures at the initiation of coating and modifying the drying (inlet air) temperature such that the temperature of the tablet core is not greater than the melting point of the batch of additives used.


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