Problems and Remedies for Tablet Coating

During the coating, process tablets pass through many stages during those stages tablets face many problems so for avoiding these problems we use a few techniques to solve this type of coating problem, in this article we do a detailed discussion on tablet coating problems and remedies for their solution.

List of Tablet Coating Defects and Remedies

1. Blistering
2. Blooming/Dull film
3. Blushing
4. Chipping/ Edge Erosion
5. Cratering
6. Cracking/Splitting
7. Color variation
8. Core Erosion/ Surface Erosion
9. Discoloration
10. Logo Bridging
11. Logo In-filling /Break Lines In-filling
12. Orange Peel (Roughness)
13. Pitting
14. Peeling
15. Sticking and Picking
16. Scuffing of Film-coated Tablets
17. Twinning
18. Tablet Breakage

Details discussion on the tablet coating problems and remedies:

BLISTERING 

It is local detachment of film from the substrate forming blister. 

Reason: 

Entrapment of gases in or underneath the film due to overheating either during spraying or at the end of the coating run. 

Causes and Remedies:

Causes		Remedies
1.	Entrapment of gases or vapor in or under the film due to overheating either during spraying or at the end of the coating.	Use mild drying conditions.
2.	Effect of temperature on the adhesion, elasticity, and strength of the film.	Use mild drying conditions.

BLOOMING / DULL FILM 

It is a coating defect where the coating becomes dull immediately or after prolonged storage at high temperatures. In other words, it is the fading or dulling of a coated tablet color after a prolonged period of storage at a high temperature.

Reason: 

To high processing temperature low molecular weight of Plasticizer.

Causes and Remedies:

Causes		Remedies
1.	High concentration of plasticizer.	Use lower concentration of plasticizer.
2.	Low molecular plasticized coating system.	Use high molecular weight of plasticizer.

BLUSHING 

It is a coating defect where whitish specks or haziness in the film appear. In other words, it is a haziness or appearance of white specks in the film coated tablet.

Reason:  Excessively high coating temperature.

Causes and Remedies:

Causes		Remedies
1.	Precipitation of coating polymer due to use of a poor solvent or high coating temperature.	Decrease the drying air temperature.
2.	Use of sorbitol in coating formulation which causes major fall in the thermal gelation temperature of the HPMC (Hydroxy Propyl Methyl Cellulose), HPC (Hydroxy Propyl Cellulose), Cellulose ethers, and Methyl Cellulose.	Avoid the use of sorbitol with HPMC (Hydroxy Propyl Methyl Cellulose), HPC (Hydroxy Propyl Cellulose), Cellulose ethers, and Methyl Cellulose.

CHIPPING / EDGE EROSION

It is a coating defect where the film becomes chipped or worn away and dented usually at the edges of the tablet as the coating is being applied.

Causes and Remedies:

Causes		Remedies
1.	Excessive pan speed	Decrease pan speed
2.	Low solids content in coating liquid	Increase solids content of coating liquid
3.	Worn tablet punches	Refurbish or replace punches
4.	Low tablet hardness/friability	Improve mechanical strength of core by increasing compaction force, changing tablet shape, modifying core formulation or changing process by which core is produced (e.g. use granulation process instead of direct compaction)
5.	Pan loading with insufficient amount of core tablets.	Pan fill with a sufficient amount of core tablets.
6.	Inappropriate baffles design.	Use appropriate baffles design
7.	Worn tablet punch/tooling.	Replace tablet tooling.
8.	Sharp edge on tablets.	Use the proper tablet shape.
9	Low spray rate.	Use an optimum spray rate.
10.	Low-coating suspension solid.	Increase suspension solid level.
11.	High degree of attrition related to the coating process.	Increase the hardness of the film by increasing the molecular weight of the coating polymer.

CRATERING

It is a defect of film coating whereby volcanic-like craters appear 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.  

Causes and Remedies:

Causes		Remedies
1.	Penetration of coating solution at the surface of the tablet, often at the crown where the surface is more porous causing localized disintegration of the core.	Decrease the spray rate and use optimum drying conditions.
2.	Inefficient drying.	Use efficient and optimum drying conditions.
3.	Application of higher rate of the coating solution.	Decrease spray application rate.
4.	Low the viscosity of the coating solution.	Increase viscosity of coating solution to decrease spray application rate.

 

CRACKING / SPLITTING

It is defect in which the film either cracks across the crown of the tablet (cracking) or splits around the edges of the tablet (Splitting). 

Reason: 

Internal stress in the film exceeds the tensile strength of the film.

Causes and Remedies:

Causes		Remedies
1.	Higher internal stresses in the film than the tensile strength of the film.	Adjusting the type and concentration of plasticizer.
2.	High molecular weight polymeric blends or polymers.	Use lower molecular weight polymeric blends or polymers.
3.	Coating and core have different thermal expansion properties.	Avoid mineral type excipient.
4.	Expansion of core due to overheating.	Avoid overheating the tablet core.
5.	Low mechanical strength of the coating.	Use coating with proper mechanical strength.
6.	Inadequate coating formulation.	Use proper coating formulation.
7.	Insufficient plasticization or too much pigmentation.	Use adequate plasticizer or pigment.

COLOUR VARIATION

A defect that involves variation in color of the film.  

Reason: 

Alteration of the frequency and duration of appearance of tablets in the spray zone or the size/shape of the spray zone.

Causes and Remedies:

CORE EROSION / SURFACE EROSION

It is a coating defect where tablet surfaces erode as tablets tumble during the coating process. 

Reason:

Especially this problem has happened with the presence of inappropriate logo.

Causes and Remedies:

Causes		Remedies
1.	Hygroscopic tablet core.	Reduce the amount of hygroscopic excipient such as disintegrant.
2.	Spray rate is too high or low.	Use the appropriate spray rate.
3.	Pan speed too high.	Use the correct pan speed.
4.	High friability of core tablet.	Use less friable tablet.
5.	Insufficient film strength to provide resistance to edge damage.	Use high film strength.
6.	Poor punch design.	Use high-quality punch.
7.	Punch wear.	Use a non-wear punch.
8.	Poor logo design or placement.	Use proper logo design.
9.	Low-coating suspension solid.	Increase suspension solid level.

DISCOLORATION

It is a rare coating defect where discoloration is appeared either through or on the coating caused by interactions of ingredients in the core or by heat from the process causing ingredients in the core to migrate through the coating. The problem is commonly seen with nutraceutical products.

Causes and Remedies:

LOGO BRIDGING

• This occurs when the coating fills in the lettering or logo on the tablet and is typically caused by improper application of the solution, poor design of the tablet embossing, high coating viscosity, high percentage of solids in the solution, or improper atomization pressure. 
• During drying, the film may shrink and pull away from the sharp corners of an intagliation or bisect, resulting in a bridging of the surface. 
• This defect can be so severe that the monogram or bisect is completely obscured. 

Causes and Remedies:

Causes		Remedies
1.	Poorly plasticized coating system.	Use optimal plasticizer coating.
2.	Low adhesion coating system.	Use high-adhesion coating.
3.	Low adhesion core ingredients.	Use high-adhesion core ingredients.
4.	Spray rate too high.	Reduce spray rate.
5.	Product temperature too low.	Increase temperature.
6.	Poor logo design.	Improve logo design.

LOGO IN-FILLING / BREAK LINES IN-FILLING

It is a coating defect where the engraved logo or break line filled with either particle of dried polymer or solidified foam. Coating droplets are over-dried before contacting tablet surface, increasing tablet surface roughness and filling the logo.

Causes and Remedies:

Causes		Remedies
1.	High atomizing air pressure.	Reduce atomizing air pressure.
2.	High drying air temperature.	Reduce drying air temperature.
3.	High coating suspension solid.	Use a low viscosity coating suspension.
4.	Too high gun to bed distance.	Optimize gun to bed distance.
5.	Poor spray gun design.	Use proper spray gun design
6.	Turbulent air flow.	Minimize pan depression.
7.	Aeration of coating suspension.	Use optimized suspension preparation to avoid aeration.
8.	Poor logo design.	Improve logo design.
9.	Excessive foaming of the coating solution.	Improve the mixing process.
10.	Foam and Bubble formation because of air spraying of a polymer solution.	Add alcohol or use spray nozzle capable of finer atomization.
11.	Over drying of spray.	Optimum drying of spray.

ORANGE PEEL/ROUGHNESS 

It is surface defect resulting in the film being rough and nonglossy. Appearance is similar to that of an orange. 

Reason: Inadequate spreading of the coating solution before drying.

Causes and Remedies:

Causes		Remedies
1.	Rapid Drying.	Use mild drying conditions.
2.	Too high coating suspension viscosity.	Reduce the viscosity coating suspension.
3.	Too low atomizing air pressure.	Increase Atomizing air pressure.
4.	High spay rate/ Over-wetting.	Decrease spray rate.
5.	Poor spray gun performance.	Use better spray guns.
6.	Inadequate distribution of the coating suspension.	Decrease in viscosity of coating suspension.
7.	Very close gun to bed distance.	Adjust gun to bed distance.

PITTING 

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.

Causes and Remedies:

Causes		Remedies
1	Inappropriate drying (inlet air) temperature.	Use appropriate drying (inlet air) temperature.
2	Melting or dissolution of lubrication on tablet surface or melting of core ingredient due to high core temperature.	Use control drying temperatures

PEELING

It is a coating defect where peels off the tablet during or after the coating process. Peeling refers to the formation of uneven or rough irregularities on the surface of a coating film applied on a core tablet surface.

Causes and Remedies:

Causes		Remedies
1.	Low adhesion core ingredients.	Use high-adhesion core ingredients.
2.	Low mechanical strength of the coating.	Use proper composition with improved mechanical strength
3.	Low adhesion coating system.	Use high adhesion coating.
4.	Coating is rubbing off due to excessive attritional effects.	Reduce pan speed.

STICKING & PICKING

It is defect where isolated areas of film are pulled away from the surface when the tablet sticks together and then part. 

Reason: 

Conditions similar to cratering that produces an overly wet tablet bed where adjacent tablets can stick together and then break apart. 

Causes and Remedies:

Causes		Remedies
1.	Inadequate drying.	Use optimum drying conditions.
2.	High viscosity of coating solution.	Increasing viscosity of coating solution.
3.	Spray rate too high.	Reduce spray rate.
4.	Too low drying air volume	Increase air volume.
5.	Too low drying air temperature.	Increase the inlet air temperature.
6.	Pan speed too low.	Increase pan speed.
7.	Low atomization pressure.	Increase atomization pressure.
8.	Poor spray gun set up.	Improve spray gun setting.
9.	Surface solubility.	Increase gun to bed distance.
10.	High tablet surface porosity.	Improve core characteristics.

SCUFFING 

It is a coating defect in which gray to black marks on the surface of white (or lightly colored) film-coated tablets appear. 

Scuffing is appeared when tablets are more sliding rather than trembling.

Causes and Remedies:

TWINNING

It is a coating defect where tablets stick together especially when two tablets stick together.

Causes and Remedies:

Causes		Remedies
1.	Commonly flat surfaces along the edges of capsule-shaped tablets.	Change tablet shape.
2.	Excessively tacky coating formulation.	Use coating formulation which is less tacky.
3.	Inappropriate tablet shape.	Use appropriate tablet shape.
4.	Pan speed too low.	Increase pan speed.
5.	Spray droplet size too large.	Increase atomizing air pressure.
6.	Spray rate too high.	Reduce spray rate.
7.	Insufficient drying.	Use optimum drying conditions.

TABLET BREAKAGE

It a defect where tablets break apart during pan loading or while tumbling in the pan or during the coating process, or during unloading of the coating pan.

Causes and Remedies:

Causes		Remedies
1.	Core tablets are too soft.	Change core formulation.
2.	Tablets are too brittle.	Change core compression parameters.
3.	Poor tablet shape for coating.	Use the appropriate tablet shape.
4.	Poor baffle design.	Change baffle design.
5.	Too high pan speed.	Reduce pan speed.
6.	Loading and unloading tablets from the coating pan.	Carefully load and unload tablets.
7.	Less binder used in the formula.	Use optimum binder quantity.

ALSO READ: What is Maximum Compression Force?