Tablet Coating Problems and Solutions in Pharmaceuticals

Tablet Coating Problems and Solutions in Pharmaceuticals.

There are various problems that occur in the coating of pharmaceutical tablets. In this article, we will discuss some of these problems and their possible solutions to these problems.

Poor Tablet Design (e.g. fine embossing, flat surfaces)
Physical Damage (e.g. overloading the coater, loading/ unloading operations)
Over-wetting - The suspension droplets hit the tablet surface whilst still wet and the drying air does not dry quickly enough. This can lead to surface pitting.
Spray Drying - The suspension droplets hit the tablet surface after the moisture has been removed. This can lead to a lack of / poor adherence to the coat.

Picking/Sticking

Possible Causes and Solutions

Spray rate too high - Reduce spray rate
Inadequate drying conditions - Improve drying conditions
Pan speed too low - Increase the pan speed
Inadequate atomization of coating - Increase atomizing air pressure/volume liquid
Poor distribution of coating liquid - Select formulation for improved adhesion characteristics

Tablet-to-Tablet Color Variation

Possible Causes and Solutions

Too little coating applied - Increase the quantity of coating applied
Inadequate mixing of tablets during coating - Increase the pan speed and/or improve baffle system
Poor opacity (or hiding power) of coating - Reformulate coatings to improve opacity or use an opacified white pre-coat (for coloured tablet cores)
Solids content of coating liquid is too high - Reduce solids content of coating liquid
Insufficient number of spray guns - Increase number of spray guns
Poor spray pattern bed distribution - Ensure gun position is correct and provides optimum bed coverage
Pan speed too low - Increase the pan speed

Twinning

Possible Causes and Solutions

Spray rate too high - Reduce spray rate and/or increase efficiency atomizing
Pan speed too low - Increase pan speed
Inappropriate tablet shape - Select new tablet shape that minimizes chances of flat surfaces coming into contact during application of coating liquid (e.g. avoid capsule-shaped tablets with straight edges or thick side walls)
Tacky coating formulation - Reformulate the suspension.
Spray guns too close to tablet bed - Increase gun to tablet bed distance

Cracking

Possible Causes and Solutions

Lower mechanical strength of coating, inadequate plasticizer or pigmentation - Select formulation with improved mechanical strength characteristics.
Core has significantly different thermal expansion characteristics than coating - Avoid use of mineral-type fillers (e.g. carbonate, calcium sulphate, carbonate) where possible.
Extended elastic recovery of core after submitting them to compaction - Extend holding period for tablets prior to coating process

Peeling

Possible Causes and Solutions

Low mechanical strength of coating - Select formulation with improved strength
Poor adhesion of coating to tablet surface - Select formulation for improved adhesion characteristics
Excess lubricant usage in formulation - Use an excipient with good lubrication properties like STARCH 1500

Orange Peel Roughness

Possible Causes and Solutions

Viscosity of coating liquid too high - Reduce solids content of coating liquid /viscosity
Poor atomisation of coating liquid - Increase atomizing air pressure / volume (n.b. excessive atomisation may exacerbate spray drying)

Logo Bridging

Possible Causes and Solutions

Inadequate adhesion of the film coating - Select formulation for improved adhesion characteristics
Surface characteristics of the product - Modify core formulation to include more coated (e.g. hydrophobic substrate) hydrophilic ingredients (where possible)
Inappropriate design of logo(e.g. too detailed or too fine) - Select a different logo design (with increased area within the debussing and modified angles)
Insufficient plasticiser in film / high internal stress - Reduce spray rate / increase drying rate

Logo Infilling

Possible Causes and Solutions

Inappropriate design of logo (e.g. too detailed or too fine) - Select a different logo design (with increased area within the debussing and modified angles)
Logo "disappearance" can be due to erosion of tablet surface around logo of tablet - Reduce erosion potential by reformulating core, changing logo design or modifying curvature of faces
Logo Bridging - See solutions for Logo Bridging
In-filling of logo with spray-dried coating material - Reduce spray-drying potential by: - increasing spray rate - reducing inlet air temperature / air flow - reducing atomising air pressure - reducing distances between spray guns and surface of tablet bed

Core Erosion

Possible Causes and Solutions

Inherent softness or high friability - Improve mechanical strength of core by increasing of core compaction force, modifying core formulation or changing process by which core is produced (e.g. use granulation process instead of direct compaction
Excessive pan speed in coating process - Reduce the pan speed
Spray rate too low - Increase the spray rate
Low solids content of spray - Select formulation with high solids solution content
Premature swelling of hydrophilic disintegrant in formulation - Replace super disintegrant with less hydrophilic super disintegrant like STARCH 1500

Edge Chipping/Erosion

Possible Causes and Solutions

Low mechanical strength of coating - Select formulation with improved mechanical strength
Excessive pan speed - Decrease pan speed
Low solids content in coating liquid - Increase solids content of coating liquid
Low spray rate - Increase spray rate
Sharp edges on tablets - Replace or use modified punch design (use dual radius punch tips)
Worn tablet punches - Refurbish or replace punches
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).