Tablet Coating problems and there solutions

1. Blistering:

Blistering is detachment of film from the substrate and it forms blister.

 

Possible reasons:

Overheating, either during spraying or at the end of the coating run, this overheating affects elasticity strength and adhesion of the film. 

Solutions:

Milder the drying conditions.

2. Chipping:

Film becomes chipped and dented, usually at the edges of the tablet.

 

Possible reasons:

This is because the too much attrition during coating process.

Solutions:

• Decrease speed of rotation of the drum or fluidizing air in pan.
• Increase film hardness by increasing the molecular grade of polymer.

3. Cracking/Splitting:

When film cracks across the crown of the tablets is called cracking. When film splits around the edges of the tablet is called Splitting. It happens when internal stress of the film is more than tensile strength of the film.

Possible reasons:

• Use of high molecular weight polymers or polymeric blends.
• Insufficient or improper concentrations of plasticizer or pigment.
• Different thermal expansion characteristics of core and coating.

Solutions:

• Select and adjusting proper plasticizer or Pigment type and concentration.
• Use lower molecular weight polymers or polymeric blends.
• Avoid mineral types of fillers.

4. Capping:

When the upper or lower part of the tablet separates, either partially or completely from the tablet and comes off as a cap, during ejection from the tablet press, or during subsequent handling. It happens due to the air- entrapment in a compact during compression process or because of tablet expansion during ejection of a tablet from a die.

Possible reasons:

• Low moisture content.
• Inadequate drying of granules.
• Too much fines in the granulation.
• Inadequate amount or not proper type of binder.
• Inadequate amount or not proper type of Lubricant.
• High speed of turret
• Seep –off blade not adjusted properly.
• Improper setting of lower punch.

Solutions:

• Increase moisture content of granules by adding hygroscopic martial like PEG, sorbitol etc.
• Ensure proper drying of granules
• Remove excess fines from granules
• Adjust adequate amount or change type of binder.
• Adjust adequate amount or change type of Lubricant.
• Adjust speed of turret.
• Adjust Seep –off blade properly.
• Ensure proper setting of lower punch.

5. Picking and Sticking:

When coating removes a piece of the tablet from the core, it happens sometime when tablets sticks together.

Possible reasons:

• Inefficient drying.
• Over wetting, due to high spray rate of coating solution or too low pan speed.
• Improper distribution of coating liquid.

Solutions:

• Improve drying conditions.
• Adjust proper spray rate and pan speed to avoid over wetting. 
• Adjust proper viscosity of coating solution to ensure proper distribution of coating solution.

6. Color Variation:

It involves variation of color of film.

Possible reasons:

• Inadequate coating
• Improper mixing during coating
• Poor spray pattern
• Migration of dyes during drying.

Solutions:

• Adjust and ensure proper coating.
• Increase pan speed to ensure proper mixing.
• Adjust gun position and ensure optimum number of guns to cover the tablet bed properly.
• Adjust plasticizers type and concentration, reduce drying temperature.

7. Mottling:

Uneven distribution of color on a tablet, with light or dark spots.

 

Possible reasons:

• Inadequate mixing of colored binder solution.
• Migration of dye to the granule surface during drying.
• When drug substance is colored and Excipients are white or colorless.

Solutions:

• Ensure proper mixing of colorant and adhesives.
• Reduce drying temperature, change binder or solvent system.
• Use suitable colorants

8. Twining:

When two tablets sticks tighter. It usually happens with capsule shaped tablets.

 

Possible reasons:

• High spray rate and low pan speed.
• Low distance between spray guns and tablets bed.
• Inappropriate shape of tablets.
• Over lapping of spray
• Low Bed/Exhaust temperature 

Solutions:

• Reduce spray rate and increase pan speed.
• Increase distance between spray guns and tablets bed.
• Slight modification in design of tooling.

Some Common Tablet Coating Problems & Solutions in Pharmaceuticals:

There are various problems in pharmaceutical tablet coatings we are discussing some of them here and providing their possible solutions.

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 lack of / poor adherence of the coat.

TABLET-TO-TABLET COLOR VARIATION

Possible Causes and Solutions:

Too little coating applied - Increase quantity of coating applied
Inadequate mixing of tablets during coating – Increase 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 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:

Low 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 atomization of coating liquid – Increase atomizing air pressure / volume (n.b. excessive atomization 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 plasticizer in film / high internal stress - Reduce spray rate / increase drying rate.

PICKING / STICKING

Possible Causes and Solutions:

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

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 atomizing 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 pan speed

Spray rate too low - Increase 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)