Tablet Coating – A Comphrensive overview

Tablet Coating -Guide

This article contains comphrensive review on Tablet Coating with Challenges and Solutions

Table of Contents

Tablet Coating overview with Challenges and Solutions

INTRODUCTION :

Tablet coating is a operation in pharmaceutical manufacturing which involves applying a fine layer of a coating material on whole Tablet surface. The coating material can be a polymer, sugar, or cellulose-based substance that provides several benefits, including masking the taste and odor of the drug, protecting the drug from moisture, light and air in addition to that it is controlling the release of the drug in the body.

The tablet coating process involves several steps, including preparing the coating solution, coating the tablets using a coating machine, and drying the tablets to remove any excess moisture. The coating machine applies the coating material onto the tablets by spraying or dipping the tablets into the coating solution.

Tablet coating can be done using different techniques, such as film coating, sugar coating, and enteric coating, each with its advantages and limitations. Film coating is the most commonly used technique in modern pharmaceutical manufacturing because it offers several advantages, such as being easy to apply, providing uniform coating thickness, and allowing for different colors and logos to be printed on the tablets.

Advantages of Tablet Coating Operations :

Improved appearance: Coated tablets have a more attractive appearance, making them easier to distinguish from other medications. Coatings can also be used to add color or logos on tablets.
Better taste and odor: Coatings can mask the unpleasant taste or odor of some medications, making them easier to swallow and improving patient compliance.
Increased stability: Improving product stability by protecting components from atmospheric degradation. This can be extend the shelf life of the medication.
Controlled release: Coatings can be used to control the release of the medication in the body, ensuring that the drug is delivered at the right time and in the right place.
Reduced gastrointestinal irritation: Some medications can cause gastrointestinal irritation. Coatings can be used to protect the stomach and intestines from these medications, reducing the risk of irritation.
Easier to swallow: Coatings can make tablets smoother and easier to swallow, which can be particularly beneficial for patients who have difficulty swallowing tablets.
Packing Productivity Improvement : Facilitating handling, particularly in high speed packing lines.

Coating process has been developed from an art to a more technological advanced and controlled such that compliance with Good Manufacturing Practices is facilitated. The design of new equipment development and new coating materials with the recognition of the impact of applied coating on subsequent release of the drug from the dosage form has contributed to improve products.

Types of Tablet Coating :

The methods for applying coatings to solid dosage form vary according to the purpose of dosage form. These are :

Sugar coating
Film coating
Enteric coating

FILM COATING :

Process of film coating involves application of thin film (20-200 µ) of Polymer based coatings on the tablets under following conditions:

Balance and control between coating liquid spray rate and drying process.
Uniformity of distribution of coating liquid across the surface of product being coated.
Optimization of the visual and functional quality of the final coated product.

Process of Film Coating :

In film coating process, spray atomization technique is used.

In spray application process, bulk coating liquids are finally atomized and delivered in such a state that, droplets of coating liquid retains sufficient fluidity to wet the surface of the product being coated, spread out and coalesce to form a film.

The highly tacky nature of the partially dried droplets causes sticking and picking. So it is always better that the droplets of coating liquid dry almost instantaneously the moment they contact the tablet surface. Due to this an appropriate rate and drying process is needed.

Factors affecting Tablet Coating Process :

Rapid drying will be takes place during application of film coatings so, following factors are to be considered for Tablet coating process :

Uniformity of distribution of coating solution controlled by :

Uniformity of application of coating liquid i.e. the number of spray guns used.
Fineness of atomization of coating liquid.

Uniformity of Tablet bed mixing controlled by :

Pan speed,
Baffle design,
Tablet size and shape.

Advantages of Film Coating :

Polymeric films are finding an ever-increasing range of application in Pharmaceutical Research Development and dosage form design. The advantages of film coating are :

1.2.4.1 Weight rise is less than 3 % of the core tablet weight.

1.2.4.2 Better resistant to chipping.

1.2.4.3 Increased tablet strength.

1.2.4.4 Lesser product cost.

Why Film coating is more preferable than the Sugar coating ?

Process time,
Manpower requirement,
Material,
Cost,
Stability of Product.

What are the Ingredients used in Film Coating ?

Following are some basic ingredients used in the Tablet coating :

TYPE EXAMPLE

1.3.5.1 Film former HPMC E-15, Eudragit

1.3.5.2 Filler Talcum, Shellac

1.3.5.3 Opaquent Titanium dioxide

1.3.5.4 Plasticizer Propylene glycol, Castor oil, PEG-6000

1.3.5.5 Colour Different types of Lakes and Dyes

1.3.5.6 Preservative Methyl paraben, Propyl paraben

1.3.5.7 Vehicle Water, Isopropyl alcohol, Methylene chloride

What is mean by Ready – Mix Coating ?

Ready – Mix is a pre-dispersed powder containing all of the ingredients of the film coating formulation, except the solvent. It contains Polymer, Plasticizer, Opacifier pre-blended in the optimum proportions for good film coat performance.

What are Benefits of Ready – Mix Coating ?

Rapid suspension preparation time – The suspension is ready to spray within 45 minutes.
Single material to Purchase, control, dispense, analysis and storage compared with four and five materials in a traditional system.
Colour uniformity and reproducibility – which are guaranteed by the manufacturer.

What are the key factors for Good Film Coating ?

Tablet Film Coating influenced by following various factors :

Tablet Core Factors.
Coating suspension Factors.
Equipment & Accessories Factors.
Process Factors.

Tablet Core Factors :

High Hardness.
Low Friability.
Good surface adhesion property.
Logo design and position.
Moisture Content.
Condition of Tooling.
Ingredient (Active & Excipient) source & quality.

Coating suspension Factors :

Stirring / Mixing prior to spraying.
Low settling rates.
Good Flowability.
Polymer Hydration / Salvation time.
Low particle size of suspended pigments and opacifier.
Degree of Pigment De-aggregation.
Solid content.
Temperature & Viscosity
Aeration & Storage time.

Equipments & Accessories Factors :

Pan size, shape, baffles and angle.
Number of Spray guns.
Spray nozzle diameter, air cap design.
Drying air system.
Exhaust air system.
A sight negative pressure inside pan.
Stirring mechanism for the Tablet coating suspension.
Good air handling unit for the area.
Flame and explosion proofing where necessary.

Process Factors :

Tablet load in the Pan.
Pan RPM.
Quantity & Temperature of Drying air.
Quantity & Temperature of Exhaust air.
Drying air Temperature variation.
Tablet Bed Temperature profile.
Atomizing air Pressure.
Spray rate of suspension.
Compressed air water content.
Distance & Position of Spray gun.
Position of Bed Temperature probe.
Tablet Bed Temperature.
Suspension stirring speed.
Coating time.
Drying Temperature after coating.
Drying Time after coating.

Process Parameters that can influence coating uniformity include :

Spray Rate.
Coating Pan Speed.
Mixing efficiency in the coating Pan.
Solids Content of the coating Suspension.
Atomization of coating Suspension.
Number of Spray guns used.
Type of Spray Pattern.
Efficiency of the Tablet coating Process.
Quantity of coating Suspension used.

Types of Film Coating :

Depending upon the vehicle used for coating solution, film coating can be categorized as :

Aqueous Coating : Where only Purified water is used as vehicle.
Hydro-alcoholic Coating : Where Purified water & organic solvents/alcohols are used as vehicle.
Non-aqueous Coating : Where only Organic solvents are used as vehicle.

Film coating has historically been carried out using organic solvents, typically mixtures of Methylene chloride and alcohol or alcohol and water. But the disadvantages of these methods are as follows :

Safety is a prime consideration. Organic solvents are toxic by inhalation and in some cases by skin absorption. There is also an explosion or flammability risk with many organic solvents.

Controlling of Exhaust air (containing solvent vapour) is very critical, and it is necessary as per cGMP to avoid problems of environment pollution, global warming etc.

Organic solvents are more expensive than water. Considering cost of provision of explosion-proof handling areas, solvent vapour control to avoid operator contact and solvent vapour elimination from exhaust systems, Aqueous coating is considerably less expensive.

Aqueous film coating has been an available technology within the pharmaceutical industry for about 25 years. In Europe and the US, the majority of tablet film coating has already been transferred to Aqueous Technology. Solvent coating may retained for the occasional old product but is not used for any new product developments.

Advantages of Aqueous Coating :

Improved Safety
Reduce Pollution
Reduce Costs
No residual solvents in Product

Disadvantages of Aqueous Coating :

Longer Process Time
Moisture sensitive products
Inadequate Equipment
Poor quality of core tablet

Solutions for above problems :

Longer Process Time – Simply changing the process conditions, for example increasing the flow rate and temperature of drying air, will reduce Tablet coating time. Use of modern high-solids, low viscosity Tablet coating formulations can provide an easy method of reducing process time; if these process changes alone are insufficient, then it will be necessary to upgrade the Tablet coating equipment.

Select correct drying conditions
Improve existing equipment
Invest in modern equipment
Use correct Tablet Coating formulation

Moisture Sensitive products – Even the most moisture sensitive tablets can be successfully coated with aqueous method. If the correct conditions are established in the Tablet coating process, drying is instantaneous and there is no opportunity for the moisture sensitivity to cause any problem. Some times the available Tablet coating equipment has not been designed specifically for aqueous systems & the drying capability required needs to be improved. Existing equipment can very often be upgraded at minimal expenses to provide the required process conditions.

Not only drying capability of the equipment but the drying rate of the Tablet coating system is equally important. Upgrading will enable existing equipment to be used but the best results will always come from the use of equipment, which has been purpose-designed for aqueous film coating.

Select correct drying conditions
Improve existing equipment
Invest in modern equipment
Use correct Coating formulation

Inadequate Equipment – Inadequacies in the equipment can be overcome by optimizing of process conditions. The usual problems with equipment are insufficient air flow or insufficiently high drying air Temperature. Very often increasing the sizes of fans or enlarging the heater banks can overcome these problems. The best results in Terms of shortest processes and highest finished product quality will always be obtained by the use of purpose-designed equipment.

Optimize Process conditions
Increase Fan size
Invest in modern equipment
Sugar coating in side vented Pans

Poor quality Tablet core – Most core problems are related to friability, giving rise to erosion of the Tablet surface during Tablet coating, which spoils the appearance of the finished product. Simply changing the shape of the compression tooling often provides sufficient improvement. Where changing the tablet shape does not provide sufficient improvement, minor core formulation changes, such as adding small quantities of compressible excipients, reducing the amount of lubrication or using a more compressible disintegrant will normally be successful.

Change of Shape
Minor formulation change

Adhesion between film coating and tablet surface is often reduced when using an aqueous process. In order to compensate for this effect it is important to choose excepients, which offer the higher levels of adhesion, particularly in formulations where the quantity of excepients is high.

The influence of Tablet face curvature on potential erosion problems during film coating can be compared with flat faced or shallow convex tablet and deep convex tablet.

Friability of the core tablet gives rise to erosion or attrition of the surface.

Attrition is related to tablet shape and careful selection of compression Tooling can be a key factor in obtaining successful results.

Ideal Tablet shape for aqueous film coating is, a shallow convex shape, logo at periphery and beveled edge is the most suited shape, the worst shape being a flat-faced, elliptical shape.

What are the factors which affects the Aqueous Coating Process ?

Distribution of coating material.
Continuous mixing.
Continuous drying.
Removal of vapor.
Design of air atomized spray gun.
Tablet bed to gun distance.
Suspension flow rate balance between guns.

Balancing mixing efficiency, attrition and baffle design can optimize continuous mixing in side vented pans with well-designed baffles.

Drying can be controlled by drying air quantity, drying air temperature, exhaust air quantity, atomizing air pressure and gun to bed distance.

The correct balance between Inlet and Exhaust airflow rates is also important. The balancing should be such that there is always a slight negative pressure in the coating chamber. The negative pressure should never be excessive or the coating efficiency will decrease significantly.

Suitable equipment’s for aqueous film coating are :

Conventional pans (Baffles, sufficient drying air),
Side vented pans (Sufficient drying air),
Other machine (modified conventional pans, Fluid bed coaters).

Following are the consideration related to formulation affecting Aqueous Coating :

Since aqueous film coating has problems as slow drying, high solids – high viscosity, high viscosity – high poor quality, high viscosity – process problems, the conventional approach is to reduce the molecular weight of the polymer being used.

The high surface tension of water reduces the adhesion between the film and the tablet surface. Being less volatile, water is retained in the film as it solidifies; this gives rise to a shrinking stress in the coating. The combination of low adhesion and shrinking stress makes aqueous systems much more prone to logo bridging.

In addition water is a poorer solvent for most of the common film coating polymers. This causes a reduction in film strength. In order to understand what is happening, we need to be able to accurately measure the important film parameters of tensile strength, elastic modulus and adhesion.

Utilities in Tablet Coating Area :

Following utilities required for Tablet Coating Operation :

Potable Water
Steam
Compressed Air
Filtered Air

Potable Water : It is used for cleaning of utensils and equipments. It is supplied from overhead tanks on service floor.

Steam : It is used for cleaning of equipment parts and Tablet coating process, which is supplied at ≅ 3.5 kg/sq.cm. and is filtered through 10 µ

Compressed Air : Moisture and oil free compressed air is used for dry cleaning of equipment parts and in Tablet coating process. It is filtered through 1 µ & 0.1 µ .

Filtered Air : Filtered air will be generated through the Air handling system

Air Handling System in Tablet Coating Area :

Air Handling Units (AHU) are provided in Tablet Coating section to :

Keep the area clean by minimizing the dust in area.
Avoid cross contamination by maintaining negative pressure.
Maintain the temperature within required range (normally 19°C to 25°C)

Separate AHU is provided for different areas . Washing area is provided with Ventilation system .

The pressure differential between coating and corridor is maintained in such a way that Tablet coating area are under negative pressure as compare to corridor. The differential pressure limit is between 1.5 to 5.0 mm hg of water gauge.

Air Handling Unit (AHU)

The air supplied to Tablet coating area, inspection area through 10 µ filters and then through 5µ and then 0.3µ filters of AHU. The supply of air in Tablet coating area is from grills mounted on the false ceiling.

The return is through the return risers which are located on walls.

LIST OF EQUIPMENTS REQUIRED FOR TABLET COATING OPERTIONS ARE AS BELOW:

Coating Machine : For coating of Tablets
Stirrer : For preparation of coating suspension
Feed Chamber : For storing the Coating solution during spraying operations
Colloidal Mill : For Milling of some excipients during preparation of Coating solution in case of conventional coating option

Machine Performance :

The new machine put in to use after Installation Qualification, Operation Qualification, Performance Qualification & Safety aspects.

The load trials are conducted after certifying the equipment suitable for use.

The performance of the machine is evaluated using the data from machine log, which is filled for each batch operation.

Machine Operations:

All machines should be operated as per Standard Operating Procedure. The setting and recording of various parameters should be done as per Product SOP.

Machine Cleaning :

The cleaning of machine is done as per standard cleaning procedure. The standard cleaning procedure are given in respective SOP of the machine.

Which Tubing used for coating solution transfer in tablet coating area ?

Generally most of the companies using Silicon Tube for coating solution Transfer from feed chamber to Gun Nozzle.

Specifications of spray gun used in Tablet coating operations :

There are various types of spray guns used in Tablet coating process one specification example provided for information only.

Gun body : Corrosion resistant aluminum.

Material nozzle : Hardened Steel located in body.

Nozzle size : 1.0 mm/1.20 mm/1.50 mm

JOB RESPONSIBILITIES :

Machine Engineer :

Daily cleaning of Tablet coating area by using disinfectant solution.
Machine cleaning as per SOP during product and batch changeover.
Machine setting as per SOP.
Safely operating the machine with minimum rejections and maximum output and maintaining quality standards.
Inprocess checks and recording them on BMR as per product SOP.
Operating and down time recording on machine log.
Checking cleanliness of empty containers and fixing of status label with signature.
Weighing of coated tablets and recording on label and BMR.
Coated tablet container pellet transfer to tablet storage area.
Machine maintenance as per schedule.

Production Officer :

Work allocation.
Supervision of Tablet coating area and co-ordination with Quality control, Maintenance, Stores, Compression & Packing.
Line clearance during Product changeover or Batch changeover.
Cleanliness of area and proper housekeeping.
Checking of machine setup and certification as per product SOP.
Inprocess control checks and recording on BMR as per product SOP.
Record maintenance related to Section.
Calibration of balances and other equipments.
Adherence to cGMP and SOP.
To maintain discipline in Section.
Submit Daily Production Report and completed BMR to Production executive or Section/Department Head.

Section/Department Head :

To complete the production schedule as per accepted plan with quality and optimum utilization of resources.
Co-ordination with other Section & Units.
Scheduling of work.
Planning of Manpower requirement.
Procurement of change parts and spares.
Checking of documents and updation of SOPs in consultation with the Officers.
Preparation of preventive maintenance schedule.
Conducting training programme for Machine Engineer, Officers & Workmen.
To under take the Sectional activities in absence of Officers.

PROBLEMS AND REMEDIES :

Tablet Coating Process Problems & Remedies :

SR.NO.	PROBLEM	CAUSE	REMEDY
1.	Picking / Sticking	1. Very high spray rate	1. Reduce spray rate
		2. Low dry air temperature	2. Increase temperature
		3. Tacky films	3. Add anti adherents
		4. Lack of adhesion to tablet surface	4. Add fillers like MCC or HPC in tablet core
2.	Orange Peel effect or Roughness on surface	1. Spray drying	1. Readjust spray distance increase solvents
		2. Fast drying rate	2. Reduce rate / temperature of dry air
		3. High atomization	3. Reduce atomization pressure
3.	Bridging	1. High spray rate coupled with equally fast drying	1. Control spray rate and drying rate
3.	Bridging	2. Deep wedges on tablets	2. Redesign tablet tooling to avoid this
4.	Chipping	1. Low film strength	1. Increase film former concentration
		2. Inadequate Plasticizer	2. Increase Plasticizer concentration
		3. High pan RPM	3. Reduce pan RPM
		4. Low spray rate	4. Increase spray rate
		5. Very sharp tablet edges	5. Redesign tablet tooling
5.	Twinning	1. High spray rate	1. Reduce spray rate
		2. Low pan RPM	2. Increase pan RPM
		3. Tablets with more flat edges	3. Redesign tablet tooling
6.	Colour variation	1. Improper mixing of bed	1. Move gun close to tablet bed
6.	Colour variation	2. Uneven spray pattern	2. Adjust spraying pattern
7.	Uniformity	1. Low pan RPM2. Low atomization	1. Increase pan RPM2. Increase atomization
8.	Mottling	1. Unsoluble solids2. Low atomization	1. Include insoluble solids2. Increase atomization
9.	Splitting	1. Weak & brittle film due to excessive solids	1. Change solid content
10.	Fading	1. Low quantity of colour	1. Increase colour quantity
11.	Blistering	1. Low bed temperature2. High spray rate3. Low dry air CFM	1. Increase bed temperature2. Decrease spray rate3. Increase CFM if possible
12.	Edgewear	1. Improper sliding of tablets in the pan causing abrasion on the leading edges	1. Extra side bar and baffles necessary for proper sliding of tablets inside the pan

Spray Gun Faults and Remedies :

SR.NO.	FAULT	CAUSE	REMEDY
1.	Gun drips	1. Material control needle or Material outlet nozzle fouled	1. Remove and clean
		2. Material control needle or Material outlet nozzle damaged	2. Replace
		3. Foreign particle present in material outlet nozzle	3. Remove and clean
		4. Packing gland excessive tight	4. Slake Packing gland slightly
2.	Material leaks from packing gland	1. Needle seal worn	1. Replace

SAFETY :

Safety norms have to be followed strictly while working for betterment of product qualities. Hence it is necessary that each and every person must know about safety norms. Safety in Tablet Coating Section is as follows :

Personal Safety

Machine Safety

PERSONAL SAFETY :

In order to work accident free personnel safety has to follow strictly. Personnel safety is built by following way :

Before starting operation earthing must be ensured.

Ear muffs, Nosemask, Gloves should be used while handling the product and special type of Nosemask like 3 M should be used while handling of potent drugs.

While handling solvents during solution preparation rubber handgloves and safety goggles should be used.

MACHINE SAFETY :

While working on any equipment limit switches should not by passed.

In order to avoid oil spillage in material handling in equipment like stacker, pallet trucks pressure release valve should be open so as to keep the fork at lower most position.

Abnormal noise must be thoroughly investigating and necessary action should be taken immediately.

MACHINE MAINTENANCE :

Regular preventive maintenance should be carried out as per schedule.

In order to reduce wear and tear and friction of parts, oiling and greasing should be done at regular intervals as per maintenance schedule.

While operating the equipments the control panel should be free from any obstructions.

Functioning of emergency switches should be checked regularly.

While cleaning the machines care should be taken to avoid contact of water with electrical panel, switches & cables.