Film Coating Process Of Tablets In Pharma Manufacturing:

Tablet Coating:-

Why Coating is required ?

The application of coating to Tablets, which is an additional step in the manufacturing process, to increases the cost of the product; therefore, the decision to coat a tablet is usually based on one or more of the following objectives, such as:

1. To mask the taste, odour, elegance and appearance of the drug.

2. To provide physical and chemical protection for the drug from atmospheric effects like temperature, humidity and light.

3. Functional coating (Control release, enteric coat, sugar syrup coating).

4. To protect the drug from the gastric environment of the stomach with an acid-resistant enteric coating.

5. To incorporate another drug or formula adjuvant in the coating to avoid chemical

incompatibilities or to provide sequential drug release.

6. To improve the Pharmaceutical elegance by use of special colours and contrasting printing on tablets, which improves the appearance of the product.

Types of Tablet Coating:

There are three main processes for tablet coating:

1. Sugar coating,
2. Film coating - Enteric coating 
3. Compression Coating

Sugar Coating process involves following steps:

1. Sealing.

2. Sub coating.

3. Syruping.

4. Finishing

5. Polishing

Film coating is a thin polymers layer on uncoated tablets to mask the odour, taste, elegance by film forming materials.

Enteric coating process involves following steps:

1. Sub Coating / Seal coating

2. Enteric Coating

3. Polishing (Optional)

Compression coating is also known as press coating or dry coating, which has been developed to produce tablets containing incompatible drugs and to develop modified-release products. It involves the compaction force of the dry coating excipients around the tablet cores that have been produced on the same compression machine.

Film Coating Process in Pharmaceuticals:

Types of spray guns, spray rates, coating pan load, Spray gun position, setup and important parameters of film coating process in pharmaceuticals.

Film Coater basics:-

A typical perforated pan tablet coater machine, as shown in below figure, is comprised of the following basic components, such as:

• A coating pan (or drum) with internal mixing baffles;
• A spray system consisting of one or more atomizing spray guns mounted inside the coating pan and connected via a hosepipe and pump to a solution tank;
• A process air handling system, including a fan or blower, an inlet filter, heating elements, and a dehumidification unit with cooling coils and/or desiccant; and
• An exhaust air handling system, including a fan or blower and an exhaust filter.

Mixing Baffles :- Baffles provide adequate mixing and help avoid "dead spots" or conversely "wet spots".

There are many types of baffles are used, such as:

1. Plough share
2. Rabbit ear
3. Fisher
4. Shoe horn
5. Tubular
6. Unique mixing baffles.

Film coating process:

Film coating process plays a very important role in film coating and number of problems in tablet coating is related with process setup. There are number of variables which should be controlled in a process and we will understand how these variables are crucial for film coating process. We will also study various kinds of equipments, which are available for film coating.

Basic requirements of film coating:

· Distribution

· Mixing 

· Drying

· Removal of solvent

First requirement of film coating process is that coating suspension should be evenly distributed throughout the tablet bed. Spray guns are most commonly used for distribution of coating suspension.

There are two different types of spray gun are used such as:

1. Airless spray guns
2. Air atomized spray guns.

One of the problems with airless spray guns are that spray rate cannot be controlled independent of air pressure. Thus in aqueous film coating air atomized spray guns are used.

There are three different types of guns, which are used such as:

1. 2 port guns
2. 3 port guns 
3. 4 port guns.

2 port guns:- In 2 port guns, on port is for liquid and other port is for air pressure. In this gun actuation pressure to operate gun & atomization pressure is controlled by one pressure and thus both cannot be controlled independent of each other.

3 port guns:- In three port guns, there are 2 ports for air pressure & 1 port for liquid. In this gun actuation pressure and atomization pressure can be controlled independently. In 3 port guns, spray width is controlled by a screw provided on gun which when opened allows air to flow and increase spray width. This can lead to variation in spray width from batch to batch.

4 port guns:- In 4 port guns, there are 3 ports for air and 1 port for liquid. Thus actuation pressure, atomization pressure and spray width all three can be independently controlled by air pressure. This ensures uniformity of spray width from batch to batch.

Film coating process-spray rate:

Spray rate monitoring is critical in aqueous film coating and if there is variation of spray rate in coating process, number of problems is experienced. Spray rate is primarily controlled through the use of peristaltic pumps. Peristaltic pumps are the simplest & easiest to clean. Spray rate can be controlled by the rpm of the pumps. Spray rate also depends upon tubing size used in pump thus depending on spray rate desired, appropriate tubing size should be selected.   

Spray rate in a coating process can be easily monitored by keeping coating suspension on balance so that spray rate per minute can be calculated. It is very important to monitor at least first 3-5 batches and derive rpm of peristaltic pump, which gives required spray rate. 

During the film coating, pan load should be optimum or maximum. If coating pan load is partial, lot of inlet air can go straight into exhaust duct and lot of energy is wasted. Due to partial load exhaust plenum is not fully covered. This reduces drying efficiency of coating pan. The process time with partial load is more compared to full load. Mixing efficiency of coating pan also decreases with partial load and it requires more pan rpm than normally used. This is more critical in functional coatings such as enteric coating where edge wear or damage can result into failure of product in disintegration test.

In order to solve the problems associated with partial load, many machine manufacturers now use multi-pan approach. Thus with one coating pan different drum sizes are given to accommodate various batch sizes.

For e. g. Interchangeable of Pan as per Batch size and as per requirements. For example 36" pan change to 24" pan when batch size minimum.

Spray Gun Position and Setup:

In coating pan where several guns are used it is very important to setup guns properly. Thus gun to gun distance and gun to bed distance is very important & need to be set properly to get required spray width.

Gun to bed distances may differ according to the liquid used as the base. Water demands a longer distance than a quickly evaporating solvent.

Idel distance of guns, such as: Usually for aqueous coating (Water based solvent)

Gun to bed distance - 8" or 10"

Gun to gun distance - 5.5" to 6" (Tip to Tip)

For processes using solvent-based solutions (Isopropyl Alcohol & Methylene dichloride solvents), that gun to bed distance should be decreased to 6 to 8 inches.

If guns are too closed kept and spray width is narrow, it leads to non-uniform coating. It requires more coating material to get uniform coating.

If guns are too wide and spray widths are wide, they overlap each other. In this situation there is localized over wetting in the spray overlapping zone. Some spray also goes to pan wall, door & baffles. This also wastes coating material. Picking & sticking can take place.

In an ideal setup spray pattern meets at the tablet surface and also covers almost entire tablet bed.

Gun Validation and Spray Rate Calculation:

Gun validation is very important and should be done before starting spray. It is important to setup gun so that there is minimum variation between guns. In the beginning spray rate rather in terms of ml or weight for each gun should be recorded and guns should be adjusted if there is more than 10% variation. 

Procedure of Gun Validation:-

• First of all, install all the spray guns on the coating gun rod & connect all the guns with silicone tubing & compressed air pipe, connect the coating suspension pipe with the guns & place other end to the coating suspension container or vessels after passing through the peristaltic pump.
• Take empty tarred container or polybag
• Wrap a poly bag around each gun with the help of a rubber band or tie clip & tare weight of each polythene bag must be known or also take empty tarred containers instead of polybag.
• The spray rate Calculations mainly depend on the number of spray guns & speed of the peristaltic pump.
• Suppose we have 6 spray guns & peristaltic pump RPMs are 20.
• Run the machine for 1 minute at zero atomization air pressure.
• When the system is turned on & the spray process is started, the suspension from guns is collected in each polythene bag.
• After 1 minute completion of spraying process collect the sample container's or sample polybag & weight it on the weighing Balance to determine the quantity of collected suspension.
• It meets the expected spraying.
• Suppose at 20 RPM following quantities are obtained from each gun, such as:

            Coating Gun A = 50.2 gm

            Coating Gun B = 49.8 gm

            Coating Gun C = 50.4 gm

            Coating Gun D = 49.6 gm

            Coating Gun E = 50.6 gm

            Coating Gun F = 49.4 gm

Note:- Variance not more than ± 10 ml between each guns.

To calculate the average spray rate use the following formula:

      A+B+C+D+E+F/6 = 300/4 = 50 gm./Min.

So our average spray rate in one minute for 6 guns at 20 rmp of the peristaltic pump is 50 gm./Min.

suppose we now want to calculate the spray rate at 30 rpm so the calculation will be done by using the unit rule formula such as,

At 20 rpm = 50 gm./min.

At 1 rpm = 50/20 

At 30 rpm = 50/20 × 30 = 75 gm./min.

In the same way, we can validate our spray guns at different rpm of the peristaltic pump e.g. at 40 rpm it will be 100 gm./Min.

Pan Pressure:

During film coating process coating pan should be operated in negative pressure and positive process should be avoided. Each machine manufacturer gives coating pan having exhaust more than 1.5 times than inlet air blower. 

There is adjustment for damper opening which allows us to set negative pressure. Negative pressure should keep between 10-25 mm during process. 

If coating pan is operated under high negative pressure, there are chances of some spray directly going into exhaust. Due to this more coating material will be required to get uniformity. As we open door of coating pan several times in process, due to high negative process room air which is at 25°C goes inside coating pan & mixes with hot air (50-55°C) and thus decreases drying efficiency of coating pan. It is also very difficult to maintain bed temperature.

If coating pan is operated under positive pressure, drying does not take place and solvents are not effectively removed. When door of coating pan is opened several times, dust & hot air from coating pan contaminates room air, which is not allowed as per GMP. Due to these reasons, coating pan should never be operated in positive pressure. 

Process Parameters in Tablet Coating:

Following are the list of process parameters that needs to be monitored throughout coating process.

· Gun geometry & Validation 

· Inlet temperature

· Exhaust temperature

· Bed temperature

· Atomization air pressure

· Spray rate

· Gun to bed distance

· Gun to gun distance

· Negative pressure in pan

· RPM of coating pan

Machine Setting process parameters for Aqueous coating:-

Inlet Temperature - 70-75°C (Approx.)

Outlet Temperature - 55-60°C (Approx.)

Bed Temperature - NMT 55°C (Approx.)

Atomization Air Pressure - 6-8 Kg./cm2

Pan RPM - 3-6 RPM

Peristaltic Pump RPM - 8-12 RPM

Machine Setting process parameters for Non-Aqueous coating:-

Inlet Temperature - 50-55°C (Approx.)

Outlet Temperature - 40-45°C (Approx.)

Bed Temperature - NMT 40°C (Approx.)

Atomization Air Pressure - 6-8 Kg./cm2

Pan RPM - 4-8 RPM

Peristaltic Pump RPM - 10-20 RPM

Film Coating Process of Tablets in Pharma Manufacturing:

Learn about Film Coating process and difference between Aqueous and Non- aqueous Solvent Film Coating.

Film Coating:

· Minimal weight gain (2-3% as opposed to 60-80 % for sugar coating)

· Coat is less likely to affect disintegration

· Single stage process an so generally quicker

· Easy to automate (less reliance on skilled operator / easier to meet GMP requirements of SOPs, validation etc) Maintains original shape of the core & allows for embossing

· Expensive equipment & plant requires large space

· High installation & energy costs.

Aqueous vs Solvent Film Coating

• Solvent based coating solutions / suspensions (e.g. alcohols, methylene chloride)

- rapid drying time due to inherent volatility

- can be applied to moisture sensitive products

- operator safety issues

- requires modification to equipment and facility (flame-proofing / intrinsically safe)

- environmentally responsible disposal is expensive (solvent recovery)

- can impart taste / smell to the product

• Aqueous based processes are now the most common

- no safety issues (e.g. can allow mobile vessels)

- can release to atmosphere

- takes longer and may lead to mechanical damage as tablets are tumbled for longer

- requires more efficient drying air plant

Advances in drying efficiency has allowed aqueous processes to be developed

even for moisture sensitive products

Film Coating - The Common Process Steps:

1. Prepration of coating solutions
2. Fixing of individual gun
3. Fixing of gun to gun assembly
4. Fixing of coating solution to inlet and return flexible hose pipe.
5. Fixing of inlet & outlet ducts
6. Pre-warm up of empty coating pan
7. Tablet loading in inch mode
8. Pre-warmed the tablets at intermittent rotation of pan.
9. Parameters set up for coating operation as per standard Batch Manufacturing Record.
10. Operation as per BMR/SOPs.

Film Coating Process:-

- The coater drum rotates and the baffles in the drum mix the tablets.

- Warm air is drawn through the bed as it enters (inlet air) and exist (outlet air) the drum through perforations

- The distance from the spray guns to the tablet bed is measured and the spray adjusted to achieve and even (non overlapping) coverage across ~75% of the bed

- The coating suspension is pumped through the spray guns with compressed (atomising) air which forms a spray of fine droplets

- As the suspension droplets hit the tablet surface they should spread into a film before the solvent is rapidly removed by the throughput air

- The aim is to gradually build up the coat as the tablets pass beneath the spray guns

Common Components of a Film Coat:

• Solvents

1. Aqueous solvent :- Water  (% Ratio 20%)
2. Non-Aqueous solvents:-

Non-aqueous solvents that is:

• Alcohols (e.g. ethanol, isopropanol, IPA) (% Ratio 20%)

• esters (e.g. ethyl acetate, ethyl lactate)

• chlorinated hydrocarbons (e.g. methylene chloride, 1:1:1 trichloroethane)

• Polymers / Film Formers

- form a clear, non-tacky, mechanically strong film

- must be soluble in the chosen solvent and also allow release of drug in the body

- must be compatible with the core contents most common example is:

1. Hydroxy propyl methyl cellulose (HPMC)
2. Methyl Hydroxy ethyl cellulose (MHEC)
3. Ethyl cellulose (EC)
4. Hydroxy propyl cellulose (HPC)
5. Povidone
6. Sodium carboxy Methyl cellulose
7. Polyethylene glycols (PEG)
8. Acrylate polymers

For aqueous solvent coating Ratio of redi-mix polymers 2% of total tablets weight and water 20%.

For Non-aqueous solvents coating ratio of redi-mix polymers 2% of total tablets weight and IPA and MDC 20% (2:3) ratio.

• Plasticisers

- modify the properties of the polymer to assist the coating process (e.g. by reducing brittleness of the coat)

- need to be miscible with the polymer e.g. propylene glycol, polyethylene glycol (PEG)

(% Ratio PEG-6000 0.3%)

• Colourants

- water insoluble pigments or water soluble dyes e.g. yellow or red iron oxide, aluminium lake dyes

(% Ratio Colours 0.1% to 0.2%) 

• Opacifiers

- provide a “hiding” function preventing the coat from being see through

- used to mask core colour and / or provide light protection e.g. titanium dioxide

(% Ratio Talc 0.2%, TiO2 0.2%)

FAQ for tablets Coating:

1. What is Tablet coating?
2. How many types of Tablet coating?
3. What is film coating process in pharmaceutical?
4. Difference between aqueous and non- aqueous solvents.
5. How many types of gun use in tablet coating?
6. What is mixing buffels and how many types of buffels use in coating pan?
7. How many types of common components use for tablet coating?
8. How many type of coating process parameters measure during tablet coating?
9. How to calculate spray rate?
10. How to perform gun validation?
11. How to set machine parameter for aqueous and non-aqueous coating?