Tablet coating is a process of applying a layer of polymer over rotating tablets in a coating pan.
A coating suspension of polymer with other excipients in a suitable solvent is prepared and is applied over the rotating bed of tablets in the form of fine droplets after automization using compressed air.
Heat is continually applied to the tablets so the solvent evaporates leaving behind the layer of polymer and other excipients on the tablet surface.
What Is Coating Weight Gain?
Before going into details of the main topic, what the weight gain problem is, first have a look over what is weight gain and which types of issues related to weight gain are faced in the practical operation of the tablet coating process in pharmaceutical industries.
As the name indicates weight gain is the addition of weight to the initial core or uncoated tablet weight during the coating process.
This weight gain results from an accumulation of coating materials over the tablet surface during the coating process.
Weight Gain Recommendations
As we know that tablet coating is generally of 03 types given below,
- Film Coating
- Enteric Coating
- Sugar Coating
Practically following ranges of weight gain are set for above mention coating types,
- Film Coating = 1 to 5 %
- Enteric Coating = 8 to 12 %
- Sugar Coating = 40 to 70%
The ranges may vary depending on coating formulations but as a general practice, these ranges are commonly used in most of the pharmaceutical industries.
Importance Of Weight Gain During Tablet Coating
A simple question may arise in our minds, why do we need to gain weight during the tablet coating process?
For its answer, some common reasons are discussed as,
- Weight gain during the coating process is important to give some specific properties to the core tablets.Suppose we are using film coating for a batch of core tablets to mask its taste,if the proper covering of coating material is not formed over tablets it will not result in complete taste masking.
- In the case of enteric coating if weight gain is less than the recommended values it may result in stability issues in acidic media.
- In the case of control release tablets, the required properties will not be achieved.
- Color uniformity will not be achieved due to less coating material weight gain in the case of color tablets.
Science Of Weight Gain
To understand the theme of weight gain let’s take the example of film coating.
Suppose we have 3% coating material for a batch of tablets to coat and the individual core tablet weight is 600 mg.
3% weight gain to initial tablet core weight means after coating it must be 618 mg per tablet.
Calculate as below,
3/100×600 = 18mg
add it to the individual core tablet weight
600mg +18mg = 618 mg/Tab
From the above example, it is clear that theoretically, individual tablet weight after coating must be 618 mg but practically it will be less than 618 mg per tablet due to loss of coating material through tablet coater exhaust.
Suppose our coater is efficient and coating material loss during the coating process is 20 % so our tablet coating material loss will be
20/100× 18mg = 3.60 mg
3.60 mg of the coating material will be lost during tablet coating process so our practical individual tablet weight gain after coating will be,
618 mg – 3.60 mg = 614.4mg
For simple understanding, we took 3% coating material and practically our weight gain after tablet coating is 2.4 % instead of 3%.
Calculate as below,
14.4 mg/600 mg× 100 = 2.4%
Weight Gain Conclusion
As we discussed our practical weight gain will be less than theoretical weight gain and it depends on our coating process parameters and efficiency or opening of exhaust.
Generally, about less than 1% of coating material should be exhausted during the coating process but it is better if we exhaust less than 0.5 %.
Types Of Weight Gain Problem
Weight gain problems during coating operations are of generally two types which are given a bow,
- Reduction In Weight Gain
- Increased Weight Gain
In case 1 suppose we have 3% coating material (as calculation above) and theoretically weight gain of 600 mg tablet must be 618 mg and practical not less than 2% mean 612 mg.
But after coating our required weight gain is not achieved and it is less than 2%.
So there is less weight gain in case 1 from our standard values.
We will discuss the reasons & solutions to overcome this tablet coating defect.
In case 2 we have the same scenario like 3% coating material for 600 mg tablet weight and the theoretical weight gain must be 618 mg per tablet.
But after the coating process our individual tablet weight is 620 mg which is more than 3%. How is it possible? We will discuss it in detail.
Factors Affecting Tablet Coating Weight Gain
First, we will discuss the factors or coating parameters which may result in a reduction in weight gain.
These are given as below,
- Inlet Air Temperature/Hot Air Volume
- Peristaltic Pump Speed
- Tablet Bed Temperature
- Coating Guns to Tablet Bed Distance
- Exhaust Opening
Inlet Air Temperature/Hot Air Volume
Inlet air temperature is the temperature of processed air which is flowing inside the coater from outside after the pretreatment and heating.
Hot Air volume is the volume of hot air which is coming inside the coater per hour and is represented by the unit m³/hr
Both these parameters are controlled by the coating operator during the coating process to achieve the required results.
We will discuss how and why these parameters or factors result in a reduction of weight gain & what is the solution.
If our inlet air temperature is very high it will result in a reduction in tablet weight gain during the coating process.
If our air flow is very high along with high temperature it will also result in a reduction of tablet coating weight gain.
The higher temperature/increased hot air volume results in rapid evaporation of the solvent and it dries before reaching to the surface of tablets so dust is generated & more powder loss occurs through exhaust.
Due to higher wastage of coating material less amount of coating material is deposited over the tablet surface so the required weight gain is not achieved.
- Decrease the temperature of inlet air.
- Do not use too much hot air during the coating process if our peristaltic pump speed is low.
- Increase Spray rate along with temperature to prevent product loss.
Peristaltic Pump Speed
Peristaltic pump speed controls our spray rate means the rate at which coating suspension is supplied to the spray guns. Let’s discuss the reasons and solutions for weight gain reduction due to peristaltic pump speed or RPMs of the pump.
If our peristaltic pump speed is low means our spray rate is less and our inlet air temperature/hot air volume is high it will result in a reduction of weight gain.
The reason is,due to less spray rate our solvent is evaporated and wastage of coating material occurs.
- Synchronize the spray rate with applied heat to counter the effect of rapid evaporation.
- Increase the peristaltic pump speed or decrease the inlet air temperature.
Tablet Bed Temperature
Tablet bed temperature means the temperature of rotating tablets which is measured by the sensor installed in the tablet coater or by the IR gun.
If our tablet bed temperature is high which is related to the increased temperature of inlet air it results in a reduction of tablet coating weight gain due to the fact the hot surface of the tablets results in rapid drying of solvent & less accumulation of coating material over the tablet surface.
- Increase the spray rate to reduce the tablet bed temperature.
- Decrease the inlet air temperature.
Coating Guns to Tablet Bed Distance
The distance between spray guns to the surface of the rotating tablets is called gun-to-bed distance.
If spray guns to tablet bed temperature is increased it results in a reduction of weight gain during the tablet coating process.
The reason is that when distance is increased,the spray droplets have to travel more distance and if we have same inlet air temperature, tablet bed temperature & spray rate then solvent evaporated before tablet surface and results in reduction of tablet weight gain
- Always Maintain a suitable distance between the tablet bed & spray guns.
- Usually, a distance of 8 to 9 inches is recommended but it may vary depending on the type of coater used and formulation.
- Increase spray rate if you want to maintain a long distance between guns & tablet bed.
Exhaust or return air valves play an important role during tablet coating & have a great impact on tablet weight gain.
Let’s discuss how exhaust parameters result in less tablet coating weight gain.
If the exhaust is opened more than the recommended values then the loss of coating material will be high which results in less weight gain.
More exhaust opening results in creation of more negative pressure and if spray gun angle is not appropriate it will pull more spray towards exhaust & results in material loss & reduction of coating weight gain.
- Reduce the opening of exhaust to prevent product loss.
Now we will discuss case number 2 where our coating weight gain is increased than the theoretical values.
If our theoretical weight is 618 mg per tablet but we achieve 620 mg per tablet after the coating process then it is only possible when our solvent is penetrated inside the tablet core & possible reasons are given below which may result in solvent penetration in the tablet core,
- Low Inlet Air Temperature
- Low Tablet bed temperature
- High spray rate
- Less distance between tablet bed & spray guns.
All the above factors result in improper evaporation of solvents due to low temperature so it can be resolved by using the following,
- Increase Inlet air temperature.
- Increase Tablet Bed Temperature
- Reduce Spray Rate
- Increase Distance between spray guns and tablet bed.
Tablet coating is a very critical process & to achieve the required tablet weight gain all the parameters of the tablet coater must be synchronized.
Monitor tablets weight after specified time intervals during the coating process to ensure that proper weight gain is achieved & our process parameters are accurate.
If we want to change one parameter according to practical processing conditions we will have to adjust other parameters accordingly to ensure the smooth running of the tablet coating operation.