You have fine-tuned the coating formula. You run a validated process. However, the tablet batch still shows issues. If this sounds familiar, it’s probably not the coating; it’s the environment during tablet manufacturing. 

This blog aims to explain how environmental factors can affect tablet coating. 

Effects of Environmental Factors on Tablet Coating:

To understand this better, let’s categorize environmental factors into four main areas: air, water, heat, and contaminants. 

Effect of air on tablet coating: 

Air plays a core part in the coating process. It affects the evaporation of the solvent, drying, and even controls temperature. 

If the airflow is either too fast or too weak, drying becomes inconsistent. 

• Rough surface: When the airflow is too strong, spray droplets dry mid-air before hitting the tablet. This leads to a coarse, gritty structure.
  
• Orange peel effect: High air temperature leads to excessive drying. When the coating dries too quickly, it creates small bumps on the surface, resembling the skin of an orange. This can trap air or moisture under the coat, affecting the tablet’s stability, its shelf life, and consistent drug release.
  
• Cracks in the Coating: Hot air dries the outer layer too quickly, making it brittle, while the inside remains soft. This causes cracks, exposes the tablet core, and leads to stability issues and poor bioavailability.
  
• Patchy coating: Excessive airflow dries the spray mid-air, causing patchy coating and poor colour uniformity.
  
• Twinning: Insufficient airflow can lead to tablets sticking together(twinning) and a wet appearance.
  
• Inconsistent batch: Uneven air distribution can result in inconsistent coating throughout the batch.

Effect of Water on Tablet Coating Quality:

Water in your environment (ambient humidity), when not controlled, is guaranteed to cause defects. It affects your coating solution for tablets in the following ways:

• Twinning: High humidity keeps tablets moist for too long. As a result, the coating remains tacky, causing the tablets to stick to one another.
  
• Mottling: Moist air slows solvent evaporation, causing pigments to settle unevenly. This leads to patchy or blotchy colour.
  
• Cracking & Peeling: Low humidity dries coating too fast. The outer film becomes brittle and cracks as tablets move.
  
• Roughness: Fluctuating humidity or poor water quality causes poor film formation.
  
• Blooming: High humidity causes plasticizer migration in coating, leading to a dull or hazy appearance on the surface.
  
• Inconsistent Batch Quality: Due to inconsistent moisture control, some tablets dry properly, while others don’t. This results in mixed results in the same batch.
   
• Picking: The coating gets pulled off one tablet and sticks to another due to the presence of more moisture. 

Effect of Heat on Tablet Coating Quality:

The right heat speeds up drying. Too much or too little ruins the coat. The coating defects caused by heat and moisture often overlap because both directly affect the film’s drying rate. Common defects include:

• Blistering: High drying temperature causes trapped moisture or solvent to expand. This leads to the formation of bubbles or blisters under the film.
  
• Blooming: Improper drying allows humidity to be trapped inside. This makes the tablet appear hazy or dull.
  
• Cracking: The outer film dries while the inner layer is still soft. This leads to cracks in the coating.
  
• Pitting: Overheating tablets before the coating process begins leads to small holes or pits on the coating surface.
  
• Chipping: Due to excessive drying, the coating breaks or chips during the formulation process.
  
• Bridging: When the coating dries too fast, it flows into the logo or scoreline and hardens there. The details get covered instead of being outlined.

Air, water, and temperature often just affect the aesthetics of tablet coating. However, the contamination is much more than a cosmetic issue. It can threaten patient safety, product efficacy, and regulatory compliance. 

Effect of Contaminants on Tablet Coating Quality: 

Contaminants can enter from the air, water, raw materials, or equipment. Even small traces can compromise coating quality, jeopardise safety, or lead to batch rejection. Common defects caused by contamination include:

• Black spots on the tablet: Dust, microbes from the surrounding area, or microbial contamination or cross-contamination from other products can lead to spotting on the tablet.
  
• Rough surface: Untreated or non-purified water may cause minerals or particles to deposit on the coating, resulting in a rough surface. This can cause uneven film formation and a gritty texture on the tablet surface.
  
• Inconsistent colour: Impurities or contaminants in water or raw materials can cause uneven pigment distribution. This leads to a patchy or blotchy appearance on the tablet. 

• Bacterial or fungal growth: Using contaminated water or poor hygiene practices in equipment can introduce microbes into the coating solution.
  
• Shorter shelf life: Contamination weakens the coating’s protective barrier. This allows moisture and air to penetrate the tablet core faster, reducing the product’s stability and shelf life.

A defective tablet coating solution can lead to product failure, harm patients, and damage a brand’s reputation. Hence, quality control during tablet coating is essential. 

Environmental Checklist for Avoiding Tablet Coating Defects

•  Is the airflow rate calibrated before each batch?
•  Are you avoiding excessive airflow 
•  Is the atomization air pressure optimally set?
•  Is the inlet air temperature within the validated range?
•  Is bed temperature monitored throughout the process?
•  Is heat applied gradually?
•  Is there a hold time for pre-warming tablets?
•  Is RH (Relative Humidity) in the coating room controlled (ideal < 50%)?
•  Is purified or demineralized water used for all solutions?
•  Are drying parameters adjusted for seasonal humidity changes?
•  Is the coating equipment cleaned and disinfected before each batch is processed?
•  Is the air HEPA-filtered and particle-controlled?
• Is water tested regularly? 
•  Is the coating area under GMP-controlled conditions?

Conclusion:

Most defects aren’t due to poor materials, but rather to minor lapses in environmental control. Tablet coating quality affects the patient’s experience, brand image, and the efficacy of the product. Hence, avoiding these defects is a must. 

With over 40 years of experience as a trusted manufacturer of tablet coating materials, Novo Excipients strongly recommends prioritising environmental control to ensure consistent quality control of tablet coating.

We provide high-quality excipients, including ready-to-use premixes, to eliminate the variability and guesswork in tablet coating solutions. Additionally, our technical team can help define validated environmental ranges tailored to your specific production setup.