Tablet Coating Process: Film Coating

Film coating is a contemporary and widely used process for coating oral solid dosage forms in the pharmaceutical industry. It is a process that draws on technologies associated with polymer chemistry, industrial adhesives and paints, and chemical engineering.

The process of film coating involves the deposition of a thin, but uniform polymer-based formulations onto the surface of solid dosage forms such as tablets, capsules, powders, granules, or pallets under conditions that permit:

1. Balance between, and control of, the coating liquid delivery rate and drying process
2. Uniform distribution of the coating materials across the surface of the product being coated
3. Optimization of both visual and functional quality of the final coated product.

The function of the coating may be to improve esthetics, mask unpleasant taste and odour, separate incompatible materials, or modify drug release profile.

The film coating process of today was introduced in the early 1950s as an improvement on the traditional sugar-coating process. The process has proved successful as a result of the many advantages offered, including but not limited to:

1. Substantial reduction in the quantity of coating applied
2. Minimal weight increase
3. Increased process efficiency and output
4. Significant reduction in processing times
5. Increased flexibility in formulations as a result of the availability of suitable film forming polymers.

In spite of these advantages, there are still some areas of concern in the film coating processes and operations. These concerns arise from the fact that most solvents used in film coating formulation can cause health and environmental hazards. Fortunately, significant advancement in process technology and equipment design, have facilitated the introduction of aqueous-based coating formulations. The result is that less volatile, and safer, solvents such as water can be easily accommodated by the process.

Description of film-coating process

Film coating involves the deposition of a thin layer of film-forming polymeric onto a product substrate. The process can be carried out using conventional panning equipment but nowadays more sophisticated equipment is used to achieve efficient drying, high degree of automation and coating time.

The coating formulation contains a polymer solubilized in a suitable solvent together with other additives like plasticizers and pigments. This solution is sprayed onto a rotating or fluidized tablet bed. The drying conditions cause removal of the solvent, leaving a thin deposit of coating material around each tablet core.

Schematic representation of the film-coating process

Ideal characteristics of film coated tablets

• Film-coated tablets should have even coverage with uniform colour coverage across the surface of each dosage unit within a batch, and from batch to batch.
• Film-coated tablets must be free from defects which affect functionality and stability of the finished product.
• Film-coated tablets must be compliant with finished product specifications and any relevant compendial requirements.

Types/ classification of film coatings

Film coatings may be classified in a number of ways but it is a common practice to do so in terms of drug delivery system.  Hence film coatings may be classified as either immediate-release film coatings or modified-release film coatings.

a. Immediate-release film coatings

Immediate-release film coatings also known as non-functional or conventional film coatings are typically reserved for situations in which it is necessary to improve product appearance, ease of swallowing, product stability, and for taste masking. The coatings have no measurable effect on biopharmaceutical properties of the coated dosage forms.

b. Modified-release film coatings

Modified-release (or functional) film coatings are used when drug release characteristics need to be modified. Modified-release film coatings may be further categorized as either delayed-release (e.g. gastro-resistant/ enteric coating) or extended-release coatings.

Types of film coating

Raw materials used in film coating formulations

A typical film-coating formulation contains the following materials:

1. Polymer
2. Plasticizer
3. Colourants
4. Solvent/ Vehicle

1. Polymers

Polymers are substances whose molecules have high molar masses and are composed of many repeated subunits. They are formed by chemical reactions in which a large number of molecules called monomers are joined sequentially, forming a chain.

In the majority of film-coating formulations, the polymer is the major component in the coating solution. Consequently, this material will have the greatest impact on the final properties of the coating. Some of the key attributes that the film-coating polymer must possess include:

1. Solubility in a wide range of solvent systems
2. Solubility requirement for the intended use e.g. free water-solubility, slow water solubility or pH-dependent solubility
3. Stability against light, oxygen, moisture, heat and the substrate being coated
4. Continuous film formation capability with adequate mechanical properties
5. High compatibility with other film-coating additives and the tablet being coated
6. Low viscosity at the preferred concentration (for adequate atomization)
7. Nontoxic with no pharmacological activity
8. Capacity to produce an elegant looking product even in the presence of additives

Polymers used in film coating fall into the following categories.

1. Cellulose ethers e.g., Hydroxy Propyl Methyl Cellulose (HPMC), Hydroxy Propyl Cellulose (HPC), Ethyl Cellulose (EC), Methyl Cellulose
2. Vinyl polymers e.g., polyvinyl pyrrolidone
3. Glycols e.g., high molecular weight polyethylene glycol
4. Acrylic acid polymers e.g., Eudragits

Polymers for immediate-release film coating formulations

Polymers for enteric film coating formulation

Polymers for Sustained (controlled)-release film coating formulation

2. Plasticizers

Plasticizers are relatively low molecular weight materials which are added to film-coating formulations to modify the physical properties of polymers. This is necessary because most acceptable film-coating polymers are essentially amorphous, and as such, exhibit a reasonably well-defined glass transition temperature, Tg (a fundamental characteristic of polymers that has a profound effect on polymer properties that can also influence film formation, especially when using aqueous polymer dispersions).

Plasticizers act by weakening intermolecular attraction between polymer chains and facilitating coalescence of discrete polymer spheres of aqueous dispersed systems during film formation. This reaction results in increased film flexibility and reduced residual stresses within the coating as it shrinks around the core during drying.

Examples of plasticizers commonly used in film coating processes include:

1. Polyols, such as glycerol (glycerin), polyethylene glycols (PEG 200 – 6000 grades) and propylene glycol.
2. Organic esters, such as Diethyl phthalate (DEP), Dibutyl phthalate (DBP), Dibutyl sebacate (DBS), Triethyl citrate (TEC), Acetyltriethyl citrate (ATEC), Acetyltributyl citrate (ATBC), Tributyl citrate (TBC), and Triacetin (glyceryl triacetate; TA).
3. Oils/ glycerides, such as fractionated coconut oil, castor oil, and distilled acetylated monoglycerides (AMG).

Summary of Common Effects of Plasticizers on the Properties of Film Coatings

3. Colourants

Colourants are included in many film-coating formulations to:

1. Improve product appearance and enable product identification
2. Modify the gas permeability of a film
3. Decrease the risk of counterfeiting the product
4. Protect the active ingredient against light by optimizing the opacifying properties of pigments.

Common Types of Colourant Used in Film-Coating Formulations

While it is possible to use either water-soluble colourants (known as dyes) or water-insoluble colourants (known as pigments), the water-insoluble colourants are preferred in film-coating formulations based on the fact that they:

1. Exhibit better light stability
2. Provide better opacity and covering power
3. Provide a means of optimizing moisture barrier properties of the applied film coatings
4. Do not suffer from the disadvantageous phenomenon of mottling (caused by solute migration) that can be observed with water-soluble colourants.

As with sugar coating colours, film-coating colourants must comply with regulations promulgated by the national legislation of the country where the products are to be marketed.

Effects of Pigments on the Properties of Film Coatings

4. Solvents/ Vehicles

Solvents are used to dissolve or disperse coating materials and convey them to the surface of the tablet core. Initially, film-coating processes were very much dependent on the use of organic solvents in order to achieve the rapid drying characteristics demanded by the process. Unfortunately, concerns with operator’s safety, environmental, and cost-related issues have provided the momentum for the current utilization of aqueous-based film coating as the preferred option. However, the use of solvents has continued, especially when:

1. The coating process will not accommodate the use of water (i.e., drying is poor);
2. The adhesion achieved with aqueous systems is unacceptable;
3. Certain critical ingredients (e.g., polymer) are neither water-soluble nor available as a latex system; and
4. Exposure to an aqueous process would cause stability problems for the product being coated.

Common solvents used in film coating

Miscellaneous coating solution component

While polymers, plasticizers, colourants, and solvents constitute the major ingredients in film-coating formulations, other materials might be used occasionally in low concentrations for specific formulations.

Flavours and sweeteners may be added to mask unpleasant odour of some drugs or to make them more palatable.

Surfactants or dissolution enhancers such as polyoxyethylene sorbitan derivatives may be added to

1. Emulsify water-insoluble plasticizers
2. Improve substrate wettability and enhance spreadability of the film during application
3. Stabilize suspensions

Additionally, some film coatings may also contain preservative/ antimicrobials (e.g., carbamates, alkylisothiazloinone, benzothiazoles etc.), adhesion enhancers (such as polydextrose, maltodextrin, and lactose), antifoaming agents (e.g., dimethylpolysiloxane), antioxidants (e.g., oximes, phenols etc.), pore – forming agents (e.g., sucrose or sodium chloride with ethylcellulose-coated salicylic acid tablets) and waxes. In rare instances, the film coat itself may contain active drug substance.

All ingredients used in film-coating formulations must comply with the relevant regulatory and pharmacopoeial requirements current in the intended marketing area.

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