INTRODUCTION:- Tablet is a pharmaceutical solid dosage form, comprising a mixture of active substances and excipients, commonly in powder form, pressed or compacted right into a stable. Capsules Dosage form is one of a most desired dosage form everywhere in the world. Almost all drug molecules may be formulated in a tablet and procedure of producing of drugs is very simple, and may be very bendy. Coating is a manner by using which a basically dry, outer layer of coating material is implemented to the surface of a dosage shape to achieve precise benefits. Coating may be implemented to a wide range of oral solid dosage shape, along with capsules, capsules, multi particulates and drug crystals. While coating composition is carried out to a batch of capsules in a coating pan, the tablet surfaces grow to be protected with a tacky polymeric film. Before the tablet floor dries, the carried-out coating adjustments from a sticky liquid to tacky semisolid and sooner or later to a non-sticky dry surface pans. Many stable pharmaceutical dosage forms are produced with coatings, both on the external surface of the tablet, or on substances dispensed inside gelatin tablets. The tablet ought to launch the medicament steadily and the drug should be to be had for digestion. The coating procedure can be in particular formulated to adjust how speedy the tablet dissolves and in which the lively tablets are to be absorbed into the body after ingestion.

 

DIFFERENT METHODS OF COATING

  1. SUGAR COATING: – Tablet coating developed originally from the use of sugar to mask the taste and provide an attractive appearance to at the core. The process of tablet coating consists of several steps, which are described below:
  • Sealing: – A seal coat is applied over the tablet to prevent moisture penetration into the tablet core. Shellac was previously used as a sealant. But due to polymerization problems, it was replaced by zein (a corn protein derivative).
  • Sub coating: – This step is done to round the edges and increase the tablet weight.
  • Syrup Coating: – The imperfections in tablet surface are covered up and the predetermined size is achieved. This step requires the maximum skill.
  • Coloring: – Gives the tablet its final color.
  • Polishing:- Powdered wax (beeswax or carnauba) is applied to provide a desired luster.

 

  1. FILM COATING:- As the sugar coating process is very time consuming and is dependent on the skills of the coating operator, this technique has been replaced by film coating technology. The process involves spraying of a solution of polymer, pigments and plasticizer onto a rotating tablet bed to form a thin, uniform film on the tablet surface. The choice of polymer mainly depends on the desired site of drug release (stomach/ intestine), or on the desired release rate. Some of the non-enteric coating polymers are Hydroxyproply methyl cellulose (HPMC), Methyl hydroxyethyl cellulose, Ethylcellulose, Povidone, etc., while the commonly used enteric coating polymers are Cellulose acetate phthalate, Acrylate polymers (Eudragit L& Eudragit S), HPMC phthalate, etc. An ideal film coating material should possess the following characteristics: –
  • It should be soluble in a solvent of choice.
  • It must produce an elegant coat.
  • It should be stable in presence of heat, light or moisture.
  • It should not possess disagreeable color, taste or odor.
  • It should be non-toxic and pharmacologically inert.
  • It should be compatible with coating additives.

 

  1. ORGANIC FILM COATING: – Currently, the most common technology for coating solid dosage forms is the liquid coating technology (aqueous based organic based polymer solutions). In liquid coating, a mixture of polymers, pigments and excipients is dissolved in an organic solvent (for water insoluble polymers) or water (for water soluble polymers) to form a solution, or dispersed in water to form a dispersion, and then sprayed onto the dosage forms in a pan coater (for tablets) and dried by continuously providing heat, typically using hot air, until a dry coating film is formed. Organic solvent based coating provides a variety of useful polymer alternatives, as most of the polymers are soluble in the wide range of organic solvents. However, there are certain disadvantages like they are flammable, toxic, and costly and possess environmental issues. ICH guidelines also prefer the avoidance of organic solvents in pharmaceutical dosage formulations considering products safety profile. So, Pharmaceutical industries are now paying much attention in developing formulations with aqueous film coating.

 

  1. AQUEOUS FILM COATING: – All above problems with organic solvents resulted in shift to use of water as the preferred coating solvent. Aqueous-based coatings have been increasingly used compared with organic-based coatings. The conversion from organic solvent based coating to aqueous based coating makes the coating process more economical, though initially it may need a little investment to upgrade the coating facility. The need of this up-gradation arises due to the need of higher drying capacity (the latent heat of water is 2200kJ as compared to 550kJ for methylene chloride). This implies that one would require 4 times more energy as compared to organic solvent.

 

RECENT TECHNOLOGIES IN COATING

 

  1. COMPRESSION COATING :- These sorts of tablet have elements like, inner core and surrounding coat. The core is small porous tablet and organized on one turret. For preparing final tablet, a bigger die hollow space in every other turret is used wherein first the coat cloth is crammed to half of after which center tablet is robotically transferred, again the last space is filled with coat fabric and ultimately compression force is applied. Often, the coat is water soluble and disintegrates without difficulty after swallowing.
  • Preparation of the compression-coated:- A carefully weighed quantity of powder mixture was positioned inside the die and compressed on a Carver Press at a acknowledged force with the tooling proven to produce a cup-fashioned tablet. The cup changed into left in the die, and a recognized quantity of both a model drug and a mix containing the drug changed into located inside the cup and tamped gently with the punch in a prolonged role. A weighed amount of the coating combo changed into positioned on top of the die contents, and the cup changed into compressed for a second time with the punch in a retracted position at a regarded pressure to provide the final compression-coated tablets.

 

  1. ELECTRO STATIC COATING :- In electrostatic powder coating, we spray powder via a place of strong electric area and excessive unfastened-ion concentration. Passing through this location, the particles are charged as discussed earlier. The process of powder particles charging within the electric area of corona discharge is governed with the aid of Pauthenier’s equation. Charging is most strongly laid low with field power, powder particle size and form, and the length of time the particle spends within the charge area.

 

 

  1. ELECTROSTATIC DRY COATING:- An electrostatic dry powder coating method for tablets become evolved for the first time with the aid of electrostatic dry powder coating in a pan coater gadget. The optimized dry powder coating method produces capsules with smooth surface, correct coating uniformity and release profile that are corresponding to that of the tablet cores. This novel electrostatic dry powder coating technique is an opportunity to aqueous or solvent based coating technique for pharmaceutical products.

The electrostatic coating method is extensively useful in paint technology, food technology, coating of residing cells, metal coatings and coating of capsules in addition to drugs. The precept of electrostatic powder coating states that spraying of a mixture of finely grounded particles and polymers onto a substrate floor without the usage of any solvent and then heating the substrate for curing on oven until the powder mixture is fused into film.

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According to the charging mechanism, there are two types of spraying units:

  1. a) Corona charging
  2. b) Tribo charging.

a)Mechanism of corona charging:- On this mechanism, the electrical breakdown and ionization of air with the aid of enforcing high voltage on a sharp pointed needle like electrode at the hole of the gun. The powder debris picks up the negative ions on their manner from the gun to the substrate. The movement of particles between the substrate and the charging gun is performed by way of the aggregate of electrical and mechanical forces. The mechanical forces generated by using the air blow the powder closer to the substrate from the spray gun. The electrical forces are derived from the electrical subject between the earthen substance and the charging tip of the spray gun, and from the repulsive forces between the charged particles. The electrical discipline may be adjusted to direct the control sample size, shape, powder’s drift, and powder density as it is released from the gun.

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b)Mechanism of Tribo charging: In the tribo charging, it makes using the principle of friction charging associated with the dielectric properties of solid substances and so that no loose ions and electric area might be present between the spray gun and level-headed substance. For tribo charging guns, the electric forces are only regarded to the repulsive forces among the charged particles. After spraying, charged particles come into the distance adjoining to the substrate and the attractive forces among the grounded substrate and the charged particles makes the particle to deposit on the substrate.

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4.MAGNETICALLY ASSISTED IMPACTION COATING (MAIC):- A method is advanced for estimating the coating time in a magnetically assisted impaction coating tool. The combination of the host, guest and magnetic particles is assumed to stay in a fluidized state in which the distribution of velocities is Maxwell-Boltzmann kind. It is assumed that the collisions happen among the particles are critical for impinging the guest debris onto the host debris, and for that reason forming a semi-permanent coating on the surface of host debris. The coating time is depending upon several parameters, consisting of the quantity density of host substances, the diameter ratio of the host and visitor debris, the height of the fluidized particle bed and the properties of materials of the host and guest particles.

 

Mechanism of coating in the MAIC process: –

Stage-I: Excitation of magnetic particles.

Stage-II: De-agglomeration of guest particles.

Stage-III: Shearing and spreading of guest particles on the surface of the host particles.

Stage-V: Magnetic-host-host particle interaction.

Stage-VI: Magnetic-host-wall interaction and

Stage-VI: Formation of coated products

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 5.AQUEOUS FILM COATING TECHNOLOGY: – The sugar coating procedure could be very consuming and its miles depending at the abilities of coating operator, this approach has been replaced through advancement in film coating technology. This approach was begun with use of organic solvents like methylene.

6.SUPER CELL COATING TECHNOLOGY: – Supercell coating technology is an innovative tablet coating that accurately deposits controlled quantity of coating materials on drugs even supposing they’re extremely hygroscopic or friable. Due to the fact the tablets are loaded in huge rotating pans and vented for decent air drying, edges of tablets can get grounded off, intagliations can get filled in via coating material, and edges and corners may not be covered with identical thickness as the tablet faces.

 

Unique functions of super cell coating technology

  1. Continuous coating
  2. Short processing time
  3. Flexible modular design
  4. No scale-up to parameters
  5. Production capacity of 6 cells coats 200K tph of 120 mg tablets
  6. R&D batch size (Minimum batch size of 30 grams)
  7. Enhancing technology
  8. Multi-layer coating
  9. Difficult-to-coat shapes
  10. Friable tablets
  11. “Low humidity process” suitable for moisture sensitive materials
  12. Enabling technology
  13. Accuracy of coating

7.DIP COATING:- Dip coating is a famous way of creating thin films for research functions. Uniform films may be implemented onto flat or cylindrical substrates. For commercial strategies, spin coating is used greater often. The dip coating process can be separated into five stages:

a) Immersion: – The substrate is immersed inside the solution of the coating material at a constant pace.

b) Start-up:- The substrate has remained inside the solution for a while and is starting to be pulled up.

c) Deposition: – The thin layer deposits itself at the substrate even as it is pulled up. The retreating is carried out at a constant velocity to avoid any jitters. The speed determines the thickness of the coating.

d) Drainage:- Extra liquid will drain from the surface.

e) Evaporation:- The solvent evaporates from the liquid, forming the thin layer. For volatile solvents.

8.VACUUM FILM COATING (FLUID BED COATING):- Spray coating can be used for all fluid bed systems, be it in batch or continuous operation or if the film is applied from a sprayed solution, suspension or hot melt. For this processing option the parameters have to be chosen to avoid agglomeration, i.e. liquid bridges between the air suspended particles. If spraying a solution or suspension the liquid only serves as a vehicle to deliver the coating material to the surface of the substrate. For hot melt coating the droplets must be small enough not to form solid bridges.

9.TOP SPRAY COATING PROCESS: – Despite the fact that the top spray fluid mattress system is extensively used for granulation its use for coating is restrained. Top spraying is the most effective method and gives the best capability and lowest capital cost. It could be used for numerous manipulations of taste covering and many others. This method alternative is often used in the food, feed and chemical industries as the characteristic of the film mainly serves to enhance the general managing situations. An ideal film is typically no longer required for this function, however care should be taken that the droplets do not emerge as too viscous before touching the substrate, so that you can hold accurate spread ability.

10.BOTTOM SPRAY COATING PROCESS:- The maximum normally recognized fluid-bed system for coating inside the pharmaceutical industry is bottom-spray technique. The precise functions of bottom-spraying are an air distributes plate and a partition that arrange fluidization of particles of debris via the partition. The nozzle is installed at the bottom of the product box and is focused in the coating zone. The quick distance between the coating materials and particles at some point of the coating technique minimizes spray drying and contributes to excessive uniformity and coating performance.

11.TANGENTIAL SPRAY COATING PROCESS:- This processing approach is with its physical concepts pretty much like bottom-spray coating handiest that the production motion is supplied by a motor driven rotor disc. Otherwise, the quality producing parameters are the same:

a.Uniform statistical residence time is warranted by defined rotor revolution speed.

b.The coating material is sprayed simultaneously within the rotating product.

c. The rolling movement of the particles affords an even higher separation force, as such preventing agglomeration. The benefits of this processing option are mainly for the layering and subsequent film coating of pellets

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Reference links

 https://www.researchgate.net/publication/285856849_Tablet_Coating_techniques_Concepts_and_recent_trends

 https://pdfs.semanticscholar.org/5506/d589121529349aa6095f4b24ab6ee39d7236.pdf?_ga=2.20190072.1953997122.1505893017-743696644.1505893017