Tablet Compression and Coating Processes PDF

TABLET COMPRESSION A tablet is a solid pharmaceutical dosage form, consisting of a small quantity of compressed powder. There are several types of tablets on the market, including sublingual, chewable, effervescent, film- coated and enteric-coated. This dosage form is acquired for correct recommende...

TABLET COMPRESSION A tablet is a solid pharmaceutical dosage form, consisting of a small quantity of compressed powder. There are several types of tablets on the market, including sublingual, chewable, effervescent, film- coated and enteric-coated. This dosage form is acquired for correct recommended dosage, cleanliness, ease of packaging and transportation and less wastage. To achieve the tablet form, powder needs to be compressed and include the right formulation of excipients The process of tablet compression is split into four different stages. 1. Filling The transfer of raw materials into position for tablet compression. The homogenized powder is poured into the machine hopper, either manually or via an automated system, whereby the powder flows into the tablet compression chamber. 2. Metering The removal of excess powders from the compressing machine. As powder reaches this stage, it passes through the feeder system, which comprises of the feeder housing and feed peddles. The feeder housing feeds the powder from the hopper into the die system, where the feed peddles ensure consistency and accuracy in the amount of powder fed into the die. The exact weight of compressed powder is enforced through the adjustable height of the lower punch. Any excess is scraped from the surface and into the next punch. 3. Compression The formation of the tablet, where the upper and lower punches come together under pressure within the die. As the punches move between two large compression rollers, a sufficient amount of force is exerted, where air from between the granules is expelled prior to the commencement of compression. The adjustable height of the punches control thickness and hardness of the produced tablet. When the particles are in sufficiently close proximity, they are bonded. Particles bond as a result of mechanical interlocking, which is described as entanglement of the particles, phase transition at the points of contact, and intermolecular forces, namely the van der Waals force, hydrogen bonding, and ionic bonding 4. Ejection The removal of the tablet from the lower punch-die station. The upper punch retracts whilst the lower punch simultaneously rises in the die, pushing the tablet upward to the surface of the die table. The take-off blade deflects the tablets through the discharge chute and into the collection container, usually through a deduster and metal detector. Tablet compression may appear simple, however it is an intricate process, where many issues can arise. These defects are based on choice of excipient, in-process or machine errors. The most susceptible are due to excipients, where the addition of excipients is inaccurate or errors occurring due to the granulation process. Consequently, the product needs to be regularly monitored for any variations and maintained to fulfil specifications. TABLET COATING Tablet coating can be described as a process of applying an edible paint on the surface of a pharmaceutical dosage form to achieve specific benefits. This is an additional process in tableting which causes an increase in the cost of tablet production. Coating can be applied to several kinds of solid dosage forms like tablets, pellets, pills, drug crystals, etc. Objectives of Tablet Coating 1. Mask the taste, odor or color of the drug 2. Provide physical and chemical protection for the drug 3. Control release of drug from tablet 4. Protect drug from gastric environment of the stomach with an acid-resistant enteric coating. 5. Incorporate another drug or formula in the coating to avoid chemical incompatibilities or provide sequential drug release. 6. Improve pharmaceutical elegance by using special colors and contrasting painting. Coating process It is most desirable that the coating should be uniform and should not crack under stress. Hence, various techniques were designed for the application of the coating on the tablet surface. Generally, the coating solutions are sprayed onto the uncoated tablets as the tablets are being agitated in a pan, fluid bed, etc. As the solution is being applied, a thin film is formed which sticks to each tablet. The liquid portion of the coating solution is then evaporated by passing air over the surface of the tumbling pans. The coating may be formed either by a single application or may be developed in layers through the use of multiple spraying cycles. Rotating coating pans are often used in the pharmaceutical industry Types of tablet coating Generally, three methods are used for tablet coating: A) Sugar coating. B) Film coating. C) Press coating. A. Sugar coating Traditionally sugar coatings formed the bulk of coated tablets but today film coatings are the more modern technology in tablet coating. Tablets suitable for sugar coating should be Smooth, rounded and polished to a high gloss. The process involved are separated into 6 operations. 1. Sealing tablet core- This involved the application of one or more coats of a waterproofing substance in the form of alcoholic spray, such as pharmaceutical Shellac (traditionally) or synthetic polymers, such as cellulose acetate phthalate (CAP) and polyvinyl acetate phthalate (PAP). This process is important because sugar-coatings are aqueous formulations which allow water to penetrate directly into the tablet core and thus potentially affecting product stability and possibly causing premature tablet disintegration. Application of many coats of partially or completely water-insoluble polymers in this step, enables sugar-coated product to exhibit modified- release pattern (extended release or delayed "enteric"- release characteristics). 2. Sub coating: Large quantities of sugar-coatings are usually applied to the tablet core (typically increasing the tablet weight by (50- 100%). This is necessary in order to round off the tablet edge. Much of this material build-up occurs during this stage and is achieved by adding a bulking agent such as Calcium carbonate, to the sucrose solution. Antiadherents e.g. Talc may be added after partial drying to prevent sticking of the tablets together. 3. Smoothing process -The subcoating stage results in tablets with rough surfaces. To facilitate the color application (which requires smooth surface), subcoated tablets are smoothed out by a thick sucrose syrup coating. 4- Coloring: Color coatings usually consist of thin sucrose syrup containing the requisite coloring materials. (water-soluble dyes or water-insoluble pigments may be used) This step must be done in a clean pan deprived of any residues from the previous operations. 5. Polishing: After the coloring step, the tablet surfaces tend to be smooth but somewhat dull in appearance. To achieve glossy finish, final stage involving application of waxes (beeswax, carnuba wax) is employed. 6- Printing: Different tablets could be identified by manufacturer' logo, product name, dosage strength or other appropriate code. For sugar-coated tablets, such identification could be only achieved by printing process using special edible inks. B. 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, solvent 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. Polymers Solvent 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 film coating 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. 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. Plasticizers It is used to modify the quality of the film in order to reduce brittleness. Plasticizing techniques involve internal plasticizers and external plasticizers. Internal plasticizers involve Chemical modification of the basic polymer that alters the physical properties of the polymers while Chemical plasticizers involve Additives of the Coating solution to achieve the desire effect of the film (flexibility, tensile Strength, adhesive properties). Level of plasticizers ranges from 1-50% by weight of film former. Examples are Castor oil, Propylene glycol, Glycerin, Surfactants, Polysorbate (tweens), sorbitan esters (spans), organic acid esters. Colourants It is to provide the distinct color and Elegance to the dosage form. To achieve the proper distribution of suspended colorants in the coating solutions requires, Use of fine powdered colorants (