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Coating Formulation Calculations

(1) Calculation of Polymer QuantitiesSince a certain layer thickness has to be achieved in film coating, the amount of coating material must berelated to the surface area of the substrate. For this reason it is expressed in mg of dry polymer substanceper cm2 of surface area.

If we divide the surface area of a substrate A (mm2) by its weight w (mg), we immediately obtain therequisite coating quantity in %, i.e. the polymer consumption in kg of dry polymer substance per 100 kgof substrate for a coating of 1 mg of dry polymer substance per cm2. If lower or higher coating weightsare specified for certain dosage forms, we must multiply by this factor l = mg polymer per cm2.

Coating weight (%) =

Note that A in this formula refers to the surface area and mg per cm2 to the amount of film former. Bothquantities are linked by the factor 100, which leads to the result in percent.

Depending on the desired function of a coating, the following values can be used for the calculation ofthe required amount of polymer:

Enteric coatings: 4 – 6 mg for round tablets5 – 10 mg for oblong-shaped tablets5 – 20 mg for gelatin or HPMC capsules

Taste-masking coatings: 1 – 2 mg for round tablets1 – 4 mg for oblong-shaped tablets

Moisture protection: 1 – 6 mg for round tablets2 – 10 mg for oblong-shaped tablets5 – 10 mg for gelatin or HPMC capsules

A(mm2) • l(mg/cm2)w(mg)

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(2) Calculation of Surface Area(2.1) Simplified Calculations

The surface areas of some pharmaceutical dosage forms can be calculated according to the followingsimplified formulas, assuming that the tablet has the shape of a circumscribed cylinder:

A = surface (mm2), D = diameter (mm), H = overall height (mm),L = length (mm), Bw = band width (mm)

Tablets: A= π • (D • H + 0.5 • D2)=mm2

Capsules, oblongs: A=π • L • Bw=mm2

Spherical shapes (microtablets, pellets, granules): A= π • D2=mm2

Following the simplified calculations, the following values for the surface area of tablets and capsulescan be used:

Tablets

Diameter [mm] 5 6 7 8 9 10 12 142 70 95 120 1503 85 115 145 175 210 250 3404 100 130 165 200 240 280 380 4855 185 225 270 315 415 5306 300 345 450 5707 505 615H

eigh

t [m

m]

8 660

Surfacearea[mm2]

Capsules

Capsule size 5 4 3 2 1 0 00 000Surface area [mm2] 175 235 290 350 410 500 610 800

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(2.2) Exact CalculationsThe following formulas can serve for the exact calculation of the tablet surface (Bauer et. al.“Coated Pharmaceutical Dosage Forms”, medpharm GmbH Scientific Publishers, Stuttgart, 1988)

a) Round Biconvex Tablet

(A=2 π • (rB+r2+Ch2)

“CH” can be calculated by means of the following formula: Ch =

A = Surface areaD = Diameterr = RadiusCh = Curvature heightB = Band heightH = Overall heightWR = Curvature radiusBW = Band width

b) Oblong-Shaped TabletsX= 2 π • (rB +r2 + Ch2)

Z=2B • (L-2r)A=X+Y+Z

The individual calculation steps are based on the hatched areas in the schematic drawing:X = round endsY = 2x top-hat segmentZ = band (2x faces)

Top view

Side view

BH

rD

X X

Y

Y

Z

B

Side view Side view

Top viewBWL

Ch

}H

H – B2

Ch

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(3) Estimating the Surface Area of Small ParticlesWhere irregularly formed crystals or granules have to be evenly coated, their surface area has to beestimated with sufficient accuracy to permit calculation of the polymer requirement and the requisitelayer thickness of the diffusion shell. It is then also easier to obtain a consistent release rate when theparticle size distribution and surface texture have changed.

A fast and simple approach is the permeability methodaccording to Blaine (ASTM Des. C 205-55), whereas bothnitrogen adsorption and mercury intrusion are much morecomplicated and time-consuming.

Blaine's method serves for quick routine assessment of thespecific surface area (in cm2 or cm2 of core volume) of smallparticles. It is based on the mathematical model of laminarflow through capillaries arranged in parallel, as establishedby Kozeny-Carman. This model states that the time taken bya constant air volume to flow through a defined product bedis proportional to the square of the specific surface area ofthis powder, i.e. inversely proportional to the diameter of theequivalent sphere.The apparatus, originally developed by Blaine for measuringmicronized ceramic powders, had to be modified forpharmaceutical purposes to serve for larger particles as well.

Air volume (Friedrich manometer) and powder bed (granulate attachment according to Gupte) wereincreased to extend the measuring range to specific surface areas up to about 100 [cm2/cm3].

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Of critical importance for accurate measurement is a defined porosity of the core bed, which should bekept constant if a particular product is measured several times. This is most easily achieved by tappingthe sample in a granulate attachment fastened to a tapping volumeter by means of an adapter.The vacuum required for the airflow is produced by keeping a fluid of known density (e.g. water) out ofequilibrium in a U-shaped tube, using a rubber bulb or automatic pipette. For calculation, the measuredairflow time has to be corrected by subtracting the no-load value of the apparatus.

For better reproducibility, time should be measured electronically with the aid of light barriers activatedby the liquid meniscus.

The following values can be used to estimate the surface for spherical particles:

Spherical Particles

Diameter [mm] 0.5 0.8 1 2 3 4 5 6Surface area mm_] / piece 0.8 2 3 12.5 30 50 80 120

For spherical particles with diameters in a range of 0.5 – 1.2 mm the following % polymer weight gainscan be used as a guideline:

Enteric coatings: 10 – 30%

Sustained-release coatings: 5 – 20%

Taste-masking coatings: 5 – 10%

Moisture protection: 10 – 30%

Depending on the solubility of the active, surface structure, size of the particles and mechanical stability,quite different amounts may be needed. Therefore it is recommended to start with a coating trial inwhich samples at different polymer weight gains should be taken and tested in order to determine therequired amount of polymer.

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This information and all further technical advice is based on our present knowledge and experience. However, it implies no liability or other legal responsibility on our part, including with regardto existing third party intellectual property rights, especially patent rights. In particular, no warranty, whether express or implied, or guarantee of product properties in the legal sense is intendedor implied. We reserve the right to make any changes according to technological progress or further developments. The customer is not released from the obligation to conduct carefulinspection and testing of incoming goods. Performance of the product described herein should be verified by testing, which should be carried out only by qualified experts in the soleresponsibility of a customer. Reference to trade names used by other companies is neither a recommendation, nor does it imply that similar products could not be used.

Röhm GmbH & Co. KG is the owner of patent rights covering the use of EUDRAGIT® polymers in compositions, procedures and/or applications which may be subject to license agreements.Compositions, procedures and/or applications falling within the claims of patents related to EUDRACOL™ and EUDRAPULSE™ will always require separate license agreements.

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EUDRAGIT = reg. Trademark of Röhm GmbH & Co. KG, Pharma Polymers, Darmstadt, Germany