Triacetin

Introduction

Plasticizers are chemicals that modify the physical properties of a basic polymer. Plasticizers are chemically and thermally stable compounds, ranging from solids to liquids. They are used in plastics to impart flexibility, resilience, and softness to various polymers, and to reduce brittleness. The most common uses of plasticizers in pharmacy are in soft gelatin capsules, gelatin-based suppositories, ointments, film-coated tablets, and plastic parenteral containers (bags).

When plasticizers are used in the correct proportion to the polymer, they impart flexibility by relieving the molecular rigidity of the material. The type of plasticizer, or combination of plasticizers, and its ratio to the polymer can be optimized to obtain the desired properties. The plasticizer and the polymer must be at least partially soluble or miscible in each other.1

A soft gelatin capsule shell basically consists of gelatin, a plasticizer, and water, along with additional ingredients such as preservatives, colorants and opacifying agents, flavorings, sugars, acids, and medications. Plasticizers used with gelatin in the preparation of soft gelatin capsules include glycerin and sorbitol. The ratio of plasticizer to gelatin determines the hardness of the gelatin shell, eg, a ratio of dry glycerin:dry gelatin of 0.4:1 will produce a harder shell used for oil-based or shell-softening products and for preparations used in hot, humid environments; a ratio of 0.6:1 will produce a medium hard capsule for oral or vaginal oil-based, water miscible-based or shell-hardening products and for preparations used in temperate areas; a ratio of 0.8:1 is used for tube, vaginal watermiscible based or shell-hardening products and for preparations used in primarily cold, dry areas.1

A gelatin-based suppository will be either hard and tough or soft and mushy if glycerin is not added as a plasticizer to obtain the desired characteristics of softness and resilience. Plasticizers also provide a medium for water to penetrate the suppository matrix and aid in dissolution and release of the active drug.

To film-coat tablets, a thin, skin-tight coating of a plasticlike material (polymer) is applied to the surface of the tablet so that the tablet is essentially close to the same weight, shape, and size as the original compressed tablet. The coating is usually thin enough to reveal any identifying monograms embossed in the tablet during its compression. A plasticizer will produce flexibility and elasticity of the coating, provide durability, and prevent the film from becoming brittle, thereby reducing the risk of chipping. A typical aqueous film-coating formulation contains the film-forming polymer (7% to 18%), plasticizer (0.5% to 2.0%), colorant and opacifier (2.5% to 8%), and vehicle (qs to 100%).

In plastics, more than 80% of all plasticizers are used with polyvinyl chloride (PVC); the rest go into such plastics as cellulosics, nylon, polyolefins, and styrenics. Phthalates are the most popular plasticizers in plastics. Plasticizers can migrate to the surface of a plastic container and can be extracted into a drug solution contained within the container.2

In general, plasticizers can include such agents as castor oil, propylene glycol, glycerin, low-molecular-weight polyethylene glycols (PEGs), surfactants, and organic acid esters. The oils are primarily used for organic solvent-based products (coating solutions) and the surfactants, propylene glycol, and PEGs are used for aqueous coating where water-soluble plasticizers are needed.

Plasticizers currently used include acetyltributyl citrate, acetyltriethyl citrate, benzoyl benzoate, castor oil, chlorobutanol, diacetylated monoglycerides, dibutyl sebacate, diethyl phthalate, glycerin, mineral oil and lanolin alcohols, petrolatum and lanolin alcohols, PEGs, propylene glycol, sorbitol, triacetin, tributyl citrate, and triethyl citrate.

Plasticizers in Current Use

Acetyltributyl Citrate NF (C^sub 20^H^sub 34^O^sub 8^, MW 402.48) occurs as a clear, practically colorless, oily liquid that is insoluble in water but freely soluble in alcohol, isopropyl alcohol, acetone, and toluene.3

Acetyltriethyl Citrate NF (C^sub 14^H^sub 22^O^sub 8^, MW 318.32) occurs as a clear, practically colorless, oily liquid that is insoluble in water, freely soluble in alcohol, isopropyl alcohol, acetone, and toluene.3

Benzyl Benzoate USP (C^sub 14^H^sub 12^O^sub 2^, MW 212.24, benzyl phenylformate) is used as a solubilizing agent and nonaqueous solvent in intramuscular injections (0.01% to 46%) and as a solvent and plasticizer for cellulose and nitrocellulose. It occurs as a clear, colorless, oily liquid with a slightly aromatic odor. When cooled below 17 deg C, it occurs as clear, colorless crystals. It is practically insoluble in water and glycerin but is miscible with 95% ethanol, ether, and oils. It is incompatible with alkalis and oxidizing agents. It should be stored in tight, well-filled light-resistant containers, and exposure to excessive heat should be avoided.3,4

Castor Oil USP is obtained as a fixed oil from the seed of Ricinus communis Linne (Fam. Euphorbiaceae). It occurs as a pale yellowish or almost colorless, transparent, viscid liquid with a faint, mild odor and a bland, characteristic taste. It is soluble in alcohol and miscible with dehydrated alcohol, glacial acetic acid, and ether. It should be stored to avoid exposure to excessive heat.3

Chlorobutanol NF (C^sub 4^H^sub 7^C^sub 13^O, MW 177.46, acetone chloroform) is an antimicrobial preservative in pharmaceuticals and cosmetics and a plasticizer for cellulose esters and ethers. It occurs as volatile, colorless or white crystals with a musty, camphoraceous odor. It is volatile and readily sublimes.3,5

Diacetylated Monoglycerides NF is glycerin that is esterified with edible fat-forming fatty acids and acetic acids. It occurs as a clear liquid that is very soluble in 80% (w/w) aqueous alcohol, vegetable oils, and mineral oils; it is only sparingly soluble in 70% alcohol.3

Dibutyl Sebacate NF consists of esters of n-butyl alcohol and saturated dibasic acids, principally sebacic acid, consisting of not less than 92.0% of C^sub 18^H^sub 34^O^sub 4^. It occurs as a colorless, oily liquid with a very mild odor. It is soluble in alcohol, isopropyl alcohol, and mineral oil; it is very slightly soluble in propylene glycol and practically insoluble in water and glycerin.3,6

Diethyl Phthalate NF (C^sub 12^H^sub 14^O^sub 4^, MW 222.24, DEP) is used as a plasticizer for film coatings on tablets, beads, and granules (10% to 30% w/w) and is used as a solvent. It is also used as an alcohol denaturant, a solvent for cellulose acetate, and a perfume fixative. It occurs as a clear, colorless, oily liquid that is practically odorless or has a very slight aromatic odor with a bitter, disagreeable taste. It is miscible with ethanol and many other organic solvents but is practically insoluble in water. It is incompatible with strong oxidizing materials and should be stored in a cool, dry place.3,7

Glycerin USP (Glycerol, 1,2,3-propane triol) occurs as a clear, colorless, odorless, viscous, hygroscopic liquid with a sweet taste about two thirds as sweet as that of sucrose. Some of its uses are as an antimicrobial preservative, an emollient and humectant, an ingredient in ophthalmic formulations, a plasticizer in film coating for tablets, a parenteral solvent, and a sweetening agent in alcoholic elixirs. Glycerin is used as a plasticizer of gelatin in the production of soft gelatin capsules and in the preparation of gelatin suppositories. It is miscible with water, methanol, and 95% ethanol and is practically insoluble in oils and chloroform and slightly soluble in acetone. It is hygroscopic and should be stored in a cool place. Incompatibilities include use with strong oxidizing agents such as chromium trioxide, potassium chlorate, and potassium permanganate, which may cause it to explode. When mixed with zinc oxide or basic bismuth nitrate and exposed to light, it will form a black discoloration. When mixed with phenols, salicylates or tannin, a darkening of the mixtures may occur because of an iron contaminant in the glycerin. It will also form a strong acid complex, glyceroboric acid, when mixed with boric acid.3,8

Mineral oil and lanolin alcohols (Amerchol L-101, Protalan M16, Protalan M-26, liquid paraffin and lanolin alcohols) is an oily liquid that is used as an emollient, emulsifying agent, and plasticizer. It consists of lanolin alcohols dissolved in mineral oil and occurs as a pale yellow-colored, oily liquid with a faint characteristic sterol odor. It is soluble 1 in 2 parts of chloroform, 4 parts of castor oil, and 4 parts of corn oil. It is practically insoluble in 95% ethanol and water. It is incompatible with coal tar, icthammol, phenol, and resorcinol. It should be stored in a cool, dry place.9

Petrolatum and lanolin alcohols (Amerchol CAB, Forlan 200, petrolatum and wool alcohols, white/yellow soft paraffin and lanolin alcohols) consists of lanolin alcohols blended together with petrolatum. It occurs as a pale, ivory-colored, soft solid with a faint, characteristic sterol odor. It is used as an absorption base component and as an emollient and plasticizer in ointments (5% to 50%). It is soluble 1 in 20 parts of chloroform or 100 parts of mineral oil. It may be dispersed in isopropyl palmitate, and it forms a gel with castor oil and corn oil. It is listed as incompatible with coal tar, icthammol, phenol, and resorcinol. It should be stored in a cool, dry place.10

Polyethylene Glycol NF (Carbowax, PEG, polyoxyethylene glycol) is an addition polymer of ethylene oxide and water. It is used as ointment bases, plasticizers, solvents, suppository bases, and tablet and capsule lubricants. The effectiveness of tablet binders can be enhanced using the higher-molecularweight PEGs, and they can impart plasticity to granules. The solid grades are also widely used as plasticizers in conjunction with film-forming polymers; they are also used as plasticizers in microencapsulation to avoid rupture of the coating film when the microcapsules are compressed into tablets. At room temperature, PEGs with molecular weights of 200 to 600 are liquid, and those with molecular weights greater than 1000 are solid. The liquid PEGs are clear, colorless, or slightly yellowcolored viscous liquids with a slight but characteristic odor and a bitter, slightly burning taste. Solid PEGs are white or offwhite pastes or waxy flakes. Those with molecular weights greater than 6000 are available as free-flowing powders. The melting points for the PEGs are as follows: PEG 1000 (37 deg C to 40 deg C), PEG 1500 (44 deg C to 48 deg C), PEG 1540 (40 deg C to 48 deg C), PEG 2000 (45 deg C to 50 deg C), PEG 3000 (48 deg C to 54 deg C PEG 4000 (50 deg C to 58 deg C), PEG 6000 (55 deg C to 63 deg C ), PEG 8000 (60 deg C to 63 deg C), PEG 20,000 (60 deg C to 63 deg C ). The PEGs are all soluble in water and miscible in all ratios with other PEGs. Listed incompatibilities include some colors and possible discoloration in the presence of iron. Penicillin and bacitracin may experience a loss of antibacterial activity, and the parabens may lose some preservative efficacy. Physically, PEG bases may soften when mixed with phenol, tannic acid, or salicylic acid. Sulfonamides and dithranol may discolor, and sorbitol may precipitate from mixtures with PEGs. Polyethylene, polyvinyl chloride, and cellulose-ester membranes (filters) may soften or be dissolved by the PEGs. When applied topically, especially to mucous membranes, they may cause irritation or stinging.3,11

Propylene Glycol USP (C^sub 3^H^sub 8^O^sub 2^, MW 76.09) occurs as a clear, colorless, viscous, practically odorless liquid somewhat resembling glycerin, with a sweet taste. It is miscible with acetone, chloroform, 95% ethanol, glycerin, and water. It is not miscible with fixed oils or light mineral oil; however, it will dissolve some essential oils. Propylene glycol is used as a plasticizer in aqueous film-coating formulations; as a humectant in topicals; as a preservative in solutions and semisolids; and as a solvent or cosolvent in aerosols, oral solutions, parenterals and topicals. Propylene glycol is actually a better solvent than glycerin. Since propylene glycol is hygroscopic, it should be stored in an airtight container and protected from light. Incompatibilities include potassium permanganate.3,12

References

1. Hostetler V. Capsules. In: Lachman L, Lieberman HA, Kanig, JL. The Theory and Practice of Industrial Pharmacy. Philadelphia, PA:Lea & Febiger; 1986:368-369, 400.

2. Anes JM, Nase RS, White CH. Use of plastics for parenteral packaging. In: Avis KE, Lieberman HA, Lachman L. Pharmaceutical Dosage Forms: Parenteral Medications. New York:Marcel Dekker, Inc; 1991;1:397.

3. US Pharmacopeial Convention, Inc. United States Pharmacopeia 25/National Formulary 20. Rockville, MD:US Pharmacopeial Convention, Inc; 2001:217, 325, 808-809, 1480, 1741, 2363, 2367, 2369, 2371, 2374, 2391, 2394,2397,2401,2625.

4. Daskalakis SA. Benzyl benzoate. In: Kibbe A, ad. Handbook of Pharmaceutical Excipients. 3rd ed. Washington, DC:American Pharmaceutical Association; 2000:44-45.

5. Nash RA. Chlorobutanol. In: Kibbe A, ed. Handbook of Pharmaceutical Excipients. 3rd ed. Washington, DC:American Pharmaceutical Association; 2000:126-128.

6. Kennedy SW, Wheatley TA. Dibutyl sebacate. In: Kibbe A, ed. Handbook of Pharmaceutical Excipients. 3rd ad. Washington, DC:American Pharmaceutical Association; 2000:178-179.

7. Julian TN, Wheatley TA. Diethyl phthalate. In: Kibbe A, ed. Handbook of Pharmaceutical Excipients. 3rd ed. Washington, DC:American Pharmaceutical Association; 2000:182-183.

8. Price JC. Glycerin. In: Kibbe A, ed. Handbook of Pharmaceutical Excipients. 3rd ed. Washington, DC:American Pharmaceutical Association; 2000:220-222.

9. Fazzi AA, Kibbe AH. Mineral oil and lanolin alcohols. In: Kibbe A, ed. Handbook of Pharmaceutical Excipients. 3rd ed. Washington, DC:American Pharmaceutical Association; 2000:349.

10. Beasley MW. Petrolatum and lanolin alcohols. In: Kibbe A, ed. Handbook of Pharmaceutical Excipients. 3rd ed. Washington, DC:American Pharmaceutical Association; 2000:365-366.

11. Price JC. Polyethylene glycol. In: Kibbe A, ed. Handbook of Pharmaceutical Excipients. 3rd ed. Washington, DC:American Pharmaceutical Association; 2000:392-398.

12. Dandiker Y. Propylene glycol. In: Kibbe A, ed. Handbook of Pharmaceutical Excipients. 3rd ad. Washington, DC:American Pharmaceutical Association; 2000:442-444.

13. Nash RA. Sorbitol. In: Kibbe A, ed. Handbook of Pharmaceutical Excipients. 3rd ed. Washington, DC:American Pharmaceutical Association; 2000:515-518.

14. Palmieri A. Triacetin. In: Kibbe A, ed. Handbook of Pharmaceutical Excipients. 3rd ed. Washington, DC:American Pharmaceutical Association; 2000:570-571.

15. Kennedy SW. Triethyl citrate. In: Kibbe A, ed. Handbook of Pharmaceutical Excipients. 3rd ed. Washington, DC:American Pharmaceutical Association; 2000:574-575.

Copyright International Journal of Pharmaceutical Compounding Mar/Apr 2003
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