Composition of Glass
Table of Contents
The chemical composition of glass is mainly of sand (SiO2), soda ash (Na2CO3), limestone (CaCO3), and cullet. Cullet is broken glass that is mixed with sand, soda ash, and limestone to acts as a fusion agent for the entire mixture. The composition of glass varies and is usually adjusted for specific purposes. The most common cations found in pharmaceutical glassware are:
- Boron, (borosilicate glass)
- Sodium,( soda-lime glass)
- Ferrous (amber colored glass)
- Zinc, and
The only anion of consequence is oxygen. Many useful properties of glass are affected by the kind of elements it contains. Reduction in the proportion of sodium ions makes glass chemically resistant; however, without sodium or other alkalies, glass is difficult and expensive to melt. Boron oxide is incorporated mainly to aid in the melting process through reduction of the temperature required. Lead in small traces gives clarity and brilliance but produces a relatively soft grade of glass. Alumina (aluminum oxide), however; is often used to increase the hardness and durability and to increase resistance to chemical action.
1. Colored Glass – Light Protection: Glass containers for drugs are generally available in clear flint or amber color. For decorative purposes, special colors such as blue, emerald green, and opal may be obtained from the glass manufacturer. Only amber glass and red glass are effective in protecting the contents of a bottle from the effects of sunlight by screening out harmful ultraviolet rays. The USP specifications for light-resistant containers require the glass to protect 290 to 450 nm of light. Amber glass meets these specifications, but the iron oxide added to produce this color could leach into the product.
2. Glass for Drugs: The USP and NF describe the various types of glass (type I, type II, type III, and type NP) and provide the powdered glass and water attack tests for evaluating the chemical resistance of given glass. These tests are the measures of the amount of alkalinity leached from the glass by purified water under controlled elevated temperature conditions. The powdered glass test is performed on crushed grains of a specific size and is meant for type I, type III, and type NP glass. The water attack test is conducted on whole containers and is used only with type II glass.
The chemical stability of glass for pharmaceutical use is given by the resistance of the glass to the release of soluble minerals into water contacting the glass. This is known as hydrolytic resistance.
Glass as a Packaging Material:
Glass is the preferred packaging material. Glass does have several advantages:
- It is inert to most medicinal products.
- It is resistant to air and moisture.
- It allows easy inspection of the container contents as it is transparent.
- It can be colored (amber colored) to protect contents from harmful wavelengths of UV light.
- Easy to clean and sterilize by heat.
- It is moldable in variously shaped containers.
Disadvantages of Glass:
- Fragile (it is easily broken): Glass fragments and cracks.
- Costlier in comparison to plastic.
- As it is heavy transport cost is high.
- Certain types of glass release alkali into the container contents.
Types of Glass
- Type-I glass
- Type-II glass
- Type-III glass
1. Type-I Glass:
Composition: Neutral glass, borosilicate glass [silica (silicon dioxide, SiO2)] and boron oxide).
- It possesses a high hydrolytic resistance.
- It is the most inert type of pharmaceutical glass.
- It has the lowest coefficient of thermal expansion (and hence is suitable for sterilization by heat for ampoules and vials).
- It has a very high glass transition temperature so needs complicated processing.
- And therefore expensive.
- Type I glass is suitable for packing all pharmaceutical preparations.
- It is widely used as glass ampoules and vials to package fluids for injection.
- In contrast to the other types of glass (type II and III), this type has no/little amounts of basic oxides, so it is used to package solutions that could dissolve basic oxides in the glass.
2. Type-II Glass:
Composition: soda-lime-silica glass.
Soda (Na2CO3) is used to decrease the glass transition temperature of silica. However, soda would increase the water solubility of silica, so lime (CaO) is used to increase the hydrolytic resistance. This type would also contain other oxides.
- This glass has a lower melting point than Type I glass. It is thus easier to produce and consequently cheaper.
- High hydrolytic resistance due to surface treatment of the glass.
- Type II glass is used to package aqueous preparations.
- However, as it contains basic oxides, it is not used to package parenteral formulations with a pH < 7 (i.e. acidic); this would increase the pH of the formulation and could affect the drug stability and potency.
- It is the glass used to produce containers for eye preparations and other dropper bottles.
3. Type-III Glass:
- Composition: soda-lime-silica glass: It has a similar composition to Type II glass but contains more leachable oxides.
- Properties and uses: Type III glass offers only moderate resistance to leaching and is commonly used to produce dispensary metric medical bottles. It is also suitable for packaging non-aqueous parenteral products and powders for injection.
- Type NP: General-purpose soda-lime glass.
- Containers made of soda-lime glass are supplied for non-parenteral products.
Although glass exhibits many advantages over other packaging materials, it has two principal faults namely the release of alkali and the release of insoluble flakes to liquids stored in the container. By decreasing the soda content in the glass or replacing the sodium oxide with other oxides, it has been possible to overcome the property of glass to release alkali cations into the solution.
Several approaches have been used to enhance the resistance of glass to alkali release by surface treatment.
1. Treating the surface of the soda-lime glass to produce a fire-polished skin of silica, which is more resistant than the inner layers of glass.
2. Treating the surface of the glass with sulfur dioxide in the presence of water vapor and heat. This causes the surface alkali to react with the sulfur dioxide, and the glass becomes more resistant. The stability of the drugs with high potency and consequently of low dosage can be readily affected by the release of soluble alkali from glass containers. As a safety factor, whenever the dosage form is liquid, the solution is buffered to eliminate any effect due to possible change in pH if some alkali were released from the glass.
The type of glass employed plays a major role in whether flake formation takes place. For example, flake formation may occur in non-borosilicate glass immediately after autoclaving, whereas, in borosilicate glass, it occurs at temperatures much higher than those normally used for autoclaving. Glass containers may possess various additives such as oxides of boron, sodium, potassium, calcium, iron, and magnesium which alter physical and chemical properties of the glass. For example, when formulating sulfate salts (drug substances or antioxidants) the glass container should have minimal amounts of calcium and barium to prevent the formation of insoluble inorganic salts. Many pharmaceutical preparations exhibit physical or chemical changes due to the radiant energy of light. Light radiations can cause color development or color fading and initiate an oxidation-reduction reaction resulting in drug degradation, rancidity of oil formulations, flavor, and odor loss.
Flint glass, which is the most widely used multipurpose container material, has the disadvantage of being transparent to light rays above 300 mµ. As a result amber glass, which has the property of shutting out certain portions of the light spectrum, has been used extensively by the pharmaceutical industry. The transmission curves for flint and amber glass show that although flint glass transmits significantly from 300 mµ, amber glass does not begin to transmit to any appreciable extent until 470 mµ.
Types of Glass Containers
- These are either amber metric medical bottles or ribbed (fluted) oval bottles. Both types are supplied with a screw closure.
- Amber metric medical bottles are used for packaging a wide range of oral medicines.
- Ribbed oval bottles are used to package various products that should not be taken orally; this includes liniments, lotions, inhalations, and antiseptic solutions.
2. Containers for Parenteral Products:
- Small-volume parenteral products, such as subcutaneous injections, are typically packaged in various containers made of Type I glass.
- Powders and semi-solid preparations are generally packed in wide-mouthed cylindrical jars made of clear or amber glass.
4. Dropper Bottles:
- Eye drop and dropper bottles for ear and nasal use are hexagonal-shaped amber glass containers fluted on three sides. They are fitted with a cap, rubber teat, and dropper as the closure.
- Packaging materials.
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