Physical Properties of Glass

Physical Properties of Glass

Reprinted with permission of Wheaton Science International

Typical Composition (Approx. % by Weight) of Some Glass Labware
Typical Properties of Some Glass Labware Glass Formulations
Glass Standards

Typical Composition (Approx. % by Weight)
of Some Glass Labware

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Material Corning® 7740
Pyrex® Borosilicate
Corning® 7913
Vycor® Borosilicate
Wheaton 180, 200, 400
Wheaton 800, 900
SiO2 80.6% 96.4% 69.15% 72.3
Na2O 4.0% — 8.6% 13.4%
B2O3 13.0% 3.0% 10.8% 0.3%
Al2O3 2.3% 0.5% 5.9% 2.7%
CaO — — 0.8% 10.3%
K2O 0.1% — 1.2% 0.5%
BaO — — 2.5% 0.2%
Fe2O3 — — < 0.05% 0.04%
MgO — — 0.4% 0.3%
ZnO — — 0.6% —
Misc. Traces — 0.1% — —

Typical Properties of Some Glass Labware

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Properties Corning® Wheaton
7740 7913 180 200 400 800 900
ASTM Glass Type I —* I I — — —
ASTM Class A —* A A — — —
USP Glass Type I —* I I I III III
EP Glass Type — — I I I III III
Strain Point (deg C) 510 890 515 515 520 507 496
Annealing Point (deg C) 560 1020 565 565 560 546 536
Softening Point (deg C) 821 1530 820 820 740 727 713
Coefficient of Expansion
(x 10-7 cm/cm/deg C)
32.5 7.5 33 33 63 88 91
Light Protection No No No No No No Yes
*Corning 7913 (Vycor®) is manufactured only by Corning Inc. No federal or ASTM standards.

Glass Formulations

Glass products are made from many different variations and formulations. Following are brief definitions of some glass types and descriptions of their characteristics. Use this information as a guideline only—all materials should be tested under actual conditions before used in specific applications.

Corning® Pyrex® 7740 is a borosilicate, low expansion Type I glass designed for use in all products requiring very high resistance to strong acids or alkalis and in products intended for use in heat applications such as autoclaves, hot plates, and open flame. Examples include Pyrex® beakers, burettes, centrifuge tubes, cylinders, desiccators, dishes, flasks, fritted ware, funnels, and jars.

Corning® Vycor® 7913 is formed as a borosilicate type glass. It is then subjected to a chemical treatment that removes most of the elements in the glass except silica (SiO2). Glass is then reheated to eliminate the microscopic holes caused by the chemical treatment. Only quartz has a higher silica content. It is designed for use in all products that must withstand very high temperatures or thermal shock. Since Corning is the only company that makes VYCOR® there are no federal or ASTM standards. Examples include evaporating dishes 34587-20 and -22.

Corning® PyrexPlus® laboratory glassware is Pyrex® brand borosilicate glass labware which has been coated with a tough, transparent plastic vinyl. The coating, which is applied to the outside of the vessel, helps prevent exterior surface abrasion. It also helps minimize the loss of contents and helps contain glass fragments if the glass vessel is broken. The recommended temperature range for PyrexPlus® labware is 10 deg C to 80 deg C.

Wheaton 180 is an exceptionally clear borosilicate glass of high chemical durability which has been especially formulated for the lowest background count while still at a reasonable cost. Great care has been taken to select only those ingredients for the batch that would not cause unwanted background count or color. Potassium as a separate element has been excluded from the batch to minimize K40. Special controls ensure high quality and batch to batch uniformity. Examples include scintillation vials 08918-01, -02, -03, -05 and -06, and sample vial 08918-20.

Wheaton 200, also referred to as Wheaton-33® low extractable borosilicate glass, is a borosilicate glass with exceptional thermal endurance that meets the requirements of Federal specification DD-G-541b, glass (laboratory), for both USP and ASTM Type I, borosilicate glass Class A. This glass meets all sterilization requirements. Examples include sample bottles 08913-15, -25, -45 and -55, and large sample vials 08918-22, -23 and -24.

Wheaton 400, also known as No-Sol-Vit, is a borosilicate glass that falls well within the limits for USP Type I chemically resistant borosilicate glass, as specified in the XXIII revision of the U.S. Pharmacopoeia. Examples include safety-coated wide-mouth bottles 34501-20, -50 and -70.

Wheaton 800 is a superior soda-lime flint glass that meets requirements for USP Type III soda-lime glass as specified in the XXIII Revision of the U.S. Pharmacopoeia. Examples include clear safety-coated bottles 08922-50 and -60.

Wheaton 900 is similar in formulation to Wheaton 800 glass except it is amber colored which provides light sensitivity. Examples include amber safety-coated bottles 08922-55 and -65.

Glass Standards

USP Types

Various standards govern the classification of glass into types suitable for specific uses. These standards include ASTM (American Society for Testing and Materials), EP (European Pharmacopoeia), and USP (United States Pharmacopoeia).

The following classifications were determined by USP criteria. The applicability of a particular container to an end use cannot be determined by this criteria alone. Other general criteria has been developed to assist with the use of the USP classifications in selecting containers.

USP Type I borosilicate glass is the least reactive glass available for containers. It can be used for all applications and is most commonly used to package water for injection, UN-buffered products, chemicals, sensitive lab samples, and samples requiring sterilization. All lab glass apparatus is generally Type I borosilicate glass. Examples of Type I borosilicate glass include Corning® Pyrex® 7740 and Wheaton 180, 200, and 400. Though Corning® Vycor® 7913 is not classified as a Type I glass, it does meet or exceed USP Type I requirements.

In most cases Type I glass is used to package products which are alkaline or will become alkaline prior to their expiration date. Care must be taken in selecting containers for applications where the pH is very low or very high, as even Type I glass can be subject to attack under certain conditions. Although Type I borosilicate has the least pH shift of any glass, there still may be some sensitivity with certain packaged products.

Surface treatment is not usually required, however it will further enhance the desirable characteristics of an already superior container. This surface enhancement may become especially important for small containers because of the high ratio of container surface area to the volume of the container contents.

USP Type II de-alkalized soda-lime glass has higher levels of sodium hydroxide and calcium oxide. It is less resistant to leaching than Type I but more resistant than Type III. It can be used for products that remain below pH 7 for their shelf life.

USP Type III soda-lime glass is acceptable in packaging some dry powders which are subsequently dissolved to make solutions or buffers. It is also suitable for packaging liquid formulations that prove to be insensitive to alkali. Type III glass should not be used for products that are to be autoclaved, but can be used in dry heat sterilization. Examples of Type III soda lime glass include Wheaton 800 and 900.

USP Type NP soda-lime glass is a general purpose glass and is used for non-parenteral applications where chemical durability and heat shock are not factors. These containers are frequently used for capsules, tablets and topical products. Examples of Type NP glass include Wheaton 810 and 910.

Factors other than USP Type

  1. The filling processing steps that the container has to withstand are important. If lower thermal expansion of the container is required in the process, several options are available. A typical tubing formed container with thinner and more uniform walls will withstand thermal shock better than a molded glass container in the same expansion range. The physical design of the container also plays a part in the amount of thermal and mechanical shock resistance it exhibits. It is frequently necessary to make a compromise between high resistance to mechanical shock and high resistance to thermal shock.
  2. Light sensitivity: If the products to be packaged are light sensitive, amber glass must be used.
  3. If a product is sensitive to the presence of a particular ion, the composition of the glass container should be considered. For example, if a container was precleaned for environmental sampling and used to test for metal ions, it would not be feasible to measure low levels of metals such as sodium or calcium as the ions are in the container matrix and would eventually bloom back to the surface.
  4. The interaction of glass and aqueous solutions is extremely complex. Extractable and corrosion resistance as well as chemical resistance need to be addressed.