Glass or plastic: The tough decision
Selecting the appropriate test tubes for your application is not as easy as it may seem. There are test tubes specialized for centrifugation, PCR, culturing, and storing. For centrifugation, there are filtration/concentration, microcentrifuge, conical and round bottom, as well as reusable, glass, and sterile tubes. Also, you have to keep in mind the obvious considerations such as cost, heating temperature, ease of cleaning, the flexibility of use, size, and if you require a disposable solution. In addition, it’s important to review the chemical compatibility of glass and plastic because both have physical properties that can be affected when subjected to certain chemicals.
Effects of chemicals on glass and plastic
Chemicals may affect the weight, strength, color, dimensions, flexibility, and surface appearance of the labware. The basic models of interaction that cause these changes are:
(1) chemical attack on the polymer chain, with the resultant reduction in physical properties including oxidation, the reaction of functional groups in or on the chain, and depolymerization.
(2) physical change including absorption of solvents resulting in the softening and swelling of the plastic, permeation of solvent through the plastic, or dissolution in a solvent
(3) Stress-cracking from the interaction of a “stress-cracking agent” with molded-in or external stresses.
The reactive combination of compounds of two or more classes may cause a synergistic or undesirable chemical effect. Other factors affecting chemical resistance include:
- Internal or external stresses (such as centrifugation)
- Length of exposure to and concentration of the chemical
- As temperature increases, resistance to attack decreases
Physical properties of glass
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 these criteria alone. Other general criteria have 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.
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.
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.
Factors other than USP Type
The filling processing steps that the container has to withstand are important. If a 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.
- Light sensitivity: If the products to be packaged are light sensitive, amber glass must be used.
- 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.
- The interaction of glass and aqueous solutions is extremely complex. Extractable and corrosion resistance, as well as chemical resistance, need to be addressed.
Physical properties of plastics
Physical properties of plastics (cont’)
*Radiationgamma irradiation at 2.5 mrad with unstabilized plastic.
**Sterilizing reduces mechanical strength. Do not use PC vessels for vacuum applications if they have been autoclaved.
ttPUR tubing is not autoclavable.
tttSome PVC tubing may be autoclaved; see specific product information.
HalarReg TM Allied Corp.
KynarReg TM Elf Atochem North America
TefzelReg TM E. I. Du Pont de Nemours & Co.
TPXReg TM Mitsui Petrochemical Industries, Ltd.