Glass vs Plastic Test Tubes

Kinesis® round bottom test tubes.

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:

  • Temperature
  • Pressure
  • 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

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 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.

  1. Light sensitivity: If the products to be packaged are light sensitive, amber glass must be used.
  2. 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.
  3. 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

Resin Max use Brittleness Transparency Sterilization
temp temp
(°F/°C) (°F/°C) Autoclave Gas Dry Radiation* Disinfectants
heat
ECTFE, ETFE 302/150 1.4952381 Translucent Yes Yes Yes No Yes
FEP 401/205 1.68148148 Translucent Yes Yes Yes No Yes
HDPE 248/120 1.48 Translucent No Yes No Yes Yes
LDPE 176/80 1.48 Translucent No Yes No Yes Yes
NYL 194/90 32/0 Translucent No Yes No Yes Yes
PPCO 250/121 1 Translucent Yes Yes No No Yes
PC 275/135 1.56296296 Transparent Yes** Yes No Yes Yes
PETG 158/70 <-40/<-40 Transparent No Yes No Yes Yes
PFA 482/250 1.68148148 Translucent Yes Yes Yes No Yes
PK 220 -40 Opaque Yes Yes — Yes Some
PMMA 122/50 — Transparent No Yes No Yes Some
PMP 347/175 68/20 Transparent Yes Yes Yes** No Yes
PP 275/135 32/0 Translucent Yes Yes No No Yes
PS 194/90 68/20 Transparent No Yes No Yes Some
PSF 329/165 1.48 Transparent Yes Yes Yes** Yes Yes
PTFE 500/260 1.68539326 Translucent Yes Yes Yes Yes Yes
PUR 180/82 1.34285714 Transparent Yes**tt Yes No Yes Yes
PVC 158/70 0.73333333 Transparent No**ttt Yes No No Yes
PVDF 230/110 1.29032258 Translucent No Yes No No Yes
TPE 250/121 — Opaque Yes Yes No — Some
XLPE 212/100 1.52542373 Translucent No Yes No Yes Yes

Physical properties of plastics (cont’)

Resin Specific Flexibility Permeability (approximate Water
gravity cc-mm/m2-24hr-Br absorption
(g/mL) N2 O2 CO2 (%)
ECTFE, ETFE 1.7 Rigid — — — <0.03
FEP 2.15 Excel 4960 11,625 34,100 <0.01
HDPE 0.95 Rigid 651 2868 8990 <0.01
LDPE 0.92 Excel 2790 7750 41,850 <0.01
NYL 1.13 Rigid — — — 1.3
PPCO 0.9 Mod 698 3100 10,075 <0.02
PC 1.2 Rigid 755 4650 16,663 0.35
PETG 1.27 Mod 155 388 1240 0.15
PFA 2.15 Excel 4511 13,656 35,030 <0.03
PK 1.24 Rigid — 3.1 25 0.45
PMMA 1.2 Rigid — — 310 0.35
PMP 0.83 Rigid 17,050 69,750 — 0.01
PP 0.9 Rigid 744 3720 12,400 <0.02
PS 1.05 Rigid 853 4650 17,825 0.05
PSF 1.24 Rigid 853 4650 10,850 0.3
PTFE 2.15 Rigid — — — 0.01
PUR 1.2 Excel — 1162-5068 6975-25,575 0.03
PVC 1.34 Rigid 31-310 62 62 0.15-0.75
PVDF 1.75 Rigid 140 217 7828 0.05
TPE 1.2 Excel 481-2248 1317-10,013 13,950-133,827 <0.01
XLPE 0.93 Rigid — — — <0.01

*Radiation—gamma 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.
Halar—Reg TM Allied Corp.
Kynar—Reg TM Elf Atochem North America
Tefzel—Reg TM E. I. Du Pont de Nemours & Co.
TPX—Reg TM Mitsui Petrochemical Industries, Ltd.

 

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