Glass transition temperature (T
) and brittle temperature
The glass transition temperature (Tg) of a polymer is the temperature at which the polymer goes from a hard, rigid state to a rubbery, flexible state.28–31 The brittle temperature is defined as the temperature at which the material becomes brittle.32–34 The correlation between the brittle temperature, the glass transition temperature, and mechanical behavior at cold temperatures
is not obvious, especially in the case of multilayer structures. In one study, it was shown that failure of a disposable bag
in a frozen state did not correlate to the value of the bag material's glass transition temperature.35 As shown in Figure 2, a low Tg does not always mean a good resistance at cold temperatures.
Figure 2. Relationship between bag failure and Tg in a drop test at frozen state (–70°C). There is no direct relationship between a low Tg and a low failure rate (Reference 1).
Water loss.36–39 Water loss is important when the concentration of the products inside a container is critical. For drug delivery containers,
acceptable losses are described in the ICH Q1A guideline.40
Gas Permeability.41–43 Permeability to gas, particularly O2 and CO2, is important when the product inside the container is sensitive to oxidation. (If the solution is not oxidizable, the oxygen
contained in the headspace above the solution will not be consumed.) Because the permeation phenomenon is driven by the partial
pressure equilibrium on both sides of the film,44 no oxygen will enter the container if oxygen is not consumed by the solution. In such a case, a low barrier film could be
sufficient. Regarding the barrier to CO2, if the quantity of carbon dioxide contained in the air is below 380 ppm, the main reason to examine this property is if
the product will be shipped in dry ice.
Transparency / Haze
Transparency, also known as haze, is evaluated by measuring the specific light transmitting and wide-angle light-scattering
properties of the materials, and is performed on a planar section of film. This criterion applies to essentially transparent
plastic materials. This property can depend on whether the film is wet or dry.
Technical requirements of the final product configuration
Chemical compatibility. The purpose of container–solution interaction studies is to determine the possible chemical attack the solution might have
on the film. Very few standard tests exist for this purpose. An adaptation of the ASTM D543 test47 can be used to "grade" the compatibility of the film. The signs of chemical attack on the film can be evaluated by measuring
the following aspects:
- Change in thickness
- Change in the film's physical properties
- Change in color (absorption, extraction, chemical reaction)
- Change in weight (absorption, extraction)
- Change in surface quality (cracking, transparency)