Most polymers produced by condensation polymerisation, such as polycarbonate are hygroscopic.  They absorb water from direct exposure or from the water vapour present in the air.  Parts fabricated from hygroscopic resins will also absorb water.

this behaviour is important to understand because:

• Moisture absorbed by a finished part will affect the performance of the part to some small extent.
• Moisture absorbed by a resin before fabrication, unless removed by drying before the processing begins, can cause serious degradation of properties during the moulding process.

Weatherability

Ultraviolet stabilisation can greatly extend the retention of key physical properties such as impact strength and appearance.

Chemical Resistance

The practical chemical resistance of material relates not only to exposure to the reagent, but also to the amount of stress on the part, the environmental temperature, and the duration of exposure.  You should consider these factors in any application involving adverse environments.  If the application involves unusual exposures or severe chemical environments, test samples or prototype parts should be fabricated, exposed to the actual materials and conditions of use, and then evaluated before a full production run is started.

Critical Stress

When a part made from a thermoplastic resin is designed for working in an environment involving chemical exposure, the amount of stress on the final part is critical.  The more highly stressed the part is, either from moulded-in stress or from external loading in the final application, the more susceptible the part will be to chemical attack.

The critical stress - the maximum stress the material can withstand in air at 23 Degree Celcius, and 50% RH - is a reference point which can vary significantly with many factors.  Among these factors are: temperature, the nature of the solvent, whether the strain is introduced by internal or external stress, and the melt flow rate of the resin.

Solubility

Most chemical handbooks include solubility data for chemicals in general use.  Such data can be used to estimate how a thermoplastic part will be affected by exposure to a particular chemical, by comparing the solubility of the thermoplastic to that of the chemical under consideration.

Gas Permeability

Gas permeability or transmission, often must be taken into account when designing packaging and certain other associated applications.  The value is determined by measuring the anount of a specific gas which passes through a volume of the material being tested, in a given time and under predetermined conditions.  The gases most frequently  used in testing are nitrogen, oxygen and carbon dioxide.