(1)The two main types of basic moisture conditioning are:
Fixed conditioning, where a test specimen is exposed to a conditioning environment for a specified time; and
Equilibrium conditioning, where a specimen is exposed until the material reaches equilibrium with the conditioning environment.
The first technique is routinely employed for screening purposes. This approach results in non-uniform moisture distribution through the thickness of the test specimen. In principle, test data obtained from specimens conditioned in this manner are only considered suitable for comparing different batches of the same material or for quality control tests. This approach, however, is widely used for generating engineering data. It is essential that test specimens used in this manner are identical in dimensions and have similar surface finishes.
Ideally, comparative studies of water absorption properties of materials should be carried out only using the equilibrium moisture content of polymeric materials exposed to identical conditions. Comparisons between composite systems with different moisture absorption characteristics are possible if the materials are conditioned to equilibrium. The thicker the material the longer the time required to reach equilibrium, hence the use of relatively thin specimens to determine the “through-the-thickness” moisture diffusion coefficient. The large timescales involved, even under accelerated testing conditions, can make this approach impractical.
The International standard BS EN ISO 62 [1] describes a procedure for determining the moisture absorption properties and/or diffusion coefficients in the “through-the-thickness” direction of flat and curved solid plastics. BS EN ISO 62 is suitable for use with polymer and composite specimens. The method can be applied to vapour exposure and liquid immersion.
Conditioning usually consists of exposing pre-dried specimens to a steady-state environment (i.e. constant temperature and constant moisture level) and measuring the moisture gain (i.e. average moisture content) for a prescribed period of time or until the specimen reaches moisture equilibrium. The amount of water absorbed by the test specimen is determined by measuring its change in mass (i.e. difference between initial mass and the mass after exposure). All surfaces are in contact with the test environment. It is recommended that when determining moisture absorption properties that square shape specimens be used for homogeneous plastics. In this case, specimen dimensions and tolerances should comply with ISO 294-3 [2].
It is recommended that specimens be pre-dried in an oven maintained at 50 ± 2 °C until the specimen weight reaches a constant value. The temperature of the drying oven should not exceed the maximum operating temperature of the polymeric-material. Specimens are removed at fixed intervals (typically 24 hrs) and allowed to cool to room temperature in a desiccator before being weighed.
After weighing, the specimen is returned to the oven and the process is repeated until the mass of the specimen is constant (zero datum level). Specimens should be free of voids in order to ensure accurate moisture absorption measurements. To minimise moisture uptake prior to conditioning, specimens are stored in a desiccator at room temperature. It should be noted that under standard laboratory conditions many polymers absorb significant levels of moisture.
Conditioned specimens need to be wiped with a clean cloth to ensure all surface water is removed prior to weighing. Damage may accumulate during long-term conditioning, and hence handling and monitoring of test specimens should be minimal. This is particularly pertinent to those specimens used for generating engineering data. Travellers are required to monitor specimen moisture content throughout the environmental history (i.e. manufacture, storage, conditioning and testing). Traveller specimens should have identical material properties, geometry and processing history as the test specimen.
Conditioning is often carried out at elevated temperatures or humidities to accelerate moisture uptake. Care should be taken to avoid exceeding the glass transition temperature Tg of the material. The recommended maximum conditioning temperatures are 45°C and 60°C for 120°C and 180°C cure systems, respectively 3]. These temperatures should not be exceeded.
The rate of moisture uptake is fairly rapid in the early stages of conditioning with the rate of moisture uptake decreasing with time (see Figure 4). It is therefore necessary to make frequent weight measurements in the early stages (3-4 measurements on day one) followed by at least two readings per day for the remainder of the first week. At least one reading per day is required per day for the second week, followed by a gradual decrease in frequency as the rate of weight gain diminishes. It is recommended that weighing be carried at approximately equal intervals of (time)1/2. The percentage uptake of water by weight, M , can be determined as follows:
(1)
The wet and dry weights are denoted by WWET and WDRY . A precision analytical balance capable of reading to within 0.0001 g is required. Accurate records need to be kept on the conditioning sequence including relative humidity, temperature and time, measured weights and derived moisture contents.
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