Plastics exposed to water are affected in several ways:
However, exposure to moisture, immersion in water, and exposure to boiling water can result in significantly different material responses. Equilibrium moisture content can be used to compare the amount of water absorbed by different types of plastics when exposed to moisture. Moisture content determined under non-equilibrium conditions can be used to compare different batches of the same material and to determine the diffusion constant of the material, determined under carefully controlled non-equilibrium moisture exposure conditions and when using plastic samples of specified dimensions.
The standard "ISO 62 Plastics - Determination of water absorption," developed by the International Organization for Standardization (ISO), describes a procedure for determining the through-thickness moisture absorption properties of solid plastics in flat or curved forms. This standard also describes procedures for determining the amount of water absorbed by plastic samples of defined dimensions when immersed in water or exposed to moist air under controlled conditions.
The through-thickness moisture diffusion coefficient can be determined for a single-phase material by assuming Fickian diffusion behavior with constant moisture sorption properties throughout the thickness of the test specimen. In Fickian diffusion, there is a flow from a higher concentration level to a lower concentration level. This model is valid for homogeneous materials and reinforced polymer-matrix composites tested below their glass transition temperatures. However, some two-phase matrices, such as cured epoxies, may require a multiphase sorption model not covered by this standard.
Ideally, the best comparison of water absorption properties or diffusion coefficients of materials should be made using only the equilibrium moisture content of plastics subjected to the same conditions. Comparing materials using their moisture equilibrium properties does not assume, and therefore is not limited to, single-phase Fickian diffusion behavior.
Alternatively, water absorption of plastic samples of defined dimensions subjected to immersion or moisture for an arbitrary period under controlled conditions can be used to compare different batches of the same material or for quality control testing of a specific material. For such comparisons, it is essential that all test samples be of the same dimensions and have, as far as possible, the same physical properties (e.g., surface smoothness, internal stresses). However, moisture equilibrium is not achieved under these conditions. Therefore, the results from such tests cannot be used to compare the water absorption properties of different types of plastics. Simultaneous testing is recommended for more reliable results.
The results obtained using the methods described in this international standard are valid for most plastics, but not for cellular plastics, granules, or powders, which may exhibit additional absorption and capillary effects. Exposure of plastics to moisture under controlled conditions for specific periods allows for relative comparisons between them. The tests described for determining the diffusion coefficient may not be applicable to all plastics.
Test specimens are immersed in distilled water at 23°C or boiling distilled water for specified periods, or are exposed to 50 percent relative humidity at specified temperatures. The amount of water absorbed by each test specimen is determined by measuring its change in mass—that is, the difference between its initial mass and its mass after exposure to water—and this change is expressed as a percentage of the initial mass. If necessary, the amount of water lost after drying of the specimens can also be determined. In some applications, relative humidity levels of 70 to 90 percent and temperatures of 70 to 90°C may be necessary. Higher relative humidity and temperatures than those recommended in this standard may be used by agreement between the parties involved. When relative humidity and temperature conditions other than those recommended are used, a full description of these conditions (with appropriate tolerances) must be included in the test report.
The following equipment is mainly used in these tests:
At least three test specimens must be tested for each material being evaluated. Test specimens of the desired dimensions can be prepared by molding or extrusion. The methods used to prepare all test specimens must be included in the test report.
The results of this method may be affected by surface effects. For some materials, different results may be obtained between molded samples and samples cut from a larger sheet. Any contaminants on the test sample surface that could affect water absorption should be removed with a cleaning agent that is not harmful to the plastic. The degree of damage should be determined according to ISO 175.
After cleaning, the samples should be allowed to dry for at least two hours at 23°C and 50% relative humidity before starting the test. Clean gloves should be worn when handling the test samples to prevent contamination. The cleaning agent should have no effect on water absorption. The effects of the cleaning agent can be ignored in determining the equilibrium moisture content.
Samples must be carefully dried before testing. For example, at 50 degrees Celsius, a drying time of one to ten days is required; the exact time depends on the sample thickness. At least 8 ml, but not less than 300 ml, of distilled water is used for each square centimeter of the total surface area of the test sample. This prevents excessive concentration of any extraction products in the water during the test. Each set of three test samples is placed in a separate container and either completely submerged in water or exposed to 50 percent relative humidity.
If multiple samples of the same composition are to be tested, the samples can be placed together in the same container by adding at least 300 ml of water. However, in this case, significant surface contact between the samples or with the container walls is unacceptable and should be avoided. Stainless steel grids can help maintain an acceptable separation between samples. For samples with a density lower than that of water, the samples are immersed by placing them in a stainless steel wire basket connected to an anchor weight by a stainless steel wire. Surface contact of the anchor weight with the samples should be avoided.
According to this standard, the following test methods are applied:
Our organization, which has been supporting businesses across all sectors for years through a wide range of testing, measurement, analysis, and evaluation activities, has a strong team of employees who closely follow global developments in science and technology and are constantly improving themselves. In this context, we also provide testing services in accordance with the "ISO 62 Plastics - Determination of Water Absorption" standard.
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