Chemical resistance
The chemical resistance, the resistance of the materials or materials against attack from chemicals is generally described. In contrast to corrosion while no material removal takes place, which for plastics and elastomers is especially typical.
Since the chemical resistance refers to the pure material, it can in real applications may be limitations on the suitability of materials come (eg, stress cracking or permeation). Therefore, one can speak also of chemical resistance, since these explicitly indicating the potential of the material to resist chemical attack.
Classification
The subdivision is usually in three simple categories:
- Chemical resistance: The material maintains its mechanical characteristics (e.g., resistance), physical (for example, color) and chemical (for B composition ) properties in spite of any length contact with the chemical substance to be tested unchanged. Since this ideal state virtually never occurs, in art, a material perfectly still considered " resistant ", which is attacked only very slowly.
- Limited chemical resistance: The material retains its characteristic properties (see above) for a limited period of time acceptable for the intended use or within specific limits of the operating conditions.
- Chemically unstable: The material loses its characteristic properties (see above) within a very short time - or faster than the application allows it. For example, for some adhesives, the chemical instability of plastics is utilized with respect to a solvent in which the material is dissolved in the adhesive body ( loss of mechanical strength ) to give a mixture of the material of the two adhesive parts is made possible. After the solvent has evaporated, the splice is cured again, and it is a fixed connection. The plastic would be for the construction of a container for the particular solvent, however, completely unsuitable.
Phenomenology of plastics
In most cases, insufficient chemical resistance in a swelling or softening, which can lead to loss of serviceability is expressed. The molecules of the medium to diffuse into the space between the polymer chains and slide them apart. Since diffusion processes are temperature-dependent, data valid on the chemical resistance always only for the specified temperature. This is of particular importance when tabulated data the chemical resistance available for room temperature, the resin is to be used but at higher temperatures.
More commonly known also for metals stress cracking. This form first microcracks ( stretched zones, also called crazing ) that can grow under mechanical stress to large crack networks. Only in a few cases it may lead to a chain degradation (see Oxidative degradation).
Unlike the corrosion of metals, there are rarely chemically induced removal of material for plastics.
Determination
The chemical resistance is usually determined in the immersion test. Methods for determining the Spanungsrissbeständigkeit are the ESC test ( Environmental Stress Cracking) and the creep test in media. This and other tests in the field of environmental simulation allow the selection of a suitable material.
The proof of the chemical resistance of a material or surface can 2812-1 ( immersion method ) or ISO 2812-4 ( Tropf-/Fleckverfahren ) be determined in accordance with ISO. In this case, the material or the surface is charged over a longer period of time defined by the corresponding chemical and the loaded position then assessed microscopically. Be changes that can be detected:
- Discoloration
- Change the degree of gloss
- Softening
- Swelling
- Detachment of coatings
- Blistering
Following exposure time defined the material surface of Prüfflüssigkeitsresten is released and reviewed and evaluated for visible changes in accordance with DIN EN ISO 4628-1 to -5. To determine a possible regeneration time the assessment is carried out immediately after and one hour after removal of the test liquid. According to DIN EN ISO 4628-1, the following criteria are divided into classes 0-5: Amount of damage ( N), size of the damage ( N) and intensity of the changes (I). Other observed irregularities are also mentioned. The evaluation is performed according to the following scheme: " Blistering, N2 -S2 " or " discoloration, I1 " A possible concentration of a test liquid on a surface by drying in a clean room environment of the real needs to be considered. Through this testing procedure can be detected whether a material or surface over a certain period, against a specific chemical " resistant ", " partially resistant " or " not resistant " is.