Chemical resistance

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As chemical resistance , the resistance is generally of materials to the action of chemicals , respectively. In contrast to corrosion , there is no material removal, which is particularly typical for plastics and elastomers .

Since the chemical resistance relates to the pure material, it may In real applications, there may be limitations in the suitability of the material (e.g. stress cracks or permeation ). Therefore, one can also speak of chemical resistance , as this explicitly indicates the potential of the material to withstand chemical attack.

Classification

They are usually divided into three simple categories:

  • chemically resistant: The material retains its characteristic mechanical (e.g. strength ), physical (e.g. color ) and chemical (e.g. mixture ) properties unchanged despite any length of contact with the chemical substance to be tested. Since this ideal state practically never occurs, a material is still considered to be “resistant” in technology, which is only attacked very slowly .
  • limited chemical resistance: The material retains its characteristic properties (see above) for a limited period of time that is 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 intended use allows. For example, with some adhesives, the chemical instability of plastics to solvents is exploited by partially dissolving the material in the area of ​​the bond (loss of mechanical strength), which enables the material of the two adhesive parts to mix. After the solvent has evaporated, the glue joint hardens again and a firm connection remains. The plastic, however, would be completely unsuitable for building a container for the solvent in question.

Resistance of plastics

In the case of plastics, insufficient chemical resistance ( degradation ) manifests itself in swelling or softening, which can lead to loss of usability. The molecules of the medium diffuse into the space between the polymer chains and push them apart. Since diffusion processes are temperature-dependent , information on chemical resistance only applies to the specified temperature. This is of particular importance if tabulated data on chemical resistance are only available for room temperature , but the plastic is to be used at higher temperatures.

Stress cracking , which is also known from metals, occurs more frequently . In the process, microcracks (stretched zones, also called crazes ) initially form , which can grow into large networks of cracks when subjected to mechanical stress. Chain degradation only occurs in a few cases (see oxidative degradation ).

In contrast to the corrosion of metals, there is seldom chemically induced material removal from plastics.

Resistance of glass

Glass is very resistant to most liquids or gases. If a glass is unstable, it runs differently than that of metals, but is also defined by material removal. The chemical resistance of glass can be divided into water resistance, acid resistance and alkali resistance.

The water resistance of glass can be determined by the so-called "glass gravel test". Here, 2 grams of glass is crushed to a grain size of 300 to 500 μm and heated to 98 ° C in a beaker with 50 ml of water over a period of one hour. The amount of ions in hydrochloric acid (Na 2 O equivalent) released from the glass by the water during this time is measured with the aid of titration. The released ions are analyzed in hydrochloric acid (0.01 mol / l) as a standard solution per g of glass grit ml / g. Methyl red solution is used as an indicator. If only up to 31 μg are released, the type of glass corresponds to water resistance class 1, i.e. it has the highest chemical resistance. The classification is based on a total of 5 water resistance classes. The following applies: the more ions released, the worse the water resistance class. Glasses with high water resistance are mainly used for the production of pharmaceutical primary packaging, since most drugs are stored in an aqueous solution.

The acid resistance of glass is divided into three classes according to the DIN 12 116 method, depending on the weight loss caused by the attack by the acid. To do this, the surface to be tested is boiled in hydrochloric acid for six hours . If half the weight loss of the surface removal is less than 0.7 mg per 100 cm 2 , the type of glass is considered acid-resistant class S1. The more that is dissolved out of the glass, the worse the acid resistance class. Depending on their composition, borosilicate glasses (e.g. Duran or Pyrex ) and soda lime glasses (e.g. AR glass) have a high resistance to acids. They are therefore classified as S1 glasses.

To determine the alkali resistance, the respective glass surface is boiled for 3 hours. The aqueous solution used for this must consist of equal parts of sodium hydroxide solution (1 mol / l) and sodium carbonate (0.5 mol / l). If the weight loss of the glass is a maximum of 75 mg / 100 cm 2 , it is a glass of alkali resistance class 1. The higher the weight loss, the worse the classification of the alkali resistance.

detection

The chemical resistance is mostly determined in an immersion test. Methods for determining the Spanungsrissbeständigkeit are the ESC test ( E nvironmental S tress C racking) and the tensile creep test in media. This and other tests from the field of environmental simulation allow the selection of a suitable material.

Evidence of the chemical resistance of a material or a surface can also be determined based on ISO 2812-1 (immersion method) or ISO 2812-4 (drip / spot method). The material or the surface is exposed to the corresponding chemicals over a defined period of time and the exposed area is then examined microscopically. Changes that can be determined are:

  • Discoloration
  • Change in gloss level
  • softening
  • Swellings
  • Removal of coatings
  • Blistering

After the time-defined load, the material surface is freed from test liquid residues and checked for visible changes in accordance with DIN EN ISO 4628-1 to -5 and evaluated. To determine a possible regeneration time, the assessment is carried out immediately afterwards and one hour after the test liquid has been removed. According to DIN EN ISO 4628-1, the following criteria are divided into classes 0 to 5: amount of damage (N), size of damage (S) and intensity of changes (I). Other irregularities observed are also mentioned.

The evaluation takes place according to the following scheme:

"Blistering, N2-S2" or "Discoloration, I1"

A possible concentration of a test liquid on a surface due to drying in the real clean room environment must be taken into account. This test procedure can be used to prove whether a material or a surface is "resistant", "partially resistant" or "not resistant" to a certain chemical over a certain period of time.

Individual evidence

  1. ↑ ISO 2812 Paints and varnishes - Determination of resistance to liquids - Part 1: Immersion in liquids other than water (ISO 2812-1: 2007); German version EN ISO 2812-1: 2007.
  2. ↑ ISO 2812 Paints and varnishes - Determination of resistance to liquids Part 4: Drip / stain method (ISO 2812-4: 2007); German version EN ISO 2812-4: 2007.
  3. ↑ ISO 4628-1 Paints and varnishes - Assessment of coating damage - Assessment of the amount and size of damage and the intensity of uniform changes in appearance - Part 1: General introduction and evaluation system (ISO 4628-1: 2003); German version EN ISO 4628-1: 2003.
  4. ↑ ISO 4628-2 Coating materials - Assessment of coating damage - Assessment of the amount and size of damage and the intensity of uniform changes in appearance - Part 2: Assessment of the degree of blistering (ISO 4628-2: 2003); German version EN ISO 4628-2: 2003.
  5. ↑ ISO 4628-3 Coating materials - Assessment of coating damage - Assessment of the amount and size of damage and the intensity of uniform changes in appearance - Part 3: Assessment of the degree of rust (ISO 4628-3: 2003); German version EN ISO 4628-3: 2003.
  6. ↑ ISO 4628-4 Paints and varnishes - Assessment of coating damage - Assessment of the amount and size of damage and the intensity of uniform changes in appearance - Part 4: Assessment of the degree of cracks (ISO 4628-4: 2003); German version EN ISO 4628-4: 2003.
  7. ↑ ISO 4628-5 Paints and varnishes - Assessment of coating damage - Evaluation of the amount and size of damage and the intensity of uniform changes in appearance - Part 5: Evaluation of the degree of peeling (ISO 4628-5: 2003); German version EN ISO 4628-5: 2003.

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