Standard Atmospheres

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Standard atmospheres

Test specimen preparation and standard climate

The reproducibility of measurement results requires error-free production of test specimens and sufficient constancy of the test climate in terms of ambient temperature and humidity. The constancy of the internal test specimen condition must also be guaranteed with regard to the moisture content. Minor changes in the ambient temperature and/or the moisture content of the test specimens lead to changes in the characteristic value level. For this reason, so-called ‘standard climates’ were defined as test conditions, which fulfil the average conditions of the temperate European climate zone in terms of temperature and humidity and thus simulate practical conditions. For the characterisation of the material properties at room temperature, the standard climate according to ISO 291 with an air temperature of 23 °C and a relative humidity of 50 % (labelling: climate 23/50) must be used for the normalisation of the test specimens and the performance of the tests [1]. This standard specifies two different classes of standard climates corresponding to the different deviation ranges. Class 1 requires a permissible deviation in temperature of ± 1 °C and relative humidity of ± 5 % and class 2 requires a permissible deviation in temperature of ± 2 °C and relative humidity of ± 10 %.

The simplest method of ensuring a constant test climate is to air-condition the entire test laboratory.

Conditioning of plastics

To determine characteristic values using the standard test methods, it is often sufficient to adjust the test specimen to the corresponding standard climate. The test specimens are conditioned for this purpose. In this process, the test specimens assume the temperature of the surrounding air, whereby the duration of the conditioning depends on the initial temperature and the geometry, especially the thickness of the test specimen. A state of equilibrium is established between the moisture content of the test specimens and the environment, depending on the diffusion constant of the plastic. Today, climatic chambers are the preferred method for adjusting the humidity. Another option is the use of exsiccators, where the relative humidity can be specifically adjusted using various saturated salt solutions at different temperatures [2].

Special conditions apply to some plastics, such as polyamide (abbreviation: PA), as PA test specimens are highly hygroscopic under standard conditions. To normalise the spray-dry test specimens, accelerated conditioning at 70 °C and 62 % humidity can be carried out in accordance with ISO 1110 [3] while controlling the increase in weight.

Closed temperature chambers must be adapted to universal testing machines (see material testing machine) to determine the temperature dependency of the mechanical properties of plastics, for example. Experience has shown that approx. 30 min is sufficient for pre-tempering multipurpose test specimens with a thickness of 4 mm. If a moisture value is specified at the same time, a media chamber must be used.

Medial-thermal resistance

The requirements for the ageing resistance (see ageing) of plastics have also become increasingly important. For this reason, in many cases it is necessary to know about changes in strength, stiffness and toughness as a function of ageing in different media. To determine the medial-thermal resistance, the test specimens are exposed to different temperatures and media (oils, water vapour, washing lye, etc.), e.g. to determine the medial resistance of plastics in lye containers for up to 5,000 h, and then the characteristic value level is determined in comparison to the initial state. In these long-term tests, storage of the same type is required in order to avoid mutual influence due to interaction effects (e.g. degradation products due to ageing).

Brown [4] provides a comprehensive compilation of conditioning times for reaching temperature equilibrium as a function of specimen geometry for prismatic and cylindrical specimens.

See also

• Test climate
• Water absorption
• Ageing

References

[1]	ISO 291 (2008-05): Platics – Standard Atmospheres for Conditioning and Testing
[2]	Bierögel, C.: Specimen Preparation and Conditioning. In: Grellmann, W., Seidler, S. (Eds.): Polymer Testing. Carl Hanser, Munich (2022) 3rd Edition, pp. 33–36 (ISBN 978-1-56990-806-8; E-Book: ISBN 978-1-56990-807-5; ePub ISBN 978-156990-808-2; see AMK-Library under A 22)
[3]	ISO 1110 (2019-05): Plastics – Polyamides – Accelerated Conditioning of Test Specimen
[4]	Brown, R. (Ed.): Handbook of Polymer Testing: Physical Methods. Marcel Dekker, New York Basel (1999), (ISBN 978-0824701710; see AMK-Library under C 5)