Aging (chemistry)
Aging is a change in the physical and chemical properties of a substance after prolonged storage or use. Many aging processes can be accelerated or slowed down by increased temperature (higher than room temperature or than the prescribed maximum storage temperature) as well as by physical and / or chemical influences. High temperatures and the effects of oxygen in the air, moisture, light, ultraviolet radiation ( UV light ) or X-rays, etc. can accelerate the aging process. A protective atmosphere consisting of inert gases ( inert gases , e.g. argon or pure nitrogen) can slow down the aging process.
metallic materials
Metallic materials can change their properties through precipitation processes. As a result of such processes, steel and iron are less malleable and tough than before. At the same time, the hardness and brittleness increase. Aging-sensitive steel is particularly susceptible to stress corrosion cracking and intergranular corrosion, which can lead to considerable stability problems in prestressed concrete structures (e.g. bridges).
Sometimes precipitation is also a desirable process that results in an improvement in certain properties (e.g. hardness during steel hardening ).
Non-metallic materials
Inorganic substances
Non-metallic, inorganic materials (such as mortar, concrete and building materials made from natural stone) age to a greater extent than metallic materials. In the aging of gels such as B. silica , there is often a regrouping of the randomly arranged molecules in the coagulate with the formation of a fairly complete crystal lattice. Similar processes apparently take place with the aging of precipitation : Freshly precipitated iron hydroxide can be dissolved in colloidal form , aged iron hydroxide cannot.
Inorganic dyes are generally significantly more stable to aging than organic dyes.
Organic substance
Plastics (e.g. polymers and especially elastomers ) are usually even more susceptible to aging processes , especially at higher temperatures. Then a degradation of the macromolecules or a stronger crosslinking accompanied by cyclizations can take place, whereby the physical and mechanical properties are clearly influenced. In some cases such degradation reactions are desirable and are brought about in a targeted manner in order to improve the processing properties of a material. Another motive for artificially accelerated aging is based on the fact that one would like to minimize the aging of the material during practical use. Last but not least, accelerated aging is also used for qualification purposes. In this way, the aging of materials or devices (e.g. electrical and control technology), which they suffer during their entire service life or duration of use, can be impressed in a short and therefore practicable time. Subsequent tests (function test, property tests, ...) enable statements to be made about the design and qualification.
Organic amines - particularly in the liquid state of aggregation - aged at light and air. This can be seen from the fact that freshly distilled, mostly colorless amines turn brown. Hydrohalides ( hydrochlorides , hydrobromides, etc.) and other salts of these amines, on the other hand, age much more slowly and are therefore significantly more stable in storage.
The lightfastness of dyes is an important quality criterion. Lightfast dyes are more resistant to aging than dyes that fade when exposed to light.
The Maillard reaction plays a role in the aging of organisms.
The embrittlement of rubber is due to an aging process.
Lithium-ion batteries age mainly due to chemical reactions of the electrolyte on the electrode surfaces. The aging manifests itself here as a loss of capacity and an increase in internal resistance .
Aging in vegetable and animal oils and fats
If the triglycerides of oils and fats contain a high proportion of unsaturated fatty acids , they are particularly sensitive to aging. Such oils and fats become rancid with aging . On the other hand, triglycerides with a high proportion of saturated fatty acid residues can be stored for much longer.
Prevention of aging
To reduce the aging of plastics and lubricants are anti-aging agents used. The anti-aging agents can vary widely in their chemical structure. Some anti- aging agents act as antioxidants , light stabilizers or radical scavengers .
In order to protect vegetable and animal oils and fats at least temporarily from aging, they are stored under light protection (storage in dark bottles or metal containers) at low temperatures in the refrigerator. Additional storage under protective gas (usually nitrogen in practice ) inhibits the aging process.
See also
Individual evidence
- ↑ a b Otto-Albrecht Neumüller (Ed.): Römpps Chemie-Lexikon. Volume 1: A-Cl. 8th revised and expanded edition. Franckh'sche Verlagshandlung, Stuttgart 1979, ISBN 3-440-04511-0 , p. 145.
- ^ Fritz Röthemeyer, Franz Sommer: Kautschuktechnologie , Carl Hanser Verlag Munich Vienna, 2nd edition, 2006, pp. 521-523, ISBN 978-3-446-40480-9 .
- ^ Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, pp. 51-52.
- ↑ Hans Beyer and Wolfgang Walter : Organische Chemie , S. Hirzel Verlag, Stuttgart 1984, p. 643ff, ISBN 3-7776-0406-2 .
- ^ Siegfried Hauptmann : Organic Chemistry , 2nd edition, VEB Deutscher Verlag für Grundstoffindustrie, Leipzig 1985, p. 749, ISBN 3-342-00280-8 .
- ^ Hans Beyer and Wolfgang Walter : Organische Chemie , S. Hirzel Verlag, Stuttgart 1984, p. 433, ISBN 3-7776-0406-2 .