Degradation of plastics
In plastics, degradation refers to their breakdown or disintegration. This often means aging , combined with swelling , embrittlement , cracking and loss of strength . The degradation is usually an undesirable process.
Degradation can be environmentally relevant. Partially degraded individual particles or molecular main, intermediate and end products as well as "bleeding" additives are often undesirable in the environment.
Types of decay
The decomposition takes place either chemically, physically or through a combination of both types of degradation.
Chemical degradation
The chemical resistance of plastics can be lost through chemically induced decomposition or decomposition through the effects of light or temperature. The disintegration takes place through the breaking of molecular chains, partly through depolymerization . At higher temperatures chain breaks occur through pyrolysis or at lower temperatures mostly through the action of free alkyl radicals (these can arise through thermal energy, radiation, mechanical energy under load or through the influence of metal ions), starting from the surface or homogeneously. A tendency to depolymerization occurs with weak CC bonds in the main polymer chain, which can lead to changes in the molecular structure (chain splitting off and molar mass reduction, chain crosslinking or chain branching). Any functional groups present in the macromolecular substance can be converted into others, e.g. B. can hydrolyze esters . Elimination of low molecular weight groups can also occur. In addition, photochemically induced reactions by ultraviolet light are known. If chemical bonds are broken, the molar mass of polymers is reduced and / or low molecular weight degradation products are formed. After chain cleavage, the molecules can crosslink or form a ring (e.g. methathesis).
Chemical degradation is associated with changes in physical properties (such as tensile and flexural strength, elongation at break, impact strength).
Physical degradation
Physically induced disintegration can begin as a result of water absorption (swelling) or the breaking up of the material or the leaching of additives such as plasticizers . Alternating shrinkage and swelling due to weathering can lead to cracking, precipitation can lead to erosion . With some plastics ( thermoplastics ), the melting that occurs under the influence of heat is reversible.
recycling
Repeated recycling can lead to the degradation of plastics due to shear forces and high thermal stress.
Degradation in sea water
The degradation of plastics in seawater proceeds similarly (slowly) as in the air, only more brittle materials (such as expanded polystyrene) are mechanically torn into smaller parts more quickly by waves and in the surf.
Biological degradation
Most plastics are not biodegradable , with the exception of biodegradable plastics . Degradation of biodegradable plastics is usually a desirable process when they are disposed of. The biodegradable plastics are among the biodegradable materials and include various synthetic biopolymers such as bio-based plastics and biodegradable plastics. Factors for biological degradation are the accessibility for microorganisms and their enzymes , the moisture permeability and the compostability in a moist environment. Pure hydrocarbons break down to CO 2 and H 2 O. The addition of starch as a filler to conventional plastics does not lead to the desired complete degradability of the plastic.
In by seating of algae , lichens and fungi resulting biofilms can by release and concentration of organic acids biocorrosion arise.
Protection against degradation
A partial protection against degradation is achieved by various methods of conservation . A coating with paints leads to decreased corrosion. UV-absorbing coatings and additives in the plastic reduce damage caused by UV light.
literature
- Ehrenstein, Pongratz: Resistance of plastics , Hanser Verlag, Munich 2007, ISBN 978-3-446-21851-2 , p. 38 ( limited preview in the Google book search).
- Achim Pfeil and 19 co-authors: Biodegradable plastics , expert-Verlag, Renningen-Malmsheim 1994, ISBN 3-8169-0963-9 , p. 81 ( limited preview in the Google book search).
Individual evidence
- ↑ Wolfgang Seidel: Werkstofftechnik, Werkstoffe - Properties - Testing - Application , Hanser Verlag, Munich 2008, 7th edition, ISBN 978-3-446-40789-3 , p. 243.
- ↑ a b c d Gottfried W. Ehrenstein, Sonja Pongratz: Resistance of plastics , Volume 1, Hanser Verlag, ISBN 978-3-446-21851-2 .
- ↑ J. Hepperle: Damage Mechanisms in Polymers , Polymer Preparation , 2002 - technical progress to increase performance and product quality, VDI-Verlag, Düsseldorf 2002, quoted in: Ehrenstein, Pongratz: Resistance of Plastics , Hanser Verlag, Munich 2007, ISBN 978-3 -446-21851-2 , p. 38 ( limited preview in Google Book search).
- ↑ Sebastian Kotzenburg, Michael Maskus, Oskar Nuyken: Polymers - Synthesis, Properties and Applications , Springer Spectrum, 2014, p. 333, ISBN 978-3-642-34772-6 .
- ↑ Ehrenstein, Pongratz: Resistance of Plastics , Hanser Verlag, Munich 2007, ISBN 978-3-446-21851-2 , p. 38 ( limited preview in Google book search).
- ↑ Littek, Schneider, Huber, Schöppner: Measurement of the material degradation of polypropylene , Zeitschrift Kunmststofftechnik / Journal of Plastics Technology, 8 (2012).
- ^ Plastics and their degradation mechanisms .
- ↑ Anthony L. Andrady (Editor): Plastics and the Environment , John Wiley & Sons, Hoboken, New Jersey, 2003, ISBN 0-471-09520-6 , p 379 ( limited preview in Google Book Search).
- ↑ Dietmar Stephan: Nanomaterials in construction. P. 134 ( limited preview in Google Book search).