Polymer degradation

Plastics are chemicals, after all they are obtained through processes involving distillation, mixing of substances and polymerization. And like most of these products, they are subject to chemical reactions when exposed to inclement weather (heat, moisture and UV radiation) or simply through contact with other materials.

It is therefore important to know not only the physical properties of each polymer but also its chemical resistance, a factor that may be crucial in a development.

In most situations where a particular plastic is attacked by aggressive chemicals, changes in behavior are usually only physical (change in visual appearance, increase or loss of rigidity, etc.), but in some cases dissolution of molecules may occur, and/or initiation of polymerization, is what we call degradation.

Degradation can be initialized through a thermal, mechanical, photochemical, chemical, radiation or stress-cracking reaction; causing various changes in the structure of the polymer.

Regardless of the degradation reaction, it is always initiated in the same way, by breaking a covalent bond (two atoms sharing a pair of electrons), generating in most cases free radicals that will be responsible for the propagation of the process.


DEGRADATION REACTIONS

Thermal

The main disadvantage of the polymers is their low thermal stability. While many inorganic materials resist temperatures above 3632°F (2000°C), the polymers have their processing temperature in the range of 212-392°F (100-200°C) and even lower usage temperatures. This is because their atoms are bound by covalent bonds whose dissociation energies are in the range of 300 to 850 kJ mol-¹ and can be broken with heating.

Photochemical

Photochemical reactions are those that occur with the participation of a chemical molecule or species in an excited electronic state. This type of degradation occurs mainly with the exposure of the polymers for long periods of sunlight, which reaches the surface of the Earth with a wavelength range that goes from infrared to ultraviolet. Some fluorescent lamps also let emission lines pass through the ultraviolet region, so certain polymers can undergo electronic transitions by absorbing light from this source by initiating degradation.

High-energy radiation

It occurs mainly in cases where the polymer needs to be sterilized, such as in food packaging, hospital equipment and pharmaceuticals.

As an example, PVC (used in the manufacture of the cited products), upon receiving doses of radiation such polymer acquires an increase in light absorption and breakage of C-C bonds in the main chain, causing a reduction in the mechanical resistance of the material.

Bibliography:
HARPER, Charles A.; PETRIE, Edward M. Plastics Materials and Process: A Concise Encyclopedia. Hoboken: John Wiley & Sons, Inc., 2003.
CANEVAROLO JR., Sebastião V. Ciência dos Polímeros: Um texto básico para tecnólogos e engenheiros. 2.ed. São Paulo: Artliber Editora, 2002.
WIEBECK, Hélio; HARADA, Júlio. Plásticos de Engenharia: Tecnologia e Aplicações. São Paulo: Artliber Editora, 2005.
Article posted in May 8, 2017
About the author: Daniel Tietz Roda is Plastics Technologist graduated from the FATEC/ZL and Mechanical Design Technician from ETEC Aprígio Gonzaga, in São Paulo, Brazil. Roda worked 5 years with technical assistance and development of plastics in industries and nowadays is the publisher of this website.
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