1. Biological corrosion of plastics by microorganisms is mainly produced in three ways:
(1) Microorganisms degrade directly by using plastic components as nutrient sources;
(2) Indirect degradation or alteration of the appearance of plastic products by microbial metabolites such as acids, enzymes and pigments;
(3) Surface effect caused by sedimentation on the surface of plastic products without microbial degradation.
Plastic products are often invaded by fungi at first. Fungi degrade plastic most, but bacteria also feed on additives. Once an additive, such as a plasticizer, begins to biodegrade and turn into smaller organic compounds, other microorganisms enter and cause greater biological corrosion. Biological corrosion often leads to the decrease of durability and service life of plastic products.
The main impacts of microbial activities include:
It is caused by intracellular pigments (such as penicillin, which can dye plastics pink) or extracellular dyes (white or black discoloration caused by colored metabolites released by microorganisms, such as the production of ferrous sulfide FeS).
(2) Changes in electrical properties:
Due to the invasion of microorganisms, the electrical properties of products, especially the insulation properties, will change. This phenomenon is caused by the microorganism deposition on the plastic surface, but the material does not deteriorate in this process. When macromolecules such as exopolysaccharides are produced by microorganisms, the corresponding degradation of electrical properties will be enhanced.
(3) The mechanical properties are reduced:
The metabolism of plasticizers caused by bacteria and/or fungi will make them brittle and shrink, and ultimately lead to the loss of tensile strength and integrity. Destruction of plasticizers into smaller molecules also promotes the growth of microorganisms, which metabolize plasticizers.
(4) Improving the absorption rate of sewage:
Because the plastic surface is attacked by microorganisms, the roughness of the plastic surface is increased, which makes the dirt more easily accumulated on the surface, thus causing a vicious circle, accelerating the deterioration and abandonment of plastics, and increasing the shortening of product life. This phenomenon is usually due to the degradation of plasticizers resulting in surface changes.
(5) Permeability of solvents and gases:
Decreased mechanical properties of plastics can also lead to increased permeability of solvents and gases, and reduce the storage or isolation of even ineffective liquids and gases.
Microbial metabolism can lead to the production of amines, ammonia or hydrogen sulfide, which can produce unpleasant odors.