Corrosion of Polymers

Plasticizers and other additives. Most polymers are formulated with plasticizers and chemical additives. These materials have a tendency to migrate to the surface and may even soften the coating and destroy adhesion. The specific polymer should be checked to determine whether the coating will cause short- or long-term softening or adhesion problems. 

Other factors. The long-term adhesion of the coating is affected by the properties of the polymer such as stiffness or rigidity, dimensional stability, and coefficient of expansion. The physical properties of the paint film must accommodate to those of the polymer. 

Corrosion of metallic materials takes place via an electro-chemical reaction at a specific corrosion rate. Consequently, the life of a metallic material in a particular corrosive environment can be accurately predicted. This is not the case with polymeric materials. 

Polymeric materials do not experience specific corrosion rates. They are usually completely resistant to a specific corrodent (within specific temperature ranges) or they deteriorate rapidly. Polymers are attacked either by chemical reaction or solvation. Solvation is the penetration of the polymer by a corrodent that causes swelling, softening, and ultimate failure. Corrosion of plastics can be classified in the following ways as to attack mechanism  

Disintegration or degradation of a physical nature because of absorption, permeation, solvent action, or other factors 

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It is convenient to consider the corrosion of polymer coated metals as a process that consists of several distinct stages ... 

Having defined the various air pollutants, we now consider briefly the types of effects which are possible in environmental corrosion of polymers. 

The particles in the air over an urban area are of a variety of sizes, shapes and chemical composition, ranging from tiny, spherical metal particles from metallurgical fumes to huge, porous conglomerates of sooty carbon, soil particles, fly ash, and fly dust of all types.The size and shape of the particles will almost totally determine the surface corrosion of polymers, plastics, the polymer in paints and lacquers. The important question is how these particles behave in the air, how far and how fast the wind carry them, and what effects they can have on polymeric materials. 

The photochemical smog contains several oxygen species such as atomic oxygen (0), ozone (0 ) and singlet oxygen ( O2) which are highly reactive with several polymers. The concentration of 0 and 0 is low and their effect is considered to be less important in the total corrosion of polymers than the effects of ozone, nitric and sulfuric acids. 

The service life of plastics and other polymeric materials is a particular problem in Los Angeles and Tokyo because of the special climatic conditions for formation of photochemical smog. This situation is by no means confined to the USA and Japan. Photochemical smog and environmental corrosion of polymers is commonly observed in some European cities, notably Madrid, Athens and London. In Stockholm where air pollution is at a low level these problems are presently not seen so serious. 

The corrosion of polymer matrix composites is also affected by two other factors the nature of the laminate, and in the case of thermoset resins, the cure. Improper or insufficient cure will adversely affect the corrosion... 

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