BOX 1-2

Degradation of Polymers and the Education of Engineers

There are many reasons for the growing use of polymers and composites, only one of which is their perceived enhanced resistance to the degradation of their properties by the environment. The inherent degradability of an organic resin must be modified by compounding it with necessary additives. Absent this important step, the performance and safety of the material will be severely compromised. This susceptibility to degradation varies widely according to the chemistry of the polymer and the environment in which it is used. For example, the fundamental carbon–carbon bond is sensitive to solar ultraviolet radiation, especially in the presence of oxygen. Because of this susceptibility, polymers are seldom employed in the neat form, especially for load-bearing applications. The intrinsic properties of a polymer, including its degradability, can almost always be modified by including one or more additives, which can be selected from a wide range to suit the particular circumstance. Such modification can retard the damage caused, for example, by oxidation.

The action of solvents is another reason for property degradation. Solvents may cause undesirable plasticization (lowering the glass transition temperature) and a loss of mechanical strength. Even small amounts of adventitious water, for example, have been known to cause problems in epoxy resins. In addition, certain solvent-polymer combinations may be peculiarly prone to stress crazing and stress cracking. Although polymers in general are relatively immune to biological attack, they are not universally so. Thus aliphatic polyesters are known to be susceptible to certain forms of microbial attack, with negative consequences. To combat these and similar phenomena, a thorough knowledge of polymer properties needs to be acquired by engineers working in the organic materials field.

In structural applications where enhanced mechanical or other properties are required, composite materials have become increasingly important. In the present context, a composite can be understood to refer to a multicomponent system in which a high-modulus fiber (say, carbon or glass) is dispersed in a stress-transmitting plastic matrix, frequently one of the thermosetting class of polymers, such as epoxies. The integrity of the composite depends on the integrity of both the matrix and the fibrous component. Technological advances in such composite materials have led to their increasing use in multiple transportation applications, in industrial and domestic infrastructure, and even in small to medium-size bridges.

While much of the committee’s data gathering has been in the field of the corrosion of metals, the committee recognizes that environmentally modulated degradation is a pervasive phenomenon that affects all classes of materials. The growing penetration of organics—principally plastics and composites—into the materials arena means that they are also of concern. Property deterioration in organic materials, commonly referred to as degradation rather than corrosion, is, as with metals, a phenomenon that can be alleviated with currently available science and technology, provided that this knowledge is appropriately applied. The fact that cases of polymer and composite failure caused by some form of environmental interaction continue to occur in significant number suggests that education in this area is entirely inadequate. Many but not all engineering curricula pay relatively scant attention to the properties of organic materials, and the phenomena associated with their degradation are often taught only at a superficial level. Even in undergraduate and graduate programs that are dedicated to organic polymeric materials, education in degradation and its mitigation is decreasing, a situation that parallels what is taking place in the metals field.



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