Moisture Degradation Mechanism

Even though epoxy adhesives are insoluble in water, they are not immune to water attack. Moisture can affect the strength of the epoxy adhesive joint by 

FIGURE 15.11 Effect of outdoor weathering on the strength of aluminum joints bonded with epoxy-polyamide.30 

Moisture degradation of adhesive bonds occurs within the bulk adhesive material, at the adhesive-adherend interface, and within certain substrates. These degradation mechanisms are discussed below. Particularly insidious is the effect of the combined elements of moisture, stress, and temperature. Unfortunately, this synergistic effect occurs at relatively low temperatures, and such a service environment is common to many adhesive applications. For these reasons, this combined environment is given special focus in Sec. 15.5.2. 

Effect on the Bulk Property of the Adhesive. Moisture can alter the properties of the bulk material by changing its glass transition temperature, inducing cracks, or chemically reacting with the polymer—a process called hydrolysis. But before these mechanisms occur, the moisture must first find its way into the bulk polymer. 

Internal degradation within the bulk adhesive or sealant occurs primarily by absorption of water molecules into the polymer structure. All polymers will absorb water to some extent. Moisture can also enter by wicking along the adhesive-adherend interface or by wicking along the interfaces caused by reinforcing fibers and the resin. Deterioration may occur more quickly in a 100 percent relative humidity (RH) environment than in liquid water because of more rapid permeation of the vapor. 

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Ambient moisture can also affect certain types of uncured adhesive, either as it is being mixed and applied to a substrate or as it is stored in a container waiting to be applied. The degradation mechanism before cure of the adhesive is discussed in Chap. 3. 

For most adhesive bonded metal joints that must see outdoor service, corrosive environments are a more serious problem than the influence of moisture. The degradation mechanism is corrosion of the metal interface, resulting in a weak boundary layer. Surface preparation methods and primers that make the adherend less corrosive are commonly employed to retard the degradation of adhesive joints in these environments. 

Hydrolysis is the principal degradation mechanism for the condensation polymers. From the point of view of chemistry, the equilibrium molecular weight of these polymers is determined by the H O concentration at given temperature, T. However, owing to the moisture absorption from the air, the reaction equilibrium is shifted toward depolymerization. The rate of hydrolytic depolymerization depends on the moisture content, T and the presence of catalyst. Since these polymers are also subject to free-radical and oxidative processes (that lead to formation of unsaturations, hence the... 

J A Hough, F R Jones and Z D Xiang, Thermally enhanced moisture absorption and related degradation mechanisms in composite materials . Proceedings 4th International Conference Deformation and Fracture of Composites, London, Institute of Materials, 1997 pp 181-190. 

Whilst fading is more of an aesthetics issue, the loss in many other properties is crucial when these composites are to be used for outdoor applications. Both UV light and moisture degrade LPCs independently, but they can also act synergistically. The effects of weathering are measured in terms of retained physical, chemical and mechanical properties, by using techniques such as mechanical testing, microscopy, FTIR spectroscopy, and DSC analysis. 

Although the reinforcing fibres are protected from direct exterior aggression by the embedding polymer matrix, they may experience substantial chemical and physical attack due to the ingress of moisture, alkaline or salt solutions in the FRP composite. In addition, these effects may be emphasized if additional sustained loads are applied to the composite materials. In this section, the various degradation mechanisms are briefly recalled for the main types of fibres used in construction. 

Kaelble DH, Dynes PJ, Crane LW, Mans L (1975) Interfacial mechanisms of moisture degradation in graphite-epoxy composites. J Adhes 7(l) 25-54... 

The latter has been cited as the more important mechanism of the two for promoting optimum adhesion [16]. Bonds formed between the coupling agent and a substrate are especially resistant to moisture degradation, and hence long-range ageing effects on the composite material are reduced. 

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