Adhesive joints environments

Silicone adhesives are generally applied in a liquid and uncured state. It is therefore the physical and chemical properties of the polymers, or more precisely of the polymer formulation, that guide the various processes leading to the formation of the cured silicone network. The choice of the cure system can be guided by a variety of parameters that includes cure time and temperature, rheological properties in relation with the application process, substrates, the environment the adhesive joints will be subjected to and its subsequent durability, and of course, cost. 

A surface is that part of an object which is in direct contact with its environment and hence, is most affected by it. The surface properties of solid organic polymers have a strong impact on many, if not most, of their apphcations. The properties and structure of these surfaces are, therefore, of utmost importance. The chemical stmcture and thermodynamic state of polymer surfaces are important factors that determine many of their practical characteristics. Examples of properties affected by polymer surface stmcture include adhesion, wettability, friction, coatability, permeability, dyeabil-ity, gloss, corrosion, surface electrostatic charging, cellular recognition, and biocompatibility. Interfacial characteristics of polymer systems control the domain size and the stability of polymer-polymer dispersions, adhesive strength of laminates and composites, cohesive strength of polymer blends, mechanical properties of adhesive joints, etc. 

Samples constructed from adherends which had been alkaline cleaned, lubricated or left untreated exhibited similar joint strength values and durability trends (Figure 10). Adhesive joints placed in the room temperature control environment or the 23 C water bath retained lOOZ and 92% of initial joint strength, respectively. Failure remained cohesive within the adhesive for all of the control samples and for the first 20 days of exposure in the 23 C water bath. After 20 days, some failure began to initiate at both the primer/steel and primer/topcoat interfaces. The adhesive/topcoat interface proved to be more durable than those found between the substrate/primer/topcoat layers. Samples exposed to the more severe salt fog, 60 C water bath and cycle tests were able to retain 70% to 50% of their initial strength over a 60-day exposure period. 

Effects of Corrosive Environments on the Locus and Mechanism of Failure of Adhesive Joints... 

Although numerous studies (1-3) have described work aimed at establishing criteria for the durability of adhesive joints, a thorough understanding of effects of the chemical and mechanical properties, on the durability of adhesive bonds is lacking. More specifically, the effects of surface preparation and dynamic loading, especially under environmental service conditions, has not been explored in detail for automotive structures. In this paper, a description of the effects of environment on the durability of adhesive bonds is presented. Particular attention is given to... 

In all adhesive joints, the interfacial region between the adhesive and the substrate plays an important role in the transfer of stress from one adherend to another [8]. The initial strength and stability of the joint depend on the molecular structure of the interphase after processing and environmental exposure, respectively. Characterization of the molecular structure near the interface is essential to model and, subsequently, to maximize the performance of an adhesive system in a given environment. When deposited on a substrate, the silane primers have a finite thickness and constitute separate phases. If there is interaction between the primer and the adherend surface or adhesive, a new interphase region is formed. This interphase has a molecular structure different from the molecular structure of either of the two primary phases from which it is formed. Thus, it is essential to characterize these interphases thoroughly. 

Great caution must be exercised in exposing any adhesive joint to the simultaneous effects of environment and stress. The stress can act to accelerate the degradation caused by the environment, and vice versa. Joints that will be exposed to both high-humidity environments and high load at the same time are especially vulnerable, and prototype specimens need to be tested. This degradation mechanism and the performance of several epoxy adhesive systems to combined environmental stress conditions are discussed in Chap. 15. 

In applications where possible degrading elements exist, candidate adhesives must be tested under simulated service conditions. Standard lap shear tests, such as ASTM D1002, which use a single rate of loading and a standard laboratory environment, do not yield optimal information on the service life of the joint. Important information such as the maximum load that the adhesive joint will withstand for extended periods and the degrading effects of various chemical environments are addressed by several test methods. Table 15.2 lists common ASTM environmental tests that are often reported in the literature. 

Mechanical stress accelerates the effect of environment on the adhesive joint. A great amount of data is not available on this phenomenon for specific adhesive systems because of the time and expense associated with stress-aging tests. However, it is known that moisture, as an environmental burden, markedly decreases the ability of an adhesive to bear prolonged stress, especially at slightly elevated temperatures. 

By far, the most detrimental factors influencing adhesives aged in a nonseacoast environment are heat and humidity. The reasons why warm, moist climates degrade many adhesive joints were presented in the last section. Near the seacoast, corrosion due to salt water and salt spray must also be considered when one is designing an adhesive joint. Thermal cycling due to weather, oxygen, ultraviolet radiation, and cold are relatively minor factors with most structural adhesives. 

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. 

There are two properties of adhesive joints that protect them from exposure to chemical or solvent environments high degree of crosslinking and low exposure area. 

From the information reported in the literature with regard to aging of adhesive joints in chemical environments, it can be summarized that... 

Comyn, J., Adhesion Science, Chapter 10, Adhesive Joints and the Environment, Royal Society of Chemistry, Cambridge, 1997. 

Falconer, D. J., et al., The Effect of High Humidity Environments on the Strength of Adhesive Joints, Chemical Industry, July 4, 1964. 

Orman, S., and Kerr, S., Effect of Hostile Environments on Adhesive Joints, in Aspects of Adhesion, vol. 6, J. Alner, ed., University of London Press, London, 1971, p. 64. 

Landrock, A. H., Effect of Environment on Durability of Adhesive Joints, Chapter 9 of Adhesives Technology Handbook, Noyes Publications, Park Ridge, NJ, 1985. 

McCrudy, R. M., and Rambosek, G. M., The Effect of Gamma Radiation on Structural Adhesive Joints, SAMPE National Symposium on the Effects of Space Environment on Materials, St. Louis, MO, May 1962. 

High-humidity and salt spray environments have been found to cause the greatest decrease in bond strength of magnesium adhesive joints. Of the several surface treatments that have been evaluated, the Dow 17 surface preparation (ASTM D 2651 Method C) provides the best overall performance with all adhesives under these types of environmental... 

R.D. Adams, J. Comyn, W.C. Wake, Effect of the environment on structural adhesives, in Structural Adhesive Joints in Engineering, Chapman Hall, London, 1997, Chap. 8. 

Depending on the environment of an adhesive joint, moisture or electrochemi-cal/corrosive reactions at the interface which lead to a chemical degradation of bonds could lead to a failure of the joint In a corrosive environment, electrochemical reactions dominate, in particular on reactive metals at room temperature, whereas the electrode potential characterizes the reactivity of the interface. 

Poor shear strengtii of the adhesive joint, particularly in aqueous environments in both medical and nonmedical applications... 

Whereas the fracture of bonded joints is due to the presence of critical discontinuity in a field of stress, the stress intensity or crack initiation energy of bonded joints and materials at which fracture occurs is also a function of the properties of the wood and the adhesive, the environment at a given time, changes in the environment, and external forces on the joint or bonded material. These relationships are explored in the following sections. 

The ability of a structural joint to maintain satisfactory long-term performance, often in severe environments, is an important requirement of a structural adhesive joint, as the joint should be able to support design loads, under service conditions, for the planned lifetime of the structure. A number of factors determining the durability of structural adhesive joints have been identified and can be grouped in three categories environment, materials and stresses. All of which are discussed next. 

Because it is more corrosive than fresh water, salt water not only attacks metals but also degrades even the so-called inert polymers polyethylene, polypropylene, polystyrene, polymethyl methacrylate, etc. (26). Indeed, salt water is very corrosive to adhesive joints. For example, it was reported (27) that exposure to a 5% salt spray for three months had a more severe effect on aluminum joints (stressed DCB) than exposure to a semi-tropical environment for three years. In Table 1, McMillan (27) compares the severity of salt spray with a natural, semi-tropical environment and various in-service conditions. 

The last severe environment on our list is external stress. External stresses (14,33) especially affect the water uptake or saltwater penetration of adhesive bonds. However, some adhesives may appear insensitive to stresses. Presumably, the critical stress level (34) is so high that no environmental attacks can take place below that level. Thus, it is difficult to discuss the effect of external stresses without considering the fracture mechanics of adhesive joints. In general, external stresses accelerate bond degradation when the joint is exposed to severe environments. 

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