Humidity and water immersion

Moisture can affect adhesive strength in two significant ways. Some polymeric materials, particularly ester-based polyurethanes, will revert , i.e., lose hardness, strength, and in the worst cases, liquefy during exposure to warm, humid air. Water can also permeate the adhesive and preferentially displace the adhesive at the bond interface. This mechanism is the most common cause of adhesive-strength reduction in moist environments.  

The rate of hydrolytic reversion depends on the chemical structure of the base adhesive, the type and amount of catalyst used, and the flexibility of the adhesive. Certain chemical linkages, such as ester, urethane, amide and mea, can be hydrolyzed. The rate of such attack is fastest for ester-based linkages. Such linkages are present in certain types of polyurethane- and anhydride-cured epoxies. In most cases, amine-cured epoxies provide better hydrolytic stability than anhydride-cmed types. The reversion rate of hydrolytic materials is also dependent on the amount of catalyst used in the formulation. The best hydrolytic properties are obtained when the 

Environmental effects on adhesive joints are accelerated by stress. Few data are available on this phenomenon, however, because of the time and expense involved with stress aging tests. It is recognized, however, that the moisture environment significantly decreases the ability of an adhesive to bear prolonged stress. 

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Not only is low tensile shear strength noticed on moisture aging, but also the mode of failure changes from one of cohesion to adhesion. Table 7.6 shows the effect of humidity and water immersion on an epoxy-nylon adhesive compared to a nitrile-phenolic adhesive. Substrate primers have been used with epoxy-nylon adhesives to provide improved moisture... 

TABLE 7.6 Effect of Humidity and Water Immersion on the Tensile Shear Strength of Epoxy-Nylon and Nitrile-Phenolic Adhesives 10... 

D-1151. Test Method for Effect of Moisture and Temperature on Adhesive Bonds. Procedure for taking previously described basic tests and measuring strength loss as a function of time after exposure to defined temperature/ humidity combinations. Some 22 sets of exposure conditions, ranging in temperature from —70°F to +600 F and variously including low humidity, high humidity, and water immersion, are suggested. 

Organic coatings are commonly evaluated using salt water immersion, salt fog or spray, modified salt exposure tests (e.g., salt fog with added SO2), and various cyclic exposure tests. Humidity exposure and water immersion, and, for many applications, physical resistance tests (adhesion, impact resistance, etc.) are widely used preliminary tests. Standard methods for most of these tests are given in compilations of standard tests such as the Annual Book of ASTM Standards (16). Test methods have been extensively reviewed (e.g., 17-23). 

Composite materials must survive in the environment to which they are subjected at least as well as the conventional materials they replace. Some of the harmful environments encountered include exposure to humidity, water immersion, salt spray, jet fuel, hydraulic fluid, stack gas (includes sulfur dioxide), fire, lightning, and gunfire as well as the combined effects of the space environment. 

In many cases, even the method of conditioning prior to test will influence the ratings. For example, flexural tests run on standard conditioned specimens (50% relative humidity and 73.5°F) may rank materials differently from tests conducted on specimens which have been immersed in water or which have been heated to some elevated temperature after outdoor exposure. 

An interesting point arises from this method. The particles of material are exposed to water vapour at a set humidity and hence the results equate to that humidity. The measured equilibrium absorption is substantially the same as the equilibrium absorption that would be obtained by immersion in an aqueous solution which would maintain the test humidity. As this implies, the equilibrium water absorption is reduced if the water is not pure. Hence, exposure to aqueous solutions should be made at the concentration of interest. As the humidity approaches 100% even small amounts of a salt have a significant effect on equilibrium absorption. 

The relative aggressiveness of the environments proved to be consistent for all substrates, with the room temperature control the least hostile (virtually no loss of adhesion), and the cycle tests the most aggressive (up to 100% loss of adhesion within 60 days). Humidity cabinet exposure and 60°C water immersion yielded very similar values. As a result, for reasons of clarity, only water immersion data is actually presented here. Joint strength data obtained from either the Ford APG or Fisher Body Cycle Tests were identical, and were therefore also represented by one set of data points. The relative aggressiveness of the host environments toward... 

A simple and very useful method of studying humid aging consists of exposing an initially dry sample in a medium at constant temperature and relative humidity (or water activity in immersion) and recording weight changes. Various types of gravimetric curves can be obtained the most frequent ones are shown in Fig. 14.1. 

Standard tests of water immersion, humidity resistance, corrosion, flexibility, adhesion, and weight are conducted on other specimens of the same coating system. 

All nylons absorb some moisture from environmental humidity. Moisture absorption characteristics must be considered in designing and joining these materials. They absorb from 0.5 to 2 percent by weight of moisture after 24-h water immersion. Freshly molded objects contain less than 0.3 percent moisture since only dry molding powder can be successfully molded. Once molded, these objects absorb moisture when they are exposed to humid air or water. The amount of absorbed moisture increases until an equilibrium condition is reached based on the relative humidity of the environment. Equilibrium moisture contents of two commercial nylon resins for two humidity levels are as follows ... 

Electrical Properties. In its electrical insulating properties Trogamid T is similar to other polyamides. Its dielectric properties are not particularly striking. The electrical properties undergo slight change in relation to humidity, and prolonged water immersion has no appreciable... 

Behavior in Water and Aqueous Solutions. Like all polyamides Trogamid T absorbs water when stored in humid air or immersed in water. This absorption, however, is lower than that of nylon 6 and nylon 6/6 (Figure 3 and Table X). 

Permeability. As a diflFusion barrier, SC is most eflFective when dry, less eflFective when hydrated, and still less eflFective when treated with solvents such as dimethylsulfoxide (DMSO) (16, 92). The hydrating eflFect of increased relative humidity, occlusion, or immersion can be visualized as separation of hygroscopic and protein elements to create diflFusion channels containing free water (20, 69, 71). Obviously, water and its solutes should be more mobile in free-water channels than in bound water. The degree of hydration also can be influenced indirectly by organic solvents that hold water (glycol, DMSO) (18) or that modify surfaces (surfactants) (16, 18). 

Difficult environments for sealant materials are long-term immersion in water or continuous exposure to high humidity. A sealant immersed in water or exposed to saturated water vapour will be subject to physical, chemical and biochemical changes. The useful life of a sealant will depend on the relative levels of these effects, which in turn will depend on the type of polymer used and the formulation. The changes may occur in bulk or on the surface, and may lead to a loss of adhesion, which is the most damaging result of attack by water (Lee et al., 1992). 

FIG U RE 1.7 Dependence of modulus elasticity of furfurol-acetone polymer concrete samples on exposition time at humidity of environment 1 50%-60%, 2 S5%-95%, 3 water immersion. (From Yu. Borisov, Yu. Potapov, O. Figovsky, and D. Beilin, Water Resistance of the Polymer Concretes, . /. Scientific Israel Advanced Technology 14, no. 3 (2012) 84-91. With permission.)... 

Water Measurements and Conditioning. The samples were suspended from glass T s above saturated salt solutions or distilled water which produced different humidities at each temperature (30,45,60°C). The humidity chambers were immersed in circulating water baths or forced air ovens to maintain constant temperature. Samples were also immersed in distilled water at each temperature. Periodic weighings were made and results recorded as detailed elsewhere (2 3). Desorption of water from selected samples was performed by holding the samples over dessicant at the same temperature as sorption had been performed. 

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