Лако-красочные материалы — производство

In the non-loaded condition, the properties of bonded joints may change under environmental influences, as mentioned above at the end of Section 7.7.4 in

connection with the wedge impact strength. In the loaded condition (i. e. in service) the effectiveness of environmental influences can be further increased under certain circumstances. The resistance ofbonded joints to environmental influences is called ‘age resistance’, although this is not entirely correct because in other fields aging may also mean a change due to the influence of time alone, without further physical or chemical influences. A hostile environment generally causes an irreversible deterio­ration ofthe mechanical characteristics ofa bonded joint. The deterioration depends on the type and intensity of the external influences, as well as on the quality of the manufacture process — that is, the conditions prevailing during the creation of the bonded joint. In the literature, with regards to the durability ofbonded joints, water or humidity are referred to as the most hostile environmental influences. Hostile media enter the bond-line via diffusion processes or capillary attraction along existing microcracks in the polymer. This is considered to be the most frequent cause of poor durability of bonded joints, particularly if the adherent material is inorganic (e. g. metal or glass). The following, mainly humidity-induced, damage mechanisms are discussed:

• Water uptake by the adhesive: The adhesive polymer is plasticized by the uptake of water. This process is partly reversible upon redrying and generally results in a cohesive failure within the bond-line. The change in strength behavior of the bonded joint stops as soon as the bond-line is saturated with water. Irreversible impairment of the bond-line by moisture rarely occurs with modern adhesives used in industry, unless the adhesives are deliberately detachable on demand.

• Detaching ofthe adhesive from the adherent surface: A failure of adhesion between the adhesive and the substrate surface is explained by competitive adsorption ofwater instead of polar groups of the polymer molecules, or by hydrolysis of chemical bonds in the boundary layer.

• Water that diffuses into the interface: When this happens, the adherent surfaces are changed by solvolysis, for example in the form of a hydration of oxides on metals, and the bonding surface is weakened. Solvolytic reactions may generate byproducts in the interface that may further weaken the polymer near the interface.

• The degradation process: This is accelerated by an enhanced water uptake by the adhesive due to high temperatures and loading.

• Electrochemical corrosion: This starts at the unprotected edges of the adherents and is due, for example, to corrosive substances such as chloride or sulfate ions being dissociated in water. This results in bond-line corrosion, particularly in metal bondings.

With the exception of bond-line corrosion, these aging processes generally take place very slowly. Therefore, the durability ofbonded joints is tested by means of most varied, more or less standardized aging simulations (see Section 7.3) in which several external influences are superposed and simultaneously affect the assembly, thereby accelerating the failure mechanisms. To achieve this acceleration effect, compared to a natural aging process with actual climatic conditions, almost all artificial aging tests

share an increase in the intensity of the test parameters (temperature, salt exposure, etc.). This is one of the reasons why artificial aging cannot be exactly correlated with outdoor weathering of bonded joints.

In the following section, some examples are provided to illustrate the durability of different bonded joints as a function of the environmental conditions. The degrada­tion mechanisms induced will be explained by means of fracture surface analyses. The durability ofbonded aluminum, steel, glass and plastic materials, respectively, will be discussed separately. Whilst most of the available knowledge has been gained with bonded aluminum (owing to its importance in aircraft and automobile construction), many of the findings can be transferred to other adhesive systems [15-17].

7.7.6