Glass

21 августа, 2015

Pokraskin

In the case of glass, without using any adhesion promoter (at least with reactive structural adhesives), no long-term durability can be obtained when bonding this adherent. At present, this is assumed to be due to the fact that in a humid environment at the interface, the glass surface is irreversibly modified by corrosion that may be accelerated by the adhesive, resulting in macroscopically observable adhesion failure [22]. However, if the glass surfaces are bonded with acrylic pressure — sensitive adhesives, even without using any adhesion promoter, a relatively high water resistance (compared to structural adhesives) is obtained. This situation does not apply to adhesion, which common exists as a static, material condition. At present, pressure-sensitive adhesives are considered to build up adhesion by a variety of physical interactions rather than by any firm chemical binding to solid-state materials (this would not be expected due to their macromolecular, saturated condition). It has been noted that, when other difficult-to-bond materials (e. g. untreated aluminum, stainless steel or relatively low-energy polymer surfaces) are bonded with this type of adhesive, the assembly has a surprisingly high durability. The polarities of the adherents have a certain influence on adhesive strength (as can be determined by measurement), although as noted above, physical interactions can be destroyed by water penetration. Hence, the positive adhesive properties of pressure-sensitive adhesives can be explained only by making the assumption that, owing to high molecular mobility (even in the solid state) they progressively replace the adhesive bonds that have been destroyed by water penetration, thus ‘fighting’ against destruction. Today, we refer to this capacity as ‘dynamic adhesion’, the existence of which can be demonstrated with macroscopically conducted strength and durability testing [24]. There is a clear relationship between the degree of molecular mobility, which in turn can be controlled by the degree of crosslinking, and water resistance. It can also be shown that the build-up and regeneration of adhesion — both of which factors are dependent on time and are very pronounced in the case of pressure-sensitive adhesives — support the validity of the hypothesis of dynamic adhesion.

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