Лако-красочные материалы — производство

During the late 1950s it was known to be possible to create high-strength adhesive bonded joints between smooth metal surfaces using organic adhesives under well- defined conditions. However, this represented an important problem for adhesive research because the formation of chemical interactions between those different types of material had long been considered impossible. It was known during the late nineteenth century that in matter, physical, intermolecular forces existed as well as chemical interactions. Indeed, if those forces did not exist it would not have been possible otherwise to explain various well-known phenomena, such as the high

boiling temperature of water, or the fact that up to 100 °C, water is a liquid at all. To sum up, these intermolecular interactions have been attributed to the existence of permanent or oscillating dipoles, which interact one with another in chemically saturated systems, but which are also able to induce dipoles in other materials — a process that is particularly obvious in the case of metallic partners.

Those interactions are generally characterized by lower binding energies than chemical interactions; neither do they change the nature of matter to the same extent. Independently of the chemical compatibility, they can, however, take effect between materials of different types. De Bruyne’s polarization theory [6], for instance, was an important approach for understanding the phenomenon of adhesion. Today, those intermolecular interactions are called ‘physical bonds’ and can be divided into three important groups:

• Permanent dipoles: These are found in molecules in which one atom with a higher atomic number (e. g. oxygen) is bound to another atom with a lower atomic number (e. g. hydrogen). This homopolar linking is due to the fact that the statistical probability distribution of the electrons in the bonding orbital is shifted towards the larger nucleus, inducing electronegativity and hence a dipole. The permanent dipole is able to build electrostatic attraction forces in the form of a dipole interaction with another permanent dipole; such forces can be calculated. Hydro­gen bridge bonds are dipole-dipole bonds, and have been included in the schematic representation of Table 3.1 between physical and chemical bonds. Owing to the relatively high bonding energy and impact, the efficiency of hydrogen bridge bonds is actually assumed to approximate that of chemical interactions. Hydrogen bridge bonds are created by a hydrogen atom located between two very electronegative atoms that, in organic chemistry, essentially can be nitrogen, oxygen or fluorine having added a hydrogen atom to one of their valencies [7].

• Induced dipoles: In adjacent molecules without any own permanent polarity, permanent dipoles are able to induce counterdipoles with which they build up static attraction forces characterized by lower bonding energy compared to dipole-dipole bonds. As already mentioned, this approach especially plays a role when trying to explain the adhesion of organic substances with polar groups on bright metal surfaces.

• Dispersion forces: These may exist between molecules with nonpermanent dipoles, and are attributed to the fact that weak oscillating dipoles can be found between the involved atoms because, at least depending on time, the statistical probability distribution of the binding electrons is not completely uniform. This in turn may induce weak interactions that take effect between all types of material, including also similar gas molecules. However, their bonding energy is generally lower than that of permanent dipole bonds.

The validity of the polarization theory for interpreting adhesion phenomena is indisputable as it can easily be observed, for example, that organic polymers with permanent dipoles (so-called ‘polar groups’, such as polyvinyl chloride or epoxy resin) better adhere to smooth metal surfaces than do nonpolar molecules (e. g. the

components of polyethylene or polypropylene). In contrast, an adhesive which generally contains polar groups can be shown to adhere better to a polar substrate or solid-state material (e. g. a cured epoxy resin) than to a nonpolar material such as polytetrafluoroethylene.

It may be assumed, therefore, that dipole effects contribute to all of the adhesive processes of specific adhesion. However, in some cases, their bonding energy can only barely explain high-strength adhesion, and it must be appreciated that those bonds which are responsible for physical adsorption can also be destroyed as soon as materials of a higher polarity penetrate the system and break the adhesive bonds by competitive adsorption, taking the place of the polar groups of the adhesive. Water, in particular, is one such substance with high polarity that is able to penetrate all polymers in defined quantities, owing to its short molecular dimensions and extremely high polarity. Water also migrates to the adhesive zone and considerably impairs or even destroys adhesion based on physical bonds, a process which can often be observed macroscopically.

Adhesion with a high resistance to water may occur, however, and is actually often observed between substances of very different types. Therefore, it must be assumed that there are further types of bond that are exposed to a much lesser extent (or not at all) to the attack of water. Primarily, these are chemical interactions that will be discussed later.

In terms of physics, it is indisputable that between two materials presenting different electronic configurations (e. g. a solid object made from an organic material and a metal), an electric bilayer is created as soon as both materials come into intimate contact with each other. Occasionally, for example, if a polyethylene layer is applied onto a metal, the bilayer can even be observed in the form ofan electron enrichment in the polyethylene layer [8]. If the two adhesive partners are separated from each other, refined mathematical calculations indicate that, at the moment of breakdown of the bi-layer, the order of magnitude of the energy needed for separation is similar to that of the adhesive energies measured. It can easily be shown that electrostatic or luminescent phenomena are occurring when adhesive systems are separated.

If a simple, single-sided, pressure-sensitive adhesive film, as is commonly used in every household, is peeled from a solid substrate which is close to a pocket radio and its antenna, and the tuning dial is kept in the medium waves range (AM), then some interesting ‘information’ will emerge from the loudspeaker. At peeling, there is a crackling noise, but if the peeling is performed more or less rapidly the crackling changes. If the assembly is joined together again, followed by a peeling-off, there is no longer any crackle. Alternatively, in a photographic dark room, if a modern envelope where the overlaps are pasted together with a pressure-sensitive adhesive (see Section 8.5) is opened, a distinct light emission may often be observed with the naked eye. Both, autographic and photographic methods have been used scientifi­cally to demonstrate this effect [9].

However, until now it has not been possible to provide unquestionable proof of the effectiveness of this bilayer in adhesion. If the bilayer was the dominant factor, the adhesive strength of the bonded joints created or tested in an intense electric field would differ from those created in neutral surroundings, but as far as we know this is

not the case [10]. Although there is clearly no doubt that the bilayer exists, it probably does not predominate the adhesive strength in the majority of cases.

3.2.2