20 августа, 2015 
Pokraskin In 1927, long before the development of present-day industrial high-performance bonding techniques, W. McBain and W. B. Lee [12] published the results of the following experiments. Polished steel and aluminum parts were joined together using an organic adhesive made from (all components by weight): 50 units of shellac,
5 units of creosote (oil obtained from wood tar), 0.5 unit of ammonia and 2 units of turpentine. Although, based on today’s knowledge, this was a curious binder, McBain and Lee obtained tensile strengths of 31.71 N mm~2 and shear strengths of 24.15 N mm~2 between these steel adherents, and values of 11.73 N mm~2 and 16.05 N mm~2, respectively, between aluminum surfaces. In virtually all cases, the authors referred to ‘cohesive failure’ in the adhesive, with the adhesion forces having proved to be stronger. At the time, it was assumed that the binder had a chain structure that was influenced by the surface and reached deep into the structure of the adhesive layer. This resulted in remarkable strength values, which were clearly higher when thin adhesive layers (<0.1 mm) rather than thicker adhesive layers were used.
No clear mention was made by McBain and Lee of any chemical interactions with the metal as being responsible for this high degree of adherence.
It is known today, however, that the most important ingredient of McBain and Lee’s adhesive was shellac, the two main components of which are aleuritic acid and shelloic acid. Aleuritic acid is an aliphatic organic acid characterized by long chains and OH — groups. In contrast, shelloic acid is extremely compact, has a cyclic inner structure, while the outer surface is occupied by CH2-, OH — and CH3-groups. Creosote consists mainly of guaiacol and other phenols and phenolic ethers. Currently, it is assumed that these substances form chelates with metal oxides or hydroxides or that, owing to their electronic configuration, the ring structures of shelloic acid are able to create a strong chemical bond with the metal surfaces.
These early results were largely ignored because, until the early 1960s, the opinion was that the origin of adhesion — at least between organic, higher molecular substances and metals — could not possibly be attributed to chemical interactions.
In retrospect, however, this is difficult to understand since aluminum chelates (which today are known form when organic molecules bind to aluminum hydroxide) had long been used as adhesion promoters in the dyeing of cotton, as alizarin (so-called ‘vat dye’). Iron-gallate ink is also known to be a chelate of iron and gallic acid, with such types of chelate being formed on well-cleaned metallic surfaces. Experimentally, if a piece of well-degreased steel is dipped into a solution of 10 g gallic acid dissolved 250 ml ethanol, and then removed and left in the air for some minutes, its surface becomes bluish-gray. This effect, which had been described many years ago by Pliny the Elder in his Natural History, suggested not only that a chelate metal complex had been formed but also that the surface would show excellent adhesive properties. Finally, when chromatography — especially on aluminum oxide — was used to separate chemical compounds, a high retention time was attributed to a chemical interaction. Likewise, despite there being only a rudimentary knowledge available of the surface condition of metals, it was also realized that metals would become covered with oxide
layers whilst, under humid ambient conditions, hydroxides could also be created that would be much more chemically reactive in terms of their chromatographic capacity.
The role of chemical interactions in the interpretation of adhesion phenomena changed greatly during the early 1960s, however. Among the emerging high — performance bonding technology, the adhesion behavior of metal surfaces was of particular interest and again subjected to a variety of investigations. Sandstede et al. [13] noted that it was possible to desorb only a small proportion of acetic acid when it had been adsorbed onto aluminum from the vapor phase. This indicated that, in terms of the Langmuir adsorption isotherm, acetic acid must bind irreversibly to the metal surface, a process which could only take place provided that a chemical interactions had occurred. Later, Kautsky and Barusch [14] stated that aminoamide, when adsorbed onto an oxidized iron surface, also presented low desorbability. By using an infrared spectroscopy technique that had been specially developed for the investigation of interfaces [Fourier transform infrared spectroscopy (FTIR) had not yet been invented!], Dunken [15] suggested that copper stearate was most likely formed when stearic acid was adsorbed onto copper. In a similar study, Dimter and Thinius [16] described the result of applying phenolformaldehyde resol to aluminum; the subsequent rise in temperature to approximately 130 °C indicated that an exothermic reaction must have taken place, which could be explained by the formation of aluminum phenolate.
As reported by Lewis and Forrestal as early as in 1964 [17], the adhesion of plastic materials (e. g. polypropylene) which adhered only loosely to metals was considerably improved when the plastic was grafted with chemically reactive groups; this was the case for polypropylene grafted with dihydroxy-boranyl or epoxy groups, respectively. This provided further evidence for effects caused by the formation of chemical bonds at the interface between metals or their oxides and organic substances.
As the irreversible sorption process can, in theory, be attributed to a chemical primary valency bonding to surface atoms [14], the above-described examples of chemisorptive bonds between organic materials and metal surfaces clearly give rise to the conclusion that chemical interfacial reactions do exist. This is also valid when the heat of adsorption measured is lower than the energies of chemical bonds, since chemically active adsorbate molecules must dissociate before adsorption can take place, thus consuming a portion of the energy liberated in bond formation.
Presumably, the heat of adsorption supplies the activation energy necessary to launch the formation of chemical bonds. Since virtually no other energy sources are available when an adhesive is applied, chemisorption or chemical adhesion first requires an exothermic adsorption. In fact, while adsorption takes place rather rapidly (requiring only a few minutes), chemisorption takes longer (between 20 min and 1 h) [3]. As the energy required to separate chemical bonds is considerably higher than the physical bonding energy, the heat produced during further adsorption is not adequate to separate the chemical bonds, and this explains why chemisorbed quantities may not be desorbed according to Langmuir’s adsorption theory [18].
During the late 1960s, Brockmann [19] extended the results obtained from adsorption and chemisorption experiments (most of which were based on the measurement of sorption isotherms) to adhesion taking place in bonded joints.
Brockmann showed that the sorption properties of low-molecular-weight phenolic resins on metal surfaces were clearly in analogy with the strength behavior of adhesion. He has already shown that there was no true adhesion failure when metal adherents were separated from a cured phenolic resin adhesive, despite the macroscopic appearance that the separation occurred within the adhesive layer. Rather, by using available measurement techniques, the residues of the adhesive were seen to be spread over almost the entire metal surface. These results were explained by Brockmann on the basis of his investigations into chemical interactions. In experiments with epoxy resins applied to aluminum surfaces (as described in a later publication), Brockmann also detected chemical interactions that occurred to a lesser extent [20] than in the case of phenolic resins. Based on these results, it could also be shown that the water resistance of adhesion could be improved considerably when specific chelate complexing agents (morin, hydroxychinoline) were used which would adsorb to the metal surfaces from alcoholic solutions, before the adhesive had been applied [21].
By using specific measurement procedures, such as FTIR spectroscopy, it was shown later in a variety of experiments that chemical interactions took place at least between chemically reactive adhesives — that is, adhesives curing within the bondline — and solid material surfaces. Although, today this has been established as a foregone conclusion, it has not been shown clearly whether chemical interactions predominate the effect of the adhesion between solid material surfaces and nonreactive adhesives. Whereas, Langmuir’s investigation methods made it possible only generally to detect the existence of chemical or chemisorptive bonds, current analytical techniques allow the nature of those bonds to be determined, at least in some cases. A sufficiently high bonding energy is not a prerequisite to determine adhesion strength in a simple model. However, in order to determine the resistance of an adhesive joint against moisture, knowledge of the nature of the chemical bonds can be of decisive importance. If acid-base bonds, alcoholic bonds (e. g. between an opening epoxy ring and aluminum oxide) or salt bonds (phenolates) are formed between the polymer adhesive and the solid material surface, the chemical reactivity is of no benefit to the water durability because those bonds are water-soluble. Chemical bonds only play an important role at the interface if they are resistant to hydrolysis (e. g. chelates). Indeed, this is the case if phenolic resins are used without any auxiliary agent, because hydroxymethyl phenol (an important low-molecular — weight component of this adhesive) is able to form chelates with aluminum oxides or hydroxides by itself. This is considered the main reason for the high durability of aluminum joints bonded with phenolic resins (see Sections 5.5.3 and 8.2.1).
In order systematically to optimize the adhesion process, it is essential to utilize currently available analytical methods for determining the chemical nature of substances present on the surfaces to be joined. Today, surface-sensitive X-ray photoelectron spectroscopy (XPS) analysis is particularly well suited to this purpose, because it allows not only the presence of specific atom groups to be detected but also the identification of the type of bond involved. It is important that such data are known if the type of chemical reaction that may occur with the binder is to be predicted.
One example where chemistry is used specifically in the optimization of adhesion and durability is that of glass, a material that has been grafted with reactive silane for adhesive purposes for almost 50 years. Silanes contain organoreactive groups that are thought to react chemically with polymers as soon as their silanol groups become firmly adsorbed to the glass surface [22]. Similar procedures have been successful with other materials, for example on zinc. Both, reactive silanes and chelate complexing agents (stearates) lead to considerable improvements in adhesion and durability, and consequently reactive silanes have been used as adhesion promoters and mixed in various adhesives destined for the bonding ofinorganic adherents (see Section 5.10) [23].
However, it is not absolutely safe to assume that the adhesion-promoting effect of this type of adhesion promoter is based simply on bifunctionality and the specific formation of water-resistant bonds with the solid material and the adhesive. Even if these promoters do not present any groups that may react with the adhesive, they do improve durability. Organosilanes, for example, promote adhesion on a glass surface without presenting any organofunctional groups — that is, without being able to react chemically with the polymer. The same must be assumed for chelate complexing agents (e. g. hydroxychinoline or alizarin) when using epoxy resins as adhesives. Their positive effect with regards to a higher durability may be due to reaction with the thermodynamically instable solid material surface of glass or even metals, thereby reducing instability. This can easily be demonstrated on oxidized aluminum: if an alizarin chelate is created on pickled or anodized aluminum, scanning electron microscopy (SEM) can be used to show that, owing to the formation of chelates, the tendency of the oxide to hydrate in humid ambient conditions (one of the most important factors which may lead to adhesion failure) is drastically reduced.
Whilst adhesion and durability must be considered as a problem of pure adhesive bonding, it is also important — especially in the case ofinorganic materials — to realize that the formation of chelates as well as the acidity or basicity ofthe adhesive systemused may reduce (or increase) the instability of the layers covering the basic material. With weak acids, aluminum oxides resist hydration relatively well. Thus, even in the case of cured phenolic resin adhesives, weak acidity promotes not only chelate formation but also stability within the assembly by improving the resistance of the oxides. The same applies to iron oxides that are very resistant to hydration under alkaline conditions (pH 10-11). This is the reason that, as long as no acid rain penetrates the system, there is a good resistance of adhesion between concrete and Monier steel. Otherwise, the pH will fall to 10 or even 9.5, the oxide will be allowed to hydrate, and the result will be destruction of the adhesion. In fact, this is the most important reason why long-term damage has frequently been observed in concrete structures.
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