The Process of Adhesive-Bonded Joint Formation

The reason for the lack of concordance between the theoreticMly and experimentally determined values of the adhesion strength lies in the fact that the process of achieving high adhesion strength is hindered by a number of phenomena that accompany the formation of adhesive-bonded joints. These factors that decrease the strength of adhesive-bonded joints can be divided into two groups  

Adsorption or the presence of oxide layers on the substrate surface can be demonstrated by the fact that surfaces of metals are considerably reactive only at the moment of formation of the above layers and can initiate polymerization reactions and start chemosorption interaction with polymers. 

One of the sources of dirtiness of the substrate surface is low-molecular weight substances in the substrate itself that gradu dly diffuse to the surface and accumulate there. In the case of polymers, such substances might be plasticizers, softeners, stabilizers, residual monomer, or various additives. 

The adsorption of impurities in the composition of an adhesive onto the substrate surface can be judged by the change of the system interphase tension. To study the interphase tension on the boundary 

With purified resin, alcohol decreases the interphase tension, with the exception of OP-10 (allyl phenol oxyethylated ester) for which it produces some increase (by ImN/m) [12]. These findings can be explained by the fact that the alcohol facilitates desorption of the low-molecular weight resin fractions with surface-active properties from the boundary between the resin and the mercury by increasing their compatibility with the bulk resin. The free energy advantage for alcohol adsorption on the mercury surface is less than that for low-molecular weight fractions, which is why it results in increase of the interphase tension. 

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In our view, the failure to determine correlation between the thermodynamic characteristics of adhesion and the strength of adhesive-bonded joints lies in the fact that what was being studied was the relationship of the initial compoimd properties (in the hquid state) with the strength of joints in which the adhesive was in the cured state. In addition to ignoring the essential differences of the cured adhesive from the initial liquid, there was no accoimt for irregularity of the process of formation of adhesive-bonded joints itself, which means that the most favorable adhesive structure (from the thermodynamic point of view) is not realized for kinetic reasons. The necessity for accounting for these factors has been established in previous sections. 

The fracture stress of adhesive-bonded joints (adhesion strength) is a consequence of processes that occur in the course of their formation, but all attempts to formulate a fundamental relationship between formation and failure of adhesive-bonded joints have so far been unsuccessful. This is mainly due to the lack of methods for meastuing adhesion that would permit determination of the failure equilibrium work. Accordingly, the relationship between the experimentally determined value of the adhesion strength and the thermodynamic characteristics can be one of correlation only. 

The aging of adhesives and lacquers affects the durability of structural bonds and coatings. It depends on environmental influences such as humidity, temperature, irradiation, the surrounding media, and the network state of the polymer. In addition, the network properties and therefore the aging behavior are influenced by substrates that provoke the formation of an interphase during network formation. The properties of the interphase differ from the bulk and it is often the weak spot of structural adhesive joints. Hence, to improve the lifetime of such a joint it is crucial to understand the chemical processes both in the bulk polymer and in the adhesive-adherend interphase under given envi-... 

In the course of formation of the adhesive-bonded joint, internal stresses appear in the adhesive layer. These stresses can change the process of formation of the polymer boundary layer and cause the formation of faults. With increase of the internal stresses in polystyr-... 

Thus, it is the separation in time of the formation processes of the linear polymer and the intermolecular crosslinks that produces adhesive-bonded joints with low internal stresses. In the adhesive containing 20% of gel fraction but in which the crosslinks were formed using triallyl isocyanurate along with the polymer formation, the intem d stresses were substantially higher than in the adhesive containing ATG only (see Fig. 4.8b, curve 4). Increase of the crosslink formation rate by adding triethylenediamine, as catalyst of the reaction of the isocyanate groups with water, to the adhesive simultaneously with addition of ATG cdso increased the internal stresses (see Fig. 4.8b, curve 5). 

There are close similarities between the moulding process and the formation of an adhesive joint. In both cases, one material (the polymer or adhesive), usually in liquid or semi-liquid form, is brought into close contact with another (the mould or substrate) and then sets. The phenomenon of mould release is in many respects the same as the phenomenon of adhesion a moulding is almost like an adhesive bond where very low, even zero, adhesion is desired. See Mould adhesion - measurement. 

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