Лако-красочные материалы — производство

The atoms or molecules in any solid are held together by so-called cohesion forces. These interatomic and intermolecular forces are conservative in character, i. e., not dependent upon the pretreatment of the material [4, p. 1]. After a break in the solid, the two fragments cannot be put back together in such a way as to restore the solid to its original state. Reactivation of the cohesion forces would be possible only if the original intervals between atoms or molecules could be reestablished. The same may be
said also of the joining of two different materials; a mutual convergence over the entire joint area on the radius of interaction of the molecules would result in adhesion of the same order of magnitude as the cohesion forces [5]. In practice such convergence is not possible. There are three reasons for this: first, the surface roughness of the adherents; second, a reduction in the effective surface for boundary layer reactions due to the surface tension of the adhesive; and third, the formation of the so-called weak boundary layer [130], [131]. This is a region within the adhesive film close to the boundary layer where the reactive groups of the molecules are directed to the adherent and con­sequently do not contribute to the curing process, that is, no cross-linking occurs. Therefore, the weak boundary layer is always a weak link in the adhesive joint. The function of the adhesive in the boundary zone is to compensate for the effects of surface roughness by providing optimal wetting of the adherents.

Over the years, several different theories have been developed as to the mode of action of adhesives, i. e., the mechanism of adhesion. They extend from the simple mechanical theory via the electrostatic and adsorption theory to the diffusion theory, these being the most important [6]. One feature common to all these theories is that, for some adhesion phenomena, these theories allow qualitatively satisfactory assertions to be made, but in other respects they fail totally and, in some cases, even lead to conflicting results. The adhesion of adhesive to an adherent, on which any joint is based, is clearly not a consistent and isolatable process [7] but a complex addition of various adhesion effects. Because the number of individual processes involved, apart from exceptions, is extremely difficult to estimate, definitive confirmation or rejection of individual theories is hardly possible.

Mechanical Theory of Adhesion. The oldest theory of adhesion is definitely the mechanical theory. It is based on mechanical anchorage of the adhesive in pores and irregularities in the adherent and is discussed primarily in reference to wood and similar porous materials [8].

Electrostatic Theory of Adhesion. According to the electrostatic theory [9], the adhesion forces between adherent and adhesive layer are applied by contact or transfer potentials. These transfer potentials cause the buildup of an electric double layer at the adhesive — adherent boundary and corresponding Coulomb attraction forces between the two components. In principle, the occurrence of transfer potentials is unquestiona­ble, as shown by the electrostatic discharges that can be detected in the destruction of adhesive joints. However, the practical significance of the attraction forces associated with these discharges is still being debated [4, pp. 150-153], [10].

Adsorption Theory. As its name indicates, this theory of adhesion draws upon surface forces for explaining the observed phenomena [11]. It regards adhesion as essentially a special property of phase interfaces. The forces that are responsible for adhesion in this process are the so-called secondary valence or van der Waal’s forces. These forces have three components, namely Keesom’s dipole orienting effect, Debye’s
induced dipole effect, and London’s dispersion effect [12]-[14]. For these forces to become active, the distances between the molecules of adhesive and adherent must converge toward molecular intervals. This requires complete spreading of the adhesive over the surface of the adherent. Good adhesion can be expected if the adhesive (in liquid form or in a liquid medium) wets the adherent [8]. Accordingly, the adsorption theory may be placed on an entirely thermodynamic basis. Adhesion is thus determined by the ratio between the surface energies of the adhesive and the constituent material of the adherent in the sense that the specific surface energy of the adhesive must be lower than that of the adherent. Accordingly, materials having high surface energy levels, such as metals, and those having medium surface energy levels, such as wood and paper, may be bonded relatively easily. In the case of polymers, bonding becomes increasingly more difficult [15], and finally almost impossible as surface energy decreases (polyole­fins and polytetrafluoroethylene). The remedial surface treatment of materials such as these may be interpreted as increasing their surface energy [16].

The adsorption theory also shows that the adhesion forces of two materials are not reciprocal. For example, if a liquid epoxy resin adhesive is allowed to set on the surface of polytetrafluoroethylene or polyethylene, a very weak adhesive joint is formed. If, by contrast, liquid polytetrafluoroethylene or polyethylene is applied to the surface of hardened epoxy resin, strong adhesive joints are obtained [17]. In practice, this theory is not free from contradictions either; above all, it does not answer the question whether the difference in the surface energies between two materials is indicative of the intensity of the adhesion force [18].

Diffusion Theory. The diffusion theory of adhesion is essentially applicable to the bonding of high polymers. According to this principle, adhesion is obtained by the mutual penetration of adhesive and substrate [19], [20]. This mobility is based on the fundamental properties of high polymers: their chainlike structure and resulting mo­bility, allowing the possibility that the chains possess Brownian molecular movements in a submolecular range. By virtue of their greater mobility, the adhesive molecules normally play a greater part in the diffusion process. However, if the adhesive is in solution, which is generally the case, and if the substrate is slightly soluble in the solvent, substrate molecules or parts thereof also diffuse into the spread adhesive. A diffusion bond is characterized by the disappearance of a clear boundary between the two phases and by the development of a gradual transition from one phase to the other. The mechanism of adhesion has developed into a three-dimensional process and is no longer confined to one interface.

The interdiffusion of the polymer molecules of adhesive and substrate is dependent upon various parameters, such as pressure, time, temperature, molecule size, and, of course, the reciprocal solubility, as shown by the correlation between the compatibility of the polymers and the quality of the bond [21]. Examples of bonds to which the diffusion theory is applicable include the bonding of PVC-U adherents to PVC in solvents containing tetrahydrofuran and so-called contact bonding, where the diffusion
process takes place between two adhesive layers and not between adhesive and substrate [22].

The limits of the diffusion theory of adhesion show up in the adhesion of polymers, for example, to metal or glass. In this case, this theory does not make any useful contribution to the understanding of adhesion.

Other Theories. In addition to these theories, some special cases are discussed in the literature. They include adhesion by primary valence forces, for example, in the bonding of metals [8], [23], and so-called liquid adhesion. In the latter, a thin film of a liquid of extremely high viscosity produces adhesion through a process in which separation of adherent and substrate results in a flow in the narrow gap which is only possible by overcoming considerable resistance. Liquid adhesion is particularly as­sumed in pressure-sensitive bonding [16] and in the initial tack of a liquid adhesive.

Summary. None of the theories covers every single aspect of bonding. A promising combination of the theories to bring about an improvement does not exist at present. In addition, the theory of adhesion as a physicochemical phenomenon and physically measurable adhesion are generally still a considerable distance apart, so that the adhesion theories are only reference points. Secondary factors, such as surface rough­ness, boundary layers with faults (atmosphere, impurities), joint design, type of stress, and aging also influence bond strength to a considerable extent.