Лако-красочные материалы — производство

3.1

Introduction

Adhesion, when considered macroscopically, is the two-dimensional (surface-to — surface) adherence of two similar or different types of material to each other. It is one of the most important material phenomena in nature and technology. In the realms of technology, a building erected by the Romans will still safely be held together by the adhesion between the mortar and bricks. However, a car tire will only function if adhesion holds together — in an absolutely safe manner — the rubber and fabric made from organic substances or steel cords. Corrosion-preventing lacquers can only be applied to cars because there is adhesion, and even paper — which still today is the medium on which books are printed — is made from an adhesive bonded fiber composite. If adhesion is considered as the sine qua non of all forms of adhesive joining techniques, it is reasonable to limit the general view of adhesive phenomena to the study of organic substances which, in most cases, are higher-molecular materials (the majority of adhesives), technically useful inorganic materials such as metals, glass, stone and ceramics, and organic materials such as plastics, wood and textiles.

Many years of experience gained from adhesion research have shown that it is wise to investigate the different aspects involved in the creation of adhesive joints on the one hand, and their behavior within the assembly on the other hand. Strictly speaking, adhesive systems virtually can never be destroyed nor fail where adhesion has built up in advance. There will not be any of the so-called ‘interfacial adhesion failure’ (as so often described in the literature) if the adhesion partners approach each other in sufficient order so as to build interactions between atoms or molecules, thus creating the possibility to produce a strong assembly.

Today it is well known that mechanical ‘interlocking’ has no significance in the bonding of the majority of nonporous, technically used materials. Consequently, at this point the subject of adhesion will not be analyzed by focusing on mechanical ‘interlocking’ as an origin of adhesion, although such an approach may be very useful when, for example, creating bonds between wood or paper and porous, swellable substances.

Technically useful interactions between atoms and molecules only occur if and when the adherents involved are brought together in very close approximation, generally less than 1 nm (see Table 3.1).

Other, more important, distances can only be covered by adhesive interactions in the form of charge transfers that can contribute to the formation of an electrical bilayer. The orders of magnitude of bonding energies that occur in physical bonds, hydrogen bridge bonds and chemical bonds are listed in Table 3.1. When the adhesion forces are calculated from these bonding energies, the theoretical values (given in MPa in relation to a given surface unit) are, in most cases, clearly higher than the strength of adhesives made from organic substances. Their theoretical strength, which is calculated from the bonding energy of carbon-carbon bonds, is expected to have an order of magnitude of500 MPa for a polymer packing with the highest density. Of course, the values ofstrength actually measured not only in adhesive assemblies but also for polymer materials, are lower than those calculated from theory because no ideal material combinations or structures are to be expected. If we suppose that the ratio between theoretical and practical values is approximately the same for both the adhesion and the adhesive, it is clearly possible to create a reliable bonded joint with the majority of potential adhesive interactions. Over many years of experience with bonding technology, this very simple postulate has been confirmed by the fact that, in a loaded adhesive assembly, there is virtually never any failure of adhesion (proper), as noted above. Strictly speaking, this winds up the study of ‘adhesion’ to be something of a phenomenon, although it is possible today to both understand and systematically optimize the (often very complex) behavior of adhe­sive systems. For this, the following additional basic knowledge is required.

When creating an adhesive bond, no or only very little energy is added to the system. Only if the adhesive wets the solid objects to be bonded do the two come into sufficiently intimate contact. Wetting is a phenomenon that can be observed when a drop of liquid spreads over the surface of a solid-state material. According to the surface condition, the type of liquid used and the matter to be found in the environment (which often is not taken into consideration), the drop of liquid forms a contact angle (a) between its surface and the surface of the solid-state material. This angle ranges, in theory, from 0° (complete spreading) to 180° (no wetting at all) (see Figure 3.1).

When the drop of liquid comes to rest, interfacial tensions are acting between the solid-state material and the environment, the liquid and the environment, and the liquid and the solid-state material, respectively. These interfacial tensions (or ener­gies) thermodynamically determine the angle of contact. When the drop of liquid is at rest, there is equilibrium between the interfacial tensions. Only if a is 90° or less can a sufficiently intimate molecular or atomic contact (see above) be created, and this results in adhesion between the wetting liquid and the solid-state material, which can be utilized in technical terms. This condition is only fulfilled ifthe interfacial energy between the solid-state material and the environment is equal to or higher than that between the liquid drop and the environment. It is worthy of mention at this point that the surface energies of common organic adhesives range between 30 and 60 mN m-1; water has a surface energy of 72 mN m-1, whereas inorganic materials such as glass and metals have surface energies in excess of 500 mN m-1.

Nonpolar organic materials, such as polyethylene and polypropylene, have surface energies of less than 30mNm-1. The surface energy of polytetrafluor — oethylene (PTFE) is the lowest known among solid-state materials (17 mN m-1).

Consequently, no wetting takes place between adhesives and some plastics if there are no auxiliary modifications taken; hence, these plastics are classified as being ‘difficult to bond’. With a view to wetting, several others, such as epoxy resins, phenolic resins and polyester, which have surface tensions of about 60 mN m_1, are better suited to bonding. With regards to the wetting criterion, inorganic substances do not represent any problems, provided that they are not covered with contami­nants characterized by low surface energies. Table 3.2 provides values of the surface energies of different materials; here, for the sake of completeness, the nonpolar and polar force contributions to the surface energies have been included and will be discussed later.

With regard to bonding technology it can be summarized that, in the first instance, adhesion will only take place if the adhesive wets the solid-state material. This explains why it is possible to deduce the potential adhesion between a solid-state material with unknown surface energy and a liquid with a defined surface tension, from the simple measurement of the wetting characteristics involved. It must be stated, however, that wetting is not the only prerequisite for adhesion. The reasons for this will be provided in the following section.

3.2