Лако-красочные материалы — производство

The background to acid-base interactions in adhesion science and technology has been reviewed with the emphasis on polymers, metal oxides, fillers, fibers, and pigments. When these specific exothermic interactions operate at interfaces they have a significant impact on adsorption, wettability, adhesion, and mixing as shown through some selected exam­ples. The importance of such findings has resulted in the establishment of an impressive array of methods which enable an adhesionist to determine acid-base scales for materials. This task is, however, very delicate because it requires the determination of the heat or the free energy changes of acid-base interactions of reference acidic and basic chemical species with the material. The choice of reference test acids and bases is also crucial and usually depends on the nature of the material under investigation and the experimental conditions associated with the technique used for the assessment of acid-base properties.

Contact angle measurements (CAMs), IGC, XPS, and AFM are among the experi­mental techniques most used to interrogate the acid-base characteristics of polymers and other materials at the macroscopic and microscopic scales. CAM is very useful for the determination of acid-base contributions to the surface free energy and the determination of the interfacial free energies in a liquid medium such as water. This is of paramount importance when one has to deal with protein adsorption and cell adhesion. We clearly advocated the vOCG theory although it has met several criticisms in the recent literature, but in very different situations it was very effective in determining the mechanisms govern­ing solubility, adsorption, adhesion, and deadhesion phenomena.

IGC remains one of the most versatile techniques for divided materials and fibers. However, whilst n-alkanes are universally used to determine the dispersive properties of materials, there is not a universal set of reference specific probes for the determination of acid-base properties of materials that differ markedly in nature (e. g., polymers and metal oxides). For example, metal oxides or clays are not amenable to characterization using specific probes such as alcohols, THF, or ethylacetate at temperatures in the 30-50° C range (real conditions). Consequently, it is very difficult to compare the acid-base properties of materials obtained at differing temperature ranges and using different sets of probes. We take this opportunity to point out that, at least, it will be very important that research papers report the temperature ranges in which acid-base constants were determined. This is not done systematically. Contrary to what is stated by Belgacem and Gandini [85], we believe that constants which are “temperature inde­pendent” determined at for example 80-100°C can hardly be representative of properties at room temperature (this is the case of metal oxides which can be more or less hydrated below or above about 100°C). To our knowledge this has never been checked experi­mentally.

The LSER theory combined with IGC should be applied more in the future because it permits distinction between London, Keesom, and Debye interactions in addition to the acid-base scales. This is not done in the traditional IGC studies in relation to adhesion.

XPS has been employed for many years to characterize the surface acid-base proper­ties of catalysts and metal oxides by various methodologies including Fermi level mon­itoring. We have shown since the early 1990s the potential of XPS in characterizing acid-base properties of conventional polymers using the molecular probe technique. This approach has recently found application in characterizing commercial resins, photo­initiators, and plasma-treated polymers in relation to metallization.

Finally, AFM appears as an extraordinary technique to study acid-base interactions at the molecular scale. It enables the determination of pKa for surface confined carboxylic and other Bronsted groups. With the systematic studies which have appeared over recent years using thiol-treated tips and surfaces, clearly AFM has become a very well established and powerful tool for fundamental and applied research studies on acid-base interactions in adhesion.

The importance of acid-base interactions in adhesion continues to attract several researchers, however, still there seems to be a lack of consistency in the approaches as stated by K. L. Mittal in the Preface of [19]. It is hoped that Round Tables will be organized in order to define common strategies for polymers, fillers, fibers, etc. which will permit inter-laboratory comparison.