The field of surface coatings is now so extensive, and is developing so rapidly, that no individual person could be expected to master it in its entirety. Hence, the days have gone when a lone author could compile a comprehensive treatise on the topic without overlooking many of the most significant developments. Yet the very growth and diversity of surface coatings make it essential that workers in the field should have access to up-to-date and authoritative reviews. It is to satisfy this need that we have assembled our team of authors, each of whom is an expert in his or her branch of the subject, and is well able to as'sess w: hat is most significant and exciting within that branch. They h, ave b en encouraged to develop their themes in what they have felt to be the most appropriate way, with minimal editorial interference from us. We believe that the results of their labours will be of benefit to those engaged not only in basic research, but also to those working ip product development and in testing and evaluation of coatings.

The science and technology of surface coatings continues to advance. Among the key areas are polymer chemistry, as new binders are developed to meet increasingly stringent environmental demands; testing and evaluation, as the need to understand the factors affecting coatings performance becomes ever more intense; and studies of that enduring problem, corrosion of metal substrates, from which coatings of ever improving effectiveness are emerging. We have in this present volume of the series continued to cover aspects of these numerous developments. There are chapters on waterborne paint, a subject of increasing environmental importance, by J. W. Nicholson, and by H.-J. Streitberger and R. P. Osterloh; on a new and sophisticated test method, acoustic emission (R. D. Rawlings); and on anticorrosion coatings both organic (W. Funke) and inorganic (M. C. Andrade and A. Macias). Finally, that topic of immense practical importance to paint technology, pigmentation, is covered in a chapter by the late T. Entwistle. All the authors have brought considerable experience in their chosen field of coatings technology to the preparation of their chapters, all of which are timely reviews of developing topics. We are grateful to each author for helping in the preparation of this volume, and for putting their experience at the disposal of the wide audience for whom this book is intended."

Ionic polymers, like elephants, are easier to recognise than to define. Several methods of classification have been attempted but none is wholly satisfactory because of the extreme diversity of ionic polymers, which range from the organic, water-soluble polyelectrolytes, through hydrogels and ionomer carboxylate rubbers, to the almost infusible inorganic silicate minerals. For this reason, a general classification is not only difficult, but has minimal utility. However, there are some characteristics of these materials that should be highlighted. The role of counterions is the significant one. These ions, either singly or as clusters, take part in the formation of ionic bonds which have a varying structural role. Often they act as crosslinks, but in the halato-polymers the ionic bonds form an integral part of the polymer backbone itself. Conversely, in polymers contain ing covalent crosslinks, such as the ion-exchange resins, the coun terions have virtually no structural role to play, since they dwell in cage-like structures without affecting the crosslinking, and are readily exchanged. They are, perhaps, best described as ion-containing polymers rather than structural ionic polymers. Another crucial factor is the role of water in ionic polymers. The presence of ionic bonds means that there is a tendency for these materials to interact with water. Where the ionic polymer contains a high proportion of ionic units, it acts as a hydrogel and may be highly soluble. Such interactions with water decrease sharply as the ionic content is reduced, though even then water can act as a plasticiser."

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