Лако-красочные материалы — производство

The field of dye chemistry has stimulated the publication of many books, monographs, special topics, and reviews over the years, but surprisingly, very little is mentioned about leuco dye chemistry — Color formers have been known for a long time. Many natural products are color formers and have become part of modern synthetic dyes, e. g., indigo, juglone, haematoxylin. The extensive utilization of indigo depended on the reduction of its color to a colorless water-soluble leuco form to facilitate dyeing and then regener­ation of the color by oxidation on the fiber. Over the years, during the development of the synthetic dye industry, leuco materials played two major roles. First, many synthetic schemes make use of leuco materials as isolated intermediates in the final dye formation. Second, leuco dyes have a vital function in dyeing techniques. The entire vat dyeing industry hinged on the increased solubility of vat dyes when reduced and on the decreased solubil­ity when reoxidized. Leuco forms of vat dyes were stabilized (trapped) and isolated as sulfuric acid esters and were marketed, for example, as indigo sols. Leuco dyes now play an ever-increasing role in modern applications, e. g., imaging, display, and memory technologies; and in analytical and biological sciences.

The use of the term leuco dye is a common paradox. Leuco color formers are materials that undergo controlled chemical or physical changes resulting in a shift from a colorless state to an intense color. The preparation of leuco color formers takes advantage of the very nature of colored materials themselves. The existence of extended conjugated л-system in dyes is responsible for the absorption in the visible region. The chemistry of such л-system is noted for facile reactivity, particularly to reactions such as reduction, oxidation, and hydrolysis (not hydrolytic cleavage). When л —

interaction is temporarily interrupted, the transition between the colored and colorless states is affected. The interrupted л-system is stabilized by a variety of trapping agents. The lifetime of a leuco dye (untrapped) can be milliseconds to several days or months, depending on the type of leuco dye. The use of unstable leuco dyes is restricted in modern applications such as imaging, although they can be of immense use in analytical and biological applications.

More recently, emphasis is shifting from natural dyes to synthetic dyes, to functional dyes. However, traditional organic synthesis has not caught up with the advances connected with functional dye chemistry. Hi-tech appli­cations present many challenges to the dye chemist to devise dyes to meet the often demanding criteria. The leuco dye chemist, in addition to meeting all of the criteria associated with dyes, should maintain an optimum balance between reactivity and stability of the colorless form. This is not often obvious and is very challenging in its own right.

The growth of the leuco dye industry has created vigorous research activity in recent years, prompted by the great potential for proprietary products of high profitability. Because of the absence of ready sources, and the increasing importance of leuco dyes in modern applications, this book should prove to be both timely and useful. The present volume is designed to fulfill the needs of the novice researcher, and provide the general reader with an account of some selected areas of leuco dyes—mostly synthetic details and a few selected applications of important classes. This format, I believe, will be invaluable to both industry and academics alike. The book provides some experimental details, and directs the reader to the pertinent literature references. It is possible to prepare leuco compounds from a variety of dyes, for example, indoaniline, azomethine, and aminostyryl dyes. The majority of these leuco dyes are relatively unstable and their uses are restricted. Therefore, in this volume, only those leuco dye classes that have at least reasonable stability have been selected. Because of the tremendous amount of literature on leuco dye applications, complete description of applications of leuco dyes could not be justified; instead a major part is allocated to the chemistry.

The book is divided into seven chapters. Chapter 1 describes photo­chromic materials which have critical applications in memory technology. These compounds generally are activated by light. Chapter 2 covers leuco quinones which, in many cases, when oxidized, have their absorption maxima in the near-infrared region. Chapter 3 describes leuco dyes of a common group of compounds—oxazine, thiazine, and phenazines—that have found applications in color photography. Chapters 4-6 describe arylmethine-type compounds that can be triggered to dyes by common chemistry. Chapter 7 describes a special class of leuco dyes, namely, tetra — zolium salts, which, unlike other dyes described in earlier chapters, become dyes by reduction, not by oxidation. The amount of literature available on this class is enormous, and page limitations have restricted its full description.

Readers interested in the role played by leuco dyes in hi-tech applica­tions will find this volume to be useful and a reference for the future development of new classes of dyes or leuco dyes.

Acknowledgments. It is a pleasure to gratefully acknowledge the con­tributors of this volume; without their hard work and kind cooperation this book would not have been possible. It is gratifying to acknowledge Drs. J. Kitchin and M. Matsuoka for their immense interest through the book’s inception to its completion. I am especially grateful to Drs. A. R. Katritzky and G. Sabongi for asking me to take the responsibility for this volume, and for their constant support and advice. I sincerely thank Plenum Press for their cooperation. I am indebted to 3M management, particularly Barb Cederberg, Doug Dybvig, and G. Wagner who saw the benefit of this work to the company and to the scientific community. Finally I am thankful to my co-workers Doreen Lynch, Robert Balchunis, Rick Ollman, and William Ramsden for their support.