Adhesives for Wood and Lignocellulosic Materials. R. N. Kumar

and biobased polyurethane adhesives in Chapter 7.

      Surface inactivation peculiar to wood is dealt with in Chapters 8 and 9. In order to resolve this problem, surface modification by suitable treatment is dealt with in Chapter 10. Treatment of biofibers is considered in Chapter 16. All of these chapters are very important for technologists working in the wood industry.

      In order to reduce the use of petroleum-derived phenol for the manufacture of phenolic resins, a lot of research has been carried out on the partial or whole substitution of phenol by natural polyphenols, namely tannins. Chapter 11 is an exhaustive account of the chemistry of condensed tannins. A good understanding of the chemistry of condensed tannins is very necessary for developing new adhesives based on natural polyphenols.

Chapter 12 discusses in great detail the technology of tannin adhesives, particularly new technologies for industrial tannin adhesives, lignin adhesives, protein adhesives, carbohydrate adhesives, unsaturated oil adhesives, and cardanol-based adhesives. The chapter also deals with wood welding without adhesives.

      The environmental aspects of adhesives, namely formaldehyde emission, are discussed in Chapter 13. Formaldehyde is of particular concern due to its classification as a “known human carcinogen” in the August 8, 2014 publication of the 12^th Report on Carcinogens (RoC). Therefore, formaldehyde emission standards are dealt with in detail in this chapter. Next, the rheology and viscoelasticity of adhesives is the subject of Chapter 14 and Chapter 15 discusses hot melt adhesives.

      Chapters 17 to 21 are included in Part B (Polymer Matrix Materials for Biofiber Composites) of the book. Both thermoplastic and thermosetting matrix materials are discussed in detail.

      The author (RNK) thanks Dr. V.V. Srinivasan, former Director of the Institute of Wood Science, Bangalore, for suggesting the idea of this book. The author also expresses his sincere gratitude to Prof. Pizzi for volunteering to co-author the book at a time when I had abandoned the idea of writing it. His encouragement, chapter contributions, help in editing the chapters and adding very important factual details, and particularly his great patience in arranging the references, is gratefully acknowledged. The author thanks Mr. P.K. Mayan, Managing Director, Western India Plywoods Ltd, Baliapatam, Kerala, for his encouragement. I record my thanks to my son, Dr. Suresh Nandakumar, for his helpful suggestions. The authors thank Mr. Martin Scrivener for his unequivocal support.

References

      1. Marra, G. Overview of wood as a material, J. Educ. Modules for Mater. Sci. Eng. 1(4), 699–710, 1979.

      2. Berglund L. and Rowell R.M. Wood composites, in Handbook of Wood Chemistry and Wood Composites, Routledge/Taylor & Francis, 2005.

      3. Pizzi, A. Special section: Wood adhesives. Foreword. Int. J. Adhes. Adhes. 18, 67, 1998.

Part A SUBSTRATES, ADHESIVES, AND ADHESION

Chapter 1 Wood as a Unique Adherend

      1.1 Introduction

      In order to make durable wood adhesive bonds in composite wood products, a clear understanding of the nature and uniqueness of wood as a substrate and of the distinctiveness of the wood–adhesive interaction is essential. In this context, it is necessary to mention that substantial differences exist between bonding in the case of wood on the one hand and most other materials on the other. The most obvious characteristics of wood that distinguish wood from other substrates are (a) its porosity, (b) presence of interconnected cells into which adhesive can flow, and (c) the cell walls that have the ability to allow low-molecular-weight chemicals and resins to pass through and in some cases even to react with them. All the above features are due to the special identity that wood possesses in contrast to other substrates.

      It is known that wood exhibits multiscale hierarchical structures. As reported by Gao [1], structural hierarchy is a rule of nature and can be observed in many other natural and man-made materials. In recent years, these materials have been called multiscale materials. Hierarchical solids contain structural elements that themselves have further finer structures [2]. In this respect, wood as an adherend is significantly different from other adherends such as metals and plastics.

1.2 Wood, An Adherend with Hierarchical Structure

Wood is basically a fiber-reinforced polymer matrix composite with cellulose as the fiber reinforcement and lignin along with polyoses functioning as mixed polymer matrix. Wood has been found to have a hierarchical cellular structure. The functional efficiency of wood, its exceptional mechanical properties, and other unique characteristics are attributed to the distinctive structure at all its levels of hierarchy. The complex hierarchical structure of wood manifests in a wide range operating from the macro scale in the case of tree to board to growth ring (macro scale), through multilayered cell walls, composite cellulose/hemicellulose microfibrillar structure (micro scale), and down to the structure of the three main polymer components, cellulose, hemicellulose, and lignin (nanoscale).

      Hierarchical solids contain basically structural elements that, in turn, have further finer structures; i.e., intricate structural features occur at different size scales.

      For instance, the hierarchical nature of wood showing the size scale of each structural element within wood has been illustrated by Moon et al. [3] with typical dimensions given in parentheses:

      Tree height (in meters), tree cross section (in cm), growth rings (in mm), cellular structure (in 500 μm), cell wall structure (in 25 μm), fibril-matrix structure (in 300 nm), fibril structure (10 nm), and cellulose (1 nm).

      This structural hierarchy of wood can play a significant role in influencing the phenomenon of wood adhesion [3].

      In order to maximize the strength and durability of adhesive bonds, one should understand the complex hierarchical structural elements of wood and their interactions with the molecules of the adhesives at various size scales.

      It should be appreciated that hierarchical structural elements of wood when exposed to the molecules of adhesives during bonding interact at different size scales in an extremely unique manner not encountered in the case of other adherends. Although the concept of hierarchical model of wood has not been explicitly mentioned, the importance of practical length scale of wood composite elements in wood adhesive bonding has been recognized and reported [4].

1.3 Details of Structural Hierarchy in Wood

      The structural hierarchy confers on wood very unique properties. Accordingly, wood is porous, permeable, hygroscopic, and orthotropic. It is a biological composite material of extreme chemical diversity and physical intricacy. Further, wood properties vary between species and even within the same species. They can vary even within a tree. Variability within a single species could be per se significant enough to throw challenge to an adhesive for its consistent and satisfactory performance [5].

      1.3.1 Physical Structure

There