The above developments markedly increased the percentage of adhesives used for the production of glued wood products. It should be mentioned in this context that a high percentage, maybe 80% or more, of all wood products produced today are glued, and that about 70% (by volume) of all the adhesives produced in the world today are used for application to wood [3]. These developments have led to an increase in the functional efficiency of wood products as well as an efficient utilization of wood resources, thus constituting an essential tool to directly or indirectly affect the sustainability of forestry and wood-based industries.
Although a number of books have appeared on the subject of adhesives in general and wood in particular, this book is unique because of the vast academic teaching and research experience and hands-on industrial experience of the authors. Their skills have been brought to bear on identifying very important and unique combinations of current topics constituting the essential contents of the book. Furthermore, this book, besides the adhesives for wood detailed in Part A, also deals with the polymeric matrix materials for natural-fiber-based composites in Part B. The decision to include polymer matrix materials was made in consideration of growing global interest in wood-polymer composites based on natural fibers during the past decade.
The first chapter of the book deals with the distinctiveness of wood as an adherend in the midst of other substrates such as metals, polymers, inorganic adherends like glass, etc. In contrast to other substrates, wood presents adhesives with hierarchical structural elements of different sizes which, along with its unique chemical and physical characteristics, greatly influence the wood-adhesive interaction.
Knowledge of the fundamentals of adhesion is extremely important for researchers as well as technologists in the industry, both for adhesive formulations and troubleshooting during production. The importance of establishing an intimate contact between the adhesive and wood has been emphasized for an effective performance and durability of the bonded wood products in actual service. Therefore, mechanical interlocking, coulombic (ionic) interaction, hydrogen bonding, and apolar interactions are discussed in Chapter 2. In addition, electronic or electrostatic theory, adsorption (thermodynamic) or wetting theory, diffusion theory, chemical (covalent) bonding theory, theory of weak boundary layers and interphases and interfacial forces based on specific donor-acceptor (acid-base) interactions between adhesive and substrate molecules are also discussed.
In Chapters 3 to 7, the chemistry and technology of urea-formaldehyde, melamine formaldehyde, phenol-formaldehyde, resorcinol-formaldehyde, and polyurethanes are discussed in detail. Special mention is given to non-isocyanate polyurethanes (NIPUs) 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 12th 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.
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
