Adhesives for Wood and Lignocellulosic Materials. R. N. Kumar
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-119-60543-0
This book is dedicated to my (Dr. R. N. Kumar) son, the late Dr. Gopal Kumar who evinced keen interest in my endeavor to write the book.
Preface
The dramatic increase in the production of reconstituted wood products over the past 100 years has been made possible through the systematic development of new adhesives devised to meet the growing challenges of the times. Originally, large-diameter trees were available, but as their availability decreased, followed by a corresponding increase in price, the wood industry had to meet the challenge by attempting to use small-diameter trees and comminuted wood particles, fibers and pulp mill waste. This resulted in the development of reconstituted wood produced from the comminuted particles bound together with adhesives. This development had a significant impact on meeting challenges. The development of new types of adhesives—both synthetic petroleum-based adhesives as well as adhesives of natural origin—occurred during this period.
This trend of moving away from solid wood towards the utilization of elements of regularly reduced dimensions was recognized by Marra [1], who illustrated his concept with the “nonperiodic table of wood elements” shown in Figure 1 [2].
Figure 1 Basic wood elements from largest to smallest (i.e., breakdown of solid wood into finer elementary components [1].
This concept established the future trends in the wood-based industry, namely, (1) use of smaller trees, (2) use of waste from other wood processing, (3) removal of defects, (4) use of rare and hitherto unused wood species, natural lignocellulosic fibers, (5) creation of more uniform components, (6) development of composites stronger than the original solid wood, (7) ability to make composites of different shapes and (8) glulams, OSB, LVL, etc., (9) development of natural-fiber polymer-matrix composites, (9) development of more sophisticated engineered wood products and structural elements, such as wooden I-Joist box beams, aided by the availability of new or improved wood adhesives, (10) development of sandwich composites of wood and non-wood materials such as metal- and plastic-faced wood panels, paper and metal honeycomb sandwiches, etc.
It is interesting to note that an answer is slowly emerging to the question mark in Figure 1. Exciting new opportunities are emerging in the field of biorefining to produce chemical feedstocks, syngas, and nanocrystalline cellulose. In the near future, nanocrystalline cellulose, produced as a high-value by-product from the biorefining process, could likely compete with carbon fiber for use in innovative high-strength biocomposites.
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