Technology of Adhesives and Wood-Based Panels E-Book

Manfred Dunky

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Serie: Adhesion and Adhesives: Fundamental and Applied Aspects
• Sprache: Englisch

Seitenzahl: 1446

Veröffentlichungsjahr: 2025

Beschreibung

The book provides essential insights into the critical role of adhesive bonding in maximizing the value of wood products, equipping both students and industry professionals with the knowledge necessary to enhance production processes and improve product performance.

Adhesive bonding of wood is a key factor in the efficient utilization of wood for the production of value-added wood products, such as wood-based panels. The production of wood-based panels requires high-performance bonds between wood adherends and the properties of these wooden products are largely determined by the type and performance of the adhesive used. Technology of Adhesives and Wood-Based Panels comprehensively covers wood-based panels, focusing on the technologies behind their raw materials and their production. Journey through the production process: starting with the raw materials, then application of adhesives onto the wood’s surfaces, pressing the mat to the board, and curing or solidifying the adhesive. Finally, this journey will culminate in an investigation of the properties of the bondline in wood-based panels. This volume explores important concepts, including the influence of wood materials and surface on wood bonding, the performance of wood-based panels, the production technology of panels, and the behavior of adhesives when applied to wood surfaces, making it a valuable resource to industry professionals and students alike.

Readers will find that this book:

• Introduces wood adhesives and their chemistry and applications;
• Comprehensively covers the technology of wood-based panels;
• Explores connections for properties and performance between adhesives and bonded products;
• Provides recent developments in wood adhesives and wood-based panels.

Audience

Engineers, chemists, scientists, researchers, students, production managers and technologists in the wood, wood-based panel, and adhesive industries.

Leseprobe

Scrivener Publishing100 Cummings Center, Suite 541JBeverly, MA 01915-6106

Adhesion and Adhesives: Fundamentals and Applied Aspects

Series Editor: Dr. K.L. Mittal ([email protected])

The topics to be covered include, but not limited to, basic and theoretical aspects of adhesion; modeling of adhesion phenomena; mechanisms of adhesion; surface and interfacial analysis and characterization; unraveling of events at interfaces; characterization of interphases; adhesion of thin films and coatings; adhesion aspects in reinforced composites; formation, characterization and durability of adhesive joints; surface preparation methods; polymer surface modification; biological adhesion; particle adhesion; adhesion of metallized plastics; adhesion of diamond‐like films; adhesion promoters; contact angle, wettability and adhesion; superhydrophobicity and superhydrophilicity. With regards to adhesives, the Series will include, but not limited, to green adhesives; novel and high‐performance adhesives; and medical adhesive applications.

Publishers at ScrivenerMartin Scrivener ([email protected])Phillip Carmical ([email protected])

Technology of Adhesives and Wood-Based Panels

Manfred Dunky

University Lecturer, Consultant, Chemical and Wood Industries, St. Poelten, Austria

This edition first published 2025 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA© 2025 Scrivener Publishing LLCFor more information about Scrivener publications please visit www.scrivenerpublishing.com.

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Library of Congress Cataloging-in-Publication Data

ISBN 978-1-394-17459-1

Cover image: Generated with AI using Adobe FireflyCover design by Russell Richardson

Preface

This book would not have been possible without the help and effort of countless researchers, technologists, scientists, and colleagues who have worked on the topics described in the various chapters of this book for decades. Many excellent research articles dealing with the basics of wood bonding were published as early as the mid-20th century, 60–70 years ago; these articles are written in very clear language, and what they tell is still valid today when talking about wood bonding.

To some extent, this book is a continuation (but not at all a simple revised new edition) of my book together with Peter Niemz from 2002 (M. Dunky and P. Niemz, Wood-based panels and resins: Technology and influence parameters). In that textbook, I had compiled the technology of adhesives and wood-based panels, whereas Peter Niemz had contributed to the composition, properties, and production processes for wood-based panels. Since that time, a lot of developments, changes, and improvements have taken place, which encouraged me to compile this book, showing the basic technologies as well as what has happened and been developed in these 20 years.

The structure of the book is oriented according to the production process, starting with the raw materials, application of adhesives onto wood surfaces, pressing of the mat to the board and curing or solidifying the adhesive, and, finally, investigation and properties of the bondline in the wood-based panels and the properties of the panels. The book, hence, covers the various aspects of the production of wood-based panels with strong emphasis on the technologies behind the various raw materials and production steps. This includes, among others:

the influence of the wood material and the wood surface on wood bonding and the performance of wood-based panels; regardless of the type of woodbased panel, always two pieces of wood adherends are adhesively joined; this might be on the scale of square meters in the case of plywood, on the scale of square millimeters in the case of particleboard, or even on the micrometer scale at the crossing point of two fibers bonded together;

the influence of the adhesives on the production technology of panels and on the properties of panels; each adhesion connection needs the bonding action of an adhesive; in the vast majority of cases, the adhesive is added to the adherends; in a few cases, the adhesive can also be generated directly from the adherends;

the behavior of adhesives when applied to the wood surface and in the hot-press, in interaction with the mechanical compression and partial densification of the adherends, in order to generate bond lines and close contact of the adherend surfaces to each other;

the use of recycled wood and wood-based panels as raw material for the production of new boards; in fact, for example, particleboard can and is produced based on full usage of recycled wood as wood raw material;

the production of wood-based panels can be seen as a fully interdisciplinary approach, starting in the forest, passing through wood technology and wood chemistry, organic chemistry, and finally highly sophisticated process technology.

Adhesive bonding of wood has been the key factor for the efficient utilization of wood in the production of value-added wood products, such as wood-based panels. The production of wood-based panels requires high-performance bonds between the wood adherends; the properties of these wooden products are determined largely by the type and performance of the adhesive used. Due to the highly variable and complex nature of wood as a raw material, wood adhesive technology is an advanced science that merges adhesive preparation and formulation with a variety of application technologies. The main features in wood bonding are cohesion (inherent strength of the adhesive) and interfacial adhesion (bond strength between the adhesive layer and the adherend), in combination with proper application technology and press conditions.

The main aspects of bonding are:

surface properties of wood related to bondability

adhesive characteristics concerning wetting and penetration when forming the interphases and the interface between the wood and adhesive

chemical curing or physical solidification of the adhesive

the behavior of the adhesive in the formed bondline

the microstructure and properties of the bondline as the basis for the performance of wood-based panels.

Wood-based panels are produced worldwide at a yearly volume of nearly 400 million m3;, with the main producers in Europe, North America, and China/Southeast Asia. Despite often being called a commodity, steady research and development has been ongoing for several decades to improve the production process and the properties/performance of the panels, showing the highly innovative behavior of the chemical and wood-based panel industries. It is the only industry that uses in many plants more or less 100% recycled raw materials, i.e., old and/or recycled wood. Current topics of development are (among others):

reduction of density and weight of the panels: This is beneficial especially for self-pickup customers, as well as for reducing material input and material costs. Improved bond strength is necessary to compensate for the effect of the lower overall density or lower density in certain zones of the boards;

further reduction of emissions, especially of formaldehyde: The performance of the panels must remain unchanged, which means that the adhesives and the bonding process must be improved. The reduction of emissions during production and from the boards has been a decisive topic in recent decades, with more and more stringent regulations, especially concerning formaldehyde emissions. Fulfillment of these requirements was mainly achieved by relevant development of suitable adhesives, e.g., aminoplastic adhesive resins with very low content of formaldehyde;

replacement of fossil-based wood adhesives by naturally-based adhesives: Though tremendous work has been done in the last few decades, the breakthrough in the market is still missing; such bio-based adhesives are used so far only for niche products;

further increase of the proportion of recycling material: Currently, recycled wood is mainly used for particleboards (in many plants up to 100%); in the future, recycling material shall also be used for the production of fiberboards (MDF) as well as in the core layer of OSB. The use of recycling materials also necessitates sophisticated cleaning technologies.

Despite the fact that extensive information is available on adhesives and adhesion, basic research on the adhesion behavior of wood—including the evaluation of the formed microstructure of the bondline as the main influential parameter within the produced bonded products—is still necessary. This is to be seen as a fundamental approach as well as applied to industrial bonding processes, from single laminated wood preparation in carpenter’s shops to industrial production of wood-based panels of several thousand m3; per day.

It is also surprising that, despite chemical and technical literature describing various aspects and types of wood adhesives and their application and performance for bonded products, the interaction of the two main raw materials (wood and adhesive) is often lacking. Either the existing literature concentrates on chemical aspects of adhesives, or the various steps of wood processing and board pressing are described as such, but keep the adhesive and its performance rather as a “black box.” The author of the book sees himself (and has shown this in his 40+ years career in the chemical and wood-based panels industry) as one of the experts covering this interface between chemistry and woodworking. This especially is the main topic of the book. Such overall coverage of wood bonding and production technology of wood-based panels is seldom available. Special aspects are described in literature, but a thorough scope as outlined above is covered only in very few books, such as one from 2002 by the author himself (Dunky and Niemz). Particular attention is given to a very extensive reference list, especially starting with the years around 2000. Older references are included as well if necessary.

The author expresses his thanks to Martin Scrivener and his team at Scrivener Publishing for their help in the publication of this book.

Manfred Dunky

April 2025

Acknowledgements

All necessary permissions for figures in journals reused in this volume have been granted. The author cordially expresses thanks to all those publishers. The following table summarizes the various journals (in alphabetical order) and publishers as well as the figures in the book (identified with their figure number).

Journal

Publisher

Figures in the text

ACS Applied Polymer Materials

American Chemical Society (ACS Publications)

4.54

ACS Sustainable Chemistry & Engineering

American Chemical Society (ACS Publications)

2.11, 2.12, 2.13, 5.33

Advances in Material Science and Engineering

Hindawi – John Wiley & Sons

5.74, 5.75

Angewandte Makromolekulare Chemie (Macromolecular Materials and Engineering)

John Wiley & Sons

2.26, 2.39

Applied Microbiology and Biotechnology

Springer Nature

2.7

Biomass and Bioenergy

Elsevier

2.18

BioResources

North Carolina State University

2.36, 3.13, 3.22, 4.47, 5.31

Carbohydrate Polymers

Elsevier

2.27

Cellulose

Springer Science+Business Media B.V.

2.17

Chemical Reviews

American Chemical Society (ACS)

5.73, 5.77

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Elsevier

3.4

Composites

Elsevier

5.32

Composites Science and Technology

Elsevier

4.9, 4.10,

Enzyme and Microbial Technology

Elsevier

2.19

Environmental Research

Elsevier

5.78

European Journal of Wood and Wood Products

Springer Nature

2.5, 3.17, 3.19, 3.57, 5.30, 5.52, 5.59

European Polymer Journal

Elsevier

5.2

Fibers

MDPI

2.4, 3.46

Forest Products Journal

Forest Products Society

3.56, 4.17, 4.18, 4.35, 4.65, 4.77, 4.83, 4.86, 5.10, 5.35, 5.36, 5.66, 5.67

Hemijska Industrija

Association of Chemical Engineers of Serbia (AChE)

4.43, 4.44

Holz als Rohund Werkstoff

Springer Nature

2.2, 2.3, 2.8, 2.37, 3.10, 3.14, 3.15, 3.36, 3.37, 4.4, 4.6, 4.21, 4.24, 4.98, 4.99, 5.9, 5.15, 5.38, 5.68, 5.69, 5.70

Holzforschung

De Gruyter

3.20, 3.21, 3.33, 3.34, 3.38, 3.39, 3.40, 4.7, 4.14, 4.25, 4.29, 4.80, 5.24, 5.25, 5.26, 5.27, 5.28, 5.29, 5.37, 5.49, 5.80

Holztechnologie

Institut für Holztechnologie, Dresden, Germany

5.42, 5.43, 5.44, 5.45

Industrial Crops and Products

Elsevier

3.59, 3.62

International Journal of Adhesion and Adhesives

Elsevier

3.3, 3.30, 3.31, 4.32, 4.61, 4.70, 4.71, 4.73, 4.74, 5.6, 5.11, 5.21, 5.22, 5.23, 5.34, 5.72, 5.76

International Journal of Dentistry

John Wiley & Sons

3.1

International Journal of Mathematics and Mathematical Sciences

Hindawi – John Wiley & Sons

4.102, 4.103

Journal of Adhesion

Taylor & Francis

2.23, 4.57

Journal of Adhesion Science and Technology

Taylor & Francis

2.32, 2.34, 2.38, 3.5, 3.18, 3.16, 3.23, 3.24, 3.25, 4.30, 4.31, 4.42, 4.34, 4.53, 4.58, 4.59, 4.62, 4.82, 4.104, 5.54, 5.55, 5.56, 5.57, 5.61

Journal of Applied Polymer Science

John Wiley & Sons

3.9, 4.36, 4.38, 4.41, 4.45, 4.46, 5.7, 5.8

Journal of Materials Science

Springer Nature

5.13

Journal of the American Oil Chemists’ Society

John Wiley & Sons

2.30

Journal of the Korean Wood Science and Technology

Korean Society of Wood Science and Technology

4.60

Journal of Thermal Analysis and Calorimetry

Springer Nature

4.37, 4.39, 4.40

Journal of Wood Science

Springer Nature

2.15, 2.20, 2.58, 3.41, 5.39, 5.62

Macromolecular Materials and Engineering

John Wiley & Sons

2.22

Materials

MDPI

5.17, 5.18

Microscopy and Analysis

John Wiley & Sons

3.12

Microscopy and Microanalysis

Oxford University Press; Microscopy Society of America

3.26, 3.29

Microscopy Research

Scientific Research Publishing

5.16

Mokuzai Gakkaishi

The Japan Wood Research Society

2.25, 5.63

Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences

Royal Society of London

1.1

Plasma Chemistry and Plasma Processing

Springer Nature

2.16

Polymer

Elsevier

2.24

Polymers

MDPI

2.1, 2.9, 2.10, 4.12

Progress in Polymer Science

Elsevier

2.33

Recycling

MDPI

2.21

Thermochimica Acta

Elsevier

4.81

Wood and Fiber Science

Society of Wood Science and Technology

2.28, 3.2, 3.11, 3.48, 3.49, 4.1, 4.2, 4.3, 4.5, 4.8, 4.13, 4.16, 4.20, 4.26, 4.27, 4.28, 4.52, 4.84, 4.87, 4.89, 4.90, 4.97, 5.3, 5.4, 5.5, 5.19, 5.64

Wood Material Science and Engineering

Taylor & Francis

2.14, 4.85, 5.58, 5.60

Wood Science and Technology

Springer Nature

3.8, 3.27, 3.28, 3.35, 4.19, 4.33, 4.55, 4.56, 4.100, 5.1, 5.65, 5.71

1Introduction

Bonding of timber and wood has a history of several thousand years. It has overcome the limitations given by size and shape of the naturally grown trees. The so-called solid wood, such as boards, planks, edgings and laths, square timber, or other products, is restricted to the size of the primary log originating from the felled tree. Various methods for recombining solid wood and smaller parts of the wood substance, e.g., veneers, strands, particles, and fibers, have opened new possibilities to overcome these restrictions in size. In addition, limitations due to factors decreasing strength, elasticity (stiffness), and toughness of wood, such as knots and other anatomical features, can be eliminated. Especially the use of various adhesives for re-jointing has enabled a huge diversity of applications and development of types of wood panel products.

Today, huge volumes of wood-based panels are produced (Table 1.1); the actual annual production of wood-based panels (plywood, particleboard, OSB, MDF, hardboard, and softboard) is reported on a regular basis by the Food and Agriculture Organization Corporate Statistical Database (FAOSTAT) [1]. Further information (also on global basis) is given in the Annual Reports of the European Panel Federation (EPF) [2]. For 2022, the production volume cumulated to approximately 375 million m³.

Table 1.1 Production volumes of wood-based panels (2022) [1].

Production volume 2022 (million m³)

Total

Plywood

Particleboard

OSB

MDF/HDF

Hardboard

Softboard

World

375.3

109

110.3

38.2

101.9

7.5

8.4

Africa

3.2

1.3

1.2

0

0.4

0.3

0

Americas

65.2

17

11.7

21.5

10.8

0.9

3.3

North America

45.2

10.9

6

20.9

3.8

0.3

3.3

Central America + Caribbean

1.3

0.3

0.9

0

0.03

0.1

0

South America

18.7

5.8

4.9

0.6

7

0.4

0.03

Asia

215.2

81.8

51.4

7.1

69.4

4.4

1.1

Europe

88.8

8.4

44.8

9.6

20

2

4

Oceania

3

0.6

1.1

0

1.3

0

0

The given data are partly already official numbers, partly still estimations. Data for 2023 and 2024 are not yet available.

The total amount of adhesives, including all types of condensation resins, such as UF, MUF, or PF, as well as PMDI, required for these volumes of board production can be estimated with an order of magnitude of up to 22 million tons per year in the form of delivery. The overall distribution of the total amount of adhesives and resins in the production of wood-based panels to the various chemical types can be estimated as follows:

UF resins including melamine-fortified UF resins: 80%–85%

MUF resins: 10%–15%

Phenolic resins (PF, PUF, RF, PRF): 3%–5%

Diisocyanates (mainly PMDI): 1%–2%, mainly in use for the production of OSB

Adhesives for laminated beams (glulam) are preferably MUF, EPI, PUR, and PRF (see Section 2.2).

Bonding of timber shows a high degree of flexibility in process and in preserving the naturally wooden appearance. Besides this, some limitations in technology exist, which might hinder an extensive use of bonding practices in various applications. The basic properties of wood as the, by far, dominating raw material of all types of wood panel products remain decisive for the behavior of the final product, especially concerning changes in size and shape due to variations of the moisture content of the material.

Progress in research and development in the wood-based industry and in the adhesive industry has shown many successes during the last decades. On the other hand, many industrial requirements still require considerable and important developments in this area. The main driving forces today are “cheaper,” “quicker,” and “more complex.” The first two are caused by the strong competition on the market and the attempt to minimize costs while maintaining the requested level of product quality and performance. The key word “more complex” stands for new, improved, and specialized products and processes. Adhesives play a central role in wood-based panel production. Development in wood-based panels is always linked to and determined by development in adhesives and resins. This has emerged in a considerable diversity of types of adhesives used for the production of wood-based panels. Well-known basic chemicals have been used already for a long time for the production of the adhesives and their resins, the most important ones being formaldehyde, urea, melamine, phenol, resorcinol, and isocyanate. The biggest portion of the currently used adhesive resins and adhesives for wood-based panels is based on these few raw materials. The “how to prepare the resin” and the “how to formulate the adhesive (mix)” are sophisticated key factors to meet today’s requirements of the wood-based panels industry.

Properties and performance of wood bonding and of wood-based panels are determined by the following three main parameters:

Wood, especially the wood surface, including the interphase as joint region and the interface as contact zone between the wood surface and the bondline

Adhesive

Working conditions and process parameters.

Sufficient properties of the wood-based panels can be attained only if all three parameters contribute to the necessary extent to the bonding and production process.

A myriad of original papers, summary and review papers as well as authored and edited books are given in the scientific technical and chemical literature on all aspects of wood adhesives and wood-based panels (composites). Individual references as well as indication of general literature is given throughout the text of the book.

1.1 Wood-Based Panels

Wood-based panels consist of a variety of different board types, including many hybrids between e.g., wood and wood-based panels, or between various types of wood-based panels, such as a solid wood core with a fiberboard as face layer. The target of and motivation for this broad variety is the wish and necessity to generate the optimal product version for each application, under consideration of various frame parameters, such as availability, type, quality, and consistency of raw materials, legal aspects concerning environmental and health aspects, and production costs (costs for raw materials, necessary processing times, mainly press times, see Table 1.2).

For installation of halls, bridges, and other construction elements, the classic gluedlaminated timber systems (glulam) and so-called “engineered wood products” are used. Whereas in the past it was mainly the classic glued laminated timber construction that produced beams for, e.g., hall construction, today, various newer “engineered wood products” are used as smaller beams, especially in single-family house construction.

The production takes place directly as a beamor rod-shaped product (e.g., veneer strip wood ParallamÒ) or as a flat element with subsequent separation into rod-shaped elements (laminated strand lumber (LSL), laminated veneer lumber (LVL)).

The advantages compared to solid wood are the large and variable dimensions, the avoidance of deformations due to drying stresses, and a higher strength compared to solid wood because no wood defects reduce the strength.

A variety of adhesives (see Section 2.2) is currently in use in the wood-based panel industry. Condensation resins based on formaldehyde represent the biggest volume. These resins are formed by the reaction of formaldehyde with various chemicals, such as urea, melamine, phenol, or resorcinol as well as with combinations of these substances. These thermosetting resins are mainly liquid and consist of linear or branched oligomeric and polymeric molecules in an aqueous solution, partly as dispersion of such especially higher molar mass-molecules in these aqueous solutions. During curing, the resins convert to insoluble and non-melting three-dimensionally crosslinked networks. The hardening conditions can be acidic (aminoplastic resins), highly alkaline (phenolic resins) or neutral to light alkaline (resorcinol resins, tannins). Diisocyanates, especially polymeric 4,4′-diphenylmethane diisocyanate [polymethylene-diisocyanate (PMDI)], are another important chemical group of adhesives used for various applications in the wood-based panel industry, especially OSB.

Inorganic adhesives comprise non-hydraulic binders (clay and loam, air-hardening lime, magnesia cement, or gypsum), hydraulic binders (hydraulic lime, cement, pozzolans, and latent hydraulic binders), water glass (soluble glass), and geopolymers [23].

The higher the degree of disintegration of the wood substance, the greater are the achievable homogeneity and isotropy, at least in two of the three spatial directions.

Table 1.2 Overview and short description of wood-based panels (for further details see also refs. [3–5]).

Class of panels

Details

Veneerand solid wood-based materials, engineered wood products

Laminated wood (non-densified and densified)

Laminated veneer lumber (LVL): parallel veneer orientation

A flat pressed multilayers wood panel similar to plywood composed of oriented wood veneers but differently from plywood oriented all in the same direction in all the layers and bonded by hot pressing using thermosetting adhesive resins

Glued-laminated timber, glulam [

6

]