COST Action E13 Wood Adhesion and Glued Products State of the Art

174 Pages·2004·0.79 MB·English

COST Action E13

Wood Adhesion and Glued Products

Working Group 1: Wood Adhesives

State of the Art – Report

Editors:

Manfred Dunky
Tony Pizzi
Marc Van Leemput

1st Edition - February 2002
ISBN 92-894-4891-1

COST Action E13 - WG 1 Report on the State of the Art

The authors:
(in alphabetical order)

Helena Cruz [email protected]
Laboratório Nacional de Engenharia Civil (LNEC)
Avenida do Brasil, 101
P-1700-066 Lisboa
Manfred Dunky [email protected]
dynea Austria (former Krems Chemie)
Hafenstrasse 77
A-3500 Krems
Dirk Grunwald [email protected]
Wilhelm-Klauditz-Institut (WKI)
Bienroder Weg 54e
D-38108 Braunschweig
Hervé Heinrich [email protected]
Ato Findley S.A.
Route du Bailly
BP 70219
F-60772 Ribécourt Cedex
J-W.G. van de Kuilen [email protected]
Section Steel and Timber Structures
Faculty of Civil Engineering and Geosciences
Delft University of Technology
NL-2600 AA Delft
Egil B.Ormstad [email protected]
Dyno Industrier ASA
P.O.Box 160
N-2001 Lillestroem
Frederic Pichelin [email protected]
Schweizerische Hochschule für die Holzwirtschaft
Abteilung F&E
Solothurnstrasse 102
CH 2504 Biel
Tony Pizzi [email protected]
Prof., Industrial Chemistry
ENSTIB, University of Nancy 1
27 Rue du Merle Blanc
BP 1041
F-88051 Epinal, France
Petri Pulkkinen [email protected]
Helsinki University of Technology
Laboratory of Wood Technology
P.O.Box 5100
FIN-02015 TKK
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COST Action E13 - WG 1 Report on the State of the Art

Martin Scheikl [email protected]
dynea Austria (former Krems Chemie)
Hafenstrasse 77
A-3500 KREMS
Leena Suomi-Lindberg [email protected]
Technical Research Center of Finland (VTT)
Building Technology
P.O.Box 1806
FIN-02044 VTT
Jeremy Tomkinson [email protected]
The BioComposites Centre
University of Wales
Bangor
UK-Gyynedd LL57 2UW
Magnus Walinder [email protected]
Post Doctoral Research Associate
Advanced Engineered Wood Composites Center
University of Maine
Orono, 04469
(cid:150) (cid:150) (cid:150)

Acknowledgement
Let me say thank you very much to all authors mentioned above and to all other colleagues within
Working Group 1 in COST E13, who have contributed to this State of the Art-Report. You all have
done a really great job. You all can especially be proud of the fact that you have done this work
beside of your daily business in industry, at the university, at research institutes or wherever you
fulfil your daily job. It was a pleasure for me to work with you and I am looking forward with great
pleasure and interest to the continuation of our work in Working Group 1 and in COST E13.

Manfred Dunky
Chairman of COST E13 WG 1 – Wood Adhesives
February 2001

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COST Action E13 - WG 1 Report on the State of the Art

Preface
By definition COST aims at developing cooperation in science ad technology in Europe. The
technical Committee for Forests and Forestry Products recognized the great value that adhesives
and bonding technology can apport and have apported to wood and to forest products. It is for this
reason that COST E13 on wood adhesives and wood gluing has been created. The aims of this
group are multifold, being:
1. to create a platform for building scientific cooperation and partnership across Europe and to
facilitate the development of consortia for EU funded research projects (R&D framework
programmes).
2. To assess the strength and weaknesses on given areas or disciplines concerning the field of
wood adhesives and wood bonding for the various countries, and even more important for the
European Community as a whole in relation to the global context.
3. To address such strengths and weaknesses with apt transnational projects and even facilitate
national approach of such problems.
4. To make the scientific and industrial community in the wood adhesives and wood gluing
cluster more visible and with greater lobbying influence in Europe and abroad.
We have now completed the State of the Art report. I would like to emphasize the necessary and
innovative aspects of this work. It is “Europe” with a single voice speaking for the first time
through the pages of this report, through you which have contributed to write the different chapters,
or that you have just contributed by voice or by organizing.
This is an important exercise as it is only through the dedication of all of you that from such a
document, the real, perceived, and urgent areas of need determinant for the future survival of this
speciality and of its related industries have started already to emerge. We have a couple of years to
go to define and refine these areas even better, in short to distil the numerous points we have
already defined in a series of important focuses in the adhesives, bonding and bonded products
areas. Take this seriously as you have taken extremely seriously the compilation of this State of the
Art report, as your own survival in this field might depend from it, even if you might not be able to
perceive this today. “Sitting on the fence”, and just playing at the stateman is a losing proposition
in these exercises aimed at a world already in fast forward gear: not to participate actively means
not to have your ideas taken into consideration, whatever these ideas and wishes might be:
financing from the EU, a bigger market share on the global scene, an innovative product or process,
impact on the world scientific stage, or even just survival by defending your own patch from
unfriendly, competitive raids. The message is clear: participate actively as the future is only in your
own hands. It is through this State of the Art report that each of you will also identify similarity of
interests and purpose, will then identify future partners, lobbying friends and common interests
without losing but rather gaining in competitive advantage at the cutting edge of the speciality. Do
not waste or belittle this opportunity: none of us know, but it might be the only one is ever offered
to us.

A.Pizzi
Chairman
COST E13

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COST Action E13 - WG 1 Report on the State of the Art

Table of Contents
Executive Summary............................................................................................................................................vii
1 Introduction.............................................................................................................................................1
2 Chemistry of Adhesives...........................................................................................................................3
2.1 Formaldehyde Resins................................................................................................................................3
2.2 Polyurethane Adhesives..........................................................................................................................38
2.3 Adhesives Based on Natural Resources..................................................................................................46
2.4 Casein Adhesives....................................................................................................................................66
2.5 Other Woodworking adhesives...............................................................................................................68
3 Analysis of Resins and Adhesives.........................................................................................................75
3.1 Introduction.............................................................................................................................................75
3.2 Main Adhesives Systems for Wood Based Products...............................................................................75
3.3 Characteristic Values of Adhesives and their Analysis...........................................................................76
3.4 Advanced Properties of Adhesives and their Analysis............................................................................79
4 Bonding Process.....................................................................................................................................89
4.1 Theory of Bonding..................................................................................................................................89
4.2 Process of Adhesion................................................................................................................................98
4.3 Properties of the Glue Line - Microstructure of the Glue Line..............................................................111
4.4 Binderless Gluing..................................................................................................................................113
5 Influence of the Wood Component on the Bonding Process and the Properties of Wood
Products...............................................................................................................................................121
5.1 Introduction...........................................................................................................................................121
5.2 Wood Structure.....................................................................................................................................121
5.3 Properties of Wood Surface..................................................................................................................122
5.4 Moisture Content of Wood....................................................................................................................127
5.5 Grain Orientation of Wood in the Glue Line.........................................................................................131
5.6 Temperature..........................................................................................................................................133
5.7 Bonding Properties of Different Wood Materials.................................................................................134
5.8 Summary...............................................................................................................................................141
6 Influence of the Adhesive on the Bonding Process and the Properties of Wooden
Products...............................................................................................................................................145
6.1 Introduction...........................................................................................................................................145
6.2 Basic Parameters...................................................................................................................................145
6.3 Aminoplastic Resins..............................................................................................................................148
6.4 Phenolic Glue Resins.............................................................................................................................151
7 Test Methods and Prediction of Performance..................................................................................155
7.1 Introduction...........................................................................................................................................155
7.2 Current Test Methods and Acceptance of Glues...................................................................................155
7.3 Accelerated Ageing Tests......................................................................................................................157
7.4 Prediction of Performance.....................................................................................................................158
8. Summary and Outlook........................................................................................................................161

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COST Action E13 - WG 1 Report on the State of the Art

Executive Summary
This State of the Art - Report not only summarizes the knowledge available today but also
especially addresses the main actual requirements in the development, production, application and
performance of adhesives; these requirements on the other side are based on the various driving
forces in the development, production, application and performance of the wood-based panels
themselves.

The report is divided into 8 chapters:
Chapter 1 gives an introduction and an overview on the driving forces for new and better
adhesives.
Chapter 2 describes all the various adhesives used in the wood based panels industry, dealing with
chemical principles as well as with the application of the adhesives and glue resins.
Chapter 3 summarizes the analysis methods used to characterize the composition of the adhesives
as well as their hardening and gelling behaviour.
Chapter 4 deals with the bonding principles, including the various theories of bonding, also
describing the process of adhesion (working parameters and process conditions as influence
parameters) and the properties and microstructure of the glue line.
Chapter 5 evaluates the wood component as one of the three main influence parameters in the
production of wood based panels; it especially deals with the influence of the wood surface on the
bonding process and bonding result.
Chapter 6 investigates the influence of the adhesive as the second important influence parameter.
Chapter 7 finally brings some details on test methods and prediction of performance.
Chapter 8 points out, that additional and especially sufficient resources and support is needed, to
fulfil all the ideas and requirements listed in the chapters above.

Following for the various chapters 2 – 7 an overview, especially necessary R&D shall be given.
This shall not replace a deeper contemplation of the details addressed in the questions and open
R&D topics after each chapter, which also show the great variety in wood bonding.
Chapter 2: Chemistry of adhesives
The adhesives on the one side form the bond line itself; on the other side the interactions with the
wood component and especially the wood surface are the decisive precondition for a good bonding
quality. Therefore all properties of the adhesives influence both, the possible cohesive as well as
the adhesive behaviour.
Main targets of the development of adhesives are:
• The speeding up of the formation of the glue line, disregarding the fact if it is a chemical or a
physical process
• Various application properties: e.g. easy, safe and harmless application, long storage stability
• Optimisation of the interactions with the wood surface
• Cost efficiency.
This includes special formulations and raw materials as well as new types of cooking procedures.
The price of the used adhesive always has to be seen in terms of efficiency and costs for a special
bonding task. Sometimes the use of a more expensive type of adhesive finally is the more cost
efficient way. This especially is an actual topic with the melamine fortification of UF-resins for the
production of low swelling core boards for laminate flooring or for boards with a certain resistance
against influence of humidity and water.
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Using various co-monomers gives the chance to achieve the necessary requirements and to
optimise the performance of the resin. In some cases however it is still not yet known if and how
the various monomers react with each other, e.g. in PMF- or PUF-resins.

Especially for hardening adhesives two parameters are important for the performance of the resin:
• The gelling and hardening reactivity
• The achievable degree of cross-linking.
Looking at formaldehyde based resins with their low content of formaldehyde (in order to decrease
the subsequent formaldehyde emission) both parameters became more and more critical due to the
decrease of the various molar ratios of formaldehyde to amide groups in the resins.

The molar mass distribution of the adhesive („optimal“ degree of condensation or „optimal“ molar
mass distribution) on the other side determines its behaviour during application, e.g. wetting vs.
penetration. Tailor made adhesives also try to optimise these two parameters.
Development of new adhesives also needs better and more efficient analysis methods, as it is
described in chapter 3.
A special task for further work is the prediction of properties of the bond line and the bonded
products based on the results of chemical and physical analysis of the adhesive. This includes the
investigation of the various influence parameters like the degree of condensation (viscosity) or the
chemical composition.

Safety and environmental issues become more and more important. This includes questions like
waste materials, effluents and emissions during the production and the application of the adhesives
as well as questions concerning worker’s safety in production and application, e.g. exothermic
behaviour of a PF-cook. Other questions concern content of monomers in the adhesive and residual
monomers after application.

Polyurethane adhesives, especially PMDI, are an alternative for formaldehyde-based resins for the
production of wood based panels. Main interest in development is the reduction of the ability to
stick to the press plates, the reduction of the amount of MDI and the enhancement of the reactivity
(low press temperatures, cold-setting applications).

The use and application of adhesives based on natural and renewable resources by industry and the
general public is often thought of as new approach that requires novel technologies and methods to
implement. Despite the increasing trend toward the use of synthetic adhesives, processes based on
the chemical modification of natural products offer opportunities for producing a new generation of
high performance, high quality products. However, more efficient and lower cost methods of
production will be precondition for wider use. Manufacturers need to have confidence that a
continual uninterrupted supply of raw material can be sustained throughout the life cycle of a
product. It is of equal importance that the feedstock should not be restricted by geographical and
climatic conditions or that yield does not dramatically vary when harvested in different locations
and at a particular time of the year.
Chapter 3: Analysis of resins and adhesives
Today a good knowledge of the chemical and structural composition of condensation resins is
available. The methods used include (i) chemical tests like purity of raw materials, content of free
formaldehyde during cooking and in the finished resins, content of formaldehyde in different form
in the resins (total formaldehyde, methylol groups), content of urea, melamine and free and total
alkaline as well as the calculation of various molar ratios, (ii) physical analysis like spectroscopic
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