Optimization of Urea
for the manufacture of wood
João Miguel Macias Ferra
Dissertation presented for the degree of Doctor
Engineering by
University of Porto Supervisors
Fernão Domingos de Montenegro Baptista Malheiro de Magalhães
Luísa Maria Hora de Carvalho
Mário Rui Pinto Ferreira Nunes da Costa
LEPAE ‐‐‐‐ Laboratory of Engineering Processes, Environment and Engineering
Chemical Engineering Department Faculty of Engineering
imization of Urea-Formaldehyde resins
the manufacture of wood-based panels
João Miguel Macias Ferra
Dissertation presented for the degree of Doctor of Philosophy in Chemical and Biological Engineering
University of Porto Supervisors
Fernão Domingos de Montenegro Baptista Malheiro de Magalhães
Luísa Maria Hora de Carvalho
Mário Rui Pinto Ferreira Nunes da Costa
Laboratory of Engineering Processes, Environment and Engineering
Chemical Engineering Department Faculty of Engineering – University of Porto
A
CKNOWLEDGEMENTS
First of all, I am grateful to Fundação para a Ciência e a Tecnologia for my Ph.D. grant (SFRH/BD/23978/2005).
My sincere appreciation and thanks to my supervisors, Professor Fernão Magalhães, Professor Luísa Carvalho and Professor Mário Rui Costa, for showing me how to be thorough and independent and for offering their knowledge, experience and support for this thesis.
I would also like to express my gratitude to Professor Adélio Mendes, for his encouragement, insightful comments and hard questions. My sincere thanks also Professor Jorge Martins, for the stimulating discussions about wood-based panels industry and everything.
I am indebted to EuroResinas – Indústrias Químicas S.A., in particular to Eng. Miguel Nogueira, Dr. João Paulo Liberal, Dr. Pedro Mena, and Margarida Nogueira for the information and knowledge about UF resins and for providing the resins, indispensable to my work.
A special thank Dr. Martin Ohlmeyer for having received me at Johann Heinrich von Thuenen Institute (vTI), Institute of Wood Technology and Wood Biology (HTB), Hamburg, Germany and for the supervision during my 3 months internship. To Daniel Karspinsky which never hesitate to help me.
I thank my colleagues in the laboratory João Ranita, João Pereira, Brígida and Filipe Silva for help during parts of my work.
I am grateful to the Chemical Engineering Department at FEUP, LEPAE-Laboratory for Process, Environmental and Energy Engineering, and ARCP-Association Competence Network in Polymers for the resources availability that leads this work to a good end.
Thanks very much to the colleagues from E319 (Joana, Sofia, Klara, Daniela, Filipa, Herney, Vânia, Pedro, Filipe, Marta) for the fantastic working environment and for friendship during the 4 years. Also, I would like to thank my friends from FEUP, Diogo, Águia, Miguel, Bin, Pópulo, Pedro Silva, and all people of the FEUPSal team for the nice moments in coffee breaks, lunches, football and parties.
My deepest gratitude goes to my lovely family: my precious parents Agostinho and Julia and my brothers Cláudia and Oscar for all their love and encouragement. Finally, always above all and above everything, thanks to my dear wife, Mayra for supporting, for believing… for all! You have made my life more beautiful.
A
BSTRACT
The present thesis is focused on the study of urea-formaldehyde (UF) resins, which are widely used in the wood-based panels industry, an important sector of the Portuguese industrial scene. In 2009, the annual production was approximately 1.3 million m3, about 70 % of which were exported. Fundamental studies on UF resins are therefore important in order to support innovation and international competitiveness. There is still a limited amount of information available in this area and new studies have to deal with chemical and physical characterization of the resins and optimization of synthesis processes. In addition, the relation between the operating conditions of the polymerization reactors and the performance of the resins in the bonding process is still unclear.
A new challenge has risen after the reclassification of formaldehyde as a carcinogen agent by the International Agency for Research on Cancer (IARC), in 2006. This forced the manufacturers of UF resins to reduce substantially the F/U molar ratio, originating a significantly decrease in UF resin performance and therefore evidencing the need for the study and optimization of the synthesis strategies. The first part of this work is dedicated to the morphological study of the UF resins. In a first approach, the resin was studied using different microscopic techniques, which allowed the visualization of colloidal structures in the UF resins and the formation of large aggregates during ageing. The disperse phase and the continuous phase were separated by centrifugation in order to be studied independently. Two mechanisms for colloidal stabilization were suggested, based on the results obtained with flocculation test and particle size distribution. After a fundamental study, methods for the characterization of UF resins by HPLC and GPC/SEC were developed. These methods were validated by the analysis of several tens of commercial resins from EuroResinas-Indústrias Químicas S.A. and other European producers. The effects of ageing on the molecular weight distribution, as well on the monomeric fraction of UF resins were evaluated using the developed
methods in GPC/SEC and HPLC, respectively.
An experimental design methodology was applied to optimize the internal bonding and formaldehyde emission of particleboards produced from several resin formulations. The number of urea additions, the time between urea additions, and the pH of the condensation step were the three factors studied. The results obtained showed that the sequential addition of the urea in the condensation step affects the properties of UF resins, as well as the pH of condensation, which is the factor that most affects the rate of the condensation reaction. Then the best results concerning the global performance of the resins were obtained conjugating the three factors studied.
Finally, new processes for UF resins synthesis were studied and implemented. The best resins were produced according to the so-called strongly acid process. The curing behaviour of laboratory and commercial resins was studied using two recent techniques: Integrated Pressing and Testing System (IPATES) and Automated Bonding Evaluation System (ABES). The information provided by each test system is discussed. The optimized resin obtained using the design of experiments methodology always presented the best performances.
S
UMÁRIO
A presente tese é focada no estudo das resinas ureia-formaldeído (UF), que são amplamente usadas na indústria dos painéis derivados de madeira, um importante sector do tecido industrial português. Em 2009, a produção anual foi de aproximadamente 1.3 milhões de m3, sendo que cerca de 70 % foi exportada. Estudos fundamentais sobre as resinas UF são portanto importantes, para suportar inovação e competitividade internacional. Há ainda uma limitada quantidade de informação disponível nesta área e novos estudos tem de estar relacionados com a caracterização química e física destas resinas, e optimização o processo de síntese. Além disso, a relação entre as condições operatórias dos reactores de polimerização e o desempenho das resinas no processo ainda não é clara.
Um novo desafio surgiu após a reclassificação do formaldeído como agente carcinogénio por parte da International Agency for Research on Cancer (IARC) em 2006. Isto forçou as empresas produtoras de resinas UF a reduzir substancialmente a razão molar F/U, originando uma significativa redução do desempenho das resinas UF, evidenciando a necessidade de estudar a optimização das estratégias de síntese.
A primeira parte deste trabalho é dedicada ao estudo morfológico das resinas UF. Numa primeira abordagem, as resinas foram estudadas através de técnicas microscópicas, que permitiu a visualização de estruturas coloidais nas resinas UF e a formação de grandes agregados durante o envelhecimento. A fase dispersa e a fase contínua foram separadas por centrifugação de forma a serem estudadas independentemente. Dois mecanismos para a estabilização coloidal foram sugeridos, fundamentados nos resultados obtidos com testes de floculação e distribuição de tamanho de partículas. Após um estudo mais fundamental, foram desenvolvidos métodos de caracterização de resinas UF por HPLC e GPC/SEC. Os métodos foram validados pela análise de várias dezenas de resinas industriais
provenientes da EuroResinas-Indústrias Químicas S.A. e outros produtores Europeus. O efeito do envelhecimento na distribuição das massas moleculares bem como na fracção monomérica das resinas UF foi avaliado com recurso aos métodos desenvolvidos em GPC/SEC e HPLC, respectivamente.
Aplicou-se uma metodologia de planeamento de experiências para optimizar a resistência interna e emissão de formaldeído dos painéis de aglomerado de partículas produzidos pelas várias formulações de resinas. O número de adições de ureia, o tempo entre adições de ureia e o pH da etapa condensação foram os três factores estudados. Os resultados obtidos mostraram que a adição faseada da ureia na etapa de condensação afecta muito as propriedades das resinas UF, assim como o pH de condensação, que é o factor que mais afecta a velocidade da reacção de condensação. Assim, os melhores resultados considerando o desempenho global das resinas UF foram obtidos conjugando os três factores estudados.
Finalmente, novos processos de síntese de resinas UF foram estudados e implementados. As melhores resinas foram produzidas de acordo com o denominado processo fortemente ácido. O comportamento de cura das resinas do laboratório e as resinas comerciais foi estudado com recurso a duas recentes técnicas: Integrated Pressing and Testing System (IPATES) e o Automated Bonding Evaluation System (ABES). A informação obtida por cada sistema teste é discutida. A resina optimizada obtida através da metodologia de planeamento de experiências apresenta sempre o melhor desempenho.
R
ÉSUMÉ
La présente thèse porte sur l´étude des résines urée-formaldéhyde qui sont très utilisées dans l’industrie des panneaux à base de bois, une filière très important de l’industrie portugaise. En 2009, la production annuel a été de approximativement 1.3 million de m3, et environ 70 % de la production a été exportée. Donc, des études fondamentales concernant les résines UF sont important pour soutenir l’innovation et la compétitivité internationale. L’information existante dans ce domaine est encore limitée et des nouvelles études devront porter sur la caractérisation chimique et physique des résines UF et optimisation du procédé de synthèse. En outre, la relation entre les conditions opératoires des réacteurs de polymérisation et les propriétés des résines lors des procédés de collage est encore peu claire.
Un nouveau défi a émergé après la reclassification du formaldéhyde comme substance cancérigène par l’IARC (International Agency for Research on Cancer), en 2006. Par conséquence, les producteurs des résines UF ont été forcé à réduire substantiellement le rapport molaire F/U, ce qui a provoqué une diminution significative de la performance de la résine et donc attestant la nécessité d’un étude et optimisation des stratégies de la synthèse.
La première partie de ce travail est dédiée à l´étude morphologique des résines UF. Dans une première approche, la résine a été étudiée par différentes techniques microscopiques, ce qui a permet de visualiser des structures colloïdales dans la résine UF et la formation de grands agrégats pendant le vieillissement. La phase disperse et la phase continue ont été séparées par centrifugation pour permettre que les deux phases d’une forme isolé. Deux mécanismes de stabilisation colloïdal ont été suggérés, basés sur des résultats des essais de floculation et mesure des distribution de poids moléculaires. Après l’étude fondamentale, des méthodes pour la caractérisation des résines UF par HPLC et GPC/SEC ont été développées.
Ces méthodes ont été validées para l’analyse de dizaine de résines de l’entreprise EuroResinas- Indústrias Químicas S.A. et d’autres producteurs européens. Les effets du vieillissement sur la distribution des poids moléculaires, ainsi que sur la fraction
monomérique des résines UF ont été évalués avec les méthodes développées en GPC/SEC et HPLC, respectivement.
Une méthodologie originale des plans d’expériences a été appliquée pour optimiser la cohésion interne et l’émission de formaldéhyde des panneaux de particules produits par différentes formulations. Les trois facteurs étudiés ont été: le nombre d’additions d’urée, le temps entre additions d’urée et le pH de l’étape de condensation. Les résultats obtenus ont montrés que l’addition séquentiel d’urée affecte les propriétés des résines UF, ainsi que le pH de condensation, qui est le facteur qui influence le plus la vitesse de la réaction de condensation. Alors, les meilleurs résultats concernant la performance globale de la résine ont été obtenus en combinant les trois facteurs étudiés.
Finalement, des nouveaux procédés pour la synthèse des résines UF ont été étudiés et mis en œuvre. Les meilleures résines ont été produites selon le procédé fortement acide. Le comportement lors du durcissement des résines produites au laboratoire et des résines commerciales a été étudié par deux techniques récentes : Integrated Pressing and Testing System (IPATES) et Automated Bonding Evaluation System (ABES). L’information fournie par chaque système de test est discutée. La résine optimisée obtenue moyennant l’utilisation d’une méthode de plans d’expériences a présenté toujours la meilleure performance.
T
ABLE OF
C
ONTENTS
CHAPTER 1 1 INTRODUCTION ... 3 1.1 Urea-Formaldehyde resins ... 3 1.1.1 Raw materials ... 7 1.1.2 Chemical reactions ... 13 1.1.3 Cure ... 15 1.1.4 Characterization ... 161.1.5 Storage stability - Ageing ... 18
1.1.6 Formaldehyde emissions ... 19
1.2 Wood-based panels industry ... 21
1.2.1 Manufacture of Particleboard ... 22
1.2.2 Market ... 23
1.2.3 Environmental Impact ... 25
1.3 Motivation and Outline ... 27
1.4 References ... 29
CHAPTER 2 2 THE COLLOIDAL NATURE OF UF RESINS AND ITS RELATION WITH ADHESIVE PERFORMANCE ... 35
2.1 Introduction ... 36
2.2 Materials and Methods ... 38
2.2.1 Materials ... 38
2.2.2 Methods ... 39
2.3 Results and discussion ... 42
2.3.1 Disperse phase morphology ... 42
2.3.2 Particle size distribution ... 49
2.3.3 Molecular weight distribution ... 54
2.3.4 Curing behaviour and bonding strength... 60
2.4 Conclusions ... 63
2.5 References ... 64
CHAPTER 3 3 EFFECT OF AGEING ON UF RESINS ... 69
3.1 Introduction ... 69
3.2 Materials and Methods ... 72
3.2.1 Resins preparation ... 72
3.2.2 GPC/SEC analysis ... 73
3.2.3 HPLC analysis ... 74
3.3 Results and discussion ... 75
3.3.1 Characterization of UF resins ... 75
3.3.2 Monitoring the ageing of UF resins ... 81
3.3.3 Determination of water tolerance ... 87
3.4 Conclusions ... 88
3.5 References ... 89
CHAPTER 4 4 OPTIMIZATION OF THE SYNTHESIS OF UF RESINS USING RESPONSE SURFACE METHODOLOGY ... 93
4.1 Introduction ... 94
4.2 Materials and Methods ... 98
4.2.1 Experimental design ... 98
4.2.2 Synthesis of UF resins ... 100
4.2.3 GPC/SEC analysis ... 101
4.2.4 Preparation of laboratory-made particleboards ... 102
4.2.5 Particleboard Testing ... 102
4.3 Results and Discussion ... 103
4.3.1 Characteristics and performance of the UF resins produced ... 103
4.3.2 Model fitting ... 108
4.3.3 Effect of all three factors on the measured responses... 110
4.3.4 Optimization of Operating Conditions ... 112
4.4 Conclusions ... 114
4.5 References ... 115 CHAPTER 5
5.2.2 GPC/SEC analysis ... 125
5.2.3 HPLC analysis ... 125
5.3 Results and discussion ... 126
5.3.1 Characteristics of the produced resins ... 126
5.3.2 Monitoring of UF synthesis ... 126
5.3.3 Comparison of the two resins ... 133
5.4 Conclusions ... 136
5.5 References ... 137
CHAPTER 6 6 EVALUATION OF UF ADHESIVES PERFORMANCE BY IPATES AND ABES MECHANICAL TESTS ... 141
6.1 Introduction ... 142
6.2 Materials and Methods ... 143
6.2.1 Raw materials ... 143
6.2.2 Methods ... 145
6.3 Results and discussion ... 148
6.3.1 Integrated Pressing and Testing System (IPATES) ... 148
6.3.2 Automated Bonding Evaluation System (ABES)... 152
6.4 Conclusions ... 158
6.5 References ... 159
CHAPTER 7 7 GENERAL CONCLUSIONS AND FUTURE WORK ... 163
7.1 General Conclusions ... 163
L
IST OF
F
IGURES
Figure 1.1. Most important events in UF resin history (source [14]). ... 6
Figure 1.2. Raw materials for the UF resin. ... 7
Figure 1.3. Scheme of UF resin reactor. ... 8
Figure 1.4. Model of chemical structure of urea. ... 8
Figure 1.5. Main countries producing urea in 2007 (adapted from [23]). ... 10
Figure 1.6. Model of chemical structure of formaldehyde. ... 10
Figure 1.7. Product Tree for Formaldehyde (source [25]). ... 11
Figure 1.8. Current formaldehyde indoor exposure limits for various countries (source [26]). ... 12
Figure 1.9. UF resin synthesis: (a) formation of monomethylolurea; and (b) condensation reactions of methylolureas to form methylene ether bridges and methylene bridges (adapted from [32])... 14
Figure 1.10. Example of structure of crosslinked UF resin. ... 15
Figure 1.11. Effect of storage temperature on storage time (viscosity) of UF resin(adapted from [12]). ... 18
Figure 1.12. History of wood-based panel industry (source [73]). ... 22
Figure 1.13. Particleboard process diagram (source [75]). ... 23
Figure 1.14. Evolution of the production of wood-based panels in world. ... 24
Figure 1.15. Evolution of the production of wood-based panels in Portugal. ... 25
Figure 1.16. Cycle of Wood based panels (source [78])... 27
Figure 1.17. A schematic diagram of linkage between various Chapters present in this thesis. ... 29
Figure 2.1. Optical microscopy image of resin B: a) diluted in water (1.6 wt % resin in water) and b) diluted in 10 wt % urea solution (1.3 wt % resin in urea solution). ... 43
Figure 2.5. SEM image of UF resin B sprayed and cured on a metal plate. ... 47 Figure 2.6. Elementary analysis for UF resin B cured on a metal plate. a) particle surface, and b) in continuous phase. ... 48 Figure 2.7. Particle size distribution for three UF resins stored at 5 °C. a) PSD in volume for 30 days, b) PSD in number for 30 days – note that the curves for samples B and C are nearly superimposed, c) PSD in volume for 170 days, and d) PSD in number for 170 days. ... 51 Figure 2.8. Particle size distribution in volume for UF resin A-2 and supernatant. a) resin stored for 20 days at 25 °C, and b) resin stored for 50 days at 25 °C. ... 53 Figure 2.9. Particle size distribution in volume for UF resin D and supernatant stored for 20 days. ... 53 Figure 2.10. Chromatogram for UF resin A-2 with 20 and 50 days, sediment and supernatant diluted 3 % in DMSO. a) normalized response of RI sensor for 20 days, b) response of RALLS sensor for 20 days, c) normalized response of RI sensor for 50 days, and d) response of RALLS sensor for 50 days. ... 55 Figure 2.11. Chromatogram for UF resin D with 20 days, sediment and supernatant diluted 3 % in DMSO. a) normalized response of RI sensor, and b) response of RALLS sensor. ... 56 Figure 2.12. DSC curves for UF resin A-2, supernatant and sediment. Resin had been stored for 50 days... 61 Figure 3.1. Chromatograms for UF-R5 and UF-R2 diluted 3 % in DMSO and stored for 5 days at 25 °C). a) normalized weight fraction (Wt Fr), and b) RALLS response. ... 77 Figure 3.2. Chromatogram obtained for resin UF-R5. ... 78 Figure 3.3. Peak areas normalized by total chromatogram area for UF-R5 and UF-R2 stored at 25 °C. ... 79 Figure 3.4. Chromatograms for five UF resins from different manufactures in Europe. a) normalized weight fraction, and b) RALLS response. ... 80 Figure 3.5. Ratios of peak areas / total area of urea (U), monomethylolurea (MMU) and dimethylolurea (DMU) for five UF resins from different producers. ... 81 Figure 3.6. Change of viscosity of UF-R5 and UF-R2 with storage time at 25 °C. ... 82 Figure 3.7. Chromatograms for UF-R2 diluted 3 % in DMSO, for different storage periods at 25 °C. a) normalized weight fraction, and b) RALLS response. ... 83 Figure 3.8. Chromatograms for UF-R5 diluted 3 % in DMSO, for different storage periods at 25 °C. a) normalized weight fraction, and b) RALLS response. ... 84 Figure 3.9. Evolution of the ratios of peak areas / total area of the urea (U), monomethylolurea (MMU), dimethylolurea (DMU) and three other oligomeric species for
UF-R2 stored for various periods at 25 °C. ... 86 Figure 3.10. Evolution of the ratios of peak areas / total area of the urea (U), monomethylolurea (MMU), dimethylolurea (DMU) and three other oligomeric species for UF-R5 stored for various periods at 25 °C. ... 86 Figure 3.11. Chromatograms for UF-R5 aged for 5 days, diluted in DMSO and very diluted (flocculated) in water. ... 88 Figure 4.1. Condensation reaction time versus pH of the condensation step for the resins produced. ... 103 Figure 4.2. Normalized response of RI sensor for UF resin run 5 diluted 3 % in DMSO. ... 105 Figure 4.3. Relation between insoluble aggregates and internal bond strength. The dashed lines represent the 90 % confidence intervals. ... 107 Figure 4.4. Relation between insoluble molecular aggregates and F content. The dashed lines represent the 90 % confidence intervals. ... 108 Figure 4.5. Experimental and calculated results of the responses considered. Y1 – internal bond strength and Y2 – formaldehyde emission. ... 109 Figure 4.6. Response surface for internal bond strength as a function of: a) time span between urea additions and number of urea additions (for different pH values of condensation step), b) pH of condensation step and time span between urea additions (for different numbers of urea additions) and c) pH of condensation step and number of urea additions (for different time spans between urea additions). ... 111 Figure 4.7. Response surface for formaldehyde emission as a function of: a) time span between urea additions and number of urea additions (for different pH values of condensation step), b) pH of condensation step and time span between urea additions (for different numbers of urea additions) and c) pH of condensation step and number of urea additions (for different time spans between urea additions). ... 112 Figure 5.1. Reaction temperature (—) and pH (---) histories for resin UF-Exp7. These simplified history curves are based on the experimentally measured values. The urea addition (Ui) and sample collection (Si) times are also indicated in the graph. ... 123 Figure 5.2. Reaction temperature (—) and pH (---) histories for resin UF-W6. These simplified history curves are based on the experimentally measured values. The urea addition (Ui) and sample collection (Si) times are also indicated in the graph. ... 125 Figure 5.3. Monitoring of UF-Exp 7 synthesis by GPC/SEC: a) samples collected during
Figure 5.5. Condensation of the methylolureas and urea to form methylene-ether bridges
and methylene bridges. ... 129
Figure 5.6. Monitoring of UF-W6 synthesis by GPC/SEC: a) 1ª condensation step and 1ª methylolation step, b) 2ª condensation step, and c) 2ª methylolation step and final resin. ... 131
Figure 5.7. Condensation reactions of urea and formaldehyde to form methylolureas that form methyleneureas and urons. ... 132
Figure 5.8. Chromatogram for four resins (UF-W6 and UF-Exp7) with 5 days (stored at 25 °C). ... 134
Figure 5.9. Ratios of peak areas / total area of the urea (U), monomethylolurea (MMU) and dimethylolurea (DMU) for UF-W6 and UF-Exp7 resins, with 5 days (stored at 25 °C). ... 136
Figure 6.1. Overview and schematic of the IPATES machine. ... 145
Figure 6.2. Particleboard mat during the pressing and subsequent testing. ... 146
Figure 6.3. Core layer temperature of the board at different pressing temperatures. ... 147
Figure 6.4. Schematic of the ABES test procedure. ... 148
Figure 6.5. Bond strength curves for particle mats pressed at 130 °C and 160 °C for UF-W6 and UF-Exp7 (8 % resin and 3 % hardener). a) 130 °C, and b) 160 °C. ... 150
Figure 6.6. Bond strength curves for particle mats pressed at 130 °C and 160 °C for UF-R8 and UF-R2 (8 % resin and 3 % hardener). a) 130 °C; b) 160 °C. ... 151
Figure 6.7. Bond strength curves for particle mats at 130 ºC with 8 % (•) and 10 % (○) of adhesive (3 % hardener). a) UF-W6 resin, b) UF-Exp7 resin, c) UF-R8 resin, and d) UF-R2 resin. ... 152
Figure 6.8. Shear strength evolution with time for UF-W6 resin at pressing temperature of the 80 °C (•) 90 °C (○), 100 °C (▼), 110 °C (∆) and 130 °C (■). ... 153
Figure 6.9. Shear strength evolution with time of four resins at pressing temperature 80 °C (•) 90 °C (○), 100 °C (▼), 110 °C (∆) and 130 °C (■). a) W6 resin; b) Exp7 resin; c) UF-R8 resin and d) UF-R2 resin. ... 154
Figure 6.10. ABES-derived Arrhenius plots for four adhesive systems. ... 155
Figure 6.11. Shear strength evolution with time UF-W6 resin with two hardener ratios, 1.5 % (•) and 3.0 % (○), at 110 °C... 156
Figure 6.12. Breaking load in shear mode for four resins by the ABES method at 100 ºC and 80 s (ten tests were performed for each resin to evaluate the error). ... 158
L
IST OF
T
ABLES
Table 1.1. Advantages and disadvantages of the main adhesives used in the production of
wood based panels (adapted from [2]) ... 4
Table 1.2. Limits of formaldehyde emissions for particleboard (adapted from [71]) ... 21
Table 2.1. Technical data for UF-resins analysed in this work ... 39
Table 2.2. Solids content of original resins, sediment and supernatant phases for UF resins A-2 and D ... 52
Table 2.3. Values of Mn, Mw, polydispersity, and parameters f1 and f2, obtained by SEC for resin, sediment and supernatant of the UF resin A-2 with 20 and 50 days and UF resin D with 20 days storage at 25 °C ... 57
Table 2.4. Cure temperatures and enthalpies for UF resin A-2, supernatant and sediment 61 Table 2.5. Values of tensile shear strength and % of cohesive failure within wood for UF resin A-2, supernatant and sediment ... 62
Table 3.1. Technical data of UF-resins ... 73
Table 3.2. Technical data on UF-resins used from different producers... 73
Table 3.3. Values of Mn, Mw, polydispersity (Mw/Mn), and parameters f1 and f2, obtained by SEC for UF-R5 and UF-R2 stored for 5 days at 25 °C ... 77
Table 3.4. Values of Mn, Mw, polydispersity (Mw/Mn), and parameters f1 and f2, obtained by GPC/SEC for UF-R2 and UF-R5 stored for different days at 25 °C ... 85
Table 4.1. Current classification of formaldehyde by some organizations ... 94
Table 4.2. Experimental levels of the three factors ... 98
Table 4.3. Central composite design matrix of experiments generated by the DoE tool ... 99
Table 4.4. Characteristics of UF resins produced ... 104
Table 4.5. Experimental results for the three measured responses ... 106
Table 5.2. Identification of different stages during the synthesis (UF-Exp 7) ... 127
Table 5.3. Values of Mn, Mw, polydispersity (Mw/Mn) obtained by GPC/SEC for samples collected during the synthesis of UF-Exp7 ... 129
Table 5.4. Identification of different stages during the synthesis ... 130
Table 5.5. Values of Mn, Mw, polydispersity (Mw/Mn) obtained by GPC/SEC for samples collected during the synthesis of UF-W6 ... 133
Table 5.6. Values of Mn, Mw and parameters f1 and f2, obtained by GPC/SEC for resins (UF-W6 and UF-Exp7) ... 135
Table 6.1. Technical data of UF-resins ... 144
Table 6.2. Parameters evaluated by IPATES ... 146
Table 6.3. Parameters evaluated on ABES ... 148
1 Introduction
1.1
Urea-Formaldehyde resins
Urea-formaldehyde (UF) polymers have been for decades the most widely used adhesives in the manufacture of wood-based panels, such as particleboard and medium density fiberboard (MDF) (both consuming 68 % of the world’s UF resins production) and plywood (consuming 23 %) [1]. Melamine-urea-formaldehyde (MUF), phenol-formaldehyde (PF) and polymeric 4, 4’-diphenylmethane diisocyanate (pMDI) polymers are also used in production of wood-based panels, but their use in particleboard and MDF is relatively small when compared with UF resins. pMDI is used mostly in the manufacture of moisture resistant panels. Table 1.1 summarizes the advantages and disadvantages of the four main wood adhesives concerning a few significant parameters [2].
The properties of wood-based panels depend on three factors: the wood species and origins, particularly the interface between the wood surface and the adhesive; the adhesive; and operating conditions and process of the production of boards [3].The adhesive is the most expensive component in the panel cost structure. According with SRI Consulting [1] the global production of UF resins in 2008 was approximately 14 million ton. Their consumption increased 2.8 % in 2008, and is expected to grow an average 3.2 % per year from 2008 to 2013, and just under 2 % per year from 2013 to 2018.
Table 1.1. Advantages and disadvantages of the main adhesives used in the production of wood based panels (adapted from [2])
Proprieties
Adhesive
UF MUF PF pMDI
Price Low Medium to
high Medium High
Cure temperature Low Medium High Low
Press time Short Medium Medium to
long Medium
Susceptibility against
wood species High Medium Low Low
Efficiency Low Medium to
high
Medium to
high High
Manipulation Easy Easy Easy Difficult
Resistant against
hydrolyzed No
Medium to
high High High
Use in wet conditions No Partly yes Yes Yes
Formaldehyde emission E1, Carb I possible E1, Carb II possible Very low emission No
The main reasons for the wide utilization of UF resin in wood based panels are their high reactivity, low cost and excellent adhesion to wood [4-5]. One the other hand, the most important drawbacks are the low moisture resistance and the formaldehyde emission during panel manufacture and service life [4-5]. Although the free formaldehyde content on these resins has been decreased during the last decades, the recent reclassification of formaldehyde by International Agency for Research on Cancer (IARC) as “carcinogenic to humans”, is forcing resin producers to develop systems that lead to a decrease in its emissions to levels as low as the present in natural wood [6-7]. This imposition has been a driving force for a considerable research effort, not only in the engineering of UF resins, but also in
producing standard particleboard and MDF [8].
Until now, the decrease on free formaldehyde emissions has been obtained by decreasing the molar ratio F/U and/or by the addition of formaldehyde scavengers (UF resin with an ultra-low molar ratio F/U well below 1.0, MUF scavenger resin, solid urea, urea solution, starch, lignin, tannin, rice husk, etc.). Both lead to a decrease on reactivity and degree of curing, harming the formation of adhesive bonds. Moreover, currently used hardeners are adapted to high F/U molar ratios and high levels of free formaldehyde in solution. Therefore, the decrease in F/U molar ratio can result in panels with low internal bond strength. The experience of wood-based panel producers is that resins with lower molar ratio F/U have lower flexibility in the control of the process variables. This is a very important factor, since producers are nowadays using mixtures made of recycled wood and wood from different origins.
Dynea AS Company, one of the most important wood adhesives producers in the world, claims that, currently, the key topics in the development and production of adhesives are: cost reduction with effective bonding solutions; broadening the operating window in terms of production processes; reduction of the formaldehyde emissions; improvement of adhesive behavior in humid conditions; and finding alternative raw materials, namely bio-based adhesives [3].
History
The first published studies about the reaction between urea and formaldehyde were the works by Tollens [9] in 1884. However, Goldschmidt authored the currently most cited work in the open literature, in 1896 [5, 10]. This researcher described the formation of precipitates from solution due to the reaction of urea and formaldehyde under acidic conditions. Later, Carlson [11] related the primary precipitate referred by Goldschmidt [10] with cyclic structures (now called urons) and empirically identified it as C5H10O3N4.
In 1931 IG-Farbenindustrie (now BASF)
production of UF resins for wood based industry
development of wood based panels industry, namely particleboard and MDF forced the growth of UF resins industries
Still later, formaldehyde was designated in 1978
giving rise to a major change in the synthesis process. The reduction of formaldehyde emissions in the wood based panel was achieved with a large decrease on molar ratio F/U. This change had a great impact on
process (higher press times and higher press temperatures) and properties (lower bonding strength and lower moisture resistance based panels. The decreasing of
formaldehyde emission, but increases
absorption (WA), as well as leads to a decrease (internal bond strength (IB) and modulus
led to an 80 % decrease in formaldehyde methods to measure the formalde
developed and the first rules and regulation were established.
Farbenindustrie (now BASF) Company in Germany started the industrial production of UF resins for wood based industry [5]. After Second World War, the development of wood based panels industry, namely particleboard and MDF,
growth of UF resins industries [5, 12].
formaldehyde was designated in 1978 as a possible carcinogenic agent, change in the synthesis process. The reduction of the wood based panel was achieved with a large io F/U. This change had a great impact on the manufacturing and higher press temperatures) and on the physical gth and lower moisture resistance) of the
wood-of F/U molar ratio leads to a decrease in aldehyde emission, but increases the thickness swelling (TS) and water as well as leads to a decrease in mechanical performance (internal bond strength (IB) and modulus of rupture (MOR)) [2, 13]. This strategy ormaldehyde emissions. In addition, new testing methods to measure the formaldehyde emissions from wood-panels were developed and the first rules and regulation were established.
In the 1980s, novel synthesis processes were developed by several com 15-17] and the number of patents on UF resins has grown rapidly. 1.1.1 Raw materials
As the name indicates, the two monomers used in manufacture of UF resins are urea and formaldehyde. The two compounds are derived from natural gas ( Figure 1.2)[18-19].
Figure 1.2. Raw materials for the UF resi
Typically the production is carried out with a batch reactor. These reactors have normally volumes about 20 to 40 m
and heating and cooling systems to control the temperature. The reactors have also a continuous addition of raw materials, namely urea, formaldehyde, acid, base, and others additives. Figure 1.3 presents
manufacture of UF resins.
In the 1980s, novel synthesis processes were developed by several companies [11, and the number of patents on UF resins has grown rapidly.
s the name indicates, the two monomers used in manufacture of UF resins are urea and formaldehyde. The two compounds are derived from natural gas (see
the UF resin.
Typically the production is carried out with a batch reactor. These reactors have normally volumes about 20 to 40 m3 and are equipped with a mechanical stirrer and heating and cooling systems to control the temperature. The reactors have ous addition of raw materials, namely urea, formaldehyde, acid, base, presents a scheme of a typical reactor for the
Figure 1.3. Scheme of UF resin reactor.
Urea
Urea is an organic compound with the chemical The synthesis of this organic compound by heat discovered by Friedrich Wöhler in
obtained from inorganic chemicals. soluble in water.
The main applications of urea are in agriculture (used in
source of nitrogen) and chemical industry (used in the production of amino resins) [19, 21].
Scheme of UF resin reactor.
Urea is an organic compound with the chemical formula (NH2)2CO (see Figure 1.4). The synthesis of this organic compound by heating ammonium cyanate was in 1828 [20-21]. It is the first organic compound obtained from inorganic chemicals. Urea is a solid, colorless, odorless, and highly
are in agriculture (used in fertilizers as a convenient source of nitrogen) and chemical industry (used in the production of amino resins)
Production and manufacturing process of urea
Urea was first produced industrially by the hydration of calcium cyanamide but the easy availability of ammonia led to the development of ammonia/carbon dioxide technology. This is a two step process where the ammonia and carbon dioxide react to form ammonium carbamate, which is then dehydrated to urea [19, 21-22].
4 2 2 3
CO
H
N
COONH
2NH
+
→
−
(1.1) O H CO ) (NH COONH N H2 − 4 → 2 2 + 2 (1.2)In the process, ammonia and carbon dioxide are fed to the synthesis reactor which operates around 180-210 °C and 150 bar. The reaction mixture containing ammonia, ammonium carbamate and urea is first stripped of the ammonia and the resultant solution passes through a number of decomposers operating at progressively reduced pressures. Here, the unconverted carbamate is decomposed back to ammonia and carbon dioxide and recycled to the reactor. The urea solution is concentrated by evaporation or crystallization, and the crystals can be melted to yield pure urea in the form of pills or granules. Pills are made by spraying molten urea from the top of a high tower through a counter current air stream. Granular urea is formed by spraying molten urea into a mixture of dried urea particles and fines in a rotating drum [19, 22].
Urea is produced worldwide at large scale. The world production in 2007 was approximately 144 million ton [23]. Figure 1.5 shows the production of the main countries producers of urea. China and Russia produce more than 50 % of urea.
Figure 1.5. Main countries producing urea in 2007 (adapted from
Formaldehyde
Formaldehyde is one of the most abundant organ
with the chemical formula HCHO (see
Hofmann, with Alexander Butlerov in 1867
isolated and purified until 1892. This was achieved by Friedrich Von Stradonitz, who also introduced the concept of chemical bonds. Formaldehyde is also called methanal and is formed by oxidizing methanol.
original state because it has a short half easily dissolves, hydrates and oligomerizes transported commercially. 0 10 20 30 40 50 60 China Russia 1 0 6x T o n
urea in 2007 (adapted from [23]).
Formaldehyde is one of the most abundant organic compounds in the Universe, HCHO (see Figure 1.6). It was discovered by Wilhelm Von with Alexander Butlerov in 1867 [18, 22, 24]. Despite this, it was not isolated and purified until 1892. This was achieved by Friedrich Von Stradonitz, who also introduced the concept of chemical bonds. Formaldehyde is also called ing methanol. Formaldehyde is rarely found in its original state because it has a short half-life in air. In water, it is very unstable and it , hydrates and oligomerizes in water, which is the form in which is
Formaldehyde is an important chemical for the global economy, widely used in t production of thermosetting resin
material in the synthesis of the
methylenebis (4-phenyl isocyanate) or MDI, and pentaerythritol), and preservation and disinfection [25-26]
million ton. Figure 1.7 shows the consumption of formaldehyde and the for several end uses. The substitution
would suffer large losses in performance
capital investments would be required to produce or utilize the substitutes. Data of Formaldehyde Council Inc. show the great impact of formaldehyde industry to the U.S. and Canadian economies [25]. In 2003 the value of sales of formaldehyde and derivative products achieved approximately $145 billion
workers is 4.2 million, which represents nearly 3.4 % of employment in private, nonfarm establishments in North America
Figure 1.7. Product Tree for Formaldehyde
Formaldehyde is an important chemical for the global economy, widely used in the
resins (UF, MUF, MF, PF), as an intermediate raw s of the chemicals (polyacetal resins, 1,4-butanediol, phenyl isocyanate) or MDI, and pentaerythritol), and for 26]. The annual world production is about 21 the consumption of formaldehyde and the percentage ubstitution of these products is very difficult; consumers performance using alternative materials, and new investments would be required to produce or utilize the substitutes. Data of Formaldehyde Council Inc. show the great impact of formaldehyde industry to the . In 2003 the value of sales of formaldehyde and derivative products achieved approximately $145 billion [25]. The number of workers is 4.2 million, which represents nearly 3.4 % of employment in private,
in North America [25].
It is highly toxic, and its exposure affects eye Organization (WHO) fixed 0.1 mg/m
concentration. However, some countries adopted national regulations with higher limits (see Figure 1.8). In 2006 the
carcinogen that causes nasopharyngeal cancer and probably leukemia
Figure 1.8. Current formaldehyde indoor exposure limits for various countries
Production and Manufacturing Process of the Formaldehyde
Industrially, formaldehyde is produced by the catalytic oxidation of methanol. The most commonly used catalysts are silver metal or a mixture of an iron oxide with molybdenum and vanadium [18, 24]
used in the world. The methanol and oxygen react at 250 formaldehyde according to the chemical
2H 2HCHO O
OH
2CH + → +
xposure affects eyes, nose and throat. The World Health WHO) fixed 0.1 mg/m3 as a limit for formaldehyde indoor-air . However, some countries adopted national regulations with higher In 2006 the IARC classified formaldehyde as a human
carcinogen that causes nasopharyngeal cancer and probably leukemia[27].
Current formaldehyde indoor exposure limits for various countries (source [26]).
Production and Manufacturing Process of the Formaldehyde
duced by the catalytic oxidation of methanol. The most commonly used catalysts are silver metal or a mixture of an iron oxide with [18, 24]. Formox process, using iron oxide is the more anol and oxygen react at 250 °C – 400 °C to produce formaldehyde according to the chemical equation [24]:
O
The silver-based catalyst is usually operated at a higher temperature, about 650 °C. On it, two chemical reactions simultaneously produce formaldehyde: the one shown above, and the dehydrogenation reaction [24]:
2
3OH HCHO H
CH → + (1.4)
Further oxidation of formaldehyde during its production usually yields formic acid, which is found in formaldehyde commercial solutions.
1.1.2 Chemical reactions
In conventional production, UF resin synthesis has been established as a two step process, involving methylolation and condensation reactions, respectively [4, 28-29]. The methylolation reaction is an addition reaction and it is performed in neutral or slightly alkaline medium. Urea reacts with formaldehyde to form methylolureas (monomethylolurea, dimethylolurea, trimethylolurea and tetramethylolurea) (see Figure 1.9). Tetramethylolurea could not be observed experimentally [30]. In the condensation reaction, the system is acidified to achieve the condensation of methylolureas by the reactions between methylol groups and their primary and secondary amides. The growth of polymer is obtained by the formation of methylene ether bridges (CH2-O-CH2) and/or methylene bridges (-CH2-). In Figure 1.9, a simplified scheme of the reactions that occurred in the UF resin synthesis is presented [31].
Figure 1.9. UF resin synthesis: (a) formation of monomethylolurea; and (b) condensation reactions of methylolureas to form methylene ether bridges and methylene bridges (adapted from [32]).
During manufacture, progress of synthesis reaction is followed by viscosity measurement; the reactions proceed until the desired viscosity is reached. At this point, the reactions are blocked by neutralization and cooling, resulting in a complex mixture of molecules with different sizes and different condensation degrees [4].
1.1.3 Cure
During UF resin synthesis, the polymer condensation is stopped by neutralization and cooling. In order to reactivate it and complete the crosslinking process, it is needed to add an acid catalyst and increase the temperature (up to 80
After curing, UF resins become an insoluble, thermoset, three (see Figure 1.10).
Figure 1.10. Example of structure of crosslinked UF resin.
In general, the acid conditions are adjusted by the addition of latent hardeners (ammonium chloride, ammonium sul
ammonium nitrate, the latent hardener
in the resin to generate nitric acid (Eq. 1.5), which
4NHO 6HCHO
NO
4NH4 3 + ↔
The production of low formaldehyde UF resins, with low levels of free formaldehyde originated a decrease
latent hardeners were originally selected and optimized
high levels of free formaldehyde. So, latent hardeners are insufficient to promote the acid medium favorable to the complete cure of UF resins
The curing of UF resins can also be
synthesis, the polymer condensation is stopped by neutralization and cooling. In order to reactivate it and complete the crosslinking process, it is needed to add an acid catalyst and increase the temperature (up to 80 °C – 120 ºC). an insoluble, thermoset, three-dimensional network
Example of structure of crosslinked UF resin.
In general, the acid conditions are adjusted by the addition of latent hardeners ammonium sulfate and ammonium nitrate). In the case of ammonium nitrate, the latent hardener reacts with the free formaldehyde present
acid (Eq. 1.5), which decreases the pH [4]:
O 6H N ) (CH 4NHO3 + 2 6 4 + 2 (1.5)
The production of low formaldehyde UF resins, with low levels of free formaldehyde originated a decrease in the performance of latent hardeners. The selected and optimized to be used with resins with maldehyde. So, latent hardeners are insufficient to promote
complete cure of UF resins [33].
acetic, oxalic, formic, hydrochloric, nitric, and phosphoric, and others) [12, 29, 33-34]. However, these hardeners originate corrosion problems in the equipments, wood degradation, and reduce considerably the pot life of the resin (stability time of the catalyzed resin) [35].
Several works [36-38] reported that the thermal curing reactivity of UF resins is significantly affected by the molar ratio F/U. The decrease of molar ratio F/U increases the gel time and cure temperature. Siimer et al. [36] reported also that UF resins with higher amount of reactive methylolureas groups present a higher reactivity.
1.1.4 Characterization
The main problem of the analysis of UF resin is related to the fact that these resins are a very complex mixture with: 1) reversible reactions, namely the methylolation reaction (release of formaldehyde), 2) structural rearrangements, 3) different types of linkages (methylene-methylene ether), and 4) different monomers (methylolation degree: monomethylolurea, dimethylolurea and trimethylolurea) [39-43].
To characterize the liquid UF resin, several techniques have been used, including 13C-NMR (Carbon Nuclear Magnetic Resonance) [44-45] and FTIR (Fourier Transform Infrared) [46-47] to investigate the structure of the resin, and GPC/SEC (Gel permeation Chromatography/Size Exclusion Chromatography) [48-49] to determine the molecular weight (MW) and molecular weight distribution (MWD). More recently, the capabilities of FT-NIR spectroscopy (Fourier Transform Near-Infrared) have been exploited. This technique has demonstrated to be useful for on-line monitoring the consumption of NH2 groups during the early stages of resin
solid-state C13-NMR [52-53] FTIR [46] allowed a better understanding of the acid cure at high temperature of UF resin. The chemical cure reaction can be monitored by DSC (Differential Scanning Calorimetry) [37, 54] allowed to estimate the degree of chemical cure as well as the heat of cure reaction. The healing mechanics can be monitored by TMA (Thermal Mechanical Analysis) [55-56], DMA (Dynamic Mechanical Analysis) [57] or ABES (Automatic bonding Evaluation System) [29, 58]. Raman spectroscopy was used by Hill et al. [59] to assess the structure of cured UF resin and by Carvalho et al. [60-61] to study the cure reaction. This technique allows the analysis of the liquid resin, cured resin or the cured resin in wood based panels. The Raman spectra of several model compounds have permitted to clarify the presence of certain characteristic groups and their quantitative determination. It was possible to observe the relative growth of the concentration of methylol and methylene groups. Raman spectroscopy was found to be very interesting for the study of the resin cure and permitted to obtain kinetic data as the basis for a simple empirical model, considering a homogeneous irreversible reaction of a single kind of methylol group and ureas with rate constants depending on their degree of substitution [61]. However, the existing fluorescence on the spectra of cured resin may affect the band relative intensities and therefore the quantitative results. An internal standard, if provided, could enable more precise results.
Another technique, very recent, which has not been used for the analysis of UF resin, is MALDI-TOF (Matrix Assisted Laser-desorption Ionization Time of Flight) mass spectroscopy, which can be used to investigate the distribution of molecular weights. In this technique the polymer is dispersed in a matrix - consisting of an UV absorber - and then bombarded by a laser. The absorbed energy is able to vaporize some of the molecules between two high voltage electrodes. The electric field between the electrodes accelerates the molecules, which will hit the detector with an acceleration inversely proportional to molecular weight. MALDI-TOF was used to analyze melamine-urea-formaldehyde (MUF) resins by Zanetti et al. [62].
1.1.5 Storage stability - Ageing
During storage, uncured urea/formaldehyde resins undergo reactions that result in structural changes. Methylene groups adjacent to secondary amino groups are formed by condensation. This reaction proceeds during storage between the free terminal hydroxymethyl and amino groups.
Physical processes also take place during ageing, with the formation of colloidal particles followed by clustering, especially in the UF, melamine-formaldehyde (MF) and MUF [62-64].
During the storage time, that is about 30 days the UF resins must be maintained constant the viscosity [12, 65]. Figure 1.11 shows the evolution of viscosity of UF resin during the storage time for two temperatures [12]. It is evident that for higher temperature the storage time is lower. The problem of storage stability is very important in Iberian Peninsula due to the high temperature prevailing in summer.
Figure 1.11. Effect of storage temperature on storage time (viscosity) of UF resin(adapted 0 500 1000 1500 2000 0 5 10 15 20 25 30 v is co si ty / c P s Time / days 24 ℃ 32 ℃
1.1.6 Formaldehyde emissions
The issue of formaldehyde emission from wood based panels is related to the use of UF resins as adhesives. The emission is originated from: unreacted free formaldehyde, formaldehyde in the form of methyIenegIycoI, retained in the panels moisture, and formaldehyde released from the slow hydrolysis of methylol groups and methylene ether linkages present in UF resins [29, 66]. The emission level depends of different factors, namely the resin, the species and origins of the wood, the pressing time, the press temperature and the moisture content of wood before and after pressing [66].
In 1979, the Chemical Industry Institute of Toxicology reported that formaldehyde caused nasal cancer in rats exposed at high levels during longs time. IARC in 2006 concluded from several case-studies that there is sufficient evidence for the carcinogenicity of formaldehyde in humans, namely nasal cancer and leukemia [27]. FormaCare, which represents key European producers of formaldehyde, aminoplast glues and polyols, disagrees with this conclusion and stated that the “weight of scientific evidence does not support such a determination” [67]. In 2005, it started an independent research in order to provide a solid database about the effect of formaldehyde in humans [6, 67]. In 2007, FormaCare organized in Barcelona, Spain the International Formaldehyde Science Conference in order to discuss the new reclassification of formaldehyde by IARC, as well as gather all newly available scientific research and study results about formaldehyde [6, 67]. The bottom line of this conference was that the common use of formaldehyde in consumer products and other applications does not pose a risk to human health [67].
The determination of formaldehyde emission can be done according to several methods. In Europe, four test methods have been approved as European standards: EN 120 (perforator method), EN 717-1 (chamber method), EN 717-2 (gas analysis method), and EN 717-3 (flask method) [6].
The perforator method is the most common and is used in particleboard and MDF industrial plants [6, 28-29, 66, 68-69]. In this process the formaldehyde is extracted from test pieces by means of boiling toluene and then is absorbed by water. The formaldehyde content of this aqueous solution is determined by photometrical detection or fluorescence spectroscopy and expressed in weight (mg) per 100 g of oven dried board. The perforator values apply to boards with moisture contents of 6.5 %. In the case of boards with different moisture content (in the range of 3 % ≤ H ≤ 10 %) the perforator value shall be multiplied by a factor F = − 0,133 H + 1,86. The value of formaldehyde content obtained can be used to estimate the actual formaldehyde emission using correlations available in open literature. The mostly used nowadays was developed by Risholm-Sundman et al. [70] to convert the EN 120 test values (mg HCHO / 100 g) to EN 717-1 values (mg HCHO / m3). The disadvantages of the perforator method are the use of toluene and the low efficiency/reproducibility for very low formaldehyde emissions. In the future, the perforator method should be replaced by the gas analysis method due to the shorter analysis times and simpler procedures. Since 2009, gas analysis method is an approved alternative, small-scale, quality control test accepted by California Air Resources Board (CARB).
In recent years, national regulations for formaldehyde were established and/or reformulated in some countries limiting the formaldehyde emission from wood-based panels. Table 1.2 presents the limits of formaldehyde emissions for particleboard applied in Japan, Europe and EUA according to different regulations and the equivalent values for different test methods [71-72]. The test methods used as reference in each national regulation are different and require the use of correlations for establishing relationships between the existents regulations. The
Table 1.2. Limits of formaldehyde emissions for particleboard (adapted from [71])
Method
Japan Europe IKEA EUA
F*** F**** E1 E0.5 CARB P1 CARB P2
EN 120 (mg / 100 g o.d board) ≤ 4.5 1 ≤ 2.71 ≤ 8.0 4.0 max 11.31 max 5.61 max EN 717-1 (mg / m3 air) ≤ 0.054 1 ≤ 0.0541 ≤ 0.124 ≤ 0.050 max 0.1761 max 0.0881 max ASTM E1333 (ppm) ≤ 0.055 1 ≤ 0.0351 ≤ 0.1271 0.051 1 max 0.180 max 0.090 max JIS A 1460 (mg / L) ≤ 0.5 ≤ 0.3 ≤ 0.9 1 0.41 max 1.31 max 0.61 max 1
The values are estimating using correlations
1.2 Wood-based panels industry
Wood based panels are manufactured from wood materials having various geometries (e.g., fibers, particles, strands, flakes, veneers, and lumber) combined with an adhesive and bonded in a press. The press applies heat (if needed) and pressure to activate (cross-link) the adhesive resin and bond the wood material into a solid panel, lumber, or beam having good mechanical and physical properties (strength, stiffness, form, dimensional stability, etc.
The most used wood-based panels are plywood, particleboard, MDF and oriented strand board (OSB). Plywood, made by gluing together several hardwood veneers or plies, was the first type of wood-based panel produced in the world. Only 60 years later particleboard panels were produced. Figure 1.12 shows the main milestones in the development of the wood-based panels industry.
Figure 1.12. History of wood-based panel industry
1.2.1 Manufacture of Particleboard Particleboard is manufactured from
recycled woodchips [74]. Typically, it is made in three laye
layers consist of finer particles and sawdust, while the core layer is made of coarser material.
The manufacture of particleboard has five main steps: (1) furnish preparation, (2) resin application, (3) mat formation, (4) hot pressing,
is prepared by refining the raw materials into small particles and drying them to achieve a desired moisture content, about 2 to 7 %
particleboard depends of the characteristics desired, but normally UF resin is used. The resin/wood ratio, based on resin
usually 6 to 9 % [4, 74]. Additives to enhance characteristics moisture resistance can be applied at this stage.
particles and the adhesive system, the
forming system and is then hot-pressed under pressures between two and thr MPa and temperatures between 140 °C and 220 °C
complete, the panel is transported
sawing into finished panel sizes and sanding.
based panel industry (source [73]).
Manufacture of Particleboard
om wood chips, sawdust, waste materials and . Typically, it is made in three layers. The two external layers consist of finer particles and sawdust, while the core layer is made of coarser
The manufacture of particleboard has five main steps: (1) furnish preparation, (2) resin application, (3) mat formation, (4) hot pressing, and (5) finishing. The furnish prepared by refining the raw materials into small particles and drying them to achieve a desired moisture content, about 2 to 7 % [74]. The type of resin used in particleboard depends of the characteristics desired, but normally UF resin is used. based on resin dry solids content, and particle dry weight , is Additives to enhance characteristics like fire retardancy or
moisture resistance can be applied at this stage. After mechanically mixing the
, the material goes through a continuous mat-pressed under pressures between two and three MPa and temperatures between 140 °C and 220 °C [4, 74]. After the press cycle is complete, the panel is transported to a board cooler, and then hot-stacked to wait
Figure 1.13. Particleboard process diagram (
1.2.2 Market
The increase of the world demand for wood
of the tree role in the global ecosystem are driving the use of recycled wood and wood from different sources/species in the formulation of wood composites The variability of available wood
compatibility/adequacy of the resin (bi
resin can be appropriate to glue a certain wood species, but not appropriated for others [3].
Food and Agriculture Organization of the United Nations (FAO) reported that in 2007, approximately 106 million m
China manufacture 20 %, 10 %, 9 % and 8 % respectively), 56 million m (China, Germany and EUA manufa
Particleboard process diagram (source [75]).
The increase of the world demand for wood-based composites and the awareness global ecosystem are driving the use of recycled wood and wood from different sources/species in the formulation of wood composites [60]. available wood creates difficulties concerning the compatibility/adequacy of the resin (binding agent) with the wood [3]. One UF a certain wood species, but not appropriated for
Agriculture Organization of the United Nations (FAO) reported that in approximately 106 million m3 of particleboard (EUA, Germany, Canada and hina manufacture 20 %, 10 %, 9 % and 8 % respectively), 56 million m3 of MDF (China, Germany and EUA manufacture 45 %, 8 % and 6 % respectively) and 76
million m3 of plywood (EUA, Germany, Canada and china manufacture 20 %, 10 %, 9 % and 8 % respectively) were manufactured in the world (see Figure 1.14) [76]).
Figure 1.14. Evolution of the production of wood-based panels in world.
European Panel Federation (EPF) reported that the wood-based panels industry was affected by the economic crisis in 2008 [77], in particular the production of particleboard and MDF, that decreased in 2008 by, 8.7 % and 8 % respectively [77]. Production of wood-based panels in Portugal has been approximately stable in the last decade. In 2007, the production volume was 850 000 m3 for particleboard, 330 000 m3 for MDF and 21 000 m3 for plywood (see Figure 1.15) [76]. The production of plywood in Portugal is small due to the lack of raw material (veneers must be produced from large diameter logs).
0 20 40 60 80 100 120 1961 1965 1970 1975 1980 1985 1990 1995 2000 2005 1 0 6 x m 3 Particleboard Plywood MDF
Figure 1.15. Evolution of the production of wood-based panels in Portugal.
1.2.3 Environmental Impact
The European woodworking industry stands for about 100 000 companies, two million employees and an annual turnover of 150 billion € [78]. Furthermore, forests and forest-based industries provide direct employment to three million people throughout the EU, especially in remote areas [78]. They represent 10 % of the total production value of the EU manufacturing industry [78]. According to European woodworking industry, these businesses invest continuously in sustainable forest management, deflorestation and reforestation activities to ensure reliable wood availability.
Wood plays a major role in fighting climate changes. This was the conclusion of a multi-disciplinary working group of the European Commission. The better use of wood sources stimulates forest expansion and reduces greenhouse gas emissions. The recycling process has a great paper in future of wood-based panels industry. In 2004 the proportion of recycled wood used in manufacturing of particleboard was
0 100 200 300 400 500 600 700 800 900 1000 1 0 6 x m 3 Particle Board Plywood MDF
23 % [79]. Roffael et al. in 2009 [80] studied the use of recycled fiberboards with raw material to making MDF. They concluded that the use of waste fiberboards up to 33 % does not have effect on the mechanical properties of the panels.
Wood is formed by photosynthesis of CO2 and water, thereby blocking carbon in a durable way. During growth a tree absorbs, through photosynthesis, approximately the equivalent of 1 ton of CO2 for every m3 growth, while producing the equivalent of 0.7 ton of oxygen [81].
Wood products require less energy for manufacturing (up to 6000 MJ/m³) than alternative raw materials, hence contributing even more to the reduction of fossil fuel consumption. By using the full potential of wood (sink and substitution effects) in buildings, Europe could reduce emissions of CO2 with 300 million ton or 15 to 20% [78].
In 2003 the European Woodworking Industries, Pulp and Paper Industries and the European Commission created a work group for discuss the use of the wood sources with energy and wood products [81]. The main recommendation was to consider “wood-based products as carbon sinks under the Kyoto Protocol, thereby acknowledging the contribution of wood-based products to climate change mitigation and the carbon cycle (see Figure 1.16), and recognize their superior eco-efficiency versus other materials, as well as their outstanding properties in recycling with minimal energy use” [81].
Figure 1.16. Cycle of Wood based panels
1.3 Motivation and Outline
This work started within the scope of the Optimização de Resinas de Ureia
Compósitos de Madeira de Diferentes Espécie Inovação. The goals of this project
characterization of UF resin and optimize the synthesis process, focusing in three key variables, temperature, pH, and sequential addition of raw materia
project has an academic partner LEPAE (Porto, Portugal) and an industrial partner Euroresinas - Indústrias Químicas, S.A. (Sines, Portugal).
The present thesis is divided into seven chapters, including this introduction. Chapter II, “A study on the colloidal nature of
of UF resins was exhaustively studied
implications of the colloidal phase. Industrial resins with different molar ration F/U were studied during long time spans in order to
Cycle of Wood based panels (source [78]).
the scope of the project entitled “UFMadeira - Optimização de Resinas de Ureia-Formaldeído para a Produção de Materiais Compósitos de Madeira de Diferentes Espécies/Origens” funded by Agência de The goals of this project were to develop competences in the characterization of UF resin and optimize the synthesis process, focusing in three variables, temperature, pH, and sequential addition of raw materials. This project has an academic partner LEPAE (Porto, Portugal) and an industrial partner
Indústrias Químicas, S.A. (Sines, Portugal).
The present thesis is divided into seven chapters, including this introduction. In the colloidal nature of UF resins”, the two- phase character of UF resins was exhaustively studied in order to understand the formation and colloidal phase. Industrial resins with different molar ration F/U spans in order to assess the changes of the colloidal