Thermochemical Properties of Methylol Phenol Monomers and Phenol Formaldehyde Resoles

Transcription

1 Thermochemical Properties of Methylol Phenol Monomers and Phenol Formaldehyde Resoles A Thesis Submitted for the Degree of Doctor of Philosophy By Livia Tonge, B.Sc., B.Eng. (Hons.) Faculty of Engineering and Industrial Sciences Swinburne University of Technology September 2007

2 Abstract The principal aim of the present research is to investigate the thermochemical characteristics of individual methylol phenol monomers, which are the first addition products in the making of phenol formaldehyde (PF) resoles, in the temperature range up to 250 C. The second aim of the research is to study the cure properties of PF resoles as a whole with a particular focus on the dependence of the reaction kinetics on the degree of the cure up to 250 C. Differential scanning calorimetry (DSC) and the model-free kinetic analysis approach were used to monitor the thermochemical properties of both the monomers and PF resoles as a function of concentration of sodium hydroxide, a common basic catalyst used in the making of the resoles. A. The cure properties of methylol phenol monomers A key mechanism that has been suggested to operate during the cure of the monomers in the presence of Na is the formation of the sodium ring complex that diminishes the capacity of the monomers to participate in condensation reactions, particularly those involving ortho-methylol groups. At a particular Na level, the monomer molecules may have a range of reactivity, depending on whether they are associated with Na +. Such variation in the reactivity and the different condensation possibilities of the monomers are critical factors governing the cure behaviour. Another important mechanism that has been suggested to operate during the cure is the limitation on molecular diffusion that has the effect of slowing down the condensation reactions of the monomers. The effect of the diffusion limitation mechanism is more pronounced with increases in the amount of the methylol groups in the monomers and in the levels of Na. The advancement in the extent of cross-linking is another factor that exacerbates the significance of this mechanism as the cure proceeds. ii

3 Differences in the effects of these mechanisms between different samples are manifested in differences in a number of parameters including the shape of the DSC curves, the dependence of apparent activation energy E a on the degree of conversion and the heat of reactions ΔH T. These differences, together with the established chemistry of condensation reactions, are used to elucidate possible pathways that condensation reactions may proceed. In particular, the partial contributions of reactions to form para-para and ortho-para linkages, as well as ortho-ortho linkages in rare occasions, at different stages of the cure have been proposed for each monomer at different Na levels. B. The cure properties of PF resoles The outcomes of both studies of the monomers and the resoles are complementary to each other and provide a consistent overall picture of relevant mechanisms operating during the cure process. In particular, the sodium ring complex mechanism that has the retardation effect on the cure kinetics of the resoles is demonstrated independently by both gel time measurements and DSC data. It is suggested that the operation of this mechanism is not confined to 2-mono-methylol phenol, but also applies to other methylol phenols present in the resoles. On the basis of the data on the dependence of E a on the extent of conversion, it is suggested that the cure of the resoles proceed through two stages. The first stage is characterized by an ascending trend of E a up to conversion of , followed by the second stage which exhibits a descending trend of E a to the end of the cure process. It is proposed that the partial contribution of reactions to form the parapara linkages are dominant at low conversions and that contribution of the orthopara linkage reactions become more significant as the cure proceeds. The descending trend of E a is attributed to the increasing importance of the diffusion limitation mechanism in the second stage of the cure. The effect of this mechanism is more extensive for the resoles having higher Na / P ratio. This is attributed to higher degree of methylol substitution and higher amount of Na present in these resoles, both of which are shown in the study of monomers to have the effect of exacerbating the severity of the diffusion limitation mechanism. iii

4 The findings in the present study have practical implications in the development of PF resole adhesive systems capable of curing faster at lower temperatures. Clearly, for PF resole formulations with a particular F / P molar ratio, there is an optimal level of Na / P molar ratio where the cross-linking reactions are encouraged and the diffusion mechanism is minimised. The present results indicate that for a system with a F / P molar ratio of 2, which is commonly used in the industry, a Na / P ratio of 3 is sufficient to produce resoles with fully cross-linked networks. Higher Na / P ratios would slow down the cure reactions due to increasing importance of both the sodium ring complex and the diffusion limitation mechanisms. It is suggested that future work should involve the use of complementary techniques such as NMR and FTIR to investigate the chemical structure of the products at different stages of the cure of different monomers and PF resoles. This is necessary to confirm the possible pathways for condensation reactions proposed in the present study. As well, the issue of the effects of F / P molar ratio on the cure properties of PF resoles should be revisited using the model-free DSC method, given the effectiveness of this method in revealing possible complex sequences of the cure reactions. These additional data would add to the knowledge obtained in the present study and aid in the development of PF resole systems capable of bonding under a wide range of gluing conditions and curing faster at lower temperatures. iv

5 Acknowledgements I wish to acknowledge and thank the Forest and Wood Products Research & Development Corporation for their financial sponsorship of this PhD project. I would like to thank my supervisors Mr Aaron Blicblau, from Swinburne University of Technology, Dr Jonathan Hodgkin from CSIRO Molecular Science, and Dr Yoshi Yazaki, from CSIRO Forestry and Forest Products. I am particularly indebted to Aaron for the enormous help and scientific guidance he extended to me during the course of this project, especially his patience and willingness to assist when problems arose. I am grateful to CSIRO staff Ms Mary Reilly, Mr Peter Collins, Mrs Touba Nikpour, Dr Russell Varley for their assistance throughout this project. In particular, I would like to highlight Mary for her dedication and very special support. I thank Dr Jim Gonis from Perkin Elmer for his considerable help and advice regarding the commissioning and operation of the DSC. My deep gratitude also goes to Gerry Scheltinga for your friendship, practical assistance, encouragement, and steadfast interest in my progress. To my family, Anyu, Johnnybacsi, and to Duy, I extend my eternal gratitude for your enduring love, patience and encouragement over the years. Without Duy s unwavering caring guidance and support, this thesis would not have eventuated. This thesis is in loving memory of my dad, Eric. v

6 Declarations The work described in this thesis has never previously been submitted for a degree or diploma in any University and to the best of my knowledge and belief contains no material previously published or written by any other person except where due reference is made in the thesis itself. Parts of the work described here have previously been reported in the following publications: Effects of Initial Phenol-Formaldehyde (PF) Reaction Products on the Curing Properties of PF Resin L. Y. Tonge, J. H. Hodgkin, A. S. Blicblau and P. J. Collins in Journal of Thermal Analysis and Calorimetry, 64 (2), (2001). Thermal Behaviour of Phenol-Formaldehyde (PF) Compounds L. Y. Tonge, Y. Yazaki and A. S. Blicblau in Journal of Thermal Analysis and Calorimetry, 56 (3), (1999). Cure Kinetics of Phenol-Formaldehyde (PF) Resins L. Y. Tonge, Y. Yazaki, A. S. Blicblau and J. H. Hodgkin in Proceedings of the 8 th Asian Chemical Congress, November vi

7 Table of Contents Title page Abstract Acknowledgement Declaration Table of Contents List of Tables List of Figures i ii v vi vii xii xiv Chapter 1 Introduction Background General The production of PF resoles The Issues The Objectives Structure of the Thesis References 6 Chapter 2 Literature Review of Thermochemical Behaviour of PF Resole and Its Monomers PF Resoles Background History Application of PF resoles in the wood industry PF Resole Chemistry Formaldehyde addition to phenol to form monomers General Reactivity of methylol phenols with formaldehyde Condensation reactions to form resole Condensation reactions 14 vii

8 Effects of alkalinity on the condensation reactions Cure reactions of resole General Reactions during the cure of resole Effects of Formulation Parameters on Properties of PF Resoles The Use of DSC to Study the Cure Behaviour of PF Resoles Concluding Remarks References 31 Chapter 3 Methodology and Experimental Details Methodology System parameters Methyl phenol monomers Reaction conditions Additional experimental parameters Thermal analysis by DSC General Principle of DSC Analysis of DSC experimental data Effective activation energy E α obtained from the model-free method Experimental Details Materials Synthesis of 2,4-DMP Synthesis of 2,6-DMP Synthesis of TMP Characterisation of 2,4-DMP, 2,6-DMP and TMP DSC runs References 64 viii

9 Chapter 4 Cure Properties of Mono-Methylol Phenols Introduction Effects of Scan Rate on DSC Thermograms Peak temperature T p Fractional conversion α p at T p Heat of reactions ΔH T Effects of Na on DSC Thermograms Peak temperature T p Fractional conversion α p at T p Enthalpy of reactions ΔH T Effects of Na on the Evolution of Activation Energy E a MMP MMP Summary References 86 Chapter 5 Cure Properties of Di-Methylol Phenols Introduction Self-Condensation Reactions of DMP DSC Thermograms ,4-DMP and 2,6-DMP at molar ratios equal or less than ,4-DMP at molar ratios higher than ,6-DMP at molar ratios higher than Enthalpy of Reaction ΔH T Effects of Na on the Evolution of Activation Energy E a ,4-DMP ,6-DMP Summary References 108 ix

10 Chapter 6 Cure Properties of Tri-Methylol Phenols Introduction Self-Condensation Reactions of TMP DSC Thermograms Enthalpy of Reactions ΔH T Effects of Na on the Evolution of Activation Energy E a Summary References 125 Chapter 7 Comparison of Effects of Na on the Cure Properties of Mono-, Di- and Tri-Methylol Phenols Introduction MMP MMP MMP DMP ,4-DMP ,6-DMP TMP Summary References 138 Chapter 8 Cure Properties of PF Resoles Introduction Experimental Resole synthesis GPC Gel time 142 x

11 8.2.4 DSC experiments Results and Discussion GPC Gel time DSC curves Enthalpy of reactions ΔH T Effects of Na / P molar ratio on the evolution of activation energy E a Summary References 158 Chapter 9 Conclusions and Future Work 162 xi

12 List of Tables Table Page Table 2.1: Reaction products from the self-condensation reactions of monomers as observed by Yeddanapalli and Francis 17 Table 2.2: Reaction products from the self-condensation reactions of monomers as observed by Grenier-Loustalot et al 19 Table 3.1: Reaction models used to describe thermal decomposition in solids 47 Table 3.2: 1 H-NMR chemical shifts 58 Table 3.3: 13 C-NMR chemical shifts 59 Table 4.1: Effects of scan rate on: the peak temperature of cure (T p ); total enthalpy of reactions (ΔH T ); and the extent of the cure reactions at the peak of the exotherm (α p ) for Na : 2- MMP molar ratios of 0.0 and Table 4.2: Effects of scan rate on: the peak temperature of cure (T p ); total enthalpy of reactions (ΔH T ); and the extent of the cure reactions at the peak of the exotherm (α p ) for Na : 4- MMP molar ratios of 0.0 and Table 4.3: The DSC Pyris computer software generated values of T α at four scan rates (for Na : 2-MMP molar ratio 0.45) and the corresponding values for the dependent and independent variables for equation Table 5.1: The DSC Pyris computer software generated values of T α at four scan rates (for Na : 2,4-DMP molar ratio of 0.45) and the corresponding values for the dependent and independent variables for equation xii

13 Table 6.1: The DSC Pyris computer software generated values of T α at four scan rates (for Na : 2,4,6-TMP molar ratio of 0.45) and the corresponding values for the dependent and independent variables for equation Table 8.1: The DSC Pyris computer software generated values of T α at four scan rates (for Na / P molar ratio of 0.30) and the corresponding values for the dependent and independent variables for equation xiii

14 List of Figures Figure Page Figure 2.1: Reaction paths for the addition of formaldehyde to phenol 13 Figure 2.2: Formation of dimethylene ether and methylene bridges 15 Figure 2.3: Condensation reactions of TMP from ph 3 to ph 5 21 Figure 2.4: Condensation reactions of TMP from ph 5 to ph Figure 2.5: Condensation reactions of TMP above ph Figure 2.6: Three- dimensional cross-linked state 23 Figure 3.1: The five initial intermediate monomers 39 Figure 3.2: Power compensated DSC 42 Figure 3.3: DSC dynamic scan peak 43 Figure 3.4: A dynamic DSC thermogram in the scanning mode depicting an exothermic reaction 45 Figure 3.5: Reaction steps for the synthesis of 2,4-DMP 52 Figure 3.6: Schematic for the synthesis of compound II 52 Figure 3.7: Schematic for the synthesis of 2,4-DMP 53 Figure 3.8: Reaction steps for the synthesis of 2,6-DMP 55 Figure 3.9: Figure 3.10: Figure 3.11: Figure 3.12: Figure 3.13: Figure 3.14: 1 H-NMR spectra of 2,4-DMP 60 1 H-NMR spectra of 2,6-DMP 60 1 H-NMR spectra of 2,4,6-TMP C-NMR spectra of 2,4-DMP C-NMR spectra of 2,6-DMP C-NMR spectra of 2,4,6-TMP 62 xiv

15 Figure 4.1: Figure 4.2: Figure 4.3: Figure 4.4: Figure 4.5: Dynamic traces for 2-MMP at varying scan rates in the absence of Na 68 Dynamic traces for 4-MMP at varying scan rates in the absence of Na 69 Dynamic traces of 2-MMP in the presence of varying Na : 2-MMP molar ratios at 10 C min -1 scanning rate 72 Dynamic traces of 4-MMP in the presence of varying Na : 4-MMP molar ratios at 10 C min -1 scanning rate 73 Fractional conversion α p as a function of Na : MMP molar ratio for 2-MMP and 4-MMP 74 Figure 4.6: ΔH T as a function of Na : MMP molar ratio for 2- MMP and 4-MMP 75 Figure 4.7: Graph of ln(φ/t 2 α ) vs. 1/T α between α = 0.05 and α = 0.95 and the corresponding square of the correlation coefficient (r 2 ) values for 2-MMP sample with Na : 2- MMP molar ratio of Figure 4.8: Effects of Na on the evolution of apparent activation energy E a for 2-MMP as a function of the degree of conversion 79 Figure 4.9: Condensation reactions of 2-MMP 80 Figure 4.10: The sodium ring complex 81 Figure 4.11: Effects of Na on the evolution of apparent activation energy E a for 4-MMP as a function of the degree of conversion 82 Figure 4.12: Self-condensation of 4-MMP 83 Figure 4.13: Addition reaction of O to 4-MMP 83 Figure 5.1: Condensation reactions of 2,4-DMP 88 Figure 5.2: Minor condensation reaction of 2,4-DMP 89 Figure 5.3: Condensation reaction of 2,6-DMP 89 xv

16 Figure 5.4: Para and ortho quinoid structures of 2,6-DMP and 2,4- DMP 90 Figure 5.5: Dimethylene ether linkage formation 90 Figure 5.6: Figure 5.7: DSC thermograms for the self-condensation reactions of 2,4-DMP in the presence of varying Na : 2,4-DMP molar concentrations obtained at 10 C min -1 scan rate 91 DSC thermograms for the self-condensation reactions of 2,6-DMP in the presence of varying Na : 2,6-DMP molar concentrations obtained at 10 C min -1 scan rate 92 Figure 5.8: ΔH T as a function of Na : DMP molar ratio for 2,4- DMP and 2,6-DMP 97 Figure 5.9: Graph of ln(φ/t 2 α ) vs. 1/T α between α = 0.05 and α = 0.95 and the corresponding square of the correlation coefficient (r 2 ) values for 2-MMP sample with Na : 2,4-DMP molar ratio of Figure 5.10: Figure 5.11: Effects of Na on the evolution of apparent activation energy E a for 2,4-DMP as a function of the degree of conversion 101 Effects of Na on the evolution of apparent activation energy E a for 2,6-DMP as a function of the degree of conversion 104 Figure 6.1: Condensation reactions of TMP 111 Figure 6.2: Figure 6.3: Chemical structure of trimer following condensation reactions of TMP 111 DSC thermograms for the self-condensation reactions of TMP in the presence of varying Na : TMP molar concentrations obtained at 10 C min -1 scan rate 113 Figure 6.4: ΔH T as a function of Na : TMP molar ratio for TMP 115 Figure 6.5: Graph of ln(φ/t 2 α ) vs. 1/T α between α = 0.05 and α = 0.93 and the corresponding square of the correlation coefficient (r 2 ) values for 2,4,6-TMP sample with Na : 2,4,6-TMP molar ratio of xvi

17 Figure 6.6: Figure 6.7: Figure 8.1: Figure 8.2: Figure 8.3: Figure 8.4: Effects of Na on the evolution of apparent activation energy E a for TMP as a function of the degree of conversion 119 Fractional conversion as a function of temperature for TMP samples with various Na molar ratios 122 The weight-average molecular weight (M w ) and the polydispersity (M w /M n ) of PF resoles as functions of Na / P molar ratio 143 The gel time of PF resoles as a function of Na / P molar ratio 144 DSC thermograms of the PF resoles having different Na / P molar ratios obtained at 10 C min -1 scan rate 146 Fractional conversion of the cure reactions of the resoles as a function of temperature 147 Figure 8.5: ΔH T as a function of Na / P molar ratio for PF resoles 149 Figure 8.6: Graphs of ln(φ/t 2 α ) vs. 1/T α between α = 0.05 and α = 0.95 and the corresponding square of the correlation coefficient (r 2 ) values for a resole sample having Na / P molar ratio Figure 8.7: Effects of Na / P on the evolution of apparent activation energy E a for the resoles as a function of the degree of conversion 155 xvii

18 Chapter 1 Introduction 1.1 Background General The production of reconstituted wood products has become increasingly important as the demand for wood and wood-based products continues to increase and the availability of high quality large diameter logs continue to lessen due to logging restrictions and environmental concerns. Reconstituted wood products consist of products where wood chippings, shavings or course saw dust are bonded together by adhesives to form a larger piece of solid wood product such as wood flake boards, particle boards, wood fibre boards and plywood. The viability of the reconstituted wood product industries greatly depends on the understanding of and the use of suitable wood adhesives. In fact, wood bonding is one of the largest-volume uses of adhesives, particularly in the softwood plywood industry. Some of the commonly used wood adhesives in Australia are, phenol formaldehyde (PF), resorcinol formaldehyde (RF), melamine urea formaldehyde (MUF), and urea formaldehyde (UF). Of these products, PF resoles are preferred especially for external and structural applications because they are structurally the most durable and can provide high quality wood bonding suitable for all climatic conditions. They are also environmentally more acceptable due to negligible formaldehyde emission. Apart from timber applications, PF resoles have also been used extensively because of their high temperature resistance, high char yield and moderate flame resistance in many areas, especially in coating applications, carbonless copy paper, air and oil filters and in other composites. 1

19 Despite these advantages, conventional PF resoles have relatively slow cure rates, require high cure temperature and are less tolerant of variations in anatomical features and wood substrate properties such as moisture content and density, which limit their allowable gluing conditions. Extensive research and development over the last few decades have gone into making better performance PF resoles. One of the most important research fields is the investigation of the mechanism and kinetics of the cure behaviour because PF has a very complicated cure process that involves many reactions that occur simultaneously, each of which is profoundly influenced by reaction conditions. Detailed information on the applications, advantages and general issues regarding the limitations of PF resoles can be found in a number of references [1-10] The production of PF resoles Detailed information on the production of PF resoles can be found in references [2-6]. Generally, PF resoles are produced by base-catalysed reaction between phenol and formaldehyde and step-growth polymerisation. The resoles produced consist of low to medium molecular weight reactive intermediates which are stable at room temperature, but are thermo-sensitive and can readily be transformed into three dimensional, cross-linked, insoluble, and infusible polymers by the application of heat during the curing process. The first isolable products of the reaction between phenol and formaldehyde are methylol phenols. The position of the methylol groups on the phenol ring and the ratio of various methylol derivatives formed largely determine the rate of polymerisation, as well as the structure and properties of the subsequent higher molecular weight products. It is generally agreed that the first phase, methylolation, involves the addition of methylol groups exclusively at the active ortho and para positions of the phenol ring to form two mono-methylol phenols, which further react with formaldehyde to form two di-methylol phenols, followed by one tri-methylol phenol. Molar ratio of formaldehyde to phenol, type and concentration of catalyst, temperature and ph are important factors that influence the nature and composition of the methylol phenols formed during this phase [11-14]. As the process advances to the second phase with the application of heat, condensation reactions of the methylol phenols occurr 2

20 to form methylene and/or ether bridges. The condensation reactions of individual methylol phenols vary considerably, leading to a large number of reaction products with varying reactivity and thermal properties. As the process proceeds further to higher temperatures during the cure phase, more complex condensations and rearrangements of the pre-polymer intermediates occur, leading to highly condensed infusible network structures. The formation and nature of this network structure determine the properties of the fully cured product. 1.2 The Issues Various studies have been carried out to investigate the thermochemical characteristics of the entire PF reaction cycle that involves the addition of formaldehyde to phenol, the condensation reactions of methylol phenol monomers and the subsequent curing reactions. However, the kinetics and mechanisms governing the entire PF reaction cycle remain relatively unclear, not only due to the complexity of the system, which involves many consecutive or integrated processes, but also the profound effect of temperature, ph conditions and the molar ratio between phenol and formaldehyde on the reaction system [see, for example, 15-20]. Extensive efforts have gone into elucidating the reaction pathways by simplifying the system and starting with the first addition products. For the purpose of simplification, the use of individual methylol phenol monomers, rather than the complex PF resoles as a whole, has been advocated as a legitimate approach to the mechanistic study and can be very useful in providing empirical parameters for modelling and controlling the PF reaction cycle. However, these efforts generally concerned themselves with the mechanisms and kinetics of the reactions occurring during individual stages, rather than with the entire PF process. Hence, there is very limited published information regarding the thermochemical properties of methylol phenols for the entire PF cycle [see, for example, 21-31]. Apart from the approach of using individual methylol phenol monomers, many research efforts have been dedicated to the investigation of PF resoles as a whole. Whilst differential scanning calorimetry (DSC) is often used to study the cure 3

21 properties of the resoles, these studies were often limited to the interpretation of the DSC curves, rather than focusing on kinetic analysis to obtain relevant kinetic information [see, for example, 32]. Where kinetic analysis was carried out, it was often mistakenly assumed that the activation energy of the thermal reaction was constant and did not change with the extent of the cure. A number of studies have addressed this issue and demonstrated the complex dependence of the reaction kinetics on the degree of the cure. Despite these encouraging efforts, the use of DSC to obtain insights into mechanisms of the cure of PF resoles is still limited [see, for example, 17, 18, 33-38]. 1.3 The Objectives The principal aim of the present research is to investigate the thermochemical characteristics of the individual monomers in the temperature range up to 250 C. As opposed to the common approach of focusing on individual curing stages, this temperature range captures the kinetics throughout the entire PF cure cycle which is identified to be the least well understood. The experiments incorporate the initial lower temperature cross-linking reactions of the monomers to form the pre-polymer compounds, through to the fully cure reactions that lead to solid network structures occurring at higher temperatures. The second aim of the research is to study the cure properties of PF resoles as a whole with a particular focus on the dependence of the reaction kinetics on the degree of the cure up to 250 C. This focus aims to address the problems created by the common mistaken assumption in the published literature, that the activation energy of the cure reactions did not change with the extent of the cure. It also recognises the importance of a changing reaction medium as the cure proceeds that may induce significant variations in the reaction kinetics of the resoles. The thermochemical properties of both methylol phenol monomers and PF resoles are monitored as a function of concentration of sodium hydroxide, a common basic catalyst used in the making of the resoles. DSC is employed as the major analytical tool to obtain relevant kinetic information using isoconversional analysis. The use of the isoconversional method allows the activation energy to be determined as a 4

22 function of the extent of the cure and/or temperature without making any assumptions about the reaction model, thus eliminating the uncertainties involved in the traditional model-fitting approach. These kinetic data, together with relevant established chemical information, form the basis upon which the reaction pathways throughout the entire cure cycle will be elucidated. The outcomes of the research serve as a contribution to efforts aiming to improve the understanding of the cure mechanism of PF resoles, and from here, to aid in the development of PF resole adhesive systems capable of bonding under a wide range of gluing conditions and curing faster at lower temperatures. 1.4 Structure of the Thesis The body of the thesis is presented in 9 chapters. Following the current chapter which introduces the background to the research, chapter 2 is a literature review of the chemistry and thermochemical behaviour of PF resoles and their monomers. The effects of formulation parameters on the properties of the resoles, as well as the use of DSC to study their cure behaviour, will also be briefly reviewed in chapter 2. Chapter 3 presents the methodology and experimental details for the study of the monomers. The experimental results and discussion for mono-methylol phenols, dimethylol phenols and tri-methylol phenol are presented separately in chapters 4, 5 and 6, respectively. Chapter 7 provides a summary of the findings and compares the thermochemical properties of individual methylol phenols in an effort to provide a consistent overall picture of relevant mechanisms operating during the cure process. Chapter 8 focuses on the study of PF resoles as a whole and the effects of sodium hydroxide concentration on the properties and cure behaviour of the resoles. The outcomes of the monomers study are used in the interpretation of the results. Chapter 9 concludes the thesis and proposes directions for future research. 5

23 1.5 References 1. T. Sellers Jr., Wood Adhesive Innovations and Applications in North America, Forest Prod. J. 51, (2001). 2. A. Pizzi, Wood Adhesives, Marcel Dekker, New York, A. Knop, and L.A. Pilato, Phenolic Resins Chemistry, Applications and Performance, Springer-Verlag, Berlin, A. A. Whitehouse, E. G. K. Pritchett, G. Barnett, Phenolic Resins, Iliffe: London, A.A. Marra, Technology of Wood Bonding: Principles in Practice, Van Nostrand Reinhold, Y. Yazaki and P. J. Collins, Adhesion Science and Technology, in Proceedings of the International Adhesion Symposium, Japan, 1994, p N. J. L. Megson, Unsolved Problems in Phenol Resin Chemistry, Chem.- Ztg. 96(1-2), (1972). 8. A. Pizzi, in Handbook of Adhesive Technology, A. Pizzi, K.L. Mittal (ed.), Marcel Dekker, New York, A. Gardziella, L.A. Pilato, A. Knop, Phenolic Resins: Chemistry, Applications, Standardization, Safety, and Ecology, 2nd ed., Springer- Verlag, New York, M.F. Grenier-Loustalot, G. Raffin, B. Salino and O. Païssé, Phenolic resins Part 6. Identifications of Volatile Organic Molecules During Thermal Treatment of Neat Resols and Resol Filled with Glass Fibers, Polymer 41(19), (2000). 11. J. Bouajila, G. Raffin, H. Waton, C. Sanglar, J.O. Paisse, M-F. Grenier- Loustalot, Phenolic Resins - Characterizations and Kinetic Studies of Different Resols Prepared with Different Catalysts and Formaldehyde/Phenol Ratios, Polymers & Polymer Composites 10, 341 (2002). 6

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25 22. M.F. Grenier-Loustalot, S. Larroque and P. Grenier, Phenolic Resins: 1. Mechanisms and Kinetics of Phenol and of the First Polycondensates Towards Formaldehyde in Solution, Polymer 35(14), (1994). 23. M. Higuchi, T. Urakawa and M. Morita, Condensation Reactions of Phenolic Resins. 1. Kinetics and Mechanisms of the Base-Catalyzed Self- Condensation of 2-Hydroxymethylphenol, Polymer 42, 4563 (2001). 24. J.H. Freeman and C.W. Lewis, Alkaline-catalyzed Reaction of Formaldehyde and the Methylols of Phenol; A Kinetic Study, J. Am. Chem. Soc. 76, (1954). 25. L.M. Yeddanapalli and D.J. Francis, Kinetics and Mechanism of the Alkali Catalysed Condensation of o- and p-methylol Phenols by Themselves and with Phenol, Die Makromolekulare Chemie 55, (1962). 26. D.J. Francis and L.M. Yeddanapalli, Kinetics and Mechanism of the Alkali Catalysed Condensations of Di- and Tri-Methylol Phenols by Themselves and with Phenol, Die Makromolekulare Chemie 125, (1969). 27. R.T. Jones, The Condensation of Trimethylol Phenol, J. Polym. Sci. 21, 1801 (1983). 28. Grenier-Loustalot, S. Larroque and P. Grenier, Phenolic Resins: 4. Self- Condensation of Methylolphenols in Formaldehyde-Free Media, Polymer 37(6), (1996). 29. N. Kamo, M. Higuchi, T. Yoshimatsu, T. Yoshimatsu, Y. Ohara, M. Morita, Condensation Reactions of Phenolic Resins III: Self- Condensations of 2,4-Dihydroxymethylphenol and 2,4,6- Trihydroxymethylphenol, Journal of Wood Science 48(6), (2002). 30. N. Kamo, M. Higuchi, T. Yoshimatsu, M. Morita, Condensation reactions of phenolic resins IV: self-condensation of 2,4-dihydroxymethylphenol and 2,4,6 trihydroxymethylphenol (2), Journal of Wood Science 50(1), (2004). 31. N. Kamo, J. Tanaka, M. Higuchi, T. Kondo, M. Morita, Condensation reactions of phenolic resins VII: Catalytic Effect of Sodium Bicarbonate for 8

26 the Condensation of Hydroxymethylols, ), Journal of Wood Science 52(4), (2006). 32. J. Monni, L. Alvila, J. Rainio, T.T. Pakkanen, Novel Two-Stage Phenol- Formaldehyde Resol Resin Synthesis, J. Appl. Polym. Sci. 103 (1), (2007). 33. P.W. King, R.H. Mitchell, and A.R. Westwood, Structural Analysis of Phenolic Resole Resins, J. Appl. Polym. Sci. 18, (1974). 34. A.W. Christiansen and L. Gollob, Differential Scanning Calorimetry of Phenol-Formaldehyde Resols, J. Appl. Polym. Sci. 30, (1985). 35. G. Carotenuto and L. Nicolais, Kinetic Study of Phenolic Resin Cure by IR Spectroscopy, J. Appl. Polym. Sci. 74, (1999). 36. B.D. Park, B. Riedl, Y.S. Kim and W.T. So, Effect of Synthesis Parameters on Thermal Behaviour of Phenol-Formaldehyde Resol Resin, J. Appl. Polym. Sci. 83, (2002). 37. G. He, B. Riedl and A. Ait-Kadi, Model-Free Kinetics: Curing Behavior of Phenol Formaldehyde Resins by Differential Scanning Calorimetry, J. Appl. Polym. Sci. 87, (2003). 38. J. Monni, L. Alvila, T.T. Pakkanen, Structural and Physical Changes in Phenol-Formaldehyde Resol Resin, as a Function of the Degree of Condensation of the Resol Solution, Industrial & Engineering Chemistry Research 46(21), (2007). 9

27 Chapter 2 Literature Review of Thermochemical Behaviour of PF Resole and Its Monomers 2.1 PF Resoles Background History In 1910, synthetic resins formed by the condensation of phenols with formaldehyde were the first resinous products to be commercially produced entirely from simple compounds of low molecular weight. They remain one of the more important products of the plastics industry as moulding and impregnated products and insulation materials, particularly for electrical insulation. Early difficulties were the tendency for the product to be brittle, crack, blister easily, and the violent nature of the condensation reaction made it difficult to control. However, in 1907, Baekeland provided the real solution of making quick-curing mouldings under controlled conditions without the problems of cracking and blistering [1]. He showed that acids and bases were chiefly catalytic in action and could be used in very small proportions, whereas previously equi-molar or even larger amounts had been used. With small proportions of an acid catalyst and a low molar ratio of formaldehyde to phenol, permanently fusible resins soluble in common solvents, such as alcohol and acetone, were obtained and called novolaks. This type of adhesive resin is not important as a wood adhesive because the faster cure of the novolak compounds can only result in linear molecules which result in a permanently fusible resin [2]. On the other hand, resinous compounds obtained with a basic catalyst and high molar ratio of formaldehyde to phenol were different in character and were called resoles. Once fully cured, they have the ability to form infusible, insoluble, three dimensional cross-linked network structures which provide highly desirable performance properties such as high modulus and tensile strength, good 10

28 dimensional stability and solvent resistance as well as being relatively low cost [3]. For these reasons, the ability to characterise the cure of the PF resole is of great benefit from an application standpoint, since the degree of cure will significantly influence the properties of the cured resin. In the 1930s, resole adhesives became widely used in the wood products industry for the manufacture of particleboard and plywood and then for the manufacture of oriental strand board (OSB) since its introduction in the 1970s. Today, the PF resoles continue to dominate composite wood adhesives and are a major cost factor in the industry [4] Application of PF resoles in the wood industry Generally, PF resoles are produced and applied in the wood industry in three stages [4]: Stage A: Is obtained by reacting phenol and formaldehyde with basic catalyst. The resin may be solid, liquid or semi-liquid, and is soluble in solvents. It can be stored until applied to the wood components. Stage B: The wood components and resin are then placed in a hot press, with temperatures ranging between 130 C to 140 C and high pressures between (300 to 700) kpa. During this stage the PF resin becomes solid and insoluble, but may swell in common solvents such as acetone or alcohol. Stage C: On further heating, the resin in Stage B is converted to the final Stage C, which is infusible and insoluble in organic solvents. This cure stage is normally effected in 5 to 10 minutes. Volatiles, mainly water and insignificant amounts of formaldehyde are eliminated during the cure process. Major advances have been made in clarifying the mechanisms of each of the three stages, particularly when methods were developed for simplifying the systems by using pure phenol alcohols in place of the complex mixtures found in typical resoles. There were some criticisms of early workers using the model phenol alcohols because it was contended that the results might not be applicable to 11

29 commercial resins. However, this approach was later recognised to be sound as it was accepted that functional groups generally undergo the same reactions in monomeric and polymeric systems [see, for example, 4-6]. 2.2 PF Resole Chemistry Three reaction sequences must be considered in relation to PF resole production and application: formaldehyde addition to phenol to form monomers, condensation reactions to form resole, and finally the cross-linking reactions or cure of the resole Formaldehyde addition to phenol to form monomers General The first step in the formation of resole is the addition of formaldehyde to phenol to form monomers. This reaction is carried out at around 60 C using molar excess formaldehyde and in the presence of alkaline metal hydroxides, commonly sodium hydroxide, at ph The reaction paths are shown in Figure 2.1. Essentially, the formaldehyde attacks exclusively at the active ortho and para positions of the phenol ring, adding methylol groups to these sites to form two mono-methylol phenols (MMP), then two di-methylol phenols (DMP), followed by one trimethylol phenol (TMP) (compounds 1 to 5 respectively) [5, 7, 8]. Meta substitution does not occur. The objective during this step is to react as much of the phenol with the formaldehyde to obtain as many methylol groups attached as possible, which is important for structural as well as environmental reasons. The methylol functional groups on the monomers tend to react by condensation. However, at 60 C and below, condensation reactions are negligible, thus giving the phenol an opportunity to react relatively completely with the formaldehyde [9, 10] Reactivity of methylol phenols with formaldehyde Freeman and Lewis [11] first performed the most complete study to determine the reactivity of individual methylol phenols with formaldehyde by reacting them at 30 C with an amount of formaldehyde equivalent to the total number of reactive 12

30 phenolic sites, so that complete conversion to TMP could be achieved in each case. Using paper chromatography technique, they followed the reaction paths of individual monomer compounds until full conversion to TMP occurred and determined their individual rate constants. As most commercial resoles are prepared at higher temperatures and lower formaldehyde concentrations, the findings of Freeman and Lewis may not be strictly applicable to commercial resoles. However, their results provided a starting point for a discussion of the problem and in fact were used as a basis by later researchers [see, for example, 9, 12-16]. 1 + O HO 4 + O P + O HO 5 + O O + O Figure 2.1: Reaction paths for the addition of formaldehyde to phenol [11]. They found that the reactions are second-order and that there are significant differences in both positional and molecular reactivity. In particular, whilst an ortho position in the phenol is slightly less reactive than the para, the introduction of an ortho-methylol group on to the phenol enhances the reactivity of the remaining active positions. An introduction of a methylol group in the para position of a phenol retards further activity. These effects are multiplied in the dimethylol analogs with 2,6-DMP being the most reactive and therefore readily converted to TMP, whereas 2,4-DMP has very low reactivity. The observed 13

31 differences in reactivity between the para and ortho methylol compounds are attributed to the effect of hydrogen bonds in the ortho methylol compounds. With these results, Freeman and Lewis predicted that in a reaction between phenol and formaldehyde, 4-MMP and 2,4-DMP are the major components, 2-MMP is a minor component, 2,6-DMP will be below the limits of detection and the relative amounts of TMP and residual phenol are determined by the amount of formaldehyde available. More recently, Grenier-Loustalot et al. [17] conducted a series of studies to determine the reactivity of individual methylol phenols with formaldehyde in conditions of resole synthesis (60 C, catalysed by Na at ph 8). Using a range of techniques including HPLC, 13 C NMR, FTIR and chemical assays, they monitored the kinetic and mechanistic changes in each monomer as a function of time and obtained rate constants by simulating kinetic curves during the first hours using a second-order equation of the type dx/dt = kc 0 (1-x) 2. Their results supported the reaction path of each monomer as found by Freeman and Lewis. They also confirmed that 2,6-DMP is the most reactive compound and 2,4-DMP the least and will likely to accumulate in the mixture. However, the results for the reactivity of 2-MMP and 4-MMP seemed to contradict those of Freeman and Lewis. In the experimental conditions chosen, they classified the reactivity of each monomer as: k 2,4-DMP < k 2-MMP < k 4-MMP < k 2,6-DMP. Besides the addition of formaldehyde to the phenol, some condensation reactions occurring between the monomers to form dimers and trimers were also observed Condensation reactions to form resole Condensation reactions As heating is continued in the range from above 60 C to 100 C, the reaction advances to the second stage of the process, which involves the condensation reactions of the methylol phenols [6]. This may occur via three possible reaction mechanisms to form ether and / or methylene linked chains as shown in Figure 2.2 [17]. In the case of the ether bridge, the mechanism involves the reaction between two methylol groups and the release of one molecule of water with the creation of a 14

32 dimethylene ether bridge (Scheme IV, Figure 2.2). Ether formation is favoured under neutral or acidic conditions. The formation of the methylene bridge involves the reaction of a methylol group either with another methylol group with the simultaneous release of one molecule of water and one molecule of formaldehyde, or with a proton on the aromatic ring (ortho or para) with the release of one molecule of water (Schemes V and VI, respectively, Figure 2.2). The resultant resole has low degrees of polymerisation and consists of a complex mixture of species such as unreacted phenol, formaldehyde, water and various monomers and dimers with a substantial proportion of reactive methylol groups reacted. + HO O + H 2 O IV + HO + H 2 O + O V + HO + H 2 O VI Figure 2.2: Formation of dimethylene ether and methylene bridges. The kinetic and mechanistic aspects of the condensation reactions of the individual methylol phenols have been investigated in a number of studies [14, 18-23]. Whilst the reaction conditions and the method of analysis between these studies are different, there are differing results, but also some similarities. One of the early experiments to shed an insight into the mechanism was conducted by Reese in a series of experiments by individually heating the five monomers in alkaline 15

33 solutions at 70 C [21]. The monomers were heated alone for a predetermined period following which the products of the reaction were separated by twodimensional chromatography. Reese found that for both 2-MMP and 2,6-DMP, the condensation involved the reaction of a methylol group with a free para hydrogen on the ring of the coupling monomer to form an (o,p) methylene link. No loss of formaldehyde was observed and hence the reaction proceeded via Scheme VI (Figure 2.2). On the other hand, 4-MMP and 2,4-DMP and 2,4,6-TMP, coupled preferentially at the para position and formed (p,p) methylene links with the loss of formaldehyde as in Scheme V (Figure 2.2). He also observed small quantities of 2,4-DMP formed when 4-MMP was condensed, which may be attributed to the addition of the released formaldehyde to a free nuclear position. Yeddanapalli and Francis [20, 22] carried out a series of studies to determine the kinetics and mechanisms of the self-condensation reactions of the five monomers in alkaline solution. They heated the reaction mixture in a reaction vessel isothermally at (70, 80, 90) C and samples of the mixture were removed at regular intervals and the course of the reaction was followed by quantitative paper chromatography to analyse and identify the reactants and products (Table 2.1). It was noted that other reactions also appeared in minor amounts, but could not be identified. In regard to the relative reactivity, their results indicated that the paraposition of 2-MMP appeared to be twice as reactive as the ortho-position of 4- MMP. This is in agreement with the generally recognised fact that the ortho position is less reactive than the para in electrophilic substitution reactions. They also obtained values for the activation energy from the plot of the first-order log rate constants against reciprocal of temperature (Table 2.1). The self-condensation reaction of the 2-MMP in the absence of a catalyst was observed to be second-order with an activation energy of 83.7 kj mol -1. Similar to Reese, small quantities of 2,4-DMP formed when 4-MMP was condensed. 16

34 Table 2.1: Reaction products from the self-condensation reactions of monomers as observed by Yeddanapalli and Francis [20, 22]. Monomer Reaction Product + H 2 O Linkage / Mechanism ortho - para VI Rate constant* (k) s -1 Ea kj mol x ortho-para + H 2 O VI 1.67x HO + H 2 O + O para-para V + 2,4-MMP HO HO + H 2 O ortho-para VI 6.23x H 2 O ortho-ortho VI HO HO HO + H 2 O ortho-para VI 8.56x HO HO HO HO HO + H 2 O + O para-para V *Rate constant for the disappearance of the monomer. 17

35 A more recent study carried out by Grenier-Loustalot et al. [17] simulated the condensation reactions for each of the five substituted phenol monomers as alkaline solutions without formaldehyde and in similar conditions of resole synthesis (60 C, catalysed by Na at ph = 8) in order to determine the reaction mechanisms and the reactivity during condensation of each of the monomers. Using 13 C-NMR and HPLC, they followed the changes during the selfcondensation reaction of these monomers in the absence of formaldehyde. Their results, similar to those in the work of Reese and Yeddanapalli, showed only the formation of methylene bridges under these particular experimental conditions (Schemes V or VI, Figure 2.2) and that two parameters affecting the reactivity of the monomers were the position and the number of methylol groups on the aromatic ring. Table 2.2 summarises their observations in terms of the mechanism and type of linkage formed during the self-condensation of the monomers. The study by Grenier-Loustalot et al. showed that no ortho ortho linkage was formed and that a methylol group in the para position preferentially reacted with another para methylol, rather than with an ortho methylol, to form para para methylene bridges. This may be due to intra-molecular interactions between methylol groups in ortho position and the hydroxyl group of the aromatic ring, or to steric hindrance preventing the sites from reacting [17, 22]. The reactivity of the monomers towards themselves was also shown to increase with increasing methylol substitution. Furthermore, the reactivity of 2-MMP toward itself was about five times less than that of 4-MMP. These results mostly corroborate with those of Yeddanapalli and Francis. 18