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Studies of Pf Resole / Isocyanate Hybrid Adhesives

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Studies of Pf Resole / Isocyanate Hybrid Adhesives STUDIES OF PF RESOLE / ISOCYANATE HYBRID ADHESIVES by Jun Zheng A Dissertation submitted to the Faculty of Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Wood Science and Forest Products Approved by: Dr. Charles E. Frazier Chairman Dr. Frederick A. Kamke Dr. Wolfgang G. Glasser Dr. Richard F. Helm Dr. Garth L. Wilkes December 17, 2002 Blacksburg, Virginia Keywords: phenol-formaldehyde, isocyanate, adhesives, solid-state NMR, fracture testing, spectral decomposition, urethane, weathering, wood adhesion Copyright 2002, Jun Zheng STUDIES OF PF RESOLE / ISOCYANATE HYBRID ADHESIVES by Jun Zheng Charles E. Frazier, Chairman Wood Science and Forest Products ABSTRACT Phenol-formaldehyde (PF) resole and polymeric diphenylmethane diisocyanate (PMDI) are two commonly used exterior thermosetting adhesives in the wood-based composites industry. There is an interest in combining these two adhesives in order to benefit from their positive attributes while also neutralizing some of the negative ones. Although this novel adhesive system has been reportedly utilized in some limited cases, a fundamental understanding is lacking. This research serves this purpose by investigating some of the important aspects of this novel adhesive system. The adhesive rheological and viscometric properties were investigated with an advanced rheometer. The resole/PMDI blends exhibited non-Newtonian flow behavior. The blend viscosity and stability were dependent on the blend ratio, mixing rate and time. The adhesive penetration into wood was found to be dependent on the blend ratio and correlated with the blend viscosity. By using dynamic mechanical analysis, the blend cure speed was found to increase with the PMDI content. Mode I fracture testing of resole/PMDI hybrid adhesive bonded wood specimens indicated the dependence of bondline fracture energy on the blend ratio. The 75/25 PF/PMDI blend exhibited a high fracture energy with a fast cure speed and processable viscosity. Exposure to water-boil weathering severely deteriorated the fracture energies of the hybrid adhesive bondlines. More detailed chemistry and morphological studies were performed with cross- polarization nuclear magnetic resonance and 13C, 15N-doubly labeled PMDI. A spectral decomposition method was used to obtain information regarding chemical species concentration and relaxation behavior of the contributing components within the major nitrogen resonance. Different urethane concentrations were present in the cured blend bondlines. Water-boil weathering and thermal treatment at elevated temperatures (e.g. > 200oC) caused reduced urethane concentrations in the bondline. Solid-state relaxation parameters revealed a heterogeneous structure in the non-weathered blends. Water boil weathering caused a more uniform relaxation behavior in the blend bondline. By conducting this research, more fundamental information regarding the PF/PMDI hybrid adhesives will become available. This information will aid in the evaluation of, and improve the potential use of PF/PMDI hybrid adhesives for wood-based composites. iii DEDICATION To Julia -my lovely daughter iv ACKNOWLEDGEMENTS The work presented in this dissertation would never have been accomplished without many people’s contributions. They are the ones who deserve the credit. Dr. Chip Frazier served as Chair of my advisory committee and has provided guidance and inspiration to which I will forever be indebted. His encouragement, patience and confidence in me have made my Ph.D. study a smooth and enjoyable experience. I am proud to be one of his students. I would like also to express sincere gratitude to my other committee members, Dr. Fred Kamke, Dr. Wolfgang Glasser, Dr. Garth Wilkes and Dr. Rich Helm for their invaluable discussions and suggestions. I would like to thank my fellow graduate students and group members for their help and companionship during my study. They made my everyday lab work a happy experience. Scott Renneckar, Sudipto Das and especially Nicole Brown devoted significant amount time in helping edit my writing, including my dissertation. Many thanks also go to the faculty and staff members of the Wood Science and Forest Products Department. Mr. Carter Fox conducted the fracture testing work in this research as an undergraduate researcher and I appreciate his excellent work. Mr. Tom Glass provided invaluable assistance in my NMR experiments. Dr. Judy Riffle generously allowed me to use her Q1000 DSC. This project was funded through the Wood-Based Composites Center at Virginia Tech. I would like to express my appreciation to the Center Directors (Dr. Fred Kamke and Linda Caudill) and the supporting industrial member companies, especially Huntsman Polyurethanes, Dynea, Georgia-Pacific Resins and Dow Chemical for providing adhesives for my research. I was named the Huntsman Polyurethanes Fellow from 1999 to 2002. I also thank Dr. Todd Miller for his introduction to Borden and Dr. Greg Nieckarz for offering me an exciting job at Borden Chemical, Inc. in Springfield, Oregon. Lastly, but by no means least, I am indebted to my family, especially my wife Weiwen, my parents and my brother. Their constant love, support and encouragement have been the driving force during this endeavor. v TABLE OF CONTENTS ABSTRACT...................................................................................................................... II DEDICATION.................................................................................................................IV ACKNOWLEDGEMENTS ............................................................................................ V TABLE OF CONTENTS ...............................................................................................VI LIST OF FIGURES ........................................................................................................IX LIST OF TABLES .........................................................................................................XV CHAPTER 1. INTRODUCTION.................................................................................... 1 CHAPTER 2. LITERATURE REVIEW........................................................................ 3 2.1. PHENOL-FORMALDEHYDE (PF) ................................................................................. 3 2.1.1. Overview ..................................................................................................... 3 2.1.2. PF Resole Synthesis .................................................................................... 4 2.1.3. Resole Cure Chemistry ............................................................................... 8 2.1.4. Wood/PF Interactions............................................................................... 10 2.2. POLYMERIC DIPHENYLMETHANE DIISOCYANATE (PMDI)...................................... 10 2.2.1. Overview ................................................................................................... 10 2.2.2. PMDI Synthesis......................................................................................... 12 2.2.3. Isocyanate Reactions ................................................................................ 17 2.2.4. Wood/PMDI Bonding Mechanism ............................................................ 20 2.3. RESOLE/ISOCYANATE HYBRID BINDERS ................................................................. 22 2.3.1. Overview ................................................................................................... 22 2.3.2. Resole/Isocyanate Chemistry.................................................................... 24 2.3.3. Thermal Degradation and Hydrolysis of Polyurethanes.......................... 30 2.4. CHARACTERIZATION OF THERMOSETS .................................................................... 34 2.4.1. Overview of Thermoset Cure .................................................................... 34 2.4.2. Conventional and Modulated DSC ........................................................... 37 2.4.3. Dynamic Mechanical Analysis (DMA) ..................................................... 42 2.4.4. Rheological Techniques............................................................................ 43 2.5. SOLID-STATE NMR SPECTROSCOPY ....................................................................... 46 2.5.1. Overview ................................................................................................... 46 2.5.2 Solid-state NMR Signal-Enhancing Techniques....................................... 49 2.5.3. NMR Relaxation Spectroscopy of Polymers ............................................. 52 2.5.4. CP/MAS NMR Studies of Wood Adhesives ............................................... 55 2.6. REFERENCES ........................................................................................................... 59 CHAPTER 3. RHEOLOGICAL CHARACTERIZATIONS..................................... 71 3.1. INTRODUCTION ....................................................................................................... 71 3.2. EXPERIMENTAL....................................................................................................... 73 3.2.1. Materials................................................................................................... 73 3.2.2. Methods..................................................................................................... 74 3.3. RESULTS AND DISCUSSION.....................................................................................
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