THE DEVULCANIZATION OF UNFILLED AND CARBON BLACK FILLED ISOPRENE RUBBER VULCANIZATES BY HIGH POWER ULTRASOUND A Dissertation Presented to The Graduate Faculty of the University of Akron In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Ximei Sun May, 2007 THE DEVULCANIZATION OF UNFILLED AND CARBON BLACK FILLED ISOPRENE RUBBER VULCANIZATES BY HIGH POWER ULTRASOUND Ximei Sun Dissertation Approved: Accepted: Advisor Department Chair Dr. Avraam I. Isayev Dr. Sadhan C. Jana Committee Member Interim Dean of the College Dr. Ernst D. von Meerwall Dr. George R. Newkome Committee Member Dean of the Graduate School Dr. Sadhan C. Jana Dr. George R. Newkome Committee Member Date Dr. Erol Sancaktar Committee Member Dr. Michael Cheung ii ABSTRACT The effects of ultrasound on virgin gum isoprene rubber (IR) and on the devulcanization of unfilled and carbon black (CB) filled IR were studied. Ultrasonic treatment altered the structure and properties of gum IR by creating low molecular weight tails which broadened the molecular weight distribution and improved processability. Ultrasonic devulcanization of IR vulcanizates resulted in a reduction of gel fraction and crosslink density. Increasing the ultrasonic amplitude yielded a further reduction, regardless of CB loading, in the IR vulcanizates. This is contrary to the previous work on natural rubber (NR), the natural counterpart of IR which showed a minimum gel fraction and crosslink density at an intermediate ultrasonic amplitude. The devulcanization of filled IR resulted in more main chain scission than in unfilled IR due to the immobility of bound rubber at the filler surface which leads to lower properties in revulcanized rubbers than in virgin rubber. Upon blending the devulcanized IR with virgin IR, properties comparable to those of virgin rubber were obtained at certain blending ratios. A cure kinetics model with reversion adequately predicted the evolution of state of cure in curing and reversion stages under isothermal and non-isothermal conditions. The higher reversion observed in filled IR than in unfilled IR was consistent with the difference of reversion rate constant obtained in simulation. iii NMR proton transverse relaxation technique was unable to differentiate the contribution of short component mobility between physically entangled (heavy sol) and chemically crosslinked (gel) networks. Ultrasound severed both the chemical crosslinks and the main chain, creating dangling chain ends, with no generation of additional fragments of oligomeric species. Simulation of network structures using the Dobson-Gordon theory of network statistics indicated crosslinks were easier to break than main chains under ultrasonic exposure. Unfilled IR and NR had similar rate constant ratios of main chain scission and crosslink rupture. An increase of CB loading increased this ratio for both IR and NR with higher ratio in IR. The addition of processing oil in the filled IR compounds reduced this ratio. iv ACKNOWLEDGEMENTS I would like to express my sincere gratitude to many people without whom this work could have never been accomplished. First of all, I would like to thank Dr. Avraam I. Isayev for being my mentor for the past years. I am very grateful to work under his thorough guidance and consistent encouragement. I would also like to thank Dr. Ernst von Meerwall not just for his serving on my committee but more for his inspiring discussions on NMR work with me. My sincere appreciation extends to the remaining committee members: Dr. Sadhan C. Jana, Dr. Erol Sancaktar and Dr. Michael Cheung for their thoughtful advice and suggestions. I cherish the tremendous help from Theresa Schillig, Tayba Tahir and Marcile Pendleton of the Akron Polymer Training Center (APTC, Univ. of Akron) who made my life and work in Akron a whole lot easier. I appreciate the help provided to me by Bob Seiple (Polymer Science Dept.) and Henry Pawlowski (Alpha Technologies ) in using the APA 2000 and by Jon Page (Polymer Science Dept.) for GPC experiments. Many thanks to Mr. Tirtha Joshi (Physics Dept.) for the NMR experiments we did together. My earnest gratitude goes to my parents for giving me the wisdom and devotion for work. Especially I want to thank my husband, Guofeng Huang, and my son, Roy Huang, for their love and keeping me motivated to try my best all the time. Last but not the least, thanks go to the Goodyear Tire and Rubber Company and the Akrochem Corporation for the material support in this research work. v TABLE OF CONTENTS Page LIST OF TABLES ……………………………………………………………………...xii LIST OF FIGURES …………………………………………………………………….xiv CHAPTER I. INTRODUCTION..................................................................................................... 1 II. LITERATURE SURVEY AND BACKGROUND .................................................. 4 2.1 Natural Rubber (NR)............................................................................................... 4 2.2 Synthetic Isoprene Rubber (IR) .............................................................................. 7 2.3 Comparison of NR and IR..................................................................................... 10 2.4 Carbon Black (CB)................................................................................................ 11 2.5 Carbon Black Filled Rubber.................................................................................. 14 2.6 Vulcanization ........................................................................................................ 16 2.6.1 Sulfur Vulcanization ..................................................................................... 17 2.6.2 Mechanism of Accelerated Sulfur Vulcanization ......................................... 22 2.7 Rubber Recycling.................................................................................................. 26 2.7.1 Landfills and Waste Rubber Utilization........................................................ 27 2.7.2 Grinding Methods......................................................................................... 27 2.7.3 Rubber as a Fuel Source and Pyrolysis ......................................................... 28 2.7.4 Chemical Method.......................................................................................... 29 2.7.5 Microwave Method ....................................................................................... 30 vi 2.7.6 Biotechnological Method .............................................................................. 30 2.7.7 Ultrasonic Method......................................................................................... 32 2.7.8 Summary of the Recycling Methods............................................................. 33 2.8 Recycling of Isoprene Rubber – Current Studies.................................................. 34 2.9 Application of Ultrasound in Polymers................................................................. 36 2.9.1 Ultrasonic Treatment of Polymers ................................................................ 39 2.9.2 Ultrasonic Devulcanization Mechanism ....................................................... 40 2.9.3 Modeling of Ultrasonic Devulcanization Process......................................... 41 2.10 Molecular Mobility Analysis by Solid-State NMR. ............................................. 45 1 2.10.1 Proton Transverse Relaxation ( H T2)........................................................... 46 2.10.2 Pulsed Gradient Spin-Echo (PGSE) Diffusion.............................................. 47 III. EXPERIMENTAL .................................................................................................. 50 3.1 Materials................................................................................................................ 50 3.2 Compounding........................................................................................................ 52 3.3 Vulcanization and Vulcanizates Grinding ............................................................ 54 3.4 Revulcanization..................................................................................................... 55 3.5 Ultrasonic Treatment and Devulcanization Equipment ........................................ 55 3.5.1 Ultrasonic Treatment of the Virgin Gum IR ................................................. 58 3.5.2 Devulcanization of the Vulcanizates............................................................. 59 3.6 Characterization Methods..................................................................................... 59 3.6.1 Vulcanization Kinetics.................................................................................. 60 3.6.2 Dynamic Properties....................................................................................... 60 3.6.3 Gel Fraction................................................................................................... 62 3.6.4 Crosslink Density.......................................................................................... 62 vii 3.6.5 Mechanical Properties................................................................................... 67 3.6.6 Molecular Weight Determination.................................................................. 68 3.6.7 Thermal Properties........................................................................................ 69 3.6.8 T2 Relaxation and PGSE Diffusion............................................................... 69 IV. CURE KINETICS STUDY