Durability of Adhesive Joints Subjected To Environmental Stress by Emmett P. O’Brien Dissertation for Doctor of Philosophy in Chemical Engineering Virginia Polytechnic Institute and State University Blacksburg, Virginia 24060 Committee Chair: Thomas C. Ward Committee Members: Dr. Richey Davis Dr. David Dillard Dr. Eva Marand Dr. Ravi Saraf August 28, 2003 Key Words: polymer, adhesion, environmental degradation, epoxy, water, interface, durability, diffusion, shaft-loaded blister test, sub-critical crack growth, electrochemical impedance spectroscopy (EIS), dielectric spectroscopy, ink 1 Title: Durability of Adhesive Joints Subjected to Environmental Stress Author: Emmett P. O’Brien Abstract Environmental stresses arising from temperature and moisture changes, and/or other aggressive fluid ingressions can degrade the mechanical properties of the adhesive, as well as the integrity of an adhesive interface with a substrate. Therefore such disruptions can significantly reduce the lifetime and durability of an adhesive joint.1-4 In this research, the durability of certain epoxy adhesive joints and coatings were characterized using a fracture mechanics approach and also by constant frequency impedance spectroscopy. The shaft-loaded blister test (SLBT) was utilized to measure the strain energy release rate (G) or adhesive fracture energy of a pressure sensitive adhesive tape. In this study, support for the value of the SLBT fracture mechanics approach was obtained. The SLBT was then used to investigate the effects of relative humidity on a model epoxy bonded to silicon oxide. Lastly, the effects of water and temperature on the adhesion of a commercial filled epoxy bonded to silicon oxide was characterized and interpreted. A novel impedance sensor for investigating adhesion was developed in a collaborative effort between Virginia Tech and Hewlett-Packard. Utilizing the technique of constant frequency impedance spectroscopy, the distribution and transport of fluids at the interface of adhesive joints was measured. A broad spectrum of adhesives was tested. In addition, the effects of hygroscopic cycling on the durability of adhesive coatings were measured for the commercial filled epoxy using the device. Lastly, recommended modifications of the experimental set-up with the new sensor are proposed to improve the technique. 2 Acknowledgements I would like to sincerely thank the following people. You have made the time at Virginia Tech well spent. Dr. Ward, thank you for all your advice and support throughout the years (personal and professional) and giving me the opportunity to obtain a Ph.D. Thank you for giving our group the intellectual freedom and resources to pursue our own scientific interests. Thank you opportunity to attend numerous conferences, visit interesting places, and meet many important people in industry. My committee members: Dr. Richey Davis, Dr. David A. Dillard, Dr. Eva Marand and Dr. Ravi Saraf for their tough questions, valuable suggestions and even more valuable time. I would like to particularly thank David A. Dillard for teaching me about adhesives and adhesives research from a both a practical and fundamental standpoint. Thank you for guiding me through fracture mechanics and giving me the chance to earn a living at breaking things. Thank you also for introducing me to many people in the adhesives industry and for the opportunity to network in places like Yosemite, CA. Thanks to all my friends and professional colleagues, especially Sandra Case who graduated right before me and showed me the way. Thanks to Amy Eichstadt for keeping my shoes. Thank you Rob Jensen for mentoring me in the embryonic stage of graduate school. Thanks to Dave Porter for teaching me to rig stuff up. Thanks to Taigyoo Park for introducing me to the SLBT. Corey Reed and Jeremy Lizotte for several championships. Thanks to everyone else: Kermit Kwan, Jennifer Robertson, Ojin Kwan, Chitra Subraminiam, Jaime Kalista, Hailing Yang, Kalpana Viswanathan, Ron Defelice, Mitch and Brenda Jackson, Mike Bortner, Eric Scribben, Scott Trenor, Jeremy Lizotte, Lee Williams, The Hunsuckers, Matt O’Sickey, Dave Godshall, Chris Robertson, Martha McCaann, Bradford Carmen, Doug Crowson, Anders DiBiccari, Holly Grammer, and Derek and Kelly. iii I would like to thank every one in the Hewlett-Packard project including Dr. John G. Dillard, Dr. James. E. McGrath, Sumitra Subraminian, Johnny Yu, Sankar, Hitendra Singh, Shu Guo, David Xu, and Zuo Sun. I would like to thank the folks at Hewlett-Packard: Thank you Paul Reboa and Thomas Lindner for great project management and the numerous hefe-weizens. Thank you Marshall Field, Dan Pullen, and David Markel for design and production of the impedance sensors. Thank you Jim McKinnel for the numerous tutorials on electrical engineering. Thanks to Josh Smith for his big brain. Despite what people say, you are not really a jackass. Thanks to Brad Benson. Thanks to Ellen Chappell. Thanks to the Center for Adhesive and Sealant Science: Dr James P. Wightman, Tammy Jo Hiner, and Linda Haney. Thanks to the Adhesive and Sealant Council and all the great people that I have met through the ASC. Great folks like Malinda Armstrong, Wendy Yanis, Dave Jackson, Rick Barry, Robert Lefelar, Jim Sorg, Kay Peters, Norman Pfeifer, Darrel Bryant, Rick Jihnston, Dwight Lynch The guys from Franklin International Adhesives: Mark Vrana, Larry Owen, Larry Gwin, Matt McGreery, Chusk Shuster, Dale and Evan Williams. The folks at Adhesives Research Incorporated: Ranjit Malik and Brian Harkins. Thank you, Professor J. E. “Big Jim” McGrath for all your help, friendship, and nights at the Karaoke bar. Thanks to Professors Judy Riffle and Alan Esker. Thanks to the friends and folks at TA Instruments: Gary Mann and Steve Aubuchon. Thanks to the following SURPs: Kevin Doyle, Stefan Adreev, and Wes. Thank you Michael Vick and Corey Moore. Thanks to all my undergrad friends: Ray and Tammy, Kirrukk, Kollman, Hinett, Rich, Castello, Sujan, Maharshi, Guarav, Raindeer, Ceasar, Cindy, and Laura. Thanks to Millie Ryan, Laurie Good, and Esther Brann who were always so nice and willing to lend an ear or a cup of coffee. Thanks to Frank Cromer for hanging out and letting me use the sputter coater. Thanks to Jim Coulter for his electrical engineering tutorials and to Travis for helping with computer woes. Thanks to Al Shultz, Dr. Garth Wilkes, Diane Cannaday, Dr. William Conger, and Diane Patty. Thanks to the Physics Machine Shop rednecks: Scott, John, Melvin, Melvin’s cousin and even Fred (O.D.B.). Thanks to Jane for keeping our lab tidy. Thanks to the boys in Boulder, Co. and Aaron iv Campbell. Thanks to people in Dart League, early morning tailgaters, and NASCAR people. Thanks to the late Francis VanDamme. I have saved thanking the most important people in my life for last. I would like to thank my mom, Esperanza R. O’Brien for all her love and support over the years. Mom, I finally got a real job. Thank you, my wife, Kristen W. O’Brien for your love, good times, and improving my writing skills. Thank you to all of Kristen’s family for being such great people and raising such a special woman. Good luck to all the new graduate students, you will need it. v Table of Contents DURABILITY OF ADHESIVE JOINTS SUBJECTED TO ENVIRONMENTAL STRESS..............................1 1 INTRODUCTION..............................................................................................................................................1 2 BACKGROUND ON ENVIRONMENTAL DEGRADATION OF ADHESIVE JOINTS AND COATINGS..................................................................................................................................................................3 2.1 FACTORS THAT INFLUENCE WATER TRANSPORT AND SUBSEQUENT ADHESION LOSS................................3 2.2 GENERAL TRENDS REGARDING LOSS OF ADHESION...................................................................................4 2.3 HYPOTHESIS AND MECHANISM FOR LOSS OF ADHESION BY WATER...........................................................5 2.3.1 Water Accumulation at the Interface.....................................................................................................5 2.3.2 The Heterogeneous Nature of Adhesive Bond: Water Transport and Accumulation.............................7 2.3.3 Osmotic Forces and Lateral Growth by Continued Condensation and Blistering................................7 2.3.4 Physisorption of Water at the Interface.................................................................................................8 2.4 METHODS TO INCREASE THE LIFETIME OF ADHESIVE BONDS.....................................................................8 2.5 DIFFUSION IN POLYMERS AND ADHESIVE JOINTS......................................................................................10 2.5.1 Fickian Diffusion.................................................................................................................................10 2.5.2 Non-Fickian Diffusion.........................................................................................................................12 2.5.3 Interfacial Diffusion and Diffusion in Adhesive Joints........................................................................13 2.5.4 Diffusion in Epoxies.............................................................................................................................15 3 ADHESIVE STUDIES USING THE SHAFT-LOADED BLISTER TEST................................................17 3.1 INTRODUCTION..........................................................................................................................................17 3.2 STRAIN ENERGY RELEASE RATES OF A PRESSURE SENSITIVE ADHESIVE TAPE MEASURED BY THE SHAFT- LOADED BLISTER TEST............................................................................................................................................18 3.2.1 Abstract................................................................................................................................................18 3.2.2 Introduction to Adhesive Testing of Thin Coatings..............................................................................19 3.2.3 Shaft-Loaded Blister Test Theory........................................................................................................24 3.2.4 Experimental........................................................................................................................................28 vi 3.2.5 Results and Discussion........................................................................................................................29 3.2.6 Effects of Fluids at the Interface..........................................................................................................38 3.2.7 Conclusions..........................................................................................................................................40 3.2.8 Appendix: Experimental compliance calibration.................................................................................41 3.2.9 Figures.................................................................................................................................................46 3.3 MOISTURE DEGRADATION OF EPOXY ADHESIVE BONDS MEASURED BY THE SHAFT-LOADED BLISTER TEST 61 3.3.1 Abstract................................................................................................................................................61 3.3.2 Introduction.........................................................................................................................................61 3.3.3 Experimental........................................................................................................................................63 3.3.4 Results and Discussion........................................................................................................................66 3.3.5 Conclusion...........................................................................................................................................67 3.3.6 Figures.................................................................................................................................................68 3.4 ENVIRONMENTAL STUDIES OF ADHESIVE BONDS USING THE SUB-CRITICAL SHAFT-LOADED BLISTER TEST 71 3.4.1 Abstract................................................................................................................................................71 3.4.2 Introduction.........................................................................................................................................71 3.4.3 Experimental........................................................................................................................................76 3.4.4 Results and Discussion........................................................................................................................81 3.4.5 Summary..............................................................................................................................................88 3.4.6 Figures.................................................................................................................................................90 3.4.7 Appendix: Measuring Diffusion Coefficients.......................................................................................99 4 INVESTIGATION OF THE DURABILITY OF ADHESIVE JOINTS AND COATINGS USING CONSTANT FREQUENCY IMPEDANCE SPECTROSCOPY........................................................................101 4.1 INTRODUCTION........................................................................................................................................101 4.2 BACKGROUND ON IMPEDANCE SPECTROSCOPY.......................................................................................103 4.2.1 Constant Frequency Impedance Measurements................................................................................104 4.2.2 The Relationship between Capacitance and Fluid Concentration.....................................................105 vii 4.2.3 Estimating the Area of Debonding.....................................................................................................108 4.2.4 Figures...............................................................................................................................................110 4.3 A NOVEL IMPEDANCE SENSOR DESIGN FOR MEASURING THE DISTRIBUTION AND TRANSPORT OF FLUIDS IN ADHESIVE JOINTS..............................................................................................................................................111 4.3.1 Abstract..............................................................................................................................................111 4.3.2 Introduction.......................................................................................................................................111 4.3.3 Experimental......................................................................................................................................113 4.3.4 Results and Discussion......................................................................................................................116 4.3.5 Summary............................................................................................................................................117 4.3.6 Figures...............................................................................................................................................119 4.4 DIFFUSION OF WATER AND SUBSEQUENT DEBONDING OF A PSA TAPE..................................................125 4.4.1 Abstract..............................................................................................................................................125 4.4.2 Materials and Experimental..............................................................................................................125 4.4.3 Results................................................................................................................................................125 4.4.4 Conclusion.........................................................................................................................................126 4.4.5 Figures...............................................................................................................................................127 4.5 INTERFACIAL DIFFUSION OF AGGRESSIVE FLUIDS INTO EPOXY ADHESIVE JOINTS AND COATINGS MEASURED BY CONSTANT FREQUENCY IMPEDANCE SPECTROSCOPY...................................................................128 4.5.1 Abstract..............................................................................................................................................128 4.5.2 Research Objectives...........................................................................................................................128 4.5.3 Experimental......................................................................................................................................129 4.5.4 Results and Discussion......................................................................................................................133 4.5.5 Conclusions........................................................................................................................................137 4.5.6 Figures...............................................................................................................................................139 4.6 MOISTURE DIFFUSION FROM THE EDGE IN BONDED UV CURABLE PRESSURE SENSITIVE ADHESIVES MEASURED BY CONSTANT FREQUENCY IMPEDANCE SPECTROSCOPY...................................................................146 4.6.1 Abstract..............................................................................................................................................146 4.6.2 Experimental......................................................................................................................................146 viii 4.6.3 Results and Discussion......................................................................................................................148 4.6.4 Conclusions........................................................................................................................................152 4.6.5 Figures...............................................................................................................................................153 4.7 DURABILITY OF ADHESIVE COATINGS SUBJECTED TO HYGROSCOPIC CYCLING MEASURED BY CONSTANT FREQUENCY IMPEDANCE SPECTROSCOPY..............................................................................................................160 4.7.1 Abstract..............................................................................................................................................160 4.7.2 Introduction.......................................................................................................................................161 4.7.3 Experimental......................................................................................................................................165 4.7.4 Results and Discussion......................................................................................................................172 4.7.5 Summary............................................................................................................................................182 4.7.6 Figures...............................................................................................................................................184 4.8 RECOMMENDATIONS AND MODIFICATIONS FOR CONSTANT FREQUENCY INTERFACIAL IMPEDANCE SPECTROSCOPY......................................................................................................................................................197 4.8.1 Abstract..............................................................................................................................................197 4.8.2 Sources of Signal Noise.....................................................................................................................197 4.8.3 Recommended Modifications.............................................................................................................197 5 CONCLUDING REMARKS.........................................................................................................................200 6 REFERENCES:.............................................................................................................................................201 ix Table of Figures FIGURE 3-1 SCHEMATIC OF THE PULL-OFF TEST...........................................................................................................46 FIGURE 3-2 SCHEMATIC OF THE SHAFT-LOADED BLISTER TEST....................................................................................46 FIGURE 3-3 STRESS-STRAIN CURVE FOR KAPTON® PSA..............................................................................................47 FIGURE 3-4 SCHEMATIC OF THE EXPERIMENTAL SET-UP OF THE SHAFT LOADED BLISTER TEST....................................48 FIGURE 3-5 SCHEMATIC OF VIEW OF BLISTER RADIUS PROPAGATION..........................................................................49 FIGURE 3-6 LOAD (P) VS. CENTRAL SHAFT DISPLACEMENT (W0) FOR N = 1, 2 AND 4.....................................................50 FIGURE 3-7 DEBONDING RADIUS (A) VS. CENTRAL SHAFT DISPLACEMENT (W0) FOR N = 1, 2 AND 4..............................51 FIGURE 3-8 SCHEMATIC OF THE ACTUAL BLISTER PROFILE...........................................................................................52 FIGURE 3-9 LOADING AND UNLOADING CYCLES FOR KAPTON® PSA TAPE BONDED TO POLISHED ALUMINUM. THE LOADING PORTION OF THE CURVE IS SHOWN IN FILLED SYMBOLS AND THE UNLOADING PORTION IS SHOWN IN UNFILLED SYMBOLS............................................................................................................................................53 FIGURE 3-10 PLOT OF THE FINAL LOAD PMAX OF EACH CYCLE VS. W0 FOR KAPTON® PSA TAPE BONDED TO POLISHED ALUMINUM..........................................................................................................................................................54 FIGURE 3-11 PLOT OF EFFECTIVE MEMBRANE STRESS AS A FUNCTION OF CRACK LENGTH (A) AND R............................55 FIGURE 3-12 PLOTS OF PREDICTED SCALING LAWS BASED ON LOAD, HYBRID, AND DISPLACEMENT EQUATIONS..........56 FIGURE 3-13 LOAD (P) VS. CENTRAL SHAFT DISPLACEMENT (W0), AND CRACK LENGTH (A) VS. CENTRAL SHAFT DISPLACEMENT CURVES (W0), FOR KAPTON® PSA TAPE BONDED TO TEFLON (N = 1).........................................57 FIGURE 3-14 SCHEMATIC OF THE EXPERIMENTAL SET-UP USED TO MAKE IN-SITU MEASURE OF LIQUID INFLUENCE THE STRAIN ENERGY RELEASE RATE..........................................................................................................................58 FIGURE 3-15 LOAD (P) VS. CENTRAL SHAFT DISPLACEMENT (W0) FOR VARIOUS CONCENTRATIONS OF METHANOL IN WATER (0, 40, 60, 80, 100 WT. %).......................................................................................................................59 FIGURE 3-16 LINEARIZED PLOT OF EQUATION 7, P A2 VS. W03 FOR N = 1(□), 2 (О), AND 4 (◊)........................................60 FIGURE 3-17.SCHEMATIC OF THE SHAFT-LOADED BLISTER TEST..................................................................................68 FIGURE 3-18 SCHEMATIC OF SHAFT-LOADED BLISTER TEST SPECIMEN.........................................................................68 FIGURE 3-19 SUCCESSIVE LOAD AS A FUNCTION OF CENTRAL SHAFT DISPLACEMENT CURVES FOR 98% RELATIVE HUMIDITY. THE FILLED IN SYMBOLS ARE THE LOADING CURVES AND THE UNFILLED SYMBOLS ARE THE UNLOADING CURVES...........................................................................................................................................69 x