1. Materials and Methods (cont.)
Obtaining controlled adhesive bond strength through modified surface treatments Kenneth L. Proctor Mentored by Dr. James M. Sands and Dr. Daniel B. Knorr Jr.
The ability to effectively join materials with reliable strength and durability offers the engineering design community the capability to fabricate light-weight structures that meet performance requirements. In the coming transition from metallic structures to advanced materials such as composites, the traditional joining technique of using through-thickness bolts introduces increased weight and focuses stresses, leading to degradation of performance.
Replacing mechanical fasteners in flight-critical components with adhesives is currently not an option, as bond strength cannot be validated without bond destruction. An interest was taken in designing an adhesive interface that can be demonstrated to have a replicable design strength after a number of conditions, validating bond strength without the use of mechanical fasteners.
To accomplish this goal, surface treatments involving custom primers based on functional silane groups that bond strongly to the substrate, but have varying degrees of coupling with the adhesive were tested for feasibility. It is anticipated that through modifying the ratios of multiple silane surface treatments, bond strength can be altered, allowing for joint failure at a specific load, aiding the fabrication of breakaway structures for the purpose of damage mitigation.
Introduction
Materials and Methods (cont.)
Conclusion
Another characterization method utilized was x-ray photoelectron spectroscopy (XPS), in which the surface is bombarded with x-rays of a single energy. This provides information on the chemical makeup of a substrate at the surface through measuring the quantity and kinetic energy of electrons that are ejected. This was done to compare the amount of C-O bonds to C-C bonds at the surface. This is useful because molecular interactions and surface reactions dictate coupling strength at the interface.
The results of the contact angle study (Graph 1) show a direct correlation between surface treatment composition and contact angle. As the concentration changes from 0% to 40% GPS, the contact angle decreases systematically from ~93 to ~55 degrees. The measurement of water contact angle on a surface reflects the hydrophobicity/hydrophilicity of the surface. The 0% surface (all PTMO) has little or no affinity for water, and thus a large contact angle due to the hydrocarbon chain on the PTMO. The 100% GPS surface has some water affinity (moderate contact angle) because the terminal epoxy group can interact with water through hydrogen bonding. At 40%, the contact angle stops changing. This could be caused by the reaction rate of GPS being higher than PTMO, leading to any amount of GPS in the solution fully covering the surface before PTMO has a chance to bond to the surface.
XPS data (Graph 3) shows a clear relationship between surface treatment composition and the ratio of C-O to C-C bonds. This makes sense due to the fact that there are two O atoms participating in C-O bonding in GPS, but none in PTMO. The data point at 50% is an outlier in the trend, and not a physically possible value because it suggests there are more C-O bonds than C-C bonds in GPS (See Figures 1 & 2 for structures). This may be due to possible contamination, or ambient oxidation.
The similarities between the contact angle and XPS results support the conclusion that GPS and PTMO mixed layers can be applied to alter surface chemistry in such a way that one could reasonably expect to see a change in bond strength with treatment composition, paving the way for subsequent bonding studies.
Results
The contact angles taken from each potential treatment formulation were compared to their expected contact angles. It was found that a mixture of 200 mL ethanol, 10 mL H2O, ½ mL acetic acid, and 1 mL PTMO produced contact angle measurements in the range of standard PTMO deposition methods. This formulation produced values consistent with previous research (Barroeata-Robles, Cole, & Sands, 2010) when PTMO was swapped out for equimolar GPS.
Graph 1 relates contact angle to surface treatment composition.
The XPS data in Graphs 2A & 2B display data from a surface with 0% GPS (upper graph) and 90% GPS (lower graph). Each graph was fit with two curves, one centered on the known binding energy of C-O bonds, and the other centered at the energy of C-C bonds (Biesinger, Lau, Gerson, & Smart, 2010). The area under each curve (relative amount of those bonds) was then found. Graph 3 displays the ratio between the amount of C-C bonds to the amount of C-O bonds as a function of treatment composition.
Materials and Methods
The surface treatments used were trimethoxy(propyl)silane (PTMO, Figure 1), which does not form a bond to the substrate, and (3-glycidyloxypropyl)trimethoxysilane (GPS, Figure 2), which bonds strongly to both the substrate and adhesive. These treatments were chosen based on the widespread use of GPS in the engineering design community, and the compatibility of the two.
Graph 1: Contact angle vs. composition.
References
Treatment formulation was determined through testing of various solvents, water content, solvent acidity, and deposition times. After a treatment protocol had been determined based on preliminary testing, a number of surface analysis techniques were used to provide insight into what occurred at the interface. The first measurement was contact angle, measured between the surface and the tangent of a liquid where it contacts the surface. The result is a consequence of the energy and geometry of the surface, both of which play a role in determining bond strength.
Figure 1: PTMO polymer. Si O
Barroeata-Robles, J., Cole, R., & Sands, J. M. (2010). Proceedings from Society for the Advancement of Material and Process Engineering Technical Conference: Development of controlled adhesive bond strength for assessment by advanced non-destructive inspection techniques, Seattle, WA.
Biesinger, M. C., Lau, L. W.M., Gerson, A. R., & Smart, R. St.C. (2010). Resolving surface chemical states in XPS analysis of first row transition metals, oxides and hydroxides: Sc, Ti, V, Cu and Zn. Applied Surface Science, 257(3),
Electrons per Second
(Arbitrary Unit)
Graph 2A
Figure 2:
GPS polymer. O Si
Acknowledgements
I would like to extend my thanks to Mr. Dan DeSchepper, Mr. Dave Flanagan, Dr. Rob Jensen, and Mr. Jim Wolbert, all of the U.S. Army Research Laboratory, for working alongside me in completing my project.
Graph 2B
Graphs 2A - 2B: XPS data of 0% GPS and 90% GPS.
Graph 3: Relative amounts of bonds.