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Method for Locally Measuring Strength of a Polymer-Inorganic Interface During Cure and Aging



OBJECTIVE: Develop and demonstrate a method to locally measure quality of the interface in an adhesive system (metal substrate/polymeric resin) during resin curing and during aging under hot/wet conditions. 

DESCRIPTION: Surface treatment processes dominate the durability of interfaces in adhesively bonded joints, fiber reinforced composites, and polymer encapsulated electronics in military and commercial applications. 1. These surface treatments may include abrasion (e.g., grit-blasting), chemical etching, polishing, chemical functionalization (e.g., with coupling agents), and they are used to control the wettability, chemical functionality, and morphology of the interface between an inorganic substrate (e.g., aluminum) and an adhesive/polymer encapsulant (e.g., an epoxy resin with a diamine curing agent). The wettability, chemical functionality and morphology all influence (1) the initial strength of the interface during curing of the polymer, and (2) the long-term durability of the bond under hot/wet conditions experienced in theatre. The durability of these bonds is often dictated by the ability of the interface of the cured polymer system to resist moisture infiltration and the corresponding degradation of the adhesion between the adhesive and substrate (e.g., bond breakage, corrosion). 2. Furthermore, local defects are known to provide points of stress concentration that can locally serve as the "weakest link" in the polymer system, leading to premature failure. 3. Despite the central importance of the surface treatment in these systems, there is currently no commercially available method for locally (<100µm) measuring the quality of the polymer-substrate interaction (1) during curing (initial strength) and (2) during aging under hot/wet conditions (e.g., in liquid water at 60ºC). There are, however, a few existing techniques that can likely be modified for such measurements including: modulated microscopy techniques, surface forces apparatus techniques, and small-scale mechanical testing techniques. Modulated scanning probe measurements using lock-in techniques[4] could potentially be used to monitor contact stiffness in situ. Additionally, the surface force apparatus technique[5] has been developed to the point that it can be used to monitor adhesive forces within various liquid environments, making it an option as well. Finally, micron-scale mechanical testing, which was developed for solder testing may be applicable as well. Thus, we seek development of novel techniques or novel use of existing instruments that can be used to measure the quality of the interface (e.g., adhesion, interfacial shear strength, contact stiffness, or some other acceptable metric) both during curing and over time (after cure) under hot/wet conditions. Such a method would allow for demonstration of the utility of new surface treatments, allow for simulation of local defects, and provide a means of evaluating strategies to mitigate defect formation. 

PHASE I: The offeror(s) shall develop a technique to monitor the change in the interface quality during polymer curing. The offeror(s) shall demonstrate the use of this method to measure interface quality during room temperature and heated (>50ºC) curing of a model substrate/resin system. The suggested model substrate is aluminum oxide, and the suggested model resin is a stoichiometric cure of diglycidyl ether of bisphenol A and Jeffamine® D230 - see properties in Tables 3 and 4 of Lenhart et al [7]. The offeror(s) shall also develop a technique using the same instrument to measure the change in the quality of the interface of this same model substrate/resin system as a function of time in the presence of liquid water at the interface in separate tests at room temperature and at 60ºC for at least one week each. 

PHASE II: The offeror(s) shall implement the method developed in Phase I to investigate the influence of multiple factors on initial strength of the interface and the durability (hot/wet testing) using the chosen model system. These factors will include: (1) surface roughness (RMS roughnesses of ~10nm to ~1µm), (2) chemical treatment (e.g., etches in various acids), (3) functionalization (e.g., silane coupling agents like 3-aminopropyltriethoxysilane and 3-glycidoxypropyltriethoxysilane). The offeror(s) shall extend the use of the method to determine the influence of localized defects (e.g., large/sharp surface asperities or air bubbles). The offeror(s) will validate their results against lap-shear tests according to ASTM D1002-10 using the same resin and a comparable substrate. In addition, the offeror(s) will demonstrate the utility of the technique on substrates used in other systems of interest to the military that require polymer encapsulation. Examples include substrates similar to those encountered in glass-fiber reinforced composites (e.g., silicon oxide), and substrates in electronics applications (e.g., indium tin oxide). 

PHASE III: The offeror is expected to aggressively pursue opportunities to market the method developed herein for use in evaluating and testing adhesives, surface treatments, coupling agents, passivation methods, and substrate preparation methods for adhesive systems, fiber reinforced composite applications, and electronic encapsulants in both military and commercial applications. Of particular interest is the establishment of an industry-wide standard method (e.g., ASTM or equivalent) for predicting the success or failure of proposed changes in surface preparation methods in meeting military specifications. 


1: Jensen, R. E.; McKnight, S. H.; Quesenberry, M. J. Strength and Durability of Glass Fiber Composites Treated with Multicomponent Sizing Formulations; Laboratory, U. S. A. R.2002.

2: Bradley, W. L.; Grant, T. S. The effect of the moisture absorption on the interfacial strength of polymeric matrix composites. Journal of Materials Science 30 (21), 5537-5542.

3: Hobbiebrunken, T.; Fiedler, B.; Hojo, M.; Tanaka, M. Experimental determination of the true epoxy resin strength using micro-scaled specimens. Composites Part A: Applied Science and Manufacturing 2007, 38 (3), 814-818.

4: Sills, S.; Overney, R. M.; Chau, W.; Lee, V. Y.; Miller, R. D.; Frommer, J. Interfacial glass transition profiles in ultrathin, spin cast polymer films. Journal of Chemical Physics 2004, 120 (11), 5334-5338.

5: Israelachvili, J.; Min, Y.; Akbulut, M.; Alig, A.; Carver, G.; Greene, W.; Kristiansen, K.; Meyer, E.; Pesika, N.; Rosenberg, K.; Zeng, H. Recent advances in the surface forces apparatus (SFA) technique. Reports on Progress in Physics 2010, 73 (3).

6: Kwon, S.; Lee, Y.; Han, B.; Asme. Advanced micro shear testing for solder alloy using direct local measurement; Amer Soc Mechanical Engineers: New York, 2003. p 537-542.

7: Bain, E. D.; Knorr, D. B.; Richardson, A. D.; Masser, K. A.; Yu, J.; Lenhart, J. L. Failure processes governing high-rate impact resistance of epoxy resins filled with core-shell rubber nanoparticles. Journal of Materials Science 2015, 51 (5), 2347-2370.


KEYWORDS: Surface Treatments, Composites, Manufacturing Processes, Fabrication, Surface Chemistry, Coupling Agent, Durability, Adhesion 

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