Adhesives that are blends of commercially available urethane and silicone adhesives have been found to be useful for bonding metal parts that flex somewhat during use. These urethane/silicone adhesives are formulated for the specific metal parts to be bonded. The bonds formed by these adhesives have peel and shear strengths greater than those of bonds formed by double-sided tapes and by other adhesives, including epoxies and neat silicones. In addition, unlike the bonds formed by epoxies, the bonds formed by these adhesives retain flexibility.
In the initial application for which the urethane/silicone adhesives were devised, there was a need to bond spring rings, which provide longitudinal rigidity for inflatable satellite booms, with the blades that provide the booms' axial strength. The problem was to make the bonds withstand the stresses, associated with differences in curvature between the bonded parts, that arose when the booms were deflated and the springs were compressed. In experiments using single adhesives (that is, not the urethane/ silicone blends), the bonds were broken and, in each experiment, it was found that the adhesive bonded well with either the ring or with the blade, but not both. After numerous experiments, the adhesive that bonded best with the rings and the adhesive that bonded best with the blades were identified. These adhesives were then blended and, as expected, the blend bonded well with both the rings and the blades.
The two adhesives are Kalex (or equivalent) high-shear-strength urethane and Dow Corning 732 (or equivalent) silicone. The nominal mixture ratio is 5 volume parts of the urethane per 1 volume part of the silicone. Increasing the proportion of silicone makes the bond weaker but more flexible, and decreasing the proportion of silicone makes the bond stronger but more brittle.
The urethane/silicone blend must be prepared and used quickly because of the limited working time of the urethane: The precursor of the urethane adhesive is supplied in a two-part form, comprising a resin and a hardener that must be mixed. The resulting urethane adhesive has a working time of 3 to 5 minutes. To prepare the urethane/silicone blend, one must quickly add the silicone to the urethane adhesive and mix it in thoroughly within the working time of the urethane.
Once the urethane/silicone blend has been mixed and applied to the bond surfaces, it takes about 2 hours for the adhesive to cure under pressure. However, it takes about 24 hours for the adhesive to reach full strength.
This work was done by Paul D. Edwards of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Materials category.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:
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Refer to NPO-30737, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Urethane/Silicone Adhesives for Bonding Flexing Metal Parts
(reference NPO-30737) is currently available for download from the TSP library.
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Overview
The document is a Technical Support Package from NASA's Jet Propulsion Laboratory, focusing on Urethane/Silicone Adhesives for Bonding Flexing Metal Parts, referenced as NPO-30737 in NASA Tech Briefs. It aims to disseminate the results of aerospace-related developments that have broader technological, scientific, or commercial applications.
The document outlines the significance of urethane and silicone adhesives, particularly in applications where flexibility and strength are crucial, such as in aerospace engineering. These adhesives are designed to bond metal parts that experience flexing, which presents unique challenges due to the differing properties of the materials involved. The document emphasizes the importance of developing adhesives that can withstand the stresses and strains associated with flexing components, ensuring durability and reliability in critical applications.
A notable innovation discussed is the creation of "Edwards Silicon," a new compound formed by mixing two existing silicone adhesives: Dow Corning 732 Multi-Purpose Sealant and Kalex Urethane adhesive. This compound exhibits an exceptionally strong semi-flexible bond between metallic surfaces, outperforming individual adhesives and other tested silicone options. The development was motivated by the need to bond a metal spring ring to a carpenter blade with different curvatures, a task that traditional adhesives struggled to accomplish due to bond failure under stress.
The document details the experimental process that led to the successful formulation of Edwards Silicon. It highlights the necessity for adhesives to possess high peel and shear strength while maintaining flexibility. Through rigorous testing, it was found that combining the best bonding properties of two silicones resulted in a superior adhesive capable of withstanding the mechanical demands of the application.
Additionally, the document provides information on further resources available through NASA's Scientific and Technical Information (STI) Program Office, including contact details for assistance. It also includes a disclaimer regarding the use of the information provided, clarifying that the U.S. Government does not assume liability for its application.
Overall, this Technical Support Package serves as a valuable resource for understanding the advancements in adhesive technology, particularly in the context of aerospace applications, and encourages further exploration of these innovations for commercial use.
