Special-purpose balloons and other inflatable structures would be constructed as flexible laminates of multiple thin polymeric films interspersed with layers of adhesive, according to a proposal. In the original intended application, the laminate would serve as the envelope of the Titan Aerobot — a proposed robotic airship for exploring Titan (one of the moons of Saturn). Potential terrestrial applications for such flexible laminates could include blimps and sails.
In the original application, the multi-layered laminate would contain six layers of 0.14-mil (0.0036-mm)-thick Mylar® (or equivalent) polyethylene terephthalate film with a layer of adhesive between each layer of Mylar®. The overall thickness and areal density of this laminate would be nearly the same as those of 1-mil (0.0254-mm)-thick monolayer polyethylene terephthalate sheet. However, the laminate would offer several advantages over the monolayer sheet, especially with respect to interrelated considerations of flexing properties, formation of pinholes, and difficulty or ease of handling, as discussed next.
Most of the damage during flexing of the laminate would be localized in the outermost layers, where the radii of bending in a given bend would be the largest and, hence, the bending stress would be the greatest. The adverse effects of formation of pinholes would be nearly completely mitigated in the laminate because a pinhole in a given layer would not propagate to adjacent layers. Hence, the laminate would tend to remain effective as a barrier to retain gas. Similar arguments can be made regarding cracks: While a crack could form as a result of stress or a defect in the film material, a crack would not propagate into adjacent layers, and the adjacent layer(s) would even arrest propagation of the crack.
In the case of the monolayer sheet, surface damage (scratches, dents, permanent folds, pinholes, and the like) caused by handling would constitute or give rise to defects that could propagate through the thickness as cracks or pinholes that would render the sheet less effective or ineffective as a barrier. In contrast, because damage incurred during handling of the laminate would ordinarily be limited to the outermost layers, the barrier properties of the laminate would be less likely to be adversely affected. Therefore, handling of the laminate would be easier because there would be less of a need to exercise care to ensure against surface damage.
For the Titan Aerobot, the laminate is required to retain its physical properties (especially flexibility and effectiveness as a barrier) to a sufficient degree at temperatures as low as that of liquid nitrogen. To evaluate this laminate and other candidate materials, a flex testing apparatus (see figure) has been used to repeatedly flex samples of the materials with a 45° twist and a 2-in. (≈5-cm) compression while the samples were immersed in liquid nitrogen. After having been flexed a set number of cycles, samples were examined by use of an apparatus that can easily detect gas leaks from through pinholes as narrow as 10 µm in diameter. In this test, a six-layer polyethylene terephthalate laminate as described above survived more than 3,400 flex cycles in liquid nitrogen without developing through pinholes — performing significantly better than did a monolayer polyethylene terephthalate sheet of equivalent overall thickness.
To evaluate these materials for utility as terrestrial balloon materials, the flexing and pinhole tests were performed at room temperature. As in the liquid-nitrogen tests, the laminate performed better than did the monolayer sheet. In a contemplated improvement on the basic laminate design, a layer (or layers) of reinforcing fabric would be laminated with the layers of polymeric film and layers of adhesive. At the time of reporting the information for this article, evaluation of candidate materials for use in such fabric augmented laminates was in progress.
This work was done by Andre Yavrouian, Gary Plett, and Jerami Mannella of Caltech for NASA’s Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free online at www.techbriefs.com/tsp under the Materials category. NPO-40636
This Brief includes a Technical Support Package (TSP).
Multi-Layer Laminated Thin Films for Inflatable Structures
(reference NPO-40636) is currently available for download from the TSP library.
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Overview
The document discusses the advancements in multi-layer laminated thin films for inflatable structures, particularly focusing on a novel hexa-laminate of Mylar® proposed for the Titan Aerobot, a robotic airship designed to explore Titan, Saturn's largest moon. Developed by NASA’s Jet Propulsion Laboratory, this multi-laminate structure offers significant advantages over traditional monolayer films of similar thickness.
Key benefits of the hexa-laminate Mylar® include improved flexing properties, better barrier performance, reduced crack propagation, and less pinholing during handling and use. The design localizes damage to the outermost layer, which minimizes the risk of defects propagating through the entire material. This characteristic is crucial for maintaining the integrity of the envelope material, especially under extreme conditions, such as the liquid nitrogen temperatures required for the Titan Aerobot.
To evaluate the performance of the hexa-laminate, a modified flex testing apparatus was employed, allowing for rigorous testing of the material's durability under repeated flexing and compression while submerged in liquid nitrogen. The hexa-laminate demonstrated exceptional resilience, surviving over 3,400 flex cycles without developing pinholes, significantly outperforming equivalent thickness monolayer Mylar® films.
The document also highlights the ease of handling associated with multi-laminate materials. Damage during handling, such as scratches or dents, is confined to the outer layer, preventing the creation of defect sites that could compromise the overall barrier properties of the material. This allows for less stringent handling procedures compared to monolayer films, which are more susceptible to damage that can lead to failure.
Future developments aim to enhance the multi-laminate films by laminating them with reinforcing fabrics, further improving their structural integrity and performance. The selection and evaluation of the best materials for this purpose are currently underway.
Overall, the document emphasizes the potential of multi-layer laminated thin films to revolutionize inflatable structures in aerospace applications, providing enhanced durability, flexibility, and reliability, which are essential for missions in extreme environments like Titan.
