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Robust, Low Permeability, Water-Filled Microcapsules



OBJECTIVE: Identify and develop a long-lasting (i.e., mechanically robust, withstand temperatures from -40°F to 135°F without rupture, and have low to zero permeability for 20+ years) dry powder of water-filled microcapsules that can be mixed into viscous pre-polymer liquids without breaking, and remain intact after rubber curing and fuel bladder manufacturing and maintenance, but will break and release their contents when mechanically shocked (i.e., shot with a .50 caliber bullet). Enable the incorporation of the microcapsules into a self-sealing material for fuel bladders solution under development by Naval Air Systems Command (NAVAIR). 

DESCRIPTION: The Navy currently needs to develop robust and long-lasting water-filled microcapsules for a self-sealing material that is activated when it comes into contact with water. For the desired fuel bladder application, no external water source will be required to activate the material after the fuel bladder has been penetrated. Current water-filled microcapsules contain water for days to weeks before the water permeates the microcapsule shell; however, fuel bladders need to be able to self-seal during 20 years of fleet usage. Microencapsulation of liquids is a process used in many industries (e.g., coatings, pharmaceuticals, cosmetics, consumer goods, agriculture); however, microencapsulating water has been a challenge due to the small size of the water molecule that causes it to permeate through the shell of the microcapsule at room temperature. For this project, the microcapsules should contain greater than 80% water by volume as determined by microscopy or a chemical reaction of a ruptured microcapsule and the particle size (in the range of 50-200 microns in diameter) needs to be tightly controlled and nearly monodisperse,(i.e., a coefficient of variation (standard deviation of particle diameter divided by mean particle diameter) of 3% or less) and analyzed with a particle analyzer for each batch of material produced, in order to provide consistent breaking strength from batch to batch (a coefficient of variation (standard deviation of breaking strength divided by mean breaking strength) of 5% or less. The breaking strength range of the microcapsules should be characterized at room temperature (75 degrees F), -40 degrees F, and 135 degrees F by a suitable method, such as Atomic Force Microscopy (AFM) with the load-deflection curves, rupture force and deformation, and Young’s Modulus as significant outputs of the analysis. A large majority of the microcapsules (>99%) must remain intact when exposed to a temperature range of -40 degrees F to 135 degrees F, which is the temperature range of the crashworthy and self-sealing fuel bladder specification MIL-DTL-27422F gunfire test. This may be evaluated by temperature exposure followed by Scanning Electron Microscopy (SEM) and statistical evaluation of the images (i.e., ratio of ruptured to intact microcapsules). The water-filled microcapsules must be resistant to rupture during mixing in viscous (in the range of 100 to 25,000 mPas) pre-polymers, so a lab scale mixing test must be developed where the ratio of ruptured microcapsules before and after mixing is evaluated, again likely through SEM. The microcapsules must be resistant to JP-5 and JP-8 fuel, so their mechanical strength should be evaluated by AFM or other suitable means after exposure to fuel. The composite of the microcapsules with the cured polymer will need to withstand lab scale handling tests including a fold over test (180-degree bend) to simulate a bladder being folded during installation into an aircraft according to MIL-DTL-27422F paragraph Stress Aging; an Impact Resistance drop test (1 lb. blunt chisel dropped from several heights) to evaluate small mechanical shocks according to MIL-DTL-27422F Paragraph 4.5.6 Impact Resistance; and quasi-static compression (up to 300 lbs./inch) to simulate maintainers walking and crawling on fuel bladders. An accelerating aging test (see MIL-STD-810G section 520.3 Temperature, Humidity, Vibration, and Altitude and section 524 Freeze/Thaw) will need to be developed to determine the likelihood of the microcapsules surviving in the fuel bladder for 20+ years. This will likely need to consist of thermal cycling and mechanical bending and compression cycling tests, while looking for activation of the microcapsule/polymer composite (i.e., the self-sealing material). An evaporation test will need to be performed by monitoring and tracking the weight loss of several ounces of microspheres in a drying oven over the course of weeks to months to determine a water evaporation rate. The microcapsule/polymer composite will need to be incorporated into a MIL-DTL-27422F Phase I test cube to undergo gunfire testing. Since the construction of a fuel bladder test cube is not a trivial task, it is recommended that the proposer partner with a fuel bladder manufacturer in the final stages of this project. 

PHASE I: Define and develop a concept for a microcapsule that will meet the Description above and determine the feasibility of producing a prototype both at lab scale and on a large scale. Develop concepts for robustness (mechanical, thermal, chemical), longevity (accelerated aging and evaporation), and shock tests. Develop a concept for incorporating the microcapsules into a polymer matrix. The Phase I effort will include prototype plans to be developed during Phase II. 

PHASE II: Develop, on a lab scale, prototypes of the microcapsule and microcapsule/polymer composite based on concepts developed in Phase I. Implement the test concepts using them as quality checks on the products in the lab scale process. Refine the tests, as required. Once the lab scale process for producing the microcapsules and the microcapsule/polymer composite and all of the tests are mature, generate a concept for large scale production of the microcapsules and microcapsule/polymer composite along with a concept for integration of the microcapsule/polymer composite into a fuel bladder. 

PHASE III: Produce fuel bladders or partner with another company that already produces fuel bladders so that the microcapsule/rubber composite self-sealing layer may be evaluated in a Phase I (not STTR Phase I) fuel bladder cube gunfire test according to MIL-DTL-27422F. After passing the gunfire test, incorporate the self-sealing technology into a fuel bladder production process to bring the technology to the fleet. Successful development of this technology could benefit fuel bladder manufacturers by giving them the ability to meet the fuel bladder self-sealing requirements. The microcapsules could be used in the pharmaceutical and consumer products industries for encapsulating aqueous medicines, activating water curing adhesives like cyanoacrylate (superglue), and self-healing coatings. The self-sealing layer that incorporates the water filled microcapsules can be used in other self-sealing applications, including pneumatic tires and inflatable rafts and life vests. 


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KEYWORDS: Microcapsule; Microsphere; Microencapsulation; Dry Water; Nanosphere; Powdered Water 

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