Description:
OBJECTIVE: Mechanical closures of the hook and loop type used in Army uniforms are the critical sources of leaks in protective clothing/equipment, limiting the protective capability of the ensemble. To address this problem, new closure systems need to be developed to provide both the macroscopic adhesion strength obtainable from the hook and loop closures while also allowing for hermetic sealing against any vapor permeation through the closure. No existing type of closure systems can accomplish these objectives and new concepts need to be developed. This topic addresses the technical challenges and innovative solutions sought to achieve a hermetic sealing closure system for protective clothing ensemble. DESCRIPTION: Chemical protective fabrics used in clothing are designed to be impervious to chemical and biological agents while allowing for thermal comfort to the wearer by permitting moisture transport. The chemical protective nature of the garment ensemble is however compromised by the use of conventional mechanical closure of the hook and loop type (Velcro is a commercial example), which has macroscopic contact regions through which significant gas transport is possible. Different types and classes of hook and loop closures are used in Army uniforms and the specifications are described in the General Services Administration commercial item description A-A-55126B [1]. The typical minimum peel strength is in the range 0.5 to 1.0 lbs/inch (75 to 150 N/m). The typical lap shear strength is in the range 5 to 30 lbs/sq. inch (35 to 210 kPa). While there are other properties that can be important, this solicitation will be focused on adhesive strength as indicated above, measured by the ASTM peel strength and lap shear strength measurement methods. One may speculate that adhesive contact surfaces with nanoscopic roughness may provide larger resistance to gas transport, compared to the hook and loop system with macroscopic roughness. Further, the possibility of generating significant adhesion between surfaces possessing nanoscale contacts has been recognized from a study of biological systems [2,3]. Based on this concept, one may consider coating polyelectrolyte multilayers on substrates since the multilayers have numerous polymer contact elements in the nanoscopic range and can provide adhesion between surfaces [4]. In a recent study the adhesion and hermetic sealing of such a multilayer closure was investigated [5]. This study showed that the resistance to air flow through the multilayer closure system is approximately 20-800 times larger than that possible with conventional hook and loop type closure systems, at all humidity levels (from 5 to 95% relative humidity), as measured by the Dynamic Moisture Permeation Cell (DMPC) apparatus [6]. However the adhesive strengths of the polyelectrolyte multilayer closure systems evaluated in this study are an order of magnitude smaller than the hook and loop closure and therefore the multilayer system as developed cannot be employed for closure application. As shown in these studies, new closure concepts are possible and they are the focus of this topic. Some obvious approaches built on the work described here are as follows: One option is to develop polymer systems that can provide significantly large adhesive energies while maintaining hermetic sealing. One recent study shows that moisture retention in the multilayers leads to significant increase in the lap shear strength [7]. Another approach to achieving large adhesive strength based on surface patterning has recently been proposed, inspired by biomimetics [8, 9]. Another option is to use the hook and loop system for providing the adhesion strength and integrate it with another system such as the multilayer to achieve hermetic sealing. Entirely new approaches can be considered as well. PHASE I: Conduct research on novel concepts for closure system to achieve both hermetic sealing and minimum adhesion strength needed. Upon completion of Phase I, samples of the closure system developed on any flexible substrate should be made available for independent evaluation. The system should show peel strength and lap shear strength in the range specified under Description and an air flow resistance at least 100 times larger than that from the typical hook and loop system (with their backs sealed to prevent air leakage) over the humidity range of 5 to 95% RH, as measured by the DMPC. The hook and loop system for the comparative study will be identified by the Technical POC. PHASE II: The closure system developed should be integrated with a fabric such as NYCO. It should be possible to produce the integrated closure system in large scale. At the end of Phase II, closure samples on a swatch of fabric should be made available for independent evaluation. The closure samples should provide the adhesion strength and air flow resistance specified under Phase I goals over the entire relative humidity range and these properties must be retained after laundering at least 3 times under military laundering conditions. PHASE III: A closure system demonstrated in Phase II successfully should be integrated into chemical and biological protective clothing ensemble. PHASE III DUAL USE APPLICATIONS: First responder and anti-terrorism personnel would also benefit from use of improved protective garments providing hermetic sealing. In addition to protective clothing, other applications such as for respirator face seals, vehicle doors and windows, and attachment of tent and other shelter modules are possible. Further, the closure can be used for applications such as face masks in hospitals, schools, and other buildings when high levels of protection from indoor contaminated air are desired. REFERENCES: 1. Commercial Item Description: Fastener Tapes, Hook and Loop, Synthetic, A-A-55126B, September 7, 2006 2. Tian Y, Pesika N, Zeng H, Rosenberg K, Zhao B, McGuinnen P, Autumn K, Israelachvili JN, Adhesion and friction in gecko toe attachment and detachment, PNAS 103, 19320-19325 (2006) 3. Arzt E, Biological and artificial attachment devices: lessons for material scientists from flies and geckos. Materials Science and Engineering C, 26, 1245-1250 (2006). 4. Gong H, Garcia-Turiel J, Vasilev K, Vinogradova OI, Interaction and adhesion properties of polyelectrolyte multilayers. Langmuir 21, 7545-7550 (2005) 5. Marcott SA , Ada S, Gibson P, Camesano TA, Nagarajan R, Novel application of polyelectrolyte multilayers as nanoscopic closures with hermetic sealing, J ACS Appl. Mater. Interfaces 4, 1620-1628 (2012) 6. Gibson P, Rivin D. Cyrus K. Convection/diffusion test method for porous materials using the dynamic moisture permeation cell. Natick Technical Report; Natick/TR-98/014, 1997. 7. Matsukuma D, Aoyagi T, Serizawa T, Adhesion of two physically contacting planar substrates coated with layer-by-layer assembled films. Langmuir 25, 9824-9830 (2009) 8. Chan EP, Greiner C, Arzt E, Crosby AJ, Designing Model Systems for Enhanced Adhesion, MRS Bulletin, 32, 496-503 (2007) 9. Boesel LF, Greiner C, Arzt E, del Campo A, Gecko-inspired surfaces: A path to strong and reversible dry adhesives, Adv. Mater. 22, 21252137 (2010)
