Liquid Armor: Nano Particle Technology

Over the years, scientists and military specialists have continued to work on improving the functionality of body armor. Many of this work has created revolutionary advancements, however, adverse effects such as comfort and mobility of the wearer continue to be a challenge.

Body armor used by military personnel continue to be extremely heavy, inadvertently limiting the mobility and agility of the wearer and often causing injury or even death. There is a great need for high performance body armor with minimum thickness and weight for the wearer.

Liquid body armor is bullet proof and stab resistant armor vests developed by incorporating shear thickening fluid (STF) in the fabric. Liquid body armor is flexible, lighter and thinner (45%) than conventional Kevlar vests. In addition to its use in body armor, STF fabrics have several additional applications including, suspicious package covers, bomb blankets and ankle supports for boots which become rigid on impact.

Figure.1- Comparison of stab resistance for neat Kevlar fiber and liquid armor

Shear thickening fluid (STF) is a non-Newtonian colloidal suspension which behaves non-linearly upon the application of shear stress, i.e. its shear strain increases non-linearly with respect to the applied shear stress. The presence of dispersed nano particles in the solvent enables STF to inhibit shearing of the fibers in the fabric at higher shear rates. Most of the shear thickening colloidal suspensions contain optimum sized nano particles which remain suspended in the solvent (dispersing medium). Solute particles in the STF form hydro-clusters under applied stress to prevent the penetration of bullets and sharp objects. Above a certain shear rate (critical shear rate), there is a sharp increase in the viscosity of the shear thickening fluids due to nano-particle dynamics. Shear thickening is a reversible process which allows the fabric-STF structure to return to its original state after the impact.

Figure.2- Reorientation of particles in shear thickening fluids

High impact resistant fibers are normally used in liquid body armor. Various kinds of combinations are employed for manufacturing the armor but the most common variants are manufactured using:

1. 1
   Fibers impregnated with Nano-powder such as Kevlar, nanofibers, M5 (magelaen) and Dyneema.

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2. 2
   Fibers filled with the mixture of polymer & nano particles

   .

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The following fibers and nano particles are generally used in the development of liquid armor such as Platelets, Tubes, hexagons, Al-Si zeolytes, cubicles, nano coated metal/ ceramic particles and chitosan particles

Figure. 4- Fibers impregnated with STF containing silica nanoparticles (a) Kevlar Fibers (b) Ultra high molecular weight polyethylene (UHMWPE) fibers

STF is mixed with ethanol solvent to form the Shear thickening impregnation fluid. Ethanol improves the capacity of the fibers to absorb the STF colloidal suspension. Kevlar or other fabrics are immersed in the ethanol-STF mixture. After absorption of the solution in the fabric, ethanol is removed by heating the fabric in a convection oven. The texture of the fabric remains unaltered after the application of STF, it looks exactly the same as it was before the treatment.

STF impregnated fabrics exhibit exceptional results for several quality tests. Liquid body armor provides more flexibility and comfort along with enhanced quality and level of protection when compared to other fabrics. Three tests are performed to approve liquid body armor: 1. Stab Test 2. Flexibility Test 3. Ballistic Test

Figure.3- Testing apparatus for liquid armor (a) Stab Test (b) Flexibility test (c) Ballistic test

1. 1
   Stab Test

   .

   Impact properties of STF-fabric structure play an important role in defining the efficiency of liquid body armor. A universal testing machine is used for stab testing of liquid armor.  

   1. The machine includes a spike (impactor), fitted in a movable load cell.
   2. The test sample of the armor is placed under the spike on a backing slab.
   3. The backing slab contains four neoprene sponge layers (5.8 mm each), one polyethylene foam layer (31 mm) and two rubber layers (6.4 mm each).
   4. The impactor is dropped down onto the shear thickened fabric from 30 mm distance at an optimum rate of 500 mm/min.

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2. 2
   Flexibility Test

   .

   The flexibility of the STF-fabric samples is tested through a flexibility test.  

   1. The composite fabric and the load cell of the universal testing machine are connected with a wire.
   2. Tensile force is applied to the wire which forces the fabric to bend at a certain angle.
   3. The angle and displacement of the sample edge are measured to calculate the flexibility of the composite fabric.

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3. 3
   Ballistic tests

   .

   Ballistic tests are performed with fragment simulating projectiles (1.1 g bullets).  

   1. The bullets are assessed in terms of V50 test, which measures the ballistic limit of the bullet.
   2. The ballistic limit is the minimum speed at which a certain projectile completely penetrates the armor of a given thickness and physical properties at a definite oblique angle (MIL-STD-662F, 1997).
   3. The bullet is fired at the composite fabric at a specific speed and the deformation of the fabric is measured from the indentation depth in the backing material (cement-glass material).

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