Brian Spencer and Dr. Anton Bowden
Introduction and Objective
Body armor is one of the most important tools used in the protection of soldiers and other armed forces around the world. Although the current style of armor, using Kevlar fiber and ceramic inserts, is generally effective, it is bulky and heavy and can cause overheating and lack of mobility. Our research attempts to remedy these issues by using shear-thickening fluid (STF) impregnated Kevlar XP combined with corrugated plastic to create and test lighter, more effective body armor.
Materials
Most of the materials used in our research were those used to make the STF-impregnated Kevlar. The fluid was prepared using guidelines established by the developer of the process, Dr. Norman Wagner from the University of Delaware. The materials needed, as published in Dr. Wagner’s study are colloidal fumed silica, ethylene glycol, ethanol, and Kevlar XP. In a later attempt at making STF, aqueous surfacefunctionalized silica and molecular sieves were also used.
Methods
We began our research by conducting many hours of research on body armor, Kevlar, and on how to create a shear-thickening fluid. Though the process to create the appropriate shear-thickening fluid seemed straight-forward, we were met with several challenges during our experimentation. The first hurdle was the fact that the initial process published by Dr. Wagner made use of a silica solution that was no longer available in its exact form. After some research on preparing the solution ourselves, we ordered and attempted to create a solution from which to prepare the desired STF. This proved unsuccessful due to the solution reaching saturation long before the correct proportion of silica to ethylene glycol was attained. We contacted the supplier who had previously manufactured the discontinued solution concerning a suitable substitute and were recommended a similar product that may or may not have been entirely equivalent. We acquired a sample of this solution and used it to prepare a sort of STF, though it did not exhibit the characteristics that we needed in order to create the STF-impregnated Kevlar required for our actual experimentation. In the end, as the novel concept of our project dealt more with the effect of layering the STF-Kevlar between a plastic backing material, we decided to proceed with testing with the substandard STF that we were able to create. The details of our attempts at creating silica-ethylene glycol STF are discussed in the following paragraphs.
In our first attempt at making the STF, we tried to follow the guidelines as established by Dr. Wagner. We used a table top mixer to mix the appropriate amounts of colloidal fumed silica, ethylene glycol, and ethanol. We then placed the solution in the oven to bake out the ethanol and leave the perfect ratio of silica and ethylene glycol. Although at first promising, the end results of this process did not yield a shear thickening fluid; it was obvious we were missing a step or steps somewhere in the process as evidenced by the many problems we were experiencing. First of all, the mixing of the materials was difficult to do and the correct proportions required an immense volume of colloidal fumed silica. In fact, the solution would reach saturation long before the correct proportion of silica to ethylene glycol was attained. Also, when we attempted to bake out the ethanol at the temperature and time specified, we were left with a hard, dry substance. We attempted to contact Dr. Wagner for help but never received a response. After several attempts at this process, and several attempts using derivatives of this process, all yielding the same results, we decided to look for a new way of creating the STF.
After further online research and several conversations with chemistry professors, we decided to try a mixture using a new form of silica, aqueous surface-functionalized silica. Surface-functionalized silica is more soluble than normal colloidal silica and would allow us to pass the previous saturation point that had frustrated us on our initial attempts. After mixing the aqueous solution and ethylene glycol in the correct proportions, we used molecular sieves to remove the excess water from the solution. Conceptually, this would create a STF with the consistency and properties of Dr. Wagner’s experiments. This process and several similar processes, however, yielded undesirable results. Most of our difficulties resulted from the fact that we needed to remove more water than is normally attempted by the use of molecular sieves. Also, when removing the sieves from the solution, it was nearly impossible not to remove at least some of the solution together with the sieves. One professor suggested that we suspend the sieves above the solution in a sealed container so the sieves could absorb the water as it escaped as a vapor into the atmosphere directly over the container. Though a sound idea, the sheer amount of water that would have to be removed using this method made it entirely impractical. After several attempts with similarly unsuccessful outcomes, we began searching for another method of creating the STF.
Upon conducting further online research, we found an article detailing another way of making STF. Using this method, we filled several test tubes with the aqueous silica solution and centrifuged them three times for three hours each time. After each three hour block, we would remove the water from the top of the mixture and replace the removed water with an equal amount of ethylene glycol, eventually ending with the correct silica-to-ethylene glycol ratio. This method proved to be somewhat successful as we were able to create a small, mildly shear-thickening solution. It was obvious, however, that the process we were using was not equivalent to the process that produced the STF created by Dr. Wagner, as we were unable to create a STF with the properties desired for our experiments. As this attempt was as close as we had yet come, we decided to use this STF to conduct our ballistics test.
Using the STF developed by the method described in the previous paragraph, we coated 4 pieces of Kevlar approximately 10cm by 10cm. As a preliminary test, we set up the 4 layers Kevlar against 4 layers of corrugated plastic. This setup was shot with a 9 mm pistol at a distance of approximately 15 feet. The five layers of STF impregnated Kevlar and corrugated plastic showed virtually no resistance to the bullet. The Kevlar layers were then doubled to 8 and shot again. Once again, the Kevlar and plastic offered little resistance against the bullet. In an effort to see how much Kevlar was required to slow the bullet, the layers were doubled again to 16, which, coincidentally, is the number of layers of plain Kevlar normally used in bulletproof vests. This time, the bullet did not penetrate all the way through the Kevlar. Our tests showed that the STF we were able to develop contributed negligibly to ballistic resistance.
Conclusion
We were unable to test the concept we wished to explore as we were unsuccessful in creating the required STF. Had we been able to secure the appropriate STF, testing would have been much more enlightening as to the proof or disproof of the layering concept. Further research should be conducted with the expertise of those more familiar with the molecular mechanics of STF’s and the preparation of such, as the lack of this knowledge was the main source of difficulty and, ultimately, failure in our efforts. We remain confident in the soundness of the ballistics concepts and testing outlined in our proposal until they are proven or disproven by actual testing.