Funding was obtained by Dr. Larry Wadsworth during 2001-2002 from the Army Research Office (ARO) to produce melt blown (MB) thermoplastic polyurethanes (TPUs) for possible protective barriers for soldiers. ARO has focused much interest on developing protective clothing that provides protection while also being lightweight, comfortable, stretchable and affordable. Technical cooperation was also received from the U.S. Army Soldier Biological Chemical Command, Natick, MA (Dr. Heidi Schreuder-Gibson and Phil Gibson) and from Noveon Inc., Cleveland, OH, (Susan Hemphill and Joe Vontorcik), and Noveon also generously supplied the TPU resins. The potential to melt spin TPUs promises important advantages compared to conventional polyurethanes which must be solvent spun to produce fibers. Unfortunately, it is generally known that melt spinning elastic fibers is difficult because of their tendency to retract in the spinline during fiber diameter attenuation. This problem is even more challenging for the MB process since aerodynamic drag forces that attenuate filaments vary markedly through time and spinline position.
Preliminary studies were conducted on the 6-inch MB line at TANDEC and presented at the Joint INDA-TAPPI INTC in Baltimore in September 2001 (1). A series of optimization studies with different TPU resins were then conducted on the 20-inch Accurate Products MB line at TANDEC. We were successful in melt blowing film grades of TPU in the first series of trials, but the average fiber diameters were relatively large, which led us to reduce the MB die orifice from 20 and 18 mils to 14.5 mils and the air knife gap from 60 to 30 mils (with a corresponding die tip setback of 30 mils). Since thermal degradation was not apparent in fibers produced in the first project phase, we also increased melt and air temperatures from 390 ºF to 420 ºF, resulting in mean fiber diameters = 6 µm (2). These findings led to further improvements in small fiber diameters, stronger webs and controllable air permeability with both film grade (Noveon Estane 58245) and fiber grade (Estane 58280) TPU resins (3). The figure below is taken from the presentation at the October 2002 Fiber Society Meeting.

 

As can be seen in the figure, increasing air flow rate rapidly overcame the elongational viscosity, elasticity of the filaments and the effect of gravity (horizontally oriented die) to rapidly attenuate the filaments down to about 8 µm. Then the reduction in fiber diameter to 5 µm is much slower with increasing air flow rate. This work in optimizing MB processing conditions for a family of new TPUs was useful to Moldex-Metric, Culver City, CA, which injected activated carbon granules into MB TPU, under contract with ARO, to produce 145 g/m2 MB TPU webs with 0, 18%, 35%, and 49% added carbon content, as well a 30 g/m2 MB PP plus 73% added carbon. Carbon leakage from these nonwovens was found to be 2-4 times lower for the TPU/carbon fabrics than for MB PP/carbon fabric. In comparison to the carbon containing liners in the older Battledress Overgarment (BDO) and the more recent Joint Service Lightweight Integrated Suit Technology (JSLIST), the MB TPU with 35% carbon was found to have the best overall performance properties in terms of water vapor diffusion, air permeability and protection in terms of organic vapor flow-through and methyl salicylate, a mustard agent simulant.(4).
The recent donation from Shell Chemical Company of the Berkstorff Model ZE40 40MM Twin Screw Extruder in 2002, valued at $250,000, through the efforts of Larry Wadsworth, will provide new opportunities for introducing additives to modify and strengthen MB TPUs and other polymers. The twin screw extruder is expected to be installed during summer 2003 for use with the 20-inch die in the new melt blowing bay under construction at TANDEC.

References

1. Wadsworth, L. C., Q. Sun, D. Zhang, R. Zhao, H. L. Schreuder-Gibson, and P. Gibson, “Process-Properties Study of Melt Blowing Polyurethane for Elastic Military Protective Apparel Garments,” Proceedings, INTC 2001, Baltimore, MD, September 5-7, 2001.
2. Wadsworth, L. C., Y. Lee, R. R. Bresee, H. L. Schreuder-Gibson and P. W. Gibson, “Melt Blown Thermoplastic Polyurethane for Elastic Military Protective Chemical Liners,” Proceedings, Joint INDA-TAPPI INTC 2002, Atlanta, GA, September 24-26, 2002.
3. Wadsworth, L. C., Y. Lee, R. R. Bresee, H. L. Schreuder-Gibson, and P. W. Gibson, “Melt blown and Spunbond Thermoplastic Polyurethanes for Elastic Military Protective Chemical Liners and for Other Possible Military Applications,” Proceedings of Abstracts, The Fiber Society Fall Techical Meeting, Natick, MA, October 16-18, 2002
4. Schreuder-Gibson, H. L., P. Gibson, D. Rivin,, J. Varnum and L. C. Wadsworth, “Direct Incorporation of Granular Carbon into Melt Blown Thermoplastic Polyurethane—an Approach for Lightweight Chemical Protective Fabrics,” Proceedings, 12th Annual International TANDEC Nonwovens Conference, The University of Tennessee, Knoxville, TN, November 19-21, 2002.

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