Multicomponent fibers contain two or more distinct polymers within their cross section. They have been tailored to suit esthetic and functional requirements for numerous applications. For example, the ‘islands in the sea’ cross section, where a polymer is fed in individual streams into a ‘sea’ composed of another polymer (see Figure 1), has been studied extensively.1 The sea polymer is subsequently dissolved, generally after the fibers have been knit or woven into fabric. A typical island-to-sea ratio, e.g., of high-strength, 50nm-diameter nanofibers, is 80:20.
Figure 1.Cross section of bi-component ‘islands in the sea’ fibers produced through spinning.
In a multicomponent fiber extruder, the fiber cross sections are created by forcing melted polymers through an ensemble of plates called a ‘spin pack.’ Spin-pack hardware components have historically been manufactured by conventional methods such as milling and/or drilling. Alternatively, more modern systems use techniques similar to those used in printing circuit boards. Spin-pack components can very accurately distribute polymers in the extremely small area available to produce high-resolution cross-sectional features.
There is general interest in developing fiber geometries that can provide a number of functionalities—such as a high-strength textile fiber that can resist wear and tear—and at the same time exhibit a useful optical response. Consider a textile made from a highly reflective fiber. It would provide a first responder with a measure of safety for use at night and help others identify the wearer. It is also conceivable that the fiber could function as a sensor to identify the presence of dangerous chemicals that may be present at an emergency scene. The high degree of geometric control available in multicomponent-fiber extrusion allows realization of such fibers.
At our High Performance Fiber Facility, we recently used our tricomponent fiber extruder to create a directionally reflective fiber with retro-reflective properties (see Figure 2).2Its cross section is comprised of a series of right angles. These are retroreflection features that run along the entire fiber length. This right-angle feature has the property of directing light back to the source as well as refracting light, similarly to a prism. In addition, multiple reflections occur at the fiber surface, thus enhancing the response of a photoreactive material that may be located at the feature surface.
Figure 2.Cross section of directionally reflective, bi-component fiber. The colors represent the two polymers used. TYP: Typical.
An essential first step in the fiber-fabrication process was to identify polymers that are thermally, chemically, and mechanically compatible. In addition, to maximize the fiber's reflective properties, the indices of refraction of the two polymer components had to be as different as possible. This maximizes the degree of reflection that occurs at the 90° feature interface. We identified polypropylene, polyester, and polyvinyl alcohol as acceptable materials and used them to fabricate a retro-reflective fiber (see Figure 3).
Figure 3.Scanning-electron-microscope image of retro-reflection fiber cross section (Diameter: 390μm).
We studied its optical response by first wrapping it around a nonreflective planar surface, followed by illuminating the fiber bundle with a laser, and observing the back-reflected light pattern (see Figure 4). The latter clearly shows a series of discreet lines resulting from reflection and refraction from individual fibers within the bundle. The results indicate that the fiber responds as designed. In addition, we have shown that the fibers can be drawn to further reduce their diameters, while still maintaining the 90°retro-reflective features.
Figure 4.The well-defined lines within the back-reflected light pattern from the retro-reflection fiber are indicative of reflection and refraction from optically planar surfaces.
Future work will be undertaken to spin the fibers into yarns and then weave them into a textile swatch that can be further evaluated for its optical response. The multicomponent-fiber-extrusion process provides an exciting opportunity to reconsider the nature of fiber-based textile systems. Using this highly flexible process, we intend to introduce additional functionalities into our directionally reflective fiber to create fabrics and textiles with unprecedented multifunctional properties.
Brian R. Kimball, Francisco Aranda, Deana Archambault, Lauren Belton, Joel Carlson, David Ziegler
Nanomaterials Science Team US Army Natick Soldier Research, Development, and Engineering (RD&E) Center
Brian Kimball is a former Wyman-Gordon Foundation Fellow at Worcester Polytechnic Institute, a 2005 Nano-50 Award winner, and twice the recipient of the Department of the Army Research and Development Achievement Award.
US Army Natick Soldier RD&E Center
Fiber Processing and Technology Team/Warfighter Science, Technology, and Applied Research (WarSTAR)
2. F. J. Aranda, J. Perry, D. Archambault, L. Belton, J. Carlson, D. Ziegler, B. Kimball, Optical properties of a retro-reflection fiber cross section formed via tri-component fiber extrusion, Proc. SPIE
7781, pp. 778107, 2010. doi:10.1117/12.859298