#White Papers
Transparent composites
I was intrigued when I ran across Patently Apple’s posting, “Apple Exploring new Transparent Fiber Composite Materials for Future Devices Including Wearable Computers”
The site relates that Apple Inc. (Cupertino, Calif., USA) has been experimenting with composites since 2007, and has now invented a method for manufacturing relatively transparent fiber reinforced plastic structures.
Interestingly, Mo-Sci Corp. (Rolla, Mo., USA) received a 2007 SBIR research funding award for “High-Strength and Optically Transparent Fiber-Reinforced Composites”, which describes “a need for mechanically strong composite materials of high optical quality and transparency equivalent to window glass” and achieving this “by layering a polymer matrix reinforced with glass ribbons (micron-size glass fibers with rectangular cross section) and a tough compliant polyurethane film.” Apparently Mo-Sci had already licensed technology from U.S. Patent 5,665,450 awarded to the University of Missouri and produced research quantities of glass ribbon-reinforced epoxy for high-strength window applications. The patent shows the technology’s foundation of prior art and accompanying issues, which include difficulty in matching the refractive index of materials due to variations with temperature. The Army Research Lab report, “Transparent Composite Utilizing Nonlinear Optical Polymers” specifically noted that “index-matched systems are transparent only over a narrow temperature range.”
In its application US 20140030522 A1, Apple describes matching the refractive index of the glass fiber, the sizing applied to the fiber and the resin matrix which the fiber reinforces so that the difference between them is less than .005. It also describes forming a transparent fiber-resin composite by injection molding chopped glass fibers ranging in length from 0.25 to 0.50 inch (6 to 13 mm), which can be mixed into the resin prior to injection. This method is said to enable high volume production and even distribution of the fibers in the resin. The patent does NOT disclose how Apple has overcome the RI matching challenges faced by previous developers.
A search on “transparent composites” reveals many different approaches, including bacterial nanofibers, plant nanofibers, chitin particles and polymer nanofibers, and fibers made from polymer ribbons. Transparent composites have also been made using hollow nanofibers electrospun from nylon and polyacrylonitrile (PAN), the common precursor for carbon fiber). The nylon is embedded into an epoxy matrix and the PAN into poly methyl methacrylate (PMMA). Applications claimed include protective armor and aircraft windows.
