By Jan H. Schut
Imagine 4000 nanolayer film. Now imagine stacking 200 pieces of it to make 800,000 nanolayer sheet. What would that look like? What could that do? Micro layer coextrusion is a powerful product development tool, but commercializing it isn’t easy. Commercializing microlayers has consumed nearly 40 years of R&D since the mid ‘60s when Dow Chemical Co. (www.dow.com) invented the first layer-multiplying feedblock (U.S. Pat. # 3239197).
3M Co. (www.3m.com) bought layer-splitting technology from Dow (U.S. Pat. # 5094788) and developed a portfolio of highly successful +500-layer light-enhancing films for windows and electronics. Nike Inc. (www.nike.com) took another big step forward with 76-layer nitrogen barrier bladders for running shoes (U.S. Pat. # 6082025). Only three years ago the first commercial micro layer packaging films emerged for down-gauged stretch and shrink films. These notably include the first commercial micro-layer blown film from Cryovac/Sealed Air (www.cryovac.com) with 29 layers (see June 2011 blog).
Throughout those nearly 40 years of product development Case Western Reserve University’s polymer program (www.case.edu) also generated a steady stream of intriguing micro layer materials research under professors Eric Baer and the late Anne Hiltner. The Case program often partnered with Dow, but built its own unpatented layer-multiplier, simpler than Dow’s but arguably more flexible. Where Dow’s device splits an extruded tape into four equal strips and stacks them, quadrupling even layers, Case’s device splits the extrudate in two, either equally or unequally. Besides research simplicity, this binary split allows different micro layer thickness profiles for specific functions.
The Baer/Hiltner research group generated dozens of micro layer patents over the years, some for Case, some for Dow and other partner institutions. The group also authored around 100 scholarly papers on micro-layer technology out of about 600 polymer research papers in all. Many of the micro-layer papers were published by the Society of Plastics Engineers’ annual ANTEC conferences. SPE’s Thermoplastic Materials and Foams division also gave Hiltner its annual outstanding achievement award in 2004, while Baer was elected to the Plastics Hall of Fame in 2000. But until very recently none of all the Case work was close to commercial.
That’s changing. Starting in 2006 the National Science Foundation set up a special CLiPS (Center for Layered Polymeric Systems) program headed by Case (stc-clips.org), but also including the University of Texas at Austin; Fisk University in Nashville; the University of Southern Mississippi in Hattiesburg; and the U.S. Naval Research Lab on the Chesapeake. With annual funding of $4 million/year for 10 years, CLiPS’s goal is to commercialize promising micro layer technologies like 800,000 nanolayer sheet, which makes powerful, light-gathering lenses. The initial line-up also includes the first roll-to-roll data storage film and improved dielectric films for capacitors.
4000-NANOLAYER FILM FOR 800,000 NANO-LAYER SHEET
The first commercial enterprise to emerge from the CLiPS incubator is PolymerPlus LLC in Valley View, Ohio (www.polymerplus.net). Set up in 2010, it has 10 employees and already makes 800,000-nano-layer composite sheet on a prototype fabrication line in a 10,000-level clean room for test lenses for the U.S. military. PolymerPlus’s patented technology (U.S. Pat. # 7002754), licensed from Case, uses a binary micro-layer feed block to create a profile of nano and micro layers for what Case calls GRIN (gradient index optics) technology.
GRIN alternating layers are PMMA (polymethyl methacrylate) and SAN (styrene acrylonitrile) copolymer with different refractive indexes—1.49 for PMMA and 1.57 for SAN. The nano-layered film’s refractive index can be tuned to intermediate values between 1.49 and 1.57 by altering relative nano layer thickness.
PolymerPlus’s prototype line has three extruders. Two coextrude a two-layer tape of PMMA and SAN, which is then split and recombined 11 times in succession to make 14-in. wide film with over 4000 micro layers—4096 to be exact. Layers range between 10 and 40 nanometers thick. A third extruder adds a protective peel layer of LDPE in the die to prevent disruption of the surface layers.
The peel layers are removed, and up to 200 individual 4096-layer films are stacked together to make a single composite sheet with up to 819,200 nano layers. A loose stack of 200 films may be 7 to 15 millimeters high, depending on the desired GRIN optic thickness. Case literature describes rotating micro-layer films randomly when stacking them to reduce birefringence in the lens sheet. A Case paper published in Optics Express in 2008 says that “the (refractive) index profile is determined by the order in which the individual films are stacked.” The patent, for example, shows micro-layer films stacked with wider layers in the middle graduating to narrower layers on the outside.
The stack of film is then compressed in an autoclave to create 3-5 mm thick sheet. The 800,000-layer sheet can be cut and thermoformed into lenses for night vision goggles for the military or molded between glass mold halves to form solid light focusing rods or cylinders for solar concentrators, optics and imaging systems. The patent even describes A-B-C micro-layer structures with three alternating layers of different refractive indexes.
Since setting up the prototype fabrication line, PolymerPlus has made over 200 prototype lens sheets for testing. When the process is commercial by late 2013, the company expects to make nano-layered optic film up to 3 feet wide with outputs of about 500 sheets a month. PolymerPlus reports that its lens technology focuses 3.5 times more sharply than glass lenses and weighs a lot less. Controlled gradient lens material also allows variable focal length viewing from a single lens, which isn’t possible with glass.
As difficult as it is to produce, the 800,000 layer sheet represents a big manufacturing efficiency. It can be used to mold and mill a variety of lenses of different sizes for different systems, speeding development of new products, rather than having each application require custom lens molds and tooling. The lenses are expected to find other applications in night vision optics and cameras for unmanned aircraft because of their light weight.
MICROLAYER DATA STORAGE AND DIELECTRIC FILMS
Two other technologies from Case are on the commercial horizon, but without clear timelines. They include optical data storage films for DVDs and dielectric films for capacitors, under development with separate partnerships at Case. Roll-to-roll production of an optical data storage medium could be dramatically less expensive than current technologies. Micro-layer capacitors would be more expensive, but potentially more efficient and more reliable than current technologies.
Micro-layer optical data storage film has the potential to increase storage beyond current Blu-Ray discs, which hold about 50 gigabytes of image data, to terabyte capacity. Current Blu-Ray discs have only two layers, a spin-coated organic dye layer and a reflective metal layer, sputtered on. Case’s patent-pending micro-layer coextruded film technology can produce 128-layer optical data storage film with 64 potential active storage layers. Currently, optical data can be read on 23 of those 64 layers—the most ever reported. Images could be read on the 23rd layer even after film had been stored for two years at ambient temperature. “Hardware under development will allow writing in many more layers,” says Case professor Kenneth Singer, a physicist rather than a polymer chemist.
Micro layer optical data storage film uses an active layer of PETG doped with an organic florescent dye (Chromophone C18-RG) paired with an inactive buffer layer of PVDF, a high gas barrier polymer. PETG data storage layers are 0.3 nanometers thick; PVDF protective layers are ten times thicker at 3.1 nanometers. PETG and PVDF are coextruded at the same temperature, 230 C, at which they have the same viscosity. The developmental 64-active-layer film is 200 nano meters thick and co-extruded with a protective peel layer.
One innovative part is coextrusion of nano layers with dye next to micro layers without dye without having the dye bleed through. This is done by using polymers with different solubility and builds on previous work incorporating fluorescent dyes in coextruded micro layers for all-polymer lasers (U.S. Pat. # 7936802 and 8144744). But the optical data storage application now has commercial priority over lasers. In mid June the optical data storage research program received a grant of $50,000 from ICORPS (innovation corps) from the National Science Foundation to help start a company in the near future. As added incentive, the grant expires December 31, if they haven’t.
Micro-layer capacitor films under development at Case are also tagged by CLiPS as commercially promising. Capacitor films act like a battery to hold and release a charge quickly, and are used for electrical noise reduction and filtering in most electrical equipment. The patented micro layer dielectric film developments (U.S. Pat. Applic. # 20100172066) are funded by the U.S. Naval Research Laboratory, but belong to Case.
The technology is intended to make films with higher energy density and lower energy losses than current state-of-the-art capacitor films. Current capacitors use a PP or PET film substrate, which is either metalized with vapor metallization or laminated to aluminum foil. Micro-layer coextrusion would be more expensive, but potentially offers higher energy density and greater reliability.
Developmental micro-layer capacitor films use alternating layers of PC (polycarbonate) and PVDF-HFP (poly vinylidene fluoride-co-hexafluoropropylene). PVDF-HFP is the active layer because of its very high dielectric constant of 12, but PVDF-based polymers have low “breakdown” strength. Electrical breakdown strength means resistance to forming holes–the point at which the capacitor is no longer insulating and storing electricity, but passing current through. PC has very high break down strength, but relatively low dielectric constant of 3 vs. PVDF’s 12.
Combining the two materials could make film with low energy dissipation and high break down strength. Case researchers tested film with both blends and coextrusion of the two materials, testing from 2 up to 256 alternating layers and found a sweet spot. The optimum number and thickness of active layers was 32 layers of PVDF-HFP 350-nanometer-thick. Commercial partnerships are pending.