Patent Application
20170362740
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The present invention relates generally to composite materials. More particularly, the present invention pertains to flexible ceramic fibers, and their use to form composite materials and methods of making the same. Even more specifically, the present invention relates to the fabrication of composite materials comprising flexible ceramics micro and nanofibers and polymers using electrospinning, forcespinning and blowspinning methods.
Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
The following description of the art related to the present invention refers to a number of publications and references. Discussion of such publications herein is given to provide a more complete background of the principles related to the present invention and is not to be construed as an admission that such publications are necessarily prior art for patentability determination purposes.
Composites are materials that are made from two or more constituent materials with significantly different physical or chemical properties that, upon being combined, produce a resulting “composite” material with different characteristics than the constituent materials. The goal in making composites is usually to obtain a material with certain enhanced properties or characteristics when compared to the constituent materials.
The prior art reveals numerous examples of ceramic and polymer composite materials, including different combination rations of a ceramic and a polymer. Ceramic materials used in those composite examples include, but are not limited to: alumina, silica, zirconia, yttrium stabilized zirconia, Titania, and titanium carbide. Polymer materials used in those composite examples include, but are not limited to, polyvinyl alcohol, polymethyl methacrylate and polydimethylsiloxane.
The electrospinning process consists of driving a polymer solution jet through a high electric filed rendering a meso-scale fluid jet into nano-scale fibers. The electrospinning process as a method to manufacture “fine” fibers dates back to the early 1900s work of Morton (U.S. Pat. No. 705,691) and Cooley (U.S. Pat. No. 692,631). Morton's and Cooley's patents, as refined by Formhals (U.S. Pat. No. 2,158,416) in 1939, were marred by then current technology limitations. Consequently, the methods developed by Morton, Cooley and Fromhals did not teach a way to make nanofibers.
In 1995, Soshi and Reneker reintroduced the electrospinning process, as we know it today, by using then available scanning electron microscope (“SEM”) techniques thus resulting in the production of nanofibers. Further, Soshi and Reneker identified numerous applications for electrospun nanofibers in a myriad of fields like structures, textile, membrane and biomedical engineering. (See Sakar, et al., Materials Today, Vol. 13, No. 11, 2010).
Polymer micro and nanofibers fabricated with electrospinning were first reported decades ago. However, the first ceramic nanofibers produced from electrospinning were produced relatively recently in 2003. Nevertheless, those nanofibers were not flexible. Flexible ceramic materials comprising electrospun nanofibers have been previously reported even more recently, starting around 2006 (See U.S. Pat. Publication No. 2006/034948 to Reneker, et al).
More recently, a method to make nanofibers from a wide range of materials has been developed. That method is known as forcespinning. Forcespinning uses centrifugal force instead of electrostatic forces to spin into nanofibers solutions or solid materials (dissolved or melted). Forcespinning emerged as a faster and cheaper alternative to electrospinning. One can make ceramic nanofibers using forcespinning. Another method to fabricate micro and nanofibers of polymers and ceramics is blowspinning as disclosed and claimed in U.S. Pat. No. 8,641,960 to Medeiros. Blowspinning uses pressurized air to spin solutions into nanofibers. However, applicants could not find prior art examples of flexible ceramics being made using forcespinning or blowspinning.
Nonetheless, there are prior art examples of other small thickness flexible ceramics being made by using very thin depositions or growths of ceramic materials (See http://www.enrg-inc.com and http://www.camnano.com).
One of the main objectives of the invention embodied in the present application is to provide free standing, flexible and continuous ceramic films using either electrospinning, blowspinning or forcespinning. That material which is the subject of the present application will be referred to hereinafter as Flexiramics™. Specifically, electrospinning of ceramics normally yields rigid, non-woven mats of ceramic micro and nanofibers. Those mats are not continuous and flake shaped. In addition, a substrate that serves as mechanical support is needed. The present invention overcomes all of those shortcomings of the prior art.
Because of its physical and chemical properties, the free standing, flexible and continuous surface ceramic films, and the composites using the film of the invention embodied in the present application meet or exceed the requirements for many practical, industrial and commercial uses. For example, the material described and claimed herein can be used to: (1) replace the currently used flexible printed circuit boards substrates which are usually made using Polylmide (aka Kapton or PI) or PI with low ceramic fillers; and (2) replace some polymeric protective layers used for cable insulation (polyethylene with aluminum hydroxide filler).
Normally, Applicants work with 3% yttria-stabilized zirconia. However, Applicants have found that silica and Titania, thin layers of several metal oxides including, but not limited to alumina, zinc oxide, and perovskites can also become flexible.
The film composites of the present invention are bendable to a bending radius close to 0 as shown in
Objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings. The objects, advantages and novel features, and further scope of applicability of the present invention will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
A principal objective of the invention embodied in the present application is to make Flexiramics™ and to improve the mechanical strength (tensile) of the Flexiramics™ by using the method disclosed herein to make composite materials with several polymers. The polymers can be thermosets (a curing temperature is needed to create a full polymer cross-linking, typically between 20 and 300° C.) or thermoplastics (a melting temperature is needed to soften and make the polymer fluid, typically between 100 and 400° C.).
Specifically, Applicants have used polydimethylsiloxane (PDMS) as liquid polymer with Flexiramics™ to make composites. Applicants have prepared PDMS/Flexiramic™ composites, ranging from weight ratios of 0.1 to 99.9 of PDMS to Flexiramics™. Applicants achieved the very low ratios by using diluted PDMS precursor solutions. Dilution percentages typically ranged 70 and 90%. The preferred solvents used to dilute the PDMS precursors were toluene and hexane. The viscosity of the resulting diluted solution was typically between 80 to 200 Millipascal per Second (mPa·s).
Generally, to achieve that desired viscosity range, a pre-crosslinking at 60° C. was needed. Due to the low viscosity of the solution, the Flexiramic can be easily impregnated by applying the solution over the top of a sample extended on a flat and rigid surface. This can be achieved using a casting knife or a spray coating gun. Due to capillarity and gravity, the Flexiramic became completely impregnated with the solution.
Next, the polymer was thermally cured by placing the sample into an oven at temperatures between 60° C. and 90° C. The curing step can be achieved at temperatures as low as 20° C. with the only effect being longer curing times. The resulting maintained the desired fibrous structure due to the fact that Applicants applied the PDMS as a thin coating on every individual ceramic nanofiber. In the preferred embodiment of the invention, the coating was in the range of a few tenths to a few hundred nanometers.
Applicants achieved the desired high PDMS/Flexiramic ratios by embedding the Flexiramic in non-diluted PDMS precursor solutions. The non-diluted solutions' preferred viscosity range between 1500 and 15000 (mPa·s). Applicants then casted the non-diluted solutions on flat and rigid surfaces with preferred thickness between 0.1 to 5.0 millimeters (mm).
Next, the Flexiramic was deposited on top of the casted solution, thus allowing the solution to permeate through the entire sample via capillarity forces.
Next, the sample was thermally cured by placing the sample into an oven at temperatures between 60° C. and 90° C. The curing time was directly proportional to the thickness of the sample. For example, at 60° C., the curing time was one (1) hour.
The cured sample comprises a thick PDMS layer on one side (between 0.1 to 5.0 mm), and a thin PDMS layer of few micrometers on the other side, typically from 1 to 50 μm. On an alternative embodiment of the invention, Applicants prepared samples with thicker layers on both sides by casting an extra PDMS precursor layer on top of the thin PDMS layer. Applicants can easily control the thickness of that layer by modifying the viscosity of the PDMS precursor solution. In order to increase the viscosity of the PDMS, short thermal treatments at moderate temperature (between 30° C. to 60° C.) can be performed. Alternatively, Applicants can decrease the viscosity of the PDMS by mixing small amounts of toluene or hexane (1% to 100%) with the PDMS precursor solution.
In yet another alternative embodiment of the invention, polyethylene (PE) was used to prepare composite materials with the Flexiramic. In that embodiment, PE was melted at temperature above its melting point of 135° C. The melted PE was then applied on top of a Flexiramic applying sufficient pressure (typically between 1 to 10 kiloNewtons) for a complete embedding of the PE onto the Flexiramic. This was done using a hot-press melt equipment, which resulted in the application of sufficient pressure. The composite was then allowed to cool down to room temperature resulting different thickness ranging from 0.1 to 5.0 mm. The gradual calibration of the amount of PE results in being able to control the thickness of the layer. Therefore, a wide range of PE/Flexiramic ratios can be achieved.
Another embodiment of the invention comprises the use of polyurethane (PUR) for making composite materials. In that embodiment, the PUR precursor is melted under temperatures above 200° C. The melted PUR precursor is then applied on top of a Flexiramic using a pistol equipped with a slot die head. Next, the resulting PUR precursor/Flexiramic sample is thermally cured inside an oven at 100° C. The resulting composite embodiment has a thicknesses ranging typically from 0.1 to 5.0 mm.
Another embodiment of the invention can be achieved by double side coating after the curing of the first layer. The thickness of the layers can be controlled by adjusting the opening of the slot die, and by manipulating the viscosity of the molten PE by increasing or decreasing the temperature used to melt the polymer. Those controlling steps result in a broad range of PE/Flexiramic ratios that can be predictably modified depending on the application.
Another embodiment of the invention can be obtained by using Polyimide (PI) as the polymeric material for the fabrication of composite materials with the Flexiramic. In order obtain that embodiment of the invention, Applicants dissolved poly(amic acid) in N-Methyl-2-pyrrolidone (NMP) resulting in the precursor solution with typical viscosities of 1000 to 10000 mPa·s. In order to obtain alternative embodiments of the precursor solution, Applicants used solvents like NMP and γ-butyrolactone. The solution was then casted on a flat and solid surface and the Flexiramic was deposited on top, thus allowing the solution to penetrate through the entire sample via capillarity forces. Next, the sample was dried at 80° C. typically for 1 h and then was thermally dried by applying heat up to 300° C. using a hot plate or a furnace, typically for 30 minutes. Upon allowing the sample to cool down, it presents polyimide films on both sides of the Flexiramic, typically ranging from 1 to 100 μm. That thickness can be modified by casting thinner or thicker poly(amic acid) films.
The method of the present invention can be executed using a pistol with a slot die head, as well as other techniques like the doctor blade or the casting knife. The resulting samples were dense but the fibrous structure of the Flexiramic can be maintained by diluting the poly(amic acid) with higher amounts of solvents in order to decrease the viscosity down to a range of e.g. 50 to 300 mPa·s. Then, nanofibers could be individually coated with thin polyimide coatings as described above for the PDMS.
These composite materials can also be prepared with different polymers like polypropylene (PP), polyether ether ketone (PEEK), Polyethylenimine (PEI), cyanate esters, epoxy resins, polyesters, vinyl esters, urea-formaldehyde, allylics, polyphthalamide (PPA), polyphenylene sulfide (PPS) and polytetrafluoroethylene (PTFE). The techniques applied would be the same than before, namely, spray coating, pistol with slot head die, doctor blade, casting knife and hot press melt.
This composite materials retain their flexibility and can be bended to very low bending radius without breaking or being damaged, even when the polymeric content does not even exceed 5%. Additionally, these composites present a great enhancement of the thermic properties as compared with the polymers themselves. For example, the composite made with PDMS catches fire two times slower than freestanding PDMS foil (of the same thickness) when exposed to a methane flame. The composite made with PE can even retard the flame at least twice and up to one order of magnitude more than free standing PE of the same thickness (see video). Furthermore, when the composite material is burning, there is no dripping of any part, preventing the fire to spread. Instead, a protective crust is formed. Another example to illustrate the excellent thermic properties of the composite material prepared with polyimide is that the sample can resist temperatures as high as 500° C. without losing flexibility and flatness when the ceramic content is 25%. Instead, a freestanding polyimide film of the same thickness, starts wrinkling at temperatures around 300° C. or higher because the glass transition of the polyimide is surpassed.
In general, Flexiramics can be used to create bendable composites with higher thermal endurance and better flame retardancy. The weight ratio of ceramic/polymer can range from very low, being a dense polymer film with very low content of ceramic fibers, to very high, being a porous films (non woven) with the ceramic fibers individually coated with polymer.
The Flexiramic-based composite material of the present invention is flexible in a macroscopic scale (as a mat) and at a single fiber scale. The mechanical properties of the material of the present invention can be attributed to several factors:
The composite materials of the present invention comprising non-woven ceramic micro and nanofibers (Flexiramics) and polyimide present optimal thermal stability. At temperature as high as 400-500° C., the composite does not wrinkle nor loses flexibility and therefore, increases the temperature threshold at which it can be used. Additionally, the material is light and has a low density (10-40 g/m2).
The composite materials of the present invention comprising Flexiramic and polyethylene present optimal fire retarding properties. Applicants have found that it takes at least twice as long for that material to start catching when compared with materials of the prior art being used for similar purposes. Additionally, once the material of the present invention starts combusting, no parts drip and the fire can be contained because a crust of the calcined material is formed and held onto the fibers. That crust also prevents the flame from propagating through the material.
The preferred embodiment of the present invention is prepared by the method comprising the following steps:
1. Preparation of a precursor solution, the precursor solution comprising the metallic ions or inorganic polymer (sol) that will form the final metal oxide (ceramic), as well as polymer to increase the viscosity.
2. Spinning the precursor solution by using forcespinning or electrospinning.
3. Annealing the fibers obtained from the spinning process which are not ceramic after the spinning. Instead, the spun fibers are polymeric fibers comprising ionic metal or inorganic polymer.
The accompanying drawings, which are incorporated into and form a part of the specification, illustrate an embodiment of the present invention and, together with the description, serve to explain the principles of the invention. The drawings are only for the purpose of illustrating a preferred embodiment of the invention and are not to be construed as limiting the invention. The scope of the invention can only be limited by specific limitations contained in the appended claims.
Simple sketches that allow one not necessarily familiar with the technical area to which this application pertains to gain a visual understanding of the invention.
