Patent Application
20120216975
Non-woven mats are used in many applications, such as battery separators that may be placed between electrodes in various types of batteries. In some battery applications, very thin mats may be useful to increase battery performance. Other applications may also benefit from non-woven mats as well.
A non-woven mat may be generated using synthetic wood pulp and short glass fibers. The non-woven mat may be made in very thin, light weight configurations for various uses. The synthetic wood pulp may serve as a dispersant to disperse the glass fibers in a mixture prior to using paper making techniques to form the mat. A heat flux may bind the glass fibers by melting the synthetic wood pulp to form a mat. The mat may be calendared. In some embodiments, the mat may have a secondary application of a polymer, such as PVDF or PMMA which may further strengthen the mat.
This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
In the drawings,
A mat may be formed from short glass fibers bound by a synthetic wood pulp. The synthetic wood pulp may act as both a dispersant to help keep the glass fibers from clumping prior to mat forming, as well as a binder to bind the glass fibers together after mat forming.
The synthetic wood pulp may be a polyolefin, polyethylene or polypropylene fibrous material that may have a high moisture content when mixed with short glass fibers in a liquid carrier. In many cases, the liquid carrier may be water which may or may not have a thickening agent added. The mixture may be mixed or processed to disperse the glass fibers and synthetic wood pulp, then a non-woven glass mat may be formed by passing the mixture through a screen to deposit the fibers.
Once a mat has been formed, the mat may be heat stabilized to bind the glass fibers using the synthetic wood pulp. The mat may be subsequently processed using calendering, additional heat treatments, and additional polymer coatings.
Specific embodiments of the subject matter are used to illustrate specific inventive aspects. The embodiments are by way of example only, and are susceptible to various modifications and alternative forms. The appended claims are intended to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the claims.
Throughout this specification, like reference numbers signify the same elements throughout the description of the figures.
When elements are referred to as being “connected” or “coupled,” the elements can be directly connected or coupled together or one or more intervening elements may also be present. In contrast, when elements are referred to as being “directly connected” or “directly coupled,” there are no intervening elements present.
Embodiment 100 is an example of a manufacturing process that may produce a mat of glass or other fibers that are bound with a polymer matrix. In some embodiments, the mat may be very thin and largely porous. Such embodiments may be as thin as 0.001 in or less and have an areal weight of 5 to 12 grams/meter squared.
Forming mats from short glass fibers can be challenging for several reasons. Short fibers, such as fibers that are 0.25 in, 0.50 in, 1.0 in or less may not intertwine or entangle to create effective mats. Such fibers may form a mat when a binder is used to adhere or bind the fibers together.
The process of embodiment 100 creates a mixture in which glass fibers and a binder material are dispersed, then passes the mixture through a screen to deposit the fibers and binder material. The fibers and binder material are heated to flux the binder material and form the mat. The mat may then be post processed.
Embodiment 100 creates a first mixture and a second mixture. The first mixture may use a thickening agent to increase viscosity of the liquid to more effectively disperse the fibers and binding agent. The second mixture is much more dilute and may be used to lay the fibers onto a screen.
In block 102, the first mixture may be formed. The first mixture may include both the glass fibers and the binding agent. In some embodiments, the glass fibers may be chopped glass fibers of 0.25 in, 0.50 in, 1.00 in or less. In some embodiments, the fibers may be longer or shorter.
The binding agent may be a synthetic wood pulp. Synthetic wood pulp may be a polyethylene, polypropylene, polyolefin, or other polymer that may be produced in a manner that resembled fibrous wood pulp. Synthetic wood pulp may be commercially available under the trade name Fybrel by Mitsui Chemicals in a variety of materials and densities.
The addition of synthetic wood pulp to short glass fibers in an aqueous solution causes the glass fibers to disperse. Without the synthetic wood pulp, the glass fibers may tend to clump, which may lead to uneven mat formation.
The ratio of synthetic wood pulp to glass fibers is preferably between 50% to 70%, but some embodiments may use a ratio of 40% to 80%.
The first mixture may be an aqueous solution that includes 1%, 2%, or more of polyethylene oxide. The polyethylene oxide may serve to increase the viscosity of the mixture and better hold the dispersed fibers in suspension.
After creating the first mixture and mechanically stirring the mixture, the first mixture may undergo ultrasonic agitation in block 104. Ultrasonic agitation may further enhance dispersion in some embodiments.
In block 106, the first mixture may be further diluted with water to create a second mixture. The dilution in block 106 may yield a mixture with 75-150 mg of fibers to 800 ml of liquid. Other embodiments may have ratios of fibers to liquid that are 1:10, 1:20, 1:50, 1:100, 1:200, or higher or lower ratios.
While in the second mixture, the mixture may be continually mixed prior to mat forming. The glass fibers may have a tendency to clump or group together if the mixture is kept too long without agitation.
The mixture may be passed across a screen in block 110 to remove the liquid and capture the fibers and binding agent. The process of block 110 may be a wet dip paper making operation, Fourdrinier process, or other mechanism. At the end of block 110, the glass fibers and binding agent may be dispersed across the surface of the screen.
The thickness and structure of the finished mat may depend on how much fiber and binding agent has been deposited on the screen in block 110. For a thicker mat, more fibers and binding agent may be deposited and for a thinner mat, less fibers and binding agent may be deposited. The amount of fibers deposited on the mat may be varied by changing the amount of mixture passed through the screen as well as by changing the ratio of fibers and binding agent to the liquid.
In block 112, a first heat step may be applied to the mat. The first heat step may soften the binding agent to the point that at least some of the glass fibers may be held together. In some embodiments, the first heat step of block 112 may be performed while the fibers and binding agent are still on the screen from block 110 and before removing the mat from the screen in block 114. In other embodiments, the mat may be removed from the screen and into a heated roller or other mechanism for applying the first heat step.
In the case of an embodiment with polypropylene synthetic wood pulp, the first heat step may raise the temperature of the mat to 100 to 120 Celsius.
The first heat step may serve to at least partially form the mat. The partially formed mat may then undergo a second heat step to fully flux the binding agent. In an embodiment with two heat steps, the first heat step may be sufficient to hold the mat together for processing until the second heat step may be applied.
In the case of an embodiment with polypropylene synthetic wood pulp, the second heat step may raise the temperature of the mat to 140 to 160 Celsius.
The second heat step of block 116 may cause the binding agent to fully melt to hold the glass fibers together to form the mat.
The mat may be post processed in block 118. The post processing may include calendering, embossing, slitting, or other secondary processes.
In some embodiments, the mat may have a second polymer coating applied in block 120. The second polymer coating may add a different polymer than the binding agent. The second polymer coating may serve as a sizing, adhesive, finish, or other purpose for the subsequent use of the mat.
In some embodiments, the second polymer coating may be PVDF, PMMA, or other polymer. The second polymer coating may be formed by dissolving the second polymer in a solvent and diluting the solution, then applying the solution to the mat, and drying the solution to leave the polymer coating on the mat.
For example, a PMMA coating may be performed by creating a solution of 20-25% PMMA to toluene and then diluting the solution with acetone to create a 2-4% solution. The solution may be applied by dip coating, spraying, or other method to the mat. The mat may be air dried to from a PMMA coated mat.
In another example, a PVDF coating may be performed by creating a solution of 10-40% PVDF to NMP and further diluting the solution with acetone to create a 2-4% solution. The solution may be applied by dip coating, spraying, or other method to the mat. The mat may be air dried and heated to form a PVDF coated mat.
The process of embodiment 100 may be used to form very thin mats in some cases. The mats may be as thin as 0.005 in or thinner, such as mats that are 0.003 in, 0.002 in, 0.001 in or thinner. In many embodiments, the mats formed by embodiment 100 may also be thicker.
The mats produced by embodiment 100 may have very low areal weights, such as areal weights of 50, 20, 15, 10, 5, or lower grams/meter squared. In many embodiments, the mats formed by embodiment 100 may be heavier.
The mats produced by embodiment 100 may be used in various applications. The mats may be used alone or incorporated into other materials.
In some uses, the mats may be used alone as air permeable or liquid permeable filters.
In other uses, the mats may be coated with a polymer or other coating that may form a porous material. In such uses, the mat may provide some structural integrity for the porous material that may be desired for mechanical handling of the porous material during manufacturing or as a component within a device using the porous material. For example, when used as a filter, the mat may provide some strength to avoid puncturing the filter during use.
In one embodiment, the mat may be coated with a PVDF solution that may form a microporous structure around the mat. The PVDF microporous material may be inherently structurally weak, but the mat of embodiment 100 may provide sufficient strength for processing the PVDF structure as well as post processing when the coated mat is used to assemble products.
One such product may be an electrochemical cell, such as a battery or supercapacitor, where a microporous PVDF structure formed over a mat may act as a separator between electrodes. The microporous material may be saturated with electrolyte in which ions may flow from one electrode to another during charging and discharging.
In such an application, the very light areal weights of the mats of embodiment 100 may provide large areas where ions may pass. Because the fibers of the mat may be sparsely spread out, the mat fibers may not severely inhibit ion flow. As the density of the fibers increases, the ion flow may be restricted. Thus, the light areal weights may be preferred for battery separators or similar applications.
The mats of embodiment 100 may give some mechanical structural properties to the delicate PVDF microporous material. The mechanical properties may help during manufacturing of the PVDF material as well as during assembly of a battery or similar device.
The combination of a very light areal weight but with enough bonding of the fibers in the mat may provide a good tradeoff between mechanical strength during manufacturing against the performance of the electrochemical device.
Several samples have been manufactured. Two of the samples are listed in Table 1 with various physical properties. The separators are manufactured from Synthetic Wood Pulp (SWP) and glass fibers using a PVDF binder.
Additionally, the separators have been used in several lithium ion batteries. The discharge power capabilities of some of the batteries are shown in
Specifically, JOA1001 uses a 61% glass/39% SWP nonwoven web. JOA1021 uses an 80% bi-component fiber/20% SWP. JOA1023 uses a 60% bi-component fiber/40% SWP. JOA1025 uses 80% glass/20% SWP, and JOA1027 uses a 70% glass/30% SWP composition.
Each of the JOA10xx samples use a non-woven web having the various compositions coated with a PVDF-based porous material.
From the figure, the batteries manufactured using the SWP-based non-woven web produce acceptable results.
The foregoing description of the subject matter has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the subject matter to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments except insofar as limited by the prior art.
| Number | Date | Country | |
|---|---|---|---|
| 61446985 | Feb 2011 | US |
