WO2015195103A1

WO2015195103A1 - Highly loaded flexible sorbent and method of making

Info

Publication number: WO2015195103A1
Application number: PCT/US2014/042801
Authority: WO
Inventors: Stanislav E. Solovyov; Brian Kenneth BRADY
Original assignee: Multisorb Technologies, Inc.
Priority date: 2014-06-17
Filing date: 2014-06-17
Publication date: 2015-12-23

Abstract

A highly loaded flexible sorbent includes first and second layers of ethylene vinyl acetate copolymer or low density polyethylene film; and a compressed mixture of micronized sorbent and micronized ethylene vinyl acetate (EVA) copolymer binder disposed between the first and second layers of film.

Description

HIGHLY LOADED FLEXIBLE SORBENT AND METHOD OF MAKING

BACKGROUND FIELD

[0001 3 This disclosure relates generally to flexible sorbents and more particularly to a highly loaded non-dusting flexible sorbent in the form of a sheet or film, and a method for making the sorbent.

DESCRIPTION OF RELATED ART

[00023 There is a need for flexible sorbents for a variety of applications including particularly applications in the electronics field. Electronic devices such as computer hard disk drives (HDDs) and other packaged precision electronic instruments and components often require a controlled atmosphere within the enclosure in order to provide their reliable operation for the duration of their expected service life. Moisture and volatile organic compounds (VOCs), present initially or generated during the extended device operation, often adversely affect the precision electro-mechanical parts (such as magnetic heads) and electronic components within the device, by contamination, fouling, corrosion, etc., thus shortening their useful life and eventually causing a device to fail. Due to internal heating inherent in such device operation, the packaging often does not completely seal the device from the environment and some kind of a breather filter is then used to control air exchange between the device and the environment as well as to relieve gas pressure buildup. Even hermetically sealed devices with modified atmosphere within the enclosu re, where pressure buildup is not a concern, may still suffer from VOC vapors, e.g. generated due to friction and elevated temperature of lubricated internal moving parts.

[0003] Activated carbon (AC) has been extensively used in construction of filtering and absorbing media to control harmful moisture and VOCs in design of HDDs and other electronic devices. While effective in removing VOCs and moisture, AC presents its own problems such as fine carbon dust particles potentially present in a filter construction. These particles, either present initially or generated due to abrasion and rough handling of the filter assembly, can become easily detached from the filter structure and contaminate the device headspace. The presence of loose AC particles can lead to a premature failure of the device being protected.

[0004] In addition to AC, sorbents include desiccants and sorbents for absorbing VOCs including volatile organic compounds. While sorbents in the form of loose powder packaged in sachets have been used in such applications, sachets, especially sachets made from nonwoven materials such as those sold under the trademark Tyvek have some disadvantages including dusting. Dusting is the escape of very fine sorbent particles from the packet in which they are contained, and is a problem, potentially a serious problem in electronic applications where the presence of fine particles can be damaging to the product being protected. High-speed HDDs are an example of this.

[0005] Finely divided AC, especially micronized carbon having a particle size in the range of 1 0 to 20 μ is an effective sorbent. For other applications, where desiccant capabilities are required, finely divided silica gel and to a lesser extent molecular sieve can be employed.

[0006] Some conventional methods produce highly porous sintered AC structures via burn off of binder resin. Others rely on expanded PTFE as a binder to create a highly porous AC containing composite structures. US Patent Nos. 5,792,51 3, 6,077,588, and 6,022,436 describe mixing powdered binder resin with active particles (like AC) and heating the mixture on a suitable substrate above softening point of the binder resin but below melting point of the substrate in order to fuse the binder and AC to a substrate.

[0007] Sintered AC suffers from brittleness and insufficient flexibility , thus preventing production of clean, dust-free, flexible filtering media due to difficulties in forming and cutting thin webs. Highly porous AC webs made with thermoplastic binders like PTFE still suffer from significant dusting due to abrasion and dusting from cut edges.

[0008] Use of thermoplastic binders to form a continuous resin phase in a non-porous AC composite is limited by the requirement to have a large absorbing capacity in a small volume available within a device. As a result, the AC content of such a composite has to be as high as 80-95 wt.% to be cost effective. Conventional melt extrusion of thin sheets using such highly loaded composites is out of question due to a lack of a contiguous resin phase and resulting poor quality and breakdown of the extruded sheet. Fusing AC-binder mixture to an unmodified substrate also requires the use of specially prepared porous webs (such as specialty paper and various non-wovens).

[0009] Given very low apparent density of AC particles (defined as a particle mass divided by its apparent volume excluding internal porosity), it is often physically impossible to obtain a full encapsulation of AC particles by the binder at high weight ratios above approximately 80 wt.% and correspondingly high volume ratios in highly loaded AC composites. Requirement of high porosity of the filter media is not really a limiting factor in filter design: solid AC composites with contiguous resin phase can only be produced when the AC volume fraction does not exceed about 70 vol.%. At higher volumetric AC loadings, there is not enough binder resin in the composite to completely fill all gaps between the particles. Above 80 vol.% AC loading (corresponding to approximately 60 % by weight, depending on the specific gravity of the binder resin), significant porosity of an AC composite is unavoidable, thus preventing manufacture of solid composites with limited breathing capabilities. The resulting discontinuous morphology of such AC composites, lack of mechanical strength and a lack of AC particle encapsulation by a binder resin are often cited as disadvantages of using thermoplastic binders in filter media design. Dusting of such AC composites is also cited to increase as a result.

BRIEF SUMMARY

[001 0] In one aspect of this disclosure, a flexible sorbent in the form of a non-dusting web comprises first and second relatively thin layers of a material that is permeable to the compound being absorbed and a relatively thicker layer of sorbent in a major proportion combined with a minor proportion of a binder that is the same as or compatible with the material forming the first and second layers.

[001 1 ] In accordance with another aspect of this disclosure, the sorbent may be micronized activated carbon, and the first and second layers may be ethylene vinyl acetate,

[001 23 In accordance with another aspect of this disclosure, the thermoplastic binder is used and processed in such a way as to eliminate most of the cited disadvantages of AC composites based on them and produce flexible, essentially non-dusting, high volume fraction carbon webs, suitable for manufacturing breather filter media. The prepared mixture of fine polymer powder such as EVA with micronized AC (supported and transported through the process with permanent or temporary plastic webs) is repeatedly compressed and heated above the binder melting temperature via passage through a series of preheated temperature-controlled compression rollers. The repeated compression, plastic deformation and melting of the binder causes the low volume fraction of a binder to essentially flow under stress, fill the gaps between AC particles and fuse them together. The result of consecutive downgauging is a thin AC composite sheet that may exhibit at least one of increased apparent density, reduced dusting, significant remaining porosity, and improved flexibility and crack resistance. After calendering, the sheet may be characterized by highly evolved and deformed plastic binder morphologies with significantly improved interparticle adhesion, however without fully encapsulating the AC particles. In some implementations, the produced AC web can be further processed into individual filter pieces by cutting it with preheated dies. This technique allows one to smooth and seal the cut edges of the individual pieces and prevent subsequent dusting.

[001 3] Low melting temperature polymeric webs can be used for supporting and encapsulating the AC composite during thermal processing. The advantage of using polymeric films with melting temperatures comparable to the softening temperature of the thermoplastic binder is that the supporting webs will be partially molten during compression and heating cycle. If the supporting web is thin enough, its contiguous morphology may also be controllably disrupted, resulting in partial web breakdown and the increased porosity of the composite article surface, while retaining the flexibility and non- dusting property of the composite. Control of the solid encapsulating film surface disruption level may be achieved via one or more of applied pressure, roller temperature and dwell time. Partial web surface breakdown causes formation of micron scale openings in the surface film, thus increasing its permeability and at the same time retaining encapsulation integrity to prevent dusting. [001 4] In other implementations, temporary su pporting webs that do not adhere to the AC composite during processing and that are eventually removed to leave a pure AC web as a product may be used.

[001 5] This disclosure is not limited to production of encapsulated carbon webs: other active ingredients such as micronized silica gels, clays, molecular sieves and other natural and synthetic zeolites can be encapsulated via the described techniques. In this case, the end product (die cut strips and/or other shapes) can be used to control moisture levels within the packaged device.

[001 6] Novel aspects of this disclosure are set forth with particularity in the appended claims. The disclosure itself, together with additional objects and advantages thereof, may be better understood by reference to the following detailed description of certain embodiments thereof taken in conjunction with the accompanying drawing in which:

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[001 7] Figure 1 is a section view of an absorber in accordance with an aspect of the invention.

[001 8] Figu re 2 is a diagrammatic view of an apparatus for making an absorber in accordance with an embodiment of this disclosure.

DETAILED DESCRIPTION [001 9] Referring now to Figures 1 A and I B, an absorber l OOis illustrated in a cross-sectional view. Figure 1 A shows a mixture indicated generally at 1 02 of sorbent particles 1 04 and binder particles 1 06 before calendering. While the mixture is generally homogeneous, the particles are not bound together. Figure I B shows the absorber 1 00 after calendering between two films 1 08 and 1 1 0 that are compatible with the binder. The mixture of sorbent particles 1 04 and binder particles 1 06 is bound together so that the sorbent particles cannot escape. This substantially reduces or eliminates dusting by the activated carbon particles by bonding and encapsulating the activated carbon/binder blend between solid film lalyers. The calendered layers are still flexible.

[0020] In order to produce the desired thin webs from mixtures of powdered materials via the described invention and develop the described morphologies of AC composites, the starting powders need to be sufficiently fine. For the webs with the targeted thickness within 1 00-500 micron, this means preferably having the mean particle sizes of the processed powders below 20 micron and more preferably below 1 0 micron. The preferred particle sizes of AC powder are within 1 - 20 micron. That usually means having the source AC powder micronized via a suitable milling process. It is also advantageous in some implementations for the thermoplastic binder powder particle sizes to be comparable or smaller than that of AC powder. This may facilitate deformation and melting of the smaller binder particles trapped between larger AC particles and also increase the number of contact points. Due to plasticity, low melting temperature and stickiness of common binder resins, it is often very challenging and costly to produce a powder of such binder resin of sufficiently small mean particle size. Often, these particle sizes cannot be further reduced by standard size reduction techniques due to particle melting, agglomeration and fusion problems. Commercially produced binder resins such as EVA and phenolic resins can be obtained with mean particle sizes within 1 0-20 micron. This fact correspondingly puts a lower limit on the mean size of milled AC powder around the same numbers. Smaller (< 5- 1 0 micron and nanoscale powders) sizes of active ingredients and binder particles can be used in the described process if a thinner finished web is desired (e.g. thinner than 50 micron).

[0021 ] Materials:

[0022] Micronized activated carbon powder (1 - 20 micron mean particle size), which may be coconut shell carbon.

[0023] Flexible binder: micronized EVA powder, phenolic resin powder, etc. (1 - 20 micron mean)

[0024] Potential additives: compatibilizers, surfactants, surface treatments and treatment agents for improving interparticle contact and reducing surface tension of molten binder particles. These additives can be incorporated into the binder resin at the manufacturing stage. Alternatively, the binder resin powder can be treated with one or more of the additives before mixing with the sorbent. Suitable compatiblizers include: functionalized organic oligomers terminated with functional groups such as carboxylic acids, esters, amines, and tackifiers, such as acrylics.

Encapsulating film: 1 mil thick EVA film (0.5 - 2 mil range).

[0025] Temporary web supports: flexible plastic films such as PET, nylon and others with melting temperature higher than the AC composite processing temperature.

[00263 Process:

[0027] Making the web flexible requires multiple contact points between the binder and the solid, high affinity between the carbon and the binder, and high flexibility of the binder itself at the conditions of use. Some residual porosity of the finished web product is inevitable due to a high solids content and a low fraction of deformable binder which cannot fill all volume between AC particles. Advantageously, this porosity contributes to the improved finished sheet flexibility and higher rates of absorption.

[0028] Combining micronized carbon powder with micronized polymeric binder (such as fine EVA powder) via batch mixing and calendering type processing can induce formation of multiple contact points between the binder and carbon particles, however, many carbon particles will be left uncontacted by the binder. The resulting web will still be dusty. [0029] Referring now to Figure 2, a system 200 for making an absorbent web 202 will be described. The system 200 includes a supply of a first web 204, a supply of a second web 206, and a hopper/dispenser 208 containing a mixture 21 0 of the binder particles and carbon. The dispenser 208 places the mixture on the second web 206, and the first web 204 is subsequently placed over the mixture 21 0 to sandwich the mixture 21 0 between the first and second webs 204, 206. Using multiple heated rollers 21 2 , the system 200 compresses, densifies, and melts the binder particles between the first and second webs 204, 206. The rollers 21 2 may cause deformation of the binder and allow it to flow into interparticle (i.e., between the activated carbon particles) spaces, which may result in partially encapsu lating the carbon particles. The mixture compression and rearrangement during rolling and binder melting phase may also serve to collapse the pores and voids between the particles and increase the apparent specific gravity of the web. The compressive force on each set of heated rollers 21 2 is preferably balanced to maximize interparticle space reduction but simultaneously to prevent collapse of carbon particle internal pores, so as to not affect its capacity as an adsorbent. Subsequent cooling of the heated rolled web at cooling rollers 21 4 will solidify the developed morphology which may be characterized by one or more of the partial binder melt flowing into the interparticle spaces (deformed and starburst-like binder patterns with significantly reduced interparticle free volume), higher specific gravity of the web vs. cold pressed carbon-binder mixtures, and /or greatly reduced dusting due to inducing binder contact with majority of the carbon particles (which is impossible to achieve in cold pressing operations such as tableting).

[0030] In accordance with another aspect of this disclosure, small amounts of the sorbent/binder mixture may be placed in spaced apart locations between the sealing sheets prior to calendering. This will produce spaced apart sorbent regions on the sheet that can be cut into individual elements. Preferably, to manufacture sorbents in this way, a pocketed calendering roller would be used. This would permit discrete products to be made without the need for cutting through the sorbent/binder blend and resealing the edges. The discrete products would already be sealed at their edges.

[0031 ] Another process implementation includes a batch mixing first step to combine the AC and binder powders and disperse the binder in a mixture. The second step is the web forming step in which the powder mixture is placed between two webs. The webs may be the webs 204, 206 described above, which will become part of the sorbent, as in Figure I B, or they may be non- sticking support webs arranged for subsequent removal. In the latter arrangement, only compressed mixture remains. In another arrangement, the mixture may be encapsulated in a sleeve type support. The web/mixture or sleeve/mixture is then drawn through a series of vertically arranged heated roller pairs 21 2 (top and bottom roller) with a preset gap between each pair. Each pair of heated temperature-controlled rollers will have a reduced gap relative to the previous pair in order to build up the web morphology and downgauge the web thickness. The roller temperature is to be set above the melting temperature of the binder resin. The cooling rollers 21 4 can also be used in the end of the forming process. The supporting web can be removed at a later stage or by the final user.

[0032] Example 1

[0033] A blend of the activated carbon (such as that commercially available from Carbon Activated Corp.) with EVA powder (such as that commercially available as Equistar Microthene FE53200) was weighed in an 85 / 1 5 ratio of C/EVA (carbon/EVA) in a small lab mixer until well blended. In the first example, 5 grams of this blended powder was put onto a l mil thick EVA sheet that was on a Teflon sheet on the flat plate of a mold. This mold was 4 inches in diameter. The second sheet of EVA was put over the powder that had been spread evenly over the bottom sheet. A second sheet of Teflon sheet was put over the top and then the top circular part of the mold was put on top. The assembly was put into a heated Carver Press that had been heated to 400T (204^°C). The force on the press was then slowly increased to 24,000 lbs. and held for 1 0 sec. The force was removed, the mold removed from the press and the sample removed from the mold. Other variations were done with 3.5-8.0 grams of material, temperatures were varied from 300T to 425T and the force was varied between about 20,000lbs. to 24,000lbs. (the maximu m of this particular press). [0034] The surface modification binder resin additives for improving carbon particle encapsulation by the binder can also be used if the porosity of the resulting web is still high and the starburst-type binder dispersion pattern has not been obtained.

[0035] The final web can provide an adsorbing capacity equal to that of the pure carbon powder of the same volume, since nearly all binder powder in the web is distributed between the carbon particles without affecting their packing density.

[0036] Self-adhesive backing can be applied to one or both web surfaces to facilitate adhesion of die cut parts to the package interior if desired.

[0037] While the invention has been described in connection with certain presently preferred embodiments thereof, those skilled in the art will recognize that many modifications and changes may be made therein without departing from the true spirit and scope of the invention which accordingly is intended to be defined solely by the appended claims.

Claims

CLAIM OR CLAIMS

1 . A highly loaded flexible sorbent comprising: first and second layers of ethylene vinyl acetate copolymer or low density polyethylene film; and a compressed mixture of micronized sorbent and micronized ethylene vinyl acetate (EVA) copolymer binder disposed between the first and second layers of film.

2. The sorbent of claim 1 in which the sorbent comprises micronized activated carbon.

3. The sorbent of claim 1 in which the mixture comprises between about 85 and 90% by weight of the micronized sorbent and between about 1 5 and 1 0% by weight of the binder.

4. The sorbent of claim 1 in which the first and second layers have a thickness of about one mil.

5. The sorbent of claim 1 in which the first and second layers have a low melting temperature.

6. The sorbent of claim 5 in which the melting temperature of the polymeric material is comparable to the softening temperature of the thermoplastic binder.

7. The sorbent of claim one in which the micronized sorbent is selected from the group consisting of activated carbon, silica gel, clay, molecular sieve, and natural and synthetic zeolites.

8. The sorbent of claim 1 having a thickness in the range of 1 00 to 500 micron.

9. The sorbent of claim 1 in which the mean particle sizes of the micronized sorbent and micronized EVA binder is less than about 20 microns.

1 0. The sorbent of claim 1 in which the sorbent has a particle size between 1 and 20 microns.

1 1 . The sorbent of claim 1 in which the binder particle size is less than or approximately equal to that of the sorbent powder.

1 2. The sorbent of claim 1 in which the sorbent comprises coconut shell carbon.

1 3. The sorbent of claim 1 in which the binder comprises phenolic resin powder.

1 4. The sorbent of claim one further comprising one or more of compatiblizers, surfactants, surface treatments, and treatment agents for improving interparticle contact and reducing surface tension of molten binder particles.

1 5. A method of making a highly loaded flexible sorbent comprising: providing first and second layers of ethylene vinyl acetate film; disposing a mixture of micronized sorbent and micronized ethylene vinyl acetate (EVA) copolymer binder between the layers of film; and calendering the film by simultaneously applying heat and pressure.

1 6. The method of claim 1 5 in which calendering the film comprises repeatedly compressing, plastically deforming, and melting the mixture of micronized sorbent and micronized EVA binder between the layers of film.

1 7. The method of claim 1 6 further comprising processing the film into individual pieces by cutting the film with preheated dies to smooth and seal the cut edges of the individual pieces.

1 8. The method of claim 1 5 in which calendering the film comprises passing the film through a plurality of pairs of heated rollers separated by decreasing distances.

1 9. The method of claim 1 8 further comprising passing the film through at least one pair of chilling rollers.