Patent Application
20170072350
The invention relates to a method for adapting a filter medium to predefinable parameters, such as
A method and means for rendering germproof air filter materials hydrophobic, which are preferably used in the areas of anesthesia and surgery, is known from DE 38 19 583 A1. The hydrophobic means in question, with which the surface of the aforementioned filter materials is treated, includes silicones and silicone compounds. These are brought into contact with the filter material by spraying or dipping processes or vapor deposition. The hydrophobic means in the form of a solution or emulsion or in pure form is said to improve the air flow of the air filter material, to be physiologically harmless and to meet medical-hygienic requirements.
A fuel filter or tank filler neck insert is known from DE 199 32 383 A1 as a molded part having a body made of an electrically non-conductive plastic, the surface of which is coated entirely or partially with a striped, linear, lattice or net-like structure made of an electrically conductive material, preferably in the form of an electrically conductive polyacetate composition, wherein different areas of this structure are selectively electrically conductively connected to one another. This molded part is advantageously produced according to the monosandwich method or the sequential injection molding method.
A filter element for filtering liquids, in particular, in the form of an oil filter, is known from DE 197 35 993 A1, having a zigzag-like foldable filter strip made of a non-woven filter material. The non-woven filter material may contain metal fibers, glass fibers and/or polyester fibers, wherein the zigzag-like foldable filter strip is produced from pre-stamped panels of non-woven filter material or from a rolled strip of non-woven filter material. The filter element, if it is constructed of glass fiber non-woven materials, may be advantageously laminated or coated on one or both sides with polyester fibers. In this way, a type of grooving or other imprinting may be easily carried out and the sensitive glass fiber non-woven material may be processed to form a star-like pleated filter element without additional protective non-woven material on the inflow side or the outflow side and, if applicable, also with no backing.
Thus, it is state of the art to modify, in particular, to finish known filter media for air, fuel and oil filtration using corresponding coating methods, to thereby create improved characteristics for the respective filter medium. However, this does not include a targeted adaptation to pre-definable desired parameters; instead, depending on the parameter or requirement profile, the respective filter medium is coated more or less randomly and in a trial and error fashion with materials from which there is hope of corresponding improvements in the operation of the filter medium.
Based on this prior art, the object underlying the invention is the targeted adaptation of the filter medium to the exemplary parameter values described above. Such an object is achieved by a method having the features of Claim 1 and a filter medium according to the feature configuration of Claim 9, each in its entirety.
The adaptation, because it is made according to the characterizing portion of Claim 1 to at least one of the aforementioned parameters by a targeted influencing of the surface energy of the filter medium, no longer takes place in a random and experimental fashion, but rather targeted, by considering the respective surface tension of fluids, such as hydraulic oil, fuel or air, and/or of solid bodies, such as filter element materials (filter medium). This consideration of the surface energy is based on the finding that the surface energy of the respective filter medium can be divided into a disperse fraction and a polar fraction, and that by appropriate selection of coating methods and of coating and infusion media, the respective proportion of the disperse and polar fractions are adjustable relative to one another in such a way that the filter medium treated in this manner is adapted to the predefinable parameters optimal for its operation.
To determine the respectively prevailing surface energy, in addition to its disperse or polar fractions, a contact angle measurement is preferably used, specifically, on the filter medium itself and/or on the fluid flowing through the filter medium. If, for example, the determining parameter for the filter medium is the electrostatics, in the sense that an electrostatic charge, which may damage the filter medium material, is avoided, the magnitude of the difference between the surface tension of the hydraulic oil to be filtered and the surface energy of the filter material or filter medium is the crucial factor in the charging of the oil. It has been shown in such case that the more similar the materials are in terms of their surface energy, the less oil charging is expected during operation. Thus, based on the tribo-electrical series known for materials, the material of the filter medium, in terms of the surface energy to be determined, must be designed in a way similar to the hydraulic oil to be filtered and the coating material used.
The flow rate behavior, which is reflected in the differential pressure, may likewise be decisively influenced by adjusting the surface energy of the filter medium. As the hydrophobicity of the material increases, so too does the so-called bubble point, i.e., the pressure that is required to push the fluid for the first time into the pores of the filter medium, as based on the so-called Washburn equation. If the material is wetted, the flow rate resistance decreases as a result of the “repulsion of the liquid” by the hydrophobic coating at the boundary surface between the filter medium and the liquid. Areas of application include primarily air filters, in addition to hydraulic filters. Comparable considerations also apply if the intention is to decisively influence the so-called water entry pressure (dynamic pressure) by adjusting the surface energy of the filter medium.
The fraction filtration efficiency or the coalescence factor as an additional parameter may also be influenced by adjusting the surface energy of the filter medium. As the hydrophobicity of the filter medium (filter material) rises, the fraction filtration efficiency is able to increase significantly. The filter medium is then significantly more resistant to penetration of liquid droplets from the air, so that such droplets can be more easily separated from an air flow. This effect may be employed as a barrier, for example, for the separation of oil mist, in the case of air filters, ventilation filters or so-called coalescence filters.
In preserving a constant quantity and quality of fluid additives, the undesirable separation of oil additives from the fluid may be avoided, which counteract oil ageing, for example, specifically, again by manifest adjustment of the surface energy. Additives dispersed in oil present as non-dissolved substances, for example, in the form of silicon droplets as anti-foam additives for the oil, are separated on the surface of the filter medium as a result of boundary surface effects, such as Van-der-Waals forces and the like. These effects are reduced, if not outright prevented, by a targeted change of the surface energy. Accordingly, surface modified filter media may be “smoother” during filtration and help to avoid intrusions of oil in the additive packet.
Based on the targeted split of the surface energy into polar and disperse fractions, theoretical combinations for complete wetting may be represented by predefinable reference curves (wetting envelope). Based on these, it is possible to reliably estimate the wetting behavior or other effects of the filter medium in relation to the liquid to be wetted.
Additional advantageous embodiments of the method according to the invention are the subject matter of the dependent claims.
The subject matter of the solution according to the invention is also a filter medium, wherein the filter medium is produced at least in part from glass fibers, which are encased in a non-bonding manner at least partially by polymerized silicon dioxide molecules, or the glass fibers form a covalent bond with fluoropolymers, polyethylene or polypropylene. In addition, the following coatings have proven advantageous for modifying the surface energy of the filter medium, for example, in the form of polydimethylsiloxanes (PDMS) or in the form of poly-3,4-ethylenedioxythiophene (PEDOT).
The solution according to the invention is explained in greater detail below with reference to various embodiments. In the drawings, depicted schematically and not to scale,
Filter media, in particular, in pleated form, and filter elements constructed therefrom, in particular for the filtration of fluids, such as hydraulic oil, are sufficiently known and described in detail, for example, in the applications for patent rights DE 10 2013 000 932, DE 10 2013 000 933, DE 10 2013 000 934, DE 10 2013 000 939, DE 10 2013 020 752, etc. of the property rights owner, so that these are not discussed in further detail at this juncture and the filter medium according to the invention, along with its respective method of production, will be described below only insofar as it concerns the finishing, in particular, by the targeted influencing of the surface energy of the filter medium.
Filter media, even when they are constructed in multiple layers and include protective non-wovens and support screens, regularly exhibit a high percentage of glass fibers for the filtration of fluids, such as air, gases, process water, fuel and lubricants as well as hydraulic oil, etc. The aforementioned glass fibers for filtration, in addition to silicon dioxide, also consist of other oxides, such as aluminum (Al), barium (Ba), boron (B), calcium (Ca), potassium (P), magnesium (Mg), manganese (Mn) and zinc (Zn), in various compositions. In addition to the commonly known borosilicate glass, special glass compositions are also used, such as low alkali, ferrous glass (E-glass); acid-resistant high carbon glass (C-glass); low heavy metal glass; low boron glass or quartz glass. The components are mixed and melted together and cooled to form glass beads or pellets. By re-melting and pressing the liquid glass through nozzles, the glass is stretched and deposited on a metal screen. As a further type of production, glass rods are heated and stretched by means of a glass flame and subsequently deposited on a screen.
The resultant glass fibers are further processed in a paper machine to form filter material. In the process, the glass fibers are dispersed in water in a pulper. To lend strength to the glass fibers, the fibers are bonded using an acrylate or epoxy resin. Before finally being deposited, such a binder is placed on the wet paper web under a vacuum on a screen or at the end of the screen. The glass fiber medium is subsequently dried in a drying stage and is suitable for use as a filter material.
Another filter material used very frequently for a filter medium consists of so-called melt-blown fibers. Melt-blown fibers are regularly produced in a process, in which non-woven materials are produced directly from granulate. A specialized weaving method is used in combination with high-velocity hot air in order to produce fine fibered materials having different structures. Granulate melted in an extruder is fed directly into a nozzle block via a spinning pump. The polymer, once it has exited through the nozzle tips, is drawn by means of compressed process hot air. The resultant microfiber non-woven is deposited on an air-permeable sieve belt. To produce the laminates, the melt-blown unit is extended by an uncoiler in front of and behind the nozzle. A calander bonds the fed materials to one another. Potential feed materials include polypropylene (PP), polybutylene terephthalate (PBT), polyethylene (PE), polyamide (PA) and other comparable plastics. Each filter material produced in this way is then pleated with additional filter layers (wire mesh, non-wovens, nets) in knife pleating machines or rotary pleating machines and further processed to form a complete filter element. The production methods described above likewise form part of the prior art.
In the following, it will now be described how such known fiber material of the filter media is correspondingly finished. As a departure from the production solution for a filter medium described above, at least one finishing step for the filter material or filter medium is carried out before the medium is pleated. Thus, the completely fabricated glass fiber filter material is finished prior to pleating using a spray technique. For this purpose, a so-called sol-gel spray method is carried out in an appropriate device according to the illustration in
Preferably polymerized silicon dioxide molecules are used as a coating material for this wet chemical sol-gel method of application. The molecules may be used in combination with other chemical substances, for example, iron, magnesium, calcium, fluorine, etc. and, in particular, as further modifications of the oxides of silicon. The thickness of the coating may be individually adapted to the filtration task from several nanometers up to a micrometer by the adaptation of the parameters during the application process.
Whereas the individual silicon molecule is reproduced in the manner of a structural formula in
The main focus of attention of the present coating is in the increase in hydrophobicity (oleophobicity). The surface energy may be determined by measuring the contact angle CA (sessile drop) (for example, using the Owens, Wendt Rable, Kälbe, Wu methods, etc.). Additional test methods involving the determination of the surface energy are the Du Pont test methods or the 3M water repellency test.
When measuring the contact angle, it is possible to determine indirectly the surface energy or surface tension of the filter material. Untreated filter materials made of glass fiber plastics generally have a contact angle CA relative to water (H2O) and/or oil of <90°, which is shown in
The entire device system is moved into a low pressure vacuum and an initial plasma activates the surface by forming locations of free radicals on the surface of the substrate in the form of the glass fiber material. A coating material made of monomer gas introduced into the chamber 26 forms covalent bonds on the surface of the substrate or filter material 18 at the locations of the free radicals. Thus, the coating material is anchored on the surface and is extremely resistant. A subsequently produced pulsating plasma 30 activates the applied coating monomer and causes the polymerization of the material at the surface of the filter medium 18. The internal pressure of the chamber is then returned to ambient pressure and the filter material is coated so as to be hydrophobic/oleophobic. The monomer gas, before entering the actual vacuum chamber 26, is stored in an antechamber 32 and the vacuum chamber 26 may be fitted on its output side with a type of exhaust gas post-treatment device 34. Since the plasma separation can release very high amounts of energy, a box-shaped cooling device 36 is attached to the underside of the transport path 14 in question for cooling the filter medium or filter material 18.
Preferably all fluoropolymers are considered as coating material for this method of finishing from the gas phase. However, the following monomers in particular, are suited to be polymerized as coating material in this method:
Final products belonging to the group of fluoropolymers as coating material are, in particular:
The thickness of the coating may again be adapted individually from several nanometers up to a micrometer by the adaptation of the parameters during the application process. Comparable contact angles CA also result, as they are shown in
In addition to the plasma-enhanced chemical gas phase separation, as introduced above, other chemical gas phase separations may also be used. Physical gas phase separations are equally suitable, in plasma-enhanced form as well. In addition to the coating materials described above, polydimethylsiloxane (PDMS) has also proven very reliable, as well as polythiophene of various types. Fibers made of plastic materials, including cellulose materials, which may also be impregnated with phenolic resin, may also be used in place of the glass fiber materials and the melt-blown fibers.
For an exact determination of the surface tension of liquids and solid bodies, the former is divided into a polar fraction and a disperse fraction. Whereas the polar fraction of the surface tension results from a dipole-dipole interaction and from the hydrogen group bonding or the Lewis-acid-base interaction, the disperse fraction of the surface tension is properly characterized by a Van der Waals interaction.
By practical trials or by basic theoretical considerations associated with each filtration task along with the desired parameter, for example, it is possible to determine a favorable curve profile of the surface tension, and then, based on these findings obtained, to select the finishing process for any arbitrary filter material or filter medium in such a way that the predeveloped wetting envelope-curve (
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2014 003 314.7 | Mar 2014 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2015/000307 | 2/12/2015 | WO | 00 |
