The invention relates to a process for producing filtration-active fibers, in which an aqueous suspension of a hydrophilic polymer which is both free of additives and which also has no organic solvents is heated in a tank and then discharged from the tank. The invention also relates to a device as claimed in claim 14.
Various processes are known for producing fiber materials. Thus, for example grinding of natural fibers or staple fibers which can be fibrillated in disk refiners or cone refiners is current technology in the paper industry. Grinding influences dewatering behavior on a Fourdrinier paper machine and the strength of the paper web. In particular, wet compaction action by convolution of the fibrils is current practice in the production of deep filter media for liquid filtration. For this application, in addition to natural fibers (cellulose fibers) also staple fibers from cellulose regenerates which are characterized by high fibrillation capacity, for example Lyocell fibers (WO 95/35400), have been used. Ideally the parent fibers first remain, while the fibrils are peeled sideways up to complete fibrillation of the fibers.
In this process it is disadvantageous that the extensive fibrillation of the fibers requires long grinding times. As the grinding time increases however shortening of the parent fibers occurs more intensely up to pulverization of the fibers. Exact adherence to the grinding conditions under which shortening to the benefit of fibrillation is for the most part repressed, can be accomplished in practice only with uneconomically low material concentrations and high demands for homogeneity of the initial fibers. In addition the process for natural fibers is limited to fiber types with suitable cell wall architecture and for synthetic fibers is limited to suitable materials (for example acrylonitrile polymers).
Production of extremely fine fiber structures from melts of polymer materials is used within the framework of melt-down technology in an industrial scope for producing deep filter materials (for example U.S. Pat. No. 5,681,469). In this process a polymer melt is extruded through a special nozzle. A hot air flow is superimposed on the emerging polymer flow at the nozzle output; in the turbulence field of the hot air flow the polymer filament solidifies as it stretches and frays. The fiber morphology can be influenced via the temperature, delivery speed of the polymer and the air flow intensity. The fibers produced in this way can be used without further processing and can be immediately deposited after spinning as nonwoven or filter elements ready for use.
Since the turbulence forces for fiber spinning can act only on the outer surface of the extruded polymer filament and at the same time with impact of the gas flow the solidification of the polymer is caused, finer fiber diameters require a disproportionally stronger air flow. The unduly high energy requirement and cost which are necessary for fiber diameters of dp<3 microns oppose the good availability of the fiber material.
Furthermore, the process is limited to materials which when heated pass into a homogenous, low-viscosity melt and are relatively sensitive to thermal stress.
In the splicing process a binary polymer melt is extruded. The extrudate contains two polymer fibers which are deposited within and next to one another, with diameters below the diameter of the extrudate. The individual filaments can be isolated by mechanical treatment (grinding) or by application of compressed air to the extrudate. U.S. Pat. No. 3,402,231 describes the coextrusion of PAN and hydroxymethyl cellulose from a hot suspension. In a similar process the target polymer is extruded jacketed by an auxiliary polymer. Stretching the extrudate drastically reduces the diameter of the inside filament of the target polymer.
Those polymers which have mutual adhesive forces which are low enough to enable subsequent exposure of the extremely fine fibers by mechanical action are suited to the process. Additives are necessary to prevent overly intimate connection of the polymers.
Processes which work with precipitation from a homogenous solution are known for polystyrene (PS), vinyl chloride (VC) and their copolymers (U.S. Pat. No. 4,224,259) and mainly cellulose acetate (for example, U.S. Pat. No. 5,071,599, DE-OS 196 16 010). For reasons of better reprocessing, preferably highly volatile solvents are used (methylethyl ketone, dioxane, tetrohydrofuran) in these processes. A homogenous solution of the polymer is combined in a shear field with a nonsolvent (“precipitation agent”). Nonsolvents and solvents are completely miscible with one another, so that at the instant of meeting of the polymer solution and the precipitation agent instantaneous precipitation of the polymer takes place as a result of dilution of the solvent. Under suitable conditions the polymer precipitates with strong stretching as the extremely fine fiber. The required shear field can be effected by spraying the solution into a precipitation bath (U.S. Pat. No. 3,441,473, U.S. Pat. No. 4,047,862), the use of Venturi nozzles (U.S. Pat. No. 4,192,838, U.S. Pat. No., EP-OS 533 005) or rotor-stator systems (Dispax reactors; U.S. Pat. No. 4,224,259, DO-OS 196 16 010). The attainable fiber morphology corresponds to the requirements for an auxiliary agent in liquid filtration.
The disadvantage of this process is that in the first process step large volumes of solvent-containing fiber pulp are produced. In this stage the solvent is in a low concentration in the liquid, but in part also in the fibers. Due to the remaining content of solvent until its final removal there is always the danger of influencing the fiber morphology. Careful separation methods are therefore required (steam distillation).
The cost for solvent recovery is reduced with the flash spinning process in which fiber formation is achieved by vaporization of the solvent. Depending on the pretreatment of the polymers different process sequences can be distinguished. The polymer is placed in solution or is dissolved in situ under process conditions. The polymer solution is heated to temperatures above the boiling point of the solvent or solvent system and relieved through a suitable nozzle system using at least the autogenous pressure (expansion spinning).
U.S. Pat. No. 3,740,940 describes for example expansion spinning of cellulose acetate (CA) to highly fibrillated yarns with high water absorption capacity. The CA is first homogeneously dissolved at an elevated temperature and at high pressure in an alcohol/freon mixture and then released through a 0.4 to 0.7 micron-wide opening to atmospheric pressure. The halogenated hydrocarbon in this system does not contribute to the dissolution of the polymer, but acts as an inert propellant. According to this process fiber qualities with a specific surface up to 0.4 m2/g (BET) can be achieved.
The disadvantage is that the use of halogenated hydrocarbons for reasons of environmental protection is associated with high equipment cost (complete encapsulation of the system). In general the use of halogenated hydrocarbons should be evaluated critically with respect to climate protection.
Another version of obtaining extremely fine fibers by flash spinning processes proceeds from a polymer suspension which is heated under pressure. Adding softeners causes plasticization of the polymer without a genuine polymer solution being obtained. The suspension agent is used as a carrier substance for the softener and due to its spontaneous vaporization at the outlet causes a turbulence field which ultimately produces the extremely fine fibers. For cellulose acetate, softeners are known from the production of CA moldings.
For the preferred application of liquid filtration it is a disadvantage that well-suited polymers, especially cellulose alkanoate (cellulose acetate/butyrate), polyacrylonitrile (PAN and copolymers) and polyamides (PA-6, PA-66) can be kept in the molten state over a longer time only with the addition of additives.
A heavily studied polymer class is acrylonitrile polymerizates and copolymerizates (for example methacrylate, vinyl chloride, vinyl acetate, styrene are suitable as copolymers) which are combined below under the name PAN. There have been attempts to spin acrylonitrile polymer filaments from mixtures of acrylonitrile polymers and water. These attempts as are described in U.S. Pat. No. 3,402,231 and U.S. Pat. No. 258,544 have however led to fibrinous materials which are suitable for paper production or to strand materials of molten and sintered or foamed particles, but not to textile filaments.
Thermoplastic deformation of this material in the presence of water is described for example in DE 23 43 571. Here the process is used to produce extruded textile filaments from a superheated melt of hydrated PAN (single phase melt). In addition to water, by adding an organic solvent (0.5 to 10% by mass relative to the polymer) or inorganic components (ZnCl2 or nitric acid) plasticization at elevated temperature and pressure is achieved. P 71.39396 describes the production of fibrous structures, so-called “plexifilaments”, with a specific surface of 0.6−3 m2/g from PAN and PA 6 which are produced by heating an aqueous suspension of the polymer using a stabilizer (silica gel or aluminosilicates) and an inert tenside (for example Triton X®).
According to DE-OS 41 18 298 A1 and DE-PS 41 22 994, to produce extremely fine fibers (fiber diameters between 0.1 and 100 microns) from a supercooled melt of PAN with at least 75% acrylonitrile portion even the organic solvent can be abandoned. In doing so a mixture of PAN and water is heated to a temperature above the melting point under closed conditions to form an amorphous melt. Then the amorphous melt is cooled to a temperature below the melting point of the supercooled phase and extruded in the temperature range between the melting point and the solidification temperature of the melt to form an extrudate. Water acts as the co-melting material which interacts with the polar nitrile groups of the PAN and cancels the helical twisting of the PAN molecule chains so that this melt can be spun in the supercooled state. Additives of any type should be critically evaluated especially for applications in which release of additives from the polymer into liquid should be avoided as much as possible.
While plasticization in water is known for copolymerizates of acrylonitrile and has been used to produce textile fibers (DE OS 22 48 244), for fibers of cellulose acetate (if not noted otherwise, the term “cellulose acetate” designates an acetyl cellulose ester with an acetic acid content between 52 and 55%) a production process in which softeners or organic solvents can be abandoned is not known to date. Generally fibers are obtained by precipitation from a homogenous solution or by extrusion of a melt which contains at least proportionally an organic solvent, softener or stabilizers.
U.S. Pat. No. 3,952,081 describes a process for producing extremely fine fibers from a CA suspension which is obtained by suspension of the polymer in a mixture of water and an organic solvent (acetone, methanol, ethanol, etc). A homogeneous solution is achieved by heating the suspension. This melt can be spun into filament yarn by suitable extrusion elements (4.75 Denier/filament or 2.6 Denier/filament).
U.S. Pat. No. 4,040,856 discloses that cellulose acetate dissolves at an elevated temperature and elevated pressure homogeneously in aqueous acetone solutions in which it is insoluble under normal conditions. In doing so the polymer passes through a plasticized state in which it can be converted into fibers by spinning nozzles below the temperature at which a homogenous solution is present. The content of organic solvent components can vary over the range between 1 to 60% by mass. In addition to acetone, also methanol, ethanol, and methylethyl ketone are suitable as solvents; a comparable solution behavior is also described for other cellulose alkanoates such as cellulose triacetate, cellulose acetate/propionate or cellulose acetate/butyrate. By releasing the system via a suitable nozzle a fiber pulp can be produced from the plasticized mass. One such process requires for solvent recovery an additional low pressure space from which the solvent can be exhausted and can be sent to reprocessing, and thus high equipment cost.
The object of the invention is to devise a process and a device with which fibers, especially extremely fine fibers, can be produced for filtration purposes, and additives or organic solvents will be abandoned and the process can be carried out economically.
This object is achieved with a process in which as the polymers homopolymers of polyacrylonitrile, polyamide or cellulose acetate are used, the suspension is heated in a tank to a temperature T which is at least 60° above the boiling point of the water and at least 30° C. below the softening temperature of the polymer and at which the at least plasticized polymer mass which has been obtained in this way is discharged through at least one nozzle means at least using the autogenous pressure of superheated water.
It has been ascertained unexpectedly that under these conditions a degree of plasticization of the polymer material is achieved which allows extrusion by suitable nozzle systems. An at least plasticized polymer mass is defined as a polymer mass which can contain dissolved portions. It cannot be precluded and is in part even advantageous that at least some of the polymer under the given pressures and temperatures is in a dissolved state, the water acting as the solvent. To achieve fine fibers a dissolved state of the polymer is advantageous. But it has been found that establishing a completely dissolved state of the polymer is very complex with respect to plant configuration and process guidance. Since proceeding from a predominantly plasticized state the required fiber finenesses can be achieved, the latter state represents a more economical version. By partially dissolving the polymer material the fiber strength and thus the strength of the fiber agglomerates produced from them can be increased.
Fibers are formed at the nozzle outlet by stretching the plasticized polymer mass in a shear field of the spontaneously vaporizing water. Stabilization of shape is achieved by the instantaneous cooling of the fibers as a result of removal of the vaporization heat of the superheated water.
The fibers are thus shaped by the configuration of the nozzle means, especially a capillary nozzle via which the superheated fluid is released into a area of much lower pressure and lower temperature.
The extremely fine fibers are produced predominantly for use in filter media for liquid filtration. Accordingly the fiber quality is assessed according to the filtration behavior of the deep filter papers. Their use is not however limited to this area. Their use is conceivable in nonwoven materials, for example in filter nonwovens for air filtration or as additives to special papers. Due to their large specific surface they can be used to advantage in chromatographic processes or selective adsorption processes in which they can be used as a carrier material for specific ligands. Compared to particular carrier materials microfiber networks due to their pores through which flow can take place offer the advantage that a large portion of the surface is made accessible to the flowing medium and thus the contact time necessary for formation of specific interactions is shortened.
The application liquid filtration substantiates the minimum requirements which must be met by the fibers produced as claimed in the invention: Very fine fiber dimensions (the fiber diameter should be less than 5 microns, preferably below 1 micron) are required which go along with a high specific surface from 5 to 30 m2/g (determined using the BET process). If these fibers are combined into networks or agglomerates with dimensions which are orders of magnitude greater than the fiber dimensions, a highly porous system is formed.
A filter layer can be produced by homogenous embedding of microfibers or their agglomerates into a matrix of coarser fibers (for example, cellulose pulp) or when used as a precoat material; this filter layer allows mechanical separation of particular sediments of unstable shape (for example, microorganisms) up to a size of 0.2 microns. By adsorption the minimum separable particle size can be further reduced. A homogenous distribution of the microfibers in the matrix presupposes a narrow range of variation in fiber diameters which is maintained by the process as claimed in the invention.
In the preferred application of clear filtration it must be ensured that the filtration medium is completely inert to the filtered material. This is achieved by the use of a suspension without organic solvents and additives which does not contain components which can be eluted by aqueous or alcohol solutions, for example softeners, solvent residues, but also hydrolysis products which could lead to contamination in an adverse case. For applications in less critical domains than in the pharmaceutical or food industry also small residues of softeners among others can be tolerated. In this way the use of economical raw materials is optionally possible. The advantage of the process that no solvents need be circulated and thus cannot be contained in the product is preserved. Furthermore the polymer can be easily wetted to keep the flow resistance of the filter medium low and to ensure a homogenous flow-through and thus uniform loading of the filter material.
In the area of deep filter layer production the microfibers enable replacement of mineral auxiliary filter agents such as kieselguhr, perlites or aluminosilicates.
In applications other than liquid filtration the replacement of glass fibers in air filter nonwovens is conceivable; their very fine types represent a health hazard due to the ease of their reaching the lungs. This enumeration is intended for example to represent the use of extremely fine fibers without precluding an application beyond it.
This temperature provided as claimed in the invention is advantageous in that in this way a corresponding pressure can be built up in the pressure vessel and the water is superheated to the extent that when the plasticized mass emerges from the capillary nozzle the desired shear field is built up.
Furthermore, it is advantageous if the aqueous suspension is heated only to a maximum 30° C. below the softening temperature of the respective polymer in order to prevent overly strong melting of the polymer.
Preferably polyamide 6 or polyamide 6, 6 is used as the polyamide.
The softening temperatures of the polymers are as follows:
PAN: 305°-310° CA (diacetate): 225°-250°
PA 6: 215°-220° CA (triacetate): 290°-310°
PA 6, 6: 255°-260°
Preferably cellulose diacetate or cellulose triacetate is used as the cellulose acetate. The degree of acetylation is preferably 2.5 for diacetate and 2.9 to 3 for triacetate.
Advantageously the cellulose acetate has an acetic acid content of 52-56%.
It has been shown that by means of the process as claimed in the invention polymers such as PAN, which normally cannot be extruded, can also be extruded because the water assumes the function of a lubricant.
The suspension is preferably kept at the temperature T for a maximum 240 sec. The preferred maximum times for the individual polymers are as follows:
Cellulose diacetate: max. 60 seconds
Cellulose triacetate: max. 90 seconds
PA: max. 120 seconds
PAN: max. 240 seconds
While PAN and PA have low susceptibility to thermal damage, it is of special importance in cellulose acetate not to exceed an interval of 60 seconds and 90 seconds. In cellulose acetate decomposition by hydrolysis and oxidation occurs under certain circumstances. Hydrolysis can begin at 200° C. due to easy water repellency. Cellulose acetate at high pressures and temperatures can intercalate water molecules between the polymer molecules. Due to the presence of water it has been surprisingly found that cellulose acetate can be quickly heated even into the area of the softening point and can be extruded jointly with water without the cellulose acetate being seriously damaged.
Preferably the suspension is continuously mixed during heating.
Preferably the polymer concentration in the suspension is 1 to 10% by mass.
Preferably the fibers are discharged in a space with a pressure≦atmospheric pressure. It is complex to establish a fine vacuum and entails only few advantages with respect to turbulence for fiber formation. Practicable vacuum values are preferably up to 0.8 bar.
Advantageously the suspension is exposed to a pressure between 15 and 25 bar during heating.
Preferably a polymer powder with grain sizes between 20 and 100 microns is used. So-called flakes can also be used with dimensions up to 10 mm.
The device for continuous production of filtration-active fibers from PAN, PA or cellulose acetate has a storage tank for preparing the suspension and a mixing tank which is connected to the storage tank and which is made for heating an aqueous polymer suspension under pressure. At the output of the mixing tank there is a nozzle means for discharge of the plasticized polymer mass.
The nozzle means has at least one capillary nozzle, one pneumatic nozzle, one needle valve or one binary nozzle. In the binary nozzle the channel which discharges the suspension is surrounded by an annulus through which steam is expelled at the same time. In this way, on the one hand cooling of the polymers is slowed down and on the other hand turbulence is increased; both have a beneficial effect on fiber formation.
Preferably there are three capillary nozzles adjacent to one another; their outlet openings are aligned at an acute angle to one another. This arrangement has the advantage that the turbulence field is intensified at the site of formation of extremely fine fibers. The diameter of the capillary nozzles is preferably 0.1-10 mm.
For example, for production of fibers with a diameter of 1-10 microns capillary nozzles with a diameter of 1 mm are suited. The ratio of the length to the diameter of the capillary nozzle is preferably 30-50.
Preferably the mixing tank is an extruder.
According to another embodiment the mixing tank can have a dispersing means.
Advantageously a coarse mixer is connected upstream of the mixing tank.
Advantageously behind the nozzle means there is a discharge tank in which a pressure≦atmospheric pressure can be set.
In addition there can be another metering pump in front of the nozzle means.
Advantageously another heat exchanger can be connected upstream of the mixing tank.
The process can be carried out both in batches and also continuously. Fundamentally for both types of processes the same structure of the device can be used, and in batch operation delivery means in front of the mixing tank can be abandoned.
One sample embodiment of the device is detailed below using the drawings.