PROCESS FOR THE RECOVERY OF SYNTHETIC FIBRILS
United States Patent 3883630
A process for recovering synthetic polymer fibrils which are in the presence of vapors of an organic solvent, by depositing the fibrils on a moving porous support in a closed vessel and continuously extracting the solvent vapors from the vessel by suction.
US Patent References:
Manufacture of artificial materials
Taylor et al. - March 1937 - 2072928
Method and apparatus for spinning unwoven fabrics
Manning - December 1943 - 2336743
Process for producing fibrous products
Francis, Jr. - September 1944 - 2357392
Method of making filter cartridges, abrasive sheets, scouring pads, and the like
Manning - November 1946 - 2411660
Dry spinning system
Burke - July 1950 - 2515393
Manufacture of artificial materials
Taylor et al. - March 1937 - 2072928
Method and apparatus for spinning unwoven fabrics
Manning - December 1943 - 2336743
Process for producing fibrous products
Francis, Jr. - September 1944 - 2357392
Method of making filter cartridges, abrasive sheets, scouring pads, and the like
Manning - November 1946 - 2411660
Dry spinning system
Burke - July 1950 - 2515393
Application Number:
05/286683
Publication Date:
05/13/1975
Filing Date:
09/06/1972
View Patent Images:
Export Citation:
Assignee:
Solvay & Cie (Brussels, BE)
Primary Class:
Other Classes:
264/518, 264/87, 264/37.130, 162/157.500, 162/157.400, 162/157.300, 264/204
International Classes:
C08J11/02; D01D5/11; D04H13/00; C08J11/00; D01D5/00; B29B5/04
Field of Search:
162/102,203,211,217,157R 264/87,91,101.2,112,37,38,203-205,24F 764/DIG.75
US Patent References:
| 2906367 | De-gasifying liquids | September 1959 | Vandenburgh | |
| 3026190 | Elastomer bonded abrasives | March 1962 | McMahon et al. | |
| 3056719 | Continuous web forming machine | October 1962 | Webster | |
| 3094374 | Dry spinning process for preparing coalesced spandes filaments | June 1963 | Smith | |
| 3111368 | Process for preparing spandex filaments | November 1963 | Romano | |
| 3210452 | Dry spinning of polyethylene | October 1965 | Howard | |
| 3729374 | April 1973 | Lissalde |
Primary Examiner:
Arnold, Donald J.
Assistant Examiner:
Auville, Gene
Attorney, Agent or Firm:
Spencer & Kaye
Claims:
I claim
1. In a process for the recovery of synthetic polymer fibrils which are in the presence of vapors of an organic solvent, the improvement comprising the steps of
2. passing a movable porous support through a closed vessel which is maintained free of air and contains water vapors, which water vapors are of a liquid which is immiscible with the organic solvent and inert in relation to the polymer of which the fibrils are composed, said water vapors acting to reduce condensation of the solvent vapors in the closed vessel and having a partial pressure between 5% and 80% of the total pressure prevailing in the vessel;
3. introducing the fibrils and organic solvent vapors into the vessel and depositing the fibrils in the form of a sheet onto the movable porous support in the vessel;
4. continuously extracting the solvent vapors from the vessel by suction; and
5. continuous extracting from the vessel the sheet of fibrils formed in the vessel by passing it through a hydraulic seal constituted by water in the form of a liquid phase to remove any remaining traces of solvent from the fibrils.
6. A process according to claim 1, wherein the pressure prevailing in the closed vessel is higher than atmospheric pressure.
7. A process according to claim 1, wherein the temperature prevailing in the vessel is higher than the condensation temperature of the organic solvent vapors under the pressure conditions prevailing in the vessel.
8. A process according to claim 1, wherein the temperature prevailing inside the vessel is lower than the boiling temperature of the liquid water under the pressure conditions prevailing in the vessel.
9. A process according to claim 1, wherein the temperature prevailing in the vessel is between 50° and 100°C, and the pressure prevailing in the closed vessel is sufficiently high to prevent the water from boiling.
10. A process according to claim 1, wherein the fibrils are continuous and form a three-dimensional network.
11. A process according to claim 1, wherein the fibrils are discontinuous and have a length varying from 1mm to 5mm.
12. A process according to claim 1, wherein the fibrils are produced from a polymer selected from the group comprising the polyolefins, polyamides, polyesters, polyurethanes, polycarbonates, vinyl and acrylic polymers.
13. A process according to claim 1, wherein the organic solvent is a product selected from the group formed by the aliphatic, alicyclic, and aromatic hydrocarbons, halogenated solvents, alcohols, ketones, esters, and ethers.
14. A process according to claim 1, wherein the polymer of which the fibrils are composed is a high density polyethylene.
15. A process according to claim 10, wherein the organic solvent is hexane.
1. In a process for the recovery of synthetic polymer fibrils which are in the presence of vapors of an organic solvent, the improvement comprising the steps of
2. passing a movable porous support through a closed vessel which is maintained free of air and contains water vapors, which water vapors are of a liquid which is immiscible with the organic solvent and inert in relation to the polymer of which the fibrils are composed, said water vapors acting to reduce condensation of the solvent vapors in the closed vessel and having a partial pressure between 5% and 80% of the total pressure prevailing in the vessel;
3. introducing the fibrils and organic solvent vapors into the vessel and depositing the fibrils in the form of a sheet onto the movable porous support in the vessel;
4. continuously extracting the solvent vapors from the vessel by suction; and
5. continuous extracting from the vessel the sheet of fibrils formed in the vessel by passing it through a hydraulic seal constituted by water in the form of a liquid phase to remove any remaining traces of solvent from the fibrils.
6. A process according to claim 1, wherein the pressure prevailing in the closed vessel is higher than atmospheric pressure.
7. A process according to claim 1, wherein the temperature prevailing in the vessel is higher than the condensation temperature of the organic solvent vapors under the pressure conditions prevailing in the vessel.
8. A process according to claim 1, wherein the temperature prevailing inside the vessel is lower than the boiling temperature of the liquid water under the pressure conditions prevailing in the vessel.
9. A process according to claim 1, wherein the temperature prevailing in the vessel is between 50° and 100°C, and the pressure prevailing in the closed vessel is sufficiently high to prevent the water from boiling.
10. A process according to claim 1, wherein the fibrils are continuous and form a three-dimensional network.
11. A process according to claim 1, wherein the fibrils are discontinuous and have a length varying from 1mm to 5mm.
12. A process according to claim 1, wherein the fibrils are produced from a polymer selected from the group comprising the polyolefins, polyamides, polyesters, polyurethanes, polycarbonates, vinyl and acrylic polymers.
13. A process according to claim 1, wherein the organic solvent is a product selected from the group formed by the aliphatic, alicyclic, and aromatic hydrocarbons, halogenated solvents, alcohols, ketones, esters, and ethers.
14. A process according to claim 1, wherein the polymer of which the fibrils are composed is a high density polyethylene.
15. A process according to claim 10, wherein the organic solvent is hexane.
Description:
BACKGROUND OF THE INVENTION
The present invention relates to a process for the recovery of synthetic polymer fibrils which are in the presence of vapors of an organic solvent, and also to an apparatus which is particularly suitable for carrying out this process.
Various processes have already been proposed which enable fibrils or fibrillate structures to be produced from synthetic polymers. Thus, according to Belgian Pat. No. 568,524 issued June 11, 1958, in the name of E.I. du Pont de Nemours, continuous structures are obtained which are composed of a multitude of fibrillar strands or sections which are joined together and separated at random intervals in order to form a "unitary fibrillar plexus" by extruding a solution of a synthetic polymer, which is at a temperature higher than the normal boiling point of the solvent and under autogenous or higher pressure, through an orifice of suitable shape into a zone of lower pressure.
By this technique the instantaneous vaporization of the solvent of the polymer solution at the moment of abrupt expansion leads to the formation of continuous fibrillate structures, which are delivered at very high speed, their formation obviously being accompanied by the formation of a very large volume of solvent vapors.
In another technique, discontinuous fibrils are produced by abrupt expansion through an aperture of a two-phase liquid mixture of molten polymer and solvent which is at elevated temperature and pressure, so as to bring about the instantaneous vaporization of the solvent and to solidify the polymer when an additive fluid is introduced into the aforesaid two-phase liquid mixture before expansion is complete. The additive fluid may be identical to the solvent used to form the two-phase liquid mixture. This second technique also entails the production of a large volume of solvent vapors.
In a third technique, discontinuous fibrils are also produced by effecting the abrupt expansion of a two-phase liquid mixture of molten polymer and solvent which is at elevated temperature and pressure so as to effect the instantaneous vaporization of the solvent and to form a continuous fibrillate structure, and shredding the continuous fibrillate structure thus produced, at the moment of its formation, by a transverse current of fluid.
The fluid constituting the transverse current may be identical with the solvent used to form the two-phase liquid mixture. The production of discontinuous fibrils by this last technique also leads to the release of large volumes of solvent vapors.
It therefore appears that all known methods of producing synthetic polymer fibrils entail the use of very large amounts of an organic solvent, which is completely vaporized in the last stage of the production process.
Consequently, in order above all to reduce the production cost price of fibrils, these solvent vapors should be recovered to permit their recycling after condensation. This recovery is moreover indispensable and poses considerable problems when these solvent vapors are toxic and entail the risk of forming explosive mixtures with oxygen.
Moreover, in order to serve a useful purpose, the fibrils produced must be recovered in an easily handled form, for example in the form of a continuous sheet possessing a certain internal cohesion and capable of being subsequently treated by known conventional techniques for the production of non-woven textiles of synthetic papers.
SUMMARY OF THE INVENTION
It is therefore the object of the invention to provide a process and apparatus which accomplish the the above-described goals.
The invention involves a process for the recovery of synthetic polymer fibrils which are in the presence of vapors of an organic solvent, by depositing the fibrils in the form of a sheet on a movable porous support which permits practically complete recovery of the solvent vapors.
In the process according to the present invention the recovery of fibrils which are in the presence of organic solvent vapors is effected in a closed vessel containing vapors of a liquid which is not miscible with the organic solvent and which is inert in relation to the polymer of which the fibrils are composed, the solvent vapors being continuously extracted from this vessel by suction.
The term "fibrils" as used herein includes continuous or discontinuous fibrillar structures composed of very fine filaments, of the thickness of about 1 micron, connected together to form a three-dimensional network, the specific surface of these products being greater than 1 m 2 /g. The discontinuous fibrils usually have a length varying from 1 mm to 5 cm.
According to a preferred embodiment, the closed vessel also contains liquid which is not miscible with the organic solvent and which is inert in relation to the polymer of which the fibrils are composed.
The expression "liquid not miscible with an organic solvent" is also intended to include liquids which are slightly miscible with the organic solvent. This term will therefore include any liquid which, on being mixed with the organic solvent, gives rise, after 60 seconds rest, to the formation of two phases separated by an interfacial meniscus, each phase containing respectively at least 95% of one of the constituents of the mixture at 20°C.
In addition, the immiscible liquid must be inert in relation to the polymer of which the fibrils are composed. As a matter of preference it should dissolve less than 20g of polymer per kg of liquid at 20°C. The partial pressure of the vapors of liquid which is not miscible with the organic solvent, and which is inert in relation to the polymer of which the fibrils are composed, in the closed vessel is preferably so regulated as to be kept between 5% and 80%, particularly between 10% and 50%, of the total pressure prevailing in the vessel.
The preferred immiscible liquid is water, particularly because of its low cost and of the special advantages which it provides and which will be explained later. It is however quite obvious that other immiscible liquids may likewise be suitable for carrying out the process according to the present invention.
In order to avoid any accidental admission of air into the closed vessel, it is preferable that the total pressure prevailing in the latter should be higher than atmospheric pressure. The increase in pressure in relation to atmospheric pressure may advantageously be between 50 and 500 mm of a column of water.
In order to facilitate their recovery, it is preferred to avoid, or at least to reduce, as much as possible the condensation of the organic solvent vapors in the closed vessel. It is consequently convenient for the temperature prevailing in the vessel to be higher than the temperature of condensation of the organic solvent vapors under the conditions of pressure prevailing in the vessel.
It has been found that a temperature in the vessel of between 50° and 100°C is generally sufficient to avoid the condensation of the vapors of organic solvents usually employed for the production of fibrils.
It is also preferred that the temperature prevailing in the closed vessel should be lower than the boiling temperature of the liquid which is immiscible with the organic solvent under the pressure conditions prevailing in the closed vessel. This has the result that if, in certain particular cases of production of fibrils, the organic solvent used has a condensation temperature substantially higher than 100°C under the conditions of pressure prevailing in the closed vessel, it is preferable for the immiscible liquid used to be a liquid other than water and having a boiling point higher than the condensation point of the organic solvent under the same conditions of pressure.
The vapors of organic solvent are continuously extracted from the closed vessel by suction, mixed with vapors of immiscible liquid. They are preferably drawn by suction through the movable porous support of the fibril sheet, because the suction then cooperates in forming the sheet of fibrils and gives it better cohesion. The immiscible liquid vapors extracted from the closed vessel at the same time as the organic solvent vapors do not entail any particular problem, because they can easily be separated from the solvent by condensation and decantation.
The sheet of fibrils continuously formed on the movable porous support is continuously extracted from the closed vessel through a device which maintains the tightness of the closed vessel. This device may for example be a sealing system of labyrinth rollers. It is nevertheless preferred to extract the sheet of fibrils from the closed vessel through a sealing device constituted by a hydraulic seal formed by the liquid which is not miscibile with the organic solvent.
The process according to the invention may be applied to any process for the production of synthetic polymer fibrils in the presence of organic solvent vapors.
Among the polymers which can be used for the production of such fibrils, mention should be made of polyolefins, such as polyethylene, polypropylene, copolymers of ethylene and propylene, polyisobutylene, etc., the polyamides, polyesters, polyurethanes, polycarbonates, vinyl polymers, such as optionally post-chlorinated polyvinyl chloride, polyvinyl fluoride, etc., the acrylic polymers such as acrylonitrile homopolymers and copolymers, this list being enumerative and non-limitative.
Among the organic solvents used with these polymers, mention may be made of the aliphatic hydrocarbons, such as pentane, hexane, heptane, octane, and their homologues and isomers, the alicyclic hydrocarbons such as cyclohexane, the aromatic hydrocarbons, such as benzene, toluene, etc., halogenated solvents, such as the chlorofluoromethanes, methylene chloride, ethyl chloride, etc., the alcohols, ketones, esters, ethers, and also mixtures of these same solvents. These solvents are generally toxic and many of them are readily inflammable and entail the risk of explosion.
The process according to the invention offers the advantage of permitting total recovery of the organic solvent, while eliminating any danger of accident. Moreover, when the immisicible liquid is water or another polar liquid, the risk of explosion due to the presence of electrostatic charges is eliminated. Finally, the passage of the sheet of fibrils through the hydraulic seal permits the elimination of the last traces of solvent with which the sheet may be impregnated.
The invention also involves an apparatus which is particularly suitable for carrying out the process according to the invention. This apparatus has:
a. a closed vessel containing vapors of a liquid which is immiscible with the organic solvent and inert in relation to the polymer of which the fibrils are composed;
b. a device for introducing the fibrils and the solvent vapors into the top of the vessel;
c. a movable porous support moving continuously under this device in the vessel;
d. a suction box situated under the movable porous support, facing the device for introduction of fibrils and solvent vapors and provided for the purpose of effecting the formation of the sheet of fibrils and the evacuation of the major part of the organic solvent vapors;
e. a sealing device for continuously removing from the closed vessel the sheet of fibrils deposited on the movable porous support; and
f. a device for the introduction into the vessel of the liquid which is immiscible with the organic solvent and inert in relation to the polymer of which the fibrils are composed.
According to a preferred embodiment, the bottom of the closed vessel constitutes a reservoir intended to contain liquid immiscible with the organic solvent and inert in relation to the polymer of which the fibril is composed, in the form of a liquid phase in equilibrium with the vapors of immiscible liquid.
The device for introducing fibrils and solvent vapors into the closed vessel may for example be a chimney, a pipe, etc.
The immiscible liquid may be introduced into the vessel in any form, for example, in gas, vapor, or liquid form.
The movable porous support may be a simple rotating drum, provided that the sheet of fibrils deposited on it has sufficient cohesion to be self-supporting when it leaves this drum. This cohesion may optionally be increased by means of a pressing roller acting on the sheet so as to compact it just before it leaves the rotating drum.
In order to prevent the sheet from tearing for lack of cohesion, it is nevertheless preferred to use, as a movable porous support, a continuous porous band which accompanies and supports the sheet during its extraction through the sealing device.
This porous band may be a microperforated metal sheet, a very fine wire mesh, a felt, or a cloth. This band must be as undeformable and as wear-resistant as possible. In addition, the material of which it is composed must withstand the organic solvent vapors used. It has in particular been found that cloths made from polyester fibers such as ethylene glycol polyterephthalate fibers or polypropylene fibers, prove to be particularly advantageous because of their chemical inertness and their hydrophobic character.
The sealing device may be of any kind provided that it is effective, and it may for example as above mentioned, be composed of a system of labyrinth rollers. According to a preferred embodiment, this sealing device is composed of a hydraulic seal formed by the liuid phase of liquid immiscible with the organic solvent which lies at the bottom of the closed vessel, in a reservoir provided for this purpose.
The bottom part of this closed vessel may also be provided with heating means for the bottom layer of liquid immiscible with the organic solvent.
In order to permit the elimination of any fibrils which may be deposited on the surface of this liquid, and of other impurities such as polymers of low molecular weight, it is convenient to provide for continuous circulation of this liquid, for example by means of continuous feeding and of evacuation by means of an overflow. The liquid removed from the vessel may in addition undergo decantation treatment for the recovery of any organic solvent that it may contain.
The apparatus may also be completed by a second continuous band bearing on the sheet of fibrils deposited on the first porous band during its passage through the hydraulic seal. In this way the sheet of fibrils is sandwiched between the two continuous bands, which oblige it to pass through the sealing device. This modified embodiment is found very useful when the fibrils have a density lower than the density of the liquid constituting the hydraulic seal.
The closed vessel is provided with heating and monitoring means so that a thermostat can be used to maintain its walls at the desired temperature, depending on the organic solvent used and with the aim of avoiding condensation of the solvent.
The apparatus according to the present invention is described in greater detail in the following description of the preferred form of its construction. It is however understood that this description is given purely by way of illustration and that it does not in any way limit the scope of the present invention, because the apparatus which is described below may undergo numerous modifications within the range of those versed in the art, without thereby departing from the framework or spirit of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic view of an apparatus according to the invention.
FIG. 2 is an enlarged view of a detail in FIG. 1, showing the formation of the sheet of fibrils and its compaction.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As can be seen from FIGS. 1 and 2, the apparatus is composed of a closed vessel 1, to the top of which is connected a chimney 2 through which the fibrils in the presence of vapors of an organic solvent are introduced into the closed vessel 1.
In FIG. 1 it can be seen that this mixture of fibrils and solvent vapors is obtained by the abrupt expansion of a polymer solution through nozzle 3. It is however quite obvious that the apparatus described is in no way restricted to this particular form of production of fibrils, and that it could be just as effective for the recovery of fibrils which are in the presence of organic solvent vapors and are obtained by other processes. A suction box 4, over which passes a continuous porous band 5 composed of a cloth of ethylene glycol polyterephthalate fibers, is situated in the closed vessel 1, exactly facing the bottom opening of the chimney 2.
The continuous band 5 has a constant, controllable speed of linear movement and follows the trajectory imposed on it by the support rollers 6 to 13.
After having passed beyond the suction box 4, the continuous porous band 5 passes into the gap formed by the driving support roller 7 and the compression roller 14, which is adjustable by means of the pneumatic or hydraulic device 15.
As soon as this porous band 5 leaves the compression roller 14, a second continuous band 16 of the same nature as the first band, or if desired of a different nature, is applied progressively against the porous band 5 and accompanies the latter during part of its trajectory. This second band 16 moves between the support rollers 17 to 21 at a speed equal to that of the porous band 5.
At the bottom the closed vessel is provided with a hydraulic seal 22 composed of a liquid which is immiscible with the organic solvent and inert in relation to the polymer of which the fibrils are composed. The bottom part of the vessel also contains an overflow weir 23 connected to an evacuation pipe 24.
The two continuous bands 5 and 16 pass through the hydraulic seal 22, through which these bands leave the closed vessel 1. The path of these bands is imposed by the support rollers 7, 8, 9, and 19. The last two support rollers 9 and 19 are so disposed as to move the continuous bands 5 and 16 away from one another. After their passage out of the vessel, during which they release the sheet of fibrils, these continuous bands enter the closed vessel again, passing once again through the hydraulic seal 22, under the action of the support rollers 11, 12 and 13 in one case of the support l7, 18 and 21 in the other.
A drier 25 may optionally be provided to dry the continuous porous band 5 between the rollers 6 and 13 before it passes over the suction box 4.
This drier may be suction box provided with a slot over which the continuous band slides, this box being connected toa vacuum source not shown. In this way the vapors drawn in under local conditions of high speed and low pressure effect the elimination of the liquid wetting the band.
In a modified embodiment the continuous band 16 may also pass around the pressing roller 14 and pass through the cap between this roller and the driving roller 7.
The level of liquid immiscible with the organic solvent contained in the bottom part of the closed vessel 1 is determined by the overflow 23.
Heating means, not shown, are provided in the bottom of the closed vessel 1, so as to bring the liquid contained in this vessel to the desired temperature and to keep it at this temperature.
The device for be a the immiscible liquid into the closed vessel is composed of a pipe, likewise not shown, which to a supplies immiscible liquid continuously into the closed vessel 1 so as to produce a circulation.
The closed vessel 1 is in addition equipped with means, such as a jacket, enabling its walls and consequently its interior to be kept at a predetermined temperature. Finally, during their passage out of the closed vessel 1 the continuous bands 5 and 16 may pass over tensioning, crease-removing or centering rollers 26. In addition, they may be cleaned by rotating brushes or by water sprays.
The operation of the apparatus as described above is in addition made clear by the example of practical performance which is given below. It is however clearly understood that this example, which is given solely by way of illustration, does not in any way limit the scope of the present invention.
EXAMPLE
A solution of 150 g/kg of high density (0.950) polyethylene in hexane, this solution being at a pressure of 70 kg/cm 2 and a temperature of 180°C, are injected at a rate of flow of 50 kg polyethylene per hour through the main pipe, and hexane under a pressure of 70 kg/cm 2 and a temperature of 180°C is simultaneously injected through the secondary pipe, into the nozzle 3, which is composed of a LECHLER KSD 12 atomizer of the firm LECHLER APPARATEBAU K.G.. The volumetric flow of hexane is substantially equal to the volumetric flow of the polyethylene solution.
Through the abrupt expansion of the polyethylene solution and the instantaneous vaporization of the solvent there is obtained at the outlet of the nozzle 3 a suspension of discontinuous fibrils of polyethylene in hexane vapors.
This suspension of fibrils is directed through the chimney 2 into the closed vessel 1. This vessel has the external shape of a cube with a side dimension of about 3 meters.
The walls of the closed vessel 1 are thermostatically controlled at a temperature of 90°C, and the vessel contains a volume of water of the order of 10 m 3 , which is kept at 80°C. The continuous supply flow of water is kept at 1 m 3 per hour.
At the base of the chimney 2 of the polyethylene fibrils 27 are deposited on the continuous porous band 5 through the action of the suction box 4, which is connected to an installation, not shown, for recovery of hexane vapors. The band, which has a width of 2.3 meters, is made of ethylene glycol polyeterephthalate fibers and has a weight of 1,000 g/m 2 . The suction is so controlled that the pressure prevailing at any moment in the closed vessel 1 will be slightly higher than atmospheric pressure. The excess pressure, of the order of 300 mm water column, has the effect of preventing any accidental entry of air into the closed vessel 1. The partial pressure of the water vapor in the vessel is 0.255 atmospheres absolute and the partial pressure of hexane vapors is about 0.775 atmospheres absolute.
The presence of water vapor in the closed vessel and the temperature and pressure conditions prevailing in the latter reduce the condensation of the hexane vapors to a minimum, and consequently the fibril sheet 28 which is progressively formed on the porous band 5 is practically free from liquid hexane.
This sheet 28 of tangled fibrils 27 deposited on the porous band 5, which moves at a linear speed of 10 meters per minute, is then compacted by passing between the drive roller 7 and the pressing roller 14.
The sheet compacted in this manner is then progressively imprisoned between the continuous bands 5 and 16, which oblige it to pass through the hydraulic seal 22, where it is freed of the last traces of solvent.
On passing out of the hydraulic seal 22, the sheet 28 of compact fibrils is freed by the continuous band 5 and 16. This sheet, the weight of which is from about 80 to 110 g/m 2 can then be consolidated and converted into a non-woven product of good quality by conventional techniques, such as by calendering.
It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.
The present invention relates to a process for the recovery of synthetic polymer fibrils which are in the presence of vapors of an organic solvent, and also to an apparatus which is particularly suitable for carrying out this process.
Various processes have already been proposed which enable fibrils or fibrillate structures to be produced from synthetic polymers. Thus, according to Belgian Pat. No. 568,524 issued June 11, 1958, in the name of E.I. du Pont de Nemours, continuous structures are obtained which are composed of a multitude of fibrillar strands or sections which are joined together and separated at random intervals in order to form a "unitary fibrillar plexus" by extruding a solution of a synthetic polymer, which is at a temperature higher than the normal boiling point of the solvent and under autogenous or higher pressure, through an orifice of suitable shape into a zone of lower pressure.
By this technique the instantaneous vaporization of the solvent of the polymer solution at the moment of abrupt expansion leads to the formation of continuous fibrillate structures, which are delivered at very high speed, their formation obviously being accompanied by the formation of a very large volume of solvent vapors.
In another technique, discontinuous fibrils are produced by abrupt expansion through an aperture of a two-phase liquid mixture of molten polymer and solvent which is at elevated temperature and pressure, so as to bring about the instantaneous vaporization of the solvent and to solidify the polymer when an additive fluid is introduced into the aforesaid two-phase liquid mixture before expansion is complete. The additive fluid may be identical to the solvent used to form the two-phase liquid mixture. This second technique also entails the production of a large volume of solvent vapors.
In a third technique, discontinuous fibrils are also produced by effecting the abrupt expansion of a two-phase liquid mixture of molten polymer and solvent which is at elevated temperature and pressure so as to effect the instantaneous vaporization of the solvent and to form a continuous fibrillate structure, and shredding the continuous fibrillate structure thus produced, at the moment of its formation, by a transverse current of fluid.
The fluid constituting the transverse current may be identical with the solvent used to form the two-phase liquid mixture. The production of discontinuous fibrils by this last technique also leads to the release of large volumes of solvent vapors.
It therefore appears that all known methods of producing synthetic polymer fibrils entail the use of very large amounts of an organic solvent, which is completely vaporized in the last stage of the production process.
Consequently, in order above all to reduce the production cost price of fibrils, these solvent vapors should be recovered to permit their recycling after condensation. This recovery is moreover indispensable and poses considerable problems when these solvent vapors are toxic and entail the risk of forming explosive mixtures with oxygen.
Moreover, in order to serve a useful purpose, the fibrils produced must be recovered in an easily handled form, for example in the form of a continuous sheet possessing a certain internal cohesion and capable of being subsequently treated by known conventional techniques for the production of non-woven textiles of synthetic papers.
SUMMARY OF THE INVENTION
It is therefore the object of the invention to provide a process and apparatus which accomplish the the above-described goals.
The invention involves a process for the recovery of synthetic polymer fibrils which are in the presence of vapors of an organic solvent, by depositing the fibrils in the form of a sheet on a movable porous support which permits practically complete recovery of the solvent vapors.
In the process according to the present invention the recovery of fibrils which are in the presence of organic solvent vapors is effected in a closed vessel containing vapors of a liquid which is not miscible with the organic solvent and which is inert in relation to the polymer of which the fibrils are composed, the solvent vapors being continuously extracted from this vessel by suction.
The term "fibrils" as used herein includes continuous or discontinuous fibrillar structures composed of very fine filaments, of the thickness of about 1 micron, connected together to form a three-dimensional network, the specific surface of these products being greater than 1 m 2 /g. The discontinuous fibrils usually have a length varying from 1 mm to 5 cm.
According to a preferred embodiment, the closed vessel also contains liquid which is not miscible with the organic solvent and which is inert in relation to the polymer of which the fibrils are composed.
The expression "liquid not miscible with an organic solvent" is also intended to include liquids which are slightly miscible with the organic solvent. This term will therefore include any liquid which, on being mixed with the organic solvent, gives rise, after 60 seconds rest, to the formation of two phases separated by an interfacial meniscus, each phase containing respectively at least 95% of one of the constituents of the mixture at 20°C.
In addition, the immiscible liquid must be inert in relation to the polymer of which the fibrils are composed. As a matter of preference it should dissolve less than 20g of polymer per kg of liquid at 20°C. The partial pressure of the vapors of liquid which is not miscible with the organic solvent, and which is inert in relation to the polymer of which the fibrils are composed, in the closed vessel is preferably so regulated as to be kept between 5% and 80%, particularly between 10% and 50%, of the total pressure prevailing in the vessel.
The preferred immiscible liquid is water, particularly because of its low cost and of the special advantages which it provides and which will be explained later. It is however quite obvious that other immiscible liquids may likewise be suitable for carrying out the process according to the present invention.
In order to avoid any accidental admission of air into the closed vessel, it is preferable that the total pressure prevailing in the latter should be higher than atmospheric pressure. The increase in pressure in relation to atmospheric pressure may advantageously be between 50 and 500 mm of a column of water.
In order to facilitate their recovery, it is preferred to avoid, or at least to reduce, as much as possible the condensation of the organic solvent vapors in the closed vessel. It is consequently convenient for the temperature prevailing in the vessel to be higher than the temperature of condensation of the organic solvent vapors under the conditions of pressure prevailing in the vessel.
It has been found that a temperature in the vessel of between 50° and 100°C is generally sufficient to avoid the condensation of the vapors of organic solvents usually employed for the production of fibrils.
It is also preferred that the temperature prevailing in the closed vessel should be lower than the boiling temperature of the liquid which is immiscible with the organic solvent under the pressure conditions prevailing in the closed vessel. This has the result that if, in certain particular cases of production of fibrils, the organic solvent used has a condensation temperature substantially higher than 100°C under the conditions of pressure prevailing in the closed vessel, it is preferable for the immiscible liquid used to be a liquid other than water and having a boiling point higher than the condensation point of the organic solvent under the same conditions of pressure.
The vapors of organic solvent are continuously extracted from the closed vessel by suction, mixed with vapors of immiscible liquid. They are preferably drawn by suction through the movable porous support of the fibril sheet, because the suction then cooperates in forming the sheet of fibrils and gives it better cohesion. The immiscible liquid vapors extracted from the closed vessel at the same time as the organic solvent vapors do not entail any particular problem, because they can easily be separated from the solvent by condensation and decantation.
The sheet of fibrils continuously formed on the movable porous support is continuously extracted from the closed vessel through a device which maintains the tightness of the closed vessel. This device may for example be a sealing system of labyrinth rollers. It is nevertheless preferred to extract the sheet of fibrils from the closed vessel through a sealing device constituted by a hydraulic seal formed by the liquid which is not miscibile with the organic solvent.
The process according to the invention may be applied to any process for the production of synthetic polymer fibrils in the presence of organic solvent vapors.
Among the polymers which can be used for the production of such fibrils, mention should be made of polyolefins, such as polyethylene, polypropylene, copolymers of ethylene and propylene, polyisobutylene, etc., the polyamides, polyesters, polyurethanes, polycarbonates, vinyl polymers, such as optionally post-chlorinated polyvinyl chloride, polyvinyl fluoride, etc., the acrylic polymers such as acrylonitrile homopolymers and copolymers, this list being enumerative and non-limitative.
Among the organic solvents used with these polymers, mention may be made of the aliphatic hydrocarbons, such as pentane, hexane, heptane, octane, and their homologues and isomers, the alicyclic hydrocarbons such as cyclohexane, the aromatic hydrocarbons, such as benzene, toluene, etc., halogenated solvents, such as the chlorofluoromethanes, methylene chloride, ethyl chloride, etc., the alcohols, ketones, esters, ethers, and also mixtures of these same solvents. These solvents are generally toxic and many of them are readily inflammable and entail the risk of explosion.
The process according to the invention offers the advantage of permitting total recovery of the organic solvent, while eliminating any danger of accident. Moreover, when the immisicible liquid is water or another polar liquid, the risk of explosion due to the presence of electrostatic charges is eliminated. Finally, the passage of the sheet of fibrils through the hydraulic seal permits the elimination of the last traces of solvent with which the sheet may be impregnated.
The invention also involves an apparatus which is particularly suitable for carrying out the process according to the invention. This apparatus has:
a. a closed vessel containing vapors of a liquid which is immiscible with the organic solvent and inert in relation to the polymer of which the fibrils are composed;
b. a device for introducing the fibrils and the solvent vapors into the top of the vessel;
c. a movable porous support moving continuously under this device in the vessel;
d. a suction box situated under the movable porous support, facing the device for introduction of fibrils and solvent vapors and provided for the purpose of effecting the formation of the sheet of fibrils and the evacuation of the major part of the organic solvent vapors;
e. a sealing device for continuously removing from the closed vessel the sheet of fibrils deposited on the movable porous support; and
f. a device for the introduction into the vessel of the liquid which is immiscible with the organic solvent and inert in relation to the polymer of which the fibrils are composed.
According to a preferred embodiment, the bottom of the closed vessel constitutes a reservoir intended to contain liquid immiscible with the organic solvent and inert in relation to the polymer of which the fibril is composed, in the form of a liquid phase in equilibrium with the vapors of immiscible liquid.
The device for introducing fibrils and solvent vapors into the closed vessel may for example be a chimney, a pipe, etc.
The immiscible liquid may be introduced into the vessel in any form, for example, in gas, vapor, or liquid form.
The movable porous support may be a simple rotating drum, provided that the sheet of fibrils deposited on it has sufficient cohesion to be self-supporting when it leaves this drum. This cohesion may optionally be increased by means of a pressing roller acting on the sheet so as to compact it just before it leaves the rotating drum.
In order to prevent the sheet from tearing for lack of cohesion, it is nevertheless preferred to use, as a movable porous support, a continuous porous band which accompanies and supports the sheet during its extraction through the sealing device.
This porous band may be a microperforated metal sheet, a very fine wire mesh, a felt, or a cloth. This band must be as undeformable and as wear-resistant as possible. In addition, the material of which it is composed must withstand the organic solvent vapors used. It has in particular been found that cloths made from polyester fibers such as ethylene glycol polyterephthalate fibers or polypropylene fibers, prove to be particularly advantageous because of their chemical inertness and their hydrophobic character.
The sealing device may be of any kind provided that it is effective, and it may for example as above mentioned, be composed of a system of labyrinth rollers. According to a preferred embodiment, this sealing device is composed of a hydraulic seal formed by the liuid phase of liquid immiscible with the organic solvent which lies at the bottom of the closed vessel, in a reservoir provided for this purpose.
The bottom part of this closed vessel may also be provided with heating means for the bottom layer of liquid immiscible with the organic solvent.
In order to permit the elimination of any fibrils which may be deposited on the surface of this liquid, and of other impurities such as polymers of low molecular weight, it is convenient to provide for continuous circulation of this liquid, for example by means of continuous feeding and of evacuation by means of an overflow. The liquid removed from the vessel may in addition undergo decantation treatment for the recovery of any organic solvent that it may contain.
The apparatus may also be completed by a second continuous band bearing on the sheet of fibrils deposited on the first porous band during its passage through the hydraulic seal. In this way the sheet of fibrils is sandwiched between the two continuous bands, which oblige it to pass through the sealing device. This modified embodiment is found very useful when the fibrils have a density lower than the density of the liquid constituting the hydraulic seal.
The closed vessel is provided with heating and monitoring means so that a thermostat can be used to maintain its walls at the desired temperature, depending on the organic solvent used and with the aim of avoiding condensation of the solvent.
The apparatus according to the present invention is described in greater detail in the following description of the preferred form of its construction. It is however understood that this description is given purely by way of illustration and that it does not in any way limit the scope of the present invention, because the apparatus which is described below may undergo numerous modifications within the range of those versed in the art, without thereby departing from the framework or spirit of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic view of an apparatus according to the invention.
FIG. 2 is an enlarged view of a detail in FIG. 1, showing the formation of the sheet of fibrils and its compaction.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As can be seen from FIGS. 1 and 2, the apparatus is composed of a closed vessel 1, to the top of which is connected a chimney 2 through which the fibrils in the presence of vapors of an organic solvent are introduced into the closed vessel 1.
In FIG. 1 it can be seen that this mixture of fibrils and solvent vapors is obtained by the abrupt expansion of a polymer solution through nozzle 3. It is however quite obvious that the apparatus described is in no way restricted to this particular form of production of fibrils, and that it could be just as effective for the recovery of fibrils which are in the presence of organic solvent vapors and are obtained by other processes. A suction box 4, over which passes a continuous porous band 5 composed of a cloth of ethylene glycol polyterephthalate fibers, is situated in the closed vessel 1, exactly facing the bottom opening of the chimney 2.
The continuous band 5 has a constant, controllable speed of linear movement and follows the trajectory imposed on it by the support rollers 6 to 13.
After having passed beyond the suction box 4, the continuous porous band 5 passes into the gap formed by the driving support roller 7 and the compression roller 14, which is adjustable by means of the pneumatic or hydraulic device 15.
As soon as this porous band 5 leaves the compression roller 14, a second continuous band 16 of the same nature as the first band, or if desired of a different nature, is applied progressively against the porous band 5 and accompanies the latter during part of its trajectory. This second band 16 moves between the support rollers 17 to 21 at a speed equal to that of the porous band 5.
At the bottom the closed vessel is provided with a hydraulic seal 22 composed of a liquid which is immiscible with the organic solvent and inert in relation to the polymer of which the fibrils are composed. The bottom part of the vessel also contains an overflow weir 23 connected to an evacuation pipe 24.
The two continuous bands 5 and 16 pass through the hydraulic seal 22, through which these bands leave the closed vessel 1. The path of these bands is imposed by the support rollers 7, 8, 9, and 19. The last two support rollers 9 and 19 are so disposed as to move the continuous bands 5 and 16 away from one another. After their passage out of the vessel, during which they release the sheet of fibrils, these continuous bands enter the closed vessel again, passing once again through the hydraulic seal 22, under the action of the support rollers 11, 12 and 13 in one case of the support l7, 18 and 21 in the other.
A drier 25 may optionally be provided to dry the continuous porous band 5 between the rollers 6 and 13 before it passes over the suction box 4.
This drier may be suction box provided with a slot over which the continuous band slides, this box being connected toa vacuum source not shown. In this way the vapors drawn in under local conditions of high speed and low pressure effect the elimination of the liquid wetting the band.
In a modified embodiment the continuous band 16 may also pass around the pressing roller 14 and pass through the cap between this roller and the driving roller 7.
The level of liquid immiscible with the organic solvent contained in the bottom part of the closed vessel 1 is determined by the overflow 23.
Heating means, not shown, are provided in the bottom of the closed vessel 1, so as to bring the liquid contained in this vessel to the desired temperature and to keep it at this temperature.
The device for be a the immiscible liquid into the closed vessel is composed of a pipe, likewise not shown, which to a supplies immiscible liquid continuously into the closed vessel 1 so as to produce a circulation.
The closed vessel 1 is in addition equipped with means, such as a jacket, enabling its walls and consequently its interior to be kept at a predetermined temperature. Finally, during their passage out of the closed vessel 1 the continuous bands 5 and 16 may pass over tensioning, crease-removing or centering rollers 26. In addition, they may be cleaned by rotating brushes or by water sprays.
The operation of the apparatus as described above is in addition made clear by the example of practical performance which is given below. It is however clearly understood that this example, which is given solely by way of illustration, does not in any way limit the scope of the present invention.
EXAMPLE
A solution of 150 g/kg of high density (0.950) polyethylene in hexane, this solution being at a pressure of 70 kg/cm 2 and a temperature of 180°C, are injected at a rate of flow of 50 kg polyethylene per hour through the main pipe, and hexane under a pressure of 70 kg/cm 2 and a temperature of 180°C is simultaneously injected through the secondary pipe, into the nozzle 3, which is composed of a LECHLER KSD 12 atomizer of the firm LECHLER APPARATEBAU K.G.. The volumetric flow of hexane is substantially equal to the volumetric flow of the polyethylene solution.
Through the abrupt expansion of the polyethylene solution and the instantaneous vaporization of the solvent there is obtained at the outlet of the nozzle 3 a suspension of discontinuous fibrils of polyethylene in hexane vapors.
This suspension of fibrils is directed through the chimney 2 into the closed vessel 1. This vessel has the external shape of a cube with a side dimension of about 3 meters.
The walls of the closed vessel 1 are thermostatically controlled at a temperature of 90°C, and the vessel contains a volume of water of the order of 10 m 3 , which is kept at 80°C. The continuous supply flow of water is kept at 1 m 3 per hour.
At the base of the chimney 2 of the polyethylene fibrils 27 are deposited on the continuous porous band 5 through the action of the suction box 4, which is connected to an installation, not shown, for recovery of hexane vapors. The band, which has a width of 2.3 meters, is made of ethylene glycol polyeterephthalate fibers and has a weight of 1,000 g/m 2 . The suction is so controlled that the pressure prevailing at any moment in the closed vessel 1 will be slightly higher than atmospheric pressure. The excess pressure, of the order of 300 mm water column, has the effect of preventing any accidental entry of air into the closed vessel 1. The partial pressure of the water vapor in the vessel is 0.255 atmospheres absolute and the partial pressure of hexane vapors is about 0.775 atmospheres absolute.
The presence of water vapor in the closed vessel and the temperature and pressure conditions prevailing in the latter reduce the condensation of the hexane vapors to a minimum, and consequently the fibril sheet 28 which is progressively formed on the porous band 5 is practically free from liquid hexane.
This sheet 28 of tangled fibrils 27 deposited on the porous band 5, which moves at a linear speed of 10 meters per minute, is then compacted by passing between the drive roller 7 and the pressing roller 14.
The sheet compacted in this manner is then progressively imprisoned between the continuous bands 5 and 16, which oblige it to pass through the hydraulic seal 22, where it is freed of the last traces of solvent.
On passing out of the hydraulic seal 22, the sheet 28 of compact fibrils is freed by the continuous band 5 and 16. This sheet, the weight of which is from about 80 to 110 g/m 2 can then be consolidated and converted into a non-woven product of good quality by conventional techniques, such as by calendering.
It will be understood that the above description of the present invention is susceptible to various modifications, changes and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.