CA1049213A - Process for the manufacture of discountinuous fibrils - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE:

Process for the manufacture of discontinuous fibrils by the abrupt pressure release through an orifice 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 vaporisation of the solvent and the solidification of the polymer, the solvent not dissolving more than 50 g/litre of polymer under normal conditions of temperature and pressure and possessing a normal boiling point of more than 20°C lower than the polymer melting point. The process is characterized by the fact that a make-up fluid is introduced into the two-phase liquid mixture before the pressure release is complete. The fibrils obtained are parti-cularly suitable for the production by the usual methods of non-woven textiles and synthetic papers.

Description

~049Z~3 The pre~ent invention relates to a process for the manufacture of discontinuous fibrils by the abrupt release of pres-sure on a two-phase liquid mixture of molten polymer and solvent which i~ under elevated pres~ure and at elevated temperature.
It i9 well known that one can produce continuous fibril-lated structures or rovings by analogous proce~ses.
Thus, in United States Patent 2 372 695 of the 15th May 1940 in the name of CEIANESE CORP 0~ AMERICA there is a descrip-tion of the production of a downy mass formed of very fine fila-ment~ connected with;one another by bringing abou't an abrupt pres~urerelease through an appropriate orifice of a solution of a cellul-ose derivative which i8 at elevated temperature and pressure.
According to Belgian Patent 568 524 of the 11th June 1958 in the name of E.I. DU PON~ DE NEMOURS, continous structures consisting of a multitude of fibrillous strands or sections which come together and separate at random intervals to form a "unit fibrillous plexus" are obtained by extruding a solution of synthe-tic polymer which is at a temperature higher than the normal boil-ing point of the solvent and under autogenous pressure or under a higher pressu~e, through an orifice of suitable shape into a zone of lower pressure.
The fibrillated structures obtained according to the processes described above take the form of continuous rovings.
Moreover, as is stated in Belgian Patent 568 524, these struc-tures are produced at a very high speed (which may reach as much as ~13,;700 m./min.)j which makes it impossible to cut them up by mechanical means.
~ he sub~equent proce~sing of these continuous rovings produced at very high speed is very difficult. Moreover, for a large number of applications, it is essential to use the fibril-lated products in a shredded form, that i~ to ~ay in the ~ 049Z13 - 3-form of discontinuous structures of relatively short length, for example of the order of a few millimetres.
That is why, as can be seen from French Patent 1 246 379 of the 17th November 1959, in the name of E.I. du Pont de Nemours, one is obliged to reduce the length of the continuous fibrillated rovings by a treatment in a grinding apparatus which is harmful to the physical qualities of fibrillated structures and calls for a supplementary operation whichnecessitates tying up large amounts of capital and a considerable consumption of power.
It can be seen from this that a process of the type described above but leading to the direct acquisition of short fibrils could in numerous cases permit of a more economical and easier use of the products obtained and improve their quality.
The Applicants have now developed such a process.
The present invention relates to a process for the manu-facture of discontinuous fibrils by the abrupt pressure release through an orifice 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 vaporisation of the solvent and the solidification of the said polymer, the solvent not dissolving more than 50 g/litre of polymer under normal conditions of temperature and pressure and possessing a normal boiling point of more than 20C
lower than the polymer melting point, in which process a make-up fluid is introduced into the said two-phase liquid mixture before the pressure release is complete.
By the designation "discontinuous fibrils" the Appli-cants are designating elongated fibrillated structures consisting of very slender filaments, of a thickness of the order of a micron, connected with one another so as to form a three-dimensional network.
These fibrils which are of a fluffy appearance generally have an elongated shape. Their length varies from 1 mm to about 5 cms. and their diameter from about 0.01 to 5 mm. The specific surface area of these products is greater than 1 sq.m. per gram. These fibrils are particularly suitable for the production by the usual methodsof I B non-woven textiles and synthetic papers.

~ he process according to the invention may be carried out by using any polymer which is suitable for spin~ing.
Among the polymers which can be used one may mention the polyolefins such as polyethylene polypropylene, ethylene/
propylene co-polymers, polyisobutylene etc., polyamides, poly-e~ters, r,olyurethanes, polycarbonates, vinyl polymers such as polyvinyl chloride, which may al~o be post-chlorinated, poly-~inyl fluoride, acrylic polymers such as the homopolymers and co-polymers of acrylonitrile etc., this li~t being by way of example and not restrictive.
Nevertheless the Applicants prefer to use crystallisable polymers whose rate of crystallinity measured by X-ray diffraction is at least 10% and preferably at least 20~o~ because the stretch to which these polymer3 are subjected as a result of the action of vapours released during the abrupt pressure release imparts tb them an orientated structure which imparts high mechanical properties.
Among these polymers, the polyolefins such as high-density polyethylene, isotactic polypropylene ~nd isotactic poly-4-methylpentene-1 lead to the be3t results.
The sol~ent i~ chosen according to the polymer used as well as the following criteria. ~he ~olvent must not dissolve more than 50 g/litre and preferably not more than 10 g/litre of polymer under normal conditions of temperature and pressure (20C and 1 atmosphere). Moreover, it must possess at normal pressure a boiling point which is more than 20C and preferably more than 40C lower than the melting or softening point of the polymer u~ed. ~urthermore, it must permit of the formation of a two-pha~e liquid mixture under operational conditions just prior to the abrupt pressure release.
Among the solvents which can be used one may mention the aliphatic hydrocarbons ~uch as pentane, hexane, heptane, octane and their homologues and isomers, the ~licyclic hydrocarbons such aæ cyclohexane, the aromatic hydrocarbons such a~ benzene, toluene etc., the halogenated solvents such as the ~hloro-fluoromethanes, methylene chloride, ethyl chloride etc" the alcohois, ketones, esters and ethers.
lhe Applicants define below what is meant by the expres-~ion "two-phase liquid mixture".
When one sub~ects a mixture of suitable solvent and poly-mer, with a 8uitable concentration of polymer, to very elevated temperature and pressure, one observes that the mixture takes the form ~of a single homogeneous liquid phase. If then, whilst main-taining all the other conditions constarlt, one gradually reduces the pressure, one observe3 that as from a certain pressure onwards, which varies according to the case, the solution of polymer becomes turbid because of the establishment of a system of two liquid phases conæi8ting of a continuous liquid phaæe which i8 poor in polymer in which there is dispersed in the form of droplets a second liquid phase which is rich in polymer. The value of the pre~sure at which thi8 phenomenon makes its appearance may be determined experimentally.
In the proce-ss according to the invention it is there-fore advisable to choose the pres8ure of the mixture which is sub~ected to the abrupt preæsure release in such a way that it i8 present in the form of a two-phase liquid mixture. The same applies to the concentration of polymer and the temperature ~ In practice one may prepare a solution with a single liquid phase at a higher pressure than that at which the forma-tion of a two-phase liquid mixture takes place and then carry out a sufficient prior release of pressure to bring about the esta~
blishment of the system with two liquid phases.
~ he temperature of the two-phase llquid mi~ture subjected to abrupt pressure release must be such that the latent heat stored by the solvent and the molten polymer is sufficient to bring about the complete vaporisation of the solvent during the abrupt pres-sure release. ~his temperature must not however exceed a maximum value otherwise the quantity of heat consumed by the vaporisation of the sol~ent would be insufficient to bring about the solidifi-cation of the polymer. ~urthermore, it must enable to operate at a pressure at which the formation of the two-phase liquid mixture takes place. Finally it must be lower than the critical temperatu-re of the solvent. Generall~ speaking, the temperature of the mix-ture iB between 100 and 300C and preferably between 125 and 250C.
The concentration of polymer in the mixture used is alsoselected so as to permit of obtaining a two-phase liquid mixture.
It may vary from 1 to 500 g/kg. of mixture. However, the Appli-cants prefer to use mixtures containing from 10 to 300 g of poly-mer per kg. of mixture and preferably 50 to 200 g/kg.
~ or each particular polymer therefore it i9 necessary to choose a solvent complying with the criteria defined above and then to determine the concentration of polymer, the pressure and the ; temperature of the mixture which is subjected to instantaneous pres-sure release. These parameters are therefore chosen not only 30 as to give a two-phase liquid mixture, but also so that the solvent vaporise instantaneously and completely during the abrupt pressure release. ~hese conditions are the same as those imposed on two-phase liquid mixtures used according to Prior ~rt to manufacture continuous fibrillated rovings.
The two-phase liquid mixtures are sub~ected to an abrupt pressure release, that i3 to say their pressure is brought to a value close to atmospheric pressure, preferably to a pressure low-er than 3 kg/cm2 absolute, within a very ~hort period of time, pre-ferably less than 1 second. ~his pressure release may be brought about by pas~ing the mixture through any device which is capableof creating high load iosses, such as a diaphragm, a Venturi or a valve. However, it is preferable to use dies whose cylindrical orifices have a diameter of between 0.1 and 3 mm and preferably between 0.3 and 1 millimetre and the length/diameter ratio of which is between 0.1 and 10 and preferably be~Jeen 0.5 and 2.
It is obvious that the two-phase liquid mixture used may also contain other usual additives for polymers such as sta-bilisers to the action of heat and light, reinforcing agents, fillers, pigments, antistatic agents, nucleation agents etc.
The make-up fluid injected into the two-phase liquid mixture before the pressure release is complete may be of any kind: it may be a gas, a vapour or a liquid.
However, it will be obvious that this fluid must not exert any harmful action on the continuous fibrillated $truc-ture produced by-the abrupt pres~ure relea~ of the mixture. In particular, the use of a fluid which exerts a solvent or swelling action on the polymer u~ed at ambient temperature must be ruled out.
As has been stated above, the fluid used may be of any desired kind. In particular, the Applicants have obtained excel-lent results when this fluid was nitrogen, water vapour, water or an organic liquid. ~he Applicants have also found that it is possible to use as fluid the solvent used to make the two-phase liquid mixture.
When the make-up fluid is water under pressure, the Ap-plicants have found that it is advantageous to incorporate a-wet-ting agent in it.
~he pressure under which the make-up fluid is injected must obviously be higher than the pressure of the two-phase liquid mixture at the point of in~ection.
~he make-up fluid may be at any temperature. This tem-perature i9 preferably selected so that the supply of calories to the two-phase liquid mixture cannot hinder the instantaneous vapo-risation o~ the 301vent and the solidification of the polymer during the abrupt pressure release of the mi~ture.
~ his temperature is preferably between 20C and the boil-ing point of the fluid at the working pressure, that i9 to say at its pressure of in~ection.
When the make-up fluid is a li~uid, its temperature is preferably higher than its boiling point at the pressure of the pressure release chamber, that is to say the pressure obtaining at the outlet from the abrupt pressure release orifice.
The ratio between the volume of make-up fluid and the volume of two-phase liquid mixture may vary between 0.3 and 10.
However, the Applicants prefer this ratio to be from 0.7 to 5 and preferably from 1 to 3.
- The make-up fluid may be in~ected into the two-phase liquid mixture either before the abrupt pressure release or during this release. In the former case, the make-up fluid is injected~
into the two-phase liquid mi~ture at a point situated before or upstream of the abrupt press~re release orifice~ In the latter case, the make-up fluid is in~ected into the two-phase liquid mixture during its passage through the pressure release orifice.
The residence time of the make-up fluid in the pressure release deYice is preferably less than 2 seconds. The best re-sults are achieved when the residence time is less than 5.10 1 and preferably 10 seconds.
By ad~usting the quantity of make up fluid and its va-rio~us parameters it is possible to determine experimentally the conditions for obtaining after pressure release fibrils of the desired length.
The Applicants ha~e attempted to give a physical expla-3o nation for the phenomena which lead according to the process ofthe invention to the formation of discontinuous fibrillated struc-tures of short length.

~049213 As has been stated above, the two-phase liquid mixture prior to abrupt ~ressure release consists of droplets or bubbles of solution with a high concentration of polymer emul~ified in a mate-rial consisting of a continuous solution with a low concentration of polymer.
According to known proce~ses, during the abrupt pressure release of the two-pha~e liquid mixture, these droplets or bubbles each cause the formation of a fibrillated structure due to the a-- brupt vaporisation of their solvent and these different structure~
weld together to give the continuous structure or fibrillated roving already known.
The Applicants therefore think that the injection of a make-up fluid into this two-phase mixture prior to the pre~sure re-lease being complete probably ha~ the effect of increasing the dis-tance geparating the droplets or bubbles suspended in the dilute phase, and in this way creating a certain heterogeneity inside the ; mixture consisting of two liquid phases and leading to a subsequentpressure release of an intermittent nature bringing about discon-tinuities in the fibrillated structure produced.
However, it is possible that a more thorough 3tudy of the phenomena would ~ubsequently lead to a different explanation.
In any ca~e it is obvious that the explanation put forward cannot in any way influence the value of the present in~en-tion.
A~ has been stated, in order to carry out the process according to the invention the Applicants prefer to bring about the abrupt pressure release of the two-phase mixture by passing it through a die.
This die may be of the ~ame type as those used for the proce~ described in Belgian Patent 568 524 already cited, apart from the fact that it is advisable to provide one or more channel~
intended for the injection of the make-up fluid.

~he~e channels may open out either upstream of the abrupt pressure relea~e orifice or into the wall of this orifice according to whether one wishes to inJect the make-up fluid prior to or during the abrupt pre~sure release of the two-pha~e liquid mi~ture.
~hese channels may be arranged perpendi¢ularly in relation to the direction of flow of the two-pha3e liquid mixture or they may merely be inclined in relation to this direction.
Furthermore, these channels may be connected normally or tangentially to the pipes containing the two-phase liquid mixture. The Applicants have also observed that the tangential in~ection permit~ of a more energetic agitation and lead~ gene-rally to better results.
~he diameter of the channels for the make-up fluid at the point of injection is of the order of 0.1 to 5 mm.
The proces~ according to the invention will now be illustrated with reference to the following non restrictive examples and the accompanying drawings, wherein:
Figure 1 represent~ a cross-~ectional view of a die as used in Examples 1 and 4;
Figure 2 i~ an elevation in cro~s-section of a die as used in Example3 2 and 5;
Figure 3 is a plan section taken along line A-A' of Figure 2; and Figure 4 show6 in elevation and cross-section a die as used in Examples 3 and 6.
ExamPle 1 A die i~ u~ed as shown in cross-~ection and on a scale of 1:1 in Figure 1 of the attached drawings.
This die comprises a pre-release chamber 1 ~ituated after a prerelease orifice 2 of a diameter of 1.5 mm, the function of whlch i~ to subject the mixture of polymer and solvent to a ~ -10-B

~ 1049Z13 sufficient load 1088 to cau~o the formation of a 8y8tem with two liquid phases.
Two injection channels 3 for the make-up fluid with a diameter of 1.5 mm open at an angle of 45 into the abrupt pres~ure release channel 4. This channel hae a length of 16 mm and a diameter of 2 mm.
Through the prerelease orifice 2 one passes a mi~ture of ELTEX 54 OOl (high-den~ity polyethylene produced by the Applicants) I

-lOa-,~
`~ ~

and methylene chloride. ~his mixture which is at a pressure of 48 kg/cm2 and a temperature of 215C in the pre-release chamber 1 has a polyethylene concentration of lO~o. In this chamber the mixture is under conditions which cause the formation of two liquid phases.
The flow rate of the feed is 3 kg of polymer per hour.
Through the pipes 3 one injects simultaneously nitrogen at a pressure of 50 kg/cm2 and at a temperature of 20C at a flow rate of 80 normal cu~m. per hour.
The abrupt pressure release of the mixture at the end of the channel 4 causes the formation of discontinuous fibrils whose length is of the order of a millimetre and whose specific surface area i9 of the order of 5 to 6 sq.m. per gram. The pro-duction of fibrils is 3 kg/hr.
The product obtained is perfectly suitable for the pro-duction of non-woven textiles and synthetic papers.
When one gradually reduces the rate of flow of make-up fluid, one observes that the length of the fibrils increases to gi~e finally a continuous fibrillated structure.
Example 2 ; ~he die shown in Figure 2 and ~ of the attached drawings ~: i8 used.
Figure 2 is a sectional elevation of the die and Figure 3 is a plan section along the line A-A' of Figure 2, the two figures being on a scale-of 1:1.
In order to show the details of the device more clearly, the injection nozzle 5 for the make-up fluid is shown outside its housing 6 in the die. The orifice of the injection channel has a diameter of 1 mm.
The die contains a pre-release chamber 7 with a diameter 3 of 5 mm in which the mixture of polymer and solvent is injected tangentially by a pre-release orifice 8 having a diameter of 1.5 mm.
Ihe abrupt release orifice has a length and a diameter of lmm.

A mixture identical to that of Example 1 is passed through this die in such a way that in the pre-releaæe chamber it is under the same conditions of pressure and temperature as in Example 1.
The flow rate is 5 ~gs. per hour of polymer.
~ hrough the noz~le 5 one continuously injects at a flow rate of 35 noxmal cu.m. per hour nitrogen under a preæsure of 50 kg/cm and at a temperature of 20 C.
~ he abrupt pres~ure release of the mixture at the end of the die causes the formation of diæcontinuous fibrils the length of which varies from 1 to 10 mm and the specific surface area of which is of the order of 7 sq.m. per gram.
Exam~le 3 ` A die is used as ~hown in elevation and section in Figure 4 of the attached drawingæ.
For reasons of clarity, the various items which make-up the die æhown on a scale of 1:1 are represented in positions which are diæplaced from one another.
Thus it can be seen in this Figure that the die compriseæ
a pre-release chamber 9 provided with a housing 10 intended to receive a deflector ~l which has the effect of causing a turbulent movement in the two-phaæe liquid mixture prior to the injection of the make-up fluid.
This chamber is connected tangentially to pipe 12 with a diameter of 4 mm for the in~ection of make-up fluid.
Underneath the pre-releaæe chamber there is the abrupt release orifice 13, which has a diameter of 2 mm and a length of 1 mm. ~his orifice may be replaced if desired by a valve with ad-~ustable aperture.
The die is extended by an acceleration and shredding channel 14 of a length of 20 cms. An a diameter of 10 mm.
The deflector 11 may communicate to the two-phase liqui`d mixture either a turbulent movement in the same direction as that -]2-caused by the tangential in~ection of the make-u~ fluid, or a turbulent movement in the opposite direction. -A mixture of E~TEX 54 001 and hexane of technical quality"polymerization grade" at a temperature of 190C and with a con-centration of 180 g of polymer per kg. of mixture is passed through this die. The pressure of this mixture is regulated so that its pressure in the pre-release chamber is 40 kg/cm2, at which pres-sure this mixture presents two liquid phases.
~ hrough the channel 12 one injects at the same time at a flow rate of 240 litres per hour water under a pressure of 42 kg/cm2 and at a temperature of 190C.
By operating under these conditions one obtains 25 kg per hour of fibrils having a length of 10 mm and a specific sur-face area of 15 sq.m. per gram.
Example 4 Use i9 rnade of a die identical with that described in , Example 1. ~hrough the pre-release orifice 2 there is passed a mixture of SOLVIC 228 (a product of Solvay ~ Cie~, Brussels, Belgium, composed of polyvinyl chloride produced by polymeri2a-tion in suspension) and dichlorethane, the polyvinyl chloridebeing stabilized by means of IRGAS~AB 17 MO (tr~demark; tin-based stabilized produced by CIBA-GEIGY). ~his mixture which, in the pre-release chamber is at a pressure of 70 kg/cm2 and at a temperature of 165C, has a concentration of 150 g of polymer per kg of solution. In this chamber, the mixture is present under conditions provoking the formation of two liquid phases. ~he supply rate is 30 kg of polymer per hour.
Via the channels 3, nitrogen is simultaneously injected at a pressure of 70 kg/cm and at a temperature of 25 C at a 3 flow rate of 50 normal m3 per hour.
~ he abrupt pressure release of the mixture at the end of the channel 4 provokes the formation of discontinuous fibrils having ` 1049213 a length of the order of 5 mm and a specific ~urface area of the order of 5-10 m2 per gram. The rate of production of fibrils i~
30 kg per hour.
- Example 5 A die is u~ed which is identical with that de~cribed in Example 2. ~hrough the orifice 8 there is injected a mixture of SO~VIC 228 stabiliæed by IRGAS~AB 17 M0 and dichlorethane. This mixture whioh, in the pre-release chamber 7, is at a pres~ure of 70 kg/cm2 and a temperature of 170C, haæ a concentration of 200 g of polymer per kg of solution. ~he flow rate is 45 kg of polymer per hour.
Via the nozzle 5 is simultaneously and continuously in-~ected dichlorethane heated to 170 C and under a hydraulic pressure of 70 kg/cm2, the flow rate being 300 liters per hour.
; The abrupt pressure relea~e at the extremity of the die provokes the formation of discontinuous fibrilg having a length which ~ari~s between 5 and 15 mm and a specific surface area which ~; varies between 5 and 10 m2 per gram. The rate of production of ~- fibrils is 45 kg per hour.

Example 6 Use is made of a die identical with that described in Example 3.
A mixture of polyvinylidene fluoride and methylene chlo-ride at a temperature of 180C and a concentration of 100 g resin per ~g,of solution is passed through this die. The pressure of this mixture is regulated so that its pressure in the pre-release chamber is 35 kg/cm2, at which pressure this mixture present~ two liquid phases. The rate of deli~ery is 5 kg of resin per hour.
~hrough the channel 12 there is simultaneously in~ected nitrogen at a flow rate of 20 normal m3 per hour, the nitrogen being at 20 C and under a pressure of 40 kg/cm2.
By operating under these conditions, there is obtained 1049Z~3 5 kg of fibrils per hour, the fibrils having a length of less than or equal to 5 mm and a specific surface area of 15 m2/g.
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.

Claims (14)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Process for the manufacture of discontinuous fibrils by the abrupt pressure release through an orifice of a two-phase liquid mixture of molten polymer and organic solvent which is at elevated temperature and pressure so as to bring about the instantaneous vaporisation of the solvent and the solidification of the said polymer, the organic solvent not dissolving more than 50 g/litre of polymer under normal conditions of temperature and pressure and possessing a normal boiling point of more than 20°C lower than the polymer melting point, characterized by the fact that a make-up fluid is introduced into the said two-phase liquid mixture before the pressure release is complete.

2. Process for the manufacture of discontinuous fibrils in accordance with claim 1, characterized by the fact that the make-up fluid is introduced into the two-phase liquid mixture before the abrupt pressure release of the latter.

3. Process for the manufacture of discontinuous fibrils in accordance with claim 1, characterized by the fact that the make-up fluid is introduced into the two-phase liquid mixture during the abrupt pressure release of the latter.

4. Process for the manufacture of discontinuous fibrils in accordance with claim 1, characterized by the fact that the make-up fluid is a gas.

5. Process for the manufacture of discontinuous fibrils in accordance with claim 4, characterized by the fact that the make-up fluid is nitrogen.

6. Process for the manufacture of discontinuous fibrils in accordance with claim 1, characterized by the fact that the make-up fluid is a vapour.

7. Process for the manufacture of discontinuous fibrils in accordance with claim 6, characterized by the fact that the make-up fluid is water vapour.

8. Process for the manufacture of discontinuous fibrils in accordance with claim 1, characterized by the fact that the make-up fluid is a liquid.

9. Process for the manufacture of discontinuous fibrils in accordance with claim 8, characterized by the fact that the make-up fluid is water under pressure.

10. Process for the manufacture of discontinuous fibrils in accordance with claim 9, characterized by the fact that the make-up fluid contains a wetting agent.

11. Process for the manufacture of discontinuous fibrils in accordance with claim 1, characterized by the fact that the make-up fluid is identical with the solvent used for making the two-phase liquid mixture to be released abruptly.

12. Process for the manufacture of discontinuous fibrils in accordance with claim 1, characterized by the fact that the temperature of the make-up fluid is between 20°C and its boiling point at its injection pressure into the two-phase liquid mixture.

13. Process for the manufacture of discontinuous fibrils in accordance with claim 1, characterized by the fact that the ratio between the volume of make-up fluid injected and the volume of two-phase liquid mixture is from 0.3 to 10.

14. Process for the manufacture of discontinuous fibrils in accordance with claim 1, characterized by the fact that the polymer is selected from the group consisting of polyolefins, polyamides, polyesters, polyurethanes, polyearbonates, vinyl and acrylic polymers.