IL33120A - Continuous process for producing a spinnable polyacrylonitrile solution - Google Patents

Description

33120/2 4 Continuous process or producing a spiimable polyttcrylonitrile solution UCB (UNION CHIMIQUE«-CHEMISOHE BEDRIJVEN)fS.A C. 31306 The present invention is concerned with a method of polymerising acrylonitrile and, more particularly, with a method of producing a spinnable solution of polyacrylonitrile in an organic solvent, with the spinnable solution thus produced, and with fibres, filaments and films manufactured from said spinnable solution.

Hereinbelow, the expression polyacrylonitrile is to be understood to mean not only the homopolymer of acrylonitrile but also copolymers containing at least 8yfo by weight of acrylonitrile, 0 to 1C$ by weight of another ethylenically unsaturated comonomer and 0 to by weight of an ethylenically unsaturated comonomer for improving the tinctorial properties.

In view of the commercial importance of polyacrlic fibres in the textile field, a large number of processes have already been proposed for the preparation of spinning solutions.

In the earliest processes, some of which are still widely used industrially, the polyacrylonitrile is prepared by polymerisation of acrylonitrile in an aqueous suspension in the presence of redox catalysts. The polyaorylonitrile thus obtained is separated and then dissolved in an appropriate solvent for obtaining a spinnable solution of polyacrylonitrile.

Another more recent technique, to which the present invention relates, consists in effecting directly the polymerisation of acrylonitrile in the presence of the solvent used for spinning, which constitutes an obvious simplification and economy in operation, as oompared with processes of polymerisation in aqueous emulsion.

The processes applying this new technique have in common one or more of the following points: a. The acrylonitrile is copolymerised with compounds which are ethylenically unsaturated, such as acrylic and/or methacrylic esters, vinyl acetate, styrene or the like, and also with other comonomers intended to improve the tinctorial properties of the fibre and which are receptive to acid dyes (for example, vinyl-pyridine) or to basic dyes (for example styrene-sulphonic acid, allyl- or methallyl- sulphonic acid or the like); b. The copolymerisation is effected in the presence of solvents, the most important of which are concentrated aqueous solutions of zinc chloride or sodium sulphocyanide, or organic solvents, such as dimethyl sulphoxide, dimethyl acetamide or dimethyl formamide; c. The copolymerisation is effected in the presence of free radioal- forming catalysts (azo compounds, organic and inorganic peroxides, persulphates, hydrogen peroxide or the like) or of the anionic type (organo-metallic derivatives of alkali metals, sodium cyanide, phosphines or the like) ; d. The molecular weight of the polyacrylonitrile may be regulated by chain regolatora, such as ethyl-, jt-butyl- or dodecylmercaptans, thiourea, dixanthogenate disulphide, thioglycollio acid or the like; c. The temperature of the polymerisation reaction is within the range from - 80 to + I 00C. or even higher.

Depending on the monomer/solvent ratio in the reaction medium, the following distinction is made: Polymerisation in solution, when the amount of solvent is sufficient to dissolve the polyacrylonitrile formed in the course of the polymerisation; Polymerisation in suspension, when this quantity of solvent is insufficient. ¾y way of example, when using dimethyl formamide as the solvent, the polymerisation is effected in solution when the acrylonitrile/ dimethyl formamide ratio is lower than Ο.76 (in mols) or lower than Ο.55 (by weight) and it is effected in suspension when thie ratio io higher than these two values.

Polymerisation in solution has the disadvantage that the viscosity increases rapidly with the molecular weight and with the concentration of the polyacrylonitrile; for this reason, polymerisation in solution is generally stopped when the concentration of polyacrylonitrile reaohes at most 2 ¾ by weight. Another disadvantage, due to the low concentration of monomers> is the low speed of reaction, which necessitates a polymerisation time of from 25 to 50 hours (see German Patent Specifications Nos. 1,052,687 and 1, 163, 027) .

On the other hand, in suspension polymerisation, in which the operation is carried out in the presence of a large quantity of monomer, the speed of polymerisation is much higher, varying from a few minutes to a few hours. However, the risk of gelling of the polymerisation medium is great; tha medium eooetimos gells oven at a polynor concentration as low as 6% (see U.S. Patent Specification No. 2,528, 710) .

However, if certain precautions are taken, it is possible to continue polymerisation until about 20^ by weight of polyaorylonitrile is obtained (see U.S. Patent Specification No. 2, 528, 710, British Patent Specification No. 1,095, 749» published Hatch Patent Application No. 6,506,295 and Belgian Patent Specification No. 666 ,121) .

Polymerisation in suspension is thus found to be more advantageous from the industrial point of view than polymerisation in solution because the hourly output of polymer per unit of voluue of apparatus is much higher. Nevertheless, a disadvantage is that, as indicated above, it is not possible to exceed a concentration of about 20% by weight of polymer in the reaction medium because of the excessive increase of viscosity. Attempts have been made to remedy the situation by various expedients. Thus, in Belgian Patent Specification No. 572| 665 , it is proposed to add from 4 to 20o of water in order to increase the percentage of conversion of acr lonitrile into polyaorylonitrile, while retaining a sufficiently low viscosity to permit the handling of the reaction medium obtained. The disadvantage of this method is that it is subsequently necessary to eliminate all of the water thus added, thus entailing considerable problems in respect of distillation. Again, it has also been proposed to effect the polymerisation in two stages, first a polymerisation in suspension until about 2Q¾ of polymer is obtained and then polymerisation in solution by adding an additional quantity of solvent (British Patent Specification No, 1,095,749 and published Dutch Patent Application No. 6,506, 370) . The disadvantage of this method is that it requires two different reactors, so that it is difficult to synchronise the two reactions and there is a risk of wider distribution of the molecular weights of the polymer obtained.

Prom the industrial point of view, it would, therefore, be advantageous to find a method of polymerising acrylonitrile in suspension which retains the advantage inherent to this technique of a high speed of polymerisation, which would permit a rate of conversion of acrylonitrile into polyacrylonitrile considerably greater than 2Q¾ by weight, thus improving the hourly production capacity of the installation and which would be easy to carry out, i.e. without having to employ the previously mentioned expedients which, although they enable the rate of conversion to be increased, on the other hand entail other technological problems which are difficult to solve. The object of the present invention is to provide a method of this type.

We have found that by observing certain operational conditions, it is possible to bring the concentration of the polyacrylonitrile in the polymerisation medium to values which are higher than 252 and may even attain and exceed 40 , while obtaining a reaction medium the viscosity of which remains sufficiently low to permit easy handling.

The present invention relates to a continuous method for the production of a spinnabla solution of polyacrylonitrile in a dialkylamide by polymerisation in suspension and in one stage, in whioh use is made of a reaotion medium comprising a dialkylamide, monomeric acrylonitrile, polymeric acrylonitrile and a polymerisation catalyst in a mixer reactor and which is characterised in that: a. a concentration of at least 2 The ethylenically unsaturated comonomer used in an amount of from 0 to 10 by weight, referred to the total amount of copolymer, may be a vinyl compound, such as vinyl acetate or styrene, but is preferably an alkyl ester of acrylic or methacrylic acid, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, hexyl or 2-ethylhexyl aerylate or methacrylate or the like.

The ethylenically unsaturated comonomer whioh improves the tinctorial properties and is used in an amount of 0 to 5$> referred to the total amount of copolymer, is, for example, vinyl-sulphonic acid, allyl-sulphonic acid, methallyl-sulphonic acid, jj-styrene-sulphonio acid, vitiylphenyl-ether-£-sulphonic acid, N-phenyl-maleamic-^-sulphonic acid or N-phenyl-maleimide£-sulphonic acid or the alkali metal, ammonium or amine salts of these acids, ¾y dialklamide, there is to be understood dimethyl formamide, dimethyl acetamide or the like.

For the purpose of carrying out the process according to the present invention, it is essential that the acrylonitrile-dialkylamide ratio by weight in the reaotion medium should be at least 1.63· When this ratio is lower than this critical value, it becomes impossible to maintain, in the reaction medium, a concentration of dry matter greater than 20 by weight, without the viscosity at the same time increasing excessively. With regard to the upper limit of this ratio, it is dictated solely by the minimum amount of dialkylamide necessary for maintaining the catalyst in solution. Under practical conditions, this minimum quantity is $ by weight, i.e. an acrylonitrile/dialkylamide ratio of about 15 in the reaction medium (taking into account the polyacrylo-nitrile contained in this medium).

In order to keep the aorylonitrile/dialkylamide ratio higher than 1.63, a mixture containing at least 72$ by weight of monomelic acrylonitrile and at most 28$ by weight of dialkylamide is added continuously to the reaction medium, this mixture containing at most of acrylonitrile and at least of dialkylamide. Instead of adding the acrylonitrile and dialkylamide in the form of a mixture, these two substances may also be added separately to the reaction medium, in the ratio indicated above.

With regard to the catalyst, the latter comprises an oxidising agent, which is preferably inorganic, and an organic reducing agent. In principle, the oxidising agent can be an organic substance, for example, a peroxy compound, such as benzoyl peroxide, a hydroperoxy compound, such as eumene hydroperoxide, or an azo compound, such as azo-bis-isobutyro-nitrile, but for economio reasons preference is given to inorganic oxidising agents, such as hydrogen peroxide, sodium perborate, sodium perchlorate and, in particular, ammonium persulphate. The inorganic reduoing agents conventionally used in redox systems in aqueous suspension cannot be used in a reaction medium composed of acrylonitrile and dialkylamide because they are not soluble therein. For this reason, according to the present invention the reducing agent is an organic •ompound, such as . a mereaptan, for example ethyl meroaptan, J butyl mercaptan, dodecyl mercaptan or other organo-sulphur compounds, such as thiourea, dixanthogenate disulphide, thioglycolic acid or the like. Mercaptans are preferred because they are largely recoverable on separation of the monomers from the effluent of the mixer-reactor and are recycled to the latter at the same time as these monomers. In the method according to the present invention, the oxidising agent/reducing agent molar ratio in the catalyst is from 5.0 to 0.05, preferably from 1.0 to 0.2. Further, the amount of catalyst (oxidising agent + reducing agent) in relation to the reaction medium is between 0.01 and ¾¾ by weight.

The temperature of the reactional medium in the course of the polymerisation process aocording to the present invention is maintained between about 0QC. and about 70flC. When the temperature is lower than 40°C, the speed of polymerisation becomes very low and the residence times in the mixer-reactor are lengthened accordingly. On the other hand, when the temperature is higher than about 70fiC, the latter becomes too close to the second order transition temperature (which is between 80 and 100°C), the apparent solubility of the polymer increases and swelling of the polymer by the solvent is observed, at the same time as a considerable increase in viscosity.

Other conditions remaining unchanged, the residence time of the reaction medium in the mixer reactor is dependent on the content of dialkylamide in the reaction medium and the polymer concentration which it is desired to obtain. With an increase of the dialkylamide content, the residence time increases and the higher the desired concentration of polymer the longer this residence time will be. Thus, in order to obtain of polymer in a reaction medium which contains 5A> of dialkylamide, a residence time of from 15 to 30 minutes is required. In order to obtain 40 of polymer in a reaction medium which contains 20 of dialkylamide, a residence time of from 90 to 120 minutes is required.

The process according to the present invention is advantageously carried out in equipment comprising a reaction vessel equipped with heating and cooling means, a mechanical agitator, storage reservoirs with metering pumps for continuous or intermittent introduction of the constituents of the reaction medium, optionally a tube for introducing an inert gas, such as nitrogen, and a means of continuous removal, for example a pump, in order to maintain a substantially constant volume of reaction medium. The effluent of the reactor is delivered to a dilution tank in order to receive therein an additional quantity of dialkylamide and thus to obtain a solution containing I5/2O0 by weight of dry material. This solution is continuously fed to a rectifier, into which there is introduced dialkylamide vapour which entrains, in the form of a distillate, the monomers which have not been polymerised together with the major portion of the reducing agent and dialkylamide, this distillate being returned to the reactor after correction of its concentration of monomers and reducing agent by adding fresh materials, while the residue withdrawn from this rectification consists of a polyacrylonitrile solution having a concentration of from 15 to 20f0 by weight, which is delivered to a concentrator, for example a dropping film evaporator, in order to bring the concentration of the polyacrylonitrile to the spinning concentration, which is generally from 23 to 27% by weight of polyacrylonitrile. Since, as a rule, the rectifier and the concentrator operate under reduced pressure, perfect degasification of the spinning solution is simultaneously achieved, so that this solution can be delivered direotly to the spinnerets or to a storage tank preceding the latter. The spinning solution thus obtained has a monomer content of less than 0.1$ by weight.

According to the process of the present invention, it is, therefore, possible practically to double the polyacrylonitrile content of the reaction medium, while imparting to the latter a sufficiently low viscosity to enable it to be easily handled until it is converted into spinnable solution. It will be noted that, in contrast to the previously known processes, this increase of the polyacrylonitrile content in the reaotion medium is obtained in a single stage or without having to add a diluent, such as water, in order to lower the viscosity, this diluent having to be completely eliminated subsequently and at great expense.

The following Examples are given for the purpose of illustrating the present invention; in these Examples, the molecular weight of the polymers is calculated from the intrinsic viscosity in solution in dimethyl formamide at 25fl0. in accordance with the equation: Example 1.

A 2-litre glass reactor provided with a double wall for thermostatic control, a cage-shaped agitator, a thermometer, a level indicator- and inlet and outlet connections for a current of nitrogen, is continuously fed with the mixture described hereinafter. The polymer suspension formed is withdrawn at such a rate that the volume of the stationery phase remains constant at 1.4 litres. The temperature inside the reactor is kept at ό0°0.

Feeding is effected by means of three metering pumps which, respectively, introduce per hour: a. A mixture of 661 g. of aorylonitrile, 36 g. of methyl acrylate and 3.23 g. of n-dodecanethiol; b. A mixture of 129 g. of dimethyl formamide and 0.912 g. of ammonium persulphate; and c. A mixture of 5· 3 g« of sodium methallyl sulphonate in about 10 ml. of water.

The residence time or, in other words, the time for the renewal of the contents of the reactor, is equal to 84 minutes. After 4 cycles of 84 minutes, equilibrium is reached and the stationary phase contains 32% by weight of polymer. The monomer-solvent ratio in the stationary phase is 3·5 and the conversion, referred to monomer, is 38%. The polymer has a molecular weight of 51,000 and it contains 100 milliequivalents of NaSO., groups per kg. of polymer.

At the outlet of the reaotor, the suspension is diluted with dimethyl formamide in order to lower the content of dry materials to between 15 and 20% and it is introduced into one of the top trays of a stripping column operating under a vacuum. Vaporised dimethyl formamide is introduced into one of the bottom trays and the monomers which have not reacted are recovered in a condenser. The polymer is dissolved when passing through the column and is withdrawn at the bottom tray in the form of a syrup containing from 15 to 20% of polymer and less than 1% of residual monomers. The syrup is concentrated to between 23 and 30% of dry material in order to obtain the viscosity required for spinning, by passing through a thin layer evaporation apparatus. The monomers recovered are returned to the reactor.

Example 2.

Similarly to Example 1, a reactor is fed with a mixture of 661 g. of acrylonitrile, 36 g. of methyl aerylate, 129 g. of dimethyl formamide, 5·3 £· of sodium methallyl-sulphonate, 0.684 g« of ammonium persulphate and 2 g. of dodecanethiol per hour. The residence time is 120 minutes and the volume of the reactor is 2 litres. 4% of polymer is obtained in the suspension withdrawn, so that the stationary phase contains a monomer/solvent ratio equal to 2.7» The molecular weight of the polymer is 81,000 and the content of NaSO,, groups is 28 milliequivalents per kg. The yield, referred to monomers, is 49%. This Example shows that it is possible for the polymer content to be practically doubled in relation to the previously known processes; in addition, this polymer content of 41% does not constitute the upper limit.

Example 3.

The operation is carried out as in Example 2 but the residence time is reduced to 60 minutes so that there is only a stationery volume of 1 litre and the stationary phase contains 25$ of polymer.

The polymer has a molecular weight of 53, 000 and contains 106 millie- · quivalents of NaSO. per kg. of resin. The stationary phase has a 3 monomer/solvent ratio of 3.7· The yield, referred to monomer, is - 30 . This Example shows that, by reducing the residence time, the polymer content of the reaction medium is automatically reduced.

Example 4.

The reactor is fed under the same conditions as in Example 3 but omitting the dodecanethiol. After 55 minutes, the reactor is clogged because of the gelling of the contents. The polymer content is, however, less than 10$. The polymer has a molecular weight of 141, 300.

Example 5.

In this Example, the critical minimum acrylonitrile/solvent ratio of at least I.63 is not respected and the reactor is fed with a mixture containing 0 by weight of acrylonitrile, 3$ of methyl aerylate, 47$ of dimethyl formamide and about 0.14$ of ammonium persulphate. The content of the reaotor is blocked when the amount of polymer is $· The polymer has a molecular weight of about 50, 000.

Example 6.

In this Example, polymerisation is started w th a monomer/solvent ratio higher than I.63 according to the present invention but, as the polymerisation progresses, this ratio is reduced until it falls below I.63, when it gives rise to gelling.

The reactor is fed with a mixture of 5&5 ·5 S* °f acrylonitrile, 28.4 g. of methyl acrylate, 250 g. of dimethyl-formamide, 2.84 g. of sodium methallyl-sulphonate, 0.684 g. of ammonium persulphate and I.78 g. of lauryl mercaptan per hour. The stationary volume is kept at 1 litre and the temperature at b0°C. Polymerisation starts normally but the contents of the reactor start to become viscous from the moment when there is 16$ of polymer. At this moment, the monomer/ dimethyl-formamide ratio is equal to 1.85. Shortly afterwards, as the concentration of polymer increases still further, the monomer/dimethyl formamide rstio approaches I.63 and the reactor is blocked. The polymer contains 112 milliequivalents of NaSO per kg. of resin.

Example 7» This Example describes the production of a polyacrylonitrile fibre. -litre reactor, heated to between 55 and 60°C. is fed with a mixture of 2644 g. of acrylonitrile, 144 g. of methyl aorylate, 516 g. of dimethyl formamide, 23·2 g. of ammonium methallyl-sulphonate, 3.65 g. of ammonium persulphate and 7· 2 g. of t-utyl mercaptan per hour. The polymer suspension is withdrawn from the reactor at such a rate as to maintain a residence time of 75 minutes. The polymer content in the reactor rises to 31.7 · The molecular weight is about 65,000 and the content of NHjSOj is about 100 milliequivalents per kg. of resin. The monomers are recovered in the manner described in Example 1 and the syrup is concentrated to an appropriate viscosity in a thin layer evaporation apparatus. Spinning results in the formation of white fibres which are easily dyed by basic dyestuffs and which have the following properties: titre: 2.Θ dtex before and 3.0 dtex after relaxation. Tensile strength 29.7 g./tex before and 27 g./tex after relaxation. Tensile elongation: 27 before and 3¾« after relaxation. Loop strength: 8.4 g./tex before and 11 g./tex after relaxation. Loop elongation: 4% before and 8 after relaxation.

Example 8.

This Example shows the influence of temperature.

Polymerisation carried out under the conditions described in Example 3 but working at 70°C. instead of at 6O0C. and using half the amount of catalyst, produced an increase in viscosity when the polymer content in the reactor reached 10%; at that moment, the reaction was very violent and difficult to control. Polymerisation had to be stopped at a polymer content of 12 in the stationary phase.

Example 9.

This Example shows that the reaction is no longer controllable above a certain temperature.

Repetition of Example 8 at 8O0C. with a quarter of the amount of catalyst likewise led to difficulties. The reaction was stopped before 10% of polymer was obtained in the stationary phase.

Example 10.

This Example shows that it is possible to operate at a lower temperature but with greater consumption of catalyst.

Example 2 was repeated at 50°C. with three times the amount of persulphate. The reaction takes place normally and lasts 12 hours. The stationary phase contains 31$ by weight of polymer. The polymer contains 110 milliequivalents of NaSO^ groups per kg. and has a molecular weight of 70,000.

Example 11.

In this Example, methyl methacrylate is used as comonomer.

A reactor heated to 60^0. is fed with a mixture of 6776· of acr lonitrile, 16.7 δ· of methyl methacrylate, 129 g» of dimethyl formamide, 5·58· of sodium methallyl-sulphonate in 10 g. of water, 0.912 g. of ammonium persulphate and 3·23 g« of n-dodecyl mercaptan per hour. The residence time is 96 minutes. The suspension withdrawn Θ hours later contains 36, 2 of polymer. The polymer has a molecular weight of 45» 000 and contains 115 milliequivalents of NaSO^ groups per kg. of copolymer.

Claims (1)

WHAT WE CLAIM IS: 1. A continuous method of producing a spinnable solution of polyaoiylonitrile in a dialkylamide by polymerisation in suspension in one stage, wherein use is made of a reaction medium comprising a dialkylamide, mo omerio acrylonitrile, polymeric acrylonitrile and a polymerisation catalyst in a mixer-reactor, characterised in that: a* a concentration of at least 25% by weight of polymeric acrylonitrile is constantly maintained in the reaction medium by maintaining therein a weight ratio between monomelic acrylonitrile and dialkylamide of at least I.63 by weight by the continuous addition of a mixture containing at least 72$ by weight of monomer aorylonitrile and at most 28% by weight of dialkylamide; b. use is made of a polymerisation catalyst comprising an oxidising agent and an organic reducing agent; c. the temperature of the reaction medium is maintained between 40 and 70«C; d. a residence time of the reaction medium in the mixer-reactor of from a few minutes to several hours is maintained; e. a portion of the reaction medium is continuously withdrawn and an appropriate quantity of dialkylamide added thereto so that, after elimination of unpolymerised acrylonitrile, a spianable solution of polyaorylonitrile is obtained. 2. A method according to Claim 1, wherein the polyacrylonitrile is an acrylonitrile homopolymer. 3. A method according to Claim 1, wherein the polyacrylonitrile is a copolymer containing, by weight, at least 85% of acrylonitrile, from 0 to 10% of an ethylenically unsaturated comonomer and from 0 to % of an ethylenically unsaturated comonomer for improving the tinctorial properties. 4· A method aocording to Claim 3, wherein the ethylenically unsaturated comonomer which is used in an amount of from 0 to 10 by weight, referred to the total copolymer, is vinyl acetate, styrene, an alkyl ester of acrylic acid or an alkyl ester of methacrylic acid. 0 5· A method according to Claim 3 or 4 , wherein the ethylenically unsaturated comonomer for improving the tinotorial properties which is used in an amount of from 0 to t referred to the total copolymer, is vinyl-sulphonic acid, allyl-sulphonic acid, methallyl-sulphonio aoid, jD-styrene-sulphonic acid, vinylphenyl-ether-^-sulphonic acid, N-phenyl-maleamio-£-sulphonic acid, N-phenyl-maleimide-jgi-sulphonio aoid or an alkali metal, ammonium or amine salt thereof. 6, A method according to any of the preceding claims, wherein the dialkylamide is dimethyl formamide or dimethyl acet amide. 7. A method according to any of the preceding claims, wherein the oxidising agent is an inorganio compound. Θ* A method aocording to Claim 7> wherein the oxidising agent of the polymerisation catalyst is hydrogen peroxide, sodium perborate, sodium perchlorate or ammonium persulphate. 9. A method according to any of the preceding claims, wherein the reducing agent of the polymerisation catalyst is ethyl mercaptan, Jt-butyl mercaptan, dodecyl mercaptan, thiourea, dixanthogenate disulphide or thioglyoolio acid. 10. A method according to any of the preceding claims, wherein the oxidising agent/reducing agent molar ratio in the catalyst is from 5.0 to 0.05. 11. A method according to Claim 10, wherein the oxidising agent/ reducing agent molar ratio in the oatalyst is from 1.0 to 0.2. 12. A method according to any of the preceding claims, wherein the amount of catalyst, constituted by the oxidising agent and the reducing agent, represents from 0.01 to 2?o by weight of the reaction medium. 13. A method according to Claim 1 of producing a spinnable solution of polyacrylonitrile, substantially as hereinbefore dasoribed and exemplified. 14. A spinnable solution of polyacrylonitrile, whenever produced by the method according to any of Claims I-I3.

1. . Fibres, filaments and films, whenever produced from a solution according to Claim I4. For the Applicants DR. REiNHOLD CCH 0 PARTNERS