Continuous process for preparing a spinning solution of acrylic polymers

The present invention relates to a continuous process for preparing a spinning solution of acrylic polymers.

More particularly this invention relates to a continuous process for preparing a spinning solution of acrylic polymers produced by bulk polymerization, employing, as a spinning solvent, an organic compound such as dimethyl-acetamide, dimethyl-formamide, ethylene carbonate, dimethyl sulphoxide etc,

Acrylic polymers - as the term is used herein - means long-chain synthetic pqlymers.cqnsi.stixig format, least. .8 5%. by ,weight of acrylonitrile units, the remaining amount being constituted of at least another ethylene-unsaturated monomer copolymerizable with acrylonitrile.

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As is known, acrylic polymers may be prepared by polymerization in suspension, polymerization in solution and bulk polymerization processes.

The polymerization process conducted in an aqueous suspension offers the advantages of a relatively high polymerization rate and that of providing polymer qualities such as purity and whiteness.

Still, the necessity of obtaining the polymer in the dry state, to be employed in the spinning with organic solvents, involves complicated and expensive operations,,such_as filtration, drying.and grinding.״. ________

Subsequently the dried polymer is dissolved in the spinning solvent under stirring.

When the polymerization is carried out in solution, the complexity essentially consists in the necessity of operating with a highly viscous medium in the polymerization reactor. Such property of the medium limits the heat exchange coefficients and, by consequence, the reactor dimensions.

Furthermore, since the polymeric solution is very viscous, the separation of the monomer not converted into polymers must be carried out in special and very often complicated apparatuses.

Also the bulk polymerization of acrylonitrile is known, which operates in the absence of both water and solvent. According to such technique and when the polymer is to be employed for producing fibres, films and other shaped articles, the reaction mass leaving the reactor is subjected to a polymer/monomer mechanical separation (centrifugation and/or filtration), and the polymer is dried, ground and dissolved in the solvent.

Thus, it is the object of this invention to provide a continuous process for directly obtaining the spinning solution from the polymerization slur2

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ry leaving the reactor and prepared according to a bulk polymerization process, without carrying out the polymer/ monomer mechanical separation and without drying or grinding the polymer obtained.

We have now found that such object can be attained through a process comprising the following steps:

a) adding to the polymerization slurry leaving the reactor a solvent in such quantity as to be insufficient to attack and dissolve the polymer and to result in a polymer/EKHiomer/solvent mixture having a viscosity less than 2 poises;

b) heating of the mixture thus obtained to a temperature (T) exceeding room temperature and not exceeding 170°C, said temperature (T) being such that there is no appreciable attack of the polymer by the solvent;

c) removing at least 30% of the unreacted monomer or monomers by means of one or more adiabatic evaporation stages under vacuum;

d) optional adding of additional solvent until the required polymer concentration is attained;

e) heating the mixture sub d) to a temperature not higher than 170 ^eC with a view to attacking and thoroughly dissolving the polymer; and

f) completing, if necessary, the removal of the residual monomer or monomers by thin layer evaporation.

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The present process exploits the bulk polymerization of acrylonitrile in that it allows to continuously prepare, in a simple and economically advantageous way, the polymer solution in the spinning solvent. In fact, by operating according to the process illustrated hereinabove, the removal of the unreacted monomer or monomers occurs by simple adiabatic evaporation

3a while the polymer is not yet attacked by the solvent and the mixture, by consequence, is very fluid, A further advantage of the present process consists in the possibility of utilizing the polymerization heat for carrying out said adiabatic evaporation,

The monomers subject to removal are those remaining in the slurry that leaves the reactor after the polymerization process.

Generally, such monomers essentially consist of acrylonitrile, however, in the preparation of the acrylonitrile copolymers, besides this monomer also other ethylene-unsaturated compounds copolymerizable with the acrylonitrile are present.

Such ethylene-unsaturated compounds may be : alkyl-, aryl-, and cyclo-alkyl-acrylates, such as methyl acrylate, ethyl acrylate, isobutyl acrylate etc.; alkyl-, aryl-, and cyclo-alkyl-methacrylates, such as methyl methacrylate, ethyl methacrylate, butyl methacrylate; unsaturated ketones, vinyl esters such as vinyl acetate, vinyl propionate etc.J vinyl ethers; vinyl-benzene hydrocarbons, such as styrene or vinyl toluene; vinyl halides, such as vinyl or vinylidene chloride, vinyl or vinylidene fluoride; vinyl or vinylidene bromide, methacrylonitrile, butadiene and the like.

The solvent amount to be added to the polymerization slurry in step a) depends on the operative temperature and on the concentration of the unreacted monomer or monomers present in the slurry.

Such amount, as explained hereinbefore, shall be such as not to attack the polymer, and causing its swelling and solubilization, so as to form the spinning dope, as it conversely happens during the polymerization in solution.

Figure 1 shows the curve representing the temperature at which the dis״ solution of the polymer begins as a function of the monomer concentration, ,100 ־־ c x referred to the solvent,(------—), wherein c is the polymerization conversion expressed in per cent by weight with respect to the fed monomer or mixture of monomers, and Q is the solvent amount referred to 100 parts of polymerization slurry, in the case in which dimethylacetamide .is employed as a solvent.

The temperature at which the polymer dissolution begins, for each (100-c)/ /0 value, has been determined using a rotary blade viscosimeter and heating the sample by 1°C per minute.

Thus, for each sample, a viscosity - temperature curve is obtained, which exhibits a tipically stepped trend. The value in whose proximity the first sudden viscosity variation takes place has been assumed as polymer dissolution starting temperature.

In such curve, therefore, A represents the zone in which the polymer/ monomer/solvent mixture is fluid, the polymer being not attacked by the solvent, and B represents the zone in which the mixture is thick due to the attack of the polymer with progressive dissolution.

With a view to keeping such mixture fluid during the adiabatic evaporation, it has been experimentally found that, when dimethylacetamide is used as a spinning solvent, the quantity (0) of solvent to be added to the polymerization slurry is given by the formula 5 (^,™^100) (ו-ק-c) (1) wherein Q is the solvent quantity by weight referred to 100 parts of slurry, T is the temperature in °C of the solvent/slurry mixture, and c is the polymerization conversion expressed in per cent by weight with respect to the fed monomer or mixture of monomers.

The viscosity of the polymer/monomer/solvent mixture before and after the adiqbatic evaporation, the processability of such mixture and the amount of evaporated.monomer as a function of the amount of added sol vent, of the mixture temperature and of the unreacted monomers concen tration (acrylonitrile - vinylacetate), are reported on the following table I,

TABLE 1

<td> Polymerization conversion in ? by weight</td><td> .Monomers concentr, in % by wgt.</td><td> Amount of added solvent in g, with respect to WO g of slurry</td><td> Temperature , of the monomer/polyraer/ solvent mixture in ^eC</td><td> Mixture viscosity before evaporation.in poises</td><td> Mixture viscosity after evaporation in poises</td><td> Amount of evaporated monomer in ן with respect to starting monomer</td><td> . REMS</td>

<td> ׳40</td><td> 60</td><td> 229</td><td> 40</td><td> 0.3</td><td> 0.6</td><td> 40</td><td> fluid slurry</td>

<td> 40</td><td> 60</td><td> 250</td><td> 40</td><td> 2</td><td> 100</td><td> 5</td><td> dope begins to form during evaporation</td>

<td> 40</td><td> 60</td><td> 200</td><td> י .⁴⁰</td><td> 0.1</td><td> .0.3</td><td> 50</td><td> fluid slurry</td>

<td> 45</td><td> 55 '</td><td> 91</td><td> 85 '</td><td> 0.5</td><td> 3.</td><td> 40</td><td> fluid slurry</td>

<td> 45</td><td> 55</td><td> 100</td><td> 85</td><td> 3</td><td> 3,000</td><td> 2</td><td> formation of not processable gel</td>

<td> 45 A</td><td> 55 '</td><td> 80'</td><td> 85</td><td> 0.3</td><td> 1</td><td> 50 .</td><td> fluid slurry</td>

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Consequently, as it may be inferred from the table reported hereinabove, the quantity (q) of solvent to be admixed to the polymerization slurry is critical, in the sense that if solvent amounts higher than those defined , . a .

by formula (1) are added, the slurry changes into״dope or even into a gel ן .: no longer processablejand the amount of monomer removed by adiabatic evaporation is lower than the 30% by weight of the initial one. In this case, of course, the remaining monomer amount shall be removed by evaporation on a thin layer, such operation - as is known - being very slow and expensive.

The adiabatic evaporation is conducted at a residual pressure lower than . 100 mm Hg and preferably comprised between 5 and 50 mm Hg.

The solvent is further added when the percentage of solid matters - referred to the solvent - exceeds that of the solution to be spun.

The solvent may be added at any temperature, but preferably at room temperature

After addition of the solvent, the mixture is heated, in order to attack and dissolve the polymer with a view to forming the dope, by flowing through a heat exchanger.

The temperature at which the mixture is brought is lower than 170°C and preferably ranges from 70 to 100°C.

The optional final evaporation is conducted on thin-layer evaporators of the standard type, at a temperature comprised between 50° and 120®C.

A preferred practise of the present invention consists in a continuous process for the preparation of the spinning solution, comprising the following steps :

a) adding dimethyl acetamide to the polymerization slurry leaving the reactor in an amount as indicated by formula (1) hereinabove;

b) conveying the slurry/solvent mixture thus obtained through a heat exchanger to heat the mass to a temperature between 60° and 120’C, if the mass is not yet within such temperature range;

c) subjecting the pre-heated mass to adiabatic evaporation until room temperature at residual pressure varying from 5 to 50 mm Hg;

d) optional adding to the mixture thus obtained further dimethyl acetamide as to obtain a slurry having a polymer concentration, with respect to the solvent, of from to 25% by weight;

e) heating the slurry obtained in d) in a heat exchanger to a temperature of from 70* to 100^eC; and

f) removing, if necessary, residual monomer or monomers in a thin-layer evaporator.

The polymer solutions prepared according to this invention may be transformed into filaments or fibres by the usual methods, i.e. by wet or dry spinning. The filaments or fibres obtained exhibit excellent whiteness and thermal stability.

The following example is given to better illustrate the inventive idea of the present industrial invention and the practise thereof. In such example reference is made to the numerical symbols reported on the enclosed figure 2 representing schematically the various apparatus used in the process of this invention.

EXAMPLE 1 g/h of cumene hydroperoxide, g/h of gaseous SO^ and g/h of 2-mercaptoethanol are continuously and separately fed, at room temperature, to a polymerization reactor (1), having a capacity of 2,000 cc, provided with stirrer (2), thermometer, cooler-condenser (3) and overflow pipe (4), precharged to overflow with a mixture consisting for 72% of acrylonitrile, for 23% of vinyl acetate and for 5% of S0^ (water content in the mixture = 0,3%).

After the first 10 minutes, a monomer mixture made up for 83% of acrylonitrile and for 1/% of vinyl acetate, containing 0.1% of dimethyl-acetamide in the dissolved state, is fed at a flowrate of 10,000 g/h.

The polymerization temperature is of 75’C and the heat generated is entrained by the evaporating reaction mixture} the condensed steam flows back to the reactor (1),

Under stationary conditions, a slurry having a polymer content of 42% by weight - the remaining portion consisting of the following mixture of monomers : vinyl acetate and acrylonitrile in the ratio of 24% and 76% respectively - flows from the overflow pipe (4).

The slurry is mixed with dimethylacetamide fed, at the outlet (5) of the overflow pipe, at room temperature and at a flowrate of 9,500 g/h.

The mixture thus obtained, having a viscosity of 0.5 poises, is made to flow through a heat exchanger (6) wherein it is heated to 83.5°C,

This mixture is fed, at a flowrate of 19,500 g/h, to an insulated tank (7), kept at a pressure of 20 mm Hg. By adiabatic evaporation of the monomers, the mixture temperature is brought to 30°C.

The condensed monomers, made up for 62% of acrylonitrile, for 25% of vinyl acetate, for 11.5% of dimethyl acetamide and the remaining percentage of water and SO^, flow from the top (8) of the insulated tank at a flowrate of 4,700 g/h.

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The evaporated mixture, having a viscosity of 3 poises, is withdrawn from tank (7) by a gear pump (9) at a flowrate of 14,800 g/h and is added with dimethyl acetamide fed from a lateral pipe (10) at a flowrate of 7,840 g/h. The mixture is then made to flow through a heat exchanger (11), wherein it is heated to 80^0. The solution thus obtained is introduced into tank (12), provided with stirrer (13),and then fed, by a gear pump (14)jto the top of a thin-layer evaporator (15) for a thorough removal of the monomers.

In evaporator (15) the pressure is of 50 mm Hg and the temperature of 90° C; the solution remains in said evaporator for an average time-period of 30 - 60 seconds.

The solution leaving evaporator (15) exhibits a concentration in solid matters corresponding to 25%, a viscosity of 150 poises and is practically free from monomers.

Such solution is fed by means of a gear pump (16) to a filter (17) and then to a spinneret of a spinning device for being transformed into filaments according to the known spinning techniques.