Continuous process for preparing a spinning solution of acrylic polymers

The present invention relates to a continuous process for the preparation of a spinning solution of acrylonitrile polymers.

More particularly, the invention relates to a continuous process for the preparation of a spinning solution of acrylonitrile polymers prepared by bulk polymerization using as spinning solvent an organic compound such as dimethylacetamide, dimethylformamide, ethylene carbonate, dimethylsulfoxide, etc.

Acrylonitrile polymers, as used herein, are long chain synthetic polymers consisting of at least 85% by weight of acrylonitrile units, the remainder being at least one other ethylenically unsaturated monomer copolymerizable with acrylonitrile.

As is known, acrylonitrile polymers can be prepared by suspension polymerization, solution polymerization and bulk polymerization processes.

The polymerization process carried out in an aqueous suspension offers the advantages of a relatively high polymerization rate and of providing such polymer properties as purity and whiteness.

7407644-9

However, the need to obtain the polymer in the dry state for use in spinning with organic solvents involves complicated and expensive processes, such as filtration, drying and grinding. The dried polymer is successively dissolved in the spinning solvent with stirring.

When the polymerization is carried out in solution, the difficulty essentially consists in the need to work with a highly viscous medium in the polymerization reactor. Such a property of the medium limits the heat exchange coefficients and consequently the reactor dimensions.

In addition, since the polymer solution is very viscous, the separation of the monomer which is not converted to polymer must be carried out in special and very often complicated devices.

Mass polymerization of acrylonitrile is also known which works in the absence of both water and solvent. According to such a procedure and when the polymer is to be used for the production of fibers, films and other shaped articles, the reaction mass leaving the reactor is subjected to a mechanical polymer / monomer separation (centrifugation and / or filtration) and the polymer is dried, ground and dissolved in the solvent.

An object of the present invention is thus to provide a continuous process for directly obtaining the spinning solution from the polymerization slurry leaving the reactor and is prepared according to a mass polymerization process without carrying out the mechanical polymer / monomer separation and without drying or grinding the obtained polymer.

It has now emerged that this can be achieved by a procedure comprising the following steps:

a) adding to the polymerization slurry, as it leaves the reactor, a solvent in an amount insufficient to attack and dissolve the polymer and wherein the polymer / monomer / solvent mixture has a viscosity of less than 2 poise;

b) heating the mixture thus obtained to a temperature (T) higher than room temperature and lower than 170 ° C without the polymer already being attacked by the solvent;

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

d) optionally adding an additional amount of solvent until the required polymer concentration is obtained;

e) heating the mixture id) to a temperature not exceeding 17O ° C with the intention of attacking and carefully dissolving

7W7644-9. ------------ the polymer, and

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

The present process utilizes the bulk polymerization of acrylonitrile in that it allows continuous preparation, in a simple and economically advantageous manner, of the polymer solution in the spiny solvent. In fact, by working in accordance with the above procedure, the unreacted monomer or monomers are removed by simple adiabatic evaporation while the polymer is not yet attacked by the solvent and the mixture is consequently very liquid. A further advantage of the present process consists in the possibility of using the heat of polymerization to carry out said adiabatic evaporation.

The monomers which are subjected to removal are those which remain in the slurry leaving the reactor after the polymerization process.

In general, such monomers consist mainly of acrylonitrile, but in the preparation of acrylonitrile copolymers, in addition to this monomer, other ethylenically unsaturated compounds are also copolymerizable with the acrylonitrile present.

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

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

Such an amount should, as previously stated herein, be such that it does not attack the polymer and cause its swelling and solubilization so that the spin solution is formed as the reverse happens during the polymerization in solution.

Figure 1 shows the curve representing the temperature at which the solution of the polymer begins as a function of the monomer concentration, relative to the solvent, (- °), wherein c

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................... the polymerization conversion is expressed in weight # with respect to the monomer fed or the mixture of monomers ooh Q, the amount of solvent is attributed to 100 parts of polymerization slurry in the case in which dimethylacetamide was used as the solvent.

The temperature at which the polymer dissolution begins has, for each (100-e) / 9 value, been horsepowered using a rotating blade viscometer and heating the sample at 1 ° C per minute.

Thus, for each sample, a viscosity-temperature curve is obtained which shows a typical rising tendency. The value in the vicinity of which the first sudden change in viscosity takes place has been assumed as the initial temperature of the polymerization solution.

In such a curve, therefore, A represents the zone in which the polymer / monomer / solvent mixture is liquid, the polymer not being attacked by the solvent, and.

In order to keep such a mixture liquid during the adiabatic evaporation, it has been found experimentally that when dimethylacetamide is used as a spinning solvent, the amount (9) of solvent to be added to the polymerization slurry is given by one of the formula:

<¹⁰⁰ - ^c> ' (¹) wherein 9 "is the amount of solvent in parts by weight based on 100 parts by weight of slurry, t is the temperature in ° C of the solvent / slurry mixture and c is the polymerization conversion expressed in weight # with respect to the monomer fed or the monomer mixture.

The viscosity of the polymer / monomer / solvent mixtures before and after the adiabatic evaporation, the processability of such a mixture and the amount of evaporated monomer as a function of the amount of solvent added, of the mixing temperature and of the concentration of unreacted monomers (acrylonitrile-vinyl acetate) are as follows table I. · '

7407644-9

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7407644-9

Accordingly, it can be concluded from the above table that the amount (9) of solvent to be mixed with the polymerization slurry is critical in the sense that if larger amounts of solvent than those defined by formula (1) are added, the slurry changes to a spin solution and to and to a gel which is no longer processable and the amount of monomer removed by adiabatic evaporation is less than 30% by weight of the unreacted amount. In this case, of course, the remaining amount of monomer must be removed by evaporation on a thin layer, such a process, as is known, being very slow and expensive.

The adiabatic evaporation is carried out at a residual pressure which is lower than 100 mm Hg and suitably is between 3 and 50 mm Hg. The solvent is further added when the percentage of solids - attributed to the solvent - exceeds that of it. solution to be spun.

The solvent can be added at any temperature l

but preferably at room temperature.

After addition of the solvent, the mixture is heated to attack and dissolve the polymer in order to prepare the spin solution by flow through a heat exchanger.

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

Any final evaporation is carried out on standard type thin-layer evaporators, at a temperature between 50 and 120 ° C.

A preferred application of the present invention consists in a continuous process for preparing the spinning solution comprising the following steps:

a) adding to the polymerization slurry leaving the dimethylacetamide reactor in an amount indicated by formula (1);

b) passing the slurry / solvent mixture thus obtained through a heat exchanger which heats the pulp to a temperature between 60 and 120 ° C in case the pulp is not already within such a temperature range;

c) exposing the preheated mass for adiabatic evaporation to room temperature and to a residual pressure ranging from 5 to 50 mm Hg;

d) optionally adding to the mixture of solvents thus obtained in such an amount as to obtain a slurry having a polymer concentration, with respect to solvents ranging from 15 to 25% by weight;

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e) passing the resulting slurry id) through a heat exchanger until it is circulated to a temperature between 70 and '100 ° C and

f) eliminating, if necessary, the remaining monomer or monomers in a thin film evaporator.

The polymer solutions prepared according to the present invention can be converted into threads or fibers by the usual methods, i.e. by wet spinning or dry spinning. The resulting threads or fibers exhibit excellent whiteness and heat resistance.

The invention is illustrated approaching? by means of the following examples in which reference is made to the numerical designations given in the attached figure 2 which schematically show the various devices used in the method according to the invention.

Example, g / h cumene hydroperoxide, g / h gaseous SO₂ and g / h 2-mercaptoethanol is fed continuously and separately, at room temperature, to a polymerization reactor (1) with a capacity of 2000 cm³, equipped with stirrer (2), thermometer, cooler-condenser (3) and overflow line (4), pre-charged to overflow with a mixture consisting of 72 fy acrylonitrile, 23 fy vinyl acetate and 5 fy S0₂ (water content of the mixture = 0.3 fy).

After the first 10 minutes, a monomer mixture of 83 .mu.l of acrylonitrile and 17 .mu.l of vinyl acetate, containing 0.1 .mu.l of dimethyl acetamide in solution, is fed at a flow rate of 1000 g / hour. The polymerization temperature is 75 ° C and the heat generated is absorbed by the evaporating reaction mixture; the condensed steam flows back to the reactor (l).

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

The slurry is mixed with the dimethylacetamidine feed, at the outlet (5) of the overflow line, at room temperature and a flow rate of 9500 g / hour.

The mixture thus obtained, which has a viscosity of 0.5 poise, is caused to flow through a heat exchanger (6) where it is heated to 83.5 ° C.

This mixture is fed at a flow rate of 19500 g / hour to an insulated tank (7) maintained at etc pressure of 20 mm Hg. Through

7407544-9 adiabatic evaporation of the monomers is brought to mixing temperatures. fcill 30 ° C.

The condensed monomers, consisting of 62 fy acrylonitrile, fy vinyl acetate, 11.5 fy dimethylacetamide and the remaining percentage of water and S0₂j flows from the upper part (8) of the insulated tank at a flow rate of 4700 g / hour.

The evaporated mixture, which has a viscosity of J poise, is withdrawn from the tank (7) with a gear pump (9) with a flow rate of 14800 g / hour and dimethylacetamide is added from a side line (10) with a flow rate of 7840 g / hour. The mixture is then made to flow through a heat exchanger (11) in which it is heated to 80 ° C. The solution thus obtained is introduced into a tank (12) provided with a stirrer (13) and then fed, with a gear pump (14) to the top of a thin layer evaporator (15) for careful removal of the monomers.

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

The solution leaving the evaporator (15) has a concentration in solids corresponding to 25 fy, a viscosity of 150 poise and is practically free of monomers.

Such a solution is fed with a gear pump (16) to a filter (17) and then to a spinning nozzle of a spinning device for conversion into fibers according to known spinning methods.