Method and system for producing a spinning solution for producing a polymer fiber

A method for producing a spinning solution for producing a polymer fiber, particularly a p-aramid fiber, the polymer being blended with a solvent, mixed, melted, homogenized, and degassed and then discharged, liquid sulfuric acid being used as the solvent and at least the mixing, homogenization, and degassing taking place in a preferably continuously operated single or double cell reactor (1).

BACKGROUND OF THE INVENTION

This invention relates to a method of producing a spinning solution for producing a polymer fiber, in particular an aramid fiber, which method comprises admixing the polymer with a solvent, and melting, homogenizing, devolatilizing and then discharging the mixture and also to apparatus therefor.

Manufactured polymer fibers are currently needed for many applications in the private and industrial spheres. This applies, for example, to all kinds of garments, reinforcing fabrics, industrial textiles, wear pads, filters, etc. For this purpose, the polymer in question, which is usually present in piece or powder form, is admixed with a solvent, melted, homogenized, devolatilized and then fed while still in the viscous state or remelted after intervening transportation to dies from which it is then extruded under high pressure to form the fiber.

U.S. Pat. No. 5,882,563, for example, discloses a process for making fibers from poly(p-phenyleneterephthalamide) in which sulfuric acid cooled to below its coagulation point is mixed in the form of ice with p-aramid to obtain a solid mixture. Thereafter, this solid mixture is reheated and melted and this liquid is spun through spinning orifices under pressure. The heating of this solid mixture is carried out in two separate zones, namely a melting zone and a pressure build-up zone, with the polymer being kneaded and mixed in the melting zone at least.

The object of the present invention is providing a method and apparatus of the abovementioned type which provides the product with a long residence time while at the same time increasing the shear rates in the apparatus and providing a maximum of flexibility, processing consistency and reduced apparatus components.

SUMMARY OF THE INVENTION

The object is achieved when liquid sulfuric acid is used as solvent and at least the mixing, homogenizing and devolatalizing is effected in a preferably continuously operated single- or twin-shaft reactor.

This eliminates the need for the hitherto necessary cooling of the sulfuric acid to below its coagulation point, making the entire process significantly simpler and less costly.

Hitherto, furthermore, single-shaft reactors have generally been used, but they have a limited shear rate and are also limited in torque absorption. A twin-shaft reactor, for example of the CRP or CKR type from List, as described in EP 0 517 068, however, gives significantly better results, particularly with regard to the residence time of the product in the reactor, the higher volume capacity of the reactor, the free surface area of the product, and the robustness of the system. A feature particularly worth highlighting is that the three process steps, viz., mixing, homogenizing and devolatalizing, take place in a single reactor. It is only the premixing step which may possibly require a separate mixer.

The large surface area of the product results in good devolatilization and intensive surface renewal during kneading.

In twin-shaft kneading machines, the two shafts can be operated at equal or else at different speeds, which makes the shear rate and the mixing of the product significantly better than in a single-shaft reactor. Moreover, there are scarcely any dead zones in a twin-shaft reactor where product deposits might build up and then possibly remain in the reactor and lead to degradation of the product.

A further feature of the present invention relates particularly to the sequence of inputting the liquid sulfuric acid and the polymer powder. It was determined that it is specifically the order of the metered additions that leads to significant improvements. The polymer powder, which is introduced first, can mix significantly more intensively with the sulfuric acid introduced later. The powder constituents are uniformly enveloped by the sulfuric acid.

DETAILED DESCRIPTION

Inventive apparatus for producing a spinning solution for producing a polymer fiber includes centrally a twin-shaft reactor1as described in EP 0 517 068 for example. This reactor1has two input stubs2and3, with input stub3being used for liquid sulfuric acid and input stub2for adding the polymer in powder form. The feed line for the solvent may additionally include a precooler12.

The reactor is additionally fitted on top with a vapor dome4, via which any gases still present are withdrawn during the mixing and homogenizing of the product in reactor1.

Reactor1connects on its downstream side to a single or twin-shaft discharging device5of variable speed of rotation. In the present illustrative embodiment, it takes the form of a twin-shaft discharging device. The mixed product passes out of the reactor1through the discharging device to a pump (not specifically illustrated) which then extrudes the mixed product through appropriate dies to form fibers.

The illustrative embodiment of the inventive apparatus according toFIG. 2differs from that according toFIG. 1in that polymer and sulfuric acid are premixed in a mixer6upstream of the reactor1and then the mixed product is introduced into the reactor1in an already premixed state and preferably by metering, through a single input stub11. The polymer and the sulfuric acid are introduced into the mixer6through two metering stubs7and8.

The illustrative embodiment of an inventive apparatus according toFIG. 3differs from that according toFIG. 2in that the reactor1connects on its downstream side to a single-shaft discharging screw5.1, whose axle9extends axially relative to the reactor1. This results in a pressure building up between a discharge opening10and the interior of the reactor1, making it possible for the mixed product to be discharged from the reactor1continuously.

The mode of functioning of the present invention is as follows:

Referring to the illustrative embodiment shown inFIG. 1, sulfuric acid and polymer powder, in particular p-polyphenyl terephthalamide (PPTA), are introduced in a continuous reactor1, in particular a mixing kneader from List of the CRP or CKR type, through the input stubs2and3. This reactor has two horizontally disposed shafts fitted with appropriate kneading elements, and not only the outer shaft but also the shafts and optionally the kneading element are heatable.

The polymer powder which comes into intensive contact with the solvent, sulfuric acid in this case, evolves a certain solution enthalpy. This quantity of heat combined with the contact heat transportation quantity through the reactor walls and kneading shafts leads to rapid heating and melting of the mixture.

At the same time, it is transported from the input stubs2,3in the direction of the discharging device5. During this transport, the mixed product in turn undergoes uniform devolatilization over the entire length of the kneading zone, and the gases are withdrawn through the vapor dome4. The product thus intimately commixed is then discharged through the discharging device5.

The illustrative embodiment shown inFIG. 2comprises premixing the polymer powder with the sulfuric acid. It has been determined in this connection that, with this sequence, the mixing between polymer powder and sulfuric acid becomes significantly more intensive. Only then is the already premixed product introduced into the reactor1, where it is homogenized and devolatilized. Discharge is again through the discharging device5, but it is also possible, as shown inFIG. 3, to utilize a single-shaft discharging apparatus5.1comprising a single-shaft discharging screw having a turning axle which extends axially parallel to an axis of the reactor and the screw is connected to one of the shafts.