Quenching and coagulation of filaments in an ultrasonic field

More uniform and more rapid quenching and coagulation of filaments is achieved by contacting the filaments in a chamber with coagulating liquid and generating pressure fluctuations in the liquid at high frequency sonic or ultrasonic frequencies.

BACKGROUND OF THE INVENTION 
A process for preparing m-phenylene isophthalamide fiber involves spinning 
the solution of the polymer, as prepared, including dimethylacetamide and 
by-product calcium chloride and contacting the extruded filaments with a 
hot inert gas such as nitrogen to partially remove solvent. A cold aqueous 
solution is used to quench and coagulate the filaments. Finally, the 
filaments are wash-drawn and collected. Satisfactory results have been 
achieved by this process, however, attempts to increase throughput in the 
quench-coagulation step has often resulted in nonuniformities as shown by 
opaque white streaks in the otherwise translucent filaments and by 
variations in tensile strength among the filaments. Also, fusion between 
filaments may occur as well because of slow, non-uniform cooling of some 
filaments. The present invention has applicability to processes wherein 
the freshly extruded solvent-containing filaments first contact an inert 
gas or fluid before quench-coagulation with an aqueous solution as well as 
to wet-spinnning processes wherein the solvent-containing filaments are 
spun directly into an aqueous quench-coagulation solution.

SUMMARY OF THE INVENTION 
The present invention provides an improved process for preparing fiber from 
a polymer solution which includes the steps of: 
a) extruding the solution from a spinneret to form a plurality of 
filaments; 
b) optionally passing the extruded filaments through an inert gas; 
c) treating the filaments with an aqueous liquid coagulant to quench and 
coagulate the filaments; 
d) washing and drawing the filaments; and 
e) collecting the filaments; the improvement comprising, quench coagulating 
the filaments more uniformly and more rapidly in step c) by passing the 
filaments between substantially parallel opposing walls of a chamber 
containing the aqueous liquid coagulant, the said opposing walls 
comprising the faces of ultrasonic transducers, and driving the 
transducers, in phase, at a frequency of from 5 to 100 kHz to cause 
pressure fluctuations in the liquid coagulant, the spacing between the 
said opposing walls being less than one-half the wavelength of sound 
generated by the transducers in the liquid coagulant. 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention is described below with reference to a process for 
preparing m-phenylene isophthalamide (MPDI) fiber. However, the invention 
can be applied to other processes such as the spinning process described 
in the Blades patent U.S. Pat. No. 3,767,756 for making poly(p-phenylene 
terephthalamide) fiber wherein the solvent-containing filaments leaving 
the spinneret are first passed through an air gap and then through aqueous 
liquid coagulant or a spinning process wherein the solvent-containing 
filaments leaving the spinneret are passed directly into and through an 
aqueous liquid coagulant. The process is particularly effective in the 
production of aromatic polyamide fiber, preferably aramid fiber where a 
salt is present in the spin dope. Conventional quench coagulation is 
adversely affected by the presence of salts in the spin dope, as will be 
understood to those skilled in the art. 
As-prepared MPDI polymer solution conventionally contains dimethyl 
acetamide (DMAc) or other solvent and calcium chloride or other salt in 
addition to the polymer itself. The solvent may constitute as much as 
about 80% of the solution. In the process for preparing fiber from the 
polymer, this solution or spin dope is spun or extruded through a 
spinneret to form a plurality of filamentary streams, and a flow of hot 
inert gas such as nitrogen at a temperature of about 450.degree. C. is 
passed in contact with the spun filaments. The solvent content of the 
filaments is thereby reduced. In the next step of the process, the hot 
filaments are contacted with an aqueous liquid, generally cold water, 
below 5.degree. C., which quenches and coagulates the filaments. It is 
this step which is the focus of the present invention. Streaks are the 
result of improper quenching, that is, the quench liquid is not uniformly 
distributed around the filaments when they contact the quench liquid. 
Uniform quenching produces a uniform, polymer-rich skin structure on the 
surface of the fiber. Improper quenching allows water to penetrate the 
skin structure and create voids in the surface. 
To achieve the improvement of the present process, the filaments are 
quench-coagulated in a special manner. The filaments, after treatment with 
the hot inert gas, are passed through a chamber having opposing walls 
comprising radiating ultrasonic transducer faces. The filaments in bundles 
of 15,000 denier or greater may traverse the length of the chamber at 
speeds of 200 to 250 yards per min. or even faster. Cold liquid is fed 
into the chamber generally at a rate of 80 to 120 gallons per hour, to 
quench and coagulate the filaments. The procedure can be performed as 
depicted in FIG. 2 showing a schematic side view of the chamber 1, having 
opposing walls 2. Aqueous liquid coagulant 3 enters through ports 4 to 
maintain a desired level in the chamber. Filaments 5 enter the chamber, 
are centered and flattened into a ribbon by guide 6 and pass through the 
chamber in contact with coagulant liquid 3. The opposing faces 2 of 
ultrasonic transducers 8 are driven, in phase, at a frequency of from 
5-100 kilohertz kHz. By "in phase" is meant that the two opposing 
transducer faces move towards and away from each other in synchronism. 
Magnetostrictive or piezoelectric devices may be employed as the 
transducers. Preferably, a frequency of from 20 to 70 kHz is employed. 
Vibra-Bar transducers (Crest Ultrasonics, Trenton, N.J.) at 40 or 65 kHz 
are suitable for this purpose. The distance between the two opposing walls 
of the chamber which are constituted by the radiating transducer faces 
should be less than one-half the wavelength of the sound generated by the 
transducers in the liquid coagulant. Generally, 1 inch or less is 
suitable, the specific distance limit being readily determined by the 
frequency at which the transducers are driven and the coagulant fluid 
employed, as is well-understood by the art. For example, at a frequency of 
40 kHz with water as coagulant at 4.degree. C. the faces are about 3/4 
inch apart or less. 
The transducers used in this invention are driven at a total average power 
level of 36 to 250 watts to provide average power densities of 
approximately 1 to 7 watts per square inch of radiating area and 4 to 28 
watts per cubic inch of liquid in the quench chamber. When compared to 
conventional ultrasonic cleaning baths, the maximum area power density of 
this invention is 2 to 3 times higher, while the maximum volume power 
density is 100 to 600 times higher. 
The intense sound field generated by the transducers is characterized by 
pressure fluctuations in the quench liquid that are most intense in the 
plane centered between the radiating transducer faces, which is congruent 
with the path of the ribbon of filaments. The pressure fluctuations 
produce several beneficial effects that improve the uniformity and speed 
of filament quenching or coagulation. On a macroscopic scale, the quench 
liquid is driven into and out of the filament ribbon to improve the 
uniformity of the liquid contact with all of the filaments, particularly 
those not in the surface layer of the ribbon. On a microscopic scale, 
localized, high-velocity liquid eddies and currents penetrate the filament 
boundary layers to continually carry fresh quench liquid to the filament 
surfaces. Also, cavitation bubbles form and collapse as the sound pressure 
field alternates below and above the ambient pressure, creating extremely 
localized shock waves. These microscopic phenomena combine to increase 
thermal diffusion and mass transfer rates, thereby increasing the speed of 
the quench-coagulation process. 
The treated fiber bundle and entrained liquid exits the chamber through 
port 7. The quenched-coagulated MPD-I filaments are normally subjected to 
a wash-draw where the filaments are washed and drawn and then collected 
before or after drying. 
The following example of the invention is not intended as limiting. 
EXAMPLES 
The fibers or filaments of these examples were prepared from aromatic 
polymers such as are disclosed in U.S. Pat. No. 3,063,966 to Kwolek, 
Morgan, and Sorenson; 3,094,511 to Hill, Kwolek and Sweeny; and 3,287,324 
to Sweeny, for example. Filaments were prepared from a filtered solution 
consisting of 19.2%, based on the weight of the solution, of 
poly(meta-phenylene isophthalamide) in N,N-dimethylacetamide (DMAc) that 
contains 45% calcium chloride based on the weight of the polymer. The 
polymer had an inherent viscosity of 1.57 as measured on a 0.55 solution 
in DMAc/4% LiCl at 25 degrees C. The spinning solution was heated to 
120-145 degrees C and extruded through a 3600-hole spinneret, each hole 
0.006 inch (150 microns) in diameter and 0.012 inch (300 microns) long, 
into heated spinning cells containing an inert gas. For each of the 
following examples, the speed of the just-spun filaments was in excess of 
200 ypm. 
EXAMPLE 1 (CONTROL) 
This example illustrates a prior art process, which is disclosed in U.S. 
Pat. No. 3,493,422 to Berry; this reference discloses an apparatus and 
process for efficient heat and/or mass transfer by sequentially contacting 
a moving shaped structure through a stripping liquid. The filaments, as 
spun above, (each filament being about 12 dpf as spun), were formed into a 
flat ribbon of filaments at the top of the quench zone and then brought in 
contact with a cold, approximately 4.degree. C., aqueous solution 
containing 4-12% DMAc and flowing essentially co-current with the filament 
ribbon in a serpentine manner as dictated by the shape of the quenching 
apparatus. Filaments made by this process had visible streaks, the 
quantity of which was proportional to the speed of the filament ribbon. 
EXAMPLE 2 
This example illustrates the invention of this application. The filaments, 
as spun above (each filament being about 12 dpf as spun), were formed into 
a flat ribbon at the top of the quench zone and then entered a straight 
rectangular quench chamber approximately 1 in. by 3 in. in cross-section 
and 6 in. long, said chamber containing a cold, approximately 4 degrees C, 
aqueous solution containing 4-12% DMAc and flowing co-current with the 
filament ribbon. The radiating faces of two piezoelectric transducers 
constituted the opposing wider walls of the chamber as illustrated in FIG. 
2. The width of the ribbon passed between the two opposing transducer 
faces which were vibrated in phase (moving towards and away from each 
other in synchronism) at a sonic frequency of 40 kHz, generating intense 
pressure fluctuations in the liquid in the sonic field zone. The two 
transducers were driven at a total average power level of 250 watts to 
provide average power densities of approximately 7 watts per square inch 
of radiating surface area and 28 watts per cubic inch of liquid in the 
quench zone. Essentially none of the filaments made by this process had 
visible streaks; and filament quality was not as sensitive to the speed of 
the filament ribbon.