Polyarylene sulfides (also abbreviated as “PAS” hereafter) such as polyphenylene sulfides (also abbreviated as “PPS” hereafter) are engineering plastics having excellent heat resistance, chemical resistance, flame retardant properties, mechanical strength, electrical properties, dimensional stability, and the like. PASs are commonly used in a wide range of technical fields such as electrical equipment, electronic equipment, automobile equipment, and packaging materials since PASs can be molded into various molded products, films, sheets, fibers, and the like by means of a general melt processing method such as extrusion molding, injection molding, or compression molding.
A known representative example of a PAS production method is a method of reacting a sulfur source with a dihalo aromatic compound such as paradichlorobenzene (also abbreviated as “pDCB” hereafter) in an organic amide solvent such as N-methyl-2-pyrrolidone (also abbreviated as “NMP” hereafter). An alkali metal sulfide, an alkali metal hydrosulfide, or a mixture thereof is typically used as a sulfur source. If an alkali metal hydrosulfide is used as a sulfur source, the alkali metal hydrosulfide is used in combination with an alkali metal hydroxide.
The polymerization reaction of the sulfur source and the dihalo aromatic compound is a desalting polycondensation reaction, and a large amount of a salt such as NaCl (that is, an alkali metal halide) is produced after the reaction. Since the reaction uses a sulfur-containing compound, hydrogen sulfide is also produced as a product or a by-product. Therefore, conventionally, after a PAS polymer produced by the polymerization reaction is separated from the reaction solution and collected, the collected polymer is washed using a washing agent such as water, an organic solvent, or a mixture of water and an organic solvent so as to remove salts such as NaCl, oligomers, or the like.
Washing methods are broadly classified into batch washing and continuous washing methods. Batch washing is a method of repeating a process of stirring and washing a collected polymer with a washing agent in a washing vessel all at once or in separate batches. In batch washing, a large-capacity washing vessel is often required, which results in a complex equipment and an increased construction cost, while at the same time leading to increases in the amount of washing liquid consumed, increases in the amount of drainage treated, increases in agitation power, and increases in operating costs.
A counter current washing method has been proposed as a type of continuous washing, wherein a polymer in a wet state and an organic solvent or water serving as a washing agent are brought into contact with one another by means of a counter current. For example, it is proposed in Document 1 (Japanese Unexamined Patent Application Publication No. H03-86287A) to bring a granular resin such as a PPS into contact with a washing solution by means of a counter current inside a vertically erected tubular body having static mixing elements arranged continuously in series. It is proposed in Document 2 (WO/2003/048231) to bring a polymer slurry containing a polymer into contact with a washing solution by means of a counter current using a roughly V-shaped tube device having a downward tube part and an upward tube part. In addition, a vertical solid-liquid contact device comprising a plurality of stirring chambers arranged continuously in the vertical direction is disclosed in Document 3 (Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2008-513186).
The idea of repeating continuous washing or performing batch washing prior to continuous washing is also known.
In the PAS production process, an acid treatment step of treating the PAS polymer using an acidic compound such as hydrochloric acid or acetic acid was arranged immediately after the washing step so as to completely remove impurities such as water-soluble metal salts and to remove basic residues of the polymer chain terminals of the PAS. One method of the acid treatment step is a method of stirring, for a prescribed amount of time, a reslurry prepared by adding an aqueous solution of an acidic compound to a slurry prepared by dispersing a washed PAS in an aqueous medium, as described in Document 4 (Japanese Unexamined Patent Application Publication No. S62-48728A) or Document 5 (Japanese Unexamined Patent Application Publication No. H7-118389A). In this case, after acid treatment, it is necessary to completely remove the acid components by sufficiently washing the solution with water after neutralizing the solution with a base as necessary. Therefore, as described in Document 2 above, with a method of performing acid treatment by bringing a washed PAS slurry into contact with an aqueous washing solution by means of a counter current after adding an acidic compound to the PAS slurry, the water-washing operation performed to remove acid components originating from the acidic compound after acid treatment is unnecessary, so this method is effective.
With the objective of stably producing a high-quality PAS, the present inventors began dedicated research on effectively performing a washing step, for example, a counter current washing step and/or a counter current contact/acid treatment step, in the PAS production process.
In the washing step, a slurry of a PAS polymer comes into contact with a washing solution in a washing vessel. The specific gravity of PAS particles (also called “PAS polymer particles” hereafter) is originally greater than 1, so most PAS particles move downward due to gravity. However, some particles do not migrate to the bottom of the treatment vessel in spite of having a larger specific gravity than the washing solution and continue to float on the surface of the washing solution in the upper part of the inside the washing vessel. As a result, the PAS cannot be sufficiently separated and collected after washing treatment based on the specific gravity difference between the PAS particles and the washing solution, and the present inventors were confronted with the problem that the PAS particles floating on the liquid surface leak to the outside of the washing vessel as washing drainage together with the washing solution.
In a counter current washing step or a counter current contact/acid treatment step, although most PAS particles migrate downward due to gravity as a result of the PAS polymer slurry and the washing solution coming into contact by means of a counter current, some PAS particles do not migrate to the bottom of the treatment vessel in spite of having a larger specific gravity than the washing solution, and the PAS particles sometimes migrate upward together with the washing solution and float on the surface of the washing surface in the upper part of the inside the washing vessel. As a result, as described above, the PAS cannot be sufficiently separated and collected after counter current washing or counter current contact/acid treatment, which leads to the problem that the PAS particles floating on the liquid surface leak to the outside of the washing vessel as washing drainage together with the washing solution.
The PAS particles floating on the liquid surface also include extremely fine particles that are difficult to collect with a conventional method, but most of the particles are basically PAS particles that can be used in the production process, and the fact that such PAS particles leak out without being collected as a product leads directly to a reduction in product yield. Moreover, another problem was also discovered, wherein the amount of PAS particles separated and collected is not stable, and the product yield fluctuates among the obtained experimental lots.
Decreases in product yield, fluctuations in product yield, and fluctuations in product composition associating such fluctuations could conceivably have a large direct effect on quality in terms of the particle properties such as the particle size or particle size distribution of the produced PAS, the average molecular weight or melt viscosity, the mixing stability with other raw materials or melt processing stability when obtaining a molded product, or the thermal and mechanical characteristics of the molded product, so the present inventors continued to conduct dedicated research on the causes of and solutions to the aforementioned problems associated with washing or acid treatment.
As a result, the present inventors obtained the following knowledge with regard to the cause of the floating of fine PAS particles or the like on the surface of the washing solution in the upper part of the inside of the washing vessel when performing washing treatment on the PAS or when performing acid treatment on the PAS slurry after washing treatment.
Specifically, in washing treatment such as counter current washing and counter current contact/acid treatment, a slurry of a PAS polymer fed at a relatively high temperature ordinarily comes into contact with a washing solution having a relatively low temperature. In this process, if the temperature of the washing solution having a relatively low temperature increases, the air, oxygen, or the like that had been dissolved in the washing solution may be dissociated from the liquid as a gas and may float due to a decrease in the gas solubility in liquid resulting from the temperature increase. In addition, air incorporated into the liquid phase inside the washing vessel due to stirring associated with washing treatment such as counter current washing or counter current contact/acid treatment, air infiltrating the particles during the polymerization step or separation step, gases produced at the time of acid treatment, and the like are dissociated from the liquid phase or the particles as gases and may float. This phenomenon occurs markedly in winter or in cold regions where the temperature of the washing solution is low.
The present inventors discovered that if these gases are adsorbed onto PAS polymer particles, in particular, particles having a small particle size, particles having many voids, particles having a rough surface, or the like, the PAS polymer particles with adsorbed gases tend to easily migrate toward the top of the washing vessel together with the washing solution, even if the inherent difference in specific gravity between the PAS particles and the washing solution is large, and that the PAS polymer particles eventually emerge as polymers in a floating state (also called a “floating polymer” hereafter) on the liquid surface of the washing solution in the upper part inside the washing vessel.
The present inventors further discovered that in counter current washing or counter current contact/acid treatment, in particular, the sedimentation rate of particles varies depending on the composition and temperature of the liquid phase in the slurry of the PAS polymer and the composition and temperature of the washing solution. Thus the present inventors therefore also focused attention on the fact that a floating polymer may also be formed as a result of the aforementioned gases adsorbed onto the polymer.
In the washing treatment of the slurry of the PAS polymer, the slurry of the PAS polymer is introduced from the top of the washing vessel, and the slurry settles in the washing solution due to gravity and is discharged from the base of the washing vessel. In particular, in counter current washing and in counter current contact/acid treatment in which the PAS polymer slurry is brought into contact with the washing solution using a counter current after an acidic compound is added to the PAS polymer slurry, the PAS polymer slurry is transported from top to bottom in the washing vessel, and the washing solution is transported from bottom to top so as to continuously bring the washing solution into contact with the PAS slurry using a counter current. In this case as well, the slurry of the PAS polymer is introduced from the top of the washing vessel, and the slurry settles in the washing solution due to gravity and is discharged from the base of the washing vessel. On the other hand, the washing solution is introduced from the base of the washing vessel and is discharged from the top of the washing vessel.
The floating polymer described above is in a floating state on the surface of the washing solution in the upper part of the inside of the washing vessel, so the floating polymer leaks to the outside of the washing vessel together with the washing solution as the washing solution is discharged to the outside of the washing vessel from the top of the washing vessel, which prevents the floating polymer from being separated and collected as a product. The present inventors confirmed that there are cases in which the amount of the floating polymer that leaks to the outside of the washing vessel reaches several percent of the PAS polymer produced by the polymerization reaction and discovered that in order to achieve the stable production of high-quality PAS, to improve the product yield, and to reduce the environmental load (amount of suspended substances in the waste water), it is necessary to prevent the production of a floating polymer and to prevent leakage to the outside of the washing vessel.