1. Field of the Invention
This invention relates to a process for industrially advantageously recovering a polyarylene sulfide having a low content of electrolytic ions from a mixture obtained through a reaction between a polyhalogenated aromatic compound and an alkali metal sulfide containing water in an organic polar solvent. The polymer obtained by the process of the present invention is particularly useful for use in molded articles such as electronic and electric components and films, sheets and fibers.
2. Description of the Prior Art
Polyarylene sulfide is produced by carrying out a polycondensation reaction between a halogenated aromatic compound and an alkali metal sulfide in a water-soluble polar solvent having a high boiling point and in the presence of a small amount of water, the reaction being conducted under pressure and at a relatively high temperature, i.e., 200.degree. to 280.degree. C., at which the resulting polyarylene sulfide is melted. The product of the reaction is a slurry consisting of polyarylene sulfide, an alkali metal halide, water, a small amount of unreacted matter, an oligomer which is a low-molecular weight polyarylene sulfide, and the above-described solvent. Various processes for recovering polyarylene sulfide from the reaction mixture have been proposed. For example, U.S. Pat. No. 3,478,000 discloses a process wherein the reaction mixture is heated under reduced pressure to evaporate the solvent, water and the unreacted matter, and the separated powdery solid matter is mixed with water to elute the alkali metal halide and then dried to thereby recover polyarylene sulfide.
This process suffers, however, from the following problems. Organic polar solvents which are employed for reaction are generally expensive and must be evaporated substantially completely in the course of recovery through evaporation from the viewpoint of disposal of waste water. Since such solvents have high boiling points, heating must be carried out at high temperatures. Polyarylene sulfide which is recovered by such a prior art process has a relatively high content of electrolytic ions because electrolytic ions remaining in the polymer are not readily eluted by washing with water, a process which is carried out in the subsequent step. The amount of residual electrolytic ions is demanded to be reduced in specific uses, for example, when polyarylene sulfide is used as a sealing material for ICs.
According to another conventional process, the recovery is carried out under a vacuum in order to lower the heating temperature. However, since it is necessary to produce a high degree of vacuum, the cost of the evacuation equipment is high and the running cost is increased, and if the degree of vacuum is lowered, the recovery time is increased, and this leads to lowering of productivity. Thus, this process is not economical. Although the relationship between the recovery temperature and the difficulty of eluting electrolytic ions by means of water has not yet been clarified, it may be considered that, when polyarylene sulfide is exposed to high temperatures, minute irregularities on the surface of particles of polyarylene sulfide powder are smoothed and this leads to a reduction in the surface area and, at the same time, causes metal ions to be wrapped and held in the polymer.
U.S. Pat. No. 3,687,907 discloses a process wherein the reaction product is mixed with water which is a bad solvent for polyphenylene sulfide and the temperature of the mixture is lowered to prepare a polyphenylene sulfide slurry which is then filtered to recover polyphenylene sulfide. According to this process, chloroform or the like is added to the filtrate consisting of water having an alkali metal halide dissolved therein and an organic polar solvent to cause extractive distillation of the polar solvent, thereby recovering the solvent.
Although it is considered to be easy to extract electrolytic ions remaining in the polyphenylene sulfide with water through this process, as compared to the previous process, there is an increase in the cost of recovering the organic polar solvent remaining in the extraction residue consisting mainly of water and an alkali metal halide. More specifically if it is intended to recover the residual organic polar solvent by distillation, it is necessary to evaporate a large amount of water which has a lower boiling point and a greater latent heat in evaporation than those of the solvent, whereas, if it is intended to recover the residual organic polar solvent by extraction with chloroform or the like, it costs a great deal to install equipment for extracting the organic polar solvent from a liquid of low concentration, to run the equipment, and to conduct separation of the materials for reuse.