Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...
Reexamination Certificate
1999-11-29
2001-03-06
Boykin, Terressa M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Treating polymer containing material or treating a solid...
Reexamination Certificate
active
06197924
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to polyethersulfones, and more particularly to an improved method for preparing dihydroxybiphenyl- and dihydroxybenzene-derived polyethersulfones.
Dihydroxybiphenyl-derived polyethersulfones are known in the art. A common example is the polyethersulfone derived from 4,4′-dihydroxybiphenyl and bis(4-chlorophenyl)sulfone (hereinafter sometimes “DDS”), which comprises structural units of the formula I:
The polyethersulfone of formula I is typically an amorphous, injection moldable resin with high solvent resistance and other desirable properties. It is most often prepared by a reaction of the DDS with an alkali metal salt of the dihydroxybiphenyl (hereinafter sometimes “biphenyl salt”) at high temperature (typically at least 140° C.) in solution in a dipolar aprotic solvent, as illustrated by dimethyl sulfoxide and sulfolane, which may be combined with chlorobenzene, as described, for example, in Johnson et al.,
J. Poly. Sci., Part A-
1,5, 2375-2398 (1967). When preparation is complete, it is thus necessary to isolate the polymer by removal from the solvent, typically by precipitation with a rather large amount of anti-solvent such as chlorobenzene. This creates a mixture of organic liquids (solvent and anti-solvent) which, to make preparation economically feasible, must be separated for solvent recycle owing to the high cost of such compounds. Separation procedures are expensive and burdensome.
In an alternative method of polymer isolation, the solvent is removed by volatilization, e.g., by extrusion or film-trusion. It is preferred, however, to separate by-product alkali metal chloride salt from the polymer before volatilization is undertaken. Removal by filtration is usually impracticable because of the high viscosity of the solution and the small size of the salt crystals. Removal by dissolution in water is even less feasible, since the organic solvent is miscible with water. Thus, polymer preparation in this way is not commercially advantageous.
Another method of preparation, disclosed specifically or by analogy in U.S. Pat. Nos. 5,229,482 and 5,830,974, employs a less polar, water-immiscible solvent such as o-dichlorobenzene or anisole in combination with a phase transfer catalyst. Temperatures in the range of about 125-250° C. are utilized, making it advantageous to employ a phase transfer catalyst which is resistant to high temperatures, such as a hexaalkylguanidinium halide. An advantage of this method is that the by-product salt can be removed by extraction with water, since the solvent is water-immiscible. However, such extraction is often cumbersome and inefficient since there is generally poor phase separation after introduction of water.
In various solid state methods for the preparation of aromatic polycarbonates, as disclosed, for example, in U.S. Pat. Nos. 4,948,871, 5,204,377 and 5,717,056, the normally amorphous polymer is crystallized, often by heat treatment or by contact with a solvent or non-solvent. Such crystallization operations are, however, not known to be operative with polyethersulfones.
It is of interest, therefore, to develop improved methods of preparing and isolating polyethersulfones, particularly dihydroxybiphenyl-derived polyethersulfones and the like. In particular, a method is desired which enables isolation by operations which do not include steps requiring anti-solvent precipitation or separation of phases which separate poorly.
SUMMARY OF THE INVENTION
The present invention is based on the discovery that polyethersulfones can be prepared by any method known in the art and subsequently rendered crystalline. The crystalline polymer is of a particle size which renders it easily separable in the solid state from the reaction mixture in which it was prepared, along with by-product salt which can subsequently be easily removed by extraction with water.
The invention in one of its aspects, therefore, is a method for isolating a dihydroxybiphenyl- or dihydroxyphenyl-derived polyethersulfone which comprises:
(A) treating a solution thereof in an organic solvent to effect crystallization of said polyethersulfone;
(B) separating the resulting crystallized polyethersulfone from said solution; and
(C) removing by-product salts from said polyethersulfone.
Another aspect of the invention is crystalline dihydroxybiphenyl- or dihydroxyphenyl-derived polyethersulfones having an average particle size in the range of about 5-100 microns.
DETAILED DESCRIPTION PREFERRED EMBODIMENTS
The polyethersulfones which are the subject of the present invention are typically prepared from at least one monomer unit reagent comprising an alkali metal salt of a dihydroxybenzene such as resorcinol or hydroquinone or a dihydroxybiphenyl such as 3,3′- or 4,4′-dihydroxybiphenyl, with 4,4′-dihydroxybiphenyl being preferred. The salt is typically a sodium or potassium salt. Sodium salts are frequently preferred by reason of their availability and relatively low cost. The other reagent is at least one monomer unit comprising a bis(halophenyl)sulfone such as bis(3-bromophenyl)sulfone or DDS, the latter being preferred. Equimolar proportions of these two reagents are preferably employed. It is also contemplated to employ polyethersulfones containing minor proportions, generally up to about 25 mole percent, of structural units derived from other monomers.
The condensation reaction between these two reagents is conducted under strictly anhydrous conditions. It may be conducted in a dipolar aprotic solvent, in a water-immiscible solvent or in a mixture of the two. Preparation in a solvent system that includes a dipolar aprotic solvent is generally at a temperature in the range of about 150-300° C., as described hereinabove. For simplicity of operation, however, it is highly preferred to employ a solvent system consisting of at least one water-immiscible aromatic solvent such as chlorobenzene, o-dichlorobenzene, trichlorobenzene, chlorotoluene, anisole, phenetole, diphenyl ether or another aromatic compound having a polarity no higher than those, in combination with a phase transfer catalyst and at a temperature in the range of about 125-250° C.
Phase transfer catalysts with high thermal stability, i.e., those that are stable in the range of about 125-250° C., are highly preferred. Various types of phase transfer catalysts have this property. They include quaternary phosphonium salts of the type disclosed in U.S. Pat. No. 4,273,712, N-alkyl-4-dialkylaminopyridinium salts of the type disclosed in U.S. Pat. Nos. 4,460,778 and 4,595,760, and guanidinium salts of the type disclosed in the aforementioned U.S. Pat. No. 5,229,482. Said patents are incorporated by reference herein. The preferred phase transfer catalysts, by reason of their exceptional stability at high temperatures and their effectiveness to produce high molecular weight polyethersulfones in high yield, are the hexaalkylguanidinium and &agr;&ohgr;-bis(pentaalkylguanidinium)alkane salts. Proportions of catalyst, when employed, are most often in the range of about 1-10 mole percent based on sulfone.
In step A of the method of the invention, the solution of the polyethersulfone is treated to effect crystallization of said polyethersulfone. Such treatment may be by merely cooling a solution in a water-immiscible solvent, preferably the solution in which the polyethersulfone was prepared, from the reaction temperature to one in the range of about 20-100° C., and typically to ambient temperature. Slow cooling, over a period of at least about 2 hours, is often particularly effective for crystallization.
It is also within the scope of the invention to contact the solution before or during cooling with a crystallization aid. The preferred crystallization aid is preformed polyethersulfone in its crystalline form. It is also contemplated to employ inorganic nucleating agents such as titanium dioxide and talc as crystallization aids.
The average particle size of the crystalline polyethersulfone obtained as described above is typically in the range of about 5-500 m
Boykin Terressa M.
Brown S. Bruce
General Electric Company
Johnson Noreen C.
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