Process for the isolation of polyarylene sulphide from salts contained therein

The invention relates to a process for the isolation of polyarylene sulphides, polyarylene sulphides with particularly low electrolyte content being obtained.

The invention relates to a process for the isolation of polyarylene 
sulphides, polyarylene sulphides with particularly low electrolyte content 
being obtained. 
Polyarylene sulphides, their preparation and isolation are known (e.g. U.S. 
Pat. No. 2,513,188). 
In general, to isolate polyarylene sulphides, the heterogeneous product 
solution containing salts is precipitated after the reaction. The 
inorganic salts are washed out of the precipitated polyarylene sulphide 
with water. Subsequently, the polyarylene sulphide, containing up to 500% 
by weight (referred to polymer) of water and/or other extraction media, is 
dried at elevated temperature, if appropriate in vacuum. Nevertheless, 
residual amounts of .gtoreq.1000 ppm of chloride can be detected in the 
dried polyarylene sulphide. 
It has now been found that polyarylene sulphides with very low residual 
electrolyte content and greatly improved electric properties are obtained 
if the produced common salt, unreacted sulphide and, if applicable, 
further reaction-promoting salts which are present in the solid phase in 
the reaction solution are separated off before precipitation of the 
reaction solution by processes which are known in principle, at 
temperatures above the solidification point of the solution. 
The polyarylene sulphides, which can also be branched, can be prepared, for 
example, by reaction of dihaloaromatics, if appropriate with addition of 
tri- and tetrahaloaromatics and alkali metal sulphides in organic solvents 
according to known processes, if appropriate under excess pressure. 
According to the invention, the reaction solution is filtered through a 
heated pressure filter after the polymerization reaction. As filter media, 
wire sieves or sinter plates which are stable under these conditions, are 
used preferably, the mesh sizes or pore sizes of which can be adjusted 
over a wide range and which depend on the selected filter conditions, e.g. 
viscosity of the solution, filter pressure, temperature, on the desired 
degree of purity of the filtrate, etc. The technical equipment used for 
the filtration is known, for example simple pressure filters, agitated 
pressure filters, trailing blade centrifuges and rotary filters can be 
used, amongst others. 
The invention therefore relates to a process for working up of a reaction 
solution as results after the polymerization during the preparation of 
polyphenylene sulphide, characterized in that the reaction solution after 
completed reaction is filtered, if appropriate under pressure, at 
temperatures &gt;180.degree. C., preferably &gt;210.degree. C., through a heated 
pressure filter comprising wire sieves or sinter plates the pore sizes of 
which are adjustable in the range from 0.5 to 500, preferably 10 to 200 
.mu.m. 
A preferred embodiment uses the same filter which is already covered with 
common salt, the salt-filter layer being reduced from above, before the 
second filtration step, down to a residual layer of 0.5-1 cm according to 
an advantageous embodiment. The common salt can be extracted once or 
several times with hot organic solvent, preferably the solvent of the 
polyarylene sulphide reaction, the polymer content in the common salt 
dropping to values under 0.05%. The process according to the invention 
permits more than 90%, preferably more than 98%, of the salts contained in 
the reaction solution to be separated off. 
A further advantageous procedure, especially suitable for continuous 
operation, is to feed the reaction solution to a heated decanter 
centrifuge before the actual filtration. The salts can be separated off, 
washed and dehumidified in the decanter so that a relatively low-salt 
product solution can be fed to the subsequent filtration step. A preferred 
embodiment is the remixing of the separated salts with the solvent used 
for washing and a second solid-liquid separation in the decanter 
centrifuge, the solvent being fed back to the first separation step or to 
the reaction. 
The common salt obtained can be further processed in the normal way. 
The reaction solution, freed from suspended salts, according to the 
invention, is worked up further in a known fashion. For example, it can be 
introduced into a precipitation medium such as water or a water-containing 
medium and the polyarylene sulphide thus precipitated and further 
purified. 
The precipitate obtained can be a coarse or fine powder or fibrous or 
flaky, or, if it proves to be advantageous, can be converted to a 
corresponding different form before the further processing. It is a 
particular advantage of the process that the water-soluble contaminants 
are present in the separated-off common salt, so that washing the 
separated-off polyarylene sulphide can be dispensed with, depending on the 
filtration quality. If appropriate, the precipitate is extracted with 
organic extraction media to remove the organic solvent and to ensure 
gentle drying. 
The products treated by the process according to the invention are 
distinguished by low mould corrosion and particularly good electric 
properties. They can be mixed with other polymers, pigments and fillers, 
for example graphite, metal powder, glass powder, quartz powder or glass 
fibres, or mixed with the usual additives for polyarylene sulphides, for 
example the usual stabilizers and mould release agents. They can be 
processed directly to foils, moulded articles or fibres by extrusion, 
extrusion blowing, injection moulding or other usual processing 
techniques. These products can be used in the normal way, e.g. as car 
parts, fittings, electrical parts, e.g. switches, printed circuit boards 
and electronic panels, chemical-resistant and weather-proof parts such as 
pump housings and pump impellers, caustic bath dishes, sealing rings, 
parts for office machinery and telecommunication equipment, as well as 
household appliances, valves or ball bearing parts. 
Preparation of a polyphenyl sulphide 
1,290 g of sodium sulphide trihydrate (=10.0 moles of Na.sub.2 S), 204.1 g 
of sodium acetate trihydrate (15 mole %, referred to Na.sub.2 S) and 4000 
ml of N-methylcaprolactam are placed in a mixing vessel fitted with a 
stirrer and distillation attachment and heated to 200.degree. C. A liquid 
which essentially comprises water distils off. 
1,470 g of 1,4-dichlorobenzene, dissolved in 500 ml of N-methylcaprolactam, 
is then added and the reaction mixture heated to 240.degree. C. and 
maintained at this temperature for 5 hours.

EXAMPLE 1 
The contents of the mixing vessel are placed in a pressure filter, heated 
to 220.degree. C., which is fitted with a wire mesh of mesh size 40 
.delta.m. The batch is filtered, with a nitrogen pressure of 4 bar, 
through the common salt which is depositing on the wire mesh into a 
receiver, which is heated to 220.degree. C. The NaCl content in the 
filtered solution is under 0.2%. The reaction mixture is then transferred, 
with vigorous stirring, to 30 l of initially introduced water which has 
been adjusted to pH 1 with phosphoric acid. The precipitated polyphenyl 
sulphide is then washed with water until the washings have a conductivity 
of .ltoreq.20 .mu.S. 
After drying the polyphenylene sulphide in vacuum at 140.degree. C. for 8 
hours, the chloride content of the polymer is below 1 ppm. The product is 
pressed to a 2-mm-thick, rectangular plate with dimensions 30.times.30 mm; 
the electrical breakdown voltage is &lt;100 kV/mm at 20.degree. C. 
A comparison batch the salt of which was not separated off before 
precipitation is washed in the same way and measured. The electrical 
breakdown voltage of a 2-mm-thick rectangular plate with dimensions 
30.times.30 mm is 39 kV/mm. 
EXAMPLE 2 
The 60 fold batch of a polyphenylene sulphide is prepared as described 
above in a 600 l capacity stirred kettle. The NaCl-containing suspension 
is pressed, via the central suspension feed, at a speed of 40 l/minute 
onto a decanter, heated to 220.degree. C., the drum of which rotates at a 
speed of 2000 revolutions per minute. The deposited salt is moved in the 
direction of the smaller diameter by means of a worm driven at a speed 
differing from that of the drum by 14 revolutions per minute, and is 
thrown off there. The clarified liquid runs over to the other end of the 
drum into a pressure filter, heated to 220.degree. C., which is fitted 
with a sinter plate with pore size 20 .mu.m. The batch is filtered with a 
nitrogen pressure of 5 bar into a receiver which is heated to 220.degree. 
C. The NaCl content in the filtered solution is under 0.1%. The filtered 
solution is then transferred, with vigorous stirring, into 3000 l of water 
which has been adjusted to pH 1 with hydrochloric acid. The precipitated 
polyphenyl sulphide is then washed with distilled water until the washings 
have a conductivity .gtoreq.20 .mu.S. 
After drying the polyphenyl sulphide at 140.degree. C. for 8 hours, the 
chloride content of the polymer is less than 1 ppm. An injection-moulded 
test piece with dimensions 120.times.15.times.4 mm had a creep resistance 
according to DIN 53480 of 150 and an ash content of 0.02%. 
A comparison batch the salt of which was not separated off before 
precipitation was washed, dried and measured in the same way. The creep 
resistance according to DIN 53480 amounted to 50, the ash content 0.2%.