CA1055192A

CA1055192A - Process for preparing oxymethylene polymers in a granular form

Info

Publication number: CA1055192A
Application number: CA246,680A
Authority: CA
Inventors: Gunter Sextro; Karlheinz Burg; Alwin Heller; Helmut Schlaf; Hans J. Leugering
Original assignee: Hoechst AG
Current assignee: Hoechst AG
Priority date: 1975-02-28
Filing date: 1976-02-27
Publication date: 1979-05-22
Also published as: IE42490B1; JPS5850615B2; DK85376A; DE2508886A1; FR2302316A1; LU74437A1; GB1536964A; NL179390B; IE42490L; BE839074A; JPS51109946A; NL179390C; DE2508886B2; FR2302316B1; DE2508886C3; IT1055918B; NL7601810A

Abstract

PROCESS FOR PREPARING OXYMETHYLENE POLYMERS IN A
GRANULAR FORM
Abstract of the disclosure:
Oxymethylene polymers are obtained in a granular form by cooling a solution or dispersion of an oxymethylene polymer, the temperature of which solution or dispersion is above the sintering temperature of the oxymethylene polymer, to a tem-perature just below the sintering temperature of the oxymethy-lene polymer. As dissolving or dispersing agent as well as precipitating and cooling agent there is used a mixture of methanol and water having a methanol content of at least 75%
by weight. The oxymethylene polymer obtained is suitable as engineering plastic for preparing moulded articles.

Description

H ~
. ~05S~92 The preparation of oxymethylene polymers ~POM) by copoly-mer~zation of formaldehyde or cyclic oligomers of formalde-hyde, especially 1,3,5-trioxane, with suitable comonomers, especially cyclic ethers or cyclic acetal~, i5 kno~m (cf US
Patents No~. 3,027,352 and 3,803,094). It is further known that granular oxymethylene polymers are obtained by intro-ducing a solution of oxymethylene polymers ~nto a precipitat-ing agent having a temperature ~ust below the sintering tem-perature of the oxymethylene polymer (cf. US Patent No.
3,371,066).
The present invention provides a process for preparing a granular oxymethylene polymer containing besides oxymethy-lene units from 0,1 to 20~ by weight of oxyalkylene units having from 2 to 8 ad~acent carbon atoms in the mai~ chain, by introducing a ~olution or a fine dispersion of the oxy-methylene polymer into a liquld coollng agent being k~pt in turbulent motion at a temperature of from 1 to 10C below the ~intering temperature of the oxymethylene polymer, which comprise~
a) precipitating the oxymethylene polymer by lntroducing a 3 to 35% by weight solution or dispersion of the oxymethylene polymer in a methanol/water mixture having a methanol con-tent of at least 75% by weight, the temperature of which solution or dispersion being from 5 to 65 above the sin-tering temperature of the oxymethylene polymer, into a methanol/wa~er mixture acting ~8 a cool~ng agent and ha~-~ng a methanol content of at lea~t 75% ~y we~ght, the quantlty o~ the precipltated oxy~ethylene polymer in the 29 ~u~penslon ~ormed belng at mo~t 25% by weiKht, and ~ 2 ~

lOSS1~3~
b) separating the granular oxymethylene polymer o~talned and drying it. The invention also pro~ides granular oxymeth~-lene polymers prepared by the aforesaid process Under oxymethylene polymers according to the invention there are to be understood poly(oxymethylenes) containing in the main valence chain besides oxymethylene uni~s from 0.1 to 20, preferably from 0.5 to 10% by weight of oxyalkylene units having from 2 to 8, preferablg 2, 3 or 4 adjacent car-bon atoms; oxymethylene polymers having a portion of from 1 to 5% by weight of oxyalkylene units are especially suitable.
The terminal groups of the oxymethylene polymers are prefer-ably primary alcohol groups.
The oxymethylene polymers are prepared in kno~m manner by polymerization of the monomers in buIk, suspension or solu-tion in the presence of cationicallly active catalysts, for example at a temperature of from 0 to 100C, preferably of from 50 to 90C (cf. US Patent No, 3,027,352). The cationi-cally active catalysts which can be used are:
1) protonic acids, for example perchloric acld,

2) esters of protonic acids, especially esters of perchlor~c - acid with aliphatic alcohols of low molecular weight, for example perchloric acid tertiary butyl ester, ~) anhydrides of protonic acids, especially mixed anhydrides of perchloric acid and an aliphatic carboxylic acid of low molecular welght, for example acetyl perchlorate, 4) ~ewis acids, especially halides of boron, tln, titanium, pho~phoru~, arsenic and antlmony, for example boron tri-fluorld~, tln tetrachlorlde~titan~um tetrachloride~ phos-29 phoru~ pentachlorlde, phosphorus pentafluor~de, ar3enic

- 3 -, HOE 75/F ~58 10551~'~
pentaflu~ride and antimony pentafluoride, and 5) complex compounds or salt llke compounds, preferably ether-ates or onium salts, of ~ewis acids, for example boron tri-fluoride diethyletherate, boron trifluor~de-dl-n-butyl-e~herste, triethyloxonium tetra nuoroborate, ~rimetnyloxo-nium hexafluoropho~phate, triphenylmethyl hexafluoroarse-nate, acetyltetrafluoroborate, acetylhexafluorophosphate and acetylhexafluoroaresenate.
The quantity of the catalysts used in the copolymeri-zation depends primsriiy on their efficiency and is generally from 0.1 to 2,000, preferably from 0.2 to 500 ppm, calculated on the total ~uantity of the compounds to be polymerized. Very efficient catalyst~ such as boron trifluoride are advantage-ou~ly used in an amount o~ from 10 to 150,preferably o~ from 20 to 100 ppm, calculated on the total quantity of the com-pounds to be polymerized. The corre~ponding molar quantltles sh ~ dbe used for complex compounds or ~alt-like compounds.
Highly active catalysts such as perchloric acid are used in an a~ount of from 0,2 to 10, preferably of ~rom 0,3 to 5 ppm, It i~ generally ad~i~able to use the catalysts in a di-luted form. Gaseous catalysts are diluted with an inert g~s, ~or example, nitrogen and noble gases ~uch as argon, whereas liquld or solid catalysts are dissolved ln an lnert solvent.
Suitable solvents are especially allphatic or cycloaliphatic hydrocarbons as well as nitrated aliphatlc or aromatlc hydro-carbons, for example, cyclohexane, methylene chloride, ethy-lene chloride, nitromethane and nitrobenzene. The weight ratio o~ ¢a~aly~t to diluent i~ u~ually from 1:5 to 1:10,000, pre-29 ferably from 1:10 to 1:100 Yery strongly acting cataly~ts

- 4 -", , , ,, ~, , ','''',' , ', , ' ,, , , . . . . . .. . . . .
", ", , , , ~ ,. . .

, 10551~2 are advantageously diluted in a proportion of from 1:5,000 to 1:20,000.
The polymerization is preferably effected in an inert gag atmosphere with the exclusion of moisture; suitable inert gas-es are preferably noble gases such as argon, and nitrogen.
Suitable compounds copolymerizable with trioxane especially include a) cyclic ethers 3, 4 or 5 ring members, preferably epoxides, b) cyclic acetals, preferably formals, having from 5 to 11, preferably 5, 6, 7 or 8 ring members, and c) linear polyacetals, preferably polyformals.
Suitable comonomers for trioxane are especially compounds of the formula CH2-[CR H]x-[O(CR H)z~y~0 wherein A) Rl and R2 are identical or different and represent each a hydrogen atom, an aliphatic alkyl radical having from 1 to 6, preferably 1, 2, 3 or 4 carbon atom~, or a phenyl radi-cal, and a) x is l, 2 or 3 and ~ i~ zero, or b) x i~ zero, y is 1, 2 or 3 and z i8 2, or c) x ls zero, ~ i8 1 and z i~ 3, 4, 5 or 6, or B) R is an alkoxymethyl radical having from 2 to 6, preferably 2, 3 or 4 carbon atom~, or a phenoxymethyl radical, x being 1 and ~ be~ng zero.
Bu$table cyclic ether~ ~nclude, for example, ethylene ox$de, propylene oxiae, ~tyrene oxide, cyclohexene oxide, oxa-cyclobutane and phenylglycldyl ether, and ~ultable cycl~c for-

- 5 -, ' ' ,' '' ' , HOE 75/F 0~
1(~551~Z
mals are, for example 1,3-dioxolane, 1,3-dioxane, 1,3-d~oxe-pane and 1,3,6-trioxocane as well as 4-methyl-1,3-dioxolane, 4-phenyl-1,3-dioxolane, 1,3-d~oxonane and 1,3-d~oxacyclohep-tene-~5) Suitable polyfo~mals include especially poly~1,3-dioxolanej and poly~ -dioxepane).
For preparing oxymethylene polymers having a molecul~r weight in a specific range the polymerization is advant~ge-ously carried out in the presence of a regulator. For this purpose there may be used especially formaldehyde dialkyl-acetals having from 3 to 9, preferably 3, 4 or 5 carbon atoms, for example formaldehyde dimethylacetal, diethylacetal, di-propylacetal and dibutylacetal, as well as aliphatic alcohols of low molecular weight, preferably alkanols having ~rom 1 to 4 carbon atoms, for example methanol, ethar~ol, propa~ol and butanol. The regulator is generally used in an amount of up to 0.5% by weight, preferably of from 0.005 to 0.1~ by weight, calculated on the total quantity of the compounds to be polymerized.
In order to remove lnstable portions the oxymethylene polymers are sub~ected advantageously to a controlled thermal partial degradation yielding primary terminal alcohol group~
(¢f. US Patents Nos. 3,174,948j 3,Z19,623 and 3,666,714). The thermal treatment i8 performed at a temperature of from 130 to 200C, pre~erably of from 140 to 190C, especially under non-acid conditions in aqueous/methanolic solution, advantage-ously in the presence o~ a.comp0und ~howlng a basic reaction, ~or example a tertiary aliphatlc amine of low molecular weight, such afi trlethyl amine or triethanol amlne~ or a secondary 29 al~ m~tal pho~phate such as dl~odlum hydro~en pho~hate.
, - 6 -,~ ' ,~ " , . .

, '; ' , l~SS~

A temperature of from 150 to 180C is especially advantageou~, The length of time of the thermal treatment depending on the temperature goes from 10 seconds to 2 hours, preferably from 1 minute to 60 minutes. The higher the temperature, the shorter the residence time. At a temperature of 180C about 1 to 2 minutes are sufficient, about 5 to 10 minutes at a temperature of 160C, about lO to 30 minutes at a temperature of 150C
and about 20 to 60 minutes at a temperature of 140C. The treatment is preferably carried out under substantial exclusion of oxygen.
As starting material for the process according to the in-vention there i~ used a solution or fine dispersion of an oxy-methylene polymer containing from 3 to 35, preferably from 5 to 30% by weight of polymer. A polymer solution or dispersion containing from 10 to 25% by weight of oxymethylene polymer give~ especially good re~ults.
As solvent or dispersing agent there is used a methanol/
water mixture having a methanol content of at least 75~ by weight; a mixture consisting of from 99.9 to 80% by weight of methanol and of from 0.1 to 20% by weight of water is used preferably. A mixture consisting of from 99.5 to 85% by weight of methanol and of from 0.5 to 15~ by weight of water is espe-cially advantageous, and the best results are obtained when u~ing a mixture consisting of from 99 to 90~ by weight of metha-nol and of from 1 to 10~ by weight of water.
The temperature o the solution or dispersion is 5 to 65,preferably 10 to 60~C above the sintering temperature of the oxymethylene polymer, a temperature range o~ from 25 to 55C
above ~he ~inter~ng temperature being e~peclally advantageou~.

l~Sl~
As cooling agent and prec~pitatlng agent there is also used a mixture of methanol a~d water, the proportion of the components of which being in the same range a~ the afores~id sol~ent or dispersing agent. The cooling agent has a tempera-ture of from 1 to 10, preferably o~ from 1 to 5, espec$allyof from 2 to 4C belcw the sintering temperature of the oxy-methylene polymer. The quantity of the precipitated oxymethy-lene polymer in the suspension formed by introduc~ng the poly-mer solution or dispersion ~5 at most 25, preferably in th~
range of from 5 to 15% by weight.
The methanol used within the scope of the present in~en-tion may contain up to ~0, preferably up to 10~ by weight of organic impurities soluble in me~hanol, usually formed as by-products in the synthesis of oxymethylene polymers, for ex-~mple formaldehyde, cyclic oligomers of formaldehyde, methyl-lal, glycol formal, glycol monomethgl ether, glycol dimethyl ether a~ well a~ aliphatlc alcohols of low molecular weight, al~phatic esters of low molecular weight and acetone.
The sintering temperature (Ts) ~s the temperature ~t which the solid polymer particl~s suspended in the methanol/
water mixtures ~oXten at the surface and stick together, w~th-out completely melting. It depends on the composition and the molecular weight of the polymer as well as on the nature of the solvent or di~persing agent. The sintering temperature 25 of the oxymethylene polymers used according to the in~ention is ~n the range of from 100 to 1~0, e~pecially of from 125 to 1 35C .
The proce~3 Gf the invention may be carried out, for ex-29 ample, ~n an autoclave pro~ided with a ~tirrer The cooling HOE 75/F 0~
1055~
, and precipitating agent are gi~en into this auto~la~e and the solution or di~persion of the oxymethylene pol~mer is intro-duced advantageously through a heated immersion pipe or a nozzle into the precipitating agent, the latter being kept in turbulent motion by ~tirring. The process m~y be carried out discontinuously or continuously; in the continuous method the polymer suspension obtained is withdrawn at the bottom of the autoclave at the same rate at which the solution or dispersion as well as an optional add~tional precipitating agent are added. The average residence time of the oxymethy-lene polymer in the precipitationvessel~is in the range of from 1 minute bo 12 hours, preferably of from 2 to 120 minutes.
The pressure is in the range of from 5 to 40, preferably of - from 8 to 30 bars depending on the temperature used in each case.
The separating of the granular oxymethylene polymers ob-- tained by precipitation or agglomeration is performed by the usual methods, for example by flltration, decantation or cen-trifugation. The separated oxymethylene polymer finallg ls dried at a temperature of from 20 to 1~5C, preferably of from 50 to 120C. Drying under an inert gas atmosphere, for example, under a noble gas or nitrogen a~mosphere is ad~isable.
The oxymethylene polymers obtained according to the in-vention are ~acromolecular: their reduced specific viscosity (RS~ n the range of from 0.3 to 2.0, preferably of from 0.5 to 1.5 dl/g (measured with a 0.5% by weight solution of the polymer ln ~-butyrolactone, contalnlng 2% by weight of dlphenylam~ne as ~tabllizer, at a temperature of 140 C). The 29 ¢ry~tallite meltlng polnts of the oxymethylene polymer3 are _ g _ , ~0~ 75/F 05~
,.......................................... __.
1~551~32 in the range of from 140 to 180C, their melt indlce~ M~I 150/2 are in the range of from 0.1 to 50, preferably of from 1 to 30 g/10 minutes (measured according to DIl1 = ~erman ~ndu~tr~al Standard No. 53,735 at a temperature of 190C under a loaa of 2.16 kg).
The process according to the invention enables preparing granular oxymethylene polymers having an increased conten~
of dry matter(after precipitation) and an increased apparen~
density (a~ter drying). The apparent de~sity of the gra~ular product is generally at least 300 g/l, preferably of from 350 to 550 g/l (measured with the polymer dried at a temperature of 70C under nitrogen after having been mixed homogeneou~ly for 2 minutes in a laboratory fluid mixer at a speed of 3,000 revolutions per minute).
The oxymethylene polymers obtained according to the in-vention may be stabilized additionally again~t the ac~on of heat, oxygen and/or l~ght by mlxing them homogeneously with stabilizers. Homogenizatlon i~ usually carried out in a com-mercial mixing device, for example, an extruder, at a tempera-ture abo~e the melting point of the polymer up to 250C, pre-ferably of from 180 to 210C. The total amou~t o~ the added stabilizers is in the range of from 0~1 to 10, preferably of from 0 5 to 5% by weight, calculated on the total mixture.
As stabilizers there are especially useful bi~phenol com-poun~, alkaline earth metal salt~ or carboxylic acids as wellas guanidine compound~ ~he blsphenol compound3 used are pre-ferably ~ters of monoba~lc 4-hydroxyph~nylalkanolc aci~s whlch are ~ub~itute~ ln the rlucleus once or t~/ice by an alkyl 29 ra~ical h~vlrl~ from 1 to 4 carbon atom~ and whlci~ have from 7 , '", HOE 7~1F~
10551~
to 13, preferably 7, 8 or 9 carbon atoms, with aliphatic di-~
tri- or tetrahydric alcohols having from 2 to 6, prefera~ly 2, 3 or 4 carbon atoms. Examples are esters of ~ tertiary butyl-4-hyclroxyphenyl)-pentanoic acid, B-(3-methyl-5-tertiary ~utyi-4-nyar~xy~rlerlyi)-propionic acid, ~3,5-di-tertiary ~u~yi-4-hydrophenyl) acetic acid, B-(3,5-di-tertiary butyl-4-hydrGxy-phenyl)-propionic acid or (3,5-di-isopropyl-4-hydro~yphenyl)-acetic acid with ethylene glycol, propane diol-(1J2), propane diol-(1,3), butane diol-(1,4), hexane diol-(1,6), 1,1,1-tri-methylol ethane or pentaerythritol.
Suitable alkal~ne earth metA1 salts of carboxylic ac~ds are especially alkaline earth metal salts of aliphatic, pre-ferably hydroxyl groups containing mono-, bi- or tribasic car-boxylic acids having from 2 to 20, preferably from ~ to 9 car-bon atoms, ~or example the calclum or magne~ium ~al~s or ~tea-ric acid, ricinoleic acid, lactic acid, mandelic acid, malic acid or citric acid.
Sui'cable guanidine compound~ are compound~ of the formula NC - NH - C - NH - R
~0 ~1 NH

wherein R is a hydrogen atom, a cyano ~roup or an alkyl radi-cal having from 1 to 6 carbon atoms, for example cyanoguani-dlne, N-cyano-N'-methyl~guanidine, N-cyano-N'-ethyl-gvanidine, N-cyano-l~'-isopropyl~uanidine, N-cyano-N'-tertiary butylguani-dine or N,~'-d~cyanoguanidine. The guanidine compound i~
u~ed optlon~lly in an amount of from 0.01 to 1, preforably of from 0.1 ~o 0,5% by welght, calculated on the total ~lxture 29 In addi~lon there m~y ~e ~ddetl to the oxymethylene poly-_ HOE 75/F 05~
1(~551~2 , mer prepared according to the invention Xnown llght stabilizers, for example derivatives of benzophenone, acetop~enone or tri-azine. Further usual additives, for example dyestuffs, plg-ments, reinforcing materials and fillers may al~o be used.
The oxymethylene polymers may be processed by all methods usually empl~yed for thermoplastics, for example by injection moulding, extrusion, blowing of extruded material, melt spin-ning and deep drawing. They are suitable for preparing semi-finished products and finished products such as shaped artic-les, for example bars, rods, plates, ribbons, bristles, threads, fibers, films, sheéts, tubes and flexible tubes, as well as household articles, for example dishes and cups, and machine - elements such as casings and gear wheel~ They are especially suitable as engineering plastic for preparing dimensionally stable and true to shape articles.
The following examples illu~trate the invention:
E X A M P ~ E 1:
. . _ .
3 kg of a copoly~er of 98% by weight of trioxane and 2%
by weight of e~hylene oxide having a RSV value of 0.65 dl/g, a crystallite melting point of 165C and a MFI value of 25 g/
10 minutes were mixed in an autoclave with 30 ~g of a mixture of 85~ b~ weight of methanol, 14.9% by weight of water and 0.1%
by weight o~ triethylamine. The su~pension formed was heated under nitrogen for 5 minutes to 160C, whereby the polymer dis~olved. 2 kg per minute of the ~olution obtained were pas- !
~ed continuously through an $mmersion pipe into 10 kg of a mlxture of 85% by welght of methanol and 15% by we$ght of water contalned in a further autocl~e and belng kept at a tempera-29 ture of 125C ~n turbulent motlon under nitrogen. ( At 8 s$nter-l~S1~3~
-ing temperature of the polymer of 127C the ~emperature of the precipitating agent consequently was 2C ~elow the said temperature). During the solution and preoip~tation process the pressure in the autocla~e was 17 and 9 bar~ respecti~ely.
The precipitated granular polymer was withdrawn at tne w ~om of the autoclave 7 minutes after having termina~ed the ~ntro-duction and separated by suction from the solvent and prec-ipi-tating agent; the solid product had a content of dry matter of about 40%. After drying and homogeneous mixing the poly-mer had an apparent density of 350 g/l,E X A M P ~ E 2:
Example 1 was repeated, but by using as solvent and a~
precipitating agent each time a mixture of 90~ by weight of methanol, 9.g~ by weight of water and 0,1% by weight of tri-ethylamine, The temperature durin~ the solution proce~s was170C, The granular polymer had a content of dry matter of about 38% after ~uction of the ~ol~ent and precipitating agent and an apparent density of 380 g/l after drying and homogene-ou~ly mixing, E X A M P ~ E 3:
7,5 kg of a copolymer of 98% by weight of trioxane and 2% by weight of ethylene oxide having a RSV value of 0.65 dl/g, a crystallite melting point of 165C and a MFI value of 25 g/
10 minutes were mixed in an autocla~e with 22,5 kg of a mix-Z5 ture of 96,5% by weight of methanol, 3,4% by weight of w~terand 0,1% by weight triethylene, The suspen~lon for~ed was heated under nitrogen for 5 minutes to 1~0C, whereby the polymer d~olved, The solution obtalned wa~ introduc~d conti-29 nuou~ly over a perlod of 30 mlnutes, through an lmmersion p~pe, ~IOF~ 75/F ~5~
1~5519Z

into a mixture of 96.5% by weight Gf methanol and ~,5~ by weight of water contained in a further autoclave and being kept at a temperature of 131C in turbulent motion under' n~ro-gen. (At a sintering temperature of the polymer of 1~3C the temperature of the precipitating agent was consequently 2C
below the said temperature). The pressure in the autoclave dur~ng the solution and precipitation process was ~0 and 10 bars respecti~ely. 15 minutes after having terminated the introduction the precipitated granular polymer was withdra-,m at the bottom of the autoclave and separated by suction from the solvent and precipitating agent; the dry matter content of the solid product was about 45%. After drying and homo-geneously mixing the dry polymer had an apparent density of 450 g/l.
C~M~ARA$I~
3 kg of a copolymer of 9~ by weight of trloxane and 2%
by weight of ethylene oxide having a RSV value of 0.65 dl/g, a cry3tallite meltlng point of 165C and a MFI value of 25 g/
10 minutes ~ere mixed with 30 kg of a mixture of 6'~% by weight of methanol, 39.9~ by weight of water and 0.1% by weight of triethylamine. The ~u~pension obtained was heated for 5 mlnu-tes to 160C, whereby the polymer dissolved. The solution formed thereby was cooled to a temperature of 135C. 2 kg per minute of the solution were introduced continuously through an immer~ion pipe into 10 kg of a mixture of 60~ by weight of methanol and 40~ by welght of water contairled in a further autoclave and beine kept at a temperature of 125C $n turbulent, motion under nitro~en.

` HOh 75lF t,~g -' lOS5~2 (At a sinteri~g temperature of the polymer of 127C the temperature of the precipitating agent was consequently 2~
below the said temperature.) 7 minutes after having terminat-ed the introduction the precipitated granular polymer was wi~
drawn at the bottom of the autoclave and separate~ by ~uc~on from the solvent and precipitating sgent. The conte~t of dry matter of the solid product was about 33~. After drying and homogeneous mixing the apparent density of the dry product was 270 g/l.

.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the preparation of granular oxymethy-lene polymer, the polymer comprising oxymethylene units and from 0.1 to 20% by weight of oxyalkylene units having from 2 to 8 adjacent carbon atoms in the main chain, in which (a) a solution or fine dispersion of the polymer in a methanol/
water mixture, having a methanol content of at least 75%, is formed and is maintained at a temperature of from 5 to 65°C above the sintering temperature of the polymer, (b) the solution or dispersion is introduced into a liquid cooling agent which is a mixture of methanol and water having a methanol content at least 75% by weight, the cool-ing agent being maintained in turbulent motion and at a temperature of from 1 to 10°C below the sintering tempera-ture of the polymer whereby a precipitate of the polymer is obtained in suspension, the suspension containing at most 25% of polymer, and (c) the precipitated polymer is separated in granular form and is dried.

2. A process as claimed in claim 1 in which the methanol/
water mixtures in both (a) and (b) are mixtures of from 99.9 to 80% by weight of methanol and from 0.1 to 20% by weight of water.

3. A process as claimed in claim 1 in which the methanol/
water mixture in (a) also contains a compound which shows a basic reaction.

4. A process as claimed in claim 1, claim 2 or claim 3 in which the solution or dispersion of polymer in (a) con-tains from 10 to 25% by weight of polymer.

5. A process as claimed in claim 1, claim 2 or claim 3 in which the solution or dispersion in (a) has a temperature of from 25 to 55°C above the sintering temperature of the poly-mer.

6. A process as claimed in claim 1, claim 2 or claim 3 in which the cooling agent in (b) has a temperature of from 2 to 4°C below the sintering temperature of the polymer.

7. A process as claimed in claim 1, claim 2 or claim 3 in which the polymer in granular form is dried under an inert gas atmosphere at a temperature of from 50 to 120°C.

8. A process as claimed in claim 1, claim 2 or claim 3 in which the polymer contains from 1 to 5% by weight of oxy-alkylene units having 2, 3 or 4 adjacent carbon atoms.

9. A process as claimed in claim 1, claim 2 or claim 3 in which the polymer contains from 1 to 5% by weight of ethy-lene oxide units.