IE42490B1 - Process for preparing oxymethylene polymers in a granular form - Google Patents

Abstract

1536964 Oxymethylene copolymer granules HOECHST AG 26 Feb 1976 [28 Feb 1975] 07628/76 Heading C3L Oxymethylene copolymers containing 0À1- 20% by weight of C 2 -C 8 oxyalkylene units are prepared in granular form by adding a solution or dispersion of the copolymer in methanol/ water having a temperature 5-65‹ C. above the sintering temperature of the copolymer to a turbulent methanol/water mixture having a temperature 1-10‹ C. below said sintering temperature, thereby precipitating granular copolymer; and separating and drying the granular product.

Description

The preparation of oxymethylene polymers (POM) by copolymerization of formaldehyde or cyclic oligomers of formaldehyde, especially 1,3,5-trioxane, with suitable comonomers, especially cyclic ethers or cyclic acetals, is known (cf. US Patents Nos. 3,027,352 and 3,803,094). It is further known that granular oxymethylene polymers are obtained by introducing a solution of oxymethylene polymers into a precipitating agent having a temperature just below the sintering temperature 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 oxymethylene units from 0.1 to 20$ by weight of oxyalkylene units having from 2 to 8 adjacent carbon atoms In the main chain, by Introducing a solution or a fine dispersion of the oxymethylene polymer into a liquid cooling agent being kept in turbulent motion at a temperature of from 1 to 10°C below the sintering temperature of the oxymethylene polymer, which comprises a) precipitating the oxymethylene polymer by Introducing a 3 to 35$ by weight solution or dispersion of the oxymethylene polymer in a methanol/water mixture having a methanol content of at least 75$ hy weight, the temperature of which solution or dispersion being from 5 to 65° above the sin25 tering temperature of the oxymethylene polymer, into a Methanol/water mixture acting as a cooling agent and having a methanol content of at least 75$ by weight, the quantity of the precipitated oxymethylene polymer in the suspension formed being at most 25$ by weight, and '4 3 4 9 0 b) separating the granular oxymethylene polymer obtained and drying It. The invention also provides granular oxymethylene polymers prepared by the aforesaid process.

Under oxymethylene polymers according to the invention 5 there are to be understood poly(oxymethylenes) containing in the main valence chain besides oxymethylene units from 0.1 to 20, preferably from 0.5 to 10% by weight of oxyalkylene units having from 2 to 8, preferahly 2, 3 or 4 adjacent carbon 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 preferably primary alcohol groups.

The oxymethylene polymers are prepared in known manner by polymerization of the monomers in hulk, suspension or solu tion in the presence of cationicallly active catalysts, for example at a temperature of from 0 to 100°C, preferably of from 50 to 90°C (cf. US Patent No. 3,027,352). The cationically active catalysts which can be used are; 1) protonic acids, for example perchlorid acid, 2) esters of protonic acids, especially esters of perchloric acid with aliphatic alcohols of low molecular weight, for example perchloric acid tertiary butyl ester, 3) anhydrides of protonic acids, especially mixed anhydrides of perchloric acid and an aliphatic carboxylic acid of low molecular weight, for example acetyl perchlorate, 4) lewis acids, especially halides of boron, tin, titanium, phosphorus, arsenic and antimony, for example boron trifluoride, tin tetrachloride,titanium tetrachloride, phos29 phorus pentachloride, phosphorus pentafluoride, arsenic - 3 43490 pentafluoride and antimony pentafluoride, and ) complex compounds or salt like compounds, preferably etherates or onium salts, of lewis acids, for example boron trifluoride diethyletherate, boron trifluoride-di-n-butyl5 etherate, triethyloxonium tetrafluoroborate, trimethyloxonium hexafluorophosphate, triphenylmethyl.hexafluoroarsenate, acetyltetrafluoroborate, acetylhexafluorophosphate and acetylhexafluoroaresenate.

The quantity of the catalysts used in the copolymeri10 zation depends primarily on their efficiency and is generally from 0.1 to 2,000, preferably from 0.2 to 500 ppm, calculated on the total quantity of the compounds to be polymerized. Very efficient catalysts such as boron trifluoride are advantageously used in an amount of from 10 to 150,preferably of from to 100 ppm, calculated on the total quantity of the compounds to be polymerized. The corresponding molar quantities should be used for complex compounds or salt-like compounds. Highly active catalysts such as perchloric acid are used in an amount of from 0.2 to 10, preferably of from 0.5 to 5 ppm.

It is generally advisable to use the catalysts in a diluted form. Gaseous catalysts are diluted with an inert gas, for example, nitrogeii and noble gases such as argon, whereas liquid or solid catalysts are dissolved in an inert solvent. Suitable solvents are especially aliphatic or cycloaliphatic hydrocarbons as well as nitrated aliphatic or aromatic hydrocarbons, for example, cyclohexane, methylene chloride, ethylene chloride, nitromethane and nitrobenzene. The weight ratio of catalyst to diluent is usually from 1:5 to 1:10,000, pre29 ferably from 1:10 to 1:100. Very strongly acting catalysts - 4 are advantageously diluted in a proportion of from 1:5,000 to 1:20,000.

The polymerization is preferably effected in an inert gas atmosphere with the exclusion of moisture; suitable inert gas5 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 CH,< CR1H J -f o(cr2h) J -Ο 15 L__ _ y I wherein A) R and R 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 atoms, or a phenyl radi20 cal, and a) x is 1, 2 or 3 and y; is zero, or b) x is zero, jr is 1, 2 or 5 and z is 2, or c) x is zero, χ is 1 and z is 3, 4, 5 or 6, or B) R is an alkoxymethyl radical having from 2 to 6, preferably 2, 3 or 4 carbon atoms, or a phenoxymethyl radical, x being and x being zero and R having the meaning given above. Suitable cyclic ethers include, for example, ethylene oxide, propylene oxide, styrene oxide, cyclohexene oxide, oxa29 cyclobutane and phenylglycidyl ether, and suitable cyclic for- 5 43 490 mals are, for example 1,3-dloxolane, 1,3-dioxane, 1,3-dioxepane and 1,3,6-trioxocane as well as 4-methyl-1,3-dioxolane, 4--pheny 1-1,3-dioxolane, 1,3-dioxonane and 1,3-dioxacycloheptene-{5). Suitable polyfonmals include especially poly(1,35 dioxolane) and poly(l,3-dioxepane).

For preparing oxymethylene polymers having a molecular weight in a specific range the polymerization is advantageously carried out in the presence of a regulator. For this purpose there may be used especially formaldehyde dialkyl10 acetals having from 3 to 9, preferably 3, 4 or 5 carbon atoms, for example formaldehyde dimethylacetal, diethylacetal, dipropylacetal and dibutyiacetal, as well as aliphatic alcohols of low molecular weight, preferably alkanols having from 1 to 4 carbon atoms, for example methanol, ethanol, propanol and butanol. The regulator is generally used in an amount of up to 0.55» 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 instable portions the oxymethylene polymers are subjected advantageously to a controlled thermal partial degradation yielding primary terminal alcohol groups (cf, US Patents Nos. 3,174,948, 3,219,623 and 3,666,7¼). The thermal treatment is performed at a temperature of from 130 to 200°C, preferably of from 140 to 19O°C, especially under non-acid conditions in aqueous/methanolic solution, advantageously in the presence of a.compound showing a basic reaction, for example a tertiary aliphatic amine of low molecular weight, such as triethyl amine or triethanol amine, or a secondary alkali metal phosphate such as disodium hydrogen phosphate. - 6 .42480 A temperature of from 150 to 180°C is especially advantageous. The length of time of the thermal treatment depending on the temperature goes from 10 seconds to 2 hours, preferably from minute to 60 minutes. The higher the temperature , the short cr the residence time. At a temperature of 180°C about 1 to minutes are sufficient, about 5 to 10 minutes at a temperature of 160°C, about 10 to 30 minutes at a temperature of 150°C and about 20 to 60 minutes at a’ temperature of 140°C. The treatment is preferably carried out under substantial exclusion of oxygen.

As starting material for the process according to the invention there is used a solution or fine dispersion of an oxymethylene polymer containing from 3 to 35, preferahly from 5 to 30% by weight of polymer. A polymer solution or dispersion containing from 10 to 25% by weight of oxymethylene polymer gives especially good results.

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 especially advantageous, and the best results are obtained when using 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 of the solution or dispersion is 5 to 65, preferably 10 to 60°C above the sintering temperature of the oxymethylene polymer, a temperature range of from 25 to 55°C above the sintering temperature being especially advantageous. - 7 43490 As cooling agent and precipitating agent there is also used a mixture of methanol and water, the proportion of the components of which being in the same range as the aforesaid solvent or dispersing agent. The cooling agent has a tempera5 ture of from 1 to 10, preferably of from 1 to 5, especially of from 2 to 4°C below the sintering temperature of the oxymethylene polymer. The quantity of the precipitated oxymethylene polymer in the suspension formed by introducing the polymer solution or dispersion is at most 25, preferably in the range of from 5 to 15$ by weight.

The methanol used within the scope of the present invention may contain up to 30, preferably up to 10$ by weight of organic impurities soluble in methanol, usually formed as byproducts in the synthesis of oxymethylene polymers, for ex15 ample formaldehyde, cyclic oligomers of formaldehyde, methyllal, glycol formal, glycol monomethyl ether, glycol dimethyl ether as well as aliphatic alcohols of low molecular weight, aliphatic esters of low molecular weight and acetone.

The sintering temperature (T_) is the temperature «t s which the solid polymer particles suspended in the methanol/ water mixtures soften .at the surface and stick together, withCut Completely melting. It depends on the composition and the molecular weight of the polymer as well as on the nature of the solvent, or dispersing agent. The sintering temperature of the oxymethylene polymers used according to the invention is in the range of from 100 to 140, especially of from 125 to 135°C.

The process of the invention may be carried out, for ex29 ample, in an autoclave provided with a stirrer. The cooling - 8 42 4 9 0 and precipitating agent are given into this autoclave and the solution or dispersion of the oxymethylene polymer is introduced advantageously through a heated immersion pipe or a nozzle into the precipitating agent, the latter being kept in turbulent motion by stirring. The process may 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 additional precipitating agent are added. The average residence time of the oxymethylene polymer in the precipitation vessl is in the range of from 1 minute to 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 obtained by precipitation or agglomeration is performed by the usual methods, for example by filtration, decantation or centrifugation. The separated oxymethylene polymer finally is dried at a temperature of from 20 to 135°C, preferably of from 50 to 120°C. Drying under an inert gas atmosphere, for example, under a noble gas or nitrogen atmosphere is advisable.

The oxymethylene polymers obtained according to the invention are macromolecular: their reduced specific viscosity (RSV) is in 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 in ^butyrolactone, containing 2% by weight of o diphenylamine as stabilizer, at a temperature of 140 C). The crystallite melting points of the oxymethylene polymers are - 9 42490 in the range of from 140 to 180°C, their melt indices MFI 190/2 are in the range of from 0.1 to 50, preferably of from 1 to 30 g/10 minutes (measured according to DIN = German Industrial Standard No. 53,735 at a temperature of 190°C under a load of 2.16 kg).

The process according to the invention enables preparing granular oxymethylene polymers having an increased content of dry matter (after precipitation) and an increased apparent density (after drying). The apparent density of the granular 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 70°C under nitrogen after having been mixed homogeneously for 2 minutes in a laboratory fluid mixer at a speed of 3,000 revolutions per minute).

The oxymethylene polymers obtained according to the invention may be stabilized additionally against the action of heat, oxygen and/or light by mixing them homogeneously with stabilizers. Homogenization is usually carried out in a commercial mixing device, for example, an extruder, at a tempera20 ture above the melting point of the polymer up to 250°C, preferably of from 180 to 210°C. The total amount of 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 bisphenol com25 pounds, alkaline earth metal salts or carboxylic acids as well as guanidine compounds. The bisphenol compounds used are preferahly esters of monobasic 4-hydroxyphenylalkanoic acids which are substituted in the nucleus once or twice by an alkyl radical having from 1 to 4 carbon atoms and which have from 7 - 10 434S0 to 13, preferably 7, 8 or 9 carbon atoms, with aliphatic di-, tri- or tetrahydric alcohols having from 2 to 6, preferably 2, 3 or 4 carbon atoms. Examples are esters ofiV-(3-tertiary butyl-4-hydroxyphenyl)-pentanoic acid, fi-(3-methy1-5-tertiary bulyl-4-hy(h'oxyphenyl)-propionic acid, (3,5-ώ1“ΐ®Γΐ1βι·γ butyl4-hydrophenyl) acetic acid, B-(3,5-di-tertiary butyl-4-hydroxy phenyl)-propionic acid or (3,5“di-isopropyl-4-hydroxyphenyl)acetic acid with ethylene glycol, propane diol-(1,2), propane diol-(1,3), butane diol-(1,4), hexane diol-(l,6), 1,1,1-tri10 methylol ethane or pentaerythritol.

Suitable alkaline earth me'tal salts of carboxylic acids are especially alkaline earth metal salts of aliphatic, preferably hydroxyl groups containing mono-, bi- or tribasic car boxylic acids having from 2 to 20, preferably from 3 to 9 car15 bon atoms, for example the calcium or magnesium salts or stearic acid, ricinoleic acid, lactic acid, mandelic acid, malic acid or citric acid.

Suitable guanidine compounds are compounds of the formula NC-NH-C-NH-R NH wherein R is a hydrogen atom, a cyano group or an alkyl radical having from 1 to 6 carbon atoms, for example cyanoguanidine, N-cyano-N'-methyl-guanidine, N-cyano-N’-ethyl-guanidine, N-cyano-N'-isopropylguanidine, N-cyano-N*-tertiary butylguanidine or Ν,Ν'-dicyanoguanidine. The guanidine compound is used optionally in an amount of from 0.01 to 1, preferably of from 0.1 to 0.5$ by weight, calculated on the total mixture.

In addition there may be added to the oxymethylene poly- 11 43490 mer prepared, according to the invention known light stabilizers, for example derivatives of benzophenone, acetophenone or triazine. Further usual additives, for example dyestuffs, pigments, reinforcing materials and fillers may also be used.

The oxymethylene polymers may be processed by all methods usually employed for thermoplastics, for example by injection moulding, extrusion, blowing of extruded material, melt spinning and deep drawing. They are suitable for preparing semifinished products and finished products such as shaped artic10 les, for example bars, rods, plates, ribbons, bristles, threads, fibers, films, sheets, tubes and flexible tubes, as well as household articles, for example dishes and cups, and machine elements such as casings and gear wheels. They are especially suitable as engineering plastic for preparing dimensionally stable and true to shape articles.

The following examples illustrate the invention: EXAMPLE 1: 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 165°C and a MFI value of 25 g/ minutes were mixed in an autoclave with 30 kg of a mixture of 85% by weight of methanol, 14.9% by weight of water and 0.1% by weight of triethylamine. The suspension formed was heated under nitrogen for 5 minutes to 16O°C, whereby the polymer dissoved. 2 kg per minute of the solution obtained were passed continuously through an immersion pipe into 10 kg of a mixture of 85% by weight of methanol and 15% by weight of water contained in a further autoclave and being kept at a tempera29 ture of 125°C in turbulent motion under nitrogen. ( At a sinter- 12' 4 348 0 ing temperature of the polymer of 127°C the temperature of the precipitating agent consequently was 2°C below the said temperature). During the solution and precipitation process the pressure in the autoclave was 17 and 9 bars respectively.

The precipitated granular polymer was withdrawn at the bottom of the autoclave 7 minutes after having terminated the introduction and separated by suction from the solvent and precipitating agent; the solid product had a content of dry matter of about 40%. After drying and homogeneous mixing the poly10 mer had an apparent density of 350 g/1.

EX AM Pl E 2; Example 1 was repeated, but by using as solvent and as precipitating agent each time a mixture of 90% by weight of methanol, 9.9% by weight of water and 0.1% by weight of tri15 ethylamine. The temperature during the solution process was 170°C. The granular polymer had a content of dry matter of about 38% after suction of the solvent and precipitating agent and an apparent density of 380 g/l after drying and homogeneously mixing.

EXAMPLE 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 165°C and a MPI value of 25 g/ minutes were mixed in an autoclave with 22.5 kg of a mix25 ture of 96.5% by weight of methanol, 3.4% by weight of water and 0.1% by weight triethylene. The suspension formed was heated under nitrogen for 5 minutes to 180°C, whereby the polymer dissolved. The solution obtained was introduced conti29 nuously over a period of 30 minutes, through an immersion pipe, - 13 42430 into a mixture of 96.5% by weight of methanol and 3.5% by weight of water contained in a further autoclave and being kept at a temperature of 131 °C in turbulent motion under nitrogen. (At a sintering temperature of the polymer of 133°C the temperature of the precipitating agent was consequently 2°C below the said temperature). The pressure in the autoclave during the solution and precipitation process was 30 and 10 bars respectively. 15 minutes after having terminated the introduction the precipitated granular polymer was withdrawn at the bottom of the autoclave and separated by suction from the solvent and precipitating agent; the dry matter content of the solid product v/as about 45%. After drying and homogeneously mixing the dry polymer had an apparent density of 450 g/l.

COMPARATIVE EXAMPLE: 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 165°C and a MEI value of 25 g/ minutes were mixed with 30 kg of a mixture of 60% by weight of methanol, 39.9% by weight of water and 0,1% by weight of triethylamine. The suspension obtained was heated for 5 minutes to 160°C, whereby the polymer dissolved. The solution formed thereby was cooled to a temperature of 135°C. 2 kg per minute of the solution were Introduced continuously through an immersion pipe into 10 kg of a mixture of 60% by weight of methanol and 40% by weight of water contained in a further autoclave and being kept at a temperature of 125°C in turbulent motion under nitrogen. - 14 43480 (At a sintering temperature of the polymer of 127°C the temperature of the precipitating agent was consequently 2°C below the said temperature.) 7 minutes after having terminated the introduction the precipitated granular polymer was with5 drawn at the bottom of the autoclave and separated by suction from the solvent and precipitating agent. The content 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 (6)

1. What is claimed is;

1. Process for preparing a granular oxymethylene polymer containing besides oxymethylene units from 0.1 to 20% by weight of oxyalkylene units having from 2 to 8 adjacent 5 carbon atoms in the main chain, by introducing a solution or fine dispersion of the oxymethylene polymer into a liquid cooling agent being kept in turbulent motion at a temperature of from 1 to 10°C below the sintering temperature of the oxymethylene polymer, which comprises 10 a) precipitating the oxymethylene polymer by introducing a 3 to 35% by weight solution or dispersion of the oxymethylene polymer in a methanol/water mixture having a methanol content of at least 75% by weight, the temperature of which solution or dispersion being 15 from 5 to.65°C above the sintering temperature of the oxymethylene polymer, into a methanol/water mixture acting as cooling agent and having a methanol content of at least 75% by weight, the quantity of the precipitated oxymethylene polymer in the suspension formed 20 being at most 25% by weight, and b) separating the granular oxymethylene polymer obtained and drying it.

2. Process as claimed in claim 1, which comprises using as solvent or dispersing agent and as cooling agent a mix25 ture of from 99·9 to 80% by weight of methanol and of from 0.1 to 20% by weight of water.

3. Process as claimed in claim 1, wherein the solvent or dispersing agent contains a compound showing a basic reaction.

4. Process as claimed in claim 1, wherein the cooling agent has a temperature of from 1 to 1O°C below the sintering temperature of the oxymethylene polymer.

5. · Process for preparing a granular oxymethylene polymers substanti:lly as hereinbefore described with reference to the accompanying examples.

6. A granular oxymethylene polymer whenever prepared by a process claimed in a preceding claim.