Synthesis of polyether polyols commercially suitable for the polyurethane industry, by polymerisation of polyfunctional alcohols

Abstract

The synthesis of polyether polyols suitable for the preparation of polyurethane is carried out by polymerisation of polyfunctional alcohols, using a mixture of acids as a catalyst for the polycondensation reaction, avoiding partial degradation (oxidation) of the raw materials and the consequent increase in the colour of the product. The mixture of monomers, consisting of one or more polyfunctional alcohols, is reacted under polycondensation conditions using a catalyst mixture comprising at least one oxidising acid and at least one reducing acid. The process can be used to synthesise polyether polyols with reduced colour, generally lower than 50 Hz and with a high degree of purity, in a short time and without the need for additional treatments for colour reduction.

Description

[0001] Synthesis of polyether polyols commercially suitable for the polyurethane industry, by polymerisation of polyfunctional alcohols

[0002] The present invention relates to a process for the synthesis of polyether polyols suitable for the preparation of polyurethane by polymerisation of polyfunctional alcohols.

[0003] Polyether polyols are a raw material of commercial interest due to their chemical-physical properties; they have wide applications and can be used, for example, as precursors of polyurethane.

[0004] In the polyurethane industry, polyols, and in particular polyether polyols, are the main component for the production of polyurethane. The hydroxyl groups of the polyols react with isocyanates forming the urethane bond and thus giving rise to polyurethanes. Depending on the type of final application, it is necessary to formulate the polyurethane with the most suitable polyol. For this reason, it is essential for the polyurethane market to have an increasingly wide choice of polyether polyols so as to meet all performance requirements, even the most demanding ones in terms of performance.

[0005] In particular, it is desirable and preferable that these polyether polyols exhibit the following characteristics:

[0006] - purity: typically it must be greater than or equal to 99.5%,

[0007] - residual K content: typically it must be lower than 1 ppm,

[0008] - residual Na content: typically it must be lower than 1 ppm,

[0009] - residual Fe content: typically it must be lower than 5 ppm,

[0010] - residual water content: typically it must be lower than 0.050%,

[0011] - pH: typically it must be between 4.5 and 7.5, preferably between 5.0 and 7.0,

[0012] - colour: typically it must be lower than 50 Hz, preferably lower than 35 Hz, more preferably lower than 10 Hz.

[0013] In particular, the term “colour” as used herein refers to the presence of visible colour that can be quantified by using a spectrophotometer or colorimeter, employing wavelengths of approximately 400-800 nm, and comparing it with pure water (reference: 0 Hz). The colour value is assigned according to the Hazen scale.

[0014] Current synthesis methods:

[0015] Synthesis of polyether polyols by ring-opening polymerisation

[0016] Ring-opening polymerisation is the most commonly used production method for the production of polyether polyols. The most commonly used polyether polyols are those that can be synthesised by ring-opening polymerisation of the corresponding cyclic ethers, such as polyethylene glycol (PEG), which is obtained from ethylene oxide, polypropylene glycol (PPG), which is obtained from propylene oxide, and polytetrahydrofuran (PTHF or PTMEG), which is obtained from the opening of tetrahydrofuran.

[0017] The ring-opening reaction of cyclic ethers such as ethylene oxide and propylene oxide can take place through acid catalysis or basic catalysis. The properties and composition of the resulting polyether polyol can be controlled by varying the proportion between ethylene oxide and propylene oxide and by using polyfunctional alcohols as reaction starters. Another cyclic ether usable for the synthesis of polyether polyols is tetrahydrofuran which, again through catalysed ring-opening polymerisation, allows the production of polytetrahydrofuran.

[0018] The polyether polyols obtained from these processes must be subjected to refining and purification to remove any impurities. Although the products obtained with this technology are suitable for use in polyurethane syntheses, the synthesis of polyether polyols by ringopening polymerisation has some limitations. In fact, it is not possible to obtain polyether polyols formed from monomers with 5 or more methylene units because the cyclic ethers to be used as precursors are extremely stable. This technology, therefore, allows to obtain a limited number of polyether polyols, failing to meet all the performance requirements of the polyurethane industry.

[0019] Synthesis of polyether polyols by acid-catalysed polycondensation The polymerisation of polyfunctional alcohols is a reaction that leads to the formation of a polymer chain, that is, a macromolecule, formed from repeating units deriving from the starting monomers. The starting monomers are polyfunctional alcohols. The reaction that takes place is also called polycondensation and produces water as a by-product.

[0020] Ps

[0021] The current technology for the synthesis of polyether polyols by acid-catalysed polycondensation of polyfunctional alcohols (glycols) is a process that permits to synthesise polyether polyols with extremely interesting characteristics for the polyurethane field, but it presents limitations since the obtained products have colour and purity indices not suitable for this type of application.

[0022] Over the years, various methods have been studied to improve the colour of the polyether polyols obtained with this technology, and they all include either the purification of the polyol or the pretreatment of the glycols before the polycondensation reaction.

[0023] Patent US 6,235,948 describes a pre-polymerisation process for the removal of impurities from the starting polyfunctional alcohol (which lead to colour increase) through ion exchange resins. Patent US 7,294,746 describes post-polymerisation treatment methods using adsorbents such as carbon black. Both pre- and post-treatment methods add steps to the process in order to produce a commercially acceptable polymer.

[0024] Technologies have also been developed to modify the reaction conditions and control the colour of the product. For example, patent application US 2005 / 272911 discloses methods for controlling colour formation by carrying out the polycondensation reaction in the presence of a catalyst composed of an acid and a base. Or, WO 2011 / 041348 A2 describes a process in which a base is added during the reaction in order to obtain a polyol with reduced colour. In both cases, the addition of a base reduces the catalytic activity of the acid and at the same time leads to the presence of undesired ions in the final polyol (such as Na+or K+) which make the polyol unusable for polyurethane syntheses. Metallic ions cause important interactions with the subsequent processes of the polyurethane industry and therefore require further steps to purify the product.

[0025] Another technology that uses polycondensation of polyfunctional alcohols is that described in patent EP 3 792 296 A2, which uses the combination of a Brpnsted acid with a Brpnsted base to form the catalyst, a protic ionic salt. The polyether polyols produced with this technology, however, do not have the characteristics required by the polyurethane industry. The colour of the product is high, and the protic ionic salt is extremely difficult to remove from the polyol.

[0026] The primary purpose of the present invention is to provide a process that enables the production of different types of polyether polyols, suitable for the polyurethane industry, with an efficient production process and without the need for post-synthesis treatments.

[0027] In this context, a specific object of the invention is to provide a process that avoids partial degradation (oxidation) of the raw materials and the consequent increase in the colour of the product.

[0028] Summary of the invention

[0029] In order to achieve the aforementioned purposes, an object of the present invention is a process for the synthesis of polyether polyols by acid-catalysed polycondensation of polyfunctional alcohols having the characteristics defined in the claims that follow, which constitute an integral and integrating part of the present description.

[0030] Another object of the invention lies in the use of a combination of acids, as a catalytic mixture, for the synthesis of poly ether polyols, by condensation of polyfunctional alcohols, of a catalytic mixture of acids comprising at least one oxidising acid and at least one reducing acid.

[0031] This type of catalysis allows the production of polyether polyols, from polyfunctional alcohols, commercially suitable for the polyurethane industry without the need for further treatments to reduce the colour.

[0032] The resulting polyether may be a homopolymer or a copolymer, with random or block distribution of the monomers.

[0033] Another object of the invention is a polyether polyol obtainable by the process according to the invention and having, as a result of the polycondensation reaction, a colour lower than 50 Hz, preferably lower than 35 Hz, more preferably lower than 10 Hz, in the absence of further treatments for colour reduction.

[0034] Detailed description of the invention

[0035] The polyether polyols suitable for the preparation of polyurethane to which the invention relates are poly ether polyols, obtained by polycondensation of at least one or more di- or poly- functional alcohols, with specific purity and colour specifications, which make them usable as precursors for polyurethane articles. These polyether polyols can therefore be used for numerous applications such as: polyurethane for footwear, polyurethane for flexible foams, thermoplastic polyurethane, casting polyurethane, polyurethane adhesives, hot-melt adhesives, polyurethane for synthetic leathers and polyurethane for inks.

[0036] The mixture of monomers, formed from one or more polyfunctional alcohols, is reacted under polycondensation conditions thanks to a mixture of at least one oxidising acid and at least one reducing acid.

[0037] The term polyfunctional alcohols refers to molecules containing 2 or more hydroxyl functional groups (-OH) capable of undergoing polycondensation reaction. The polyfunctional alcohols can be (but are not limited to) monoethylene glycol, diethylene glycol, triethylene glycol, 1,2-propanediol, 1,3 -propanediol, dipropylene glycol, 2,2- dimethylpropanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,9-nonanediol, 1,10-decanediol, 1,11 -undecanediol, 1,12-dodecanediol, 2-butyl-2-ethyl- 1,3 -propanediol, 1- n-propyl-2-ethyl- 1 ,3 -propanediol, 1 -isopropyl-2,2-dimethyl- 1 ,3 -propanediol, neopentyl glycol hydroxypivalate, 2-methyl- 1,3 -propanediol, 3 -methyl- 1,5-pentanediol, 1,3-butylene glycol, cyclohexanedimethanol, isosorbide, glycerine, trimethylolpropane, pentaerythritol, polyethylene glycol with MW between 200 and 10000, polypropylene glycol with MW between 200 and 10000, polypropylene glycol capped with ethylene oxide with MW between 200 and 10000, poly tetrahydrofuran with MW between 200 and 10000, polytrimethylene ether glycol with MW between 200 and 10000, or mixtures of two or more thereof.

[0038] The polyfunctional alcohols just described are the monomers that will lead to the formation of the desired polyether polyol.

[0039] According to the invention the catalytic mixture of acids is a mixture comprising at least one oxidising acid and at least one reducing acid. This mixture of acids is intended to promote the polycondensation reaction while keeping the colour low. The molar ratio between the oxidising acids and the reducing acids at the start of the polycondensation reaction may be between 0.3 and 1.5. Preferably, it may be between 0.5 and 1.2. More preferably, it may be between 0.6 and 1.0.

[0040] The acids forming the catalytic mixture may be added to the reaction monomers in various ways: already pre-mixed together or one at a time into the glycol mixture before the start of the polymer synthesis process, in the sequence reducing acid-oxidising acid or already premixed at different stages of the process, dosing the desired quantity in aliquots during processing.

[0041] The term “oxidising acid” refers to an acid in which the non-metal has the highest oxidation number it can assume. An oxidising acid is therefore a molecule, possessing an acid group, capable of being reduced, oxidising other species. The oxidising acids can be (but are not limited to): nitric acid, sulphuric acid, p-toluenesulphonic acid (as such or substituted), benzenesulphonic acid (as such or substituted), methanesulphonic acid, chlorosulphonic acid, tetrafluoroethanesulphonic acid, trifluoromethanesulphonic acid, perfluoroctanesulphonic acid, chloric acid, perchloric acid, iodic acid, chromic acid, permanganic acid or a mixture of two or more thereof. The term “reducing acid” refers to an acid in which the non-metal has an oxidation number lower than the maximum it can assume. A reducing acid is therefore a molecule, possessing an acid group, capable of being oxidised reducing other species. The reducing acids can be (but are not limited to): oxalic acid, ascorbic acid, nitrous acid, aqueous solutions of sulphur dioxide (SO2 7H2O), sulphurous acid, hydrogen sulphide, bromous acid, phosphorous acid, hypopho sphorous acid, diphosphorous acid, chlorous acid, hypochlorous acid, boric acid or a mixture of two or more thereof.

[0042] The acids forming the catalytic mixture, whether solid or liquid, will dissolve in the monomer mixture. This type of catalysis is referred to as “homogeneous catalysis”. Homogeneous catalysis is extremely effective compared to other types of catalysis; the only drawback is the need to remove the catalyst at the end of the process.

[0043] Preferred combinations of oxidising acid catalyst and reducing acid catalyst include: sulphuric acid with hypopho sphorous acid; p-toluenesulphonic acid with phosphorous acid; p-toluenesulphonic acid with hypopho sphorous acid; p-toluenesulphonic acid with boric acid; benzenesulphonic acid with phosphorous acid; benzenesulphonic acid with hypopho sphorous acid; methanesulphonic acid with phosphorous acid; methanesulphonic acid with hypopho sphorous acid;

[0044] - methanesulphonic acid with hypochlorous acid; tetrafluoroethanesulphonic acid with hypopho sphorous acid;

[0045] - trifluoromethanesulphonic acid with sulphurous acid

[0046] The polycondensation reaction is carried out according to conventional technique, keeping the reagents under stirring and with removal of the water that forms as a result of the polycondensation; the reaction temperature, which is selected according to the monomers used and the molecular weight to be achieved, may be between 160°C and 250°C. The pressure at which the synthesis is carried out may be slightly higher than atmospheric pressure (typically around +0.5 bar) in the case of a nitrogen stripping process, or the reaction may be conducted under high vacuum (typically between 30 mbar and 50 mbar of residual pressure).

[0047] The reaction time is selected according to the molecular weight desired from time to time for the polyether polyol.

[0048] The monomers may all be present from the beginning of the reaction or one or more monomers may be added at a later time.

[0049] If only one monomer is present, a homopolymer will be obtained; if multiple monomers are present from the beginning, a random copolymer will be obtained. If a monomer different from the first is added in a second phase, a block copolymer will be obtained.

[0050] The acids forming the catalytic mixture may be added to the reaction monomers in various ways: already pre-mixed together, one at a time before the start of the process, or individually at different stages of the process, dosing the desired amount all at once or in aliquots during the processing. The reaction will end once the desired molecular weight is reached. The polyether polyol will not require further treatments for colour reduction: it will only be necessary to remove the catalyst residues to stop the reaction. This latter process is carried out by adding, at the end of the reaction, a solid adsorbent capable of removing the catalyst residues from the polyether polyol and finally carrying out a simple filtration to separate the liquid phase (polyether polyol) from the solid phase (adsorbent material + residual free acids).

[0051] Example A (Reference, without reducing acid)

[0052] In a glass reactor, equipped with a stirrer and Claisen condenser, 8000 g of 1,6-hexanediol and 22 g of methanesulphonic acid are added. The reagents are heated to a temperature of 180°C under inert gas flow. The reaction is monitored in terms of molecular weight of the product and colour. The molecular weight of the product is determined by titration of the - OH terminals, while the colour is determined using a HACH Lange colorimeter on the Hz scale. Test 1:

[0053] Test 2:

[0054] Test 3:

[0055] Example B

[0056] In a glass reactor, equipped with a stirrer and Claisen condenser, 8000 g of 1,6-hexanediol and a mixture of acids composed of 22 g of methanesulphonic acid and 12 g of hypopho sphorous acid are added. The reagents are heated to a temperature of 180°C under inert gas flow. The reaction is monitored in terms of molecular weight of the product and colour. The molecular weight of the product is determined by titration of the -OH terminals, while the colour is determined using a HACH Lange colorimeter on the Hz scale.

[0057] Test 1: Test 2:

[0058] Test 3:

[0059] Example C

[0060] In a glass reactor, equipped with a stirrer and Claisen condenser, 6000 g of 1,6-hexanediol, 2000 g of monoethylene glycol and a mixture of acids composed of 20 g of methanesulphonic acid and 11 g of hypopho sphorous acid are added. The reagents are heated to a temperature of 200°C under inert gas flow. The reaction is monitored in terms of molecular weight of the product and colour. The molecular weight of the product is determined by titration of the -OH terminals, while the colour is determined using a HACH Lange colorimeter on the Hz scale. Test 1:

[0061] Test 2:

[0062] Test 3:

[0063] Example D

[0064] In a glass reactor, equipped with a stirrer and Claisen condenser, 8000 g of 1,3-butylene glycol and a mixture of acids composed of 11 g of methanesulphonic acid and 11 g of hypochlorous acid are added. The reagents are heated to a temperature of 220°C under inert gas flow. The reaction is monitored in terms of molecular weight of the product and colour. The molecular weight of the product is determined by titration of the -OH terminals, while the colour is determined using a HACH Lange colorimeter on the Hz scale.

[0065] Test 1:

[0066] Test 2:

[0067] Test 3:

[0068] Example E In a glass reactor, equipped with a stirrer and Claisen condenser, 7900 g of neopentyl glycol, 100 g of trimethylolpropane and a mixture of acids composed of 18 g of trifluoromethanesulphonic acid and 10 g of sulphurous acid are added. The reagents are heated to a temperature of 200°C under inert gas flow. The reaction is monitored in terms of molecular weight of the product and colour. The molecular weight of the product is determined by titration of the -OH terminals, while the colour is determined using a HACH Lange colorimeter on the Hz scale.

[0069] Test 1:

[0070] Test 2:

[0071] Test 3: Conclusions:

[0072] As can be seen from the comparison between Examples A and B, under identical monomer composition and synthesis conditions, the use of the catalytic mixture described in the present invention allows obtaining significantly lower colours in the final polyether polyol, consistent with the quality required for subsequent polyurethane synthesis (which requires colours below 50 Hz). Examples B, C, D, and E demonstrate how the catalytic mixture described in the present invention is also effective when using various types of monomers (glycols), whether alone or in combination with one another, offering the opportunity to generate new types of polyether polyols to be used for polyurethane synthesis.

Claims

CLAIMS1. A process for the synthesis of polyether polyols by acid-catalysed polycondensation of polyfunctional alcohols, characterised in that the polycondensation reaction is carried out using a mixture of acid catalysts comprising at least one oxidising acid and at least one reducing acid.2 The process according to claim 1 characterised in that the oxidising acid catalyst is selected from the group consisting of nitric acid, sulphuric acid, p-toluenesulphonic acid (as such or substituted), benzenesulphonic acid (as such or substituted), methanesulphonic acid, chloro sulphonic acid, tetrafluoroethanesulphonic acid, trifluoromethanesulphonic acid, perfluoroctanesulphonic acid, chloric acid, perchloric acid, iodic acid, chromic acid, permanganic acid or a mixture of two or more thereof.3 The process according to claim 1 or 2, characterised in that the reducing acid catalyst is selected from the group consisting of oxalic acid, ascorbic acid, nitrous acid, aqueous solutions of sulphur dioxide (SO2*7H2O), sulphurous acid, hydrogen sulphide, bromous acid, phosphorous acid, hypopho sphorous acid, diphosphorous acid, chlorous acid, hypochlorous acid, boric acid or a mixture of two or more thereof.

4. The process according to any one of claims 1 to 3, characterised in that the molar ratio between the oxidising acids and the reducing acids at the start of the polycondensation reaction is between 0.3 and 1.5.

5. The process according to any of claims 1 to 4, characterised by the fact that the molar ratio between the oxidising and reducing acids at the start of the polycondensation reaction is between 0.5 and 1.2.

6. The process according to any one of claims 1 to 5, characterised in that the molar ratio between the oxidising acids and the reducing acids at the start of the polycondensation reaction is between 0.6 and 1.0.

7. The process according to any one of claims 1 to 6, characterised in that the polyfunctional alcohol is selected from the group consisting of monoethylene glycol, diethylene glycol, triethylene glycol, 1,2-propanediol, 1,3 -propanediol, dipropylene glycol, 2,2-dimethylpropanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,9-nonanediol, 1,10-decanediol, 1,11 -undecanediol, 1,12-dodecanediol, 2-butyl-2-ethyl- 1,3 -propanediol, 1- n-propyl-2-ethyl- 1 ,3 -propanediol, 1 -isopropyl-2,2-dimethyl- 1 ,3 -propanediol, neopentyl glycol hydroxypivalate, 2-methyl- 1,3 -propanediol, 3 -methyl- 1,5-pentanediol, 1,3-butylene glycol, cyclohexanedimethanol, isosorbide, glycerine, trimethylolpropane, pentaerythritol, polyethylene glycol with MW between 200 and 10000, polypropylene glycol with MW between 200 and 10000, polypropylene glycol capped with ethylene oxide with MW between 200 and 10000, poly tetrahydrofuran with MW between 200 and 10000, poly trimethylene ether glycol with MW between 200 and 10000, or a mixture of two or more thereof.

8. The process according to any one of claims 1 to 7 characterised in that the mixture of catalysts is selected from the group consisting of: sulphuric acid with hypopho sphorous acid; p-toluenesulphonic acid with phosphorous acid; p-toluenesulphonic acid with hypopho sphorous acid; p-toluenesulphonic acid with boric acid; benzenesulphonic acid with phosphorous acid; benzenesulphonic acid with hypopho sphorous acid; methanesulphonic acid with phosphorous acid; methanesulphonic acid with hypopho sphorous acid;- methanesulphonic acid with hypochlorous acid; tetrafuoroethanesulphonic acid with hypopho sphorous acid;- trifluoromethanesulphonic acid with sulphurous acid9. Process according to any one of claims 1 to 8, wherein the polycondensation reaction is carried out with homogeneous catalysis at temperatures from 160°C to 250°C.

10. Use of a catalytic mixture comprising at least one oxidising acid and at least onereducing acid, for the synthesis of polyether polyols by poly-condensation of polyfunctional alcohols.

11. A polyether polyol obtainable by the process according to any one of claims 1 to 9, having, as a result of the polycondensation reaction, a colour lower than 50 Hz, preferably lower than 35 Hz, more preferably lower than 10 Hz, in the absence of further treatment for colour reduction.

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