Patent Application
20240209136
The invention relates to a novel process for preparing carbodiimides having a reduced residual isocyanate monomer content.
Carbodiimides have proven useful in many applications, for example as hydrolysis inhibitors for thermoplastics, polyols, polyurethanes, triglycerides and lubricating oils, etc.
The prior art synthesis of carbodiimides typically proceeds from isocyanates, which are carbodiimidized under basic or heterocyclic catalysis with elimination of CO2. This allows mono- or polyfunctional isocyanates to be converted into monomeric or polymeric carbodiimides.
The catalysts typically used are alkali metal or alkaline earth metal compounds and also heterocyclic compounds containing phosphorus. Corresponding catalysts are for example described in Angew. Chem. 1962, 74, 801-806 and Angew. Chem. 1981, 93, 855-866.
The removal of the isocyanates used as raw material from the product after the carbodiimidization has been performed is associated with high complexity, especially when preparing polymeric carbodiimides. The target of reducing the residual isocyanate monomer content to significantly below 0.1% by weight, without impairing the product quality (especially the colour number), is generally achieved only by repeated crystallization operations with significant losses in yield. As an alternative, as described in EP 1451239, carbodiimides containing isocyanate groups and urethane groups and having a low residual isocyanate monomer content can be prepared by repeatedly distilling off the monomers via short-path evaporators; however, this is time- and cost-intensive.
An object of the present invention was therefore that of providing an improved process for preparing carbodiimides having a reduced content of isocyanates, which to a very great extent avoids the above disadvantages of the prior art and enables high yields with economic feasibility and a high product quality.
Surprisingly, it has now been found that the aforementioned object is achieved by a process for preparing carbodiimides of the formula (I)
in which
The isocyanates used here in step a) are monomeric compounds of the formulae OCN—RI—NCO, OCN—RII and OCN—RIII.
In an alternative embodiment, in step a) compounds of the formula OCN—RI—NCO in an already oligomerized or polymerized form, i.e. as compounds of the formula OCN—RI—(—N═C═N—RI—)(n-1)—NCO, are carbodiimidized together with compounds of the formulae OCN—RII and OCN—RIII.
In further alternative embodiments, in step a) the compounds of the formula OCN—RI—NCO are used in already oligomerized or polymerized form and in a form already terminated at one end. In these cases, i.e. compounds of the formula RIII—(—N═C═N—R—)n—NCO are carbodiimidized with compounds of the formula OCN—R1, or compounds of the formula RII—(—N═C═N—RI—)n—NCO are carbodiimidized with compounds of the formula OCN—RI.
The optionally C1-C12-alkyl-substituted C6-C10 arylenes bridged via alkylene groups and having a total of 8 to 30 carbon atoms have the general structure—alkylene-arylene-alkylene-, where the alkylenes may be linear or branched and the arylene group may have up to four C1-C12 alkyl substituents, with the proviso that the total number of carbon atoms is not more than 30.
Preference is given here to C6-C10 arylenes that are bridged via alkylene groups and have no alkyl groups on the arylene group, and in which the two alkylene groups in each case have 1 to 6 carbon atoms.
In a preferred embodiment, RI is C1-C12-alkyl-substituted C6-C12 arylenes, preferably C1-C4-alkyl-substituted C6-C12 arylenes, particularly preferably mono- to tri-C1-C4-alkyl-substituted C6 arylenes, and very particularly preferably di- and/or triisopropylphenylene.
In a preferred embodiment, RII and RIII independently of one another are C1-C12-alkyl-substituted C6-C12 aryls, preferably C1-C4-alkyl-substituted C6-C12 aryls, particularly preferably mono- to tri-C1-C4-alkyl-substituted C6 aryls, and very particularly preferably di- and/or triisopropylphenyl.
In a further preferred embodiment, RI is C1-C12-alkyl-substituted C6-C12 arylenes and RII and RIII independently of one another are C1-C12-alkyl-substituted C6-C12 aryls.
In a further preferred embodiment, RI is C1-C4-alkyl-substituted C6-C12 arylenes and RII and RIII independently of one another are C1-C4-alkyl-substituted C6-C12 aryls.
In a further preferred embodiment, RI is mono- to tri-C1-C4-alkyl-substituted C6 arylenes and RII and RIII independently of one another are mono- to tri-C1-C4-alkyl-substituted C6 aryls.
In a further preferred embodiment, RI is di- and/or triisopropylphenylene and RII and RIII independently of one another are di- and/or triisopropylphenyl.
In a preferred embodiment, the carbodiimides correspond to the formula (II),
in which
Preferably, n is a number in the range from 1 to 50, preferably from 3-20.
In the above-mentioned embodiments of the invention, there may also arise mixtures of compounds of the formula (I) and/or (II) having different values for n. In this case, fractions may also result for n when determining the average value.
The carbodiimidization of isocyanates in the presence of a catalyst in step a) of the process according to the invention is typically effected in a condensation reaction with elimination of CO2, as described, for example, in Angew. Chem. 93, pp. 855-866 (1981) or DE-A-11 30 594 or Tetrahedron Letters 48 (2007), pp. 6002-6004.
The carbodiimidization can be conducted either in substance or in a solvent. It is likewise possible to first begin the carbodiimidization in substance and to add a solvent during the reaction. Suitable solvents can be easily determined by those skilled in the art. Examples of such solvents include petroleum ether, benzene and/or alkylbenzenes.
The isocyanates used are particularly preferably 1,3,5-triisopropylphenyl diisocyanate (TRIDI), 2,6-diisopropylphenyl isocyanate (DIPI) or 2,4,6-triisopropylphenyl isocyanate (TRIPI).
In one embodiment of the invention, the catalysts preferred for the carbodiimidization of the isocyanates to carbodiimides of the formula (I) in step a) are strong bases or phosphorus compounds. Preference is given to using phospholene oxides, phospholidines or phospholine oxides and also the corresponding sulfides. Further catalysts that may be used are tertiary amines, basic metal compounds, alkali metal and alkaline earth metal oxides, hydroxides, alkoxides or phenoxides, metal carboxylates and non-basic organometallic compounds. As catalyst, particular preference is given to alkylphospholene oxides such as methylphospholene oxide.
The reaction (carbodiimidization) is preferably conducted in a temperature range from 140° C. to 200° C., particularly preferably from 160° C. to 180° C.
In a further embodiment of the invention, the reaction product from step a) is filtered between step a) and step b).
The at least partial separation of the catalyst and/or monomeric isocyanate in step b) aims to achieve the removal of the majority of these substances. Taking process efficiency into consideration, it is accepted here that the monomeric isocyanate is not completely separated off. The separation can be effected by distillation or extraction, with preference being given to distillation. Any solvent possibly used in the carbodiimidization may also be separated simply in the distillation. The distillation in step b) is preferably effected at temperatures of from 140° C. to 200° C., particularly preferably at 160° C.-180° C. It is generally conducted under reduced pressure at a pressure of from 0.1 to 50 mbar, preferably 1-30 mbar, particularly preferably 10-20 mbar. In a preferred embodiment, the distillation is effected batchwise in a stirred reactor.
The addition and/or reaction of the alcohol in step c) is preferably effected at temperatures of from 140° C. to 200° C., particularly preferably from 160° C. to 180° C. Particularly preferably, both the addition and the reaction of the alcohol in step c) take place at temperatures in the range from 140° C. to 200° C., most preferably from 160° C. to 180° C.
Typically, in step c), based on the amount of carbodiimide present, 0.1%-5% by weight, preferably 0.2%-2% by weight, particularly preferably 0.3%-0.5% by weight, of alcohol(s) is added.
Preferably, the alcohols used are aliphatic and/or aromatic alcohols, preferably linear, branched and/or cyclic aliphatic C6-C18 alcohols, particularly preferably n-octanol, isooctanol, dodecanol, 2-ethylhexanol, oleyl alcohol, stearyl alcohol and/or cyclohexanol.
Step c) may be followed by a distillation in order to remove any excess of alcohol optionally present.
The carbodiimides obtained by the process according to the invention typically have a content of monomeric isocyanate of less than 1000 ppm, preferably less than 750 ppm, particularly preferably less than 500 ppm, more preferably less than 300 ppm and most preferably less than 100 ppm.
The examples which follow serve to elucidate the invention but have no limiting effect.
The residual isocyanate monomer content was determined by means of HPLC after reaction with a reagent solution (1-pyridyl-2-piperazine in 100 ml of THF). The eluent used was a mixture of ammonium acetate solution and methanol. Calibration was performed beforehand with various concentrations of the monomeric isocyanate.
Example carbodiimide: Carbodiimide of the formula (II) with R7=triisopropylphenylene, R1, R2, R3, R4, R5 and R6=isopropyl and n=approx. 18
A mixture of 1,3,5-triisopropylphenyl diisocyanate (TRIDI) and 2,4,6-triisopropylphenyl isocyanate (TRIPI) was carbodiimidized in the presence of approx. 0.2% of methylphospholene oxide at 160° C., until an NCO content of <1% was achieved. The reaction product was distilled for 2 h at 160° C. and 10 mbar.
The carbodiimide from Example 1 was distilled in a stainless steel tank for 5 h over a short distillation bridge. The temperatures in the distillation tank were in a range from 160° C. to 180° C., the final pressure was approx. 10 mbar.
The carbodiimide from Example 1 was distilled by means of a thin-film/short-path evaporator combination at approx. 180° C. and 1 mbar.
The carbodiimide from Example 1 was dissolved in toluene at 60° C. in a stainless steel tank and then recrystallized from acetone at 10-20° C. After filtration, the pulverulent carbodiimide was dried for several hours in a dryer at a reduced pressure of 10 mbar.
To the carbodiimide from Example 1 was added in a stainless steel tank 0.5% 2-ethylhexanol and the mixture was stirred at 160° C. for approx. 30 min.
To the carbodiimide from Example 1 was added in a stainless steel tank 2.0% 2-ethylhexanol and the mixture was stirred at 160° C. for approx. 30 min.
The residual content of monomeric isocyanate was determined by means of HPLC. The colour was determined according to the CIE L*a*b* method ISO 11664-4. The b value was evaluated.
The results are listed in Table 1 below.
As can be seen from Table 1, the residual isocyanate monomer content cannot be achieved in the simple batch distillation method even after very long distillation times. Moreover, this results in a marked deterioration in the product colour. The desired quality characteristics are first achieved by means of recrystallization or by means of a thin-film/short-path evaporator combination; however, these methods are complex and are associated with losses in yield. The simple reaction with small amounts of alcohol (here 2-ethylhexanol) leads to the desired results while avoiding the disadvantages of the known processes.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21169385.8 | Apr 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/059638 | 4/11/2022 | WO |
