CONTINUOUS DILUTION OF POLYISOCYANATES

Description

[0001] The present invention relates to a process for preparing a polyisocyanate composition from toluene diisocyanate, and to the polyisocyanate compositions obtainable therefrom. The invention further relates to the use of a continuous dilution process.

[0002] Polyisocyanates containing urethane groups, made from polyhydroxy compounds and toluene diisocyanate, have been known for a long time and are described, for example, in DE 870 400, DE 953 012 and DE 1 090 196.

[0003] Isocyanurates of toluene diisocyanate are produced by cyclotrimerization using various catalysts. Such reaction products have also been known for a long time and are described, for example, in DE 951168 B, DE 1013869 A, US 6,936,678 B2, DE 19523657 A1, US 4255569 A, EP 2 174 967 B1 and CN 105001701.

[0004] US 5 086 175 A discloses polyisocyanate compositions with a concentration of 75%, containing a polyisocyanate and a solvent, and free from turbidity.

[0005] US 4 419 513 A discloses polyisocyanates from mixtures of hexamethylene diisocyanate and 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane, which, in order to remove excess monomer, are subjected to distillation after preparation and then diluted with a solvent.

[0006] For a long time, there has been a desire to synthesize the known isocyanurates of toluene diisocyanate in a low-viscosity yet highly functional form. Toluene diisocyanate will also be referred to as TDI in the following.

[0007] Low viscosity is desirable, for example, to improve the application properties of paints and adhesives. Furthermore, using low-viscosity polyisocyanates as crosslinkers in paints and adhesives allows for a reduction in the solvent content of the formulation. This means that emissions of volatile organic compounds from such formulations can be reduced without negatively impacting their usability.

[0008] Furthermore, when using such polyisocyanates as crosslinkers in paints and adhesives, it is desirable for the polyisocyanates to have a high isocyanate group content. This further increases sustainability in terms of low organic solvent content and rapid crosslinking, thus ensuring high process efficiency.

[0009] Furthermore, it is desirable that the isocyanates of TDI contain a low level of free diisocyanate. Due to the toxicological concerns of monomeric TDI, this is an important condition for its universal applicability in industrially applied coatings and adhesives.

[0010] As known from DE 951168B and DE 1013869A, the conversion of TDI to polyisocyanurate yields highly viscous resins, which complicates processing or necessitates the use of larger quantities of organic solvents. Furthermore, TDI polyisocyanates exhibit a high tendency to crystallize and are only sparingly soluble in organic solvents.

[0011] Another important requirement is the solubility of oligomeric isocyanurates in common organic solvents. Incomplete solubility leads to cloudy solutions, which severely limits their use in paint or adhesive formulations.

[0012] The object of the present invention was therefore to provide a process for producing a polyisocyanate composition with which polyisocyanate compositions can be obtained that exhibit low viscosity while simultaneously having the highest possible content of isocyanate groups. Additionally, the polyisocyanate compositions should have the lowest possible color number.

[0013] This problem was solved by a process for producing a polyisocyanate composition as defined in claim 1.

[0014] The process according to the invention allows the production of polyisocyanate compositions that are more stable with respect to turbidity than those of the prior art and are therefore also suitable for applications where clear solutions are required. The range of applications is further improved by the fact that the polyisocyanate compositions have a color number of < 100 hazen, preferably < 95 hazen. The color number in hazen is determined according to DIN EN 1557:1997-03.

[0015] Thus, a process for producing a polyisocyanate composition having a color number of < 100 hazen, preferably < 95 hazen, as determined according to DIN EN 1557:1997-03, comprising the addition of at least one solvent inert to isocyanate groups to at least one polyisocyanate, characterized in that the addition of the solvent takes place in one or more stages and at least one of these stages is carried out as a continuous dilution, is also an advantageous embodiment of the present invention.

[0016] As disclosed, but not within the scope of the claims, the polyisocyanate is a polyisocyanate based on at least one aliphatic, cycloaliphatic, araliphatic, or aromatic diisocyanate other than toluene diisocyanate (TDI), preferably an araliphatic or aromatic diisocyanate other than toluene diisocyanate (TDI). The term from a diisocyanate is synonymous with based on such a diisocyanate.

[0017] Suitable aliphatic, cycloaliphatic, araliphatic or aromatic diisocyanates as disclosed, but not within the scope of protection of the claim, are selected, for example, from the group consisting of 1,4-diisocyanatobutane, 1,5-diisocyanatopentane (PDI), 1,6-diisocyanatohexane (HDI), 2-methyl-1,5-diisocyanatopentane, 1,5-diisocyanato-2,2-dimethylpentane, 2,2,4- or 1,4-diisocyanato-2,2-dimethylpentane, 2,2,4-diisocyanato-2,2-diisocyanato-2,2-diisocyanato-2,2-diisocyanato-2,2-diisocyanato-2,2,4 ...2,4,4-Trimethyl-1,6-diisocyanatohexane, 1,10-Diisocyanatodecane, 1,3- and 1,4-Diisocyanatocyclohexane, 2,4- and 2,6-Diisocyanato-1-methylcyclohexane, 1,3- and 1,4-Bis-(isocyanatomethyl)cyclohexane, 1-Isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (isophorone diisocyanate, IPDI), 4,4'-Diisocyanatodicyclohexylmethane, 2,4'-Diisocyanatodicyclohexylmethane, 1-Isocyanato-1-methyl-4(3)isocyanatomethylcyclohexane, Bis-(isocyanatomethyl)norbornane, 1,3- and 1,4-Bis(isocyanatomethyl)benzene (XDI), 1,3- and 1,4-Bis-(2-isocyanato-prop-2-yl)benzene (TMXDI), 2,4'- and 4,4'-Diisocyanatodiphenylmethane (MDI), 1,5-Diisocyanatonaphthalene, 1,3- and 1,4-Phenylene diisocyanate, or any mixtures of such diisocyanates.

[0018] Preferred diisocyanates according to the disclosure, but not within the scope of protection of the claim, are 1,5-diisocyanatopentane (PDI), 1,6-diisocyanatohexane (HDI), 1,3- and 1,4-diisocyanatocyclohexane, 2,4- and 2,6-diisocyanato-1-methylcyclohexane, 1,3- and 1,4-bis-(isocyanatomethyl)cyclohexane, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (isophorone diisocyanate, IPDI), 4,4'-diisocyanatodicyclohexylmethane, 2,4'-diisocyanatodicyclohexylmethane, 1,3- and 1,4-bis(isocyanatomethyl)benzene (XDI), 2,4'- and 4,4'-diisocyanatodiphenylmethane (MDI) or 1,5-diisocyanatonaphthalene.

[0019] In particular, the present invention was based on the specific objective of providing a process for producing a polyisocyanate composition from toluene diisocyanate, with which polyisocyanate compositions from toluene diisocyanate can be obtained that exhibit low viscosity with the highest possible content of isocyanate groups and are also available as turbidity-stable solutions. Additionally, the polyisocyanate compositions should have the lowest possible color number.

[0020] This particular problem was solved by a method as defined in claim 1.

[0021] Thus, the invention comprises a polyisocyanate based on toluene diisocyanate. The term "from toluene diisocyanate" is synonymous with "based on toluene diisocyanate".

[0022] An advantageous embodiment of the present invention is also a process for producing a polyisocyanate composition from toluene diisocyanate, which has a color number of < 100 hazen, preferably < 95 hazen, as determined according to DIN EN 1557:1997-03, comprising the addition of at least one solvent inert with respect to isocyanate groups to at least one polyisocyanate from toluene diisocyanate, characterized in that the addition of the solvent takes place in one or more stages and at least one of these stages is carried out as a continuous dilution.

[0023] In this context, toluene diisocyanate is used as a general term for the isomers 2,4-toluene diisocyanate, 2,6-toluene diisocyanate and any mixtures of 2,4- and 2,6-toluene diisocyanate.

[0024] According to the invention, references to "comprising", "containing", etc. preferably mean "essentially consisting of" and most preferably "consisting of".

[0025] In this context, "continuous dilution" means that at least two volume streams, preferably exactly two volume streams, are mixed together in such a way that the dilution occurs essentially without a concentration gradient. "Essentially without a concentration gradient" means that the solids content in the outflowing product stream can vary between ≤ 10% above and ≤ 10% below, preferably between ≤ 5% above and ≤ 5% below, and particularly preferably between ≤ 2% above and ≤ 2% below the desired solids content of the diluted product stream.

[0026] Suitable devices for continuous dilution include, for example, T-pieces, two-line systems with static mixers, and kettles or intermediate dissolving kettles. In this context, a kettle or intermediate dissolving kettle is understood to be a container into which at least two volume streams are continuously fed into a stirred volume, from which the diluted product stream flows accordingly. This product stream represents the polyisocyanate composition according to the invention, wherein, in the case of several dilution stages, the product stream obtained after the last dilution stage represents the polyisocyanate composition according to the invention.

[0027] Preferably, the continuous dilution is achieved by continuously adding at least two liquid streams, particularly preferably exactly two liquid streams, to a stirred volume, from which the diluted product stream preferably flows continuously.

[0028] Such a stirred volume can, for example, be a vessel as described above. These liquid streams typically consist of, on the one hand, the polyisocyanate to be dissolved from TDI (polyisocyanate stream) and, on the other hand, at least one solvent inert to isocyanate groups (solvent stream).

[0029] In contrast to known batch dilution, the disadvantages associated with mixing can be largely avoided in the process according to the invention, since the desired concentration is always present directly. Furthermore, the residence time can be kept as short as possible, which has a beneficial effect on the stability of the products.

[0030] Suitable solvents in polyurethane chemistry include diluents and solvents commonly used in the process, such as toluene, xylene, cyclohexane, butyl acetate, ethyl acetate, ethyl glycol acetate, pentyl acetate, hexyl acetate, methoxypropyl acetate, tetrahydrofuran, dioxane, acetone, N-methylpyrrolidone, methyl ethyl ketone, white spirit, highly substituted aromatics (e.g., Solvent Naphtha®, Solvesso®, Shellsol®, Isopar®, Nappar® and Diasol®), heavy benzene, tetralin, dekalin and alkanes with more than 6 carbon atoms, common plasticizers such as phthalates, sulfonic acid esters and phosphoric acid esters, and mixtures of such diluents and solvents.

[0031] Furthermore, polyisocyanates based on aliphatic diisocyanates, such as those described in DE-A 4 428 107, are also suitable as solvents. This allows access to dilute, low-monomer TDI trimerizates that contain no or less readily evaporating solvents and diluents.

[0032] In a further preferred embodiment of the process according to the invention, the solvent is added in at least two stages, the first stage being carried out as a continuous dilution. The second stage, which must be carried out at least once, can be carried out continuously or discontinuously. This has the advantage that the turbidity stability of the polyisocyanate composition according to the invention is further increased. In addition, the color number is also further reduced.

[0033] It is also possible to carry out a third, fourth, fifth, or multiple stage, whereby the required process engineering effort must be weighed against the potential gain in further turbidity stability. It has been found that in most cases, a two-stage addition provides the optimal balance between process engineering effort and the potential gain in further turbidity stability.

[0034] In a multi-stage or multi-stage addition of at least one solvent inert to isocyanate groups, different dilution levels (solid contents) are achieved in the sequential stages, which take place in various devices such as intermediate dissolving vessels. These dilution levels are also referred to as solid contents in the following. Here, solid content is understood to mean the weight fraction of the polyisocyanate in the polyisocyanate composition.

[0035] In a further preferred embodiment of the method according to the invention, a solids content of ≥ 30 to ≤ 90 wt.%, preferably ≥ 50 to ≤ 85 wt.%, particularly preferably ≥ 55 to ≤ 75 wt.% and most preferably ≥ 60 to ≤ 70 wt.% is set in the first stage.

[0036] In this at least two-stage addition process, it is further preferred that in the second stage a solids content of ≥ 10 to ≤ 80 wt.%, preferably ≥ 15 to ≤ 65 wt.%, particularly preferably ≥ 20 to ≤ 50 wt.%, and most preferably ≥ 25 to ≤ 35 wt.%, is achieved, wherein the solids content achieved in the first stage is reduced in the second stage by at least 15 wt.%, preferably by at least 25 wt.%. This offers the advantage that, via the at least two-stage addition, solids contents can be achieved that are as broadly suitable as possible for subsequent applications of the polyisocyanate composition obtainable according to the inventive process and are nevertheless turbidity-stable.

[0037] According to the invention, the polyisocyanate is produced from toluene diisocyanate by (i) reaction of toluene diisocyanate to form a polyisocyanate and (ii) removal of the unreacted toluene diisocyanate.

[0038] Within the scope of the present invention, the "removal of the unreacted toluene diisocyanate" is to be understood as substantially complete. "Substantially complete" here means that residual contents of monomeric toluene diisocyanate of ≤ 0.5 wt.%, preferably ≤ 0.3 wt.%, and particularly preferably ≤ 0.1 wt.%, based on the total weight of the polyisocyanate from toluene diisocyanate, are obtained.

[0039] The removal of the unreacted toluene diisocyanate in step (ii) can be carried out by any method. However, it is preferred that the removal of the unreacted toluene diisocyanate in step (ii) be carried out by at least one thermal separation process, which may have one or more stages, preferably by at least one two-stage thermal separation process, and particularly preferably by at least one downdraft evaporator and / or at least one thin-film evaporator. This has the advantage that sufficient separation of the unreacted toluene diisocyanate can be achieved even at higher throughputs.

[0040] Suitable thermal separation methods include vacuum distillation using thin-film evaporators and / or downtube evaporators. Pressures in the range of 0.1–20 mbar and temperatures of 120–250 °C are generally suitable for the removal of TDI.

[0041] The thermal separation process is preferably carried out at a heating medium temperature of ≥ 140 to ≤ 235 °C and more preferably at ≥ 160 to ≤ 215 °C. This offers the advantage that the removal of the unreacted toluene diisocyanate is gentle yet efficient. Depending on the process engineering effort, contents of monomeric toluene diisocyanate of ≤ 0.5 wt%, preferably ≤ 0.3 wt%, and particularly preferably ≤ 0.1 wt%, based on the total weight of the polyisocyanate from toluene diisocyanate, can be achieved. The lower these contents, the wider the range of applications for the polyisocyanate composition according to the invention, as workplace hygiene, especially in manual applications, is further improved. The contents of unreacted toluene diisocyanate can be determined by gas chromatography according to DIN EN ISO 10283:2007-11 with an internal standard.

[0042] If, in addition to toluene diisocyanate, other diisocyanates from the above-mentioned list are used, the residual contents and separation possibilities mentioned above refer to the total residual contents of all monomeric diisocyanates present, whereby the person skilled in the art may, for example, make minor adjustments to the process parameters in order to tailor the separation to the respective monomeric diisocyanate to be removed.

[0043] In a further preferred embodiment, a mixture of 2,4- and 2,6-toluene diisocyanate is used in step (i) of the process, which consists of ≥ 50 to ≤ 99 wt.%, preferably ≥ 70 to ≤ 95 wt.%, and particularly preferably ≥ 75 to ≤ 90 wt.%, based on the total weight of the toluene diisocyanate used, of 2,4-toluene diisocyanate. This offers the further advantage of achieving a balanced equilibrium between the selectivity of the differently reactive isocyanate groups in the 2,4-TDI and the increase in crystallization stability through at least a small proportion of 2,6-TDI.

[0044] Both 2,4- and 2,6-toluened diisocyanate, as well as mixtures thereof, are generally commercially available. They can be prepared by known methods, such as phosgenation of the corresponding toluenediamine (TDA) in the liquid or gas phase.

[0045] Toluene diisocyanates produced by gas-phase phosgenation of TDA are particularly preferred, as this method is especially efficient.

[0046] In a further embodiment, the polyisocyanate formed in step (i) of the process is a urethane-containing polyisocyanate made from toluene diisocyanate. This is preferably prepared by reacting polyhydroxy compounds with 5 to 10 times the molar amount of TDI. Suitable low-molecular-weight polyhydroxy compounds are dihydric to tetrahydric alcohols with a molecular weight of 62 to 146 and / or polyether polyols produced from them by the addition of ethylene and / or propylene oxide, in pure form or as any mixtures.

[0047] Examples of dihydric to tetrahydric alcohols include ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, neopentyl glycol, 1,6-hexanediol, 2-ethylhexanediol, glycerol, trimethylolpropane and pentaerythritol.

[0048] Suitable polyether polyols have a molecular weight of 10⁶ to 600, preferably 10⁶ to 470, which can be calculated from the hydroxyl group content and hydroxyl functionality. Polyether diols and polyether triols are preferably used. These polyether polyols can be obtained in a known manner by alkoxylation of suitable two- to four-functional starter molecules or suitable mixtures of starter molecules, wherein, in the alkoxylation, propylene oxide and / or ethylene oxide, optionally in a mixture, are used successively in any desired order. The aforementioned dihydric to tetrahydric alcohols are preferably used as starter molecules. Mixtures of trimethylolpropane and diethylene glycol are particularly preferred.

[0049] In a further preferred embodiment, the polyisocyanate of the process according to the invention is a polyisocyanurate of toluene diisocyanate having isocyanate groups, wherein the reaction of toluene diisocyanate in step (i) to form isocyanurate groups takes place in the presence of at least one catalyst and is stopped at an isocyanate group content of ≥ 30 to ≤ 46 wt.%, preferably of ≥ 34 to ≤ 44 wt.% and particularly preferably of ≥ 38 to ≤ 42 wt.% by adding at least one catalyst poison.

[0050] In this embodiment, steps (i) and (ii) are preferably carried out in the presence of ≥ 0 to < 1 wt.% liquid distillation aids boiling at least 50 °C above the toluene diisocyanate, which are inert under distillation conditions, and / or ≥ 0 to < 1 wt.%, based on the total weight of the compounds used in step (i), on compounds having one or more hydroxyl groups.

[0051] It is particularly preferred that such distillation aids are present in amounts of ≥ 0 to ≤ 0.5 wt.%, preferably ≥ 0 to ≤ 0.25 wt.%, and particularly preferably ≥ 0 to ≤ 0.1 wt.%, based on the total weight of the compounds used in step (i), and / or that the compounds having one or more hydroxyl groups are present in amounts of ≥ 0 to ≤ 0.8 wt.%, preferably ≥ 0 to ≤ 0.5 wt.%, and particularly preferably ≥ 0 to ≤ 0.1 wt.%, based on the total weight of the compounds used in step (i). Any distillation aids and / or any compounds having one or more hydroxyl groups that may be present in these amounts have no negative influence on the process according to the invention.However, it is particularly preferred if no distillation aids and / or no compounds having one or more hydroxyl groups are present in steps (i) and (ii) of the process according to the invention, with the exception of the aromatic hydroxyl groups that may be present and which are subsequently referred to as catalyst components.

[0052] In the event that solvents inert to isocyanate groups are present in steps (i) to (ii) of the process according to the invention, it is preferred that such solvents may be present in steps (i) to (ii) in ≥ 0 to ≤ 3 wt.%, preferably ≥ 0 to ≤ 1 wt.% and particularly preferably ≥ 0 to ≤ 0.05 wt.%, based on the total weight of the compounds used in step (i).

[0053] As catalysts for the formation of isocyanurate groups, hereinafter also referred to as trimerization catalysts, all known catalysts of the prior art are suitable, such as phosphines, alkali salts, alkali alkoxides, tertiary amines, fluorides, hydrogen difluorides, or hydrogen polyfluorides. Catalysts are preferably used that have N,N-dialkylaminomethyl groups bonded to aromatics and phenolic OH groups (alkyl: regardless of whether it is an alkyl chain or an alkylene chain with up to 18 carbon atoms, optionally separated by oxygen or sulfur). These groups can be distributed over several molecules or positioned on one or more benzene aromatics. Catalysts containing both hydroxyl and dialkylaminomethyl groups in a single molecule are particularly preferred.Catalysts whose dialkylaminomethyl groups (alkyl = C1 to C3 chain) are positioned in the ortho position to aromatic hydroxyl groups are particularly preferred. Examples include the following Mannich bases, such as those obtained from phenol, p-isononylphenol, or bisphenol A, for instance, by reacting 188 parts by weight of phenol with 720 parts of a 25% aqueous dimethylamine solution and 425 parts by weight of a 40% formaldehyde solution by heating to 80°C for two hours, separating the aqueous phases, and distilling the organic phase at 90°C / 1.33 kPa (10 Torr) according to DE-A 2 452 531 9. The reaction in step (i) generally takes place at temperatures between 20 and 120°C, preferably between 40 and 100°C, and particularly preferably between 60 and 90°C.

[0054] The catalysts are used as pure substances or in solution, optionally in several small portions, in step (i), the amount of which can be varied over a wide range. Preferably, the total amount of catalyst used is ≥ 0.001 to ≤ 2.0 wt.%, more preferably ≥ 0.003 to ≤ 0.5 wt.%, and most preferably ≥ 0.005 to ≤ 0.05 wt.%, based on the total weight of the compounds used in steps (i) and (ii).

[0055] The reaction in step (i) is stopped by adding at least one catalyst poison, wherein catalyst poisons can be, for example, sulfur (in the case of using phosphines as catalysts) or alkylating agents such as methyl toluenesulfonate (in the preferred use of Mannich bases as catalysts) or acylating agents such as benzoyl chloride.

[0056] The amount of catalyst poison used is selected according to the amount of catalyst used, so that the catalyst is deactivated. Preferably, a total sub-equimolar amount of catalyst poison is used, based on the equivalents of Lewis bases in the catalysts; however, amounts as low as > 20% to < 100%, based on the equivalents of Lewis bases in the catalyst used, may also be sufficient for complete deactivation of the catalyst.

[0057] Regardless of which polyisocyanate from TDI is used in the process according to the invention, in a further preferred embodiment, after adding the at least one solvent inert with respect to isocyanate groups to the at least one polyisocyanate, preferably from toluene diisocyanate, in a further step at least one polyisocyanate composition from toluene diisocyanate different from the polyisocyanate of the preceding embodiments is added, preferably at least one polyisocyanurate composition from toluene diisocyanate and / or at least one polyurethane composition from toluene diisocyanate, and optionally one or more auxiliary and additive substances.This polyisocyanurate composition from toluene diisocyanate and this polyurethane composition from toluene diisocyanate can be produced according to known methods, but also according to the methods described above for the production of the polyisocyanurate from toluene diisocyanate having isocyanate groups and the polyisocyanate from toluene diisocyanate having urethane groups.

[0058] This results in the further advantage that the physical and chemical properties of mixtures containing at least one polyisocyanate according to the invention can be specifically adjusted.

[0059] Suitable auxiliary and additive substances include, for example, the usual wetting agents, leveling agents, skin-preventing agents, antifoaming agents, solvents, matting agents such as silica, aluminum silicates and high-boiling waxes, viscosity-regulating substances, pigments, dyes, UV absorbers, and stabilizers against thermal or oxidative degradation.

[0060] A polyisocyanate composition, produced or producible according to the process of the invention, is disclosed. Surprisingly, it has been found that such a process leads to compositions with stable turbidity, whereas the addition of solvent, in which at least one step is not carried out as continuous dilution, leads to turbid compositions.

[0061] In the subject matter of this disclosure, this applies particularly to polyisocyanate compositions in which the polyisocyanate is based on 1,5-diisocyanatopentane (PDI), 2-methyl-1,5-diisocyanatopentane, 1,5-diisocyanato-2,2-dimethylpentane, 1,10-diisocyanatodecane, 2,4- and 2,6-diisocyanato-1-methylcyclohexane, 2,4'-diisocyanatodicyclohexylmethane, 1-isocyanato-1-methyl-4(3)isocyanato-methylcyclohexane, bis-(isocyanatomethyl)norbornane, 1,3- and 1,4-bis(isocyanatomethyl)benzene (XDI), 1,3- and 1,4-bis-(2-isocyanato-prop-2-yl)benzene (TMXDI), toluene diisocyanate (TDI), 2,4'- and 4,4'-diisocyanatodiphenylmethane (MDI), 1,5-Diisocyanatonaphthalene or 1,3- and 1,4-phenylene diisocyanate or mixtures of the aforementioned diisocyanates.Particularly preferred is the polyisocyanate based on 1,5-diisocyanatopentane (PDI), 2-methyl-1,5-diisocyanatopentane, 1,5-diisocyanato-2,2-dimethylpentane, 1,10-diisocyanatodecane, 2,4- and 2,6-diisocyanato-1-methylcyclohexane, 2,4'-diisocyanatodicyclohexylmethane, 1-isocyanato-1-methyl-4(3)isocyanato-methylcyclohexane, bis-(isocyanatomethyl)norbornane, 1,3- and 1,4-bis(isocyanatomethyl)benzene (XDI), 1,3- and 1,4-bis-(2-isocyanato-prop-2-yl)benzene (TMXDI), toluene diisocyanate (TDI), 2,4'- and 4,4'-diisocyanatodiphenylmethane (MDI), 1,5-diisocyanatonaphthalene, or 1,3- and 1,4-Phenylene diisocyanate is and especially preferably from toluene diisocyanate (TDI).

[0062] In addition to their advantageous turbidity stability, the disclosed polyisocyanate compositions exhibit a very low color number of < 100 hazen, preferably < 95 hazen. The color number in hazen is determined according to DIN EN 1557:1997-03.

[0063] Another aspect of the invention is the use of continuous dilution in the solution of polyisocyanates from toluene diisocyanate to prevent turbidity in the polyisocyanate composition.

[0064] The polyisocyanate composition according to the invention is very well suited for use as a crosslinking agent in an adhesive or in a coating agent, preferably in an adhesive.

[0065] The polyisocyanate compositions that can be produced by the process according to the invention are preferably used for the production of adhesives or coating materials that can be cured under the influence of atmospheric humidity. They can also be used for the production of adhesion promoters, printing inks, and polyurethane molded parts. They are particularly preferably used as crosslinkers in two-component systems with compounds known per se that are reactive towards isocyanate groups.

[0066] Another subject matter of the disclosure, but not falling within the scope of the claims, is therefore a two-component system comprising an isocyanate component A), containing at least one polyisocyanate composition according to the disclosure, and an isocyanate group-reactive component B), containing at least one isocyanate group-reactive compound, preferably at least one polyester having hydroxyl groups.

[0067] Suitable compounds reactive towards isocyanate groups include, for example, hydroxy-functional polyethers, esters, amides, carbonates, acrylates, butadienes, or mixtures of the aforementioned hydroxy-functional polymers. Low-molecular-weight di- and polyols, di- and trimer fatty alcohols, and amino-functional compounds can also be used in the two-component system according to the invention. Cyclohexanone-formaldehyde condensates, for example in castor oil, are also suitable. However, polyesters containing hydroxyl groups are particularly preferred. Additionally, other auxiliary and additive substances, such as conventional wetting agents, leveling agents, skin-preventing agents, antifoaming agents, adhesion promoters, solvents, matting agents such as silica, aluminum silicates, and high-boiling waxes, viscosity-regulating substances, pigments, dyes, UV absorbers, and thermal stabilizers, can be included in the coatings or adhesives.Oxidative degradation is used. The coating agents can be used in the form of clear lacquers as well as in the form of pigmented lacquers.

[0068] The resulting coating materials or adhesives can be used for coating or bonding any substrates, such as natural or synthetic fibers, preferably wood, plastics, leather, paper, textiles, glass, ceramics, plaster, masonry, metals or concrete, and particularly preferably paper or leather. They can be applied using conventional application methods such as spraying, brushing, flooding, pouring, dipping, and rolling.

[0069] Therefore, a composite system, produced or producible by curing the two-component system according to the invention applied to at least one substrate, is a further subject of disclosure, even if it does not fall within the scope of protection of the claims.

[0070] The invention will be explained in more detail below using examples and comparative examples. Examples

[0071] Unless otherwise stated, all percentages refer to weight.

[0072] The NCO content was determined titrimetrically according to DIN EN ISO 11909:2007-05.

[0073] The residual monomer contents were measured by gas chromatography with an internal standard according to DIN EN ISO 10283:2007-11.

[0074] The turbidity was measured according to DIN EN ISO 7027-1:2016. Example 1 (not according to the invention):

[0075] 1500 g of a toluene diisocyanate isomer mixture consisting of approximately 80% 2,4-toluene diisocyanate and 20% 2,6-toluene diisocyanate are placed in a 2 L flask at 80°C. Then, 0.52 g of a Mannich base (bisphenol / formaldehyde / dimethylamine 25% in butyl acetate / xylene 19:56) is added over 2 hours with stirring, maintaining a temperature of 78–82°C. When an NCO content of 40.4% is reached, 1 g of dibutyl phosphate is added to stop the reaction. The excess diisocyanate is then continuously removed from the resulting crude product by distillation at a temperature of 180°C and a pressure of 0.05 mbar. The resulting hot resin (370 g) is pumped directly into a stirred flask equipped with a reflux condenser, where it is diluted to 863 g of boiling ethyl acetate. This corresponds to a single-stage, batch dilution from 100% solids content to 30% solids content. A solution with the following properties is obtained: NCO: 7.1% Monomer content: 0.08% Turbidity: 21 NTU Example 2 (according to the invention):

[0076] 1500 g of a toluene diisocyanate isomer mixture consisting of approximately 80% 2,4-toluene diisocyanate and 20% 2,6-toluene diisocyanate are placed in a 2 L flask at 80°C. Then, 0.52 g of a Mannich base (bisphenol / formaldehyde / dimethylamine 25% in butyl acetate / xylene 19:56) is added over 2 hours with stirring, maintaining a temperature of 78–82°C. When an NCO content of 40.4% is reached, 1 g of dibutyl phosphate is added to stop the reaction. The excess diisocyanate is then continuously removed from the resulting crude product by distillation at a temperature of 180°C and a pressure of 0.05 mbar. The hot resin resulting from the distillation is pumped at a rate of 105 g / h directly into a stirred 500ml 4-neck flask equipped with a reflux condenser.Simultaneously, 245 g / h of ethyl acetate is added from a dropping funnel, and the continuously diluted product is transferred via a submerged riser tube by a pump at a rate of 350 g / h into a cooled receiving flask. The contents of the four-necked flask remain constant at approximately 400 g and have a temperature of approximately 80°C. A solution with a solids content of 30% and the following properties is obtained: NCO: 7.0% Monomer content: 0.03% Turbidity: 12 NTU Example 3 (according to the invention):

[0077] 1500 g of a toluene diisocyanate isomer mixture consisting of approximately 80% 2,4-toluene diisocyanate and 20% 2,6-toluene diisocyanate are placed in a 2 L flask at 80°C. Then, 0.52 g of a Mannich base (bisphenol / formaldehyde / dimethylamine 25% in butyl acetate / xylene 19:56) is added over 2 hours with stirring, maintaining a temperature of 78–82°C. When an NCO content of 40.4% is reached, 1 g of dibutyl phosphate is added to stop the reaction. The excess diisocyanate is then continuously removed from the resulting crude product by distillation at a temperature of 180°C and a pressure of 0.05 mbar. The hot resin resulting from the distillation is pumped at a rate of 105 g / h directly into a stirred 500ml 4-neck flask equipped with a reflux condenser.Simultaneously, 57 g / h of ethyl acetate is added from a dropping funnel, and the continuously diluted product is transferred via a submerged riser tube by a pump at a rate of 162 g / h into a cooled receiving flask. The contents of the four-necked flask remain constant at approximately 400 g and are at a temperature of approximately 80°C.

[0078] The contents of the receiving flask are then diluted to 30% solids in a discontinuous step using ethyl acetate. A solution with the following properties is obtained: NCO: 7.0% Monomer content: 0.05% Turbidity: 2 NTU

Claims

1. Process for producing a polyisocyanate composition comprising addition of at least one isocyanate-inert solvent to at least one polyisocyanate, characterized in that the addition of the solvent is carried out in one or more stages and at least one of these stages is performed as a continuous dilution and wherein the polyisocyanate is based on tolylene diisocyanate and is produced by (i) reaction of tolylene diisocyanate to form a polyisocyanate and (ii) removal of the unconverted tolylene diisocyanate down to a residual content of monomeric tolylene diisocyanate of ≤ 0.5% by weight based on the total weight of the polyisocyanate composed of tolylene diisocyanate.

2. Process according to Claim 1, characterized in that the addition of the solvent is carried out in at least two stages, wherein the first stage is performed as a continuous dilution.

3. Process according to Claim 1 or 2, characterized in that a solids content of ≥ 30% to ≤ 90% by weight, preferably ≥ 50% to ≤ 85% by weight, particularly preferably ≥ 55% to ≤ 75% by weight and very particularly preferably ≥ 60% to ≤ 70% by weight is established in the first stage.

4. Process according to Claim 2 or 3, characterized in that a solids content of ≥ 10% to ≤ 80% by weight, preferably ≥ 15% to ≤ 65% by weight, particularly preferably ≥ 20% to ≤ 50% by weight and very particularly preferably ≥ 25% to ≤ 35% by weight is established in the second stage, wherein the solids content established in the first stage is reduced by at least 15% by weight, preferably by at least 25% by weight, in the second stage.

5. Process according to any of the preceding claims, characterized in that the removal of the unconverted tolylene diisocyanate in step (ii) is carried out down to a residual content of monomeric tolylene diisocyanate of ≤ 0.3% by weight and particularly preferably ≤ 0.1% by weight based on the total weight of the polyisocyanate composed of tolylene diisocyanate.

6. Process according to any of the preceding claims, characterized in that the removal of the unconverted tolylene diisocyanate in step (ii) is carried out by means of at least one thermal separation process, preferably by means of at least one two-stage thermal separation process and particularly preferably by means of at least one falling film evaporator and / or at least one thin film evaporator.

7. Process according to Claim 6, characterized in that the at least one thermal separation process is performed at a heating medium temperature of ≥ 140°C to ≤ 235°C and preferably of ≥ 160°C to ≤ 215°C.

8. Process according to any of the preceding claims, characterized in that the polyisocyanate composed of tolylene diisocyanate is an isocyanate-containing polyisocyanurate composed of tolylene diisocyanate, wherein the reaction of tolylene diisocyanate in step (i) to form isocyanurate groups is carried out in the presence of at least one catalyst and is terminated at a content of isocyanate groups of ≥ 30% to ≤ 46% by weight, preferably of ≥ 34% to ≤ 44% by weight and particularly preferably of ≥ 38% to ≤ 42% by weight by addition of at least one catalyst poison.

9. Process according to any of the preceding claims, characterized in that the steps (i) and (ii) are performed in the presence of ≥ 0% to < 1% by weight of distillation aids that are inert under distillation conditions, liquid and have a boiling point at least 50°C higher than that of tolylene diisocyanate and / or ≥ 0% to < 1% by weight based on the total weight of the compounds employed in step (i) and (ii) of compounds comprising one or more hydroxyl groups.

10. Process according to any of the preceding claims, characterized in that addition of the at least one isocyanate-inert solvent to the at least one polyisocyanate is followed in a further step by addition of at least one polyisocyanate composition composed of tolylene diisocyanate which is distinct from the polyisocyanate of the preceding claims, preferably at least one polyisocyanurate composition composed of tolylene diisocyanate and / or at least one polyurethane composition composed of tolylene diisocyanate, and optionally by addition of one or more assistant and additive substances.

11. Use of continuous dilution in the dissolution of polyisocyanates composed of tolylene diisocyanate for preventing cloudiness in the polyisocyanate composition.