Distillation of polyisocyanates

Abstract

The present invention comprises: (i) modifying at least one monomeric diisocyanate to obtain a mixture containing at least one polyisocyanate and an unconverted monomeric diisocyanate; (ii) separating the mixture obtained in step (i) into at least one gaseous stream containing the monomeric diisocyanate and a liquid stream depleted of the monomeric diisocyanate; (iii) partially condensing the gaseous stream from (ii) in at least one condenser to obtain a liquid condensate and an uncondensed vapor stream; (iv) post-condensing the uncondensed vapor stream obtained in step (iii) in at least one post-condenser to obtain a post-condensate and an uncondensed off-gas; and (v) supplying the uncondensed off-gas from step (iv) to the suction part of a vacuum pump, wherein at least one post-condenser in step (iv) is operated at a post-condenser temperature and at least one condenser in step (iii) is operated at a condenser temperature, wherein The present invention relates to a method for producing a polyisocyanate having a low monomer content, characterized in that the post-condenser temperature is ≥ 1 to ≤ 168K lower than the condenser temperature.

Description

Technology Field

[0001] The present invention relates to a method for producing low-monomer polyisocyanates and also to polyisocyanates obtainable thereby, their use as starting materials for the production of polymers, and composites containing polymers. Background Technology

[0002] The modification reactions of aliphatic and cycloaliphatic diisocyanates have long been known. The polyisocyanates obtained thereby are used as crosslinking agent components in coating systems and adhesives. Conventional modification reactions involve, on the one hand, the isocyanate reacting itself to form, for example, biuret, isocyanurate, uretdione, or iminooxadiazinedione. Alternatively, the isocyanate can react with polyols or polyamines to form oligomers, thereby forming urethane, allophanate, and / or urea groups. Crucially important is the formation of high-molecular-weight adducts having a lower vapor pressure than the monomeric diisocyanate itself. Unreacted diisocyanates are removed from the reaction mixture, for example by thin-film distillation, and the polyisocyanates are left as a byproduct, which can be diluted with a solvent if desired.

[0003] The catalytic production of polyisocyanates and the removal of unreacted diisocyanates from polyisocyanate mixtures by distillation are described in detail, for example, in WO2008 / 068198A1. Monomeric diisocyanates are preferably removed by multi-stage distillation at a temperature of 90 to 220°C under reduced pressure. The pressure is preferably lowered from one stage to the next, reaching 0.1 to 10 hPa at the final stage. The devices used are flash evaporators, falling film evaporators, thin film evaporators, and / or short-path evaporators, the latter of which is also a preferred option for the final distillation stage because its design is associated with low pressure loss, allowing operation at particularly low pressures. This allows the monomeric diisocyanates to be removed under mild conditions. It is explicitly stated that thermal stress should be kept as low as possible, for example, by omitting temporary tanks and storage tanks, or by keeping the bottom volume small and the pipeline short. The distillate, i.e., the monomeric diisocyanate, is preferably recycled to the reaction step and, if necessary, may be subjected to additional treatment to improve the color index, e.g., filtration. Condensation of the distillate and related issues are not discussed.

[0004] DE102004038784A1 also describes the removal of unreacted diisocyanates from a polyisocyanate mixture by distillation. Distillation is carried out by using at least one single-path evaporator in combination with other evaporators. Process conditions are 5 mbar to 10 -4It is defined by a range of mbar and an evaporator temperature of 30 to 230°C. An internal condenser temperature is also discussed in this literature and is within a range of 5 to 150°C. In one embodiment, monomeric methylene diisocyanate is removed by distillation at 0.05 mbar and an evaporator temperature of 177°C, and the evaporated monomeric methylene diisocyanate is condensed at 50°C.

[0005] EP1426393A2 describes the preparation of low-monomer urethdione-containing polyisocyanates. The examples describe the removal of hexamethylene diisocyanate from a polyisocyanate mixture by distillation. This is carried out using a single-path evaporator combined with a pre-evaporator at a pressure of 0.1 to 0.5 mbar and a heating medium temperature of 140 to 150°C. Thereby, unreacted monomers and catalysts are removed, and the distillate is reused in the reaction. The condensation of the distillate required here is not discussed.

[0006] EP1241197A1 describes the preparation of a low-monomer isocyanate prepolymer. The crude product is preferably distilled in a short-path evaporator at a pressure in the range of 0.1 to 100 Pa and a condenser temperature of 25 to 75°C.

[0007] EP1451239B1 describes a method for preparing an MDI-based prepolymer. Monomeric diisocyanates are removed by distillation in a series of evaporators, wherein the evaporator temperature is in the range of 50 to 210°C and the condenser temperature is in the range of 15 to 55°C.

[0008] In addition to the yield and quality of the product, the value of an industrial manufacturing process also includes process reliability, which allows for stable operation without downtime in industrial manufacturing and consequently the operation of the manufacturing system with minimal possible maintenance. When removing monomeric diisocyanates from polyisocyanates via distillation under reduced pressure, it is advantageous to keep thermal stress as low as possible. In particular, for the production of polyisocyanates with particularly low monomer content, the pressure within the distillation system needs to be low. To achieve this, it is necessary to perform the condensation of the distillate at very low temperatures so that the vacuum is not limited by the vapor pressure of the low-boiling point component. As described in the prior art cited above, distillation is typically performed in a combination of evaporators, each equipped with a condenser. However, in industrial practice, the desired low operating temperature of the condenser has often been found to be associated with manufacturing problems leading to downtime due to maintenance and consequent manufacturing losses. This was caused by solid deposits formed on the condenser due to low condenser temperatures. As a result, pressure loss in the vacuum system increased, and since the essential negative pressure could not be maintained permanently, operations had to be shut down to clean the condenser in order to continue producing the desired low-monomer quality product. Attempts to solve the problem by changing the condenser temperature were unsuccessful. Quality deteriorated from the start at higher condenser temperatures because it was no longer possible to achieve the negative pressure required for distillation. Furthermore, damage to the vacuum pump used for distillation increased. The problem to be solved

[0009] Accordingly, the objective of the present invention was to provide a method for producing low-monomer polyisocyanates that can reliably produce low-monomer polyisocyanates of excellent quality. means of solving the problem

[0010] The purpose of this is

[0011] (i) modifying at least one monomeric diisocyanate to obtain a mixture comprising at least one polyisocyanate and an unreacted monomeric diisocyanate,

[0012] (ii) a step of separating the mixture obtained in step (i) into at least one gaseous stream containing monomeric diisocyanate and a liquid stream depleted of monomeric diisocyanate,

[0013] (iii) partially condensing the gaseous stream from (ii) in at least one condenser to provide a liquid condensate and an uncondensed vapor stream,

[0014] (iv) a step of performing secondary condensation of the uncondensed vapor stream obtained in step (iii) in at least one secondary condenser to provide a secondary condensate and uncondensed off-gas, and

[0015] (v) A step of supplying uncondensed off-gas from step (iv) to the suction part of a vacuum pump.

[0016] Includes,

[0017] At least one secondary condenser in step (iv) is operated at a secondary condenser temperature, and at least one condenser in step (iii) is operated at a condenser temperature, wherein the secondary condenser temperature is ≥ 1 to ≤ 168K lower than the condenser temperature.

[0018] This was achieved by a method for manufacturing low-monomer polyisocyanates. Specific details for implementing the invention

[0019] Expressions such as "first evaporator and second evaporator" or "first part (stream) and second part (stream)" or "first condenser and second condenser" must always be interpreted as an open formulation that does not exclude the existence of additional (third, fourth, etc.) evaporators, parts (streams), or condensers, unless otherwise explicitly stated.

[0020] According to the present invention, the term "comprising" or "containing" preferably means "essentially composed of," and more preferably means "composed of."

[0021] In this case, "polyisocyanate" refers to an isocyanate produced by modification from a diisocyanate, wherein at least two diisocyanate molecules are incorporated into the polyisocyanate. Such polyisocyanates are often referred to as paint polyisocyanates. Therefore, "polyisocyanate" does not explicitly refer to an isocyanate such as that obtained directly from the phosgenation reaction of a di- or polyamine.

[0022] In this case, "condenser temperature," "secondary condenser temperature," or "cooling trap temperature" should be understood to mean the flow temperature of the relevant cooling medium, unless otherwise stated in this description.

[0023] As used herein, “at least one diisocyanate” refers to one or more types, for example, 2, 3, 4, 5, 6, 7, 8, 9 or more types of diisocyanates. With respect to the components of the compounds described herein, these numbers refer to the types of components, not the absolute number of molecules. Therefore, “at least one diisocyanate” means that, for example, a single type of diisocyanate or a plurality of various types of diisocyanates may be present without specifying the amount of individual compounds.

[0024] Step (i)

[0025] In step (i), the modification of the diisocyanate is carried out according to a process known in itself and is also referred to as modification or oligomerization. The isocyanate reacts on its own or with other compounds, such as polyols, polythiols, or polyamines, under suitable reaction conditions. The reaction is preferably carried out in the presence of a catalyst, and when a specific conversion rate is reached, it is stopped by deactivating the catalyst. Deactivation can be carried out in various ways. This is preferably done by adding a compound that deactivates the catalyst. Some catalysts may also be thermally deactivated or removed from the reaction mixture to stop the reaction.

[0026] All industrially available aliphatic, cycloaliphatic, araliphatic, or aromatic diisocyanates are suitable as monomeric diisocyanates for initial modification.

[0027] Suitable aliphatic, cycloaliphatic, araliphatic, or aromatic diisocyanates are, for example, 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 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-isocyanatoprop-2-yl)benzene (TMXDI), tolylene 2,4- and 2,6-diisocyanate (TDI), 2,4'- and 4,4'-diisocyanatodiphenylmethane It is selected from the group consisting of (MDI), 1,5-diisocyanatenaphthalene, phenylene 1,3- and 1,4-diisocyanate, or any desired mixture of these diisocyanates.

[0028] Monomeric diisocyanates selected from the group consisting of 1,5-diisocyanatopentane (PDI), 1,6-diisocyanatohexane (HDI), 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (isophorone diisocyanate, IPDI), 4,4'-diisocyanatodicyclohexylmethane, 2,4'-diisocyanatodicyclohexylmethane, bis(isocyanatomethyl)norbornane, 1,3- and 1,4-bis(isocyanatomethyl)benzene (XDI), tolylene diisocyanate (TDI), 2,4'- and 4,4'-diisocyanatodiphenylmethane (MDI), or any desired mixture of these diisocyanates are particularly preferred.

[0029] Monomeric diisocyanates selected from the group consisting of 1,5-diisocyanatopentane (PDI), 1,6-diisocyanatohexane (HDI), and 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (isophorone diisocyanate, IPDI), mixtures of these isocyanates, or mixtures of these isocyanates and TDI are particularly preferred.

[0030] Monomeric diisocyanates selected from the group consisting of PDI and HDI are most preferred.

[0031] Examples of polyisocyanates produced by the modification of these isocyanates are biuret polyisocyanates, isocyanurate polyisocyanates, urethdione polyisocyanates, urethane polyisocyanates, allophanate polyisocyanates, oxadiazinetrione polyisocyanates, iminooxadiazinedione polyisocyanates, carbodiimide polyisocyanates, and / or urea polyisocyanates. Mixed forms, i.e., those containing allophanate groups and isocyanurate groups or allophanate groups and urethane groups, are also possible. These may be assigned to one group or another. They are preferably assigned to the group more frequently mentioned in relation to polyisocyanates.

[0032] In a further advantageous embodiment of the method of the present invention, the modification in step (i) is preferably a reaction in which isocyanate groups react intermolecularly to form a urethdione polyisocyanate, an isocyanurate polyisocyanate, or an iminooxadiazinedione polyisocyanate in the presence of a catalyst, particularly in the presence of a basic catalyst, and / or a reaction in which an isocyanate reacts with a previously formed urethane group to form an allophanate polyisocyanate.

[0033] This is particularly preferably trimerization into isocyanurate polyisocyanate or iminooxadiazinedione polyisocyanate or dimerization into urethdione polyisocyanate. This is very particularly preferably trimerization into isocyanurate polyisocyanate or iminooxadiazinedione polyisocyanate.

[0034] In a further preferred embodiment of the present invention, the polyisocyanate is selected from the group consisting of isocyanurate polyisocyanate, biuret polyisocyanate, iminooxadiazinedione polyisocyanate, urethane polyisocyanate, and allophanate polyisocyanate, and more preferably from the group consisting of isocyanurate polyisocyanate, biuret polyisocyanate, and iminooxadiazinedione polyisocyanate. It is precisely these isocyanates that tend to apply to the problem described in the introduction regarding the removal of unreacted monomeric diisocyanates by distillation, and for this reason, they are particularly suitable for the present method.

[0035] Step (ii)

[0036] The mixture obtained in step (i) is preferably separated into a gaseous stream containing monomeric diisocyanate and a liquid stream depleted of monomeric diisocyanate in at least one evaporator operated at a pressure generally in the range of 0.005 mbar to 400 mbar, preferably 0.005 to 100 mbar, particularly preferably 0.005 to 10 mbar, very particularly preferably 0.005 to 2 mbar and a temperature in the range of 90 to 205°C, preferably 100 to 185°C, wherein the liquid stream depleted of monomeric diisocyanate is obtained as a bottom product.

[0037] In this case, the term “depleted” relates to the corresponding content of a component contained in the starting fluid that causes the fluid to be formed. When the fluid contains the relevant component in a lower amount, preferably 0.9 times or less of the said amount, the fluid is depleted. The content is a mass content based on the total mass of each fluid in each case.

[0038] In this case, the depleted stream contains monomeric diisocyanate at a level of 0.5 times or less, particularly preferably 0.1 times or less, most preferably 0.02 times or less, based on the content of monomeric diisocyanate in the mixture obtained in step (i). In a preferred embodiment using multi-stage evaporation, the depleted stream obtained at the final stage contains monomeric diisocyanate at a level of 0.1 times or less, particularly preferably 0.01 times or less, and very particularly preferably 0.002 times or less, based on the content of monomeric diisocyanate in the feed stream of the first stage. Generally, it is not necessary for the monomeric diisocyanate to be depleted to less than 0.0005 times the above level, preferably less than 0.001 times the above level, and therefore it is impractical for economic reasons.

[0039] Suitable evaporators are preferably falling film evaporators, thin film evaporators, or short-path evaporators. In a preferred embodiment of the method of the present invention, separation is performed as a multi-stage separation, preferably in 2 to 5 stages, particularly preferably in 2 to 4 stages, very particularly preferably in 3 to 4 stages, and most preferably in 3 stages. A multi-stage design is understood to mean that a plurality of evaporators, each having at least one inlet and at least one bottom outlet, are used in series, and in each case, the bottom outlet of one evaporator serves as the inlet of the next evaporator. It is also possible to perform one or more of these evaporation stages in a plurality of evaporators operating in parallel, but this is associated with increased equipment costs.

[0040] In a preferred embodiment in which the mixture obtained in step (i) is separated into multiple stages in step (ii), it is advantageous to lower the pressure from one stage to the next, wherein at least the final stage is preferably operated at a pressure of 0.005 to 10 mbar, more preferably 0.005 to 2 mbar. Particularly to perform the separation under mild conditions, the pressure in the first stage is additionally within the range of 2 to 100 mbar.

[0041] The monomeric diisocyanate-depleted liquid stream obtained in step (ii) is a desired low-monomer polyisocyanate containing less than 0.5 wt%, preferably less than 0.3 wt%, particularly preferably less than 0.2 wt%, and very particularly preferably less than 0.1 wt% of monomeric diisocyanate based on the total mass of the stream. In a preferred embodiment using multi-stage evaporation in step (ii), the product is obtained as a bottom stream of the final evaporation stage.

[0042] It is preferable to use a single-path evaporator in the final evaporation stage. A single-path evaporator possesses the characteristic properties of an internal condenser. Accordingly, in this preferred embodiment, partial condensation of the gaseous diisocyanate-containing stream (step (iii)) is already performed in the single-path evaporator. This reduces the volumetric flow rate of the gas to the downstream device and minimizes pressure loss, thereby allowing for particularly low vacuum and consequently particularly smooth separation.

[0043] Step (iii)

[0044] Partial condensation of the gaseous diisocyanate-containing stream is performed in at least one condenser, preferably at a condenser temperature in the range of 16 to 135°C. The most suitable condenser temperature can be determined by a person skilled in the art based on existing boundary conditions, such as the monomeric diisocyanate to be removed, the selected pressure level, and the condenser and its heat-transfer surface area. Condensation is performed particularly preferably at a condenser temperature in the range of 16 to 40°C, and very particularly preferably in the range of 18 to 35°C. A device suitable for condensation is a heat exchanger, preferably a shell-tube heat exchanger, and it is particularly desirable to perform condensation in the shell space while passing a heat-transfer medium through the tubes to divert the released condensation heat away. In the case of using a single-path evaporator, partial condensation is already performed in the internal condenser of the single-path evaporator itself.

[0045] Examples of heat transfer media suitable for the operation of at least one condenser in the partial condensation of the gaseous mixture in step (iii) are water, an alcohol-water mixture, a salt-water solution, thermal oil, or an organic solvent, such as chlorobenzene. It is preferable to use water, a water-alcohol mixture, or a salt-water solution, and it is particularly preferable to use water as the heat transfer medium.

[0046] In an alternative embodiment of the method of the present invention, partial condensation of the gaseous diisocyanate-containing stream in step (iii) is performed in an air condenser. In this process, ambient air is cooled by passing it by a fan along the outside of a bundle of tubes, where cooling fins are optionally provided. Condensation is performed inside the tubes, and at the ends of the tubes, the liquid condensate and the uncondensed vapor stream initially come out together and are subsequently separated. In this particular case, the condenser temperature should be understood to mean the outlet temperature of the uncondensed vapor stream.

[0047] Step (iv)

[0048] Secondary condensation of the uncondensed vapor stream from step (iii) is performed in at least one secondary condenser, preferably at a secondary condenser temperature in the range of -33 to 130°C. The most suitable secondary condenser temperature can be determined by a person skilled in the art based on existing boundary conditions, such as the monomeric diisocyanate to be removed, a selected pressure level, and the secondary condenser and its heat-transfer surface area. Secondary condensation is performed particularly preferably at a secondary condenser temperature in the range of -20 to 25°C, very particularly preferably in the range of -10 to 17°C. The secondary condenser temperature is selected to be 1 to 168K, preferably 5 to 50K, more preferably 10 to 30K lower than the condenser temperature of the condenser in step (iii).

[0049] If necessary, for example, due to a change in mass flow rate or a change in the composition of the vapor fluid, it is possible to change the condenser temperature and / or the secondary condenser temperature during the execution of the method of the present invention while maintaining the difference between the aforementioned condenser temperature and the secondary condenser temperature.

[0050] A device suitable for secondary condensation is a heat exchanger, preferably a shell-tube heat exchanger, and it is particularly desirable to perform secondary condensation in the shell space while passing a heat-transfer medium through the tubes to send the released secondary condensation heat away.

[0051] In some cases, a portion of the secondary condensate may solidify on the cold surface of the secondary condenser at a low temperature. To prevent blockage by solidified deposits, a melting process may sometimes be performed at a higher secondary condenser temperature.

[0052] In the secondary condensation of the uncondensed vapor stream from step (iii) in step (iv), examples of heat transfer media suitable for the operation of at least one secondary condenser are water, an alcohol-water mixture, a salt-water solution, a heat transfer fluid, or an organic solvent, such as chlorobenzene. It is preferable to use water, a water-alcohol mixture, or a salt-water solution, and it is particularly preferable to use a salt-water solution or a water-alcohol mixture. The heat-transfer medium should be selected such that its freezing point is lower than the desired secondary condenser temperature of the secondary condenser.

[0053] Step (v)

[0054] The uncondensed off-gas from the secondary condenser obtained in step (iv) is supplied to the suction port of the vacuum pump. To this end, the gas space of the secondary condenser is fluidly connected to the suction port of the vacuum pump.

[0055] In a further preferred embodiment of the present invention, an additional step (iv-a) is performed between step (iv) and step (v) to further cool an uncondensed off-gas stream from a secondary condenser in an additional heat exchanger, which is a cooling trap having a cooling trap temperature at least 1K, preferably at least 5K, and more preferably at least 10K lower than the secondary condenser temperature. This cooling trap is fluidly connected at one end to the off-gas outlet of at least one secondary condenser and fluidly connected at the other end to the suction of at least one vacuum pump. The cooling trap may be operated, for example, using a cooling medium at a temperature within the range of -10 to -200°C. The temperature of the cooling medium is preferably within the range of -15 to -60°C, and more preferably within the range of -16 to -30°C. In any case, the temperature of the cooling medium is at least 1K, preferably at least 5K, and more preferably at least 10K lower than the secondary condenser temperature. Alternatively, it is also possible to perform cooling, for example, through at least one Peltier element, provided that the operating temperature of the cooling trap—that is, the temperature of at least a portion of the heat transfer surface in this particular case—is within the range described above for the temperature of the cooling medium. In the case of a multi-stage design of the distillation apparatus, the cooling trap is preferably positioned so that at least a portion of the off-gas from the first distillation stage passes through it and, after being cooled thereby, reaches the suction of the associated vacuum pump. Particularly preferably, the entire off-gas stream passes through the cooling trap before entering the vacuum pump located furthest downstream.

[0056] The vacuum pump itself may be part of a multi-stage vacuum system. Such an arrangement is described in more detail in exemplary embodiments.

[0057] Steps (i), (ii), (iii), (iv), (iv-a) and (v) described above may be supplemented by additional process steps if necessary.

[0058] One such example here is that a droplet separator can be installed at various points of the method of the present invention, for example, at the vapor outlet of an evaporator. Another example is to use a filter, for example, upstream of the inlet of a vacuum pump or at the bottom discharge of the final evaporator, i.e., in the product stream.

[0059] In a further preferred embodiment of the method, the liquid condensate from step (iii) is at least partially recycled to step (i) of the method, i.e., the modification of the monomeric diisocyanate, optionally after an additional purification step. This prevents waste and allows the method to be operated particularly economically. Optionally, in addition to or instead of the liquid condensate from step (iii), a secondary condensate may also be at least partially recycled to step (i) optionally after an additional purification step. Various embodiments exist for the recycling of the liquid condensate and the secondary condensate. For example, it is possible to recycle the liquid condensate from condensation (step (iii)) and the secondary condensate from step (iv) individually to step (i), or to combine them to form a combined condensate and then recycle them together to step (i). If the liquid condensate and / or secondary condensate undergo additional purification steps such as filtration or distillation, this may be performed only on the condensate, only on the secondary condensate, individually on the condensate and secondary condensate, or on the combined condensate.

[0060] Further preferred embodiments of the present invention are

[0061] (i) modifying at least one monomeric diisocyanate to obtain a mixture comprising at least one polyisocyanate and an unreacted monomeric diisocyanate,

[0062] (ii) a step of separating the mixture obtained in step (i) into at least one gaseous stream containing monomeric diisocyanate and a liquid stream depleted of monomeric diisocyanate,

[0063] (iii) partially condensing the gaseous stream from (ii) in at least one condenser to provide a liquid condensate and an uncondensed vapor stream,

[0064] (iv) a step of performing secondary condensation of the uncondensed vapor stream obtained in step (iii) in at least one secondary condenser to provide a secondary condensate and uncondensed off-gas, and

[0065] (v) A step of supplying uncondensed off-gas from step (iv) to the suction part of a vacuum pump.

[0066] Includes,

[0067] At least one secondary condenser in step (iv) is operated at a secondary condenser temperature and at least one condenser in step (iii) is operated at a condenser temperature, wherein the secondary condenser temperature is ≥ 1 to ≤ 168K lower than the condenser temperature, and the liquid condensate from step (iii) is at least partially recirculated to step (i) of the method, i.e., to the modification of at least one monomeric diisocyanate, optionally after an additional purification step.

[0068] This is a method for manufacturing low-monomer polyisocyanates.

[0069] Alternatively, a further preferred embodiment of the present invention is

[0070] (i) modifying at least one monomeric diisocyanate to obtain a mixture comprising at least one polyisocyanate and an unreacted monomeric diisocyanate,

[0071] (ii) a step of separating the mixture obtained in step (i) into at least one gaseous stream containing monomeric diisocyanate and a liquid stream depleted of monomeric diisocyanate,

[0072] (iii) partially condensing the gaseous stream from (ii) in at least one condenser operated at a condenser temperature of 16 to 135°C, preferably 16 to 40°C, more preferably 18 to 35°C, to provide a liquid condensate and an uncondensed vapor stream,

[0073] (iv) a step of performing secondary condensation of the uncondensed vapor stream obtained in step (iii) in at least one secondary condenser operated at a secondary condenser temperature of -33 to 130°C, preferably -20 to 25°C, more preferably -10 to 17°C, to provide a secondary condensate and uncondensed off-gas, and

[0074] (v) A step of supplying uncondensed off-gas from step (iv) to the suction part of a vacuum pump.

[0075] Includes,

[0076] The secondary condenser temperature of at least one secondary condenser in step (iv) is characterized by being ≥ 1 to ≤ 168K, preferably 5 to 50K, more preferably 10 to 30K lower than the condenser temperature of at least one condenser in step (iii).

[0077] This is a method for manufacturing low-monomer polyisocyanates.

[0078] A particularly preferred embodiment of the present invention is

[0079] (i) modifying at least one monomeric diisocyanate to obtain a mixture comprising at least one polyisocyanate and an unreacted monomeric diisocyanate,

[0080] (ii) a step of separating the mixture obtained in step (i) into at least one gaseous stream containing monomeric diisocyanate and a liquid stream depleted of monomeric diisocyanate,

[0081] (iii) partially condensing the gaseous stream from (ii) in at least one condenser operated at a condenser temperature of 16 to 135°C, preferably 15 to 40°C, more preferably 18 to 35°C, to provide a liquid condensate and an uncondensed vapor stream,

[0082] (iv) a step of performing secondary condensation of the uncondensed vapor stream obtained in step (iii) in at least one secondary condenser operated at a secondary condenser temperature of -33 to 130°C, preferably -20 to 25°C, more preferably -10 to 17°C, to provide a secondary condensate and uncondensed off-gas, and

[0083] (v) A step of supplying uncondensed off-gas from step (iv) to the suction part of a vacuum pump.

[0084] Includes,

[0085] The secondary condenser temperature of at least one secondary condenser in step (iv) is lower than the condenser temperature of at least one condenser in step (iii) by ≥ 1 to ≤ 168K, preferably 5 to 50K, more preferably 10 to 30K, and the liquid condensate from step (iii) is at least partially recirculated to step (i) of the method, i.e., to the modification of at least one monomeric diisocyanate, optionally after an additional purification step.

[0086] This is a method for manufacturing low-monomer polyisocyanates.

[0087] The present invention also provides a use of a polyisocyanate produced by the method of the present invention as a starting material for the manufacture of polymers, such as foamed plastics, polyurethane paints, coatings, adhesives, or additives.

[0088] The polyisocyanate produced according to the present invention is particularly suitable for use in the manufacture of 1- and 2-component polyurethane paints.

[0089] When the polyisocyanates of the present invention are used as crosslinking agent components in two-component coatings, they are combined with OH and / or NH components, for example, hydroxy-functional polyesters, polyacrylates, polycarbonates, polyethers, polyurethanes, and polyfunctional amines, as are generally known in relation to two-component polyurethane systems. However, they can also be used as single components for the manufacture of (partially) moisture-curing plastics and coatings.

[0090] The present invention further provides a composite comprising at least one polymer of the present invention in direct contact with at least one substrate made of metal, plastic, wood, or a mixture thereof.

[0091] Examples

[0092] All percentages are based on weight, unless otherwise noted.

[0093] The NCO content was determined using the titration method according to DIN EN ISO 11909:2007-05.

[0094] The residual monomer content was measured by gas chromatography using an internal standard according to DIN EN ISO 10283:2007-11.

[0095] crude product A:

[0096] Isocyanurate polyisocyanate prepared in a manner known in itself by the catalytic trimeration of hexamethylene diisocyanate. The residual monomer content after the reaction was stopped was about 77% hexamethylene diisocyanate based on the total reaction mixture.

[0097] crude product B:

[0098] Isocyanurate polyisocyanate prepared in a manner known in itself by the catalytic trimerization of pentamethylene diisocyanate. The residual monomer content after the reaction was stopped was about 50% pentamethylene diisocyanate based on the total reaction mixture.

[0099] crude product C:

[0100] Iminooxadiazindione polyisocyanate prepared in a manner known in itself by the catalytic trimeration of hexamethylene diisocyanate. The residual monomer content after the reaction was stopped was about 45% hexamethylene diisocyanate based on the total reaction mixture.

[0101] crude product D:

[0102] Isocyanate-terminated prepolymer prepared in a manner known to itself by the reaction of a polyether polyol with an excess of tolylene diisocyanate. The residual monomer content after the reaction was approximately 38% tolylene diisocyanate based on the total reaction mixture.

[0103] crude product E:

[0104] Polyisocyanate containing allophanate and isocyanurate groups, prepared in a manner known in itself by the catalytic reaction of hexamethylene diisocyanate and a monohydric alcohol in the presence of a trimerization catalyst. The residual monomer content after the reaction was stopped was about 40% hexamethylene diisocyanate based on the total reaction mixture.

[0105] Comparative Examples 1a-e (not in accordance with the present invention):

[0106] The crude product AE was distilled sequentially in a 3-stage vacuum distillation apparatus to remove residual monomers from the polyisocyanate. The vacuum system was also multi-stage; vacuum pumps were provided to each distillation stage, and the pressurized off-gas stream from the vacuum pump of the third distillation stage was supplied to the suction of the vacuum pump of the second distillation stage along with the uncondensed off-gas from the second distillation stage. Similarly, the pressurized off-gas stream from the vacuum pump of the second distillation stage was supplied to the suction of the vacuum pump of the first distillation stage along with the uncondensed off-gas from the first distillation stage. The pressurized off-gas stream from the vacuum pump of the first distillation stage was disposed of through the plant off-gas system.

[0107] A falling film evaporator heated with steam was used as the first distillation stage. The pressure in this first evaporation stage was 25 mbar. The steam from the falling film evaporator was condensed in a condenser operated using cooling water. The flow temperature of the cooling water was 27°C. The uncondensed fraction was removed from the distillation apparatus through a vacuum system.

[0108] The unevaporated bottom stream was fed as feed to a second falling film evaporator, where additional monomers were evaporated from the polyisocyanate thus pre-purified. This second distillation stage was also designed as a falling film evaporator, which was operated at a pressure of 7 mbar and was likewise steam-heated. The steam was condensed in a condenser operated using cooling water. The flow temperature of the cooling water was also 27°C. The uncondensed fraction was removed from the distillation apparatus through a vacuum system.

[0109] In addition, the unevaporated bottom stream from this second distillation stage was supplied as a feed to the final distillation stage. This final distillation stage was designed as a single-path evaporator with an internal condenser, wherein the heating surface of the single-path evaporator is heated using steam and the internal condenser is operated using cooling water, which also has a flow temperature of 27°C. The pressure inside the single-path evaporator was 1.4 mbar. The uncondensed vapor fraction was removed from the distillation apparatus through a vacuum system. The bottom product was cooled and analyzed by gas chromatography.

[0110] Although a residual monomer content of < 0.1% could be achieved for a short period, the vacuum pump had a very short service life of only 7 to 18 days, which meant that it had to be repeatedly stopped and restarted for repair or replacement. Furthermore, in the event of a vacuum pump failure, a sub-product with a higher residual monomer content was sometimes obtained, and ultimately, the desired residual monomer content of < 0.1% was not reliably met.

[0111] Comparative Examples 2a-e (not in accordance with the present invention):

[0112] The crude product AE was distilled in the same manner as in Comparative Examples 1a-e, except that in this example, cooling brine with a flow temperature of 4°C was used as the working medium for the condenser. The pressure in the distillation stage was initially 25 mbar (Stage 1), 7 mbar (Stage 2), and 1.4 mbar (Stage 3).

[0113] In these cases, a residual monomer content of < 0.1% could be achieved for a short period. However, in all cases, after a few days, there was a pressure rise in the distillation apparatus, and the desired residual monomer content of < 0.1% was no longer achieved in the bottom effluent of the third distillation stage. After stopping the distillation and subsequently inspecting the apparatus, it was found that solids had definitely accumulated on the cooling surface of the condenser, and except for Comparative Example 2d (distillation of crude product D, a prepolymer based on toluene diisocyanate), where the pressure rise proceeded most rapidly, the solids could not be removed by simply melting.

[0114] Examples 1a-e:

[0115] The distillation apparatus used in Comparative Examples 1a-e and 2a-e was reconfigured so that vapor from each distillation stage passes through a secondary condenser, which is a second condenser. The first condensation stage was operated using cooling water at a flow temperature of 27°C in each case, and the secondary condensation stage was operated using cooling brine at a flow temperature of 4°C. In the case of a single-path evaporator, the internal condenser was operated using cooling water, and the newly added external secondary condenser was operated using cooling brine.

[0116] The distillation apparatus modified and operated in this manner enabled the continuous production of distilled bottom product with a residual monomer content of < 0.1%. When the apparatus was inspected after operating for several weeks, no problematic solid deposits were found on the cooling surface of the condenser.

Claims

Claim 1 A method for manufacturing a polyisocyanate, comprising: (i) modifying at least one monomeric diisocyanate to obtain a mixture comprising at least one polyisocyanate and unreacted monomeric diisocyanate; (ii) separating the mixture obtained in step (i) into at least one gaseous stream containing the monomeric diisocyanate and a liquid stream depleted of the monomeric diisocyanate; (iii) partially condensing the gaseous stream from (ii) in at least one condenser to provide a liquid condensate and an uncondensed vapor stream; (iv) performing secondary condensation of the uncondensed vapor stream obtained in step (iii) in at least one secondary condenser to provide a secondary condensate and uncondensed off-gas; and (v) supplying the uncondensed off-gas from step (iv) to the suction part of a vacuum pump, wherein the at least one secondary condenser in step (iv) is operated at the secondary condenser temperature and the at least one condenser in step (iii) is a condenser A method characterized by operating at a temperature, wherein the secondary condenser temperature is ≥ 5 to ≤ 50K lower than the condenser temperature. Claim 2 The method of claim 1, characterized in that the monomeric diisocyanate is selected from the group consisting of 1,5-diisocyanatopentane (PDI), 1,6-diisocyanatohexane (HDI), 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (isophorone diisocyanate, IPDI), 4,4'-diisocyanatodicyclohexylmethane, 2,4'-diisocyanatodicyclohexylmethane, bis(isocyanatomethyl)norbornane, 1,3- and 1,4-bis(isocyanatomethyl)benzene (XDI), tolylene diisocyanate (TDI), 2,4'- and 4,4'-diisocyanatodiphenylmethane (MDI), or a mixture of these diisocyanates. Claim 3 A method according to claim 1 or 2, wherein the modification in step (i) is a reaction in which isocyanate groups react intermolecularly to form a urethdione polyisocyanate, an isocyanurate polyisocyanate, or an iminooxadiazinedione polyisocyanate, and / or a reaction in which an isocyanate reacts with a previously formed urethane group to form an allophanate polyisocyanate. Claim 4 A method according to claim 1 or 2, characterized in that the monomeric diisocyanate-depleted liquid stream obtained in step (ii) contains monomeric diisocyanate at a content of 0.5 times or less, 0.1 times or less, or 0.02 times or less of the content of monomeric diisocyanate in the mixture obtained in step (i). Claim 5 A method according to claim 1 or 2, characterized in that the liquid condensate from step (iii) is at least partially recycled to step (i) of the method, i.e., the modification of at least one monomeric diisocyanate. Claim 6 A method according to claim 1 or 2, characterized in that the separation in step (ii) is performed using at least one evaporator selected from the group consisting of a falling film evaporator, a thin film evaporator, or a short path evaporator. Claim 7 A method according to claim 1 or 2, characterized in that the separation in step (ii) is performed as a multi-stage separation in 2 to 5 stages, 2 to 4 stages, 3 to 4 stages, or 3 stages. Claim 8 A method according to claim 6, characterized by using a short-path evaporator as the final evaporator. Claim 9 A method according to claim 1 or 2, characterized in that the partial condensation in step (iii) is performed at a condenser temperature within the range of 16 to 135°C, within the range of 16 to 40°C, or within the range of 18 to 35°C. Claim 10 A method according to claim 1 or 2, characterized in that the secondary condensation in step (iv) is performed at a secondary condenser temperature within the range of -33 to 130°C, within the range of -20 to 25°C, or within the range of -10 to 17°C. Claim 11 A method according to claim 1 or 2, characterized by performing step (iv-a) between step (iv) and step (v), further cooling an uncondensed off-gas stream from a secondary condenser in an additional heat exchanger which is a cooling trap having a cooling trap temperature that is at least 1K, at least 5K, or at least 10K lower than the secondary condenser temperature. Claim 12 delete Claim 13 delete Claim 14 delete Claim 15 delete

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