CA1113032A - Process for reducing the acidity of organic polymeric isocyanates - Google Patents
Process for reducing the acidity of organic polymeric isocyanatesInfo
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- CA1113032A CA1113032A CA312,529A CA312529A CA1113032A CA 1113032 A CA1113032 A CA 1113032A CA 312529 A CA312529 A CA 312529A CA 1113032 A CA1113032 A CA 1113032A
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- polymethylene
- mixture
- polyphenylpolyisocyanate
- residue
- distillation
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Abstract
ABSTRACT OF THE DISCLOSURE
In a process for purifying and reducing the acidity of a poly-methylene polyphenylpolyisocyanate mixture wherein the mixture is subjected to a distillation step, such as by passing a flowing stream of a crude poly-methylene polyphenylpolyisocyanate mixture through a distillation column which has a reboiler means consisting of a thin film evaporator apparatus mounted therewith for rapidly heating the stream to about 190°C. to about 250°C. whereby the solvent and impurities are taken overhead, and then allow-ing the distillation residue of polymethylene polyphenylpolyisocyanate to cool, an improvement is disclosed which comprises maintaining the distilla-tion residue of the polymethylene polyphenylpolyisocyanate at a temperature minutes and then recirculating continuously a portion of the distillation residue to the distillation column for admixture with the flowing feed stream of crude polymethylene polyphenylpolyisocyanate at a recirculation rate of from about 1:1 to about 3:1 volumes of isocyanate distillation residue per volume of crude isocyanate feed. The acid level of the polymethylene poly-phenylpolyisocyanate mixture treated by the improved process of the invention is substantially reduced without adversely affecting the isocyanate equiva-ent weight, viscosity or other physical and chemical characteristics of the product.
In a process for purifying and reducing the acidity of a poly-methylene polyphenylpolyisocyanate mixture wherein the mixture is subjected to a distillation step, such as by passing a flowing stream of a crude poly-methylene polyphenylpolyisocyanate mixture through a distillation column which has a reboiler means consisting of a thin film evaporator apparatus mounted therewith for rapidly heating the stream to about 190°C. to about 250°C. whereby the solvent and impurities are taken overhead, and then allow-ing the distillation residue of polymethylene polyphenylpolyisocyanate to cool, an improvement is disclosed which comprises maintaining the distilla-tion residue of the polymethylene polyphenylpolyisocyanate at a temperature minutes and then recirculating continuously a portion of the distillation residue to the distillation column for admixture with the flowing feed stream of crude polymethylene polyphenylpolyisocyanate at a recirculation rate of from about 1:1 to about 3:1 volumes of isocyanate distillation residue per volume of crude isocyanate feed. The acid level of the polymethylene poly-phenylpolyisocyanate mixture treated by the improved process of the invention is substantially reduced without adversely affecting the isocyanate equiva-ent weight, viscosity or other physical and chemical characteristics of the product.
Description
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Background of the Invention Field of the Invention This invention relates to organic polymeric isocyanates and more particularly pertains to an improved process for purifying and reducing the acidity in methylene-bridged polyphenylpolyisocyanate mi~tures.
Description of the Prior Art Organic isocyanates are generally prepared by the phosgenation of corresponding amine compounds by one of the many well-known phosgenation processes. These phosgenation processes usually result in crude isocyanate products that contain residual acidic materials which adversely affect the reactivity of the isocyanate. The acidic material contaminants are generally those which respond as acids in standard analytical tests known and employed in the art. The acidic material contaminants are known to include hydrogen chloride and a variety of unknown by-product materials of which only some appear to be hydrogen chloride precursors. These materials also respond as acids in the aforementioned standard analytical tests.
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There are procedures known for purifying organic isocyanates to reduce acidity levels and otherwise increase their reactivity rates. Generally, the most common practice in the industry is to subject crude isocyanate mixtures to one or more distillation steps, such as by passing a stream of the crude reaction mixture of the phosgenation reaction mentioned hereinabove, which contains the organic isocyanate, solvent and impurities, through one or more distillation columns whereby the solvent employed in the phosgenation lQ reaction and acidic material contaminants are carried overhead, leaving the organic isocyanate as residue. For example, United States Patent No. 3,264,336 discloses the employment of fractional distillation as a method for removing acid contaminants from organic isocyanates.
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, However, conventional distillation techniques have left much to be desired in regard to reducing the acidity levels of organic isocyanates. Apparently, it is difficult to separate many acidic material contaminants from the isocyanate material by conventional distillation.
There are several procedures described in the literature for improving the separatlon of acidic material and other contaminants from isocyanates by distillation.
For example, a number of methods have been described which comprise treatment of crude organic isocyanates with metals, such as copper, silver, nickel, iron, zinc, co~alt, alumunum, bismuth, and the like, and then distilling the mixtures for separation. It is disclosed that the metallic compounds form materials or complexes with acidic material contaminants of the crude isocyanate which can be readily separated by distillation, thereby resulting in an isocyanate having reduced acidity. See U.S. Patent No. 3,155,69~, 3,264,336, 3,373,182 and 3,458,558.
In addition, U.S. Patent No. 3,219,~7~ discloses a process for purifying organic isocyanates for the reduction of the hydrolyzable chloride content therein which includes the steps of subjecting a crude organic isocyanate mixture that has been previously degassed and subjected to distillation for sol-vent removal (employed in the aforementioned conventional phosgenation procedure) to a temperature considerably above those tem2eratures required and used in conventior.al degassing procedures for the cleavage of organic carbamyl chlorides - formed in the phosgenation reaction into organic isocyanate and hydrogen chloride, for extended periods of time, and then distilling the mixture to separate the organic isocyanate.
It is disclosed that the heatin~ prior -to distillation apparently removes hydrolyzable chloride contaminants or those responsible for hydrolyzable chloride content which are no~ removed by simple dis-tillation of the organic isocyana-te. However, the process disclosed in U.S. Patent No. 3,219,678 leaves much to be desired from a commercial operation standpoint inasmuch as i-t would apparently require the utilization of a plurality of distillation columns and/or extensive tie-up of plant production equipment which necessarily reduce the economics of the process.
Furthermore, the above-mentioned procedures have especially left much to be desired for purifying and reducing , ` the acidity levels of polymethylene polyphenylpolyisocyanate ', mixtures. Generally, polymethylene polyphenylpolyisocyanate mixtures are prepared by the well-known procedures of mixing and reacting phosgene, in the presence of a compatible ~`~ solvent such as monochlorobenzene, with a corresponding methylene-bridged polyphenyl polyamine mixture prepared by the condensation reaction of formaldehyde and aniline or a related polyamine in the presence of a strong mineral acid or alumina-silica catalyst. Illustrative methods of the preparation of methylene-bridged polyphenyl polyamines and corresponding polymethylene polyphenylpolyisocyanates are described in U.S. Patents No. 2,~83,730; 2,950,263; 3,012,008;
3,344,162; and 3,362,979, to name a few. The primary disadvantage of employing the above-mentioned processes for purifying and reducing the acidity of polymethylene poly-phenylpolyisocyanates is the fac~ that these materials are heat-sensitive. Exposure to high temperatures for extended time periods adversely affect the chemical and physical .
- ~ -a-:-' z properties o polymethylene polyphenylpolyisocyanate mixtures, such as viscosity, isocyanate equivalent weight, weight percent free isoc~vanate, and the like. Furthermore, poly-methylene polyphenylpolyisocyanates prepared by the afore-S mentioned processes exist as mixtures of methylene diphenyl-; isocyanate and higher functionality, higher molecular weight methylene-bridged polyphenylpolyisocyanates which have variable boiling points.
For these reasons, it has heretofore been a common practice in the industry to purify and reduce the acidity of polymethylene polyphenylpolyisocyanate mixtures by a distil-lation procedure employing a fractional distilla~ion column having a reboiler consisting of a thin film evaporator means. More particularly, i.n accordance with conventional techniques, a crude polymethylene polyphenylpolyisocyanate mixture, from the phosgenation reaction containing solvent ` and impurities is initially degassed by rapidly heating the crude mixture to about 70 to about ~0C., under about 60 to about 90 mm. Hg absolute pressure, to remove unreacted phosgene and other highly volatile impurities, and then ~-passed through the fractional distillation column and thin film evaporator means where the mix~ure is subjected to high tempera~ure for only a few seconds to reduce affecting the polymethylene polyphenylpolyisocyanate mixture physical and 2S chemical characteristics. The solvent and impurities are taken overhead.
Although the distillation procedure described immediately hereinabove has been found to be effective in removing the solvent and high volatile impurities from the polymethylene polyphenylpolyisocyanate mixtures without .
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adversely a~fectin~ the physical and chemical properties of the mixt~re, it has left much to be desired in regard to reducing the acidity of the isocyanate product.
- Accordingly, it is the primary obiect of the present invention to provide an improved process for treating a polymethylene polyphenylpolyisocyanate mixture whereby the resulting isocyanate has a reduced acid level without adversely affecting the viscosity, isocyanate equivalent weight, isocyanate reactivity and like physical and chemical character-istics.
It is another object of the present invention to provide an improvement in the process for purifying and reducing the acidity level of a polymethylene polyphenyl-polyisocyanate subjected to a distil]ation step for separation ; 15 of the solvent employed in a conventi.onal phosgenation ; reaction and other contaminants.
-` It is yet another object oi the present invention ; to provide an improvement in the process for puriying and reducing the acidity of polymethylene polyphenylpolyiso~yanate mixture subjected to distillation procedures which does not require the employment of an extensive series of distillation columns and related apparatus and/or extensive tie-up of -~ plant equipment and related long time requirements.
Other objects and advantages of the present invention will become readily apparent to those having ordinary skill in the art from the following description oE the invention along with the attached drawing.
Summary of the Invention The present invention is an improvement in the process for purifying and reducing the acidity level of a '' :,' ' . .
rJ~i2 polymethylene polyphenylpolyisocyanate mixture prepared by the phosyenation of the corresponding methylene-bridged polyphenylpolyamine mixture wherein the crude phosgenated reaction product, after being subjected to conventional degassing procedures, is subjected to at least one distil-lation step to remove excess solvent and impurities therefrom, such as by passing a flowing feed stream of the degassed crude phosyenated mix-ture through a distillation column having a thin film evaporator means moùnted thérewith for heating the mixture to from about 190C. to about 250C., taking the excess solvent and impurities overhead from the distillation column, and then allowing the resulting poly-methylene polyphenylpolyisocyanate product residue from the distillation column and evaporator means to cool. The improvement of the invention comprises maintaining the polymethylene polyphenylpolyisocyanat:e product residue from the distillation column and evaporator means at a temperature ~;
: of from about 190C. to about 250C. for about 1 to 60 minutes, such as by holding in a temperature controlled : : :
. 20 holding tank, and recirculating continuously a portion of the product residue being maintained at that temperature to the distillation column for admixture with the flowing feed stream of crude polymethylene polyphenylpolyisocyanate mixture. It has been found that the maintaining and holding . 25 of the polymethylene polyphenylpolyisocyanate product resid~e at tem~eratures within the above-mentioned range ror the described times in combination with the continuous recir-culation of a portion thereof for admixture with the ~rude feed stream in the distillation column results in a sub-: 30 stantial reduction in the acid level of product mixture ., .
, 3~?~2 without adversely afecting the isocyanate equivalent weigh~, viscosity and other physical and chemical properties of the product. Moreover, the reduction of acidity of polymethylene polyphenylpolyisocyanate mixtures treated by the process of the invention i5 obtained without the use of a plurality of distillation steps heretofore usually required, thereby eliminating the necessity of employing extensive distillation equipment and/or extended process time require-ments. The process of the invention is especially effective for reducing the acidity of methylene-bridged polyphenyl polyisocyanate mixtures prepared by the aforementioned b procedures which are known to be extremely heat-sensitive and thus adversely affected by the extensive distillation procedures heretoore employed for removing impurities from crude organic isocyanate products.
Description of the Drawing The detailed description of our invention r which ` follows herein, will be further illustrated in connection with the attached drawing, which is a schematic flow sheet illustrating a preferred embodiment of the invention. In order to simplify the drawing, conventional details, such as valvesr pumpsr condensersr reboilers, flow and temperature control devices, and the like r have not been shown since the construction, operation and function thereof is known to - 25 those of ordinary skill in the ar-t.
Detailed Description of the Invention As hereinbefore mentioned, the process of the invention is particularly useful for purifying and reducing the acidity of polymethylene polyphenylpolyisocyanate mixtures.
Any polymethylene polyphenylpolyisocyanate mixture, prepared ~ 3~l~6~
by any process ~nown iIl the art can be treated by the inventive process. Inasmuch as polym2thylene polyph~nyl-polyisocyanate mixtures are well-known, and methods for their preparation and the methods for the preparation of the corresponding methylene-bridged polyphenylpolyamine mixtures employed for their preparation are well-known, as shown by ~-the above-noted patents, further detailed description thereof will not be set forth herein.
Thus, for the purposes of brevity only, the term "polymethylene polyphenylpolyisocyanate mixture" ~Jill here-after be referred to as the isocyanate mixture or product.
Referring now to the drawing, in accordance with ~;;
the process of the present invention, a crude isocyanate feed stream, obtained from any of the aforementioned phos-genation processes of corresponding methylene-bridged poly-phenyl polyamines and which contain the isocyanate, a compatible organic liquid solvent, such as monochlorobenzene, and impurities, is fed rom the phos~enation reactor 10 through line 12 through one or more flashing units 14,14a (in phantom) wherein the feed stream is heated at a temperature of from about 70 to about 90C. at about 60 to about 90 ` mm. Hg absolute pressure for about 15 minutes to about 2 hours. As the isocyanate feed stream passes -through the -flashing units 14,14a, unreacted phosgene, some solvent and some high volatile impurities are taken overhead through line 16,16a (in phantom). The liquid bottoms from the flash units 14,14a, i.e. the degassed isocyanate feed stream, then moves through line 18 and is fed to the fractionation distillation column 20, preferably at about mid-point of the column 20. The fractionation column 20 is mounted with a _g_ ~3~2 reboiler means consisting of a thin film evaporator 22. The thin film evaporator 22 is mo~lnted to the fractionation column by any conventional means, such as by lines 24 and 26. For example, the isocyanate feed s-tream passes through the fractionation column 20 and line 24 to the thin film evaporator 22 wherein it is heated to a temperature of from about 190-250C. for only a few seconds. The solvent and other impurities volatilized in the thin film evaporator 22 are then passed through line 26 back to the bottom of the fractionation column 20, passing countercurrent to the feed stream, and are taken overhead through line 28 for reuse or discard. The volatili~ed solvent and impurities are preferably taken overhead ~rom the thin film evaporator at a temperature of about 110C. to about 215C. and passed through the ~ractionation column 20 whereby the isocyana-te feed stream flowing countercurrently therewith is heated so as to provide a temperature of about 50-80C. at th~ fractionator column 20 mid-point and an overhead temperature oE about 30-50C, depending upon the particular solvent employed during the aforementioned phosgenation reaction. In addition, the fractionation column 20 is preferably operated under a pressure of from about atmospheric to about 15 mm. Hg absolute and includes reflux condenser means (not shown) to provide reflux ratios of from about 0.5:1 to about 5:1 to prevent excessive loss of the isocyanàte products.
In accordance with the process of the present inventian, the isocyanate product residue or bottoms Erom the thin film evaporator 22 is then removed therefxom through line 30 to a holding tank 32 wherein the residue is maintained at a temperature of from about 190C. to about , j, --10--3~
250~C., i.e. essen-tially the same temperature as heated to in the thin film evaporator 22, for a time period of from about 1 minute to about 60 minu-tes. The temperature can be ~-maintained in the holding tank 32 by any conventional means, such as by equipping the holding -tank 32 with appropriate insulation and/or additional heating means. The desired holding time can be provided by any conventional means such as by equipping the holding tank 22 with appropriate product removal means ~or continuously removing portions of the heated isocyanate product from the holding tank 32 when it collects and reaches a certain level. Preferably, the isocyanate product residue is maintained at a temperature of from about 210C. to about 240C. for about 1 to about 15 minutes. It has been determined that, as higher temperatures are employed, lower holding times are required.
A portion of the isocyanate product is removed ; continuously from the holding tank through line 34 and . .
continuously recirculated back to the fractionation col~nn 20 at its mid-point for intimate admixture with the crude isocyanate feed stream entering the fractionation column 20 ; through line 18 from the flash unit 14,14a. Most unexpectedly, it has been found that the continuous recirculation of the isocyanate product residue from the thin film evaporator 22 which has been maintained at the aforesaid temperature for the aforesaid time at recirculation volume ratios of from about 1:1 to as high as 3:1, temperature-maintained product residue:crude flashed feed product, results in drastic reduction of the acid content of the polymethylene polyphenyl-; polyisocyanate mixture being treated, as further shown in the examples set forth hereafter. Preferably, a recirculation : ` ~
rate of from about 2 to abou-t 3 volumes recirculated product per volume of crude feed is employed.
The isocyanate produc-t residue or evaporator bottoms recirculated through line 34 can be recirculated at the same temperature maintained in the holding tank 32 if desired. However, the product residue can be recirculated at temperatures as low as about 25C., such as by passing the recirculated residue through a cooling tank 36 (in phantom1 without adverse results so as to reduce potential equipment maintenance problems.
The isocyanate residue product is also removed from the holding tank 32 through line 38, which can include the above-mentioned product removal means (not shown). The product removal means can comprise a nozzle having an ` 15 orifice of desired size to provide the desired removal rate and holding time. It can he readily appreciated that ~he ; volume of product removed through line 38 is preferably substantially the same as the volume of crude isocyanate -~ feed fed to the fractionation column 20 through line 18 to ~0 provide processiny equilibrium~ The removed product is then .: , passed through conventional co~ling means ta storage receivers, both not snown.
The invention will be ~urther illustrated by the following specific examples which are given by way of ~5 illustration and not as limitations on the scope of this inventlon.
All of the following examples were carried out in a fractional distillation column which consisted of a ~-inch column packed with Intalox Saddles (Norton Company, Akron, Onio), A reboiler was mounted with the column which consisted ~ R~g;s~Q~e~ ~rade n~4~
of a thin film evaporator which had a heat transfer surface of 1.4 sq.ft. The column also had a 3-way valve means ~;
activated by a timer mounted therewith to provide the desired reflux, along with pressure means. A double-pipe heat exchanger was attached by appropriate conduits to the head of the column to condense vapors taken overhead. The crude isocyanate ~eed mixture to be treated, previously degassed, was introduced through a conduit connected at the mid-point o the column. For heat treatment, a small tank was attached to the bottom o~ the evaporator. The heated isocyanate product residue from the bottom of the heat exchanger entered the tank through a dip-tube mounted therein, filled the tank to a level needed to give the desired holding time, and overflowed through a no2zle and then through a cooler to rec~eivers. Holding time in the tank was adjusted by selecting one o~ multiple nozzles provided on t:he side o~ the tank.
The tank was wrapped with a coil oE tubing through which hot oil flowed to maintain a constant temperature in the iso-cyanate product residue, and the entire tank was insulated ` 20 to prevent heat loss. Recirculating means were provided for recirculating a portion of the product held in the ~olding tank back to the mid-point of the column for admixture with ; the fresh crude isocyanate feed. The recirculating means included appropriate conduits and metering devices attached ~ 25 respectively to the holding tank and distillation column at mid point for removing the isocyanate product residue from - the holding tank to the distillation column mid-point. The recirculating means included a cooling tank mounted with the conduit for reducing the temperature thereof to about 50C.
to reduce metering device maintenance. The thin film : .
evapora-tor also had appropriate conduits mounted therewith for passing the isocyanate product residue direc~ly therefrom to the cooler and receivers bypassing the holding tank This bypass means and the recirculating means both haa cut-off valves for operation when desired.
The various data relating to acid levels in the following examples were obtained as follows:
Acidity Determination 1.2 to 1.4 g. of the isocyanate mixture to be analyzed is weighed, to the nearest 0.1 mg., into each of two 250-ml. beakers. 50 ml. of methanol and then 50 ml. o~
toluene are pipetted into each beaker. A stirring bar is added to each beaker which are -then placed on a pre-heated (maximum heat) stirrer hotplate. Thermometers are placed in each solution and the solutions are heated to 60C. in less ; than 3 minutes. The beakers are then removed from the hotplate, covered with watch glasses and allowed to stand for one hour, plus or minus 5 minutes. With a pipette, the thermometers and the walls of each beaker are washed down with 10 ml. methanol. Using a pH meter with glass and calomel electrode, each solution is then titrated with 0.02N
methanolic sodium hydroxide, to pH 7. The acidity, deter-mined as HCl, is then calculated according to the following formula:
cid (Basis HCl) = ( 1. of NaOH)(N of NaOH)(3 EXAMPLE I
A crude reaction mixture resulting from the phosgenation of a polymethylene polyphenylpolyamine mixture from the condensation reaction of aniline and formaldehyde, ~ ,o~
containillg 20-25 w-t.% monochlorobenzene and 75-80 wt.%
polymethylene polyphenylpolyisocyanate mixture, previously flashed at a~out 7~C. to about 90C. at about 60 to 90 ~m.
Hg absolute pressure, was fed to the distillation column described hereinabove which was adjusted to provide holding of the isocyanate product residue in the holding tank and recirculating a portion therefrom. The crude flashed mixture was ~ed to the distillation column at a feed rate o~ 8.3 lbs./hr. The distillation column was operated under 15 mm.
Hg absolute pressure and a reflux ratio of 2/1. The holdin~
~ank was adjusted with appropriate nozzles for a holding time of 1.5 minutes. The recirculating means was adjusted .,~ . . .
;` to provide a recirculation rate of 20.7 lbs./hr. Temperatures were recorded at various points as follows:
Point Temperature Range .
Column head 27-32C.
` Column mid-point 57-81C.
Vapor from evaporator 126-212C.
; Product from evaporator 200-229C.
Holding tank 216-225C.
A portion of the product was removed from the holding tank at a rate of 8.3 lbs./hr. and passed through ` the cooler to receivers. A portion of the product was removed from a receiver and analyzed, the results of which are set forth in the following Table 1~ -Product Analysis ` Acidity, wt.~ as HCl0.054 Isocyanate content, meq/g 7.42 Viscosity, cp. at 25C. 285 MDI content, wt.% 43.5 4,4'-isomer, wt.~ of MDI 75.7 :` :
~15-E:XA~IPLE I I
A crude reaction mixture o about 50 wt. 6 mono- -chlorobenzene and 50 wt.~ polymethylene polyphenylpolyiso-cyanate, obtained ~rom the phosgenation of a polymethylene polyphenylpolyamine mixture prepared by the condensation - reaction of aniline and formaldehyde which had been previously ;~ degassed by passing through a fIash unit operated at about 70C. to about 90C. at 60 to 90 mm. Hg absolute pressure, was fed to the distillation column described hereinabove at a feed rate of 31.6 lbs./hr. The crude reaction feed mixture was the same as described in Example I except for the wt.%
monochlorobenzene present. The distillation column apparatus described hereinabove was ad~usted so as to pass the treated .
isocyanate product residue from the evaporator directly lS through the cooler to receivers bypassing the holding tank ` and recirculating means. The distillation column was operated under 15 mm. Hg absolute pressure and a reflu~ ratio of 2/1. Temperatures, measured at various points, were as follows:
Point Temperature Ran~e Column head 32-47C.
` Column mid-point 53-61C.
Vapor from evaporator 111-215C.
Product from evaporator 230-242C.
` 25 A portion of the isocyanate product residue from a receiver was analyzed, the results being set forth in the following Table 2.
- Product Analysis Acidity, wt.% as HCl 0.20 Isocyanate content, meq/g 7.60 Viscosity, cp. at 25C. 160 MDI content, wt.% 45.7 4,4'-isomer, wt.% of MDI76.6 . . .
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A comparison of the results of Table 2 to those of Table 1 illustrates the improvement ln reduction o~ acidity of organic polyisocyanate mixtures trea-ted in accordance with the process of the invention over a conventional distil-lation technique.
EXAMPLE III
The crude flashed reaction feed mixture described in Example II was fed to the distillation column described hereinabove at a feed rate of 27.4 lbs./hr. The distillation column was adjusted so that the isocyanate product residue from the evaporator would be removed to the holding tank which was adjusted to maintain the temperature of the product residue at about 172-219C. for about 11 minutes holding time. The distillation column was operated under 15 mm. Hg absolute pressure and a reflux ratio of 2/1. Temperatures were recorded at various points as follows:
Point emperature Range Column head 31-33C.
Column mid-point 58-76C.
Vapor from evaporator 150-199C.
Product from evaporator Holding tank 172-219C.
- The product passed from the holding tank at the - feed rate through a cooler to receivers. A portion of the treated isocyanate residue product was then analyzed, the results of which are set forth in the following Table 3.
Produc~ Analysls Acidity, wt.% as HCl 0.15 Isocyanate content, meq/g 7.59 Viscosity, cp. at 25C. 174 MDI content, wt.% 47.5 4,4'-isomer, wt.~ of MDI 76.0 ' .
A comparison oE tne results set forth in Table 3 ~ith the results of Table 2 shows that holding the mix-ture at the elevated temperature provides improved acidity reduction over conventional distillation. However, a comparisOn of the results of the table with those of Table 1 illustrates the unexpected improvement of acidity reduction by recirculatin~ a portion of the isocyanate product residue in accordance with the present invention.
EXAMPLE IV
', 10 A crude reàction mixture resulting from the phosgenation of a polymethylene polyphenylpolyamine mixture of the condensation reaction of aniline and formaldehyde, containing 20-25 wt.% monochlorobenzene and 75-80 wt.~ of the polymeth,ylene polyphenylpolyisocyanate mixture, which had been previously flashed by passing through a flash unit operated at about 70C. to about 90C'. at 60-90 mm. Hg absolute pressure, was fed to the dic;tillation colu~n described hereinabove. The distilla~ion column was adiusted as described in Example I to provide holding of the iso-cyanate product residue in the holding tank and recirculating ' , a portion therefrom back to the distillation column. The crude flashed mixture was fed to the distillation column at a feed rate of 7O0 lbs. per hour and the distillation column t~as operated under 15 mm. Hg absolute pressure and a reflux -, 2~ ratio of 2/1. The holding tank was adjusted with appropriate ozzles for a holding time of 1.5 minutes and the recirculating ~ ~Qans was adjusted to provide a recirculation rate of 23.0 ,, lbS./hour~ During operation, temperatures were recorded at -~ariOus points as follows:
Poin~ Temperature Range Column head 23-27C.
Column mid-point 58-77C.
Vapor from evaporator 173-200C.
Product from evaporator 174-190C.
Holding tank 194-209C.
A portion of the product was removed from the holding -tank at the feed rate of 7.0 lbs.~hour and passed through the cooler to receivers. Analysis of a portion of the product removed from a receiver provided the following results set forth in Table 4:
Product Analysis Acidity, wt.~ as HCl 0.068 Isocyanata content, meq/g 7.47 Viscosity, cp. at 25C. 80 MDI content, wt.% 66.2 4,4'-isomer, wt.% of MDI 75.8 A comparison of the result~i of Table 4 to those set forth in Tables 2 and 3 ùrther illustrate the improved reduction of acid content in polymèthylene polyphenylpoly~
isocyanate mixtures treated in accordance with the present invention.
From the foregoing description and examples of this invention, those of ordinary skill in the art may make many modifications and variations therefrom without departing `~
from the scope of the inventlon as h~reinafter claimed.
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Background of the Invention Field of the Invention This invention relates to organic polymeric isocyanates and more particularly pertains to an improved process for purifying and reducing the acidity in methylene-bridged polyphenylpolyisocyanate mi~tures.
Description of the Prior Art Organic isocyanates are generally prepared by the phosgenation of corresponding amine compounds by one of the many well-known phosgenation processes. These phosgenation processes usually result in crude isocyanate products that contain residual acidic materials which adversely affect the reactivity of the isocyanate. The acidic material contaminants are generally those which respond as acids in standard analytical tests known and employed in the art. The acidic material contaminants are known to include hydrogen chloride and a variety of unknown by-product materials of which only some appear to be hydrogen chloride precursors. These materials also respond as acids in the aforementioned standard analytical tests.
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., ' . , ' , .
There are procedures known for purifying organic isocyanates to reduce acidity levels and otherwise increase their reactivity rates. Generally, the most common practice in the industry is to subject crude isocyanate mixtures to one or more distillation steps, such as by passing a stream of the crude reaction mixture of the phosgenation reaction mentioned hereinabove, which contains the organic isocyanate, solvent and impurities, through one or more distillation columns whereby the solvent employed in the phosgenation lQ reaction and acidic material contaminants are carried overhead, leaving the organic isocyanate as residue. For example, United States Patent No. 3,264,336 discloses the employment of fractional distillation as a method for removing acid contaminants from organic isocyanates.
~' . .
, However, conventional distillation techniques have left much to be desired in regard to reducing the acidity levels of organic isocyanates. Apparently, it is difficult to separate many acidic material contaminants from the isocyanate material by conventional distillation.
There are several procedures described in the literature for improving the separatlon of acidic material and other contaminants from isocyanates by distillation.
For example, a number of methods have been described which comprise treatment of crude organic isocyanates with metals, such as copper, silver, nickel, iron, zinc, co~alt, alumunum, bismuth, and the like, and then distilling the mixtures for separation. It is disclosed that the metallic compounds form materials or complexes with acidic material contaminants of the crude isocyanate which can be readily separated by distillation, thereby resulting in an isocyanate having reduced acidity. See U.S. Patent No. 3,155,69~, 3,264,336, 3,373,182 and 3,458,558.
In addition, U.S. Patent No. 3,219,~7~ discloses a process for purifying organic isocyanates for the reduction of the hydrolyzable chloride content therein which includes the steps of subjecting a crude organic isocyanate mixture that has been previously degassed and subjected to distillation for sol-vent removal (employed in the aforementioned conventional phosgenation procedure) to a temperature considerably above those tem2eratures required and used in conventior.al degassing procedures for the cleavage of organic carbamyl chlorides - formed in the phosgenation reaction into organic isocyanate and hydrogen chloride, for extended periods of time, and then distilling the mixture to separate the organic isocyanate.
It is disclosed that the heatin~ prior -to distillation apparently removes hydrolyzable chloride contaminants or those responsible for hydrolyzable chloride content which are no~ removed by simple dis-tillation of the organic isocyana-te. However, the process disclosed in U.S. Patent No. 3,219,678 leaves much to be desired from a commercial operation standpoint inasmuch as i-t would apparently require the utilization of a plurality of distillation columns and/or extensive tie-up of plant production equipment which necessarily reduce the economics of the process.
Furthermore, the above-mentioned procedures have especially left much to be desired for purifying and reducing , ` the acidity levels of polymethylene polyphenylpolyisocyanate ', mixtures. Generally, polymethylene polyphenylpolyisocyanate mixtures are prepared by the well-known procedures of mixing and reacting phosgene, in the presence of a compatible ~`~ solvent such as monochlorobenzene, with a corresponding methylene-bridged polyphenyl polyamine mixture prepared by the condensation reaction of formaldehyde and aniline or a related polyamine in the presence of a strong mineral acid or alumina-silica catalyst. Illustrative methods of the preparation of methylene-bridged polyphenyl polyamines and corresponding polymethylene polyphenylpolyisocyanates are described in U.S. Patents No. 2,~83,730; 2,950,263; 3,012,008;
3,344,162; and 3,362,979, to name a few. The primary disadvantage of employing the above-mentioned processes for purifying and reducing the acidity of polymethylene poly-phenylpolyisocyanates is the fac~ that these materials are heat-sensitive. Exposure to high temperatures for extended time periods adversely affect the chemical and physical .
- ~ -a-:-' z properties o polymethylene polyphenylpolyisocyanate mixtures, such as viscosity, isocyanate equivalent weight, weight percent free isoc~vanate, and the like. Furthermore, poly-methylene polyphenylpolyisocyanates prepared by the afore-S mentioned processes exist as mixtures of methylene diphenyl-; isocyanate and higher functionality, higher molecular weight methylene-bridged polyphenylpolyisocyanates which have variable boiling points.
For these reasons, it has heretofore been a common practice in the industry to purify and reduce the acidity of polymethylene polyphenylpolyisocyanate mixtures by a distil-lation procedure employing a fractional distilla~ion column having a reboiler consisting of a thin film evaporator means. More particularly, i.n accordance with conventional techniques, a crude polymethylene polyphenylpolyisocyanate mixture, from the phosgenation reaction containing solvent ` and impurities is initially degassed by rapidly heating the crude mixture to about 70 to about ~0C., under about 60 to about 90 mm. Hg absolute pressure, to remove unreacted phosgene and other highly volatile impurities, and then ~-passed through the fractional distillation column and thin film evaporator means where the mix~ure is subjected to high tempera~ure for only a few seconds to reduce affecting the polymethylene polyphenylpolyisocyanate mixture physical and 2S chemical characteristics. The solvent and impurities are taken overhead.
Although the distillation procedure described immediately hereinabove has been found to be effective in removing the solvent and high volatile impurities from the polymethylene polyphenylpolyisocyanate mixtures without .
., .
adversely a~fectin~ the physical and chemical properties of the mixt~re, it has left much to be desired in regard to reducing the acidity of the isocyanate product.
- Accordingly, it is the primary obiect of the present invention to provide an improved process for treating a polymethylene polyphenylpolyisocyanate mixture whereby the resulting isocyanate has a reduced acid level without adversely affecting the viscosity, isocyanate equivalent weight, isocyanate reactivity and like physical and chemical character-istics.
It is another object of the present invention to provide an improvement in the process for purifying and reducing the acidity level of a polymethylene polyphenyl-polyisocyanate subjected to a distil]ation step for separation ; 15 of the solvent employed in a conventi.onal phosgenation ; reaction and other contaminants.
-` It is yet another object oi the present invention ; to provide an improvement in the process for puriying and reducing the acidity of polymethylene polyphenylpolyiso~yanate mixture subjected to distillation procedures which does not require the employment of an extensive series of distillation columns and related apparatus and/or extensive tie-up of -~ plant equipment and related long time requirements.
Other objects and advantages of the present invention will become readily apparent to those having ordinary skill in the art from the following description oE the invention along with the attached drawing.
Summary of the Invention The present invention is an improvement in the process for purifying and reducing the acidity level of a '' :,' ' . .
rJ~i2 polymethylene polyphenylpolyisocyanate mixture prepared by the phosyenation of the corresponding methylene-bridged polyphenylpolyamine mixture wherein the crude phosgenated reaction product, after being subjected to conventional degassing procedures, is subjected to at least one distil-lation step to remove excess solvent and impurities therefrom, such as by passing a flowing feed stream of the degassed crude phosyenated mix-ture through a distillation column having a thin film evaporator means moùnted thérewith for heating the mixture to from about 190C. to about 250C., taking the excess solvent and impurities overhead from the distillation column, and then allowing the resulting poly-methylene polyphenylpolyisocyanate product residue from the distillation column and evaporator means to cool. The improvement of the invention comprises maintaining the polymethylene polyphenylpolyisocyanat:e product residue from the distillation column and evaporator means at a temperature ~;
: of from about 190C. to about 250C. for about 1 to 60 minutes, such as by holding in a temperature controlled : : :
. 20 holding tank, and recirculating continuously a portion of the product residue being maintained at that temperature to the distillation column for admixture with the flowing feed stream of crude polymethylene polyphenylpolyisocyanate mixture. It has been found that the maintaining and holding . 25 of the polymethylene polyphenylpolyisocyanate product resid~e at tem~eratures within the above-mentioned range ror the described times in combination with the continuous recir-culation of a portion thereof for admixture with the ~rude feed stream in the distillation column results in a sub-: 30 stantial reduction in the acid level of product mixture ., .
, 3~?~2 without adversely afecting the isocyanate equivalent weigh~, viscosity and other physical and chemical properties of the product. Moreover, the reduction of acidity of polymethylene polyphenylpolyisocyanate mixtures treated by the process of the invention i5 obtained without the use of a plurality of distillation steps heretofore usually required, thereby eliminating the necessity of employing extensive distillation equipment and/or extended process time require-ments. The process of the invention is especially effective for reducing the acidity of methylene-bridged polyphenyl polyisocyanate mixtures prepared by the aforementioned b procedures which are known to be extremely heat-sensitive and thus adversely affected by the extensive distillation procedures heretoore employed for removing impurities from crude organic isocyanate products.
Description of the Drawing The detailed description of our invention r which ` follows herein, will be further illustrated in connection with the attached drawing, which is a schematic flow sheet illustrating a preferred embodiment of the invention. In order to simplify the drawing, conventional details, such as valvesr pumpsr condensersr reboilers, flow and temperature control devices, and the like r have not been shown since the construction, operation and function thereof is known to - 25 those of ordinary skill in the ar-t.
Detailed Description of the Invention As hereinbefore mentioned, the process of the invention is particularly useful for purifying and reducing the acidity of polymethylene polyphenylpolyisocyanate mixtures.
Any polymethylene polyphenylpolyisocyanate mixture, prepared ~ 3~l~6~
by any process ~nown iIl the art can be treated by the inventive process. Inasmuch as polym2thylene polyph~nyl-polyisocyanate mixtures are well-known, and methods for their preparation and the methods for the preparation of the corresponding methylene-bridged polyphenylpolyamine mixtures employed for their preparation are well-known, as shown by ~-the above-noted patents, further detailed description thereof will not be set forth herein.
Thus, for the purposes of brevity only, the term "polymethylene polyphenylpolyisocyanate mixture" ~Jill here-after be referred to as the isocyanate mixture or product.
Referring now to the drawing, in accordance with ~;;
the process of the present invention, a crude isocyanate feed stream, obtained from any of the aforementioned phos-genation processes of corresponding methylene-bridged poly-phenyl polyamines and which contain the isocyanate, a compatible organic liquid solvent, such as monochlorobenzene, and impurities, is fed rom the phos~enation reactor 10 through line 12 through one or more flashing units 14,14a (in phantom) wherein the feed stream is heated at a temperature of from about 70 to about 90C. at about 60 to about 90 ` mm. Hg absolute pressure for about 15 minutes to about 2 hours. As the isocyanate feed stream passes -through the -flashing units 14,14a, unreacted phosgene, some solvent and some high volatile impurities are taken overhead through line 16,16a (in phantom). The liquid bottoms from the flash units 14,14a, i.e. the degassed isocyanate feed stream, then moves through line 18 and is fed to the fractionation distillation column 20, preferably at about mid-point of the column 20. The fractionation column 20 is mounted with a _g_ ~3~2 reboiler means consisting of a thin film evaporator 22. The thin film evaporator 22 is mo~lnted to the fractionation column by any conventional means, such as by lines 24 and 26. For example, the isocyanate feed s-tream passes through the fractionation column 20 and line 24 to the thin film evaporator 22 wherein it is heated to a temperature of from about 190-250C. for only a few seconds. The solvent and other impurities volatilized in the thin film evaporator 22 are then passed through line 26 back to the bottom of the fractionation column 20, passing countercurrent to the feed stream, and are taken overhead through line 28 for reuse or discard. The volatili~ed solvent and impurities are preferably taken overhead ~rom the thin film evaporator at a temperature of about 110C. to about 215C. and passed through the ~ractionation column 20 whereby the isocyana-te feed stream flowing countercurrently therewith is heated so as to provide a temperature of about 50-80C. at th~ fractionator column 20 mid-point and an overhead temperature oE about 30-50C, depending upon the particular solvent employed during the aforementioned phosgenation reaction. In addition, the fractionation column 20 is preferably operated under a pressure of from about atmospheric to about 15 mm. Hg absolute and includes reflux condenser means (not shown) to provide reflux ratios of from about 0.5:1 to about 5:1 to prevent excessive loss of the isocyanàte products.
In accordance with the process of the present inventian, the isocyanate product residue or bottoms Erom the thin film evaporator 22 is then removed therefxom through line 30 to a holding tank 32 wherein the residue is maintained at a temperature of from about 190C. to about , j, --10--3~
250~C., i.e. essen-tially the same temperature as heated to in the thin film evaporator 22, for a time period of from about 1 minute to about 60 minu-tes. The temperature can be ~-maintained in the holding tank 32 by any conventional means, such as by equipping the holding -tank 32 with appropriate insulation and/or additional heating means. The desired holding time can be provided by any conventional means such as by equipping the holding tank 22 with appropriate product removal means ~or continuously removing portions of the heated isocyanate product from the holding tank 32 when it collects and reaches a certain level. Preferably, the isocyanate product residue is maintained at a temperature of from about 210C. to about 240C. for about 1 to about 15 minutes. It has been determined that, as higher temperatures are employed, lower holding times are required.
A portion of the isocyanate product is removed ; continuously from the holding tank through line 34 and . .
continuously recirculated back to the fractionation col~nn 20 at its mid-point for intimate admixture with the crude isocyanate feed stream entering the fractionation column 20 ; through line 18 from the flash unit 14,14a. Most unexpectedly, it has been found that the continuous recirculation of the isocyanate product residue from the thin film evaporator 22 which has been maintained at the aforesaid temperature for the aforesaid time at recirculation volume ratios of from about 1:1 to as high as 3:1, temperature-maintained product residue:crude flashed feed product, results in drastic reduction of the acid content of the polymethylene polyphenyl-; polyisocyanate mixture being treated, as further shown in the examples set forth hereafter. Preferably, a recirculation : ` ~
rate of from about 2 to abou-t 3 volumes recirculated product per volume of crude feed is employed.
The isocyanate produc-t residue or evaporator bottoms recirculated through line 34 can be recirculated at the same temperature maintained in the holding tank 32 if desired. However, the product residue can be recirculated at temperatures as low as about 25C., such as by passing the recirculated residue through a cooling tank 36 (in phantom1 without adverse results so as to reduce potential equipment maintenance problems.
The isocyanate residue product is also removed from the holding tank 32 through line 38, which can include the above-mentioned product removal means (not shown). The product removal means can comprise a nozzle having an ` 15 orifice of desired size to provide the desired removal rate and holding time. It can he readily appreciated that ~he ; volume of product removed through line 38 is preferably substantially the same as the volume of crude isocyanate -~ feed fed to the fractionation column 20 through line 18 to ~0 provide processiny equilibrium~ The removed product is then .: , passed through conventional co~ling means ta storage receivers, both not snown.
The invention will be ~urther illustrated by the following specific examples which are given by way of ~5 illustration and not as limitations on the scope of this inventlon.
All of the following examples were carried out in a fractional distillation column which consisted of a ~-inch column packed with Intalox Saddles (Norton Company, Akron, Onio), A reboiler was mounted with the column which consisted ~ R~g;s~Q~e~ ~rade n~4~
of a thin film evaporator which had a heat transfer surface of 1.4 sq.ft. The column also had a 3-way valve means ~;
activated by a timer mounted therewith to provide the desired reflux, along with pressure means. A double-pipe heat exchanger was attached by appropriate conduits to the head of the column to condense vapors taken overhead. The crude isocyanate ~eed mixture to be treated, previously degassed, was introduced through a conduit connected at the mid-point o the column. For heat treatment, a small tank was attached to the bottom o~ the evaporator. The heated isocyanate product residue from the bottom of the heat exchanger entered the tank through a dip-tube mounted therein, filled the tank to a level needed to give the desired holding time, and overflowed through a no2zle and then through a cooler to rec~eivers. Holding time in the tank was adjusted by selecting one o~ multiple nozzles provided on t:he side o~ the tank.
The tank was wrapped with a coil oE tubing through which hot oil flowed to maintain a constant temperature in the iso-cyanate product residue, and the entire tank was insulated ` 20 to prevent heat loss. Recirculating means were provided for recirculating a portion of the product held in the ~olding tank back to the mid-point of the column for admixture with ; the fresh crude isocyanate feed. The recirculating means included appropriate conduits and metering devices attached ~ 25 respectively to the holding tank and distillation column at mid point for removing the isocyanate product residue from - the holding tank to the distillation column mid-point. The recirculating means included a cooling tank mounted with the conduit for reducing the temperature thereof to about 50C.
to reduce metering device maintenance. The thin film : .
evapora-tor also had appropriate conduits mounted therewith for passing the isocyanate product residue direc~ly therefrom to the cooler and receivers bypassing the holding tank This bypass means and the recirculating means both haa cut-off valves for operation when desired.
The various data relating to acid levels in the following examples were obtained as follows:
Acidity Determination 1.2 to 1.4 g. of the isocyanate mixture to be analyzed is weighed, to the nearest 0.1 mg., into each of two 250-ml. beakers. 50 ml. of methanol and then 50 ml. o~
toluene are pipetted into each beaker. A stirring bar is added to each beaker which are -then placed on a pre-heated (maximum heat) stirrer hotplate. Thermometers are placed in each solution and the solutions are heated to 60C. in less ; than 3 minutes. The beakers are then removed from the hotplate, covered with watch glasses and allowed to stand for one hour, plus or minus 5 minutes. With a pipette, the thermometers and the walls of each beaker are washed down with 10 ml. methanol. Using a pH meter with glass and calomel electrode, each solution is then titrated with 0.02N
methanolic sodium hydroxide, to pH 7. The acidity, deter-mined as HCl, is then calculated according to the following formula:
cid (Basis HCl) = ( 1. of NaOH)(N of NaOH)(3 EXAMPLE I
A crude reaction mixture resulting from the phosgenation of a polymethylene polyphenylpolyamine mixture from the condensation reaction of aniline and formaldehyde, ~ ,o~
containillg 20-25 w-t.% monochlorobenzene and 75-80 wt.%
polymethylene polyphenylpolyisocyanate mixture, previously flashed at a~out 7~C. to about 90C. at about 60 to 90 ~m.
Hg absolute pressure, was fed to the distillation column described hereinabove which was adjusted to provide holding of the isocyanate product residue in the holding tank and recirculating a portion therefrom. The crude flashed mixture was ~ed to the distillation column at a feed rate o~ 8.3 lbs./hr. The distillation column was operated under 15 mm.
Hg absolute pressure and a reflux ratio of 2/1. The holdin~
~ank was adjusted with appropriate nozzles for a holding time of 1.5 minutes. The recirculating means was adjusted .,~ . . .
;` to provide a recirculation rate of 20.7 lbs./hr. Temperatures were recorded at various points as follows:
Point Temperature Range .
Column head 27-32C.
` Column mid-point 57-81C.
Vapor from evaporator 126-212C.
; Product from evaporator 200-229C.
Holding tank 216-225C.
A portion of the product was removed from the holding tank at a rate of 8.3 lbs./hr. and passed through ` the cooler to receivers. A portion of the product was removed from a receiver and analyzed, the results of which are set forth in the following Table 1~ -Product Analysis ` Acidity, wt.~ as HCl0.054 Isocyanate content, meq/g 7.42 Viscosity, cp. at 25C. 285 MDI content, wt.% 43.5 4,4'-isomer, wt.~ of MDI 75.7 :` :
~15-E:XA~IPLE I I
A crude reaction mixture o about 50 wt. 6 mono- -chlorobenzene and 50 wt.~ polymethylene polyphenylpolyiso-cyanate, obtained ~rom the phosgenation of a polymethylene polyphenylpolyamine mixture prepared by the condensation - reaction of aniline and formaldehyde which had been previously ;~ degassed by passing through a fIash unit operated at about 70C. to about 90C. at 60 to 90 mm. Hg absolute pressure, was fed to the distillation column described hereinabove at a feed rate of 31.6 lbs./hr. The crude reaction feed mixture was the same as described in Example I except for the wt.%
monochlorobenzene present. The distillation column apparatus described hereinabove was ad~usted so as to pass the treated .
isocyanate product residue from the evaporator directly lS through the cooler to receivers bypassing the holding tank ` and recirculating means. The distillation column was operated under 15 mm. Hg absolute pressure and a reflu~ ratio of 2/1. Temperatures, measured at various points, were as follows:
Point Temperature Ran~e Column head 32-47C.
` Column mid-point 53-61C.
Vapor from evaporator 111-215C.
Product from evaporator 230-242C.
` 25 A portion of the isocyanate product residue from a receiver was analyzed, the results being set forth in the following Table 2.
- Product Analysis Acidity, wt.% as HCl 0.20 Isocyanate content, meq/g 7.60 Viscosity, cp. at 25C. 160 MDI content, wt.% 45.7 4,4'-isomer, wt.% of MDI76.6 . . .
;3~
A comparison of the results of Table 2 to those of Table 1 illustrates the improvement ln reduction o~ acidity of organic polyisocyanate mixtures trea-ted in accordance with the process of the invention over a conventional distil-lation technique.
EXAMPLE III
The crude flashed reaction feed mixture described in Example II was fed to the distillation column described hereinabove at a feed rate of 27.4 lbs./hr. The distillation column was adjusted so that the isocyanate product residue from the evaporator would be removed to the holding tank which was adjusted to maintain the temperature of the product residue at about 172-219C. for about 11 minutes holding time. The distillation column was operated under 15 mm. Hg absolute pressure and a reflux ratio of 2/1. Temperatures were recorded at various points as follows:
Point emperature Range Column head 31-33C.
Column mid-point 58-76C.
Vapor from evaporator 150-199C.
Product from evaporator Holding tank 172-219C.
- The product passed from the holding tank at the - feed rate through a cooler to receivers. A portion of the treated isocyanate residue product was then analyzed, the results of which are set forth in the following Table 3.
Produc~ Analysls Acidity, wt.% as HCl 0.15 Isocyanate content, meq/g 7.59 Viscosity, cp. at 25C. 174 MDI content, wt.% 47.5 4,4'-isomer, wt.~ of MDI 76.0 ' .
A comparison oE tne results set forth in Table 3 ~ith the results of Table 2 shows that holding the mix-ture at the elevated temperature provides improved acidity reduction over conventional distillation. However, a comparisOn of the results of the table with those of Table 1 illustrates the unexpected improvement of acidity reduction by recirculatin~ a portion of the isocyanate product residue in accordance with the present invention.
EXAMPLE IV
', 10 A crude reàction mixture resulting from the phosgenation of a polymethylene polyphenylpolyamine mixture of the condensation reaction of aniline and formaldehyde, containing 20-25 wt.% monochlorobenzene and 75-80 wt.~ of the polymeth,ylene polyphenylpolyisocyanate mixture, which had been previously flashed by passing through a flash unit operated at about 70C. to about 90C'. at 60-90 mm. Hg absolute pressure, was fed to the dic;tillation colu~n described hereinabove. The distilla~ion column was adiusted as described in Example I to provide holding of the iso-cyanate product residue in the holding tank and recirculating ' , a portion therefrom back to the distillation column. The crude flashed mixture was fed to the distillation column at a feed rate of 7O0 lbs. per hour and the distillation column t~as operated under 15 mm. Hg absolute pressure and a reflux -, 2~ ratio of 2/1. The holding tank was adjusted with appropriate ozzles for a holding time of 1.5 minutes and the recirculating ~ ~Qans was adjusted to provide a recirculation rate of 23.0 ,, lbS./hour~ During operation, temperatures were recorded at -~ariOus points as follows:
Poin~ Temperature Range Column head 23-27C.
Column mid-point 58-77C.
Vapor from evaporator 173-200C.
Product from evaporator 174-190C.
Holding tank 194-209C.
A portion of the product was removed from the holding -tank at the feed rate of 7.0 lbs.~hour and passed through the cooler to receivers. Analysis of a portion of the product removed from a receiver provided the following results set forth in Table 4:
Product Analysis Acidity, wt.~ as HCl 0.068 Isocyanata content, meq/g 7.47 Viscosity, cp. at 25C. 80 MDI content, wt.% 66.2 4,4'-isomer, wt.% of MDI 75.8 A comparison of the result~i of Table 4 to those set forth in Tables 2 and 3 ùrther illustrate the improved reduction of acid content in polymèthylene polyphenylpoly~
isocyanate mixtures treated in accordance with the present invention.
From the foregoing description and examples of this invention, those of ordinary skill in the art may make many modifications and variations therefrom without departing `~
from the scope of the inventlon as h~reinafter claimed.
' ~`' .
Claims (4)
1. In a process for purifying and reducing the acidity of a polymethylene polyphenylpolyisocyanate mixture comprising distilling, in a distillation step, a crude poly-methylene polyphenylpolyisocyanate mixture resulting from the phosgenation of a corresponding polymethylene polyphenyl-polyamine mixture in a solvent to remove the solvent and impurities therefrom, said distillation step being conducted by passing a flowing feed stream of the crude polymethylene polyphenylpolyisocyanate mixture through a distillation column having a thin film evaporator means mounted therewith for heating said mixture to a temperature of from about 190°C. to about 250°C., taking excess solvent and impurities overhead from the column, and then allowing the resulting polymethylene polyphenylpolyisocyanate distillation residue from the column and evaporator means to cool, the improvement which comprises:
maintaining the polymethylene polyphenylpoly-isocyanate residue from the column and evaporator means at a temperature of from about 190°C. to about 250°C. for about 1 to about 60 minutes; and recirculating continuously a portion of said residue being maintained at said temperature for said time to said distillation column for admixture with said flowing feed stream of crude polymethylene polyphenylpolyisocyanate mixture, whereby the acidity content of the polymethylene polyphenylpolyisocyanate mixture is substantially reduced.
maintaining the polymethylene polyphenylpoly-isocyanate residue from the column and evaporator means at a temperature of from about 190°C. to about 250°C. for about 1 to about 60 minutes; and recirculating continuously a portion of said residue being maintained at said temperature for said time to said distillation column for admixture with said flowing feed stream of crude polymethylene polyphenylpolyisocyanate mixture, whereby the acidity content of the polymethylene polyphenylpolyisocyanate mixture is substantially reduced.
2. The process of Claim 1 wherein the polymethylene polyphenylpolyisocyanate distillation residue maintained at about 190°C. to about 250°C. for about 1 to about 60 minutes is recirculated to said distillation column at a ratio of from about 1 to about 3 volumes of recirculated residue per volume of said crude polymethylene polyphenylpolyisocyanate feed stream.
3. The process in accordance with Claim 1 wherein said polymethylene polyphenylpolyisocyanate residue is maintained at a temperature of from about 190°C. to about 225°C. for about 1 to about 15 minutes.
4. The process in accordance with Claim 3, wherein said polymethylene polyphenylpolyisocyanate residue maintained at said temperature for said time is recirculated to said distillation column at a rate of about 2 to about 3 volumes maintained residue per volume of crude polymethylene poly-phenylpolyisocyanate feed.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA312,529A CA1113032A (en) | 1978-10-02 | 1978-10-02 | Process for reducing the acidity of organic polymeric isocyanates |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA312,529A CA1113032A (en) | 1978-10-02 | 1978-10-02 | Process for reducing the acidity of organic polymeric isocyanates |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1113032A true CA1113032A (en) | 1981-11-24 |
Family
ID=4112506
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA312,529A Expired CA1113032A (en) | 1978-10-02 | 1978-10-02 | Process for reducing the acidity of organic polymeric isocyanates |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1113032A (en) |
-
1978
- 1978-10-02 CA CA312,529A patent/CA1113032A/en not_active Expired
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