PROCESS FOR THE PREPARATION OF POLYISOCYANATES OF THE DIPHENYLMETHANE SERIES

FIELD OF THE INVENTION The present invention relates to a process for preparing mixtures of diphenylmethane diisocyanates and polyphenylpolymethylene polyiso- cyanates, known as PMDI, having a higher HunterLab colour (L) number by reaction of the corresponding mixtures of diphenylmethanediamines and polyphenylpolymethylenepolyamines, known as PMDA, with phosgene in the presence of at least one inert organic solvent. BACKGROUND OF THE INVENTION

PMDI is an industrially important isocyanate for producing rigid polyurethane foams which are preferably used as insulation material in the building industry, as insulating foam in the refrigeration appliance industry and as sandwich panel construction material. Usually, part of the diphenylmethane 4,4'-diisocyanate, known as MMDI, present in the PMDI, is recovered by means of a suitable technological operation such as distillation or crystallization. MMDI is in turn an important constituent of polyurethane formulations for compact, microcellular and cellular polyurethanes such as adhesives, coatings, fibres, elastomers and integral foams. Likewise, various mixtures of the diisocyanate isomers in varying proportions (so-called "Mixed Isomer" products) can be prepared. Accordingly, the term "PMDI" as used herein also encompasses PMDI mixtures in which monomeric MDI, for example 4,4'-, 2,2'- and/or 2,4'-MDI, is present. Historically, PMDI was made by initial reaction of the corresponding

PMDA in an inert organic solvent with either hydrogen chloride or carbon dioxide to make a suspension of the amine salts, followed by reaction with phosgene. However, these methods are uneconomic because of the very long reaction time resulting from complete conversion of the PMDA to salt forms. PMDI is, as is known, now widely prepared industrially by direct phosgenation of the PMDA in the presence of an inert organic solvent. PMDA is in turn obtained by means of an acid catalysed aniline-formaldehyde

condensation which can be carried out industrially either continuously or batchwise. The proportions of diphenylmethanediamines and the homologous polyphenylpolymethylenepolyamines and their positional isomers in the PMDA are controlled by selection of the ratios of aniline, formaldehyde and acid catalyst and also by means of a suitable temperature and residence time profile. High contents of 4,4'-diphenylmethanediamine together with a simultaneously low proportion of the 2,4 '-isomer of diphenylmethanediamine are obtained on an industrial scale by the use of strong mineral acids such as hydrochloric acid as catalyst in the aniline-formaldehyde condensation. Use of a wide range of solid acid catalysts is also known.

The final colour of MDI products is the combined result of a number of different effects based on different chemistries. For example, the presence of colour in organic products caused by traces of halogenated impurities, especially brominated or iodinated impurities, is known, and minimising such impurities in the manufacture of MDI leads to products with improved colour (US 6900348). Reaction of oxygen with the polyamine (PMDA) precursor to PMDI can ultimately lead to the formation of quinone-imine-type impurities which are similar to highly coloured arylmethine dyes (Color Chemistry - Synthesis, Properties and Applications of Organic Dyes and Pigments, H. Zollinger, Wiley-VCR ISBN 3-906390-23-3) which can also give chromophores in the MDI after the phosgenation and work-up processes. Other impurities from process steps preceding phosgenation may also contribute to the final colour of MDI. For example, all the acid catalysed aniline-formaldehyde condensation processes described in the specialist and patent literature have in common the formation of undesired by-products, for example the formation of N-methylated and N-formylated compounds and also the formation of dihydroquinazolines. In addition, industrial PMDAs can contain residual amounts of unrearranged aminobenzylanilines which can in turn be a further starting point for further reactions. Another disadvantage is that the acid aniline-formaldehyde condensation forms chromophores which discolour the PMDA. These discolourations are reduced only insufficiently, if at all, in the subsequent neutralization of the acid condensation catalyst and

the removal of the aniline used in excess in the condensation; the same applies to the subsequent process steps of the PMDI preparation.

In the conversion of the PMDA to PMDI, the PMDA is reacted with phosgene, typically in the presence of an inert organic solvent. After suitable preparation of the various reaction components, the chemical process of converting PMDA to PMDI begins with the initial reaction of amine and phosgene, producing carbamoyl chlorides and HCl. Well known side reactions are the formation of a range of urea-group containing compounds and insoluble and heat-stable amine hydrochlorides, whose exact composition is related to the particular amine feed composition and the particular process configuration (pressure, temperature, mixing regime, etc) used at this stage. The resulting mixture may be reacted further in the same vessel or may be discharged from one reactor to a subsequent reactor for the further stage(s) of manufacture, where the thermally-sensitive carbamoyl chlorides can be decomposed to isocyanate and HCl by increasing the temperature of the mixture and solid amine hydrochlorides are converted to isocyanate by further reaction with phosgene. "Reactor" in this context can be any type of vessel (stirred tank reactors, plug-flow reactors such as tower reactors or, indeed, any device which can be used for the contacting of reactants, at this stage, the still-to-react components being carbamoyl chlorides, amine hydrochlorides and phosgene). Thus, there exist many devices and combinations of devices for carrying out the staged conversion of amine feed to the corresponding isocyanate product, by reaction with phosgene and co-formation of HCl, optionally in a solvent, and subsequent removal of excess phosgene, HCl and solvent, thermal breakdown of chlorinated impurities and removal of minor volatile impurities. For example, WO 2004/056756 and DE 10245584 describe specific process configurations which address specific issues in the complex production process, the object being to improve process operations and efficiency, rather than to improve product quality. At this phosgenation stage, the undesired by-products and chromophores in the PMDA outlined above can react with phosgene to form further compounds such as secondary carbamoyl chlorides and products of

chlorination of the aromatic ring and/or at the methylene bridge. In addition, side reactions of the phosgenation process itself form further chlorine- containing by-products such as allophanoyl chlorides and isonitrile dichlorides. The chlorine-containing compounds and chromophores are incorporated both into the low molecular weight fraction whose central constituent is the diphenylmethane diisocyanate and also into the oligomeric fractions of polyphenylpolymethylene polyisocyanate.

The technological operations which follow the phosgenation, namely removal of the phosgene used in excess, the removal of the inert solvent, the thermal treatment, the so-called dechlorination, and the removal of part of the MMDI present in the crude PMDI by distillation and/or crystallization, do not lastingly reduce the discolouration of the crude PMDI and the discolouration of the crude PMDI increases with continuing, especially thermal, stressing of the product. Discoloured PMDI is undesirable in further processing to form polyisocyanate-polyalcohol polyaddition (polyurethane) and poly(iso- cyanurate) plastics. In particular, undesirable discolourations of the PMDI can show up in the plastics prepared therefrom. Although the colour of the plastics does not have an adverse effect on their mechanical properties, light- coloured products are preferred because of their good versatility in the production process of the processor, e.g. the ability of light to pass through thin covering layers and the ability to produce a variety of colours.

There have therefore been many attempts to reduce the discolouration of PMDI in mixtures with MMDI. To lighten PMDI colour, special additional treatments of the PMDA have been proposed such as mild partial catalytic reduction (as in EP 546400 and US 5889070), reacidification (e.g. US 5386059), extra base treatment (see DE 1021 1021 ). Such additional treatments add significantly to the complexity of the PMDA preparation process and are unsatisfactory on economic grounds. To lighten PMDI colour, the addition of numerous compounds before, during or after the phosgenation reaction has also been proposed. Many examples of such added compounds can be characterised by the presence of

functional groups (especially -OH, -NH, -NH2) which react readily with phosgene and include water (US 4465639), low molecular weight monohydric or polyhydric alcohols (BP 445602), polyether polyols or alkane polyols (US 4507464), water and alcohols (US 6229043), phenol derivatives (DE 4300774), amines and/or ureas (DE 4232769), polyoxyalkylene polyalcohols (DE 4021712), hydrazine or derivatives (US 5942151). Other chemicals used include acid chlorides and chloro formates (DE 41 18914), carboxylic acids (BP 538500), dialkyl or trialkyl phosphites (DE 4006978), organic phosphorous acid (JP 3292857), acid chlorides/antioxidant (DE 4318018), special reducing agents (US 5312971). All processes which propose the addition of compounds to raw materials or products of a preparation stage for PMDI have the disadvantage of the addition of an additional agent with the inherent danger of its corrosive action on the equipment components and the formation of byproducts from precisely these added agents, which by-products can in turn have an adverse effect on the product or the equipment. Such additional treatments also add significantly to the complexity of the PMDI preparation process and are unsatisfactory on economic grounds. One patent application (WO2006/130405) attempts to overcome this additional complexity by using, as the "additional" reactant chemical, some unreacted amine hydrochlorides, by controlling the level of these intermediates as they pass from the phosgenation reaction into the work-up sections of the plant. This process suffers from the drawback that the amine hydrochlorides are insoluble in the reaction medium, and solids passing into the work-up section can cause fouling and blocking of equipment. Additionally, by deliberately not completing the phosgenation reaction, the isocyanate content of the final product is reduced.

To lighten PMDI colour, special additional treatments of the PMDI have also been proposed: hydrogenation (EP 816333, US 5583251 and US 6140382), irradiation with light (US 5994579), heat treatment with hydrogen chloride (US 5364958). Such additional treatments add significantly to the complexity of the PMDI preparation process and are unsatisfactory on economic grounds.

US Patent No. 4876380 proposes lightening the colour by extraction of a chromophore-rich PMDI fraction from the PMDI by means of pentane/hexane. Disadvantages of this process are the carrying-out of a complicated technological operation with additional steps for working up the extractant and the unavoidable formation of a reduced-quality PMDI fraction for which applications that use up equivalent amounts have to be found.

US Patent No. 6576788 proposes production of PMDI in a process where the mass ratios of phosgene to hydrogen chloride in the residence time apparatus of the second stage of the phosgenation are at the same time 10-30: 1 in the liquid phase and 1 -10: 1 in the gas phase. Disadvantages of such a process are in the complexity of simultaneously measuring and controlling the different phase compositions to achieve the lightening.

Thus, there continues to be a need for a cost-effective method of improving the colour of PMDI and PMDI-derived polyurethane and polyisocyanurate materials without the drawbacks mentioned above.

OBJECT OF THE INVENTION

It is an object of the present invention to lighten the colour of the PMDI in admixture with MMDI while avoiding the above mentioned disadvantages. In particular, the addition of additional reagents should not be necessary. SUMMARY OF THE INVENTION

We have found that this object is achieved by the staged reaction of the corresponding mixtures comprising diphenylmethanediamines and polyphenyl- polymethylenepolyamines with phosgene in the presence of at least one solvent, where in the work-up stage subsequent to the phosgenation reaction, the level of chlorine-containing impurities is reduced by the so-called heat treatment/dechlorination process to a specifically defined level before the reaction mixture is subjected to temperatures above a temperature specifically defined. Provided that this level of impurities is not exceeded, very little or no further colour deterioration occurs when the product stream is heated to higher temperatures in subsequent processing steps.

Accorgingly, in an aspect the invention provides a process for preparing mixtures comprising diphenylmethane diisocyanates and polyphenyl-

polymethylene polyisocyanates having a higher HunterLab colour (L) number. In another aspect the invention provides mixtures comprising diphenylmethane diisocyanates and polyphenylpolymethylene polyisocyanates having a higher HunterLab colour (L) number obtainable by the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION The present invention accordingly provides a process for preparing mixtures comprising diphenylmethane diisocyanates and polyphenylpolymethylene polyisocyanates having a higher HunterLab colour (L) number by reaction of the corresponding mixtures comprising diphenylmethane- diamines and polyphenylpolymethylenepolyamines with phosgene in the presence of at least one solvent, where the reaction mixture at the end of phosgenation is subjected to a staged work-up by removal of phosgene, HCl and solvent, and halogenated impurities are removed, and in said work-up the product streams are not subjected to temperatures greater than 160 ⁰ C, and preferably not greater than 140 °C, before the acidity level and hydrolysable chlorine level of the isocyanate are reduced to specific levels, namely acidity less than 1 10 ppm (m/m), preferably less than 30 ppm (m/m), and hydrolysable chlorine less than 1000 ppm (m/m), preferably less than 800 ppm (m/m).

PMDI colour has historically been quoted according to several different colour scales. Here we use the HunterLab system, where L is the Lightness. Further information on this and other colour scales is widely available in the literature, for example, in "The Measurement of Appearance", R. S. Hunter & R. W. Harold, John Wiley & Sons (ISBN 0-471-83006-2).

The level of chlorine-containing impurities is measured in various ways. Here we refer to the "acidity" and "hydrolysable chlorine" content of the product. Acidity is measured according to the ASTM method D6099. Hydrolysable chlorine is measured according to the ASTM method D4663. The phosgenation of primary amines in a mixing reactor as first stage of the phosgenation has been described a number of times. Thus, for example, US 354461 1 and EP 150435 report the phosgenation in a pressure mixing

circuit. Furthermore, EP 291819 discloses carrying out this reaction in a reaction pump. Many different designs of static mixers have been described, for example: annular slot nozzle (FR 2325637, DE 1792660), ring-eye nozzle (DE 3744001), flat jet nozzle (EP 65727), fanjet nozzle (DE 2950216), angle- jet chamber nozzle (DD 300168), three-fluid nozzle (DD 132340), coaxial jet mixer nozzle with protruding centerbody (US 2004/008572). The temperature in the first stage of the phosgenation is usually from 40 to 150°C, preferably from 60 to 130°C, particularly preferably from 90 to 120°C. By allowing the exothermic reactions taking place to increase the temperature of the mixture to above approximately 80°C, solidification of carbamoyl chlorides can be prevented (US 2006/0041 166). Careful design of the mixing device minimises urea by-product formation by minimising contacting of incoming amine with reaction products, such that formation of insoluble "polyureas" is avoided. Formation of some urea functional groups is not problematic since these will be simultaneously present in compounds also containing polyisocyanates and, thus, such "mixed functionality" compounds will be soluble in the mixture of normal polyisocyanates.

In a subsequent stage the corresponding carbamoyl chlorides and amine hydrochlorides formed in the first stage of the phosgenation can be run through many types of residence time apparatus in which the amine hydrochlorides are phosgenated to form the corresponding carbamoyl chlorides and the carbamoyl chlorides are dissociated into the corresponding isocyanates and hydrogen chloride. For example, the mixture from a previous stage of the phosgenation can be fed to a series of stirred tank reactors, tubular or column reactors or thin film devices (such as in WO 2004031 132) or combinations of different types of reactors. Batch, continuous, semi- continuous processes and combinations of these, operating at atmospheric pressure or above, are all known in the art.

The PMDI mixtures prepared by the process of the present invention usually have a diphenylmethane diisocyanate isomer content of from 30 to 90% by weight, preferably from 30 to 70% by weight, an NCO content of from 29 to 33% by weight, preferably from 30 to 32%-by weight, based on the

weight of crude MDI, and a viscosity, determined at 25 °C in accordance with DIN 51550, of not more than 2500 mPa.s, preferably from 40 to 2000 mPa.s.

Crude PMDIs having such isomer and homologue compositions can be prepared by phosgenation of crude PMDAs having corresponding product compositions in the presence of at least one solvent.

Suitable crude PMDAs are advantageously obtained by condensation of aniline and formaldehyde in a molar ratio of 6- 1.6: 1 , preferably 4-1.9: 1 , and a molar ratio of aniline to acid catalysts of 1 :0.98-0.01 , preferably 1 :0.8-0.1.

The formaldehyde can be used in any physical form (solid, liquid or gas) and is preferably used in the form of an aqueous solution, e.g. as a commercial 30-55 % strength by mass solution.

Acid catalysts which have been found to be useful are proton donors such as acid ion exchange resins or strong organic and preferably inorganic acids. For the purposes of the present invention, strong acids are those having a pKa of less than 1.5; in the case of polybasic acids, this value is that for the first hydrogen dissociation. Examples which may be mentioned are hydrochloric acid, sulfuric acid, phosphoric acid, fluorosulfonic acid and oxalic acid. Hydrogen chloride in gaseous form can also be used. Preference is given to using aqueous hydrochloric acid in concentrations of from about 25 to 33% by mass.

Suitable processes for preparing crude PMDA are described, for example, in CA 700026, DE 22271 10 (US 4025557), DE 2238920 (US 3996283), DE 24261 16 (GB 1450632), DE 1242623 (US 3478099), GB 1064559 and DE 3225125. The other starting component for preparing crude PMDI is phosgene. The phosgene can be used as liquid or gas, diluted in solvents or with other gases which are inert under the reaction conditions, e.g. monochlorobenzene, ortho dichlorobenzene (o-DCB), nitrogen, carbon monoxide, etc. The molar ratio of crude PMDA to phosgene is advantageously selected such that from 1 to 10 mol, preferably from 1 .2 to 4 mol of phosgene are present in the reaction mixture per mole of NH ₂ groups. The phosgene can all be fed into the first stage of the phosgenation or part of it can also be added to the residence time

apparatus of the subsequent stage of the phosgenation.

Suitable solvents are compounds in which the crude PMDA and the phosgene are at least partially soluble. Solvents which have been found to be useful are chlorinated aromatic hydrocarbons, for example mono- chlorobenzene, dichlorobenzenes such as o-dichlorobenzene and p-dichloro- benzene, trichlorobenzenes, the corresponding toluenes and xylenes, chloro- ethylbenzene, monochlorobiphenyl, alpha- or beta-naphthyl chloride and dialkyl phthalates such as diethyl isophthalate. Isocyanate compounds or mixtures other than MDI's or, preferably, crude or purified PMDI or other MDI material can also be used to replace some or all of the non-isocyanate solvent after the crude PMDA has been initially reacted with the phosgene. Excess phosgene can also be used to take the role of the solvent. Particular preference is given to using monochlorobenzene (MCB), dichlorobenzenes or mixtures of these chlorobenzenes as inert organic solvents. The solvents can be used individually or as mixtures. It is advantageous to use a solvent which has a boiling point lower than that of the MDI isomers so that the solvent can easily be separated from the crude PMDI by distillation. The amount of solvent is advantageously selected such that the reaction mixture has an isocyanate content of from 2 to 40% by mass, preferably from 10 to 30% by mass, based on the total weight of the reaction mixture.

The crude PMDA can be employed as such or as a solution in organic solvents. However, particular preference is given to using crude PMDA solutions having an amine content of from 2 to 45% by mass, preferably from 25 to 44% by mass, based on the total weight of the amine solution. Dependent upon the exact design of the phosgenation reaction section and the conditions of temperature and pressure selected, varying proportions of phosgene, hydrogen chloride, solvent and other components of the complex reaction mixture will be partitioned between vapor, solution and solids phases. The vapor phase may be largely or partially separated from or may be kept in direct contact with the solution and solids during different stages of the phosgenation.

Subsequent to the phosgenation stages, the reaction mixture is worked-up

such that remaining excess phosgene and hydrogen chloride and the solvent are separated from the reaction product. The work-up procedure also includes a thermal treatment step (the so-called "dechlorination") which is likewise well known in the art (GB 1080717). The crude PMDI may then be further treated to produce diisocyanate and polymeric MDI products.

In the first stage of the work-up, the majority of the phosgene and optionally part of the solvent are removed. In a second stage the remaining phosgene and the majority of the solvent are removed, and in the dechlorination stage chlorine- and bromine-containing impurities are broken down and stripped from the isocyanate product, along with any remaining solvent. Although these stages have been defined by the functions outlined above, at the operating level one or more pieces of equipment may be involved at each stage, or stages may be combined.

The dechlorination stage is conventionally carried out at 180-220 ⁰ C, with a flow of inert gas and/or boiling residual solvent used to strip out the breakdown products of the chlorinated impurities, notably HCl and phosgene (H.J.Twitchett, Chemical Society Reviews 3, 1974, pp. 209-230).

We have found that the largest deterioration in colour of the mixed MMDI and PMDI product occurs at this stage of the process. However, we have surprisingly found that the dechlorination can be carried out at lower temperatures, and provided that the temperatures used do not exceed 160 ⁰ C, and preferably not more than 140 ⁰ C, and that the concentration of chlorine- containing impurities is reduced to certain levels as defined by the analytical procedures, namely acidity below 1 10 ppm (m/m), preferably below 30 ppm (m/m), and hydrolysable chlorine below 1000 ppm (m/m), and preferably below 800 ppm (m/m), then the product becomes thermally stable to further colour deterioration, and can be heated to the higher temperatures of 180-220 °C needed to reduce the dimer content (GB 1015977) or to distil out MMDI under reduced pressure without significant changes in colour of the distillation bottoms product or de-dimerised product (beyond that caused by increase in concentration of the coloured species in the distillation residues as colourless MMDI is removed).

The manner in which the acidity and hydrolysable chlorine levels are reduced below the target levels is not critical, so long as the target maximum temperatures are not exceeded. The severity of the dechlorination procedure will depend upon the levels of chlorinated impurities created in the preceding MDA preparation and phosgenation stages. The procedure may be carried out in one or more stirred tanks, optionally with inert gas stripping, or in one or more packed columns, optionally with countercurrent inert gas flow, or a combination of such equipment.

The final MMDI and PMDI products may then be stabilized using an antioxidant based on sterically hindered phenols and/or at least one aryl phosphite. The stabilizers are advantageously used in an amount of up to max. 1% by mass, preferably from 0.001 to 0.2% by mass. Examples of suitable antioxidants based on sterically hindered phenols are: styrenized phenols, i.e. phenols which have a 1-phenylethyl group bound in the 2 or 4 position or in the 2 and 4 and/or 6 positions, bis(2-hydroxy-5-methyl-3-tert-butylphenyl)- methane, 2,2-bis(4-hydroxyphenyl)propane, 4,4'-dihydroxybiphenyl, 3,3'-di- alkyl- or 3,3',5,5'-tetraalkyl-4,4'-dihydroxybiphenyl, bis(4-hydroxy-2-methyl- -5-tert-butylphenyl)sulfide, hydroquinone, 4-methoxy-, 4-tert-butoxy- or 4-benzyloxy-phenol, mixtures of 4-methoxy-2- or -3-tert-butylphenol, 2,5-di- hydroxy-1-tert-butylbenzene, 2,5-dihydroxy-l ,4-di-tert-butylbenzene, 4-meth- oxy-2,6-di-tert-butylphenol and preferably 2,6-di-tert-butyl-p-cresol.

Aryl phosphites which have been found to be useful are tri( alkylphenyl)- phosphites having from 1 to 10 carbon atoms in the alkyl radical, e.g. tri(methylphenyl)phosphite, tri(ethylphenyl)phosphite, tri(n-propylphenyl)- phosphite, tri(isopropylphenyl)phosphite, tri(n-butylphenyl)phosphite, tri(sec- butylphenyl)phosphite, tri(tert-butylphenyl)phosphite, tri(pentylphenyl)- phosphite, tri(hexylphenyl)phosphite, tri(2-ethylhexylphenyl)phosphite, tri(octylphenyl)phosphite, tri(2-ethyloctylphenyl)phosphite, tri(decylphenyl)- phosphite and preferably tri(nonylphenyl)phosphite, and in particular triphenyl phosphite.

Using the process of the present invention leads to PMDI having lighter colour, typically having a HunterLab colour (L) number of greater than 10,

preferably greater than 25, more preferably greater than 40 and, typically, in the range 20 to 50. Colour can be determined using laboratory instruments on samples of final product or crude PMDI or by on-line instrumentation.

The invention is illustrated by the following examples. It should be noted that in the phosgenation reaction prior to work-up, the reaction mixture had not been exposed to temperatures of greater than 140 °C.

EXAMPLES

Examples 1 and 2

The bulk sample from phosgenation reaction containing about 16 % (m/m) MMDI + PMDI, o-DCB as solvent, solved phosgene, halogen-containing impurities and HCl (composition shown in Table 1) was introduced into a packed column where the majority of the solved gases (HCl and phosgene) and a part of o-DCB was removed by stripping with an inert gas at a temperature at 145 °C. The bottoms product of this column containing about 85 to 90 % (m/m)

PMDI+MMDI and about 10 to 15 % (m/m) o-DCB was introduced into a flash tank operating at a temperature specified in Table 1 under vacuum, where the remaining phosgene and the majority of the solvent were removed, and chlorine- and bromine-containing impurities were broken down and the breakdown products of the chlorinated impurities, notably HCl and phosgene, were stripped out from the MMDI + PMDI mixture, along with any remaining solvent (dechlorination). Acidity and hydrolysable chlorine levels of the PMDI mixture were monitored and the thermal treatment was continued until the acidity and hydrolysable chlorine levels of the PMDI mixture were reduced to the levels specified in Table 1.

The crude PMDI mixture was then introduced into a second flash tank operating at a temperature range of 180 to 220 "C to reduce the dimer content and to distil out MMDI under vacuum.

The solvent and phosgene contents, acidity and hydrolysable chlorine levels and HunterLab colour (L) values of the phosgenation reaction mixture, the temperatures of dechlorination, as well as the solvent and phosgene contents, acidity and hydrolysable chlorine levels of the PMDI obtained at the

end of the dechlorination step, and L values of the PMDI product as measured after removal of the MMDI by the second heat treatment are shown in Table 1.

Table 1

o-DCB content was determined by gas chromatography. Phosgene content was measured by titrimetry. Acidity was measured according to the ASTM method D6099. Hydrolysable chlorine was measured according to the ASTM method D4663. The colour (L value) was measured using a HunterLab instrument. Comparative Example

The procedure of Examples 1 and 2 was repeated under the same conditions excepting that the dechlorination was carried out at 185 °C in a conventional manner. L value and acidity level of the PMDI are shown in Table 2. The relationship between temperature of dechlorination, acidity level and colour of the PMDI is also shown in Table 2.

Table 2