Methods to produce a polyisocyanate by the reaction of a diisocyanate with an appropriate adduct, which reacts (hereafter referred to as a"reactive adduct") with the isocyanic groups to form urea bonds plus a by-product in gaseous phase are known.

Suitable diisocyanates used to produce polyisocyanates include, for example, hexamethylene diisocyanate or aromatic diisocyanates, as exemplified by toluene diisocyanate (TDI) and diphenylmethane diisocyanate (MDI). Examples of adduct reactants include water, monovalent tertiary alcohols, formic acid, hydrogen sulfide and primary monoamines.

The diisocyanate is generally reacted in excess with respect to the reactive adduct. For example at least 3 moles, preferably at least 10 moles, of diisocyanate are used per mole of reactive adduct.

The reaction of the diisocyanate with an adduct is characterized by the formation of a urea bond and by gas formation.

The urea bond thus formed subsequently reacts with one or more isocyanic groups to form a polyisocyanate with 3 or more functional groups. The resultant polyisocyanate may be either a viscous liquid or a solid that is highly soluble in organic solvents for example toluene, xylene and the acetic acid esters. Polymeric isocyanates may combine to form dimers and trimers thereof under certain circumstances.

The reaction between the diisocyanate and reactive adduct is sequential.

However, the urea dimer or the oligomers containing urea type bonds that are formed during the reaction are typically solid and not very miscible in the reaction mixture or in

the polyisocyanates produced. This may give rise to a precipitate which may hinder processing for example by clogging the production plant equipment or clouding and contaminating the final product. Desirably, there are few and preferably no free urea bonds remaining which have not reacted with an isocyanic group.

Additionally, the polyisocyanates not containing urea bonds and of high molecular weight are typically viscous and may have low solubility in other resins or solvents. Due to the poor miscibility of the high molecular weight polyisocyanates, their use in combination with the polyhydroxyl compounds, for instance polyol oils, polyethers and polyesters, is susceptible to giving poor quality products in relation to the the film-forming properties or the physical properties In order to produce polyisocyanates having low molecular weight, and having few and preferably no urea bonds, it is preferable to use an excess of diisocyanate with respect to the reactive adduct.

However, there are problems with this solution, relating in part to the recovery of the excess reactant and in part to the fact that the excess diisocyanate may undergo thermal polymerization, with the formation of at least one of uretidione, isocyanurate, and carbodiimide type rings, which may result in undesirable coloration in the polyisocyanate.

The reaction between the diisocyanate and reactive adduct is usually conducted in a batch reactor wherein the reaction is characterized by uniform residence times that favour a final product of consistent characteristics including a narrow distribution of molecular weights. However operational difficulties may arise from carrying out the reaction in a batch reactor, for example a poor rate of production and difficulties in recycling reactants. In addition, differences in quality between products made in different production runs may exist.

On the other hand, conducting the reaction in a continuous fashion may give rise to problems due to variation in the residence time of reactants and may lead to significant levels of unreacted feed stock and/or byproducts, for example, caused by further reaction of the desired product. Accordingly, a portion of the reactants could remain in the reaction vessels for shorter or longer times than others, thus giving rise to by-products that could still display urea type bonds or form products with very high

molecular weights.

Furthermore, by way of example, if all the reaction stages were conducted in a tubular reactor that provided a plug flow, for example by means of a piston, or through a series of continuous, stirred reactors to provide a flow comparable to the plug flow, difficulties would still be encountered in keeping the residence time uniform because of the presence of gas due, for example, to non-desirable mixing.

In order to reduce or overcome the drawbacks of the continuous processes, in patent US-A-4,290,969 a process for the continuous production of a polyisocyanate was proposed, wherein the initial stage of the reaction is performed in one or two continuous complete mixing type reactors until the amount of gas generated is at least about 80% of the amount of gas to be generated during the entire reaction and the subsequent stage of the reaction is performed in a plug flow reactor.

A continuous process for the production of polyisocyanates obtained by the reaction between at least one isocyanate and a reactive adduct has now been developed that, reduces or avoids the drawbacks in known continuous processes.

A first aspect of this invention provides a process for the continuous production of a polyisocyanate comprising: a) feeding at least one diisocyanate and an adduct containing at least one active hydrogen atom into a mixing zone, and mixing them together; b) performing the synthesis reaction of the polyisocyanate in an inclined coiled tubular reactor, having a reactor height/length ratio of from 2 to 50% and a L/D (length/diameter) ratio of from 10 to 1000; c) cooling the reaction product.

Suitably the reactor is inclined, relative to the horizontal, from 1 to 60° and especially from 2 to 30°.

The length/diameter ratio of the reactor is preferably 50 to 750 and especially 100 to 500.

Suitably the reactants are used in a diisocyanate/adduct molar ratio of from 5 to 50, preferably from 5 to 40, and more preferably from 8 to 30. Suitably, the reactants are pumped into the mixing zone at pressures generally of 0.2 to 10 MPa, preferably of 1 to 5 MPa. Preferably, the mixing zone and reactants in the mixing zone are kept at a

temperature of 20 to 150°C, preferably between 70 and 85°C. Suitably, the reactants are fed into the mixing zone using low or high pressure feed pumps.

Suitably the mixing zone comprises a mixing atomizer head. Preferably, the atomizer head may be of the type used in the preparation of polyurethane foams. An example of a suitable atomizer is a Cannon mixing head available from Afros Cannon of Caronno Pertusella (Milan). A static mixer head may be used in combination with, or instead of a mixing atomizer head. An example of a suitable a static mixer head is a Sulzer type static mixing head, available from Sulzer International Ltd of Winterthur (CH).

Suitably an emulsion in small droplets of the two reactants is formed by the use of a mixing zone that, by increasing the contact surface between the reactants, allows the synthesis reaction to occur with reduced occurrence and desirably avoidance of the formation of solid precipitates, for example by secondary reactions which may result in the formation of polyureas or polybiurets. To reduce or desirably avoid discontinuity of mixing, which may result in dead points (points of poor flow), the mixing zone may be directly located inside the coiled tubular reactor. Suitably the temperature of the mixing zone is maintained by a dedicated heating device, initially in order to provide sufficient heat to enable the reaction to take place and reduce the amount of incomplete reaction.

When the reaction has produced sufficient exothermic heat to maintain the reaction, the heating device stops the reaction mixture getting too hot, thereby reducing the amount of unwanted side product produced.

The coiled tubular reactor, which may be free-standing, may be inclined so as to establish a fluid dynamic regime inside the tube comparable to that obtained in a horizontal tube. In order to obtain this fluid dynamic regime, it is also preferable that the tube curvature, expressed as a ratio of coil diameter/tube diameter, is from 5 to 50, preferably from 10 to 20, whilst the tube diameter is selected to provide sufficient residence time for the completion of the reaction and to ensure low head losses, which are dependent upon several factors, for example flow rate and residence time. A laminar type fluid dynamic regime with the presence of two separate phases, with liquid in the lower part of the tube and gas in the upper part is also required so as to reduce the likelihood of discontinuity in the liquid. Suitably, this may be obtained by having a

tube diameter which allows a gas mass flow rate from 0.01 to 0.9 kgm/m2 sec, preferably from 0.05 to 0.2 kg/m2 secs.

Desirably, the presence of laminar flow is effected so there is a reduced occurrence and desirably avoidance of turbulence, except for a brief initial period, in which there may be a bubble type flow. The liquid fluid dynamic regime can be the laminar type without this prejudicing the correct distribution of the permanence/residence times. The layer in contact with the tube wall (the lowest layer) would generally be expected to be slower due to direct contact with the tube, but the lowest layer is on average at a slightly higher temperature than the upper layers and, therefore, has viscosity and density values that compensate at least in part for the greater drag with the tube's surface. Overall therefore, the velocity profile of the entire liquid layer is generally constant whatever the distance from the tube surface and therefore each liquid layer is subject to generally equal residence times and operational conditions. Suitably, the resulting overall viscosity of the process fluid remains sufficiently low such that the residence time for the reactants remains relatively constant.

The residence time is suitably as short as possible, so as to reduce the formation of by-products for example dimers of the reaction product but sufficiently long to permit reaction of the feedstocks.

Suitably, the reaction temperature is maintained from 100 to 200°C, preferably from 120 to 160°C. Suitably, the coiled tubular reactor is maintained at the desired temperature preferably through a hot diathermic oil bath kept in forced circulation by a pump. The oil bath can be used to heat the whole of the system, or just the coils to reduce the corresponding hold-up as desired.

Any diisocyanate can be used in the process of this invention. Suitable diisocyanates include the aliphatic diisocyanates for example tetramethylene diisocyanate, hexamethylene diisocyanate, 1,2-cyclohexylene diisocyanate, 1-methyl- 2,4-cyclohexane diisocyanate, 1-methyl-2, 6-cyclohexane diisocyanate, bis (4- isocyanatocyclohexyl) methane, 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethyl cyclohexane (isophorone diisocyanate) and aromatic diisocyanates, for example meta- or para-xylene diisocyanate, toluene diisocyanate and diphenylmethane diisocyanate.

The reactive adduct, containing at least one atom of active hydrogen, may be selected from water, formic acid, hydrogen sulfide, tertiary butanol, methylamine and mixtures of two or more thereof.

On leaving the coiled tubular reactor, the polyisocyanate mixture is cooled sufficiently quickly to reduce the likelihood of the formation of by-products for example by the reaction of the reaction product with itself or unreacted starting materials or intermediate products. Suitably, the mixture is cooled in an evaporative cooling tower or in a packed column. Preferably the polyisocyanate already produced and collected in, for example, a temperature controlled reservoir, is used as the coolant. The rapid cooling may reduce the time that the product remains at high temperature. As the cooling time is decreased, a higher temperature profile may be maintained, which may improve the final quality of the polyisocyanate produced, and also may assist in obtaining shorter reaction times. By carrying out the reaction at a high temperature, a short residence time may be employed but rapid cooling of the product is desirable so as to quench further reaction.

The process for the continuous production of polyisocyanate of this invention can be better understood by referring to Figure 1which represents a preferred embodiment and is not limiting.

In Figure 1, two high pressure pumps P1 and P2 feed the reactants (line 1) and (line 2) to the mixing system M inserted at the input to the coiled tubular reactor R. The reactor is enclosed in the containment box C, filled with hot diathermic oil kept circulating (line 3) by the pump P3. The hot oil also maintains the temperature of the mixing system M (line 4).

The reaction mixture circulates inside the reactor coils for the time necessary for the reactants'conversion and, thereafter, is discharged in the cooling tower T (line 5) where an evaporative distributor sprays the reaction mixture with the previously prepared polyisocyanate. The gases are discharged (line 6) from the top of T while the polyisocyanate is taken off (line 7) from the bottom and is sent to the collection tank S.

From the reservoir S, the pump P4 withdraws a stream of polyisocyanate that is fed partly (line 8) to the cooling device (air cooler) W and, subsequently, to the droplet distributor of the tower T and partly (line 9) to storage, not illustrated in the figure.

Advantageously the process for the continuous production of polyisocyanates of this invention is simpler than known processes and reduces the plant complexity with respect to known processes. The process generally produces a product having the desired quality for example with reduced occurrence and desirably avoidance of the formation of by-products. With respect to known processes, the process of the present invention combines the use of a static mixer and an inclined coiled tube reactor allowing : - the elimination of moveable mechanical stirring devices with the resultant reduction of control devices and monitoring instruments; - reduction in the number of pieces of equipment; - reduction of reaction volumes (for example 20 to 30% less with respect to the known processes); - greater ease of draining for the reactor; - absence of intermediary degassing stages that may cause incrustations in the vent lines due to the presence of solid by-products entrained by the gaseous phase; - better thermal control through an increase of the surface/volume ratio for example, this ration may increase from 500 to 600% with respect to a single perfectly mixed unit for example in a batch reactor in which the contents are generally homogeneous. This allows the system, all generally, to have a higher thermal exchange coefficient or, alternatively to use lower thermal gradients accompanied to known processes under given conditions. For safety reasons, the use of diathermic oil is preferable to the use of condensing steam which is generally necessary to give a greater heat exchange efficiency in the known cases in which there is low heat exchange surface, for the heating. The diathermic oil may be used not only for minimizing the thermal dispersion of heat from reactions, that are generally exothermic, but also to provide heat when necessary; - the whole production plant becomes easier to drain and prepare for possible different production runs, due the nature of the equipment used.

For the purpose of providing a better understanding this invention and putting it into practice, some illustrative and non-limiting examples are reported below.

EXAMPLE 1 With reference to the attached figure, a group of mixing pumps (P1, P2) at high pressure (20 bars, 2MPa) pumped toluene diisocyanate and water in the weight ratio 100/1.1 (molar ratio 10.6/1) to an atomizer head (M). The head, that sends the mixture to the reactor at a pressure slightly above atmospheric, was placed at the input to the reactor in a jacketed tubing line and heated with diathermic oil to 155°C in circulation via pump (P3). The coiled tubular reactor (R) having a L/D ratio equal to 290, tube inclination of 9 % and overall residence time of 55 minutes.

The same reactor was immersed in an oil bath present in the circular ring in which the coils and the electric resistances were housed. The oil temperature was maintained at 155°C so as to guarantee that the product temperature upon exit was close to 160°C. The TDI on input was fed at around 80°C.

The product on leaving of the reactor was cooled to 60°C in the tower (T) through washing in countercurrent with part of the polyisocyanate stored in (S). The polyisocyanate produced displayed a NCO content equal to 41% compared to a starting value (TDI) of 48.3%. The presence of dimers was 0.6%. A maintenance cleaning was performed, restricted to the input section of the reactor in which the mixing head is housed, after every 1500 hours of production. The yield of reacted adduct/input adduct was equal to around 99%.

COMPARATIVE EXAMPLE 1 For the same production capacity a train of 4 mixed reactors was used with equal volumes. So as to avoid obstruction of the lines and the seal of the mixer due to the formation of urea solids, a temperature of not less than 130°C and a residence time of at least 20 minutes for the first reactor was required so that the conversion can eliminate these intermediate products. The temperature of the last reactor was approximately 160°C. 80 minutes of residence time in total were required (30% greater than for Example 1).

The maintenance cleaning of the degassing vents needs to take place after every 150 hours of production. The yield of reacted adduct/supplied adduct was equal to around 80 to 85%.

COMPARATIVE EXAMPLE 2 Example 1 was repeated without the static mixing head. The yield of reacted adduct/input adduct was approximately 90%. In this case maintenance interventions took place on the vents around every 400 to 500 hours of production.

COMPARATIVE EXAMPLE 3 A traditional tubular reactor was used with residence time of 55 minutes and L/D ratio equal to 10. Pre-mixing and operational conditions were adopted as in the example 1.

The final product was outside specifications e. g. amount of by-products produced and incorrect viscosity levels and gave rise to serious incrustation formations.