This invention relates to a process for the continuous preparation of moderate to high viscosity reaction products requiring moderate to long reaction times (e. g. a polymer (s) including prepolymer (s)).

This invention specifically relates to a process and a unit for the continuous preparation of an isocyanate-containing polyurethane prepolymer (s) useful in the production of polyurethane articles.

Background of the invention.

The conventional method of making an isocyanate-containing polyurethane prepolymer (s) is to react a polyol and a polyisocyanate batchwise at an elevated temperature.

Batch processes for manufacturing a polyisocyanate prepolymer (s) are known and widely used. They however suffer from several drawbacks. They require long cycle time, since the entire process needs long time for filling in the reactor vessel with the first reactant (usually the isocyanate), long time for warming up, long time to add and cause to react the other reactant (the polyol) and long time to cool down to storage'temperature.

Also, they require complex and big plants, that are difficult to operate and maintain. Also, it is difficult to produce small amounts of a given product, since this kind of process is generally only economic at a larger plant scale.

There is thus a need for a process for preparing a polyisocyanate prepolymer which reduces some of these problems. Such a process may be a continuous process.

In the European Patent Application No. 480588 is disclosed a continuous process for the preparation of an isocyanate-terminated prepolymer having an NCO content in the range from 2 to 15% by weight, said process comprising the steps of: (i) continuously delivering to a reaction zone an organic polyisocyanate composition and an isocyanate reactive compound having an average molecular weight of at least 1000, the relative proportions of the isocyanate composition the isocyanate reactive compound being

appropriate for the formation of a prepolymer having an NCO content in the indicated range; (ii) allowing the isocyanate composition and the isocyanate reactive compound to react in the reaction zone to form an isocyanate-terminated prepolymer, and (iii) continuously removing the prepolymer from the reaction zone.

In the European Patent Application No. 598283 is disclosed a continuous process for the preparation of an isocyanate-terminated prepolymer, said process comprising the steps of reacting at least one diisocyanate with one or more substantially linear polyol, according to a molar ratio of 1.3: 1.0 to 15.1: 1.0. The reaction is carried out in a tubular reactor having a ratio length to diameter of at least 2, which is substantially a continuously stirred tank reactor, and with a partial recirculation of the so-obtained products back to the fresh monomer feeds, with which said so-obtained products are intensively mixed. The recirculation loop can be achieved in the reactor itself, by using appropriate stirrer, operated at an appropriate rotation speed.

In EP-A-0087817 is disclosed a process for preparing a polyurethane polymer (s) comprising combining an isocyanate and a polyol in a jacketed conduit. The jacket contains a heat transfer medium maintained at constant temperature and serves to control the peak of maximum reaction temperature. The conduit contains mixing means to produce a plug-shaped velocity profile. For example, the means can be a spiral shaped reactor.

Summary of the invention A general object of this invention is therefore to provide a continuous process which enables the preparation of medium to high viscosity compound (s) or polymer (s) requiring a moderate to a long reaction time.

A further object of this invention is to provide a unit for the continuous preparation of a compound (s) or polymer (s), which is compact, simple to realize and maintain, environmental friendly and could be incorporated into a modular design.

A specific object of this invention is therefore to provide a continuous process, which enables the preparation of a high quality prepolymer (s).

Another object of this invention is to provide a unit for the continuous preparation of a prepolymer (s), which are similar to those from a traditional batch production plant,

which can operate at medium to high viscosity with moderate to long reaction times sufficient to achieve the conversion desired.

A further object of this invention is to provide a unit for the continuous preparation of such prepolymer (s), which is compact, simple to realize, operate and maintain, and readily transportable.

Yet a further object of the invention is to provide a unit for the continuous preparation of a prepolymer (s) which is environmentally friendly.

Accordingly, the invention resides in a process, preferably adiabatic, carried out in a plug-flow like reactor, with a buffer vessel connected with it.

According to an alternate embodiment, the invention resides in a process, preferably adiabatic, carried out in a pulsed perforated plate reactor operated under pulsed conditions.

Detailed description of the invention.

A process for the continuous preparation of a chemical compound (s) or polymer (s) comprising the steps of: a) introducing the reactants into a plug flow like reactor which is a perforated plate reactor; b) causing the reactants to react in said reactor under the required pressure and temperature; c) removing the reaction products from said reactor.

A process for the continuous preparation of an isocyanate-containing polyurethane prepolymer (s), comprising the steps of: (a) introducing reactants into a plug-flow like reactor (10) said reactants being at least one isocyanate reactive compound and at least one isocyanate; (b) causing the reactants to react in said reactor under the required pressure and temperature; (c) removing the reaction products from said reactor.

The preferred plug-flow like reactor is a perforated plate reactor for the process above most preferred processes are adiabatic processes.

According to one specific embodiment, the invention provides a continuous process, preferably adiabatic for the preparation of an isocyanate-containing polyurethane prepolymer (s), comprising the steps of: (a) heating at least one isocyanate reactive compound and at least one isocyanate; (b) introducing the mixed heated reactants into a plug-flow like reactor; (c) causing the heated reactants to react in said reactor under atmospheric pressure; (d) removing the reaction product from said reactor into a buffer means (e. g. a vessel).

The invention also provides a continuous process, preferably adiabatic for the preparation of an isocyanate-containing polyurethane prepolymer (s), comprising the steps of: (a) heating at least one isocyanate reactive compound and at least one isocyanate; (b) introducing the mixed heated reactants into a perforated plate reactor; (c) causing the heated reactants to react in said reactor under pulsed conditions; and (d) removing the reaction product from said reactor.

In one embodiment of the invention, the plug-flow like reactor is a perforated plate reactor. In this embodiment, the perforated plate reactor is e. g. an elongated vertical vessel, the heated mixture is introduced at the bottom of said elongated vertical vessel and the reaction product is removed at the top of said elongated vertical vessel, preferably as an overflow. The plug-flow lilce reactor could be any type of plug reactor including a baffled reactor. The term"plug-flow like reactor"thus covers the reactor having a residence time distribution substantially close to the plug reactor (e. g. a monodisperse distribution of residence times of 25% around the median value, preferably 20%).

The term"adiabatic"means that there is no heat that is voluntarily exchanged with the reactor; there is no heating or cooling of the reactor during the reaction. This however does not prevent heating the solids into liquids prior to the reaction zone. This also does not prevent heating the reaction mixture at the beginning of the reaction in order to set the

starting reaction temperature. In other words, this term"adiabatic"means that there is no attempt to proceed according to an isothermal reaction.

In a further embodiment, the perforated plate reactor comprises a series of perforated plates regularly located inside the reactor.

In yet a further embodiment, the fractional free area of each of the perforated plates of the reactor is in the range from 5 to 70, preferably 15 to 35, and most preferably is about 25%.

According to one embodiment, the reaction is carried out under pulsed conditions.

According to this embodiment, the pulses imposed on the heated reactants mixture have a frequency of 0.1 to 10, preferably 0.5 to 2 Hz.

According to a further embodiment, the pulses imposed on the heated reactants mixture have superficial amplitude over the sectional cross area of the body of the reactor of 0.5 to 50, preferably 1 to 15 mm, most preferably 2 to 10 mm.

In one embodiment, the pulses are imposed on said heated reaction mixture while it is introduced into said perforated plate reactor.

In a further embodiment, the pulses are typically imposed on said heated reaction mixture by moving the perforated plates of said reactor and or the liquid.

According to one aspect of the invention, in step a), the heated mixture is obtained by mixing the first reactant with the second reactant and then heating the mixture.

The inlet temperature can be achieved by heating or cooling either in line or off line such as in the feed storage tank.

According to a further aspect, in step a), the heated mixture is obtained by mixing the first reactant with the preheated second reactant.

According to yet a further aspect, in step a), the heated mixture is obtained by mixing the first reactant with the preheated second reactant and then heating the obtained mixture.

According to yet a further aspect, in step a), the heated mixture is obtained by mixing the first preheated reactant with the preheated second reactant and then heating the obtained mixture.

In a further embodiment, the process further comprises a step of cooling the reaction product.

In yet a further embodiment, the process further comprises a step of filtering the (cooled) reaction product.

According to one aspect of the invention, the heated mixture entering the reactor typically has a viscosity in the range from 0.15 cP to 2000 cP, preferably from 2 cP to 1000 cP, while the mixture exiting the reactor has a viscosity in the range from 0.2 cP to 10,000 cP, preferably from 0.8 cP to 2000 cP, especially from 0.8 cP to 400 cP (cP is mPa. s).

In yet a further embodiment, the process further comprises a step of recirculating the obtained product to the feed inlet zone.

The invention also resides in a unit for the continuous preparation of products of a polymer (s) comprising (a) inlet means for the reactant (s); (b) heating means for heating at least one of the reactants; (c) mixing means for mixing the reactants; (d) an atmospheric plug-flow like reactor; and (e) a buffer vessel.

The invention finally also resides in a unit for the continuous preparation, preferably adiabatic of a compound (s) or a polymer (s) comprising: (a) inlet means for the reactant (s) (1, 2,2a, 5); (b) heating means (6,9) for heating at least one of the reactants; (c) mixing means (4) for mixing the reactants; (d) a perforated plate reactor (10); and (e) pulse generating means (11) for imposing pulses on the content of the perforated plate reactor (10).

The invention also resides in a unit for the continuous preparation of an isocyanate- containing polyurethane prepolymer (s), comprising: (a) inlet means for at least one isocyanate reactive compound and at least one isocyanate; (b) heating means for heating at least one of the reactants; (c) mixing means for mixing the reactants; (d) an atmospheric plug-flow like reactor; and (e) a buffer vessel.

The invention finally also resides in a unit for the continuous preparation, preferably adiabatic, of an isocyanate-containing polyurethane prepolymer (s), comprising:

(a) inlet means for at least one isocyanate reactive compound and at least one isocyanate (1, 2,2a, 5); (b) heating means (6,9) for heating at least one of the reactants; (c) mixing means (4) for mixing the reactants; (d) an perforated plate reactor (10); and (e) pulse generating means (11) for imposing pulses on the content of the perforated plate reactor (10).

In one embodiment of the invention, the plug-flow like reactor is a perforated plate reactor.

In another embodiment of the invention, the unit further comprises heating or cooling means for heating or cooling the reaction product either within the reactor or after exiting from the buffer vessel.

In yet a further embodiment, the unit also comprises filtering means, located after the reactor, notably after said cooling means, for filtering the (cooled) reaction product.

According to one embodiment, the unit further comprises shutdown recirculation means for (full) recirculating the (cooled) reaction product through the heating, mixing (and pulse) generating means, the (perforated plate) reactor, optionally the buffer and optionally the cooling means, at the end of the production run.

Other objects, features and advantages will become more apparent after referring to the following specification and accompanying drawings in which: Figure 1 is a block flow sheet for the process of the invention; Figure 2 is a schematic vertical cross-section along B-B of the perforated plate reactor of the unit of the invention; and Figure 3 is a schematic horizontal cross-section along A-A of the perforated plate reactor of the unit of the invention.

The process and the unit of the invention are particularly suitable for mildly exothermic, neutral or very mildly endothermic addition reactions, which have moderately high viscosity reactants and reaction products.

By mildly exothermic is herein intended to designate those reactions which generate heat up to a point where the temperature is below the temperature at which some products would begin degrading (e. g. hazing, yellowing, etc.). In general, temperature increase can be up to 50°C (or even more), while generally it is up to 20°C.

By mildly endothermic is herein intended to designate those reactions which consume heat to an extent that does not prevent these reactions. The temperature drop is usually up to 20°C, preferably up to 10°C.

By moderate high viscosity is herein intended, in general, from 20 to 2000 cP (mPa. s).

Herinafter as an example of the process of the continuous preparation of moderate to high viscosity reaction products, an organic polyisocyanate is used.

Referring to Figure 1, a first reactant, which is an organic polyisocyanate composition is introduced through line 1, pump 2 and line 3 to a feeds mixer 4 where it is mixed with an isocyanate reactive compound introduced through line 5, pump 2a and preheated by preheater 6. The feeds mixer can be, e. g., a simple mixing valve or a dynamic mixer or a static mixer, the latter one being preferred.

The pumps operating the system are preferably of the diaphragm type. This allows to handle compositions that may contain some small solids, such as MDI at low temperature; this is an advantage over classical gear pumps. The diaphragm pumps are thus robust. They are also suited for transportation. The pumps are also preferably of the ganged type, i. e. one drive shaft (one motor) powers several pumps.

The pumps are also preferably connected directly to the feed. This avoids to have recourse to the classical burdensome circulation loop.

The conduits, especially of the isocyanate side, are preferably heated, e. g. by an electrical tracing on the feed lines.

The heaters used in the instant unit can be of any classical type. Preferably they are of the finned type, i. e. a cylindrical heater in a conduit has lateral fins on its central element.

The mixer is placed preferably immediately after the feed streams meet, e. g. just before the heater. It can also be before the entry of the reactor (especially when no heating is foreseen before the reactor). This mixer can be of any type. Preferably, it is a static mixer, of the type with a packing in it.

The organic polyisocyanate composition may be selected from the group consisting of aliphatic, cycloaliphatic and araliphatic polyisocyanates, especially diisocyanates, like hexamethylene diisocyanate, isophorone diisocyanate, cyclohexane-1, 4-diisocyanate, 4,4'- dicyclohexylmethane diisocyanate and m-and p-tetramethylxylylene diisocyanate, and in particular aromatic polyisocyanates lilce tolylene diisocyanates (TDI), phenylene

diisocyanates and most preferably methylene diphenyl isocyanates having an isocyanate functionality of at least two. Methylene diphenyl isocyanates (MDI) are preferred. The methylene diphenyl isocyanates (MDI) may be selected from pure 4,4'-MDI, isomeric mixtures of 4,4'-MDI and 2,4'-MDI and less than 10 % by weight of 2,2'-MDI, crude and polymeric MDI having isocyanate functionalities above 2, and modified variants thereof containing carbodiimide, uretonimine, isocyanurate, urethane, aliphanate, urea or biuret groups. Most preferred methylene diphenyl isocyanates are pure 4,4'-MDI, isomeric mixtures with 2,4'-MDI optionally containing up to 50 % by weight of polymeric MDI and uretonimine and/or carbodiimide modified MDI. Mixtures of methylene diphenyl isocyanates with in particular up to 25 % by weight to other polyisocyanates mentioned above may be used if desired.

The isocyanate reactive compound is one that comprises a hydrogen atom that is able to react with the isocyanate, e. g. hydroxy or amine hydrogen.

The isocyanate reactive compound is preferably a polyol, which may be selected from the group consisting of polyether polyols, polyester polyols, polyesteramides polyols, polythioether polyols, polycarbonate polyols, polyacetal polyols, polyolefin polyols, and the like. A preferred polyol is a polyether polyol, especially those comprised of propylene oxide PO and/or ethylene oxide EO (tipped or random).

Other components widely used in the field can also be fed to the reactor. The skilled man will know the nature and amount of the isocyanate and isocyanate reactive compound, as well as these classical components. EP-A-480588, the content thereof being incorporated herewith by reference, comprises a description of many of these components.

Additives, if needed, are preferably introduced through line 7 and pump 8 into feeds mixer 4.

The mixture exiting from feeds mixer 4 is then heated in reactants heater 9 to a temperature in the range from 45 to more than 200°C and preferably from 60 to 130°C.

The heated mixture flows to the plug-flow like reactor 10 via unit 11 (pulsation unit) where it reacts.

The skilled man, knowing the isocyanate content and functionality and molecular weight of the isocyanate reactive compound, will have no difficulty in determining the relative amounts of components to be delivered to the reactor, in view of the desired NCO content of the prepolymer. A typical NCO content is e. g. from 2 to 31%, preferably from 5 to 25%, by weight.

Typically, the residence time is comprised between 5 min and 5 h, preferably between 10 and 90 min, most preferably between 15 and 45 min.

The time reduction is a great advantage over classical batch reactors. By way of example, the following is a side-by-side comparison (values given for the invention are exemplary only). The time is expressed in minutes. Step Prior art Invention Filling the reactor with >200- Isocyanate Warm to reaction temperature 30 2 Polyoladdition >100- Complete the reaction 90 15-45 Cool to storage temperature >150 10 The gain over the prior art is thus of more than 8 hours.

The plug-flow like reactor is exemplified here as a perforated plate reactor 10. The following description is given with respect to a perforated plate reactor.

Preferably also, the reactor is fitted with a. pulse unit 11 which gives an oscillating motion to the reaction mixture in addition its steady upwards motion. This pulse unit may comprise e. g. a pump of the diaphragm type (but without the non-return valves).

The pulses imposed on the heated reactants mixture have a frequency from 0.1 to 10 and preferably from 0.5 to 2 Hz.

The amplitude of the pulses is in the range from 0.5 to 50, preferably 1 to 15 mm, more preferably 2 to 10 mm. Amplitudes are given peak-to-peak.

The reaction product exiting from the top of perforated plate reactor 10 flows then to a buffer vessel 12. The function of the buffer vessel 12 is to provide a high flexibility to the unit of the invention. Firstly, it ensures a smoother product outlet flow. Secondly, it enables the reactor 10 to overflow, rather than be hard piped into the outlet system; hence, the reactor can be an atmospheric rather than a pressure vessel. (By atmospheric is also meant the presence of a nitrogen blanket over the reactor; the associated pressure is however negligible). This reduces the weight, the cost and the complexity of the unit, since there could be no need to fit with a complex overpressure protection system. Thirdly, the buffer vessel acts as a buffer when there is a process upset downstream of the unit, which

gives the operator time to solve the problem before the unit has to shut down. This reduces product quality risks from such upsets.

When a perforated plate reactor is used, especially under pulse conditions, the buffer vessel can be avoided (while it is preferably present especially under atmospheric conditions). A process run under a pressure higher than atmospheric pressure may also be run without the buffer vessel, even when there is no pulse conditions (although the pulse regime is preferred).

The product exiting from buffer vessel 12 is cooled by product cooler 13 (such as a spiral heat exchanger) and filtered by product filter 14.

A shutdown recirculation line 15 is provided between product cooler 13 and product filter 14. It links the exit of product cooler 13 to the inlet of feeds mixer 4. This feature minimizes the waste due to production of so-called"off-spec"material (i. e. material that is outside the desired ranges of parameters) at the end of the production run, since the content of the plant is recirculated through the equipment until the reaction is finished. Such a recirculation also enables the plant to be cooled down before emptying the plant, reducing the risk that remaining prepolymer builds up in the equipment, which would increase maintenance or cross-contamination risks.

When recirculating the plant at the end of a production run, the buffer vessel 12 acts as a reservoir. As fluid contracts when cooled, this buffer capacity of fluid aids in recirculating and cooling the fluid.

Referring now to Figure 2, it can be seen that the perforated plate reactor 10 is an elongated vertical vessel, which is in the form of a cylindrical column. It contains a series of stationary perforated plates disposed in a substantially parallel way and at substantially regular intervals along the longitudinal axis X of the column.

A perforated plate 16 is most visible on Figure 3. It comprises several substantially identical and circular perforations 17, which are regularly located in the plate 16. The number and diameter of the perforations 17 are such that the fractional free area of the plate, i. e. the sum of the areas of all perforations 17, is in general at least 5, especially in the range from 10 to 50, preferably 15 to 35, and most preferably is about 25%.

The space between two successive plates is from 10 to 500 mm, preferably from 50 to 200 mm, most preferably from 100 to 150 mm. The dimensions of the reactor can be the following: inner diameter is from 50 to 2000 mm, preferably from 100 to 500 mm; height is from 0.5 to 10 m, preferably from 2 to 6 m. The number of plates can thus be determined

by dividing the height by the spacing of the plates. The diameter of the holes in the perforated plates may vary; it is e. g. from 4 to 12 mm, preferably from 6 to 10 mm.

The perforated plates 16 need not be fixed to the internal wall of reactor 10. The plates can also be fixed to an inner axle which is itself fixed to the reactor.

In an alternate embodiment (not represented), the perforated plates can be mounted on a shaft inside the reactor 10, and the shaft is given an oscillating motion by the pulse unit 11. Thus the content of the reactor 10 also has, besides its steady upward motion, an effectively oscillating motion caused by the motion of the plates 16.

In yet an alternate embodiment, the perforated plates 16 are replaced by plates with slots, expanded metal, other baffle arrangements with free areas arranged sufficiently close together to generate cross-mixing or jet when a pulse is imposed. The plates are generally perpendicular to the axis of the reactor; inclined plates are also envisaged, e. g. to improve the free-draining properties.

The plug-flow like reactor 10 (preferably a perforated plate reactor), especially in adjunction with the pulse unit 11, brings several advantages: it enables longer reaction times with a narrow residence time distribution, hence approaching batch behavior for elements of reaction mixture passing through the reactor. The flow of the reaction mixture nearly achieves a plug-flow behavior. In addition, this is achieved with no rotating parts in contact with the process fluid, which simplifies design, improves reliability and reduces the weight and maintenance costs of the reactor. Further, the reactor also drains out well and therefore minimizes waste between products.

The pulse unit moderates the start-up temperatures, since the mass of the pulse- generating unit on the inlet to the reactor acts as a heat sink and helps achieve accurate temperature control at start-up. This also minimizes the production of"off-spec"material.

Another advantage provided by the use of the plug-flow like reactor (preferably perforated plate reactor) is that it enables the reaction to be carried out in conditions closed to adiabatic conditions, which simplifies the design of the unit, no accurate temperature control of the reactor being necessary, which in turn allows to vary the diameter of the reactor.

A further unique feature of the unit of the invention is that it can be constructed as a packaged plant. The unit can then be transported in standard transport containers and it is robust for transportation to any location where the manufacturing is to take place. It does not require complex systems at the manufacturing site to support its operation. For

example, it can take feeds from drums, feed tanks, feed vessels, etc. It can be operated by a minimum number of people, who can also undertake common maintenance tasks on the unit as well, since these are simplified.

The robust compact pumping, the preheating, the plug-flow like reactor operated under atmospheric pressure, the perforated plate reactor, the buffer vessel, the static mixer, fast reaction time and other items depicted above, are embodiments which can be used alone or in combination on a given unit (such as a traditional unit, e. g. a batch unit).

Individually, as well as together, these elements allow to obtain such a mobile unit.

The unit requires only standard site services found at many warehouse sites, i. e. electricity, optionally cooling water, compressed air (a nitrogen bottled supply is appropriate). The reaction product can be fed to a drum filling system, road tankers or bulk storage tanks.

The invention thus provides a mobile plant unit that can be operated virtually anywhere in the world.

The unit is thus unique in its ability to be easily transportable while providing high quality products, over a wide range of viscosities.

Further, as such a reactor operates at steady liquid levels, the amount of vapors displaced out during operations is much reduced when compared to a batch operation where the plant must be regularly emptied and filled, displacing vapors each batch.

Another advantage is that the equipment is also smaller, hence contains fewer vapors and displaced vapors can be very thoroughly treated to remove undesirable vapors, for example by adsorption on activated carbon.. Measures have actually shown that discharges were extremely low. This reinforces the ability of the unit to be transportable and used globally as it minimizes environmental impact.

The unit of the invention is preferably a mobile unit. It has been hitherto designed as a stand-alone unit, but it can also interface into a more complex manufacturing chain.

While a specific example is disclosed with respect to a polyurethane prepolymer (s), the invention can also be applied to other reactions characterized by moderately viscous fluids with moderate long reaction times. Especially those reactions where high conversion are required i. e. polymerisation reactions and/or biological reactions, are preferred.